制作临时的样本#

• Cut thin sections of the material to be examined 切片
• Stain 染色
• Mount the specimen on a clean glass slide 放到载玻片上
• Lower the coverslip over the specimen (to prevent the specimen drying out) 覆盖

光学显微镜和电子显微镜的对比

There are two types of electron microscope 有两种电子显微镜:

• Transmission Electron Microscope 穿透电子显微镜: 2D image, can see interior details 可以呈现出2D的图像，并且可以看到细胞内的结构。

  

• Scanning Electron Microscope 扫描电子显微镜: 3D image, can only see surface structures 可以看到细胞表面的3D结构。

  

Magnification and Resolution#

Magnification 放大倍数: the number of times larger an image is compared to the object’s size. 图像比实物大多少倍。

Resolution 分辨率: The ability to distinguish between two separate points. 能分辨出两个很近的物体的能力。

The maximum resolution can be calculated from the wave length of radiation used to view the specimen.

最大的分辨率（分辨率的极限）能通过观察光的波长计算出来，例如使用波长为500nm的光观察一个物体，那么这个物体的最大分辨率为250nm。

本章重点内容#

Mitochondria and chloroplasts

• Surrounded by double membrane 双膜结构
• Have small, circular DNA 有环状DNA
• Can synthesise proteins 可以合成蛋白质
• Have 70S ribosomes

Centrioles只在动物细胞中有。

Cell Surface Membrane / plasma Membrane 细胞膜#

• Partially permeable / semi-permeable 半透的

• Control the exchange of materials between the cell and its environment

  控制进出细胞的物质

Cell Wall 细胞壁#

• Fully permeable 全透的 - All the substances can pass through it 全部物质都可以通过

• Protect and prevent the cell from bursting when water enters by osmosis

  防止细胞内水分太多时破裂

• Resist turgor pressure 抵抗膨压

• Cell wall is relatively rigid and gives the cell a definite shape

  细胞壁相对而言比较坚固，可以给细胞提供一个稳定的形状

• Cell wall of plant cells is made of cellulose

  植物的细胞壁由纤维素*(C₆H₁₀O₅)n*构成

Plasmodesma (plural: plasmodesmata) 胞间连丝#

• A pore-like structure 孔状的结构

• Plant cells are linked to neighbouring cells by plasmodesmata

  相邻的植物细胞通过胞间连丝相连

• Contain fine strands of cytoplasm, allowing the controlled passage of materials from one cell to the other

  含有细小的细胞质链，可以控制物质在细胞间的传输

Cytoplasm 细胞质#

The contents of a cell inside the cell surface membrane, excluding the nucleus. Cytoplasm contains organelles, e.g. ribosomes or mitochondrion

细胞质指的是细胞内除了细胞核之外的所有物质以及细胞器，像是mitochondrion和robosome之类的细胞器就被包括在细胞质里。

Cytosol is the solution of the cytoplasm, in which the organelles are suspended.

细胞溶质是细胞质中的溶液，其中的细胞器都属于悬浮的状态。

如果把一个细胞想象成一杯珍珠奶茶，那么…

• 奶茶杯 = Cell Surface Membrane
• 奶茶以及加料 = Cytoplasm
• 奶茶本身 = Cytosol
• 加料 = Organelles

Nucleus (plural: nuclei) 细胞核#

• Contain the genetic material (DNA, chromatin and chromosome)

  包含遗传物质：DNA、染色质和染色体

• DNA contains the instructions / genes that control the activities of the cell, e.g. control protein synthesis

  DNA含有细胞活动的指令（或基因），可以控制包括蛋白质合成等一系列活动

• Contains nucleolus (plural: nucleoli), one or more may be present in one nucleus

  包含一个或多个核仁

• Nucleolus is the place of making ribosome 核仁负责生产核糖体

When a cell is not dividing/not during nuclear division, the material containing DNA is called chromatin. Chromatin is loosely coiled and thread-like. It’s long, thin and in a loose form.

when a cell is dividing/during nuclear division, the material containing DNA is called chromosomes. Chromosomes are condensed from chromatin. They are short and thick and in a condensed form.

通常来说一个细胞只会有一个细胞核，但是每个细胞的情况不一样，例如：

• 血红细胞为了运输氧气没有细胞核
• 肝脏里的细胞有多个细胞核
• Skeletal muscle cells有多个细胞核
• 部分真核细胞有多个细胞核

The nucleus is surrounded by two membranes called the nuclear envelope, the nuclear envelope has many small pores called nuclear pores.

细胞核被一种叫做核被膜的“双膜”结构所包围，核被膜上的小孔成为核孔。

Mitochondrion (plural: mitochondria) 线粒体#

• surrounded by two membranes (a mitochondrial envelope).

  被线粒体包膜（一种双膜结构）包围

• The inner membrane is folded to form cristae (singular: crista) to increase surface area

  内层的膜会折叠成cristae（一种凸起的结构），用于增加表面积

• the interior of the mitochondrion is called the matrix

  线粒体中的内部区域被称为matrix，是主要进行有氧呼吸的场所

• The space between the two membranes is called the intermembrane space

  两层膜之间的空间被称为膜间空间

Cells with a high demand for energy, such as liver and muscle cells, contain large numbers of mitochondria.

在需要大量能量的细胞（例如肝脏细胞和肌肉细胞）中含有大量的线粒体。

Mitochondria carry out aerobic respiration to release energy/produce ATP.

线粒体会通过有氧呼吸释放能量/产生ATP。

注意：能量不可以被产生，只能转化。Energy can only be released, not produced

ATP是能量的载体，能量在ATP中被储存为化学势能（Chemical Potential Energy）

ATP produced in mitochondria is used for energy consuming activities in cells, such as muscle contraction, protein synthesis, exocytosis and active transport.

在线粒体中产生的ATP会在肌肉收缩、蛋白质合成、胞吐或主动运输等需要能量的细胞活动中消耗掉。

ATP分子遇到水分子时会发生水解反应（Hydrolysis）：To break bonds between phosphate groups and release energy. 当分子间的化学键断裂时会释放能量。

在图片中线粒体有不同的形状，有的是圆形的，有的是胶囊形的，这是因为两个原因：

1. Mitochondria can change shape. 线粒体可以改变形状
2. They are cut at different angles. Some are cut through transverse section and some through longitudinal section. 展示的角度不一样，有的展示的是横切面，有的则是纵切面。

Many mitochondria are sausage-like. But they can change shape and divide.

大多数线粒体都是胶囊形（或者说“香肠形”的），但是它们仍然可以变换形状

Chloroplast 叶绿体#

Inside the chloroplast 叶绿体的内部

• Chloroplast envelope 叶绿体包膜
• 70S ribosome 核糖体
• Circular DNA in stroma 基质中的环状DNA
• grana made of thylakoid membranes 由类囊体膜构成的颗粒
• Starch grains 淀粉粒

Thylakoids 类囊体 - fuild-filled sacs

Chloroplasts carry out photosynthesis to produce glucose / carbohydrate. 叶绿素会通过光合作用生产葡萄糖 / 碳水化合物。

Not all plant cells have chloroplast. For example, plant cells in the root have no chloroplast.

并不是所有的植物细胞都有叶绿体，比如不需要光合作用的根毛细胞就没有。

ATP produced in chloroplast is used in photosynthesis.

ATP也会在叶绿体中产生，但是这里产生的ATP会被主要用于光合作用。

Vacuole 液泡#

Animal cells may possess small, temporary vacuoles, or called vesicle

动物细胞中也有类似于液泡的结构，被称为囊泡。只不过囊泡不是永久存在的，而且相对于液泡来说非常小

• Surrounded by tonoplas 被液泡膜所覆盖

Functions:

• Support 里面的液体会撑起整个细胞
• Lysosomal activity - Plant vacuoles may contain hydrolases and act as lysosomes. 提供和lysosome类似的功能
• Secondary metabolites 有一些生长相关，但比较次要的物质可以储存在液泡中
• Food reserves 溶解了的果糖和矿物离子会储存在液泡中
• Waste products 暂时储存一些没有用的东西
• Growth in size 水通过渗透作用进入细胞会增大细胞的体积

The solution in the vacuole is called cell sap, it contains:

• pigments
• enzymes
• sugars (glucose)
• minieral ions
• oxygen
• carbon dioxide
• …

液泡中的溶液被称为细胞液，它包含了：

• 色素
• 酶
• 糖（葡萄糖）
• 矿物离子
• 氧气
• 二氧化碳
• ……

Vacuoles help to regulate the osmotic properties fo cells (the flow of water inwards and outwards by osmosis) and support the stems of the plants.

液泡中的液体会平衡细胞的“渗透”作用，并且里面的液体会撑起细胞，乃至于让植物的茎呈现挺立的状态。

The pigments colour the petals of certain flowers and the parts of some vegetables.

液泡中的色素会赋予某些花瓣和植物丰富的颜色。

Endoplasmic Reticulum 内质网#

The endoplasmic reticulum consists of flattened sacs of membrane called cisternae.

内质网由称为囊泡的扁平膜囊组成。

Use the full name when answer is endoplasmic reticulum and do not say ‘ER’

在考试的时候尽量不要使用ER这个简写，而是使用全称Endoplasmic reticulum

There are two types of endoplasmic reticulum:

• Rough ER - covered with many ribosomes
• Smooth ER - lacks ribosomes

内质网分为两种：

• 粗面内质网 - 覆盖着许多核糖体
• 光滑内质网 - 缺乏核糖体

The membrane of rough endoplasmic reticulum is continuous with the outer membrane of the nuclear envelope, and covered by ribosomes. It’s responsible for making protein, transport protein or modify proteins (deal with proteins).

粗面内质网和细胞核的外层相连，并且上面有许多核糖体（核糖体的存在使这种内质网表面变得粗糙，这也是它名字的由来）。它负责生产蛋白质、运输蛋白质或者操作蛋白质（总之就是负责和蛋白质相关的一些事物）。

Smooth ER lacks ribosomes, the membrane of smooth ER is not continuous with the outer membrane of the nuclear envelope. It makes lipids and steroids, and it’s also the major storage site for calcium ions.

