Homeostasis#

Homeostasis

Homeostasis - the maintenance of a relatively constant internal environment for the cells within the body

The physiological factors controlled in homeostasis in mammals:

core body temperature
metabolic wastes (carbon dioxide and urea)
blood pH
blood glucose concentration
water potential of the blood

Internal Environment - refers to all the conditions inside the body of an organism

Factor	Effect on Cells	Consequences of Imbalance
Temperature	Metabolic reactions proceed at normal rates.	Too Low: Slows down metabolic reactions. Too High: Proteins (including enzymes) denature and lose function.
Water Potential	Maintains normal cell volume and metabolic activity.	Decreases: Water moves out of cells (osmosis). Metabolic reactions slow or stop. Increases: Water moves into cells. Cells swell and may burst.
Glucose Concentration	Provides fuel for respiration, supplying energy to cells.	Too Low: Respiration slows/stops; cells are deprived of energy. Too High: Water moves out of cells (osmosis), disrupting metabolism.
Metabolic Wastes	To be excreted out of body	Too High: alter the blood pH
pH	Enzymes function efficiently.	Fluctuations outside range: Enzymes function less efficiently Extreme values: Enzymes become denatured.

Homeostasis Control#

The composition of blood controls the composition of tissue fluid.

The immediate environment of body cells is the tissue fluid that surround them.

Most control mechanisms in the body use negative feedback to keep internal conditions within the range of set points

set point - the ideal value of a factor that the body controls in homeostasis

Negative Feedback#

negative feedback - a process in which a change in some parameter brings about processes which return it towards normal

the internal or external stimuli occur
the receptor receives the signal
coordination systems response to the signal (nervous system or endocrine system)

Nervous system sends the information in the form of electrical impulses through the neurones.

Endocrine system sends the information in the form of chemical messengers called hormones that travel in the blood.
effector carries out an action in response to a stimulus

This action also called corrective action, bringing the level of a factor to its set point
The factor returns to set point

Positive Feedback#

当一个人吸入过多二氧化碳时，carbon dioxide receptor会让呼吸速度更快。呼吸速度越快，吸进的二氧化碳就越多，这个receptor会反应地更加强烈，让这个人的呼吸速度越来越快。

Positive feedback cannot play any role in keeping conditions in the body constant

Excretion#

Excretion - the removal of toxic or waste products of metabolism and substances in excess of requirements from the body.

The excretory products include:

carbon dioxode
urea

Carbon dioxide is excreted out of the body through lungs

Substance	Excreted By	Source / Production
Carbon Dioxide (CO₂)	Lungs	Aerobic respiration occurring in the mitochondria
Urea	Kidneys	Liver (during the process of deamination)
Excess water & ions	Kidneys	Derived from drinks and food intake

Deamination#

Urea is produced in the liver from the deamination of excess amino acid, and transport from the liver to the kidneys in solution in blood plasma

In the liver cells, the amine group of an amino acid is removed:

\text{amino acid} \rightarrow \text{ammonia} + \text{keto acid}

$\text{ammonia}$ - The amine group together with an extra hydrogen atom, and combine to form ammonia.
$\text{keto acid}$ $keto acid$ - The rest of the amino acid becomes keto acid, that may …
- enter the Krebs cycle and be respired
- converted to glucose
- converted to glycogen or fat for storage

Ammonia is a very soluble and highly toxic compound

在很多水生动物中，ammonia会从血液中diffuse走，并溶解进水里
但是对于很多陆生动物，ammonia会增加血液中的pH值，并影响respiration和cell signaling in the brain

所以在哺乳动物中；

ammonia is immediately converted to urea
urea is less soluble and less toxic
urea cycle (a sequence of reactions) combines ammonia and carbon dioxide to from urea
$\text{ammonia} + \text{carbon dioxide} \rightarrow \text{urea}$

Urea is the main nitrogenous excretory product of humans.

