Movement Into and Out of Cells

Diffusion: particles spreading out down a gradient

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration - that is, down a concentration gradient. The word net is doing real work in that definition: individual particles move randomly in every direction, but because there are more of them on the high-concentration side, the overall result is a movement from high to low until they are evenly spread.

Nothing pushes the particles along. The energy comes from the kinetic energy of the random movement of the molecules and ions themselves, so diffusion needs no energy from respiration and would happen just as well in a dead cell.

Some substances move into and out of cells by diffusion straight through the cell membrane. Oxygen and carbon dioxide cross it freely, and small dissolved solutes such as glucose follow their gradient. A substance only diffuses into a cell if the membrane lets it through and if its concentration outside the cell is higher than inside.

• Oxygen diffuses from the alveoli into the blood, and from the blood into respiring cells, because each step is down its concentration gradient.
• Carbon dioxide diffuses out of respiring cells into the blood, and out of the blood into the alveoli to be breathed out.
• Carbon dioxide diffuses from the air into a leaf through the stomata, and oxygen diffuses out, because photosynthesis uses carbon dioxide and produces oxygen.
• Digested products such as glucose and amino acids diffuse from the gut into the blood.
• Urea diffuses out of liver cells into the blood, and is then removed by the kidneys.

The four factors that change the rate of diffusion

Cambridge limits the factors that influence diffusion to four: surface area, temperature, concentration gradient and distance. Learn each one with its reason attached, because explain questions want the mechanism, not just the name.

A larger surface area gives more places for particles to cross at the same time, so more particles get through in a given period. A steeper concentration gradient means a bigger difference between the two regions, so the net movement is greater. A shorter distance means the particles have less far to travel before they arrive. A higher temperature gives the particles more kinetic energy, so they move faster and spread out more quickly.

The same ideas explain a lot of the shapes you meet elsewhere in the course. A small cell has a large surface area to volume ratio, so substances can diffuse in and out fast enough to supply the whole cell; the flattened alveolus, the folded villus and the long thin root hair are all surfaces built to speed diffusion up.

Osmosis: water moving through a partially permeable membrane

Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane.

A partially permeable membrane has gaps that let small molecules such as water through but hold back larger solute molecules. Water can cross; the solute cannot. Water therefore always moves towards the solution with the most solute dissolved in it, which is the more concentrated solution. Osmosis is a special case of diffusion, and like diffusion it is passive - no energy from respiration is used.

Water is also the solvent in which the chemistry of life happens, which is why osmosis matters so much. In digestion, enzymes break food down into soluble products that must dissolve in water before they can be absorbed. In excretion, urea and other wastes are carried in solution to the kidneys and removed in urine. In transport, substances are carried around the body dissolved in blood plasma, and water is the medium that moves up the xylem.

• Osmosis moves water only. If a question asks about salt, sugar or ions crossing a membrane, it is not osmosis.
• Higher water potential = dilute solution = less solute. Lower water potential = concentrated solution = more solute.
• Water always moves from the dilute side to the concentrated side, whichever way round the diagram puts them.
• The cell membrane is partially permeable, which is why every living cell is affected by osmosis.

Plant cells in different solutions: turgid, flaccid, plasmolysed

A plant cell has a cell wall outside its cell membrane. In pure water, or in a solution more dilute than its own cell contents, water enters by osmosis. The cell contents swell and press outwards on the cell wall, and the cell becomes turgid.

The pressure of the water inside the cell pressing outwards on the cell wall is turgor pressure, and it is what supports herbaceous plants: it holds leaves and stems firm and upright, and it is the reason a plant that is short of water wilts.

In a solution more concentrated than the cell contents, water leaves by osmosis. The cell loses water, the contents shrink and stop pressing on the wall, and the cell becomes flaccid. If even more water is lost, the cytoplasm and cell membrane pull away from the cell wall; this is plasmolysis, and a plasmolysed cell cannot recover.

Animal cells have no cell wall: crenated and lysed

Animal cells have no cell wall, so osmosis affects them differently and the result can be fatal. In a solution more concentrated than the cell contents - a strong salt or sugar solution, for example - water leaves by osmosis, the cell shrinks, and it becomes crenated: wrinkled and spiky rather than smooth.

In pure water, or a solution more dilute than the cell contents, water enters by osmosis faster than the cell can cope with. The cell swells and eventually the cell membrane bursts. This is lysis, and the cell is described as lysed.

This is why hospital drips and rehydration drinks are made up to the same concentration as body fluids. Putting pure water straight into a vein would cause water to enter the red blood cells by osmosis until they lysed.

Compare the two kingdoms and the reason becomes clear: a plant cell in pure water becomes turgid but survives, because the cell wall resists the pressure; an animal cell in pure water swells and lyses, because it has no wall to resist it.

Active transport: moving particles against the gradient

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration - that is, against a concentration gradient. Because the particles are being moved the wrong way up the gradient, diffusion cannot do the job, and the process uses energy from respiration.

Because the energy comes from respiration, active transport only happens in living, respiring cells. Stop respiration - with a respiratory poison, or by removing oxygen or glucose - and active transport stops too, while diffusion carries on unaffected.

Special protein carriers in the cell membrane pick up the molecule or ion on one side, change shape using the energy released by respiration, and release it on the other side. The carriers are specific, so a cell takes in only the particles it needs.

Root hair cells take up mineral ions such as nitrate from the soil by active transport. The concentration of these ions inside the root hair cell is higher than in the soil solution, so diffusion would move them out of the cell rather than in. Active transport is the only way the plant can obtain them. In the same way, cells lining the gut take up glucose by active transport when the concentration in the gut is lower than in the cell, and kidney cells use active transport to reabsorb glucose from the filtrate back into the blood.

Diffusion, osmosis and active transport compared

This comparison is the single most examined idea in the topic, so learn it as a table rather than as three separate paragraphs. Diffusion and osmosis are both passive and both move particles down a gradient; active transport is the only one that goes against the gradient and the only one that needs energy from respiration.

Use one sentence to anchor it: diffusion moves any particle down its concentration gradient, osmosis moves water only down its water potential gradient through a partially permeable membrane, and active transport uses energy from respiration to move molecules or ions against their concentration gradient using protein carriers.

Watch the direction wording in every answer. Diffusion: high to low concentration. Osmosis: dilute to concentrated, or higher to lower water potential. Active transport: low to high concentration, against the gradient.