B4d - Diffusion and Osmosis

Diffusion
1. The cell membrane controls which substances enter and leave the cell. Living cells need to obtain oxygen, glucose, water and minerals from their surroundings and get rid of waste products. These substances pass through the cell membrane by diffusion - the movement of a substance from a region of high concentration to a region of low concentration.
2. There is a low concentration of oxygen inside the cell because supplies are constantly being used up in respiration; there is a high concentration outside the cell because oxygen is constantly being replaced. Particles have random movement. Diffusion is the net movement of particles from high concentration to low concentration
3. The rate of diffusion is increased when: the cell membrane's surface area is bigger, there is a bigger difference between concentrations, the particles have a shorter distance to travel.
Osmosis
1. Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
2. Water moves from a cell with a high concentration of water to a cell with a low concentration of water, until there is the same concentration of water in each cell.
3. The cell walls don't affect the movement of water or dissolved substances because they're freely permeable. The cell membranes allow the movement of water but restrict the movement of dissolved substances.
4. When predicting the direction of water movement, what matters is the concentration of the water because the solute molecules cannot pass through the membrane; only water molecules can.
5. The water particles move randomly, colliding with each other and passing through the membrane in both directions. Overall, more particles move from the area where water is at high concentration to low concentration. The effect is to gradually dilute the solution.
6. At the root hair cells, water gradually moves from the soil into the cell by osmosis along a concentration gradient.
Water in plants
1. Plants use water to: keep their leaves cool, transport materials, enable them to photsynthesise to produce glucose and keep cells firm an so keep the plant rigid.
2. Plants need to balancce the amount of water they take in with the amount they lose. Water is taken in via the roots, which are specially adapted to increase water intake by having root hair cells. They increase the surface area of the root, making a greater area for absorbtion.
3. The water travels through to leaves, along c gradient from area of high c of water to area of low c.
4. When water reaches the leaves it can be lost by evaporation through stomata. There are two adaptations that can reduce the rate at which water is lost from the leaves into the atmosphere: having a waxy cuticle on the surface, having the majority of the stomata of the lower surface of the leaf, where they are less exposed to sunlight.
Maintaining Support
1. Plant cells have inelastic cell walls, together with the water inside the cells, support the young non-wood plants.
2. The cell wall prevents cells from bursting due to excess water entering them, it also, contributes to turgidity
3. A lack of water can cause plants to wilt. If there is plenty of water in the soil, stomata open to allow transpiration and the diffusion of gases. When there is not enough water available in the soil, stomata close to try to prevent transpiration.
Osmosis in animal cells
1. The cytoplasm of a red blood cell has the same water concentration as plasma. There is no net movement of water into or out of the cell, so it can maintain its biconcave shape.
2. Water diffuses in and out of animal cells by osmosis. But animal cells do not have a cell wall, so if too much water enters a cells it could burst
3. Lysis - there is nothing to prevent the water entering the cell because unlike plant cells, there is no inelastic cell wall to resist the outward pressure of the water so they eventually burst
4. Blood cells in a concentrated solution lose water by osmosis. They shrivel + become crenated
5. Animal and plant cells respond differently to osmosis because animal cells do not have a cell wall. When plant cells absorb water they expand, but resistance from the cell wall eventually stops them from bursting. Animal cells do not have a cell wall so they absorb more until lysis.
Turgor pressure
1. When water moves into a plant by osmosis it increases the pressure inside the cell. However the cell walls are sufficiently strong to withstand the pressure and as a result the cell becomes very rigid. This is calle dturgor pressure. Cells that have sufficient supplies of water are described as maintaining their turgor
  1. When all the cells are fully turgid, the plant is rigid and upright. However, if water is in short supply, cells will start to lose water by osmosis. They lose turgor pressure and become flaccid, an the plant begins to wilt. If the cells lose alot of water, the inside of the cells contact like a deflating balloon. This is called plasmolysis.

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B4d - Diffusion and Osmosis

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	Created by Chloe.Sharland almost 10 years ago