Kidney#

Function:

excretion of urea
osmoregulation (controlled by hormone ADH, 用来调控collecting duct对水的“透性”)

After urea is produced in liver, the urea travels through:

hepatic vein -> vena cava -> heart -> aorta -> renal artery -> kidney
kidney -> ureter -> bladder -> urethra

The key structures in renal system:

fibrous capsule
cortex
medulla
renal pelvis
ureter
renal artery / vein

The key structures of nephron:

arteriole
- afferent arteriole (进入glomerulus)
- efferent arteriole (离开glomerulus)
glomerulus
Bowman’s capsule
proximal convoluted tubule
loop of Henle
- descending limb of loop of Henle
- ascending limb of loop of Henle
distal convoluted tubule
collecting duct
pelvis (the destination of urine)

Ultrafiltration [ulltra-fil-tra-tion]

Location: Bowman’s capsule (glomerulus)

Small molecules (amino acid, water, glucose, urea, inorganic ions) are filtered out of the blood capillaries
they enter the Bowman’s capsule
to form glomerulus filtrate

Selective Reabsorption

Location: Proximal convoluted tubule

Useful molecules are taken back (reabsorbed) from the filtrate
returned to the blood as the filtrate flows along the nephron

Osmoregulation [osmo-regulation]

Location: collecting duct

control the water potential by controlling the water content, and the concentration of ions, especially the sodium ion

Ultrafiltration#

Afferent arteriole has a wider lumen than efferent arteriole, so there is a higher blood pressure in glomerulus, and fluid is forced into Bowman’s capsule
On the endothelium of glomerulus, there are many holes in the endothelial cells and large gaps between these cells
A basement membrane acts as a molecular filter to prevent large plasma proteins and cells from passing through
There are filtration slits between podocytes in the epithelium of Bowman’s capsule
Red blood cells, white blood cells, and large plasma proteins cannot pass through. Small molecules, like glucose, amino acids, urea, water, and ions, pass through.

WARNING
Ultrafiltration涉及到的重要结构：

Glomerulus

Basement membrane （这不是细胞） - acts as a filter, large proteins cannot pass through

Podocytes - have filtration slits, allow filtrate to pass into the lumen (Bowman’s capsule). Filtration slits的作用是提供一个通道可以让滤液经过

Bowman’s capsule

The factors affecting glomerular filtration rate:

Glomerular filtration rate is the rate of fluid filters from the glomerular capillaries into the Bowman’s capsule

efferent arteriole lumen > afferent arteriole lumen:

there is a high blood pressure in glomerular capillaries, to raise the water potential in the blood
the solute concentration in glomerular capillaries > Bowman’s capsule:

this lowers the water potential in the glomerular capillaries

Overall, the water potential in the glomerular capillaries > Bowman’s capsule, water moves down the water potential gradient from the blood into the capsule.

Selective Reabsorption#

Selective reabsorption - only certain substances are taken back to the blood

This process occurs in there locations:

proximal convoluted tubule 主要发生在这边
loop of Henle and collecting duct 大纲上没写，算书本扩展的内容
distal convoluted tubule and collecting duct 大纲上没写，算书本扩展的内容

为了把一些特定的物质回收到血液中，有capillary network surrounds the tubule.

Reabsorption in the proximal convoluted tubule#

The features of epithelial cells of proximal convoluted tubule:

a single layer of cuboidal 立体的 epithelial cell - to reduce the transport distance
many microvilli in the luminal membrane (朝内的一侧) - to increase the surface area
many co-transporter proteins in the luminal membrane
tight junctions that hold adjacent cells together firmly, to prevent the fluid moves between the cells
there are many mitochondria to provide energy for sodium-potassium pumps in the basal membranes
folded basal membrane to give a large surface area for sodium-potassium pump proteins
many aquaporins 水通道蛋白 in the membrane and more rough ER / ribosomes for increased protein synthesis

The process of selective reabsorption:

sodium-potassium pump pumps the sodium ions out of the cell, to the blood
sodium ions in the proximal tubule lumen diffuse through the co-transporter protein, bring the glucoses and amino acid into PCT cell
transport proteins actively transport the glucose and amino acid into the blood
Water moves from filtrate into PCT cells and then into the blood by osmosis

Osmoregulation#

This process regulates the water potential in blood.

一个叫ADH的hormone可以增加collecting duct上的aquaporins，aquaporins可以增加对水的“透性”，让更多水在经过collecting duct时，通过osmosis回到血液中。

Hypothalamus (hypo-tha-la-mus) 中有两种细胞来控制ADH的产生：

osmoreceptor [osmo-receptor]

neurosecretory cells

osmoreceptor检测water potential，当water potential过低时会促使neurosecretory cell产生ADH。

产生的ADH会暂时存在posterior pituitary gland中

Negative feedback of osmoregulation#

there is a decrease in water potential
osmoreceptor in hypothalamus detects this change, and becomes shrink
this stimulates the secretion of ADH by posterior pituitary gland
ADH travels in blood stream
ADH causes the collecting duct cells more permeable to water
more water is reabsorbed by osmosis through aquaporins into blood