光滑内质网几乎没有核糖体，并且它的膜并不与细胞核相连。它负责生产脂质和类固醇，它还是储存钙离子的主要地方。

Golgi body / apparatus 高尔基体#

• Can be seen under a light microscope 可以在光学显微镜下观察到

Golgi body is made of cisternae, and constantly being formed at one end from vesicles which bud off from the rough ER, and are broken down again at the other end to form Golgi vesicles

高尔基体由囊泡构成，会在粗面内质网的一端生成，并且在另一端被分解成高尔基小泡

在Goblet cell中含有大量的Golgi bodies，Goblet cell需要产生大量的分泌物

Functions:

• modify proteins from the rough ER (sugars are added to proteins to make glycoproteins)

  进一步操作来自rough ER的蛋白质

• Make glycolipid

• package and transprot proteins in Golgi vesicles to other parts of the cell or for secretion/exocytosis

  把蛋白质运输到细胞的其他地方

• some Golgi vesicles are used to make lysosome (remain in Golgi body)

  部分高尔基小泡会被用于制作溶酶体

Golgi vesicles can…

• carry the protein to the cell membrane for exocytosis 运输功能
• remain in the cell as lysosome 作为溶酶体留在细胞中

mRNA在细胞核中产生，然后移动到Rough ER表面的核糖体上，并产生蛋白质。vesicle from rough ER会携带蛋白质到Golgi body。Proteins are modify in Golgi body and Golgi vesicle carries the modified proteins to the cell surface membrane for exocytosis.

• A: protein made on the ribosome is moving into the rough ER

  蛋白质在核糖体中产生，并移动到粗面内质网中

• B: rough ER buds of small vesicles, vesicles fuse to form the Golgi apparatus, therefore protein moves into Golgi apparatus. Protein may be modified / processed inside Golgi apparatus

  蛋白质通过运输小泡（Transport Vesicle）从粗面内质网移动到高尔基体，并在高尔基体中进一步加工

• C: Golgi apparatus buds off Golgi vesicles

  高尔基体分离出高尔基小泡

• D: Golgi vesicles travel to cell surface membrane, Golgi vesicle fuses with enzyme leaves cell or responsible for exocytosis

  蛋白质通过高尔基小泡移动到细胞膜上

• E: ribosome or mRNA leaves the nucleus

  核糖体或者mRNA来自细胞核

Ribosome 核糖体#

• Made of rRNA and proteins
• Have a large subunit and a small subunit

• Function: make proteins
• 25nm in diameter

70S ribosome has a smaller size, it can be found in mitochondria, chlroplasts and typical prokaryotic cells.

80S ribosome has a larger size, it can be found in cytoplasm and rough ER.

• Ribosomes are found in all types of cells

Lysosome 溶酶体#

• Simple sacs surrounded by a single membrane

• Contains digestive / hydrolytic enzymes, these enzymens must be kept seperate from the rest fo the cell to prevent damage.

  溶酶体中包含消化酶/水解酶，这些酶必须和细胞的其他部分隔绝，不然细胞的其他部分有可能被溶酶体中的消化酶分解

  Digestive Enzymes:

  • Amylase
  • Maltase
  • Lactase
  • Lipase
  • Sucrase
  • Proteases

  hydrolases: hydrolytic enzymes,which carry out hydrolysis reactions

  水解酶：进行水解反应的酶

• Functions: breakdown / digest unwanted substances and structures such as old organelles or even whole cells.

  分解不需要的物质，例如老旧的细胞器或者说一整个不用了的细胞

• The contents of lysosomes are acidic, pH 4 - 5 compared with 6.5 - 7.0 in the surrounding cytoplasm 溶酶体是酸性的，但属于是弱酸（pH在4到5之间）

Centrosome 中心体#

There are two centrioles (not one as it appears under the light microscope) and they are at right angles to each other to form a centrosome.

中心体由两个中心粒构成，两个中心粒互相成直角排列。

It can only be found in animal cells but not plant cells 中心体只存在于动物细胞中，植物细胞里没有

Centrosome acts as microtubule organising centres (MTOCs) that make the microtubule spindle during nuclear division.

中心体作为微管组织中心（MTOCs），在细胞核分裂过程中形成微管纺锤体。(会在细胞分裂的过程中把分裂出去的部分推开)

Microtubules 微管#

Microtubules are made of a protein called tubulin. Tubulin has two forms, α-tubulin (alpha-tubulin) and β-tubulin (beta-tubulin). 微管由一种叫做微管蛋白的蛋白质构成，微管蛋白有两种形态，α和β。

1. α- and β-tubulin molecules combine to form dimers (double molecules) α-和β-微管蛋白分子结合形成“二聚体”（一种双分子结构）

2. These dimers are then joined end to end to form long ‘protofilaments’. 这些“二聚体”随后端对端连接形成长的“原丝”。

   This is an example of polymerisation, the process by which giant molecules are made by joining together many identical subunits.

   这是一个“聚合反应”的例子，聚合反应就是许多个相同的小分子“汇聚到”一起然后形成一个大分子。

3. Thirteen protofilaments line up alongside each other in a ring to form a cylinder with a hollow centre. 十三个原纤维平行排列成环，形成一个中间空心的圆柱。

4. This cylinder is the microtubule. 这个圆柱体就是微管

Microtubules make up the cytoskeleton, which helps to determine cell shape. 微管构成细胞骨架，帮助决定细胞形状。

This diagram shows the arrangement of microtubules in two cells. The microtubules are coloured yellow.

微管相当于细胞的血管外加骨架，它对于细胞同时起到了物理上支撑的作用和功能上运输的作用。上图中的微管被标记成了黄色。

Functions of the microtubules 微管的功能：

• Support the cell structure 支撑细胞结构

• Transport secretory vesicles and other organelles and cell components 运输囊泡和其他细胞器

• Seperate the chromatids or chromosomes during the cell division 在细胞分裂期间分开染色单体或染色体

• Microtubules form part of the structure of centrioles 形成中心粒的部分结构

  Centriole’s structure:

  

  所以说：Tubulines → dimers → protofilaments → microtulube → centriole → centrosome

• Microtubules form an essential part of the mechanism involved in the beating movements of cilia and flagella. 微管是纤毛和鞭毛运动机制的重要组成部分。

During nuclear division, microtubule spindle (纺锤体) is used for the separation of chromosomes.

这是课本第五单元（第128页）的内容，现在可以不用细究，但是需要知道microtubules有这个功能。

Flagellum (plural: flagella) and cilium (plural: cilia) 鞭毛和纤毛#

这两个东西本质上是一样的（identical structure），但是由于这个东西被命名的时候还不知道它们内部的结构，所以被认为是两个不一样的东西。通常来说，它们一般是这样被区分的：

• Flagella are long and found usually one or two per cell

  鞭毛：比较长，而且一个细胞只有少量的鞭毛

• Cilia are short and often numerous

  纤毛：短而密

对于大多数真核细胞来说，它们的鞭毛有着相对来说比较复杂的“9+2”结构——即外圈有9的单元（每个单元由两个微管组成，组成一个叫microtubule doublet的东西，简称MTD），内圈有两个单元（两个独立的微管）：

在正常情况下，一个microtubule是由13个protofilaments组成的，在组成外圈的9个doublet microtubules使会有A和B之分，A microtubule有13个protofilaments，而B microtubule只有10个protofilaments。

不过中心的两个microtubules是常规的（有13个protofilaments）。

Functions:

• locomotion 移动

• Cilia cells in lungs maintain a flow of mucus which removes debris such as dust and bacteria from the respiratory tract. 通过摆动纤毛使肺中的黏液往外移动。

  注意：不要把cilia和microvilli搞混了，二者虽然都是细胞器、也都是向外衍生出去的一部分，但是cilia是类似于“手臂”之类的东西，为的是移动之类的目的；而microvilli是用来增加细胞表面积来吸收更多物质的，并不能用来移动。

Microvillus (plural: microvilli) 微绒毛#

Found in epithelial cells of small intestine, which can greatly increase the surface area of the cell surface area. 位于小肠的表层细胞上，它能极大的增加细胞的表面积。

Functions：

• For reabsorption in the proximal convoluted tubules of the kidney

  在肾脏的近曲小管中进行再吸收

• For absorption of digested food into cells lining the gut

  将消化后的食物吸收进入肠道细胞

Cell surface membrane 细胞膜#

It has three layers (trilaminar structure) - two dark (heavily stained) layers surrounding a narrow, pale interior:

The membrane is partially permeable and controls exchange between the cell and its environment. 这层膜是半透的，它能控制细胞和外界的物质交换。

关于细胞膜的更多信息会在第四章中讲到

重点信息#

• Unicellular 单细胞的

• Generally 15 μm (diameter) 直径大约在15微米

• No nucleus 没有细胞核

• Peptidoglycan / murein cell walls 肽聚糖细胞壁

• Circular (chromosomal) DNA 环状的DNA

• 70S ribosomes 核糖体（70S）

• No membrane-bound organelles (no mitochondria or chloroplasts)

  没有具有“膜”的细胞器

• Some have plasmids or flagellum 有的有质粒或者鞭毛

主要结构#

所有细菌细胞都有的结构：

• Murein cell wall 肽聚糖细胞壁
• Cell surface membrane 细胞膜
• Cytoplasm 细胞质
• Circular DNA 环状的DNA
• Ribosome 核糖体

有时候会包括的结构：

• Flagellum with simple structure 结构简单的鞭毛

• Capsule of slime layer for additional protection 药囊或黏液状的保护层

• infolding of cell surface membrane 向内部折叠的细胞膜

  may form a photosynthetic membrane of carry out nitrogen fixation

• plasmid 质粒

  several small circle of DNA 可能会有一些环状的DNA（质粒）

• Pili

  For attachment to other cells of surface; And involved in sexual reproduction.

  用于附着于其他细胞或其他表面；以及会参与有性繁殖。

Prokaryotes vs Eukaryotes#

Common structural features#

**Cell surface membrane:**essential because it forms a partially permeable barrier between the cell and its environment, regulating movement of materials into and out of the cell. This is necessary to maintain an environment inside the cell which is different from that outside the cell.

