Diffusion and Osmosis
Diffusion is the movement of molecules of a liquid or gas from a region of high concentration to a region of low concentration across a concentration gradient (it is a passive process i.e. does not require energy)
Examples of Diffusion
Selectively/ Semi- Permeable Permeable Membranes
A selectively/ semi-permeable membrane allows some but not all substances to pass through
Osmosis
Osmosis is the movement of water molecules from high concentration of water to a low concentration of water across a semi-permeable membrane
Comparing Diffusion and Osmosis
To Demonstrate Osmosis
Results: Tubing with starch solution expands and becomes more full. It gains weight - this happens because water moves due to osmosis from a high water concentration to a low water concentration in the visking tubing
Osmosis and Plant Cells
Plant cells are surrounded by cell walls which are fully permeable. However the cell membrane inside the cell wall is semi-permeable
Plant cells whose contents are more concentrated than their surroundings:
Cytoplasm of plant cells contain salts, sugars, starch, proteins and other molecules dissolved in water. If a plant cell is placed in distilled water or a solution less concentrated than its cytoplasm then water will move into the cell due to osmosis. This is the way root cells absorb water from soil.
Water will pass into the vacuole. The cytoplasm and vacuole will swell and the cell membrane will be forced against the cell wall. The cell is now much stronger or firmer due to its turgor pressure. The cell is said to be turgid. This can be compared to air being forced into a car tyre
Turor or Turgor Pressure is the pressure of the cytoplasm and vacuole against the cell wall
Animal cells whose contents are more concentrated than their surroundings:
If animal cells are placed in very dilute solutions then water will enter due to osmosis. The cell will expand and may burst as they lack a cell wall to restrain them
Amoeba has a contractile vacuole to prevent it from bursting
Plant cells whose contents are less concentrated than their surroundings:
If plant cells are placed in very concentrated solutions water will pass out of the vacuoles of cells due to osmosis
Animal cells whose contents are less concentrated than their surroundings:
If animals cells are placed in very concentrated solutions then water will pass out of the cells. The cells will lose their shape and become shriveled (crenation)
Plasmoysis
When plant cells are placed in salty water (a concentrated solutions), they lose turgor and become weaker. The cells are said to be plasmolysed. If plant cells lose turgor, plant is seen to wilt. This is temporary and can be reversed by placing plant cells in less concentrated solutions than their cell contents e.d. water them
Osmosis and Food Preservation
Bacteria and fungi both have cell walls. If they are placed in a very concentrated solution they will lose water by osmosis. This prevents them from growing and causing food to decay. There are two main examples of this process.
1. Soaking Food in very salty solutions
Preserves fish and meat such as bacon
2. High Sugar Content
Such as jam and marmalade. Preserves them from fungus decay
Examples of Diffusion
- Molecules in smells such as perfume or stink bombs spread out by diffusion
- If a cell uses oxygen in respiration it will have a low concentration of oxygen. Concentration outside the cell is higher than the concentration inside the cell so oxygen will diffuse into the cell
- If a leaf is carrying out photosynthesis then it will be producing oxygen. The concentration of oxygen in the leaf will be high. If the concentration of oxygen in the leaf is higher than concentration of oxygen outside in the air then oxygen will diffuse out of the leaf
Selectively/ Semi- Permeable Permeable Membranes
A selectively/ semi-permeable membrane allows some but not all substances to pass through
- All biological membranes are similar, this means that the cell membrane is similar to the membranes around mitochondria, chloroplasts or the nucleus
- All these membranes are selectively permeable. This means that some molecules can pass easily through the membrane while other molecules cannot easily cross the membrane
- Water, Oxygen and Carbon Dioxide can pass easily across membranes
- Salts, starch, sugars and proteins are molecules that cannot easily pass across membranes
Osmosis
Osmosis is the movement of water molecules from high concentration of water to a low concentration of water across a semi-permeable membrane
Comparing Diffusion and Osmosis
- Both diffusion and osmosis involve the movement of molecules from high to low concentration. This means the two processes are passive
- The difference between them is that osmosis requires a semi-permeable membrane while diffusion does not. This means that osmosis is a special case of diffusion
To Demonstrate Osmosis
- Soak length of visking tubing in water (visking tubing is a semi-permeable membrane and soaking it makes it soft and pliable)
- Tie a knot in one end of visking tubing
- Fill with stong starch solution whose molecules are too big to pass through the tubing
- Seal the other end of visking tubing
- Dry the tube and weigh it. Note how full it is
- Place the tubing in a beaker of distilled water
- Repeat steps 1-5 except fill the visking tubing with distilled water to act as control
- Leave apparatus for about 30 mins
- Remove the tubes from the water, dry them and weigh them. Note how full each tube is
Results: Tubing with starch solution expands and becomes more full. It gains weight - this happens because water moves due to osmosis from a high water concentration to a low water concentration in the visking tubing
Osmosis and Plant Cells
Plant cells are surrounded by cell walls which are fully permeable. However the cell membrane inside the cell wall is semi-permeable
Plant cells whose contents are more concentrated than their surroundings:
Cytoplasm of plant cells contain salts, sugars, starch, proteins and other molecules dissolved in water. If a plant cell is placed in distilled water or a solution less concentrated than its cytoplasm then water will move into the cell due to osmosis. This is the way root cells absorb water from soil.
Water will pass into the vacuole. The cytoplasm and vacuole will swell and the cell membrane will be forced against the cell wall. The cell is now much stronger or firmer due to its turgor pressure. The cell is said to be turgid. This can be compared to air being forced into a car tyre
Turor or Turgor Pressure is the pressure of the cytoplasm and vacuole against the cell wall
Animal cells whose contents are more concentrated than their surroundings:
If animal cells are placed in very dilute solutions then water will enter due to osmosis. The cell will expand and may burst as they lack a cell wall to restrain them
Amoeba has a contractile vacuole to prevent it from bursting
Plant cells whose contents are less concentrated than their surroundings:
If plant cells are placed in very concentrated solutions water will pass out of the vacuoles of cells due to osmosis
Animal cells whose contents are less concentrated than their surroundings:
If animals cells are placed in very concentrated solutions then water will pass out of the cells. The cells will lose their shape and become shriveled (crenation)
Plasmoysis
When plant cells are placed in salty water (a concentrated solutions), they lose turgor and become weaker. The cells are said to be plasmolysed. If plant cells lose turgor, plant is seen to wilt. This is temporary and can be reversed by placing plant cells in less concentrated solutions than their cell contents e.d. water them
Osmosis and Food Preservation
Bacteria and fungi both have cell walls. If they are placed in a very concentrated solution they will lose water by osmosis. This prevents them from growing and causing food to decay. There are two main examples of this process.
1. Soaking Food in very salty solutions
Preserves fish and meat such as bacon
2. High Sugar Content
Such as jam and marmalade. Preserves them from fungus decay