Movement of Substances across the Cell Membrane

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1Summary 重點摘要

Substances cross the membrane by diffusion, osmosis and active transport: the first two are passive and follow the concentration or water-potential gradient, while active transport goes against it and costs energy; water potential explains plasmolysis in plant cells and haemolysis in red blood cells.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: The membrane — a selective checkpointThe phospholipid bilayer of the cell membrane lets small molecules such as oxygen and carbon dioxide pass freely, and water molecules are small enough to slip through; but glucose, amino acids and ions are controlled by membrane proteins and cannot come and go at will. This selective permeability lets the cell keep its internal composition different from the outside world. For example, the potassium ion concentration inside a red blood cell is far higher than in plasma, a difference maintained precisely by the precise control of membrane proteins.
2. Step 2: DiffusionDiffusion is the net movement of molecules or ions from a region of higher concentration to one of lower concentration — down the gradient, a passive process that costs the cell no energy. The higher the temperature, the steeper the gradient and the shorter the distance, the faster it goes. For example, in the lungs oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli, without any energy being spent.
3. Step 3: Osmosis and water potentialOsmosis is a special kind of diffusion: water molecules cross a semi-permeable membrane from the side of higher water potential to the side of lower water potential. Water potential can be thought of as how free the water is: pure water has the highest potential, and the more concentrated a solution, the more solute it holds and the lower its potential; the membrane lets water through but holds back the solute. For example, cucumbers sprinkled with salt go soft because the salt solution has a low water potential and water flows out of the cucumber cells by osmosis.
4. Step 4: Plant cells in a concentrated solution — plasmolysisPlace a plant cell in a concentrated sucrose solution: the external solution has a lower water potential than the cell sap, so water flows out through the membrane. The vacuole shrinks, the cytoplasm and membrane pull inwards, and they peel away from the tough cell wall — this is plasmolysis. Put the cell back into water and water flows in again, reversing the plasmolysis and proving the cell was still alive. For example, over-concentrated fertiliser burns crops because the soil solution then has a lower water potential than the root cells, drawing water out of the roots.
5. Step 5: Plant cells in pure water — turgorPure water has the highest water potential. Place a plant cell in fresh water and water keeps flowing into the vacuole; the contents push outward against the membrane, which presses against the wall, and the cell becomes firm and swollen — turgid. The tough wall stops the cell from swelling until it bursts, the key difference between plant and animal cells. For example, greengrocers sprinkle water on vegetables to keep the cells turgid and the produce crisp; without turgor, plants wilt.
6. Step 6: Red blood cells and haemolysisRed blood cells have no cell wall, only a single membrane. In distilled water the outside has a much higher water potential than the cell interior, so water floods in; the cell swells until the membrane bursts, haemoglobin spills out and the solution turns a clear red — haemolysis. In concentrated salt solution, red blood cells instead lose water and shrivel. For example, intravenous drips must use solutions with the same tonicity as blood; distilled water by mistake would cause massive haemolysis with serious consequences.
7. Step 7: Active transportActive transport uses carrier proteins in the membrane to move substances from the low-concentration side to the high-concentration side, against the gradient, and every step consumes energy from respiration; when energy supply stops, active transport stops too. For example, root hair cells take up nitrate ions from the soil and small-intestine cells absorb glucose into the blood even when the outside concentration is lower — that is active transport. Cyanide stops cells from producing energy, and active transport ceases at once.
8. Step 8: PhagocytosisFor large particles such as bacteria and cell debris, cells use another method: the membrane puts out pseudopodia that surround the target, then folds inward and fuses, wrapping the particle into a vesicle inside the cell. This is phagocytosis, and it consumes energy. For example, white blood cells engulf bacteria at a wound — phagocytosis in action; the single-celled amoeba feeds the same way, showing how ancient and widespread the mechanism is.

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8Recap Recap

• Three main ways substances cross the membrane: diffusion (down the concentration gradient), osmosis (water through a semi-permeable membrane, high to low water potential) and active transport (against the gradient, energy-consuming).
• The more concentrated a solution, the lower its water potential; in osmosis water always flows from higher to lower water potential.
• Plant cells undergo plasmolysis in a hypertonic solution and become turgid in pure water; the cell wall is the key protection.
• Red blood cells burst (haemolysis) in distilled water because they have no cell wall.
• Phagocytosis lets white blood cells engulf bacteria; it relies on membrane fluidity and consumes energy.