B3.1 Exchange of Materials

  • Created by: Fiona S
  • Created on: 26-04-15 10:58


  • We see diffusion in gases and liquids
  • Diffusion moves down the concentration gradient
  • Temperature increase the rate
  • Steeper concentration gradient increases the rate
  • Larger surface area increase the rate of movement across the membrane
  • Applying more pressure will increase the rate
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Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules.

Water molecules move from an area of high water concentration to an area of low water concentration.

This is similar to diffusion as they move from a high to a low concentration and both are passive.

On the other hand, water has to go through a partially permeable membrane and diffusion is both gases and liquids whereas osmosis is only water.

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Osmosis in Cells

Animal Cells

If the concentration of body fluids is the same as the concentration inside cells, equal amounts of water enter and leave the cell by random movement. However, if the water content outside the cells is too high, water will enter the cell by osmosis. The cell swells with extra water and it may eventually burst, this is called lysis.

If a cell is placed in a concentrated solution, water leaves the cells by osmosis. The cell will shrivel, this is called crenation.


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Osmosis in Cells

Plants Cells
Plant cells also have a partially permeable membrane, but they also have a cell wall which is fully permeable and very strong. When a plant cell is placed in pure water, the water molecules enter by osmosis. The cell swells up it does not burst because the strong cell wall prevents bursting.
If a plant cell is placed in a concentrated solution, water leaves the cell by osmosis. The inside of the cell now shrivels up.


Plasmolysed - where so much water is lost the cell membrane has come away from the cell wall.

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Active Transport

Movement is said to be against the concentration gradient. As a result, cells can absorb ions from a very low concentration into the cell which has higher concentration of the ion. Important ions can be moved into cells even if the concentration of the ion is very low outside the cell. Sugars can also move by active transport. Energy released from respiration is used to carry the molecule using the transport protein carrier, across the membrane and return the transport protein back.



  • Mineral ions such as nitrates are absorbed by active transport from the soil into rot hair cells which are important to make plant proteins
  • Glucose is absorbed by active transort from the small intestine across cells lining the small intestine in your blood
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Diffusion vs. Osmosis vs. Active Transport


  • Random movement of molecules
  • No energy needed from cell - passive process
  • From higher to lower concentration; down the concentration gradient


  • Random movement of molecules
  • No energy needed from cell - passive process
  • From higher to lower concentration; down the concentration gradient

Active Transport

  • Selective movement of molecules
  • Energy needed from cell - active process
  • From lower to higher concentration; against the concentration gradient
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Sports Drinks

Sports drinks contain sugars to replace the sugar used in energy release during the activity.
They also contain water and ions to replace the water and ions lost during sweating.

The drinks are designed to help balance the concentration of body fluids and the concentrations inside cells. If the drink concentration matches the body fluid the solution is called isotonic.

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Exchanging Materials

Adaptations for exchange surfaces

The effectivenes of an exchange surface can be increased by:

1) Having a large surface area

2) Being thin, which provides a short diffusion pathway

3) Having an efficient blood supply, in animals. This moves the diffusing substances away and maintains a steep concentration(diffusion) gradient

4) Being ventilated, in animals, to make gaseous exchange more efficient by maintaining a steep concentration gradient

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The Lungs

Your lungs are specially adapted to make gas exchanges more efficient. Exchange of gases occurs in clusters of tiny air sacs called alveoli.

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Gas Exchange in the Lungs

The wall of the capilary and the alveoli are both 1 cell thick which creates a short diffusion pathway.

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Ventilating the Lungs

Complex animals need to ventilate their lungs to maintain a steep concnetration gradient. This involves moving air into and out of their exchange organs regularly. In humans, ventilation or breathing takes place in the specially adapted breathing systems. Breathing is a result of pressure changes in the thorax.