反之亦然，posterior pituitary gland不再产生ADH，使collecting duct more permeable to water，让多出来的水分通过urine的方式流失。

The cell signaling of ADH#

ADH travels in blood stream
ADH binds to the receptors in the CSM of the cells on the collecting duct
This activates an enzyme cascade, producing an active phosphorylase enzyme

enzyme cascade - used to amplify the signal
The phosphorylase causes vesicles with aquaporins to move to the CSM
The vesicles fuse with the CSM
This increases the permeability of collecting duct to water
Water diffuses through the collecting duct and enter the blood

Blood Glucose Control#

Glucose conc.	Receptor	Hormone	Effector	Process	Enzyme
High	beta cell	insulin	liver, muscle and fat cells	Glycogenesis	Glycogen synthase
Low	alpha cells	glucagon	liver	Glycogenolysis	Glycogen phosphorylase

Gluconeogenesis [gluco-neo-genesis] 糖异生 - 将不是碳水化合物的东西转化成糖

The pancreas contains groups of cells, known as the islets of Langerhans:

alpha cells - secrete glucagon
beta cells - secrete insulin

The alpha cells and beta cells act as the receptors and the central control for the homeostasis mechanism regulating blood glucose concentration.

insulin, glucagon, and ADH are small peptides (hormones)

Glucose can only enter cells by facilitated diffusion through transporter proteins known as GLUT 也就是说glucose进入细胞的量是可以被GLUT控制的

GLUT - GLUcose Transporter
Muscle cells have GULT4. The vesicles with GLUT4 protein can move to the CSM and fuse with it to increase permeability to glucose and increase glucose uptake
Liver cells have GULT2 proteins, which are always in the CSM. Liver cells increase glucose uptake by phosphorylating glucose.
This phosphorylated glucose cannot pass out through glucose transporters in the CSM 只进不出

这样一来：
- phosphorylated glucose没办法离开cell
- 不会影响到细胞内的glucose concentration

The negative feedback of glucose control#

HIGH blood glucose concentration

the alpha and beta cells in the islets of Langerhans detect the rise in blood glucose

alpha cells stop secreting glucagon

beta cells secrete more insulin
more insulin produced
liver, muscle and fat cells (adipose tissue) respond to more insulin: increase the uptake of glucose

more GLUT4 proteins move to CSM (in muscle cells)

liver cells increase glucose uptake by phosphorylating glucose
rate of respiration of glucose increases
more glucose are stored
- more glucose is converted to glycogen by glycogen synthase (this process is called glycogenesis)
- more glucose is converted to lipid
Gluconeogenesis¹ decreases
Blood glucose concentration decreases and returns to set point

LOW blood glucose concentration

the alpha and beta cells in the islets of Langerhans detect the fall in blood glucose

alpha cells secrete more glucagon

beta cells stop producing insulin
more glucagon produced
liver cells respond to more glucagon: breaking down more glycogen into glucose
- Glucagon acts as a signalling molecule and binds to receptor in the CSM of liver cells
- the 3D shape of the receptor changes (confirmation)
- G protein activated
- adenylyl cyclase activated 这个酶一定要记住
- cAMP² is produced from ATP 需要记住这个地方的second messenger的名字
- … which is second messenger
- … enzyme cascade occurs
- signal is amplified
- glycogen is hydrolysed to glucose (glycogenolysis)
- glucose is released into blood

[From syllabus] The principles of cell signalling using the example of the control of blood glucose concentration by glucagon:

Binding of hormone to cell surface receptor causing conformational change

Activation of G-protein leading to stimulation of adenylyl cyclase

Formation of the second messenger, cyclic AMP (cAMP)

Activation of protein kinase A by cAMP leading to initiation of an enzyme cascade

Amplification of the signal through the enzyme cascade as a result of activation of more and more enzymes by phosphorylation

Cellular response in which the final enzyme in the pathway is activated, catalysing the breakdown of glycogen

作为补充，hormone adrenaline也可以通过类似于glucagon的方式在liver中增加血液中glucosee的含量:

Adrenaline binds to receptors (in the cell surface membrane)
Receptor changes conformation
G proteins are activated
Adenylyl cyclase is activated
Cyclic AMP (cAMP) is made
cAMP acts as a second messenger
Activates a kinase (protein)
Enzyme cascade orrurs
Glycogen is broken down to glucose (glycogenolysis)
Glucose diffuses into the blood
This results in an increase in blood glucose concentration

Measure the Concentration of Glucose#

Test strips#

Test strips can measure the glucose concentration in the urine

It can be used to determine diabetes 糖尿病

If the blood glucose concentration increases above the renal threshold, not all of the glucose is reabsorbed from the filtrate in the proximal convoluted tubule and some will be present in the urine.

