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Transport Across Cell Membranes

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Presentation on theme: "Transport Across Cell Membranes"— Presentation transcript:

1 Transport Across Cell Membranes
National 5 Biology Transport Across Cell Membranes Mr G Davidson

2 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. Wednesday, 14 November 2018 G Davidson

3 Cell Membrane Structure
Wednesday, 14 November 2018 G Davidson

4 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. Wednesday, 14 November 2018 G Davidson

5 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. Wednesday, 14 November 2018 G Davidson

6 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. Wednesday, 14 November 2018 G Davidson

7 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. Wednesday, 14 November 2018 G Davidson

8 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

9 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

10 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

11 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

12 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

13 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

14 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

15 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

16 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

17 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

18 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

19 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

20 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

21 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

22 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

23 Cotton wool soaked in ammonium hydroxide
Diffusion High concentration Low concentration Cotton wool soaked in ammonium hydroxide Litmus paper 14 November, 2018 G Davidson

24 Transport Across Membranes
Wednesday, 14 November 2018 G Davidson

25 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. Wednesday, 14 November 2018 G Davidson

26 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. Wednesday, 14 November 2018 G Davidson

27 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. Wednesday, 14 November 2018 G Davidson

28 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. Wednesday, 14 November 2018 G Davidson

29 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.” Wednesday, 14 November 2018 G Davidson

30 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. Wednesday, 14 November 2018 G Davidson

31 A B STRONG SUGAR SOLUTION
The following experiment was set up using model cells made from visking tubing bags. A B WATER STRONG SUGAR SOLUTION 14 November, 2018 G Davidson

32 The model cells were weighed at the start and then again after ½ an hour. The following results were obtained. Initial mass (g) Final mass (g) Difference in mass (g) + or - A 33.5 37.0 B 33.0 30.8 +3.5 -2.2 Complete and explain the results. 14 November, 2018 G Davidson

33 Osmosis in Plant Cells WATER ENTERS Diagram A
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. WATER ENTERS Diagram A 14 November, 2018 G Davidson

34 Turgid Plant Cells Wednesday, 14 November 2018 G Davidson

35 Osmosis in Plant Cells WATER LEAVES Diagram B
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. 14 November, 2018 G Davidson

36 Plasmolysed Plant Cells
Wednesday, 14 November 2018 G Davidson

37 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. Wednesday, 14 November 2018 G Davidson

38 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. 14 November, 2018 G Davidson

39 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. 14 November, 2018 G Davidson

40 Active Transport 14 November, 2018 G Davidson

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