Movement of Substances Across a Plasma Membrane, worked answers

The solution has a lower water potential than the cytoplasm inside the red blood cell, so water moves out of the cell by osmosis, down the water potential gradient, across the partially permeable plasma membrane. As the red blood cell loses water it shrinks and its surface becomes crinkled, a change called crenation, because the cell has no cell wall to resist this loss of water.

lower water potentialosmosiswater potential gradientpartially permeable membranecrenation

Visking tubing is partially permeable, with pores small enough to let small molecules such as iodine pass through but not large starch molecules. Iodine molecules diffuse from a region of higher concentration outside the tubing to a region of lower concentration inside, down the concentration gradient, and react with the starch to produce a blue-black colour inside the tubing. Starch molecules are too large to diffuse out through the pores of the tubing, so the iodine solution outside stays orange-brown because no starch reaches it to react.

partially permeablediffusionconcentration gradientstarch too largeblue-black colour

Diffusion moves particles down the concentration gradient, from a region of higher to lower concentration, and needs no energy because it happens through the random kinetic movement of particles; an example is the diffusion of oxygen from the alveolus into the surrounding blood capillary. Active transport moves particles against the concentration gradient, from a region of lower to higher concentration, and requires energy from respiration in the form of ATP together with a specific carrier protein; an example is the reabsorption of glucose from the filtrate back into the blood in the kidney tubule, even when the blood glucose concentration is already higher than that in the filtrate.

down the gradientno energyagainst the gradientATPcarrier proteinkidney tubule reabsorption

Because the mineral-ion concentration in the soil solution is lower than inside the root hair cell, the ions cannot move in by diffusion, which only works down a concentration gradient. Instead, the root hair cell uses active transport: energy released from respiration in the mitochondria is used by specific carrier proteins in the plasma membrane to pump mineral ions into the cell against their concentration gradient. This is why root hair cells contain many mitochondria, to supply the ATP that active transport of mineral ions requires.

against concentration gradientactive transportrespirationATPcarrier proteinmitochondria

A smaller cube has a larger surface area to volume ratio than a larger cube of the same shape, so relative to its volume, the smallest block has more surface exposed for the dye to diffuse in across. Because the diffusion distance from the surface to the centre of the block is also shorter in the smallest cube, the dye reaches the centre faster, so the smallest block shows complete colour change first even though diffusion happens at the same rate per unit area across all the blocks.

surface area to volume ratiosmaller blockshorter diffusion distancesame rate per unit area

Raising the temperature gives particles more kinetic energy, so the dye molecules move and collide faster, increasing the rate of diffusion, and this effect on diffusion continues to rise as temperature rises within a normal range. Active transport, however, depends on carrier proteins in the plasma membrane, and proteins are made of amino acid chains held in a specific three-dimensional shape by bonds that break down at high temperature; at 60°C the carrier proteins denature and lose their functional shape, so they can no longer bind and transport glucose, which lowers the rate of active transport even though the temperature is higher.

kinetic energyrate of diffusion increasescarrier proteindenatureactive transport decreases