物质进出细胞

Diffusion 扩散 is the net movement 净移动 of particles 粒子 from a region of their higher concentration 浓度 to a region of their lower concentration. We say the particles move down a concentration gradient 浓度梯度.

The particles spread out because of their own random movement 随机运动. The energy 能量 for diffusion comes from the kinetic energy 动能 of the random movement of molecules 分子 and ions 离子 — so diffusion needs no extra energy.

Some substances pass into and out of cells by diffusion through the cell membrane 细胞膜.

Why diffusion matters. Many gases 气体 and dissolved substances (solutes 溶质) move by diffusion in living things. For example, oxygen 氧气 diffuses into cells for respiration, and carbon dioxide 二氧化碳 diffuses out.

The rate of diffusion changes with four factors:

Water as a solvent

Water is a very good solvent 溶剂 — many substances dissolve 溶解 in it to make a solution 溶液. This matters for digestion 消化 (food must dissolve before it can be used), excretion 排泄 (wastes are carried away dissolved in water) and transport 运输 (substances travel around the body dissolved in blood and sap).

What osmosis is

Osmosis 渗透 is a special kind of diffusion: the net movement of water across a partially permeable membrane 半透膜. This kind of membrane has tiny holes that let small water molecules through but hold back larger solute particles. Water moves into and out of cells by osmosis through the cell membrane.

A concentrated solution has a lot of dissolved solute and little water. A dilute solution has little solute and a lot of water.

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

Osmosis and plant cells

When you put plant tissue into different solutions, water moves by osmosis:

• In pure water or a dilute solution, water moves into the cells. The cells swell and become firm, or turgid 膨胀. The water presses outward on the cell wall; this outward push is the turgor pressure 膨压. Turgid cells make a plant stand up straight — this is how plants are supported.
• In a concentrated solution, water moves out of the cells. The cells lose their firmness and become soft, or flaccid 松软.
• (Supplement) If even more water leaves, the cell membrane pulls away from the cell wall. This is plasmolysis 质壁分离.

You can investigate osmosis using dialysis tubing 透析管 (an artificial partially permeable membrane), or using cylinders cut from a potato. Measure their length or mass before and after soaking. Cylinders in pure water or a dilute solution gain length and mass; cylinders in a concentrated solution lose length and mass; in a solution of equal concentration there is no change.

Worked example. A potato cylinder has a mass of 5.0 g before soaking and 5.6 g after soaking in a dilute solution. Find the percentage change in mass.

• change in mass = 5.6 - 5.0 = 0.6 g (a gain).
• percentage change = change ÷ starting mass × 100 = 0.6 ÷ 5.0 × 100 = +12%.

A positive answer means the cylinder gained water, so the solution was more dilute than the cell contents. Always work out the percentage change, not just the change in grams — it lets you compare cylinders that started at different masses fairly.

Active transport 主动运输 is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration — that is, against the concentration gradient. Because this is "uphill", it needs energy from respiration 呼吸作用.

(Supplement) Active transport lets a cell take in useful molecules or ions even when they are already more concentrated inside the cell. For example, root hairs 根毛 take up mineral ions from the soil by active transport. Protein carriers 载体蛋白 in the membrane pick up the molecules or ions and carry them across, using energy.

Common mistakes.

• Saying water moves from a high to a low concentration of solution. Water moves from a dilute solution (more water) to a concentrated one (less water). Say which concentration you mean, or use water potential.
• Saying the solute moves in osmosis. In osmosis only water crosses the membrane; the solute particles are too big to pass.
• Saying a plant cell bursts in pure water. The strong cell wall stops it bursting, so the cell becomes turgid. An animal cell, such as a red blood cell, has no cell wall and can burst.
• Saying diffusion stops when the concentrations are equal. The particles still move both ways; there is just no net movement.
• Saying diffusion and osmosis need energy from respiration. They use the particles' own kinetic energy; only active transport needs energy from respiration.
• Comparing potato cylinders by the change in grams. Use the percentage change, because the cylinders started at different masses.

• Always write net movement: particles move both ways, but the overall flow is down the gradient (for diffusion and osmosis).
• Osmosis moves water only, and only through a partially permeable membrane. Diffusion can move many kinds of particle.
• Only active transport uses energy from respiration, and only it goes against the gradient.
• For the potato or plant experiment: water in → turgid, longer and heavier; water out → flaccid, shorter and lighter; equal concentration → no change.
• A larger surface area, higher temperature, steeper concentration gradient and shorter distance all make diffusion faster.