5.1 .1 + 5.1.2 (Communication and homeostasis + Excretion and homeostatic control)

A-Level Biology -- OCR A

Homeostasis definition

Maintenance of constant internal conditions, irregardless of changes to the external environment

Difference between receptors and effectors

Receptors detect a change in environment and effectors provoke a change to maintain constant internal conditions

Negative feedback definition

Opposes a change

Positive feedback definition

Enhances a change

What part of the brain is the most involved in homeostasis?

Hypothalamus

Physiological reactions that endotherms have to cold temperatures

1. Shivering (spontaneous muscle contractions)

2. Vasoconstriction

3. Increase in metabolic rate in the liver (releases energy)

4. Piloerection (hairs stand up to trap a thick layer of insulating air)

Behavioural responses to temperature in endotherms

HOT

1. Find shade

2. Reduced physical activity

3. Spread out limbs

4. Evaporative cooling mechanisms (such as panting)

COLD

1. Huddling

2. Migration

3. Postural contraction (curling up)

4. Finding shelter

Behavioural responses to temperature in ectotherms

HOT

1. Conduction (pressing body to a cold surface)

2. Going into shade

3. Moving from land to water

COLD

1. Basking (spreading out in the sun to maximise surface area exposed to heat radiation from the sun)

2. Conduction (pressing body to a hot surface)

3. Going into the sun

4. Moving from water onto land

5. Huddling to reduce heat loss when it turns to night

What is a stimulus?

Any change in the environment that causes a response

What is a response?

A change in behaviour or physiology as a result of a change in the environment

What makes a good communication system?

1. Covers the whole body

2. Enables specific communication

3. Enables rapid communication

4. Enables short-term and long-term responses

5. Enables cells to communicate with each other

What are two ways that cells can communicate with each other?

1. Signalling by secreted molecules

2. Signalling by plasma-membrane-bound molecules

What are the two major communication systems?

1. The hormonal system

2. The neuronal system

What are the differences between the hormonal system and the neuronal system?

NEURONAL

1. Rapid responses

2. Involves neurotransmitters

3. Involves action potentials

4. Involves neurones

HORMONAL

1. Long-term responses

2. Involves hormones

3. Involves target cells

4. Involves the endocrine organ

5. Hormones travel in the blood

Excretion definition

The removal of toxic waste products of metabolism and substances in excess of requirement from the body

What are some of the effects of kidney failure?

Loses ability to filter waste so may be glucose or proteins in the urine or more/less concentrated urine. Symptoms include headaches, fatigue, seizures, coma, oedema (fluid retention so swelling/puffin in face/ankles/feet/hands), confusion, fatigue.

What is vasoconstriction?

Arterioles near the skin surface restrict and more blood flows through the capillaries further away from the skin. Less heat energy radiating from the skin.

What is vasodilation?

The arterioles near the surface of the skin dilating due to the contraction of muscles near the blood vessels that connect to the arterioles. This constricts the blood vessels supplying the capillaries. More blood flows through the arterioles near the skin surface so more heat is lost by radiation.

How can excretory products be used in medical diagnosis?

Tests in urine to see if there are things there that shouldn’t be or if there are things missing that should be there. Glomerular filtration rate (GFR) test - low GFR suggests that the kidneys are not working properly.

Advantages and disadvantages of kidney transplant?

ADVANTAGES

1. Longer survival

2. Better quality of life

3. No need for dialysis, which is time-consuming

DISADVANTAGES

1. Lifelong immunosuppression

2. Major surgery (which carries risks in itself)

3. Need to wait for a donor and a match

How is water potential maintained in the blood? IN DETAIL

This example is what happens when the water potential ($\psi$) is too low. The $\psi$ in the blood is lower than in cells so when the blood gets to the brain, water moves by osmosis from osmoreceptor cells in the hypothalamus into the blood and they shrink. Neurosecretory cells are stimulated to produce ADH. This travels down the axon of the nerve in the hypothalamus and into the terminal axon in the posterior pituitary gland where it is released. The ADH travels in the blood down to the collecting duct where it binds to specific membrane-bound receptors on cells in the collecting duct. This causes a chain of enzyme-controlled reactions which result in vesicles containing aquaporin fusing to the plasma membranes of the cells of the walls of the collecting duct, making the collecting duct walls more permeable to water.