Describe the structure of the Fluid Mosaic model.
- phospholipids form a continuous bilayer. The bilayer is fluid because the phospholipids are always moving.
- cholesterol molecules present within the bilayer.
- proteins scattered through bilayer. Include: channel proteins + carrier proteins which allow large molecules and ions to pass through membrane, receptor proteins on cell-surface membrane allow cell to detect chemicals released from other cells.
- some proteins able to move sideways through bilayer, while others in fixed position.
- some proteins have polysaccharide chain attached-these called glycoproteins.
- some lipids have polysaccharide chain attached- these called glycolipids.
Describe phospholipids as a part of the cell membrane.
- phospholipids have hydrophilic head and a hydrophobic tail.
- molecules automatically arrange themselves into bilayer, heads face outwards towards water on either side of membrane.
- centre of bilayer hydrophobic so membrane doesn’t allow water-soluble substances through it, it acts as a barrier to dissolved substances.
Describe cholesterol as a part of the cell membrane.
- cholesterol is type of lipid.
- present in all cell membranes except bacterial cell membranes.
- cholesterol molecules fit between phospholipids, binding to their hydrophobic tails causing them to pack more closely together. This restricts movement of phospholipids making membrane less fluid and more rigid.
- cholesterol helps maintain shape of animal cells, particularly important for cells that aren’t supported by other cells.
Explain how simple diffusion depends on concentration gradient.
the higher it it faster rate of diffusion. As diffusion takes place, difference in conc between two sides of membrane decreases until reaches equilibrium. Means diffusion slows down over time
Explain how simple diffusion depends on thickness of the exchange surface.
Thinner the exchange surface, faster the rate of diffusion.
Explain how simple diffusion depends on the surface area.
Larger surface area, faster rate of diffusion.
Eg. Microvilli give cell a larger SA. Larger SA means more particles can be exchanged in the same amount of time, increasing rate of diffusion
Explain how facilitated diffusion depends on the concentration gradient.
Higher the conc gradient, faster the rate of facilitated diffusion. As equilibrium reached rate of facilitated diffusion will level off.
Explain how facilitated diffusion depends on the number of channel or carrier proteins
Once all proteins in membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the conc gradient. So greater number of channel or carrier proteins in cell membrane, the faster the rate of facilitated diffusion.
Describe osmosis.
Osmosis is diffusion of water molecules across a partially permeable, from an area of higher water potential to an area of lower water potential.
What is meant by isotonic.
When two solutions have the same water potential.
Explain how the rate of osmosis depends on the water potential gradient.
The higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so rate of osmosis levels off over time.
Explain how the rate of osmosis depends on the thickness of the exchange surface.
Thinner exchange surface, faster rate of osmosis.
Explain how the rate of osmosis depends on the surface area of the exchange surface.
Larger surface area , faster rate of osmosis.
Describe the role of carrier proteins in active transport.
Molecule attaches to carrier protein, protein changes shape and this moves molecule across membrane releasing it to the other side.
Explain how active transport is different to facilitated diffusion.
- AT usually moves solutes from low to high concentration, in FD they always move from high to low conc
- AT requires energy, FD doesn’t
Explain the importance of ATP in active transport
ATP undergoes a hydrolysis reaction, slitting into ADP and Pi. This releases energy so that the solutes can be transported
Explain how the speed of individual carrier proteins affects the rate of active transport.
Faster they work, faster rate of transport.
Explain how the number of carrier proteins present affects the rate of active transport.
More proteins, faster rate of reaction.
Explain how rate of respiration in cell and the availability of ATP affects the rate of active transport.
If respiration is inhibited active transport can’t take place.
Describe the absorption of glucose in the mammalian ileum by co-transport.
- Sodium ions actively transported out of the ileum epithelial cells, into the blood, by the sodium-potassium pump. This creates concentration gradient. Now higher conc do sodium ions in lumen of ileum than inside the cell.
- causes sodium ions to diffuse from lumen of ileum into epithelial cells, down their concentration gradient via the sodium-glucose co-transporter proteins.
- co-transporter carries glucose into the cell with sodium. So concentration of glucose inside cell increases
- glucose diffuses out of cell, into blood, down conc gradient through protein channel by facilitated diffusion
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1 Biological Molecules88
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2 Homeostasis30
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2 Nervous Coordination And Muscles24
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2 Stimuli And Responses47
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2 Energy, Ecosystems and Nutrient Cycles38
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2 Populations In Ecosystems37
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2 Populations And Evolution32
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2 Genetics21
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2 Photosynthesis and Respiration41
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1 Diversity, Classification And Variation36
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1 DNA, RNA And Protein Synthesis13
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1 Exchange And Transport Systems55
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1 Cells And The Immune System24
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1 Cell Membranes20
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1 Cell Structure52