**Cytoplasm:**site of metabolic activity. It contains biochemicals in solution.

**Ribosomes:**sites of protein synthesis, an essential activity of all cells. (DNA controls cells by controlling which proteins are made.) Protein synthesis is a complex process involving the interaction of many molecules; the ribosome provides a site where this an happen in an organised way.

**DNA:**the genetic material. DNA contains the information which controls tha activities of the cell. It can replicate itself, enabling new cells to be formed.

Monomers and Polymers#

Monomer可以想象成一个个单独的积木，当这些相同的积木拼在一起时会组成Polymer。

或者说如果用cell、tissue和organ之间的关系来解释的话，就是：

• Elements → Cell organelles
• Monomers → Cells
• Polymer → Tissue

Monomer:

• Elements make up monomers
• Simplest repeating subunit of a polymer

Polymer:

• A marcomolecule (giant molecule) 大分子结构

• Made from similar / identical repeating subunits (monomers)

  同样的或相似的Monomers组合在一起时会形成Polymer

Monomers和Polymer之间可以通过化学反应互相转换：

• Monomers到Polymer称作Condensation

• Polymer到Monomers称作Hydrolysis

在Hydrolysis中，水分子会被添加到Polymer中每个Monomers之间的连接处，并且将它们分开；而在Condensation的过程中，每个Monomer相互连接时会排除一个水分子。

Chemical Structures#

Isomers are the molecules that have the same chemical formula but different molecular structures.

如果有两种分子的结构不同但组成它们的元素相同，它们被称为异构体。例如：

• glucose
• fructose
• galactose

它们的分子式都是C₆H₁₂O₆，但是它们结构不一样。

不同的有机分子会根据它们的**“特殊结构”**分到不同的类别中，这种特殊结构被成为functional group：

-OH: Hydroxyl group

-COOH: Carboxyl group

-NH2: Amino / amine group

Chemical bonds between different biological molecules:

• Di-/Polysaccharide: glycosidic bond 糖苷键
• Protein: peptide bond 肽键
• Lipid: ester bond 酯键
• DNA/RNA: phosphodiester bond 磷酸二酯键

Hydrogen Bond#

Covalent bond: The atoms involved share electrons. 当两个原子共享电子时，会形成“共价键”，下图中可以看到蓝色的电子和红色的电子同时存在于两个原子的轨道上。

当一个化合物中的电子分布不均匀时，会导致这个化合物的一部分带正电，或者一部分带负电。当电子集中在一端时，这一侧会产生负电（因为电子带负电荷）。

共价键会导致氧和氢之间的分子集中在一个地方：

Dipole 偶极: Used to describe the unequal distribution of charge 用于描述电子的不均匀分布。

• Electrons away from one atom, causing a partial positive charge - δ+ (delta plus)

• Electrons close from one atom, causing a partial negative charge - δ- (delta minus)

对于水分子来说，大部分都电子都集中在“靠上”的地方，导致那部分区域带有负电（左图），所以那一侧会被标记为δ-（右图）；对于氢原子的那一侧同理。

Hydrogen bond: Attraction between the negatively charged oxygen of one molecule and a positively charged hydrogen of another.

当一个水分子的δ+与另一个水分子的δ-相互吸引时，会形成“氢键”（一般在图中用虚线表示）。

Dipoles occur in many different molecules, particularly where there is an –OH, –C=O or –NH group having polar covalent bond. 由于共价键会形成于很多种分子中（例如包含–OH、–C=O或–NH的分子中），这些不均匀分布的电子会导致产生具有“极性”的。

• Dipoles → Polar → hydrophilic (water-loving 亲水的)

• Not Dipoles → Non-Polar → hydrophobic (water-hating 疏水的)

Charged ions like Cl-, Na+ and K+ are hydrophilic.

Glycosidic Bond (糖苷键)#

• Formed Between: Two monosaccharides
• Formed By: Condensation reaction
• Broken By: Hydrolysis reaction
• Found In: Di-/Polysaccharides (e.g., Maltose, Sucrose, Starch, Glycogen, Cellulose)
• Key Function / Notes: Links sugar monomers. 1-4 and 1-6 linkages create different structures (e.g., branching).

Ester Bond (酯键)#

• Formed Between: Glycerol & Fatty Acids
• Formed By: Condensation reaction
• Broken By: Hydrolysis reaction
• Found In: Triglycerides, Phospholipids
• Key Function / Notes: Forms the main structural link in lipids.

Peptide Bond (肽键)#

• Formed Between: Two amino acids
• Formed By: Condensation reaction
• Broken By: Hydrolysis reaction
• Found In: Proteins (Di-/Polypeptides)
• Key Function / Notes: The covalent backbone of all proteins. Forms between the amino group of one amino acid and the carboxyl group of another.

Hydrogen Bond (氢键)#

• Formed Between: A δ+ Hydrogen & a δ- Oxygen or Nitrogen
• Formed By: Weak electrostatic attraction
• Broken By: Heat, pH changes
• Found In: Water, Protein structures (Secondary, Tertiary, Quaternary), DNA, Cellulose
• Key Function / Notes: Crucial for stabilizing structures. Holds water molecules together, forms α-helices/β-sheets in proteins, and holds parallel cellulose molecules together.

Disulfide Bond (二硫键)#

• Formed Between: Two Cysteine amino acids (their R groups)
• Formed By: Oxidation (forms a strong covalent bond)
• Broken By: Reduction (using reducing agents)
• Found In: Protein (Tertiary, Quaternary structures)
• Key Function / Notes: A strong covalent bond that provides significant stability to the protein’s 3D shape.

Ionic Bond (离子键)#

• Formed Between: Positively & Negatively charged R groups
• Formed By: Electrostatic attraction
• Broken By: pH changes
• Found In: Protein (Tertiary, Quaternary structures)
• Key Function / Notes: Can be disrupted by changes in the surrounding pH.

Hydrophobic Interaction (疏水相互作用)#

• Formed Between: Non-polar (hydrophobic) R groups
• Formed By: Exclusion from water (this is an interaction, not a true bond)
• Broken By: Organic solvents, detergents
• Found In: Protein (Tertiary, Quaternary structures), Phospholipid Bilayer
• Key Function / Notes: Causes hydrophobic regions to cluster together inside proteins or in membranes.

Phosphodiester Bond (磷酸二酯键)#

• Formed Between: Nucleotides (Sugars & Phosphates)
• Formed By: Condensation reaction
• Broken By: Hydrolysis reaction
• Found In: DNA / RNA
• Key Function / Notes: Forms the sugar-phosphate backbone of nucleic acids.

Water as Solvent#

Water is polar , which can attracted to ions and polar molecules (include some carbohydrates and proteins). Chemicals like ions, polar molecules, gasses and waste products (e.g. ammonia and uera) dissolve in water.

It’s important for:

• transport
• removal of wastes
• secretions
• medium for enzymatic reactions

High Specific Heat Capacity#

Specific heat capacity is the amount of heat to required to raise the temperature of 1kg of water by 1°C.

比热容是“把1kg物质升高1°C”所需要的能量。

Large amount of energy is required to raise the temperature of water, due to the hydrogen bonds in water. So the water is more resistant to the changes in temperature.

The temperature within cells and within the bodies of organisims tends to be more constant than that of the air around them.

High Latent Heat of Vaporisation#

Latent Heat of Vaporisation is the measure of heat energy needed to vaporise a liquid, changing it from a liquid to gas. 将液体汽化所需的能量

H bonds have to be broken before molecules can escape as a gas.

This is biologically important because it means that living organisms can use evaporation as a cooling mechanism.

Cohesion and Adhesion#

Cohesion - tend to stick to each other

Adhesion - tend to stick to surfaces

It’s useful in the transportation of water in plant’s xylem, it creates transpirational pull inside the xylem vessel, a long unbroken column of water can move upward.

Monosaccharide#

单糖是只由一个糖分子构成的糖，它的特征有：

• Dissolve easily in water 非常容易在水中溶解
• Sweet 甜的
• Hydrogen and oxygen are present in ratio of 2:1 氢原子和氧原子之间的比例是二比一
• General formula:$(CH_2O)_n$

在Monosaccharide中，根据碳原子的数量可以分为更小的类别：

• 三个碳原子 3C：Trioses
• 五个碳原子 5C：Pentoses
• 六个碳原子 6C：Hexoses

其中Pentoses中具有代表性的糖有：Ribose和Deoxyribose

Hexoses中具有代表性的糖有：Glucose (C₆H₁₂O₆)、fructose和galactose

当一条糖分子足够长时会环成一个环，在展开时叫做Linear / Straight-chain structure，在头尾相连是被称为Ring Structure：

Isomers of glucose: same linear structure but different ring structure.

可以看到在Glucose环成Ring Structure时，OH的方向会不一样，所以会有α-glucose和β-glucose之分。

• α-glucose: OH is below the ring (DDUDU) - $\alpha \rightarrow B$
• β-glucose: OH is above the ring (UDUDU) - $\beta \rightarrow A$

从最右边的OH group开始，顺时针旋转时OH的方向可以总结成DDUDU或UDUDU

这个葡萄糖的结构并不要求会画，但是要求能分辨出一个结构是不是葡萄糖，或者是哪种葡萄糖

Linear structure and ring structure: If the chain is long enough, then it can transform into a ring structure, ring structure is more stable and more common. 如果一条分子链足够长的话，它会环成一个“环形”，环形的糖分子更加稳定，也更加常见。

单糖的功能：

• Used as a source of energy in respiration to release energy 用于在呼吸作用中释放能量
• Used to make the polysaccharides (starch, glycogen and cellulose) 用于构建更大的分子

Disaccharide (Double sugar)#

Defination of sugar: Soluble and taste sweet 可溶于水，并且是甜的。

所以说只有单糖和双糖能算是“糖”

由两个Monosaccharides通过glycosidic bond组成，最常见的有：

• Maltose (glucose + glucose) 麦芽糖

• Sucrose (glucose + fructose) 蔗糖

  Sucrose is the transport sugar in plants and the sugar commonly bought in shops.

