OCR 21st Century B4

Revision cards for OCR 21st Century Biology  B4.

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  • Created by: Sophie :)
  • Created on: 24-02-12 11:57

What is Homeostasis?

Homeostasis- maintaining a constant internal environment.


  • core body temperature- you need to get rid of excess body heat when you're hot and retain body heat when you're cold. Your body is kept at a constant temperature of about (37 degrees Celsius).
  • blood sugar levels
  • salt levels
  • water levels- you need to keep a balance between the water you gain and the water you pee, sweat and breathe out.
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Negative Feedback

Negative feedback- a system that responds to change by reversing it.

When the level of something becomes too high or too low then your body uses negative feedback to bring it back to normal.

Homeostasis is controlled by:

  • Receptors- part of the body that detects change e.g receptor detects stimulus level is too high
  • Control centres- brain & spinal cord e.g. the processing centre receives the information and coordinates a response
  • Effectors- part of body that initiates the response (muscles/glands) e.g. counteracts the change- the level decreases

Some effectors work antagonistically, e.g. one effector heats and another cools; they work at the same time to achieve a precise temperature.

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More Negative Feedback.

Some artificial systems such as incubators work in a similar way to the body's control systems- using negative feedback. Incubators have a thermostat to detect temperature changes, and a heater to correct the change by either heating or not heating.

  • A receptor in the thermostat detects that the temperature is too low.
  • The thermostat uses the information to coordinate a response.
  • The heater is switched on.
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Diffusion is the passive overall movement of particles from a region of their HIGHER CONCENTRATION to a region of their LOWER CONCENTRATION. The bigger the difference in concentration, the faster the diffusion rate.

Gas has a fast speed of diffusion because in a gas the particles move a lot faster as they get "bumped into" much faster. Liquid has a medium speed. Diffusion across cell membranes is important as it's one way our cells can obtain the things they need to function.

  • Cells produce carbon dioxide, which means that there is always a higher concentration of CO2 in the cells than in the blood. So CO2 will move out of the cells and into the blood by diffusion to be carried away.
  • Cells need oxygen and dissolved food (e.g. glucose) to function. There is a lower concentration of these in the cells than in the blood, so oxygen and dissolved food diffuse out of the blood into cells.
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Osmosis is the overall movement of water from a DILUTE to a MORE CONCENTRATED solution through a PARTIALLY PERMEABLE MEMBRANE. A partially permeable membrane is just one that only allows certain substances to diffuse through it. E.g. it may only allow small molecules like water to pass through and not larger molecules like starch.

The concentrated starch solution gets more dilute as more water moves in. The water acts like it's trying to "even up" the concentration either side of the membrane. 

  • Animal cells can't withstand big changes in the amount of water they contain
  • If too much water moves into an animal cell by osmosis, it can rupture, killing the cell
  • If too much water moves out of an animal cell, it shrivels up and can't function properly
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Enzymes are complex proteins that speed up chemical reactions- biological catalysts. Yeast cells contain an enzyme called catalase.

A substarate is a molecule that is changed in a reaction. Every enzyme molecule has an active site- where a substrate joins on to the enzyme.

For an enzyme to work the substrate has to be the right shape to fit into the active site. If the shape doesn't fit, the reaction isn't catalysed. E.g. the lock and key model; the substrate fits into the enzyme like a key into a lock.  

The reaction is slower at higher temperatures because less hydrogen eproxide breaks down, hydrogen peroxide will no longer fit the enzyme.

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Controlling Body Temperature

The reactions in your body work best at 37 degrees Celcius. Temperature receptors in the skin detect the external temperature and receptors in the hypothalamus detect the temperature of the blood. The nervous system uses a negative feedback mechanism to control the temperature:

  • Temperature receptors detect the core body temperature is too high
  • Hypothalamus (processing centre) triggers effectors
  • Effectors e.g sweat glands produce a response to counteract the change.

Different responses are produced by effectors to counteracr an increase or decrease in body temperature:

  • Vasodilation- the blood vessels near the surface of the skin are filled with blood. Energy from the warm blood is transferred down the temperature gradient to the environment.
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Controlling Body Temperature Continued

Vasoconstriction- the muscles in the walls of blood vessels near the surface of the skin contract. Less blood flows near the surface of the skin so less blood is lost to the environment.             

Heat stroke is when the core body temp reaches over 37.5 degrees C. Symptoms are dehydration, confusion, dizziness and excessive sweating. Treatment should be trying to cool the patient down by ensuring the room is ventilated, the patient has water and the body is cooled by water. When you get very hot you naturally produce more sweat and the water loss may cause you to be dehydrated

Hypothermia occurs when the core body temp is below 35 degrees C. Symptoms include shivering, low energy, confusion, memory loss and unconsciousness. Treatment is warming the patient up by slowly moving them to a warmer place, giving them warm and dry clothes and insulating blankets.

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Controlling water content

Inputs- water can be gained from drinks, food and respiration

Outputs- water can be lost through sweating, breathing, in faeces and in urine.

Kidneys help to balance substances in the body:

  • 1) they filter small molecules from the blood, including water, sugar, salt & urea
  • 2) they reabsorb various things- all glucose, some: water, salts (depending on circumstances)
  • 3) whatever isn't reabsorbed forms urine and is excreted by the kidneys and stored in the bladder.

ADH makes the walls of the kidney tubes more permeable, when ADH is absent, water is lost to the bladder.

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Controlling water content 2

Kidneys balance water levels by producing dilute or concentrated urine which depends on 3 things.

  • 1) external temp- affects the amount you sweat & sweat contains water so sweating results in water loss. When it's hot the kidneys reabsorb more water into the blood. This leaves a small amount of water so only a small amount of concentrated urine is produced.
  • 2) exercise- makes you hotter, sweat to cool down. Same effect as heat-  concentrated, small volume of urine.
  • 3) intake of fluids/salts- not enough water/ too much salt= concentrated urine. Lots of water= lots of dilute urine
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Controlling water content 3

Concentration of urine controlled by ADH (anti-diuretic hormone). Released into the blood stream by the pituitary gland. Brain monitors the water content of the blood & tells the pituitary gland to release ADH. Controlled by negative feedback.

  • Receptor detects water content is too high
  • processing centre int he brain receives impulse & coordinates a response
  • the pituitary gland releases less ADH, so the kidneys reabsorb less water

Drugs and urine                                                                                                                    

Caffeine- produces greater volume of dilute urine.                                                                                                            Alcohol- greater volume of dilute urine, makes people more dehydrated suppresses ADH production (less water is reabsorbed into the kidneys).                                                                                                                            Ecstasy- reduces the volume of urine (over heating).

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♥ Secret - Team GR


thnx .. quite helpful :)

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