Define osmosis.
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Net movement of (free) water molecules; through a partially permeable membrane; from a region of higher water potential to a region of lower water potential
Osmosis And Membrane Transport
Explain why a red blood cell placed in pure water will burst.
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Pure water has a higher water potential than the cell cytoplasm; water moves into the cell by osmosis (down the water potential gradient); the cell has no cell wall to resist the increase in volume / cell membrane bursts (lysis)
State two factors that increase the rate of simple diffusion across a cell membrane.
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Steeper concentration gradient; higher temperature; larger surface area; shorter diffusion distance (any 2)
Explain why glucose enters red blood cells by facilitated diffusion rather than simple diffusion.
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Glucose is a large/polar molecule; it cannot pass through the hydrophobic phospholipid bilayer (directly); a (specific) carrier/transport protein is required to move it across the membrane
Compare simple diffusion and facilitated diffusion.
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Both are passive / move down a concentration gradient / require no ATP; simple diffusion does not use proteins, but facilitated diffusion uses channel/carrier proteins; simple diffusion is for small/non-polar molecules, facilitated diffusion is for larger/polar/charged molecules
Define active transport.
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Movement of substances across a membrane against a concentration gradient; using energy from ATP (hydrolysis)
Explain how ATP is used in active transport.
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ATP binds to the carrier protein; ATP is hydrolysed to ADP + Pi (releasing energy); the energy causes the carrier protein to change shape (transporting the substance across the membrane)
Suggest why cells with high rates of active transport contain large numbers of mitochondria.
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Mitochondria produce/synthesise ATP (via aerobic respiration); ATP is needed/hydrolysed to power active transport
Distinguish between endocytosis and exocytosis.
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Endocytosis moves substances into the cell (by membrane invagination/forming a vesicle); exocytosis moves substances out of the cell (by a vesicle fusing with the membrane)
Explain why endocytosis is described as an active process.
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It requires ATP / energy; to change the shape of the membrane / form the vesicle / move the vesicle
State one difference between a channel protein and a carrier protein.
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Channel proteins form a pore, but carrier proteins change shape; OR channel proteins are only used in facilitated diffusion, while carrier proteins can be used in both facilitated diffusion and active transport
Explain how the structure of a carrier protein allows it to transport glucose into a red blood cell.
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The carrier protein has a specific binding site (complementary to glucose); glucose binds and the protein changes shape; glucose is released on the other side of the membrane (down its concentration gradient)
Discuss how the cell surface membrane uses different transport mechanisms to move substances into and out of the cell. Use information from this topic and your own knowledge to support your answer.
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Small/non-polar molecules (O₂, CO₂) cross by simple diffusion through the bilayer; larger/polar molecules use facilitated diffusion via channel or carrier proteins; both passive, no ATP needed, down concentration gradient; active transport uses carrier proteins and ATP to move substances against the gradient; endocytosis brings large particles in via vesicles (e.g. phagocytosis of bacteria); exocytosis releases substances out via vesicle fusion (e.g. insulin secretion); osmosis moves water through partially permeable membrane down a water potential gradient; cells use the appropriate mechanism depending on size, polarity, and direction relative to the gradient