Test strips contain the immobilized enzymes glucose oxidase and preoxidase, a small pad at one end of a test strip is dipped in urine for a brief time, when glucose is exist:

Glucose oxidase oxidises glucose in to gluconic acid and hydrogen preoxide
$\text{glucose} + \text{oxygen} \xrightarrow {\text{glucose oxidase}} \text{gluconic acid} + \text{hydrogen preoxide}$
Peroxidase catalyses a reaction between hydrogen peroxide and colourless chromogen to give colour change
$\text{hydrogen peroxide} + \text{chromogen} \xrightarrow {\text{peroxidase}} \text{oxidised chromogen} + \text{water}$

Chromogen - colourless

Oxidised chromogen - coloured

Test strips can indicate the different concentrations of glucose in the urine,

$\uparrow$ glucose $\uparrow$ darker

There are some disadvantages of using test strips:

Can’t indicate the current blood glucose concentration 无法显示当前血液中的葡萄糖浓度
Not numerical

NOTE
[Past Paper Question] Explain how dip sticks function to test for glucose in a sample of urine (8 marks):

回答角度得分点
包含什么 - stick has immobilised enzymes
- glucose oxidase
- … which is specific to glucose
- peroxidase
使用方式 - stick dipped in urine
反应过程 - glucose reacts to give hydrogen peroxide
- hydrogen peroxide reacts with chromogen
- colourless chromogen becomes coloured
比较结果 - compare with colour chart
- more glucose, darker colour

回答角度	得分点
包含什么	- stick has immobilised enzymes - glucose oxidase - … which is specific to glucose - peroxidase
使用方式	- stick dipped in urine
反应过程	- glucose reacts to give hydrogen peroxide - hydrogen peroxide reacts with chromogen - colourless chromogen becomes coloured
比较结果	- compare with colour chart - more glucose, darker colour

Biosensor#

When blood on pad is inserted into biosensor, the immobilised enzyme glucose oxidase produces hydrogen peroxide:

\text{glucose} + \text{oxygen} \xrightarrow {\text{glucose oxidase}} \text{gluconic acid} + \text{hydrogen preoxide}

Hydrogen peroxide is oxidised at an electrode that detects electron flow.

$\uparrow$ glucose $\uparrow$ electron flow

与test strips不同的是，test strips通过颜色变化来检测hydrogen peroxide的浓度（用来指示glucose的浓度），而biosensor通过电流强度来检测hydrogen peroxide的浓度

Then, a digital reading is produced, the numerical result tells the current blood glucose concentration.

NOTE
[Past Paper Question] Describe how a glucose biosensor works (4 marks):

回答角度得分点
操作方式 - dip the blood on the pad, and inserted into biosensor
内部原理 - glucose oxidase
- … produces hydrogen peroxide and gluconic acid from glucose
- electric current is detected at an electrode
- the current is proportional to glucose concentration
读取结果 - read the digital reading

回答角度	得分点
操作方式	- dip the blood on the pad, and inserted into biosensor
内部原理	- glucose oxidase - … produces hydrogen peroxide and gluconic acid from glucose - electric current is detected at an electrode - the current is proportional to glucose concentration
读取结果	- read the digital reading

Stomata Control#

Each stoma is surrounded by 2 guard cells:

The features of a typical guard cell:

最重要的特征：Thicker cell wall facing the stomatal pore 较厚的一面对着中间的孔
Cellulose microfibrils are arranged into bands around the cell
The cell walls have no plasmodesmata 没有胞间连丝
The cell surface membrane is often folded and contains many channel and carrier proteins. 有很多用于运输物质的蛋白质
The cytoplasm has a high density of chloroplasts and mitochondria
Chloroplasts have thylakoids, but they have few grana. The starch grains in chloroplasts increase in size as starch is stored at night and decrease in size during the day
Smaller vacuoles rather than one large vacuole

植物通过控制stoma中的水分来控制stoma的开关：

Water moves into the guard cells - stoma opens
Water moves out of the guard cells - stoma closes

