Homeostasis

Define homeostasis

The maintenance of a constant internal environment by the biological systems of the body

Why is homeostasis necessary?

To ensure that the cells and tissues of the body have the correct environment ot function

Define negative feedback

A response of a biological system. When a change from the optimum is detected, the system that produced the change is turned off to return the variable to its optimum level

Which component of a biological system detects a stimulus?

A recepetor

Describe the role of the coordination system in homeostasis

It receives information about stimuli from receptors and determines what the response should be by sending instructions to effectors

What are effectors (in homeostasis)?

Components of a homeostatic mechanism that produce a response to a stimulus

Describe the ways in which the body can conserve heat in a cold environment

• Shivering

• Vasoconstriction

• Raising hairs

Describe the ways in which the body can rapidly lose heat in a warm environment

• Vasodilation

• Increase in sweating

Define the term Osmoregulation and why is it important

It is the regulation of the water potential of the blood.

It is important because cells may shrink or burst if the water potential of the blood is too high or too low, due to the movement of water by osmosis

Where and how is urea formed?

In the liver from the deamination of excess amino acids

What is a nephron?

One functional unit of the kidney

List the structures of the nephron

Glomerulus

Bowman’s capsule

Proximal convoluted tubule

Collecting duct

Afferent arteriole

Efferent arteriole

Outline the blood vessels associated with the nephron

An afferent arteriole from the renal artery forms the glomerulus and the efferent arteriole takes blood away from the glomerulus

The efferent arteriole branches to form capillaries that surround the nephron tubules, these combine to form the renal vein that drains the kidney

Describe the process of ultrafiltration

In the glomerular capillaries there is hydrostatic pressure and this filters blood, forcing water, glucose, urea and ions into the Bowmans capsule to from glomerular filtrate

Proteins and cells are too big to be filtered so they stay in the blood

Describe how the structure of the Bowman’s capsule and the glomerular capillaries is suited to its function

• The capillary endothelial wall is one cell thick and there are gaps between cells to allow substances to pass through

• The renal capsule has podocytes which have space between them to allow filtrate to pass into the renal capsule

• The cbasement membrane prevents larger molecules from leaving the blood

In which part of the nephron does selective reabsorption occur?

In the proximal convoluted tube (PCT)

Water, glucose and ions are reabsorbed back into the blood

Describe the process of selective reabsorption of glucose

• Co-transport of glucose with Na+ into the cells of the PCT and glucose diffuses out of the cell into the blood

• Na+ are actively transported out of the cells to ensure there is a low concentration

• The presence of glucose and Na+ in cells reduces the water potential which draws water out of the tubule by osmosis

What happens in the loop of henle?

• Na+ and Cl- are actively transported out of the ascending limb to the medulla

• This raises concentration of Na+ and Cl- in tissue fluid

• Water is then lost from the descending limb and ion concentration increases

• Na+ and Cl- ions diffuse out of the conc solution in the lower part of the ascending limb

Describe what happens in the Distal Convoluted Tube (DCT) and collecting duct

• In the collecting duct Na+ are actively transported out of the tubule to the tissue fluid and into the blood

• However potassium ions are actively transported into the tubule

  It is the same process as what happens in the loop of henle

What is the role of the hypothalamus in osmoregulation?

It contain receptors that detect the water potential in the blood

If water potential is low ADH is secreted

Where is ADH released in the blood?

ADH from the hypothalamus is transferred to the posterior pituitary gland where it is secreted into the blood

Explain how ADH increases the water potential

• ADH binds to the receptors on the cells of the DCT and collecting duct

• It increases the number of aquaporins in the membrane of the cells in this part of the nephron

• This increases the permeability to water as it can move freely through the membrane into the tissue fluid and medulla

When is insulin secreted?

When blood glucose concentration is high

Describe how insulin reduces blood glucose concentration

• Insulin increases uptake of glucose into cells of the blood by increasing the number of carrier proteins on the membrane

• Insulin stimulates the conversion of glucose into glycogen in muscle and liver cells

What hormone is released from the pancreas if blood glucose concentration is low?

Glucagon

Describe how glucagon increases blood glucose concentration

• Glucagon stimulates liver cells to convert glycogen to glucose (Glycogenesis)

• Glucagon stimulates liver cells to synthesis glucose from non-carbohydrates sources like amino acids and lipids (Gluconeogenesis)

What effect does adrenaline have on blood glucose concentration?

It increases the blood glucose concentration by glycogenesis

Outline the second messenger model in the stimulation of liver cells by adrenaline

• Adrenaline binds to a receptor on the membrane of a liver cell

• Results in conformational change in the receptor protein and the G protein is activated

• G protein stimulates an adenylyl cyclase enzyme in the cell

• Adenyl cyclase converts ATP into cyclic AMP

• Cyclic AMP (second messenger) binds to protein kinase A and activates it

• The protein kinase phosphorylates other enzymes to amplify the signalling cascade and finally the enzyme responsible for glycogenolysis is activated

The presence of glucose and ketones in the urine is indicative of whihc disease?

Diabetes mellitus

Describe how a dipstick can measure the concentration of glucose in a urine sample

• The glucose oxidase on the stick reacts with glucose to form gluconolactone and hydrogen peroxide

• H2O2 reacts with chromogen chemical on the stick catalysed by peroxidase

• The colour produced is compared against a chart to obtain a measurement of glucose concertation

Name a method for measuring blood glucose concentration

A biosensor

• The strip of the sensor contains glucose oxidase which reacts with the glucose in the blood to produce gluconolactone

• An electric current is produced and measured by an electrode

What may be the cause of proteins in urine?

• Diseases affecting the glomeruli

• An infection to the kidney or urinary tract

• Congestive heart failure

Which cell regulate the opening and closing of the stomata?

Guard cells

In what condtions do the stomata open and close?

• Stomata are open when there is high light intensity (allows for diffusion of CO2 during photosynthesis)

• Stomata close at night, during low humidity, high temperature and water stress to limit water loss during transpiration

Explain the mechanism by which guard cells open the stomata

When the cells is turgid, the pore opens and when flaccid the pore is closed

The guard cells drive H+ out of the cell via proton pumps using ATP. This activates K+ channels, leading to a high concentration of K+ in the cell. This reduces water potential, leading to osmosis of water into the cell making it turgid

Explain the mechanism by which guard cells close the stomata

The guard cells drives H+ out of the cell via proton pumps using ATP. This activates K+ channels and their concentration increases. This reduces the water potential leading to the osmosis of water into the cell and turgidity

What is abscisic acid?

A plant hormone released during water stress

What effect does absicic acid have on the stomata?

It causes the closure of stomal pores

Explain how abscisic acid brings about the closure of the stomata

• It binds to the receptors on the cell membrane of the guard cells. This inhibits the proton pump, leading to a rise in pH and stimulates the movement of Ca2+ into the cell

• The Ca2+ act as a second messenger, opening channels to allow anions and K+ ions to leave the cell

• This increases water potential and causes water to leave the guard cell via osmosis, closing the stomata