Breathing In

  • Diaphragm - it contracts, flattening in shape to increase the size of the chest cavity
  • Rib Cage - muscles contract to pull the ribs upwards and out during inhilation
  • Thorax Volume - increased
  • Thorax Pressure - drops
  • Pressure in lungs compared to atmospheric pressure - pressure in lungs is less which causes the air to move inwardly
  • Movement of Air - into the body
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Ventilating the Lungs

Breathing Out

  • Diaphragm - it relaxes and the diaphragm domes
  • Rib Cage - starts to relax(intercostal muscles) so ribcage goes down and inwards
  • Thorax Volume - decreased
  • Thorax Pressure - increases
  • Pressure in lungs compared to atmospheric pressure - pressure in lungs greater
  • Movement of Air - out of the body
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Artificial Breathing Aids

Artificial ventilation is the process of supporting respiration by manuel or mechanical means when normal breathing is inefficient or has stopped. There are two types of atrificial venilators, Negative pressure and Positive pressure.

An 'iron lung' is a negative pressure machine. Patients lay in a metal cylinder with their head sticking out and a tight seal around the neck. This maintains the negative pressure and vacuum. The vacuum's formed when air is pumped out the chamber, causing the chest wall of the patient to move up. This increased the volume and decreased the pressure inside the chest. So air from the outside is drawn in. Then, the vacuum is switched off automatically and air moves into the chamber, increasing pressure. The ribs move down, lowering volume and increasing pressure inside the thorax. This forced air out of the lungs.

Advantages: The modern shell is just a mini-cylinder that just fits around the chest so it is much easier for the patient to use.

Disadvantages: Not used much anymore

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Artificial Breathing Aids

A positive pressure ventilator forces a carefully measured 'breath' of air into your lungs under a positive pressure. Once the lungs have been inflated the air pressure stops, the lungs deflate and the ribs move down again, forcing air back out. 

The simple method is a face mask or tube going into the trachea with a positive pressure bag ventilator held and squeezed by doctors or nurses in emergency treatments.

The full scale method is positive pressure ventiltor machines can keep patients alive through major surgery. Patients don't have to be placed in iron lung machines. They help patients stay alive for years.

Advantages: Don't have to be placed in iron lung machine, equipment can be used at home and the patient can move about.

Disadvantages: Doctor has to hold it.

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Exchange in the gut

CarbohydratesEnd product: glucose - Enzyme: Amylase
End product: amino acids - Enzyme: Protease
End product: fatty acids and glycerol - Enzyme: Lipase

The digested food molecules are now small enough to be absorbed from the inside of the small intestine, across a single layer of cells of strucutres called vili, into the blood stream. 

Villi have several adaptation to allow efficient absorbtion of food molecules.

  • Large Surface Area - Villi have microvilli which has lots of folds, which increases the surface area
  • Short Diffusion Pathway - The walls of the villus and the walls of the capillaries are both 1 cell thick, so the nutrients only have to travel across two cells
  • Maintains steep concentration gradient - Extensive blood supply, diffusion can happen at a faster rate
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Exchange in plants

Gases diffuse in and out of leaves through tiny holes called 'stomata'. The size of the stomata is controlled by guard cells which surround them. These gases are:

  • Oxygen: needed for respiration and is a waste product of photosynthesis
  • Carbon Dioxide - needed for photosynthesis and is a waste product of respiration. The movement of these gases depends upon which process is taking place the most quickly

Plants lose water vapour through the stomata by evaporation in the leaves. Leaves are flat and very thin so the gases do not need to diffuse very far. There are also internal air spaces. Water and mineral ions are taken up by the roots. The root hair cells increase the surface area of roots for the absorbtion of water and mineral ions. If the plant loses water faster than it is replaced by the roots, the stomata can close to prevent wilting.

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Structure of the Leaf


Waxy Cuticle - waterproof layer which stops water loss
Spongy Layer - cells not tightly packed, have large surface area available for gas exchange
Guard Cells - open and close the stomata to control water loss

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Plants take up water through the roots. The water passes through the plant to the leaves, where it evaporates from the leaf cells and the water vapour diffuses out through the stomata. The movement of water through the plant is called the transpiration stream.

The plant could dehydrate if the rate of evaporation in the leaves is greater than the water uptake in the roots. Evaporation is more rapid in hot, dry, windy or bright conditions. The guard cells can close to prevent excessive water loss. Wilting of the whole plant can reduce water loss. The leaves collapse and hang down which reduces the surface area.


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