水进入到stoma中会把guard cells撑开，guard cell的两个细胞壁厚度不一样，导致内部充满水时会往一侧（内侧）弯曲，从而使stoma打开

Environmental stimuli causing stomata to open 气孔开放	Environmental stimuli causing stomata to close 气孔关闭
Increasing light intensity (光照强度增加 - 促进光合作用需求)	Darkness (黑暗 - 光合作用停止，不需要 CO₂)
Low carbon dioxide concentrations in the air spaces within the leaf (叶内气腔 CO₂ 浓度低 - 需要吸入更多 CO₂)	High carbon dioxide concentrations in the air spaces within the leaf (叶内气腔 CO₂ 浓度高 - 已满足光合作用需求)
	Low humidity (低湿度 - 减少水分蒸腾损失)
	High temperature (高温 - 避免过度蒸腾导致水分流失)
	Water stress — when the supply of water from the roots is limited and/or there are high rates of transpiration (水分胁迫 - 根部供水受限或蒸腾速率过高，防止植物失水萎蔫)

	Stomata open during the day (白天气孔开放)	Stomata closed during the day (白天气孔关闭)
Advantage	Leaves gain carbon dioxide for photosynthesis 叶片获取二氧化碳以进行光合作用	Water is retained inside the leaf, which is important in times of water stress 水分保留在叶片内部，这在水少的时候非常重要
Disadvantage	Leaves lose large amounts of water by transpiration 叶片通过蒸腾作用流失大量水分	Supply of carbon dioxide decreases so the rate of photosynthesis decreases 二氧化碳供应减少，因此光合作用速率降低
总结	更多二氧化碳，但更少水分	更多水分，但更少二氧化碳

Closing of stomata at night when photosynthesis cannot occur 夜间由于无法进行光合作用而关闭气孔

reduces the rate of transpiration 降低蒸腾作用速率

conserves water 保存水分

The mechanics of opening the stoma#

proton pump pumps out the $H^+$ ions out of the cell
$K^+$ ions diffuse into the cell down an electrochemical gradient
high $K^+$ ion concentration lowers the water potential
water moves into the cell (through aquaporins)
water makes the cell turgid and the stoma opens

Closing the Stoma#

Stomata close when the hydrogen ion pump proteins stop and potassium ions leave the guard cells and enter neighbouring cells. 让最开始使stoma开启的环节停止，就可以通过一系列反应让stoma关闭。

In conditions of water stress, the hormone abscisic acid (一般用简写ABA) is produced in plants to stimulate stomatal closure.

The plant secretes ABA in dry condition
ABA binds to the receptor that on the CSM of the guard cells
This inhibits the proton pump
High concentration of hydrogen ions remains in the cell
water potential of the cell increases
water moves out of the cell through osmosis
volume of the guard cells decreases
guard cells become flaccid to close the stomata

