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- Define diffusion as the net movement of particles from a region of their higher concentration to a region of their lower concentration down a concentration gradient, as a result of their random movement.
Make sure to learn the above definition!
- Investigate the factors that influence diffusion, limited to surface area, temperature, concentration gradients and diffusion distance.
Just to recap/ expand on the definition of diffusion, when particles diffuse, they move, often across different barriers like cell membranes. I think it’s also important to note that in diffusion, the end result is usually equilibrium (as long as nothing else interferes, preventing equilibrium from happening). Equilibrium is a state in which all the particles are equally spread.
Surface area: the larger the surface area, the more space there is for particles to diffuse across, increasing the rate of diffusion.
Temperature: higher temperatures give particles more energy, allowing them to move faster. This increases the rate of diffusion.
Concentration gradients: A concentration gradient is a measure of the difference in concentration of a certain particle between two different areas. The steeper the concentration gradient, the greater the difference in concentrations. This means, more particles will move from their region of higher concentration to their region of lower concentration to achieve equilibrium, thereby increasing the rate of diffusion.
Diffusion distance: this is the distance that particles have to travel to achieve equilibrium. The greater the diffusion distance, the more time it takes to achieve equilibrium, so the lower the rate of diffusion.
- State that substances move into and out of cells by diffusion through the cell membrane.
Many (not all) substances can diffuse freely across cell membranes, and thus, diffusion is their main method of transport across cell membranes.
- State that water diffuses through partially permeable membranes by osmosis.
Osmosis is the diffusion of water across a semi-permeable membrane. Note that a semi-permeable membrane is a membrane that only allows some substances through.
Imagine there are two aqueous solutions separated by a semi-permeable membrane, where one has a higher solute concentration (so lower water concentration) and the other has a lower solute concentration (so higher water concentration). The semi-permeable membrane blocks the movement of the solute molecules – only water can pass freely through the membrane. Therefore, water will diffuse across the membrane, until both solutions are of equal concentration.
This process is osmosis.
Note that we don’t usually refer to it as ‘water concentration’ the term we use is usually ‘water potential’.
- State that water moves into and out of cells by osmosis through the cell membrane.
Since waters can move by osmosis across cell membranes, this is one of the main methods of transport of water.
- Define osmosis as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
Learn the above definition, kiddos 😊
- Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations.
Things you will need to know before we start:
Plant cells have cell walls – this is quite important when thinking about the effect of immersing plant tissue in solutions of different concentrations.
Cells are primarily made of water (on average, about 70% of total cell mass is water).
Every cell cytoplasm has its own specific concentration of solutes, and this concentration is usually pretty similar across the same type of tissue (e.g. palisade cells will have similar concentrations of solutes in their cell cytoplasms), and that the pressure that water applies in plants (i.e. the water pressure), is known as turgor pressure.
Turgidity is the state of being ‘turgid’ or swollen, especially due to high fluid content. Plants need turgid cells to help them maintain their shape and in turn, help the plant stay upright.
Water is mainly stored in the vacuole in the cytoplasm, and it is mainly this vacuole that regulates the turgidity of a plant cell.
Now, let’s move on to the main matter of this learning objective:
- When you immerse plant tissue in solutions of lower water potential (hypertonic solution) than that of the plant cells:
Water diffuses out of the cell by osmosis. This means there is less matter inside the cell.
This causes the cytoplasm to shrink, and thus the cell membrane gets ripped away from the cell wall. This process is called plasmolysis. Cells become weak and flaccid, as there isn’t enough cytoplasm to support the cell and help it maintain its shape.
- When you immerse plant tissue in a solution of equal water potential to their cell cytoplasm (isotonic solution).
Since the concentration of the solution is equal inside and outside of the plant cells, there is no net movement of water. This means the volume or shape of the plant cell is unlikely to change.
- When you immerse plant tissue in solutions of higher water potential than their cell cytoplasm (hypotonic solution).
Here, the solution inside the cells is more concentrated than solution outside, so water diffuses down its concentration gradient into the cell, by osmosis. This causes the amount of cell matter inside the cell to increase. As the cytoplasm enlarges, it pushes outwards on the cell surface membrane more and more. Normally, this would usually cause the cell surface membrane to eventually burst (once the pressure, otherwise known as turgor pressure, in this case, grows too large). However, plant cells have very strong cell walls. This holds the plant cell intact, and as the cytoplasm pushes outside, the cell simply swells to its full size and becomes rigid. This cell is turgid.
- Explain the effects on plant tissues of immersing them in solutions of different concentrations by using the terms turgid, turgor pressure, plasmolysis and flaccid.
This has already been explained in the above learning objective! 😊
- Explain the importance of water potential and osmosis in the uptake of water by plants
Water potential is the potential of water to leave a system.
This is affected by:
- water pressure
- the volume of the water relative to the volume of the system (e.g. a lot of water in a small system will force water out of the system)
- the concentration of the water
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
In the uptake of water by plants, the soil around the plant must contain water that is more dilute than the water in root hair cells. This allows water to diffuse in by osmosis (the water outside the root hair cell will have a higher water potential than the water inside the root hair cell).
- Explain the importance of water potential and osmosis on animal cells and tissues.
Unlike plant cells, animal cells do not have a cell wall to support them.
So, if you insert them in a hypertonic solution, water will move out of the cell by osmosis, causing the cell to shrivel up.
If you immerse them in an isotonic solution, there will be no net movement of water.
If you immerse them in a hypotonic solution, the water will diffuse into the animal cell by osmosis. However, there is no cell wall to keep the cell intact, so the water pressure grows so great that the cell bursts – it undergoes lysis.
Notes submitted by Sarah
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