OCR GCSE Additional Science - B4 Homeostasis

Brief notes for the Additional Science unit for OCR, B4 Homeostasis.

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  • Created by: Rory
  • Created on: 21-12-11 11:05

What is Homeostasis?

Homeostasis is the maintenance of a constant internal environment.

What factors need to be maintained? 

Temperature and water.

Strenuous exercise such as scuba diving and mountain climbing in hot and cold climates will affect homeostasis. What is effected?

  • Temperature
  • Blood oxygen levels
  • Salt levels
  • Hydration

How do artifical systems work?

  • Use receptors (to detect stimuli)
  • Processing centres (to receive information and co-ordinate responses)
  • Effectors (to produce the response automatically)
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Negative feedback

Define negative feedback

When a factor (temperature for example) goes above or below a normal level.

What is this change detected by?

The receptor. 

A correcting mechanism is then started by the effector. Factor level is now back to normal.

Some effectors work antagonistically (can do more increasing and decreasing). This allows for a more senstive response.

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Diffusion & Osmosis

What is diffusion?

Diffusion is the passive movement of molecules from areas where they are more concentrated to areas where they are less concentrated.

What chemicals move by diffusion?

Oxygen, carbon dioxide and dissolved food.

What is osmosis?

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. The membrane allows water molecules through but not solute molecules because they are too large.

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Active transport

Active transport is the movement of a chemical substance from where they are in low concentration to where they are in a high concentration using energy.

Glucose is pumped into some cells by active transport.


Key points;

  • Enzymes are proteins
  • Enzymes are affected by temperatures
  • Enzymes speed up chemical reactions inside cells
  • All enzymes need a specific, constant temperature to work efficiently.

At low temperatures - increasing temperature even a little bit speeds up the reaction a lot. This happens because as temperature rises, the frequency & energy of collisions increases.

At high temperatures - enzymes are denatured ie. they stop working.

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Lock and key model


Only molecules with the correct shape can fit into the enzyme molecule.

This means enzymes are specific (only work on 1 type of substance)

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The active site


The place where the molecule fits in is the active site.

If the shape changes too much, the enzyme cannot join with the substrate molecule. Heating above a certain temperature or extreme pH levels can change the shape of the active site. This is irreversible. 

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Keeping body temperature constant

Enzymes work best at 37C in humans. Body temperature needs to stay close to this. To maintain this, energy loss and gain must be balanced. Energy is transferred from the blood to the body tissues in cooler areas.

If the temperature of body is too high - heat needs to be transferred to the environment. This is done by sweating.

If it is too low - body starts to shiver. 

Controlling body temperature requires...

  • Temperature detectors in skin to detect external temperature
  • Temperature detector in brain to measure temperature of blood
  • Brain (a processing centre) to receive info from temp receptors in the skin and brain
  • Effectors which carry out the response.
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Vasodilation & Vasoconstriction

What is vasodilation?

Vasodilation is the widening of blood vessels so that more blood flows close to the skin surface - increasing energy loss.

What is vasoconstriction?

Vasoconstriction is when the capillaries in the skin narrow so that less blood flows to the skin surface - reducing energy loss.

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Hypothermia is when the core body temperature falls below 35C.

Causes - Being exposed to low temperatures for a long time.

Symptoms - Core temperature below 35C leads to: shivering, drowsiness, confusion and slurred speech.

Core temperature below 30C = coma

Core temperature below 28C = breathing stops

Initial treatment Insulate them, warm gently with towels and give them warm drinks (not alcoholic)

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Controlling water balance in the body


Food and drink

Water made in respiration



Faeces, urine


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The kidneys & how they work

The kidneys two main jobs;

1. Remove waste urea from blood

2. Keep a balance of other chemicals in blood; including water.

How they work

Filter out all small molecules from the blood

Reabsorbing all of the glucose

Reabsorbing as much salt as the body needs

Reabsorbing as much water as body needs

Excreting the remaining urea, excess water and salt as urine, which is stored in the bladder.

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Water balance

The concentration of blood plasma is monitored as it passes through the brain. If the blood is too dilute then kidneys excrete more water in the urine.

If the blood is too concentrated then kidneys excrete less water in the urine. The amount of water in the blood depends on: external temperature, exercise, salts and fluid intake.

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Urine concentration & alcohol and ecstasy effects

The concentration of urine is controlled by a hormone called anti-diuretic hormone (ADH). ADH is released into the blood via the pituitary gland. Controlling water balance is an example of negative feedback. 

Effects of alcohol and ecstasy

Alcohol results in a greater volume of more dilute urine. This is due to ADH suppression. This can lead to dehydration.

Ecstasy results in a smaller volume of more concentration urine. This is due to increased ADH production.

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thank you! this helped me a little more for my Biology mock exam tomorrow 

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