Keywords#

Corrective action - 纠正措施
A response or series of responses that return a physiological factor to the set point so maintaining a constant environment for the cells within the body
Deamination - 脱氨
The breakdown of excess amino acids in the liver, by the removal of the amine group; ammonia and, eventually, urea are formed from the amine group
Effector - 效应器
A tissue or organ that carries out an action in response to a stimulus; muscles and glands are effectors
Excretion - 排泄
The removal of toxic or waste products of metabolism from the body
Homeostasis - 体内平衡
The maintenance of a relatively constant environment for the cells within the body
Hormone - 激素
A substance secreted by an endocrine gland that is carried in blood plasma to another part of the body where it has an effect
Negative feedback - 负反馈
A process in which a change in some parameter (e.g. blood glucose concentration) brings about processes which return it towards normal
Positive feedback - 正反馈
A process in which a change in some parameter such as a physiological factor brings about processes that move its level even further in the direction of the initial change
Receptor - 受体
A cell or tissue that is sensitive to a specific stimulus and communicates with a control centre by generating nerve impulses or sending a chemical messenger
Set point - 设定点
The ideal value of a physiological factor that the body controls in homeostasis
Stimulus - 刺激
(plural stimuli) a change in the external or internal environment that is detected by a receptor and which may cause a response
Urea - 尿素
A nitrogenous excretory product produced in the liver from the deamination of amino acids
Afferent arteriole - 入球小动脉
Arteriole leading to glomerular capillaries
Antidiuretic hormone (ADH) - 抗利尿激素 (ADH)
Hormone secreted from the pituitary gland that increases water reabsorption in the kidneys and therefore reduces water loss in urine
Bowman’s capsule - 鲍曼囊
The cup-shaped part of a nephron that surrounds a glomerulus and collects filtrate from the blood
Collecting duct - 集合管
Tube in the medulla of the kidney that carries urine from the distal convoluted tubules of many nephrons to the renal pelvis
Distal convoluted tubule - 远曲小管
Part of the nephron that leads from the loop of Henle to the collecting duct
Efferent arteriole - 出球小动脉
Arteriole leading away from glomerular capillaries
Glomerulus - 肾小球
(plural glomeruli) a group of capillaries within the ‘cup’ of a Bowman’s capsule in the cortex of the kidney
Loop of Henle - 亨利氏袢
The part of the nephron between the proximal and distal convoluted tubules
Nephron - 肾单位
The structural and functional unit of the kidney composed of Bowman’s capsule and a tubule divided into three regionsproximal convoluted tubule, loop of Henle and distal convoluted tubule
Podocyte - 足细胞
One of the cells that makes up the lining of Bowman’s capsule surrounding the glomerular capillaries
Proximal convoluted tubule - 近曲小管
Part of the nephron that leads from Bowman’s capsule to the loop of Henle
Selective reabsorption - 选择性重吸收
Movement of certain substances from the filtrate in nephrons back into the blood
Ultrafiltration - 超滤
Filtration on a molecular scale separating small molecules from larger molecules, such as proteins (e.g. the filtration that occurs as blood flows through capillaries, especially those in glomeruli in the kidney)
Osmoreceptor - 渗透感受器
Type of receptor that detects changes in the water potential of blood
Osmoregulation - 渗透调节
The control of the water potential of blood and tissue fluid by controlling the water content and/or the concentration of ions, particularly sodium ions
Adenylyl cyclise - 腺苷酸环化酶
Enzyme that catalyses formation of the second messenger cyclic AMP
Biosensor - 生物传感器
A device that uses a biological material such as an enzyme to measure the concentration of a chemical compound
Cyclic AMP (c-AMP) - 环磷酸腺苷 (cAMP)
A second messenger in cell– signalling pathways
Glucagon - 胰高血糖素
A small peptide hormone secreted by the α cells in the islets of Langerhans in the pancreas to bring about an increase in the concentration of glucose in the blood
Gluconeogenesis - 糖异生
The formation of glucose in the liver from non-carbohydrate sources such as amino acids, pyruvate, lactate and glycerol
Glycogenesis - 糖原合成
Synthesis of glycogen by addition of glucose monomers
Glycogenolysis - 糖原分解
The breakdown of glycogen by removal of glucose monomers
Insulin - 胰岛素
A small peptide hormone secreted by the β cells in the islets of Langerhans in the pancreas to bring about a decrease in the concentration of glucose in the blood
Islet of Langerhans - 朗格汉斯岛
A group of cells in the pancreas which secrete insulin and glucagon
Phosphorylase kinase - 磷酸化酶激酶
An enzyme that is part of the enzyme cascade that acts in response to glucagon; the enzyme activates glycogen phosphorylase by adding a phosphate group
Protein kinase A - 蛋白激酶A
Enzyme that is activated by c-AMP and once activated adds phosphate groups to other proteins, including phosphorylasekinase, to activate them
Abscisic acid (ABA) - 脱落酸 (ABA)
An inhibitory plant growth regulator that causes closure of stomata in dry conditions
Electrochemical gradient - 电化学梯度
A gradient across a cell surface membrane that involves both a difference in concentrations of ions and a potential difference
Guard cell - 保卫细胞
A sausage-shaped epidermal cell found with another, in a pair bounding a stoma and controlling its opening or closure
Endocrine gland - 内分泌腺
An organ that secretes hormones directly into the blood; endocrine glands are also known as ductless glands
Endocrine system - 内分泌系统
Consists of all the endocrine glands in the body together with the hormones that they secrete

Gluconeogenesis 糖异生 - the formation of glucose in liver from non-carbohydrate sources such as amino acid, pyruvate, lactate, fatty acid and glycerol ↩
cAMP - a second messenger to activate an enzyme cascade ↩

Homeostasis#

Homeostasis Control#

Negative Feedback#

Positive Feedback#

Excretion#

Deamination#

Kidney#

Ultrafiltration#

Selective Reabsorption#

Reabsorption in the proximal convoluted tubule#

Osmoregulation#

Negative feedback of osmoregulation#

The cell signaling of ADH#

Blood Glucose Control#

The negative feedback of glucose control#

Measure the Concentration of Glucose#

Test strips#

Biosensor#

Stomata Control#

The mechanics of opening the stoma#

Closing the Stoma#

Keywords#

Footnotes#