2.12 understand definitions of diffusion, osmosis and active transport
- Diffusion = the net movement of particles from an area of higher concentration to an area of lower concentration.
- Osmosis = the net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration. (Water will move from a dilute solution to a concentrated solution.)
- Active transport = the movement of particles against a concentration gradient, from an area of lower concentration to an area of higher concentration, using the energy released during respiration.
2.13 understand that movement of substances into and out of cells can be by
diffusion, osmosis and active transport
The three processes are the ways in which substances move in and out of cells.
Diffusion occurs in both liquids and gases (particles in these can move about freely). Particles travel from an area of higher concentration to an area of lower concentration. This can be as simple as different gases diffusing through eachother, or could also be the process of particles passing through a cell membrane from their area of high concentration to an area of low concentration.
- Osmosis
Osmosis is another way through which substances move in and out of cells. The water molecules pass through a partially permeable membrane (one with very small holes in it, including the cell membrane) and will travel from its region of higher concentration to its region of lower concentration. Water molecules move about randomly all the time, so they both in both directions through the membrane however there will be more water molecules on one side than the other, so there will be a steady flow of water into the region with fewer water molecules (particles travel into area of lower concentration.)
- Active transport
Active transport is the third way through which substances move in and out of cells. In this process, dissolved molecules move across a cell membrane from an area of lower concentration to an area of higher concentration, therefore against the concentration gradient (normally molecules travel from area of higher concentration to area of lower concentration.) For example, in the digestive system, if there is a low concentration of nutrients in the gut and a higher concentration of nutrients in the blood, then nutrients will go against the concentration gradient and travel from the area of lower concentration (gut) to the area of higher concentration (blood.) Carrier proteins pick up specific materials and take them through the cell membrane to the area of higher concentration, in this case the blood. Because the particles must go against the concentration gradient, they need energy to do this and this energy can be received by living organisms by respiration.
2.13 understand the factors that affect the rate of movement of substances into and out of cells, to include the effects of surface area to volume ratio, temperature and concentration gradient.
There are three main factors that affect the rate of movement of substances into and out of cells. These are:
- Surface area to volume ratio
The rate of diffusion, osmosis and active transport is higher in cells that have a larger surface area to volume ratio. For example, if a cell has a surface area of 24cm^2 and a volume of 6cm^3, then its surface area to volume ration will be 24:6 = 4:1.
So, this cell would have a faster rate of movement of substances than one that had a smaller ratio, such as a surface area of 36cm^2 and a volume of 18cm^3, which would have a ratio of 36:18 = 2:1.
2. Temperature
As the particles in a substance are heated and get warmer, they have more energy so move about faster. So, as the temperature increases, so does the rate at which substances move in and out of cells.
3. Concentration gradient
This only applies to diffusion osmosis, not active transport. The larger the difference in concentration of the inside and the outside of the cell, the faster the substances move in and out of the cell. If there are lots more particles on one side of the cell, then there are more to move across. (This side will have the larger concentration, bigger difference in concentration and therefore will have substances moving in and out of it at a fast rate.)
2.14 understand the importance in plants of turgid cells as a means of
support
When a plant is well watered, all of its cells will draw in water by osmosis and become plump and swollen. They are said to have become turgid when this occurs. The contents of the cell will push against the cell wall - this is referred to as turgor pressure. It is this turgor pressure that occurs in turgid cells that helps to support the plant tissues.
and out of cells, to include the effects of surface area to volume ratio, temperature and concentration gradient
Surface area to volume ratio
The rate of diffusion, osmosis and active transport is higher in cells with a larger surface area to volume ratio.
Temperature
When the particles in a substance are heated (get warmer), they move around at a quicker pace and have more energy. Therefore, as the temperature increases, substances move in and out of the cells at a faster rate.
Concentration gradient
This only effects the rate of diffusion and osmosis, it does not affect the rate of active transport. The bigger the difference in concentration between the inside and the outside of the cell, the faster substances move in and out of a cell. This is because if there are lots more particles on one side than the other, there are more to move across.
2.16 describe experiments to investigate diffusion and osmosis using living and
non-living systems.
Investigating diffusion in a non-living system
- Put a few drops of coloured substance (eg. green food colouring) into a jar of water.
- Time how long it takes for the food colouring to diffuse and for all of the water to be the same colour (translucent green if green food colouring is used). Expect this to happen after roughly two minutes of adding the drops.
- In another container, again add a few drops of food colouring but this time to water that is a different temperature from the first.
- Time how long it takes for the food colouring to diffuse in this second container than in the first. If the heat of the water was raised the second time, the colour will have moved through the liquid faster and therefore have diffused at a quicker rate. If you decreased the temperature the second time, the heat will be lower so the particles will have less energy and the colour will move through the liquid at a slower rate, with the diffusion therefore having a slower rate of reaction.
1.
Investigating Osmosis in Living Systems
- Cut a potato into two roughly identical cylinders.
- Measure the length of each cylinder for later comparisons.
- Place one cylinder into a beaker that has just pure water in, and the other into a beaker that has salt water.
- After half an hour or so, remove the potato cylinders from their respective beakers and remeasure them.
- If the cylinder is now a bit longer than it was before being placed into the beaker, then they have drawn water in by osmosis. If the cylinder is now shorter, then water has been drawn out by osmosis.
Investigating osmosis in non-living organisms
- Tie a wire around one end of some visking tubing (= a partially permeable membrane) and put a glass tube in the other end, fixing the tube around it with wire.
- Pour some sugar solution down the glass tube into the visking tube.
- Then, put the visking tubing in a beaker of pure water and measure where the sugar solution comes up to on the glass tube.
- Leave the tube overnight, and the next day measure where the liquid is in the glass tube.
- Water should have been drawn into the vi=sking tubing by osmosis and this will therefore force the liquid up the glass tube.
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