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Biology Topic 3 - Movement of substances into and out of cells…
Biology Topic 3 - Movement of substances into and out of cells
Definitions
Osmosis - The net movement of free water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Active Transport - The movement of particles from a region of lower concentration to a region of higher concentration using energy from respiration.
Diffusion - the net movement of particles from a region of higher concentration to a region of lower concentration
Concentration gradient
If you are moving from a higher concentration to a lower concentration (e.g. diffusion) you
move down
or
along
the concentration gradient.
If you are moving from a lower concentration to a higher concentration (e.g. active transport) you
move up
or
against
the concentration gradient.
The word 'net' makes it clear that there is movement in both directions due to
random movement
, but that there is a overall effect in one direction.
What is diffusion and osmosis and Active Transport?
Diffusion
Small, uncharged molecules (crucially,
oxygen
and
carbon dioxide
) can diffuse freely across the cell membrane
Osmosis
Osmosis is a special type of diffusion that refers
only
to the movement of
water
through
partially permeable membranes
(i,e. the cell membrane or visking tube
The diagram below shows two sucrose solutions separated by a partially permeable membrane. There is the same number of water molecules on each side of the partially permeable membrane, but there are more sucrose molecules on the left.
There are many more free water molecules on the right-hand side because most of the water molecules on the left hand side are attracted to the sucrose molecules. Thus there is a region of higher concentration of water molecule on the right hand side and e say it has a higher water potential
We use the term water potential to describe how 'free' the water molecules are to move. Pure water has the highest possible water potential as there are no solute molecules to attract water molecules.
Active Transport
Sometimes a cell needs to take in a molecule when there is little of it outside the cell ( i.e. against the concentration gradient). To do this is uses
energy from respiration
to power a
protein pump
embedded in the membrane that moves the molecule across the membrane (e.g. mineral ion uptake in the root hair, glucose absorption in the ilium).
Factors that affect the rate of movement in and out of cells.
Surface area to volume ratio
Cell A: Consider a cell that is a cube to have exactly 1cmx1cmx1m its surface is 6cm2 and its volume is 1cm3, so its surface area to volume ratio is 6: 1.
Cell B: Consider another one, but this one to have a surface area of 24 cm2 and a volume of 8 cm3. So the surface area to volume ratio is 24:8 or 3:1.
Cell A would easily be able to obtain oxygen by diffusion as there is a large surface area to supply a small volume.
Although cell B has a larger surface area it also has a proportionally larger volume, so the rate of diffusion into the centre of the cell will be much slower.
This explains why cells that need to move substances by diffusion, osmosis and active transport maximise their surface area (for example, root hair cells having a long projection or cells lining the alveolus being very thin and flat.)
Temperature
Particles in liquids and gases have kinetic energy. This allows them to move down concentration gradients.
If you increase the temperature in a system the particles have more kinetic energy so move faster.
Concentration Gradient
For fast movement it is important to maintain a steep concentration gradient (i.e. a large difference in concentration between the higher and lower regions).
For example a steep concentration gradient between oxygen in the alveolus and oxygen in the blood is maintained by:
Circulation - Moving the oxygenated blood away along the pulmonary artery.
Ventilation - Replenishing the alveoli with fresh oxygen from the air.
Practical: Diffusion and Osmosis using living and non-living systems.
Diffusion and Surface area to volume ratios.
Drop the cubes into a tube of HCL.
Record the time taken for all the colour to disappear from the agar block.
Method: 1. Cut blocks of agar containing an indicator into cubes of different sizes.
Results: In this experiment the agar block represents a cell and the acid a gas such as oxygen diffusing into the cell. You should find that larger blocks, with lower surface area to volume ratios take longest for the colour to disappear.
Osmosis in potato rods
Method: 1. Cut 5 potato rods of equal diameter and length and find their masses.
Soak each in a salt solution of different
concentration (0%,25%,50%,75%,100% Nacl).
Leave for
30 minutes
Remove from solutions and dab dry with paper towel.
Find new masses and record percentage change.
Results: Rods soaked in pure water should gain mass, as the contents of the potato cells have a lower water potential than the external solution, so water moves in by osmosis down the water potential gradient and through the partially permeable cell membrane. Rods soaked in a strong salt solution should lose mass, as the contents of the potato cells have a higher water potential than the external solution,so water moves out by osmosis down the water potential gradient and through the partially permeable cell membrane. If a chip shows no gain or loss in mass the concentration of salt inside its cells must be equal to the concentration of salt in the external solution.
