5090 Biology - Unit 2 Movement into and out of the Cell
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Course
Cambridge O Level
Institution
Cambridge O Level
Complete note covering all aspects required for Cambridge O Levels Biology, Unit 2 Movement into and out of the Cell. Includes detailed explanations, clear tables and diagrams to help you ace your exams.
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1. Diffusion
Particles (atoms, molecules, and ions) are always on the move.
In solids, particles cannot move (only vibrate) as they are held firmly with forces of
attraction.
In liquids, particles can freely bounce and rebound on each other.
In a gas, particles are far apart, no forces of attraction between them.
As particles move freely, they tend to spread themselves out as evenly as possible.
Diffusion is a result of the random motion of particles.
In a cell H2O, O2 and CO2 can pass through the cell membrane easily.
Diffusion – is the net movement of molecules and ions from a higher concentration to the region
with a lower concentration down a concentration gradient as a result of their random movement.
Examples of Diffusion
1. Absorption of Carbon Dioxide by plants for Photosynthesis. (CO2 diffuses from the air to the leaf
through the stomata)
2. Oxygen diffuses out of the leaves to the air, after photosynthesis.
3. The gas exchange in the Alveoli.
4. Absorption of nutrients after digestion by the Ilium (intestine) happens by diffusion.
Starch Glucose absorbed by blood vessels of Villi
Proteins Amino Acids
Lipids Fatty Acids and Glycerol - Absorbed by Lacteal Villi.
5. Flowers use diffusion to spread their scent to attract pollinators.
6. During respiration by cells O2 diffuses in and CO2 out.
7. Most Living things obtain O2 through Diffusion to survive.
8. N2 diffuses into the body of divers as they go further into to the sea and when they come up
diffuses out.
Concentrated (Conc.) – has more sugar particles than water.
Dilute (Dil.) – more water than sugar particles.
Result – with time, a uniform distribution of water and sugar molecules is formed as a result of
diffusion.
, Factors That Affect the Rate of Diffusion
1. The temperature (with the increase of temperature, particles gain more energy making them
move faster which speeds up the diffusion)
2. The distance of diffusion (greater the distance, slower the diffusion)
3. The difference between the concentration gradient. (The greater the difference, faster the
diffusion)
4. Absorption by the surface area. (Larger the surface area, faster the diffusion)
2. Osmosis
Osmosis – is the passage of water molecules from a region of higher water potential to a
region of lower water potential through a partially/semi permeable membrane.
The only difference between Diffusion and Osmosis is that in Osmosis only water
molecules are allowed to pass through the semi permeable membrane unlike in diffusion
where any molecule could mix with anything.
In vertebrates, the brain checks on the concentration and is adjusted by the kidneys, so that
the cells don’t fill up excessively with water or don’t get dehydrated without water.
Partially/Semi Permeable Membrane – is a membrane that will only allow some
molecules to pass through it.
Natural Semi Permeable Membrane – Animal Cell Membrane, Plant Cell Membrane,
Egg Membrane.
Artificial Semi Permeable Membrane – Cellophane, Visking/Dialysis Tubing.
Cellophane Visking Tubing
Is a thin, transparent sheet made of regenerated cellulose. It has a low Visking/Dialysis Tubing is
permeability most molecules (including bacteria and air) and is used used in separation
for food packaging. But cellophane is highly permeable to water techniques, as it facilitates
vapour, so it is coated with a nitrocellulose lacquer to prevent this. the flow of tiny molecules in
solutions.
There is more water potential in the low
concentration side, so the water molecules
move from the low concentration side
(high water potential) to the high
concentration side (low water potential),
through the semi permeable membrane, till
both sides become uniformly concentrated.
Demonstration of Osmosis using a Visking Tubing
Observation – The visking tubing swells and becomes firm and
turgid. The level of water in the beaker decreases and the level of
liquid in the glass tube will increase.
Conclusion – the beaker has a higher water potential, and the
solution has a low water potential. The water molecules move from the
beaker to the visking tubing through the semi permeable membrane,
making the tube swell and firm.
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