  WARNING

  Sucrose不是reducing sugar，无法通过Benedict’s test，但是经过acid hydrolysis后会产生reducing sugar

• Lactose (glucose + galactose) 乳糖

Chemical formula for these disaccharide:

$$\ce{C12H22O11}$$

两个Monosaccharides可以通过condensation reaction组成Disaccharide：

Glycosidic bond: a C–O–C link between two sugar molecules, formed by a condensation reaction; it is a covalent bond 共价键.

$$\ce{α-glucose + β-glucose ->[condensation] sucrose + water}$$

当两个单糖通过condensation形成双糖时，glycosidic bond和水分子会产生。由于只有两个单糖，所以双糖并不算Polymer。

Starch 淀粉#

• Store of energy in plants (in chloroplast’s starch grain)

Starch is a mixture of two substances – amylose and amylopectin.

“1, 4 glycosidic bond” 表示一个glucose上第一个C和另一个glucose上第四个C相连：

Glycogen 糖原#

• Store of energy in animals (相当于动物的“淀粉”)

• Made from α-glucose

• 1,4 glycosidic bonds and 1,6 glycosidic bonds at branching points

• Even more branched than amylopectin

  比支链淀粉有更多分支

  

• Clumped together → forms granules

  聚集在一起 → 形成颗粒

• Abundant in liver and muscle cells (these to cells require large amount of energy supply)

  大量存在于肝脏和肌肉细胞中（这两种细胞需要很多能量）

Why does excess glucose need to be stored as starch and glycogen?#

Glucose:

soluble:

1. increase concentration in cell
2. water would enter
3. cell volume increase, animal cells may brust

Reactive - interferes with other reactions in cell.

Starch and glycogen:

• Inert - not reactive
• Insoluble - it does not diffuse easily out of cells, it does not tend to draw water into cells by osmosis
• Compact - more of them cab be stored in a small place
• Easy to transport

简单来说：

• 葡萄糖会溶解在水里，导致water potential发生改变，从而导致异常的osmosis发生（细胞可能会爆炸）。
• 葡萄糖太活跃了，容易和其他物质发生化学反应。

而淀粉和糖原：

• 是惰性的，不会轻易和别的物质发生反应。
• 不可溶的，所以不会扰乱正常的osmosis。
• 排列紧凑，可以很方便地储存和运输。

Cellulose 纤维素#

Used to make plant’s cell wall

• Made by β-glucose

  由 β-葡萄糖 构成

• linked by 1,4 glycosidic bonds

  通过 1,4 糖苷键连接

• 180° rotated between nearby glucose molecules

  相邻葡萄糖分子之间旋转 180°

• unbranched, linear structure

  无分支的线性结构

• Celluloses molecules lie parallel → microfibrils → fibres (arranged in a criss-cross manner)

  纤维素分子 平行排列 → 微纤维 → 纤维

• Cellulose fibres criss-cross to form cell wall.

  纤维素纤维纵横交错形成细胞壁

Iodine Test#

Used to test the presence of starch.

Colour change: Orange-brown → Bule Black

Benedict’s Test#

Reducing Sugar 还原糖 - The sugar can give out electrons (itself is oxidised)

氧化和还原是化学中的知识，目前来是其实只要明白Reducing Sugar可以让Benedict’s test变色就行了，具体的细节（应该）不用理解。

Colour of the solution: Blue → Brick Red.

Semi-quantitative Benedict’s test 半定量的本尼迪克测试:

1. setup standard solutions with known concentration of reducing sugar

   配置一系列已知浓度的溶液

2. use the standard as colour scale which can be used to compare with the unknow solution

   通过这些已知的溶液做出一个比色标尺，由浅到深（蓝色→绿色→黄色→橙色→砖红色）

3. use colorimeter 色度计 to measure the light absorbance 吸光度 by each solution

   可以通过色度计来准确地测量颜色（避免肉眼观察的偏差）

4. A graph of concentration against light absorbance can be plotted

   通过画图的方式来较为准确地估算浓度

Testing the Presence of Non-reducing Sugar#

Procedure:

1. Do the Benedict’s Test first, if the colour dosen’t change, so the reducing sugar is not present

   首先先对未知的溶液做Benedict’s Test，如果颜色没有改变，说明这个溶液里没有reducing sugar

2. Add acid to another sample in test tube and heat it in water bath (to hydrolyse non-reducing sugar into monosaccharides)

   将酸加入同样的溶液（但是没有做过Benedict’s Test），并在水浴锅中加热（用于水解非还原糖到单糖）

3. Neutralise the solution by adding an alkali (e.g. NaOH)

   中和第二步中产生的溶液

4. Carry out Benedict’s Test on the sample again

   对新产生的溶液重新做Benedict’s Test

Result 实验结果:

• Brick Red precipitate - Non-reducing sugar present 有非还原糖
• Remain blue colour  - Not a sugar 无非还原糖（因为还原糖和非还原糖都没有，所以这个样本中没有糖）

If the Benedict’s test shows the positive result, and then do the hydrolysis, neutralisation and Benedict’s test again, and the colour becomes even darker, we can conclude that this sample contains both reducing sugar and non-reducing sugar.

当最开始的Benedict’s Test呈现出显性的结果，并且经过“水解”和“中和”后再进行Benedict’s Test时颜色变深了，说明第二次Benedict’s Test时还原糖的浓度更高，或者说有额外的糖在“水解”和“中和”的反应中产生了，也就是说这个样本同时含有还原糖和非还原糖。

Fatty Acid#

There are two types of fatty acids:

• Saturated fatty acid 饱和脂肪酸 - No double bonds and triple bonds 没有双键和三键
• Unsaturated fatty acid 非饱和脂肪酸 - Has double bonds of triple bonds 含有双键或三键

饱和指的是所有中间的C都与4个其他原子相连（没有双键），如果有双键就是非饱和。

Animal lipids are often saturated and occur as fats, whereas plant lipids are often unsaturated and occur as oils, such as olive oil and sunflower oil.

动物脂质通常为饱和脂肪酸，以脂肪形式存在；而植物脂质通常为不饱和脂肪酸，以油脂形式存在，例如橄榄油和葵花籽油。

Formation of triglyceride:

1 glycerol + 3 fatty acids → triglyceride (linked by ester bond) + 3 water (removed by the condensation)

Ester bond formed between one hydroxyl groups of a glycerol molecule and the carboxyl group of the fatty acid molecule.

Triglycerides#

• 1 glycerol + 3 fatty acids

• Linked by ester bonds

• Formed via condensation reaction

• Can be saturated of unsaturated

• Insoluble in water (because it’s non-polar and hydrophobic)

• Soluble in organic solvents:

  • ether

  • chloroform

  • ethanol

    Both polar and non-polar can be dissolved in ethanol

Phospholipids#

1 fatty acid chain in triglyceride is replaced by a phosphate group.

相较于triglyceride，其中的一条fatty acid被替换成了phosphate group。

Phospholipids consist of …

• Glycerol - the body part

• Hydrophilic Head (Phosphate group) - charged and polar

• Hydrophobic tails (Fatty acids) - insoluble and non-polar

Phospholipid can form a phospholipid bilayer - the basic component of cell membrane

Phospholipid会形成一种双层的结构，这种结构是细胞膜最基础的结构（不止细胞外膜，还包括nuclear envelope之类的细胞膜）

Summary#

R groups#

The R group is the side chain that makes each amino acid unique. There are 20 main types, with different chemical properties: R基是氨基酸的侧链（位于氨基酸的边上），它决定了每种氨基酸的唯一性。主要有20种类型，具有不同的化学性质：

• Non-polar R groups (Hydrophobic): Repel water, often found buried inside proteins.

  非极性R基（疏水）：排斥水，通常藏在蛋白质内部。

  • e.g., Glycine (Gly), Alanine (Ala), Valine (Val)

• Polar R groups (Hydrophilic): Attract water, often found on the protein’s surface.

  极性R基（亲水）：吸引水，通常位于蛋白质表面。

  • e.g., Serine (Ser), Threonine (Thr)

• Charged R groups (Acidic/Basic): Can form ionic bonds, very hydrophilic.

  带电荷的R基（酸性/碱性）：可以形成离子键，亲水性很强。

  • e.g., Aspartic acid (acidic, Asp), Lysine (basic, Lys)

The chemical nature of the R groups determines how the protein folds and functions. R基的化学性质决定了蛋白质如何折叠并执行其功能。

总共有20种不同的R group，因为这些R group的不同，所以有20中氨基酸。

例如最简单的R group只由一个H（氢原子）构成，这种氨基酸被称为Glycine

不同的R group可以在课本的第608页附录中找到：

对于考试来说，必须要会分辨Cysteine和Glycine：

• 只含有一个H氢原子，则是Glycine

• 含有S硫原子，则是Cysteine

Polar and Non-polar Amino acids#

• Charged side - hydrophilic (溶于水)

  如果R group的一端标有+或-，说明这个部分是被电离了的。

• Polar side - hydrophilic (溶于水)

  如果R group的一端有OH或NH₂，那么这个是极性的

• Non-polar side - hydrophobic (不溶于水)

  对于上述没有提到的情况，大多数都是非极性的

Cysteine是特殊情况，它具有非常弱的极性，所以有时候会被分类成非极性的。

Formation of Peptide bond#

两个氨基酸会通过Condensation连接到一起，并产生一个peptide bond，peptide bond在上图右侧标出来了，它包括四个原子（四个元素）：C, O, N, H。

两个氨基酸通过这个condensation连接到一起时会形成Dipeptide，而很多个氨基酸通过同样的方式连接到一起时会形成Polypeptide↓。

Primary Structure#

• Sequence of amino acids

  指polypeptide中氨基酸的排列顺序

• Held together by peptide bonds

  通过peptide bond共价连接

• Determine the final 3D shape od the protein

  决定了蛋白质最终的三维形状和功能

Secondary Structure#

• In a polypeptide chain

  指多肽链主链骨架的局部空间结构，不涉及R基团。

• Held by hydrogen bonds

  主要由主链上的C=O与N-H group之间形成的氢键来稳定。

  WARNING

  只能C=O和N-H产生链接

In α-helix, hydrogens bonds form between the O of the C=O group of one amino acid and the H of the –NH group of the amino acid four places ahead of it.

