StemUp: OCR A A level Biology 5.1.1 Communication and homeostasis

Why is the ability to respond to changes in the environment important for organisms? (3)

- Helps organisms survive by allowing them to respond to changes in

- External environment: e.g., animals avoiding harmful environments that are too hot or cold

- Internal environment: ensures optimal conditions for metabolic reactions

How do plants respond to changes in their environment? (2)

- Plants grow towards light

- To maximise the amount of light available for photosynthesis

What is cell signalling? (2)

- Communication between cells that produces a response

- Receptors need to communicate with effectors

How can cell signalling occur? (4)

- Adjacent cells

- E.g., cells in the nervous system communicate by secreting neurotransmitters that send signals to nearby nerve or muscle cells

- Distant cells

- E.g., hormones released by the hormonal system travel in the blood to act on distant cells

What role do cell surface receptors play in cell signalling? (1)

- Cell surface receptors enable cells to recognise

- The chemicals involved in cell signalling

What is homeostasis? (2)

- The maintenance of a constant internal environment despite changes in the external environment

- Essential for normal cell function and preventing damage

What factors must be maintained within certain limits in homeostasis? (4)

- Temperature

- Water potential

- pH

- Blood glucose levels

What components are involved in homeostatic systems? (3)

- Receptors: Detect stimuli (specific to one type of stimulus)

- Communication system: Nervous or hormonal system

- Effectors: Cells that bring about a response to counteract the change

What is negative feedback in homeostasis? (2)

- A mechanism that restores levels to normal when they are too high or too low

- Information is communicated via the nervous or hormonal system to effectors, which counteract the change

Why does negative feedback only work within certain limits? (2)

- If the change is too big

- Effectors may not be able to counteract it

What is positive feedback, and why is it not involved in homeostasis? (2)

- Positive feedback amplifies changes rather than keeping the internal environment constant

- It is used in situations where rapid activation is needed, e.g., blood clotting or dilation of the cervix during childbirth.

What is an example of positive feedback in the body? (2)

- Blood clotting: Activated platelets release chemicals that trigger more platelets to be activated, forming a blood clot

- Childbirth: Dilation of the cervix during labor is driven by positive feedback

How do ectotherms (e.g., reptiles, fish) control their body temperature? (3)

- Ectotherms cannot control body temperature internally

- They regulate their temperature through behavioural changes (e.g., reptiles bask in the sun to gain heat)

- Many aquatic ectotherms do not need to thermoregulate because of water's high specific heat capacity

How do endotherms (e.g., mammals, birds) control their body temperature? (2)

- Endotherms control body temperature internally through homeostasis

- They can also regulate temperature by behavioural means (e.g., moving into the shade)

How does the internal temperature of ectotherms compare to external temperature? (1)

The internal temperature of ectotherms depends on the external temperature

How does the internal temperature of endotherms compare to external temperature? (1)

The internal temperature of endotherms is less affected by the external temperature (within certain limits)

How does external temperature affect the activity level of ectotherms? (2)

- Activity levels in ectotherms depend on external temperature

- Ectotherms are more active at higher temperatures and less active at lower temperatures to conserve energy

How does external temperature affect the activity level of endotherms? (2)

- Activity levels in endotherms are largely independent of external temperature

- Endotherms can remain active at any temperature (within certain limits)

How do ectotherms generate heat? (2)

- Ectotherms have a variable metabolic rate and generate very little heat themselves

- They need less food than endotherms because they use less energy regulating their temperature and can survive in habitats where food is scarce

How do endotherms generate heat? (2)

- Endotherms have a constantly high metabolic rate and generate a lot of heat from metabolic reactions

- Therefore, they need to consume more food than ectotherms of similar size to meet their metabolic needs.

How does increased sweating help reduce body temperature in mammals? (2)

- As the water in sweat evaporates from the skin's surface

- Heat is lost, cooling the blood beneath the skin

How does vasodilation help reduce body temperature in mammals? (2)

- Arterioles near the skin's surface dilate, allowing more blood to flow through the capillaries in the dermis

- This increases heat loss through radiation, lowering body temperature

How does the position of hairs help reduce body temperature in mammals? (2)

- When it's hot, the erector pili muscles relax, causing hairs to lie flat

- This reduces the insulation effect, allowing more heat to escape

How does reduced sweating help increase body temperature in mammals? (2)

- Much less sweat is secreted by sweat glands when it's cold

- Reducing the amount of heat lost from the body

How does vasoconstriction help increase body temperature in mammals? (3)

- Arterioles near the surface of the skin constrict

- Allowing less blood to flow through the capillaries in the dermis

- Reducing heat loss

How does the position of hairs help increase body temperature in mammals? (3)

- Erector pili muscles contract

- Making hairs stand up

- Trapping more air, and preventing heat loss

How does shivering help increase body temperature in mammals? (3)

- Muscles contract in spasms

- Causing shivering

- Which produces more heat through increased respiration

How do hormones help increase body temperature in mammals? (3)

- The body releases adrenaline and thyroxine

- Which increase metabolism

- Producing more heat

What anatomical adaptations help endotherms living in cold climates stay warm? (3)

- Small surface area to volume ratio to reduce heat loss

- Thick layer of insulating fat under the skin

- Hibernation to conserve energy and warmth

How does negative feedback help control body temperature in mammals? (2)

- Body temperature is maintained at a constant level by the hypothalamus

- Which receives information from thermoreceptors

Where are thermoreceptors located and what do they detect? (3)

- Thermoreceptors in the hypothalamus detect the internal temperature (temperature of the blood).

- Peripheral thermoreceptors in the skin detect the external temperature (temperature of the skin).

- A combination of the two gives greater control over body temperature

How do thermoreceptors and the hypothalamus work together to regulate temperature? (3)

- Thermoreceptors send impulses along sensory neurons to the hypothalamus.

- The hypothalamus sends impulses along motor neurons

- To effectors (e.g., skeletal muscles, sweat glands) to restore body temperature to normal