Investigating Cells

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Investigating Cells
B- Diffusion

• Standard Grade Biology

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Diffusion
Diffusion is the movement of molecules from an
area of high concentration to an area of low
concentration, until evenly spread out.
Watch the How Diffusion Works animation on the
website
http//highered.mcgraw-hill.com/sites/0072495855/s
tudent_view0/chapter2/animation__how_diffusion_wor
ks.html
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Why is diffusion important?
All living cells rely on diffusion to live. They
use it for

• getting raw materials for respiration
  (dissolved substances and gases)

• removing waste products (eg. from respiration)

• photosynthesis in plants (raw materials in,
  waste products out)

Examples
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Respiration- gas exchange
In breathing you exchange carbon dioxide (CO2)
and oxygen (O2) between alveoli in the lungs and
the blood.
This is an example of diffusion.
Have a look at skoool Biology KS3, Activity 31
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Respiration- in cells
food oxygen ? carbon dioxide water
energy
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Respiration diffusion
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Photosynthesis Diffusion
carbon dioxide water ? oxygen glucose

• Carbon dioxide diffuses in through the stomata

• Oxygen and water diffuse out of the stomata

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Osmosis
Osmosis is a special case of diffusion.
It is the net movement of water across a
selectively permeable membrane as a result of a
water concentration gradient.
Selectively permeable membrane ???
The cell membrane has very small holes in it.
Small molecules can pass through, but larger ones
cant. We say it is selectively permeable.
Water concentration gradient ???
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Water concentration gradient
In osmosis water moves both ways to balance up
the concentrations. The overall (net) movement is
to the area of low water concentration.
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Osmosis in action
Have a look at
http//zoology.okstate.edu/zoo_lrc/biol1114/tutori
als/Flash/Osmosis_Animation.htm
http//highered.mcgraw-hill.com/sites/0072495855/s
tudent_view0/chapter2/animation__how_osmosis_works
.html
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Investigating Diffusion Osmosis
Collect a workbook. Read p 17 18 and plan your
experiment.

• Useful points
• Visking tubing is selectively permeable and is
  designed to copy what a real cell membrane does.
• Small particles can get through the membrane
  (like glucose) whilst larger ones cannot (like
  starch)

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Experimental Results

• At the start and the end-
• Where is the glucose?
• Where is the starch?
• How has this happened?

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Why is osmosis important?
Plants rely on the movement of water through them

• Water always moves to areas of lower water
  concentration.

• Root hairs take in water from the soil by
  osmosis.

Have a look at skoool biology KS4 activity 5
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Plants- Water moving in
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Cell turgor pressure
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Plants- Water moving out
When water moves out of the plant cell vacuole,
by osmosis, it becomes flaccid
If a lot of water leaves the cell, the cytoplasm
starts to peel away from the cell wall and we say
the cell has undergone plasmolysis.
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Plasmolysis
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Osmosis in Plant Cells
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Activity Over a few days let a plant dry out,
then water it. Observe the changes. Explain the
changes in terms of water movement
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Animals- Water moving in
Animal cells have no cell wall to stop the
swelling, just a flexible cell membrane.
haemolysed
turgid then burst
normal
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Animals- Water moving out
haemolysed
normal
http//zoology.okstate.edu/zoo_lrc/biol1114/tutori
als/Flash/Osmosis_Animation.htm
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Quick Quiz- Osmosis

• What is a partially permeable membrane?
• Which substance moves by osmosis?
• What happens to plant cells that take up water by
  osmosis, and what do you call the cell?
• What happens to plant cells that lose water by
  osmosis, and what do you call the cell?
• What does haemolysed mean? How does it happen?

A membrane that only lets small molecules through
it
Water, H2O
The cells become full of water and are called
turgid
The cytoplasm comes away from the cell wall and
the cells are called plasmolysed.
Animals cells which have lost water become
shrunken and are called haemolysed.