• α-helix - helical structure formed by a polypeptide chain, bind by hydrogen bonds
• β-pleated sheet - a loose, sheet-like structure formed by hydrogen bonding between parallel polypeptide chain

图中的虚线代表hydrogen bond

Tertiary Structure#

一条蛋白质链在α-helix或β-pleated sheet的基础上形成的三维结构被称为三级结构。一条蛋白质链最多只能形成三级结构，如果要组成四级结构需要更多的蛋白质链加入。

• Folding and Coiling of a polypeptide chain (secondary structures → 3D shape)

  指一条多肽链在二级结构基础上进一步折叠和盘绕形成的完整三维空间结构。

• Due to interaction between R groups

• Formed by four types of bonds:

  1. hydrogen bonds

  

  单个hydrogen bond的连接强度是比较弱的，但是很多个hydrogen bond在一起时会形成一个非常牢固的结构。

  氢键容易被高温和pH变化破坏。

  1. disulfide bonds

  

  Disulfide bonds formed by oxidation, and broke by reduction.

  Very strong bond, need to use reducing agent to break down the bond. 这是一种非常强的共价键，需要还原剂才能破坏。

  1. ionic bonds

  

  The ionic bond will be broken when the pH changes

  1. weak hydrophobic interactions

  

  如果两个R group非常接近，并且它们都是hydrophobic（疏水）的，它们两个会因为都与水发生排斥，然后连接在一起。通过添加non-polar的溶液（比如酒精）就可以使这个键分开。

Quaternary Structure#

三级结构只由一条蛋白质链构成，当有一条以上的链共同形成一个三维结构时，会形成四级结构。

• Made by two or more ploypeptide chains (may also contains other structures)

  由两条或两条以上具有独立三级结构的polypeptide构成（有时候还会包括非peptide的成分，比如血红蛋白中的血红素）

• Also made by the 4 types of bonds in tertiary structure

Globular Protein - Haemoglobin#

球状蛋白是可溶的，它疏水的部分（non-polar hydrophobic R group）集中的蛋白质的中心位置，不会影响到周围的水分子；同理，它亲水的部分（hydrophilic R group）在蛋白质的外层，使整个分子是出于亲水的状态。

关于可溶性的相关内容可以回顾上面的[hydrogen bond](#Hydrogen Bond)部分。

血红蛋白是一个典型的球状蛋白，它：

• Red in colour

• Carry oxygen

• Has quaternary structure (contains more than one polypeptide chain)

• Made of 2 α-chains and 2 β-chains

  Two of the haemoglobin chains, called α chains, are made from α-globin, and the other two chains, called β chains, are made from β-globin.

  一个血红蛋白中有两种血红蛋白链：α链和β链。α链是由α-血红蛋白制成的；β链是由β-血红蛋白制成的。

Each chain has a haem group → each haem group has one iron atom → each iron atom can bond reversibly with one oxygen molecule.

血红蛋白中有四条链 → 每条链中有一个血红素 → 每个血红素中有一个铁原子 → 每一个铁原子都可以和一个氧分子O₂“可逆地”结合（铁和氧既可以连接在一起也可以分开）

所以说一个血红蛋白可以携带四个氧分子（总共8个氧原子）

Haem group is a prostatic group.

WARNING

一个血红蛋白有：

• 两个α-globin chain
• 两个β-globin chain
• 四个haem group

血红素的颜色取决于是否含有氧气：

• 含有氧气：亮红色 Bright red (含有氧气的血红素又被叫做oxyhaemoglobin)
• 没有氧气：暗红色 Bluish red

Sickle Cell Anaemia 镰刀状细胞贫血#

镰刀状细胞贫血是一种遗传性血液疾病，其根本原因在于基因突变导致血红蛋白分子结构异常。这种异常的血红蛋白在缺氧状态下会使正常的圆盘形红细胞变形为僵硬的“镰刀状”，从而引发一系列健康问题。

• One amino acid on the surface of the β chain is replaced with a different amino acid.

  一个碱基被另一个碱基所替换（Substitution）。

• This mutation will make haemoglobin less soluble

  溶解度下降使得血红蛋白分子容易相互粘连，形成长纤维并沉淀

• Cause the unpleasant and dangerous symptoms

  造成疼痛以及很多危险的症状。

The effects of the mutant sickle cell allele on the phenotype of a person with sickle cell anaemia#

The mutation leads to an altered β-polypeptide chain in the hemoglobin protein (a change in primary structure).The mutation leads to an altered β-polypeptide chain in the hemoglobin protein (a change in primary structure).

This structural change makes the hemoglobin less soluble, particularly under low oxygen condition.

The formation of fibres causes the red blood cells to deform into a sickle or crescent shape.

These sickled cells are fragile and break down (lyse) more rapidly, leading to a chronic shortage of red blood cells (haemolytic anaemia).

The sickled cells are less efficient at carrying oxygen.

The stiff, sickled cells get stuck in narrow capillaries, blocking blood flow to tissues and organs.

This blockage causes intense pain and episodes known as a sickle cell crisis.

Fibrous Protein - Collagen#

• Making up 25% of the total protein in mammals

• It is an insoluble fibrous protein found in skin, tendons, cartilage, bones, teeth and the walls of blood vessels.

  胶原蛋白是一种不溶的纤维蛋白，存在于皮肤、肌腱、软骨、骨骼、牙齿以及血管壁中。

• Relatively long, thin structure

• Generally insoluble

• Metabolically inactive

A collagen molecule consists of three polypeptide chains, each in the shape of a helix. These three helical polypeptides are wound around each other, forming a ‘triple helix’. (三条螺旋组成了一个“三螺旋”结构)

Helix with three amino acids per turn; every third amino acid is glycine. 螺旋结构每转包含三个氨基酸，每第三个氨基酸是甘氨酸（就有那个最简单的R group的氨基酸）。

These strands are held together by hydrogen bonds and some covalent bonds. 同时包括共价键和氢键。

Collagen molecules lie parallel to from a fibril, linked by covalent cross-links. Each fibre is made of many fibrils lying side by side. 胶原蛋白分子并排在一起形成纤维丝，每条胶原蛋白分子之间通过共价键连接。每个纤维由许多纤维丝并排构成。

The Structure of Collagen Molecule and Fibre#

Collagen Molecule	Collagen Fibre
Triple helix	cross links / covalent bonds between molecules
tightly wound	molecules arranged in parallel
hydrogen bonds between polypeptides	the ends are not aligned
every third amino acid is glycine	made from collagen molecules

Summary Table#

• haemoglobin contains irons that can combine with oxygen

Intracellular and Extracellular Enzymes#

并不是所有的酶都会在细胞内发生反应，有的细胞会分泌酶到细胞外面，然后在细胞外面发生催化反应。

• Intracellular - 在细胞内发生催化反应
• Extracellular - 在细胞外发生催化反应

Digestive enzymes in the gut are extracellular enzymes. Some organisms secrete enzymes outside their bodies. Fungi, for example, often do this in order to digest the food on which they are growing. 胆囊中产生的消化酶就属于在细胞外作用的酶（细胞外酶），它们被分泌到消化道中消化食物。还有些生物（比如真菌）会直接将消化酶分泌到它们要消化的食物上。

The lock-and-key Hypothesis#

Enzyme has a dent on their surface called active site. The reactant an enzyme acts on is called substrate, the substrate attaches to the active site on the enzyme, when they bind together, a enzyme-substrate complex forms.

酶的表面有一个称为活性位点的凹陷。酶作用的反应物称为底物，底物与酶的活性位点结合。当它们结合在一起时，形成酶-底物复合物。

The shape of the active site allows the substrate to fit perfectly. The substrate is held in place by temporary bonds which form between the substrate and some of the R groups of the enzyme’s amino acids.

一个substrate可以和特定的active site完美地连接在一起，substrate会和酶氨基酸中某些R groups形成一些暂时性的“键”。

一种enzyme只会针对于一个特定的substract产生作用，这是因为enzyme的active site只和一种substrate的形状契合。可以用以下词语来描述：

• The enzyme’s active site has a specific three-dimensional shape.
• This shape is complementary to the shape of its particular substrate molecule
• …allowing only that one substrate to fit perfectly.

The induced-fit Hypothesis#

这种假说在大体上来说是和“the lock-and-key hypothesis”一样的，但是不同点在于：

• The shape of the active site of the enzyme is partially complementary to the shape of the substrate.
• As the substrate enters the active site, the active site induces and changes the shape slightly.
• Then the active site is complementary to the substrate

Differences between Lock-and-Key & Induced-Fit 两种假说的区别#

关键点：现代研究支持诱导契合假说，认为酶是柔性的，结合时会发生构象变化，从而更高效地催化反应。

Describe the Graph#

• As the time increases, the total volume of $\ce{O2}$ (Dependent variable) increases
• As the time increases, the reaction rate decreases
• Address actual data to support

描述图像的时候，需要提到因变量和反应速率怎么根据时间发生改变，并且提到具体的数据来支持。

As the factor on the x-axis increases, the factor on the y-axis increases / decreases.

