National 5 Biology Transport Across Cell Membranes Mr
National 5 Biology Transport Across Cell Membranes Mr G Davidson
Plasma Membrane • All living cells have a cell membrane which is composed of protein and lipid (fat) molecules. • The exact structure is still unclear but the most popular model is known as the Fluid Mosaic Model which suggests that the membrane has constantly mobile lipid molecules known as the Mosaic. 12 March 2021 G Davidson 2
Cell Membrane Structure 12 March 2021 G Davidson 3
Cell Membrane • Because lipids are held by weak bonds, this allows them to change position which is necessary to allow some cells to move. • It is thought that oxygen and carbon dioxide diffuse freely through the lipid layer while water enters through the pores by osmosis. 12 March 2021 G Davidson 4
Movement of Substances Across the Membrane • It is essential for all cells that substances can pass in and out through the membrane. • For this to happen, a concentration gradient has to exist. 12 March 2021 G Davidson 5
Movement of Substances Across the Membrane • Any movement down this gradient is called passive transport and requires no energy, e. g. diffusion and osmosis. • If the movement is up the gradient, it is called active transport and this does require energy. 12 March 2021 G Davidson 6
Transport Across Membranes • Diffusion is the movement of molecules from a region of high concentration to a region of low concentration down the concentration gradient until the molecules are evenly distributed. 12 March 2021 G Davidson 7
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 8
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 9
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 10
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 11
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 12
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 13
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 14
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 15
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 16
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 17
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 18
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 19
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 20
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 21
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 22
Diffusion High concentration Low concentration Litmus paper 12 March 2021 G Davidson Cotton wool soaked in ammonium hydroxide 23
Transport Across Membranes 12 March 2021 G Davidson 24
Transport Across Membranes • Living cells depend on diffusion for the movement of substances into and out of the cells. • These substances must be dissolved in water. 12 March 2021 G Davidson 25
Transport Across Membranes • When a substance dissolves in water its molecules spread out and move into the spaces between the water molecules. • If the substance that dissolves has large molecules, the water molecules are pushed farther apart. 12 March 2021 G Davidson 26
Transport Across Membranes • The membranes surrounding living cells do not allow all substances to pass through them. • Some substances are made up of molecules which are too large to pass through the membranes of living cells. 12 March 2021 G Davidson 27
Transport Across Membranes • Membranes which allow all molecules to pass through them are described as permeable. • Large molecules are unable to pass through the membranes of living cells and so these membranes are described as being selectively permeable. 12 March 2021 G Davidson 28
Transport Across Membranes • “Osmosis is the movement of WATER molecules from a region of high WATER concentration to a region of low WATER concentration across a selectively permeable membrane. ” 12 March 2021 G Davidson 29
Transport Across Membranes • The water molecules can easily pass through the membrane as they are so small, but the larger molecules in a solution can’t. • Therefore in osmosis, it is only the water that moves. 12 March 2021 G Davidson 30
The following experiment was set up using model cells made from visking tubing bags. A B WATER STRONG SUGAR SOLUTION 12 March 2021 G Davidson 31
The model cells were weighed at the start and then again after ½ an hour. The following results were obtained. Difference in mass (g) + or - Initial mass (g) Final mass (g) A 33. 5 37. 0 +3. 5 B 33. 0 30. 8 -2. 2 Complete and explain the results. 12 March 2021 G Davidson 32
Osmosis in Plant Cells • When a plant cell is surrounded by water (diagram A), water passes through the cell membrane and into the cell by osmosis. • The vacuole swell and pushes the cytoplasm against the cell wall. • The cell wall is strong and holds the cells together. • The cell is now turgid. 12 March 2021 G Davidson WATER ENTERS Diagram A 33
Turgid Plant Cells 12 March 2021 G Davidson 34
Osmosis in Plant Cells • When a plant cell is placed in a strong salt or sugar solution, water leaves the vacuole and passes through the cell membrane. (Diagram B) • This causes the vacuole to shrink which pulls the cytoplasm and cell membrane away from the cell wall. • This is called plasmolysis. • The cell is now flaccid. 12 March 2021 G Davidson WATER LEAVES Diagram B 35
Plasmolysed Plant Cells 12 March 2021 G Davidson 36
Osmosis in Animal Cells • If animal cells are placed in pure water, the water enters the cells and they burst as there is no cell wall to prevent it. • If they are placed in a strong salt or sugar solution, water leaves by osmosis and the cells will shrink. 12 March 2021 G Davidson 37
Active Transport • “Active Transport is the movement of molecules from a region of low concentration to a region of high concentration against the concentration gradient. ” • This process requires energy. 12 March 2021 G Davidson 38
Active Transport • The proteins in the membrane act as ‘pumps’ to move the molecules against the gradient. • This is seen in nerve cells where sodium and potassium are actively transported across the membrane. 12 March 2021 G Davidson 39
Active Transport 12 March 2021 G Davidson 40
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