Movement of Substances Across a Plasma Membrane, revision notes

Overview

The plasma membrane controls what enters and leaves a cell. This chapter explains its fluid mosaic structure and the three ways substances cross it, diffusion, osmosis and active transport, plus how these apply in living organisms and daily life.

Getting the definitions and the direction of movement exactly right is where most marks are won.

The membrane itself is more than a passive barrier. Beyond the phospholipid bilayer, embedded and peripheral proteins act as channels, carriers and receptors, while glycoproteins and glycolipids on the outer surface let a cell recognise hormones, antigens and neighbouring cells.

This selective structure is why the membrane is described as partially permeable, small non-polar molecules such as oxygen and carbon dioxide diffuse straight through the lipid bilayer, whereas larger or charged particles such as glucose and ions must pass through a specific protein channel or carrier instead.

The rate at which particles cross the membrane is not fixed. It rises with a steeper concentration gradient, a larger surface area, a smaller particle size and a higher temperature, since molecules then move faster and collide with the membrane more often.

SPM questions often ask candidates to explain how changing one of these factors affects the rate, so learn each factor together with the reason it speeds up or slows down diffusion, osmosis or active transport, rather than only memorising the list.

This chapter is assessed heavily through Paper 2 structured questions built around a labelled diagram, a set of experimental results or a short scenario, and through Paper 1 items testing definitions and factors. A strong answer states the correct term, the direction of movement and the reason together, since examiners award marks for each linked idea rather than for the term alone.

Structure of the plasma membrane (3.1)

The plasma membrane is built on the fluid mosaic model: a phospholipid bilayer in which each phospholipid has a hydrophilic (water-loving) head facing outward and a hydrophobic (water-fearing) tail facing inward, so the two tail layers meet in the middle of the membrane. This arrangement forms a stable, flexible barrier around every cell, and because the phospholipids are not fixed in place, the whole membrane behaves like a fluid, letting proteins and lipids drift sideways within it.

Scattered through the bilayer are proteins of several kinds. Integral proteins span the full width of the membrane and include channel proteins, which form a fixed, water-filled pore for specific ions or small polar molecules, and carrier proteins, which bind a particle, change shape and release it on the other side, passively during facilitated diffusion or, using ATP, actively against a concentration gradient.

Peripheral proteins sit only on one face of the membrane and often act as enzymes or as anchors for the cytoskeleton.

On the outer surface, short carbohydrate chains attach to some proteins to form glycoproteins and to some lipids to form glycolipids. These act as recognition markers that let a cell identify hormones, antigens and neighbouring cells, which is why the membrane surface is central to immune recognition and cell signalling.

Cholesterol molecules wedged between the phospholipids add rigidity and help keep the membrane's fluidity stable across a range of temperatures.

Because of this structure, the membrane is described as partially (selectively) permeable: small non-polar molecules such as oxygen, carbon dioxide and lipid-soluble substances diffuse directly through the phospholipid bilayer, while larger molecules such as glucose and charged particles such as ions cannot cross the hydrophobic core unaided and must instead pass through a specific channel or carrier protein.

Diffusion, osmosis and active transport compared (3.2)

All three transport processes move particles across the membrane, but they differ in what moves, in what direction, and in whether energy is spent. Diffusion is the net movement of any particle, a gas, a solute or an ion, from a region of higher concentration to a region of lower concentration, down the concentration gradient, and needs no energy because it results from the random kinetic movement of particles.

Osmosis is a special case of diffusion that applies only to water molecules moving across a partially permeable membrane, from a less concentrated (dilute) solution to a more concentrated solution, or equivalently from higher to lower water potential. Active transport moves particles against their concentration gradient, from a region of lower to higher concentration, and therefore requires energy from respiration together with a specific carrier protein.

Effect of solutions on animal and plant cells

When a cell is placed in a solution, the net direction of water movement by osmosis depends on comparing the water potential (or concentration) inside the cell with that of the surrounding solution. In a hypotonic solution, which has a higher water potential than the cell, water moves into the cell; in a hypertonic solution, which has a lower water potential than the cell, water moves out; in an isotonic solution, water still crosses the membrane both ways but there is no net movement, so cell size stays constant.

Animal cells and plant cells respond differently because only a plant cell has a rigid cellulose cell wall. An animal cell in a hypotonic solution keeps absorbing water until it bursts, called haemolysis, since it has no wall to resist the pressure; the same cell in a hypertonic solution loses water and shrinks and its surface becomes crinkled, called crenation.

A plant cell in a hypotonic solution absorbs water until its cell wall exerts an equal and opposite pressure, becoming turgid rather than bursting; the same cell in a hypertonic solution loses water until the cell membrane pulls away from the cell wall, called plasmolysis, and the cell becomes flaccid.