Explain the Graph#

As the time increases…

• more substrate is broken down
• the concentration of substrate decreases
• the frequency of successful collision decreases
• fewer enzyme-substrate complexes from in unit time

在反应发生之后，反应物变少了，导致反应物的浓度变少了，导致反应发生的频率更低了，导致一定时间内产生的enzyme-substrate complexes变少。

Enzyme Concentration#

当enzyme的浓度越高，反应物的反应速率更快，也就更快达到最终的含量。

• As enzyme concentration increases, the initial rate increases
• Enzyme concentration is a limiting factor, as the enzyme concentration increases…
  • the frequency of successful collision increases
  • more enzyme-substrate complexes form in unit time

如果要描述一个变量对与反应速率的影响，可以：

• 先提到这个变量是一个limiting factor
• 描述successful collision会怎么变化（increase / decrease）
• 描述enzyme-substrate complexes会怎么变化（increase / decrease）

How to Break the Bonds in Enzyme (Protein)#

当这些tertiary structure中的键发生断裂时，enzyme的形状会发生改变，从而导致active site的形状会发生改变，使得substrate和enzyme无法组合到一起发生反应。

Temperature’s Effect#

这个图像分为两个部分，40°C以左和40°C以右的部分。对于左边的部分：

• As the temperature increases from 0 to 40°C, the rate of reaction increases
• As the temperature increases, the kinetic energy of molecules increases and they move faster,
• The frequency of successful collision increases
• More enzyme-substrate complexes form

在40°C这个地方：

• At 40°C, the rate of reaction reaction reaches the peak, it’s called the optimum temperature
• The rate of reaction is fastest

在40°C以右的部分：

• After 40°C, the rate of reaction decreases as the temperature increases.
• Above the optimum temperature, the enzymes start to denature and the active sites start to *change shape
• Fewer enzyme-substrate complexes form

当反应速率完全降到0时：

• The reaction decreases to 0 when temperature is around 55°C
• At this temperature, enzyme is completely denatured

pH’s Effect#

对于pH对反应速率的影响来说，越远离optimum pH，就会有越来越多的enzyme被denature，从而导致反应速率的下降。

以下内容为AI生成，仅供参考

How to Measure Reaction Rate 如何测量反应速率#

有两种主要方法：

1. 测量产物生成速率（如：过氧化氢酶实验）
   • 反应：(\ce{H2O2 ->[catalase] H2O + O2})
   • 方法：收集氧气，测量单位时间内的体积变化。
2. 测量底物消耗速率（如：淀粉酶实验）
   • 反应：(\ce{starch ->[amylase] maltose})
   • 方法：定时取样，用碘液测试或本尼迪克特测试，结合比色计测量颜色变化。

Using a Colorimeter 使用比色计#

• 原理：测量溶液对特定波长光的吸光度或透光率。
• 步骤：
  1. 校准：使用空白溶液（如蒸馏水）将仪器调零。
  2. 测量：将样品放入比色皿，读取吸光度值。
  3. 绘制标准曲线：用已知浓度的溶液绘制浓度 vs 吸光度曲线，用于推算未知样品浓度。

实验技巧：测量前必须校准，否则数据无效。使用红色滤光片可提高淀粉-碘复合物的测量灵敏度。

Michaelis-Menten Constant ((K_m)) 米氏常数#

• 定义：反应速率达到最大速率一半（(\frac{1}{2}V_{\max})）时的底物浓度。
• 意义：(K_m ) 值越小，酶对底物的亲和力越高（所需底物浓度越低）。
• 公式：(Km = [S] \text{ at } v = \frac{1}{2}V{\max})
• 与 (V_{\max}) 的关系：两者相互独立。(V_{\max}) 反映酶的最大催化能力，(K_m) 反映酶对底物的亲和力。

记忆技巧：低 (K_m) = 高亲和力 = 酶“更喜欢”底物，在低浓度下就能高效工作。

Comparison of Inhibitors 抑制剂比较#

End-Product Inhibition 终产物抑制#

• 类型：一种可逆的非竞争性抑制。
• 机制：代谢途径的终产物作为抑制剂，结合到途径中的第一个酶的变构部位，从而负反馈调节整个途径，防止产物过度积累。
• 意义：维持细胞内代谢物的稳定浓度，是生物体自我调节的重要方式。

例子：ATP 合成途径中，ATP 过多时会抑制早期关键酶，减少 ATP 合成，避免能量浪费。

Methods of Immobilisation 固定化方法#

除了海藻酸钙微珠法，还有：

• 吸附法：酶吸附在惰性载体（如活性炭）表面。
• 共价结合法：酶通过化学键与载体连接。
• 包埋法：酶被包裹在凝胶网格（如聚丙烯酰胺）中。

Advantages of Immobilised Enzymes 固定化酶的优点（补充）#

1. 可重复使用：酶被固定后可以多次回收利用，降低成本。
2. 产物纯度高：酶不混入产物，无需后续分离。
3. 稳定性增强：对温度、pH 变化的耐受性高于游离酶。
4. 避免终产物抑制：酶与产物可快速分离，减少抑制作用。
5. 适合连续化生产：可填充于反应柱，实现连续流动反应。

Practical Application: Lactose-Free Milk 实际应用：无乳糖牛奶#

• 酶：乳糖酶（lactase）
• 过程：
  1. 将乳糖酶固定在海藻酸钙微珠中。
  2. 牛奶流经装有微珠的反应柱。
  3. 乳糖被水解为葡萄糖 + 半乳糖。
  4. 收集流出的无乳糖牛奶。

意义：为乳糖不耐受人群提供可饮用的牛奶，同时保留牛奶的营养价值。

Types of Variables 变量类型#

How to Vary Independent Variables 如何改变自变量#

1. 浓度：使用比例稀释或系列稀释法配制不同浓度溶液。
2. 体积：使用量筒或移液器精确量取。
3. 温度：使用恒温水浴锅精确控制。
4. pH：使用缓冲溶液维持特定 pH。

实验关键：每次实验只能改变一个自变量，其他所有条件必须严格保持一致，才能得出可靠结论。

Fluid mosaic model 流动镶嵌模型#

1. Made of a phospholipid bilayer
2. Contain scattered proteins (mosaic)
3. Both the phospholipids and proteins can move about by diffusion (fluid)
4. Cholesterol molecules fit between phospholipid molecules

细胞膜不是一个固定的结构，它是一个“有弹性的”，所以科学家提出了流动镶嵌模型。小分子（比如水分子）可以直接通过细胞膜，也就是phospholipid之间的间隙。也有一些大分子会通过细胞膜上的“蛋白质通道”来进出细胞，这种通过是carrier protein或者叫transport protein。

This model can explain:

• Passive and active movement between cells and their surroundings
• Cell to cell interactions
• Cell signalling

Feature and Role of Cell Membrane#

• Partially permeable / semi-permeable
  1. water is small enough to move through
  2. non-polar, hydrophobic, lipid-soluble molecules such as $O_2$ and $CO_2$ can move across the fatty acid interior and the membrane
  3. Polar, hydrophilic, water-soluble molecules and ions cannot move across the fatty acid interior and they need transport proteins

The Factors that Influence the Fluidity#

• More Unsaturated the fatty acid tails, more fluid the membrane

  不饱和脂肪酸之间排列并不紧密，所以每个脂肪酸之间的间隙更大，也更方便移动。

• The shorter the fatty acid tails, the more fluid the membrane.

• More temperature, more kinetic energy, more fluid membrane

• Cholesterol is important for the mechanical stability of membrane at high / low temperatures:

  • At low temperature, the membrane is less fluid. Cholesterol prevents close packing of fatty acid tails, increases fluidity and maintains the correct fluidity of the membrane.
  • At high temperature, the membrane is more fluid. Cholesterol reduces mobility of phospholipids and decreases fluidity

Proteins#

1. Transport protein - active transport

   • Channel protein
   • Carrier protein

2. Enzyme - help metabolic mechanics

   • Digest
   • Catalyse

3. Some proteins attach to cytoskeleton - help to maintain the shape of the cell, or involve in changes of the shape of the cell

   Cytoskeleton is made of microtubulus

Glycoproteins and Glycolipids#

1. Receptor molecules for cell signalling

2. Act as markers or antigens, allowing cell to cell recognition

   不同的血型是因为glycoproteins和glycolipids不一样.

Diffusion#

Particles move randomly due to kinetic energy in them, eventually distribute evenly in the space. Diffusion is the net movement of molecules and ions from higher concentration to low concentration down a concentration gradient.

Net movement - 用来描述粒子们整体运动趋势的单词

• Down the concentration gradient
• Passive / Don’t require ATP
• Don’t require transport protein
• Don’t require semi-permeable membrane

Hydrophobic molecules can diffuse through the cell surface membrane, for example $O_2$ and $CO_2$

Factors affecting rate of diffusion:

1. Steepness of concentration gradient
2. Temperature - particles move faster when the temperature is high
3. The nature of the molecules or ions - some lighter molecules can move faster than some larger molecules, polar and non-polar also will affect the diffusion rate, non-polar molecules can diffuse through the cell surface membrane more easily than the polar molecules
4. Surface area : Volume ratio - larger the ratio, faster the diffusion rate
5. Distance - shorter the distance, faster the diffusion rate

如何快速计算Surface area : Volume：

• 当两个长方体的体积相同，那么比较它们的边长，更长的一边意味着更扁的形状，也就是说面积更大。

Question: Why the water can become coloured when the coloured plant cell is boiled?

Answer: High temperature or alcohol denatures proteins and damage cell membrane, so the coloured pigments can diffuse out down the concentration gradient.

Facilitated Diffusion#

Facilitated Diffusion - the diffusion of molecule or ion through a transport protein (channel protein or carrier protein) in a cell membrane down their concentration gradient.

Facilitated means making things easier to happen.

• Down the concentration gradient

• Passive, don’t require extra energy

• Use transport protein (specific to a particular ion / molecule, only allow a certain type of particle to move through)

  Channel protein has a fixed shape, usually used to transport ions.

  Carrier protein can change their shape to transport the molecules such as sugars and amino acids. 就算Carrier protein会改变形状，它也没有用ATP

• Across a cell membrane

在facilitated diffusion中，transport protein是可以改变或不改变自身形状的：

• 如果是通过channel protein运输，那么不会change shape
• 如果是通过carrier protein运输，则会改变形状

Factors affecting rate of facilitated diffusion:

1. Steepness of the concentration gradient
2. Temperature
3. Number of transport proteins available
4. surface area of the membrane (large surface area means more transport proteins, if the )

The difference between simple diffusion and facilitated diffusion:

For facilitated diffusion, the number of transport protein will become a limiting factor, and lacks the uptake rate.

Osmosis#

water potential 水势 - 可以简单理解为水分子的“浓度”，但是水所谓一个溶液不能通过“浓度”来描述，所以使用了这个特殊的词汇。溶液中溶质的浓度越高，水势越低:

• Pure water has the highest water potential: 0 kPa
• Dilute solution has a negative water potential: < 0 kPa
• Concentrated solution has a more negative water than dilute solution.

