Movement of Substances Across a Plasma Membrane, practice questions

B, A channel protein forms a fixed pore that lets specific ions or small polar molecules diffuse straight through the membrane, unlike a carrier protein, which changes shape to move a particle across.

C, In an isotonic solution, water enters and leaves the cell at the same rate, so there is no net movement of water and the cell's size remains constant.

C, Active transport moves particles against their concentration gradient and needs ATP from respiration; the other three options are all passive processes that move particles down a gradient.

C, Distilled water has a much higher water potential than the cell's cytoplasm, so water enters the red blood cell by osmosis until the cell bursts; this bursting is called haemolysis and occurs because the cell has no wall to resist the pressure.

B, A larger particle size slows diffusion because bigger particles move more slowly through a medium; a steeper gradient, a larger surface area and a higher temperature all increase the rate.

C, Glycoproteins are proteins with attached carbohydrate chains found on the outer surface of the plasma membrane, where they act as recognition markers for hormones, antigens and neighbouring cells.

C, Osmosis moves water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane, and needs no energy.

C, The dialysis machine relies on diffusion: urea and excess ions move from the blood, where their concentration is higher, into the dialysis fluid, where their concentration is kept lower, across a partially permeable membrane.

B, The high salt concentration outside creates a hypertonic environment, so water leaves microbial cells by osmosis; the dehydrated cells cannot grow or reproduce, which slows spoilage.

C, A carrier protein binds a particle, changes its own shape, and releases the particle on the other side of the membrane; this shape change can happen passively or, using ATP, actively, unlike the fixed pore of a channel protein.

• Bag X (20% sucrose) increases more in mass than bag Y.
• Bag X has a lower water potential than bag Y because it contains a higher solute concentration.
• Distilled water has the highest water potential of all, so water moves into both bags by osmosis, from the water outside (high water potential) to the sucrose solution inside (lower water potential).
• Because bag X has the lower water potential, the water potential gradient between the water outside and the solution inside is steeper for bag X than for bag Y.
• A steeper water potential gradient means a faster net movement of water into bag X, so it gains more mass in the same time.
• If both bags were immersed in 40% sucrose solution instead, this solution would have a lower water potential than both 20% and 5% sucrose solutions inside the bags.
• Water would then move out of both bags by osmosis, from the bags (higher water potential) into the 40% solution (lower water potential), so both bags would lose mass.
• Bag Y (5% sucrose) would lose mass faster than bag X, because the water potential gradient between its contents and the 40% solution outside is steeper.

• The process responsible is osmosis (loss of water from the plant cells by osmosis, leading to loss of turgor).
• As the soil solution becomes more concentrated than the cell sap, the soil solution has a lower water potential than the cell.
• Water moves out of the plant cell by osmosis, from the cell (higher water potential) into the soil solution (lower water potential), across the partially permeable cell membrane.
• As water leaves, the cytoplasm and cell membrane shrink away from the rigid cellulose cell wall, this stage is called plasmolysis.
• The cell becomes flaccid and loses its turgor pressure, so it can no longer push against the cell wall to keep it rigid, and the leaves and stem droop as a result (wilting).
• In an animal cell, there is no cell wall, so instead of plasmolysing the cell would simply shrink and its surface would crinkle, a change called crenation, rather than becoming flaccid while the membrane pulls away from a wall.

• The rate of oxygen uptake increases from 10°C to 40°C, reaching a peak at 40°C, then decreases sharply from 40°C to 70°C.
• Between 10°C and 40°C, rising temperature gives the oxygen molecules and the cell membrane's components more kinetic energy, so molecules move and collide with the membrane faster, increasing the rate of diffusion.
• This part of the trend is consistent with diffusion, which has no biological ceiling of its own within this range because it does not depend on protein shape.
• Between 40°C and 70°C, the rate falls sharply because the yeast cells' proteins, including any carrier or channel proteins and respiratory enzymes involved, begin to denature at high temperature.
• Denaturation changes the three-dimensional shape of these proteins so they can no longer function normally, which reduces the rate of any protein-dependent transport or reaction even though the temperature itself would otherwise continue to speed up simple diffusion.
• If active transport of oxygen-related ions were also occurring, this later decline would be linked most strongly to the denaturation of the carrier proteins responsible for that active transport, since active transport is protein-dependent and therefore has a temperature ceiling that pure diffusion does not share.

A phospholipid bilayer with proteins scattered through it; it is partially (selectively) permeable.

Net movement of particles from high to low concentration, down a concentration gradient, without energy.

Net movement of water molecules from a less concentrated (dilute) to a more concentrated solution across a partially permeable membrane.

Movement of substances against the concentration gradient, using energy from respiration and carrier proteins.

In hypotonic solution animal cells burst (haemolysis) and plant cells become turgid; in hypertonic solution animal cells shrink (crenation) and plant cells plasmolyse.

Osmosis and diffusion explain wilting, food preservation by salting, and root absorption of minerals by active transport.

Channel proteins form a fixed, water-filled pore that lets specific ions or small polar molecules diffuse straight through the membrane. Carrier proteins instead bind the particle, change shape and release it on the other side; they can work passively in facilitated diffusion or, using ATP, actively pump a substance against its concentration gradient.

The rate of diffusion, osmosis and active transport increases with a steeper concentration gradient, a larger surface area, a smaller particle size and a higher temperature, because particles then have more kinetic energy and collide with the membrane more often; active transport is further limited by the amount of ATP and carrier proteins available.

Water potential measures how readily water molecules move out of a solution; pure water has the highest water potential, and dissolving a solute always lowers it. Water moves by osmosis from a region of higher water potential to a region of lower water potential, which is the same as saying it moves from a less concentrated to a more concentrated solution.

A kidney dialysis machine uses diffusion across a partially permeable membrane to remove urea and excess ions from a patient's blood when the kidneys fail. Intravenous drip fluids must be isotonic to blood plasma; an incorrectly concentrated drip would make red blood cells swell and burst or shrink by osmosis.

The three mechanisms act side by side in real organisms: gas exchange at the alveoli and in root hairs happens by diffusion, water enters root hair cells and red blood cells adjust volume by osmosis, and reabsorption of glucose in the kidney tubule and mineral-ion uptake in roots depend on active transport.

This chapter's principles underpin the process of osmoregulation, covered in more detail elsewhere: freshwater organisms constantly gain water by osmosis and must expel it, while marine organisms in a saltier environment tend to lose water and must conserve it, so the surrounding environment always matters when predicting the direction of osmosis.