Investigating Osmosis
Investigating Osmosis
Factors that affect the rate of movement in and out of cells.
Concentration Gradient
- Ventilation - Replenishing the alveoli with fresh oxygen from the air.
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.
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).
Temperature
If you increase the temperature in a system the particles have more kinetic energy so move faster.
Particles in liquids and gases have kinetic energy. This allows them to move down concentration gradients.
Surface area to volume ratio
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.
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.
Cell A would easily be able to obtain oxygen by diffusion as there is a large surface area to supply a small volume.
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.)
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.
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?
Osmosis
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.
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
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
Diffusion
Small, uncharged molecules (crucially, oxygen and carbon dioxide) can diffuse freely across the cell membrane
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).
Practical: Diffusion and Osmosis using living and non-living systems.
Osmosis in potato rods
- Remove from solutions and dab dry with paper towel.
- Leave for 30 minutes
- Find new masses and record percentage change.
- Soak each in a salt solution of different
concentration (0%,25%,50%,75%,100% Nacl).
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.
Method: 1. Cut 5 potato rods of equal diameter and length and find their masses.
Definitions
Diffusion - the net movement of particles from a region of higher concentration to a region of lower concentration
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.