The Greek letter psi, ψ, is used to present water potential

Osmosis is the net diffusion of water molecules (only occurs for water) from high water potential to low water potential, down a water potential gradient, through a partially / semi- permeable membrane:

• Down the water potential gradient
• Passive
• Don’t require transport protein
• Across a partially permeable membrane

Two types of partially permeable membrane

• cell membrane 细胞膜
• Visking tubing / dialysis tubing 透析管

The pores of the Visking tubing membrane have a certain size, if the ions or molecules are smaller than the pores, the ions or molecules can move through the membrane.

• Starch is a large molecule, so starch usually can’t move through the Visking tubing’s membrane
• Glucose is a small molecule, so glucose can pass through

试管	左试管（S）	右试管（I）
试管中的溶液	Starch solution	Iodine solution
透析管中的溶液	Iodine solution	Starch solution

在左试管中，starch不能进入透析管，但是iodine可以从透析管中出来，所以二者会在透析管外发生反应，在透析管外呈现出dark blue。

在右试管中，starch不能从透析管里出来，但是iodine可以进入透析管，所以透析管内会呈现出dark blue。

As the water moves into the cells through osmosis:

• Animal Cell: shrink → swell → burst

• Plant Cell: Plasmolysed → flaccid → turgid

Incipient plasmolysis is the point at which the protoplast is about to pull away from the cell wall (the point at which plasmolysis is about to occur).

质壁分离发生的那一个点。

因为植物细胞的细胞壁比较坚固，所以当植物细胞水分丢失过多时，植物细胞的细胞膜会和细胞壁发生分离。

Active Transport#

The movement of molecules or ions through carrier proteins across a cell membrane, against concentration gradient, using energy:

• against concentration gradient (from low to high)
• active transport - require energy (ATP)
• Need cell membrane - carrier protein

Carrier protein can change its shape and take the molecule against the concentration gradient:

One example of carrier protein is sodium-potassium pump, it brings 2 $K^+$ into to cell and 3 $Na^+$ out of the cell by using 1 $ATP$ molecule each time.

Bulk Transport#

Transport of large quantities of materials into and out of the cells. The materials include large molecules such as proteins oy polysaccharides, parts of cells, or even whole cell.

• Use energy

• Need membrane

• Endocytosis - moves into the cell

  • Phagocytosis - carry solid materials 吞固体
  • Pinosytosis - carry liquid materials 吞液体

• Exocytosis - moves out of the cell

  The substances to be released (such as enzymes or hormones) are packaged into secretory vesicles formed from the Golgi body.

Bulk transport forms vesicles to transport the materials, instead of carrier proteins in active transport.

Bulk transport的活动与外界的concentration gradient没有关系。

Interphase#

Mitosis (second phase in the cycle)#

分裂成两个细胞核

• Cell growth stops during the M phase
• During mitosis, the nucleus divides to form two genetically identical nuclei

Cytokinesis#

分裂成两个细胞

• Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells
• In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed

Animal Cells:

• Cytoplasm divides into 2 new daughter cells by constriction, from edges of the parential cell.

Plant Cells:

• Cell plate forms at equator
• New cell wall forms between 2 daughter cells
• Cytoplasm divides into 2 cells

Telophase and cytokinesis often overlap. 这两个阶段通常在一起发生。

Telomere 端粒#

• The chromosome before replication has 2 telomeres, and the chromosome after replication has 4.

• Telomeres are made of DNA with short base sequences repeated many times.

• Telomeres get shorter after each DNA replication.

  This may be one of the mechanisms of aging. 端粒越来越短可能是导致老化的原因之一。

Functions:

• Telomere can prevent the loss of genes from the ends of chromosomes during DNA replication
• To allow continued DNA replication and cell division.

Telomerase can add bases to telomere to prevent telomeres fron getting shorter. But it’s NOT active in most human body cells.

In the zygote, telomerase is active so it can maintain a long telomere at the end of chromosome.

Chromatid - 染色单体#

Functions of centromere:

• Hold two sister chromatids together
• The site of arrachement fo spindle microtubules

Function of kinetochore:

• Connect centromere to the microtubules

Nucleotide#

A nucleotide consists of:

• A phosphate group - negatively charges in solution, make DNA / RNA negatively charged

• A pentose sugar - 5 carbon sugar

  • ribose in RNA nucleotide
  • deoxyribose in DNA nucleotide

• A nitrogenous / nitrogen containing base

  WARNING

  上面必须要提到base！！！

Bases in DNA and RND:

• DNA - A, T, C, G
• RNA - A, U, C, G

这些bases位于nucleotide的nitrogenous base的部分。

T和U的结构非常相似，他们之间的区别就在于T比U多一个$CH_3$的部分。

WARNING

必须要会背这些全称：

• adenine
• thymine and uracil
• cytosine
• guanine

Ribose and Deoxyribose

• 右下角含有氧原子的 - ribose
• 右下角没有氧原子的 - Deoxyribose

ATP#

Adenosine triphosphate (ATP) is the energy-carrying molecule that provides the energy to drive many processes inside living cells.

It’s also a type of nucleotide, hence it has similar structure to the nucleotides that make up DNA and RNA.

ATP molecule is a phosphorylated RNA nucleotide:

• one ribose
• one adenine
• 3 phosphate grops

Hydrolysis of ATP:

1. add water
2. water molecule can break down the bond between phosphate groups
3. the energy is released

$$\ce{ATP -> ADP + P_i}$$

$P_i$ is a single phosphate molecule: $H_3PO_4$

It is a phosphorylated nucleotide.

It can combine with varies number of phosphate groups:

1. adenosine monophosphate (AMP)
2. adenosine diphosphate (ADP)
3. adenosine triphosphate (ATP)

Purines and Pyrimidines#

Purines and pyrimidines are two different types of structure that formed by the nitrogenous base molecules:

1. Pyrimidines (single-ring structure) - cytosine, thymine, uracil

2. Purines (double-ring structure) - adenine and guanine

记忆口诀：

1. Y 的读音像one，所以pyrimidines是一个环的结构
2. Purines - Pure As Glod”，然后剩下的都是pyrimidine
3. A 和 G 是特殊的，所以它们有两个环，其他的三个都只有一个环

Purines have two rings and pyrimidines have one, purine normally pairs with pyrimidine to maintain DNA double strand width, if not, DNA double helix width will be distorted 变形扭曲.

DNA Structure#

DNA is double stranded and a double helix.

Purine normally pairs / bonds with pyrimidine, so a DNA molecule h3as the same width throughout.

• A pairs with T/U by 2 hydrogen bonds
• G pairs with C by 3 hydrogen bonds

AT立场和CG动效（AT互相连接，GC互相连接）

This is called complementary base pairing.

这个左边DNA single strand的顺序是5'到3'的，而另一边正好相反，所以说DNA两边是：antiparallel strands 相反平行链。

5'到3'表示的是：从pentose最右边的碳原子按顺时针的方向开始算，第三个或第三个碳原子与下一个nucleotide相连。

DNA两侧的结构叫做sugar-phosphate backbone, sugar和phosphate之间的bond叫做phosphodiester bond (phospho - di - ester)

DNA中的两个螺旋通过hydrogen bond连接。

RNA Structure#

• Single stranded

There are three types of RNA incolved in proteins synthesis:

1. mRNA, messenger RNA - encodes protein, it can move out the nucleus and go into the cytoplasm
2. tRNA, transfer RNA - carries amino acid
3. rRNA, ribosomal RNA - forms the sibosome

Meselson & Stahl’s Experiment#

• Carried in 1958
• Using bacteria E.coli and different nitrogen isotopes

Procedure:

1. Grow E.coli with nitrogen-15 for many generations - Result: All DNA is heavy

2. Transfer to a medium with nitrogen-14 and allow E.coli to divide to produce 1 generation

   *Result: All DNA is a Hybrid of 1 heavy strand and 1 light strand*

3. Repeat for 2 more generations and observe DNA

Use centrifuge to seperate the DNA with different weight

Key Terms#

Transcription#

Take place in the nucleus

1. A part of DNA molecule unwinds, this exposes the gene to be transcribed
2. A complementary copy of code is formed (mRNA)
   • The DNA triples of bases are transcribed as mRNA codons
   • 单链的mRNA可以通过细胞核上的小孔，但是双链的DNA不行

Translation#

Translation发生的过程：

1. mRNA’s two codons at a time are exposed to the larger subnuit (in ribosome)
2. A specific tRNA brings a specific amino acid (corresponding to the codon on mRNA)
3. Complementary base pairing occurs by hydrogen bonding
4. A second tRNA brings another aminoacid next to the first amino acid
5. Peptide bond formation between the two amino acids
6. Ribosome moves along the mRNA, one codon at a time (above process is repeated until a stop codon is reached)
7. The first tRNA leaves the ribosome

Xylem#

Xylem takes the water upward, it also can:

• support
• transport water
• transport mineral

Xylem tissue is made of xylem vessel elements (cells), which are:

• elongated dead hollow cells

• no end walls

• cellulose and lignin cell walls

  lignin is a hard, strong, and waterproof polymer

There are some non-lignified pits in wall allow sideway movement of water to other parts.

The narrow diameter of lumen between the continuous cells reinforces the effect of adhesion, which makes the transpiration more efficient.

Pits are origin from the plasmodesmata areas at original cell walls, they still have the original unthickened cellulose cell wall, instead of open pores.

Pits并不是开放的小孔，而是一层允许水分子穿过的cellulose。

Structure and it’s function:

• No end walls:
  • Provide a continuous pathway for water moving upward
• Walls contain lignin:
  • Waterproofing
  • Support the plant’s structure and prevent collapse
• Pits:
  • Allow the water’s sideway movement
• Narrow diameter:
  • Adhesion

Phloem#

Phloem is used to transport assimilates, which include sucrose and amino acids.