Factors affecting the rate of movement across the membrane

The rate of diffusion, osmosis and active transport all increase with a steeper concentration gradient, a larger surface area, a smaller particle size and a higher temperature, because particles then carry more kinetic energy and collide with the membrane more often. A steeper gradient means a bigger difference between the two sides for particles to move down; a larger surface area, such as the many folds of the small intestine wall or the branching structure of the alveoli, gives more room for particles to cross at once; and smaller particles diffuse faster because they need less energy to move through the surrounding medium.

Active transport is limited by two extra factors beyond these: the supply of ATP from respiration and the number of carrier proteins available in the membrane, since each carrier can only transport a limited number of particles per second regardless of how steep the gradient against it is. Temperature also has a ceiling effect that diffusion alone does not show, raising the temperature speeds up diffusion and osmosis without limit within a biological range, but active transport depends on carrier proteins, which are proteins that denature at high temperatures, so an excessively high temperature can slow or stop active transport even as diffusion continues.

Movement of substances in living organisms (3.3)

The three transport mechanisms act together, side by side, in real tissues. At the alveolus, oxygen diffuses from the air sacs into the surrounding capillaries while carbon dioxide diffuses the opposite way, a rapid exchange made possible by the alveolus's thin, single-cell wall, large surface area and rich capillary network.

In the small intestine, digested nutrients such as glucose and amino acids cross into the blood partly by diffusion and partly by active transport, particularly when their concentration inside the gut is already lower than in the blood.

Root hair cells absorb water from the soil by osmosis, since the soil solution usually has a higher water potential than the cell sap, while they absorb mineral ions such as nitrate and magnesium by active transport, because the ion concentration in the soil is normally lower than inside the root hair cell and diffusion alone could not move ions against this gradient. In the kidney, the nephron reabsorbs glucose from the filtrate back into the blood by active transport so that none is lost in the urine, and red blood cells rely on a delicate osmotic balance with the surrounding plasma to keep their shape and function normally.

Nerve cells maintain a resting potential and generate a nerve impulse using the sodium-potassium pump, an active-transport protein that continuously moves sodium ions out of and potassium ions into the cell against their gradients.

Applications in daily life (3.4)

A kidney dialysis machine performs the same job as a healthy kidney for a patient in renal failure: blood is passed along one side of a partially permeable membrane while a dialysis fluid with a carefully controlled composition flows on the other side, so urea and excess ions diffuse out of the blood into the dialysis fluid down their concentration gradient, while useful substances such as glucose remain balanced between the two fluids and are not lost.

Salting or sugaring food is a traditional preservation method that works by osmosis: the high concentration of salt or sugar outside microbial cells such as bacteria creates a hypertonic environment, so water is drawn out of the microbial cells by osmosis, and the cells become dehydrated and cannot grow or reproduce, which slows spoilage. Intravenous (IV) drip fluids given in hospitals must be isotonic to blood plasma; a drip that is too dilute would make red blood cells absorb water and burst by osmosis, while one that is too concentrated would make the cells shrink, so matching water potential correctly is a safety requirement in medical practice, not simply a technical detail.

Key concepts to master

• Fluid mosaic model, A phospholipid bilayer with proteins scattered through it; it is partially (selectively) permeable.
• Diffusion, Net movement of particles from high to low concentration, down a concentration gradient, without energy.
• Osmosis, Net movement of water molecules from a less concentrated (dilute) to a more concentrated solution across a partially permeable membrane.
• Active transport, Movement of substances against the concentration gradient, using energy from respiration and carrier proteins.
• Effects on cells, In hypotonic solution animal cells burst (haemolysis) and plant cells become turgid; in hypertonic solution animal cells shrink (crenation) and plant cells plasmolyse.
• Applications, Osmosis and diffusion explain wilting, food preservation by salting, and root absorption of minerals by active transport.
• Membrane transport proteins, 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.
• Factors affecting rate of movement, 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, 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.
• Osmosis and daily-life technology, 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.
• Movement of substances in living organisms, 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.
• Osmoregulation link, 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.

Quick recall checklist

1. Can you define and explain Fluid mosaic model?
2. Can you define and explain Diffusion?
3. Can you define and explain Osmosis?
4. Can you define and explain Active transport?
5. Can you define and explain Effects on cells?
6. Can you define and explain Applications?
7. Can you define and explain Membrane transport proteins?
8. Can you define and explain Factors affecting rate of movement?
9. Can you define and explain Water potential?
10. Can you define and explain Osmosis and daily-life technology?
11. Can you define and explain Movement of substances in living organisms?
12. Can you define and explain Osmoregulation link?

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