• Source - assimilate出发的地方
• Sink - assimilate到达的地方

Structure of the sieve tube elements

• Elongated living cells (no nucleus, ribosome and vacuole)
• A thin layer of cytoplasm lining inside of the wall
• The cell wall only contains cellulose, no lignin

Structure of the phloem sieve tube

• Sieve plates with sieve pores form at end walls
• Each sieve tube element has at least one companion cell beside it to form a single functional unit
• There are many plasmodesmata between sieve tube element and companion cell for communication
• Companion cells have nucleus, vacuole, many mitochondria and ribosomes (at rough ER)

Structure and it’s function:

• Elongated cells with sieve plated and pores:

  • form a continuous sieve tube for transport

• Strong cellulose cell wall:

  • To withstand the hydrostatic pressure that move the assimilates and prevent excessive cell baling under the pressure

• No nucleus, vacuole and only thin layer of cytoplasm:

  • Maximise the space and reduce resistance for movement of assimilates.

1. Transpiration#

• Loss of water vapour by diffusion down its water potential gradient from air spaces of leaves to environment

• Mainly through open stomata which open during daytime and close at night

• Through cuticle (only a very small amount of water vapour)

• Can cool the leaves

2. Water Evaporation#

Liquid water evaporates from the surface of mesophyll cells to enter air spaces and replace the lost of water vapour by transpiration.

3. Water Movement from xylem in leaf to mesophyll cells#

• To replace the lost of water by evaporation via pits

• When the water stays in the cell walls - the apoplast pathway

• or enter the cytoplasm - the symplast pathway

4. Tension and transpirational pull#

1. Transpiration and evaporation of water lower the water potential at leaves, creating tension (a negative pressure) in the xylem.
2. Tension pulls water and minerals up the xylem by mass flow from root to stem the to leaf, as a continuous, unbroken column of water due to cohesion, adhesion and the narrow diameter of xylem vessels. This pulling effect is known as transpirational pull.
3. Leaves have a lower water potential and roots have a higher water potential. There is a water potential gradient between them.
4. Tension pulls xylem walls inwards. During the daytime, when stomata open, the diameter of a tree trunk decreases.
5. As tension increases in the xylem, the pressure becomes more negative.

5. Water uptake#

• Water moves into cells by osmosis
• Mineral ions are taken up into cells mainly by active transport

Root hair cells - increase surface area for absorption

Water Pathway#

Water takes two routes down the water potential gradient through the epidermis and cortex:

1. Apoplast pathway 通过细胞壁之间的间隙传输
   • water doesn’t cross membrane and doesn’t enter the cytoplasm. Water enters the call wall
   • Water moves through the cells wall
   • Water can move from cell wall to the cell wall through the intercellular spaces
   • Water may move directly from cell wall to cell wall
2. Symplast pathway 在细胞里面传输
   • Water enters the cytoplasm by osmosis through the partially permeable cell surface membrane
   • Water moves into the sap in the vacuole, through the tonoplast by osmosis
   • Water may move from cell to cell through adjacent cell surface membranes and cell walls

1
sequenceDiagram
2
    participant Soil as Soil Solution
3
    participant Cortex as Root Cortex
4
    participant Endo as Endodermis Cell
5
    participant Xylem as Xylem Vessel
6

7
    Note over Soil,Cortex: Water & Minerals Arrive
8

9
    Cortex->>Endo: Apoplast Pathway (Cell Walls)
10
    Cortex->>Endo: Symplast Pathway (Cytoplasm)
11

12
    Note over Endo: Casparian Strip Blocks<br/>Apoplast Pathway
13

14
    Endo->>Endo: Water Forced into Symplast
15
    Endo->>Endo: Selective Mineral Uptake<br/>via Active Transport
16

17
    Endo->>Xylem: Water Enters Xylem
18
    Endo->>Xylem: Controlled Mineral Entry
19

20
    Note over Endo,Xylem: Plants Control What<br/>Enters Vascular System

How the structure of xylem vessles is suited to their function#

• Elongated dead hollow cells with no end walls and no cytoplasm
  • Joined end to end to form long hollow tubes for transport

Process of Translocation (Transport in Phloem)#

1. Active loading at the source

   • Loads the assimilates into the phloem tube elements using ATP via active transport

   • Result in…

     WARNING

     1. increment of the sucrose’s concentration
     2. decrement in the water potential in phloem sieve tubes
     3. increment in volume of sap in phloem sieve tubes

2. Hydrostatic pressure changes

   • Water enter the phloem tube element (because of the water potential gradient)
   • The hydrostatic pressure increases (source)

Active Loading of Sucrose#

1. Hydrogen ions (protons, H⁺) are pumped out of the companion cell into the cell wall by a proton pump using ATP via active transport.
2. This creates a higher concentration of H⁺ ions in the cell wall.
3. The hydrogen ions move back into the companion cell by passive facilitated diffusion down their concentration gradient, through a co-transporter protein (a type of carrier protein).
4. The co-transporter protein simultaneously transports sucrose molecules into the companion cell, even against the sucrose concentration gradient.
5. Sucrose molecules then diffuse through plasmodesmata from the companion cell into the sieve tube element.

1
sequenceDiagram
2
    participant ATP as ATP Energy
3
    participant CC as Companion Cell
4
    participant Wall as Cell Wall Space
5
    participant ST as Sieve Tube Element
6

7
    ATP->>CC: Provides Energy
8
    CC->>Wall: Proton Pump<br/>Active Transport H⁺ Out
9
    Note over CC,Wall: ATP-driven<br/>Creates H⁺ Gradient
10

11
    Wall->>CC: H⁺ Diffuses Back In
12
    Note over Wall,CC: Down Concentration Gradient
13

14
    CC->>CC: Co-transporter Protein<br/>Brings Sucrose + H⁺
15
    Note over CC: Sucrose Moves Against<br/>Its Gradient
16

17
    CC->>ST: Sucrose Diffuses Through<br/>Plasmodesmata
18

19
    Note over ST: Result:<br/>↑ Sucrose Concentration<br/>↓ Water Potential

This is a mechanism in plants for loading sucrose into the sieve tubes of the phloem. It involves energy expenditure (ATP) and proton co-transport. It takes place in the companion cells.

这是植物韧皮部装载蔗糖到筛管的一种机制，涉及能量消耗（ATP） 和质子共运输。它发生在伴随细胞（companion cells）中。

Co-transporter protein and proton pump are the carrier proteins on the cell surface membrane.

Hydrostatic pressure gradient between the source and the sink causes mass flow of phloem sap.

1. Active loading of sucrose into the sieve tube at the source greatly decreases the water potential in the tube. Therefore, water enters the sieve tube from nearby xylem down a water gradient by osmosis. This creates a hydrostatic pressure.
2. Unloading of sucrose from sieve tube into sink increases the water potential in the tube. Water moves out of sieve tube into nearby xylem down a water potential gradient by osmosis. This creates a low hydrostatic pressure.

Phloem sap can move upwards or downwards. But the movement is in one direction in one sieve tube at one time. 韧皮部中的细胞液可以向上移动或向下移动，但一次只能往一个方向移动。

Addition Circulatory Systems#

• Open circulatory system in insects 昆虫通过气体来交换物质
• Closed single circulatory system in fishes 鱼类通过鳃获得氧气

Artery (and arteriole)#

Structure

• Inner layer: endothelium

• Middle layer: collagen fibres, elastic fibres and smooth muscle

  含有两种fibres和一种肌肉

• Outer layer: collagen fibres and elastic fibres

  含有两种fibres

用“squamous”这个词来描述endothelium layer：squamous cells

Functions of artery:

• Thick wall - withstand high blood pressure 高血压
• Small lumen - maintain high blood pressure 高血压
• Elastic fibres - stretch to accommodate surges in blood flow and recoil to maintain pressure 允许回弹以适应血流中的压力
• Smooth muscle - regulate blood flow 调解血流量
• Collagen fibres - prevent rupturing or bursting 防止破裂

When the smooth muscles contract:

• Artioles become narrow (vasoconstriction: vaso_constriction)
• Reduce blood flow

When the smooth muscle relax:

• Arterioles become wide (vasodilation: vaso_dilation)
• Increase blood flow

constriction - 用来描述血管粗细

contraction - 用来描述肌肉收缩

Vein (and venule)#

Structure:

• Inner layer: endothelium

• Middle layer: elastic fibre and smooth muscle

  和artery相比没有collagen fibre

• Outer layer: collagen fibres

  相当于把middle layer中的collagen fibre挪到outer layer了

Functions of vein:

• One-way valves - close to prevent the backflow of blood 防止血液到流
• Lots of collagen - provides strength and structure 支撑结构并且防止破裂
• Thin wall - Blood can be moved by contraction of surrounding muscles 可以通过收缩来推动血流
• Large lumen - reduces resistance for blood flow at low pressure 降低血液流动的阻力
• Elastic fibres - stretch to accommodate blood flow and recoil to maintain structure 用来维持弹性结构

Capillary#

Only consists of one layer of endothelium cells.

Functions of capillary:

• Very small diameter - slows blood flow, allowing more time for diffusion 减缓血流，为物质交换提供更多的时间
• Branches between cells - enables rapid diffusion of substances between blood and cells 实现血液与细胞间物质的快速扩散
• Thin walls - lack of muscle and elastic tissue allows quick diffusion 实现快速扩散

Only one red blood cell can pass through the capillary at each time.

Keywords#

1. Plasma - 血浆
2. Red blood cell - 红细胞
3. White blood cell - 白细胞
4. Platelet - 血小板
5. Tissue fluid - 组织液
6. Antibody - 抗体
7. Urea - 尿素
8. Plasma protein - 血浆蛋白
9. Solvent - 溶剂
10. Solute - 溶质
11. Oedema - 水肿
12. Immune response - 免疫反应
13. Pathogen - 病原体
14. Blood clotting - 血液凝固
15. Haemoglobin - 血红蛋白
16. Biconcave - 双凹的
17. Monocyte - 单核细胞
18. Neutrophil - 嗜中性粒细胞
19. Lymphocyte - 淋巴细胞
20. Phagocyte - 吞噬细胞
21. Phagocytosis - 吞噬作用
22. Engulf - 吞没
23. Haem group - 血红素
24. Dissociation - 解离
25. Alveolus/Alveoli - 肺泡
26. Hydrostatic pressure - 静水压

Introduction#