Cell membranes and transport

Every cell is wrapped in a cell surface membrane 细胞膜. We describe its structure with the fluid mosaic model 流动镶嵌模型.

Red blood cells under a scanning electron microscope — each is wrapped in a cell surface membrane

The phospholipid bilayer

The membrane is built mainly from phospholipids 磷脂. Each phospholipid has a hydrophilic 亲水 ("water-loving") head and two hydrophobic 疏水 ("water-fearing") tails. There is water on both sides of the membrane, so the phospholipids line up in two layers — a bilayer 双层 — with the heads facing the water outside and inside, and the tails hidden in the middle, away from water. This arrangement forms by itself because of those hydrophilic and hydrophobic interactions.

The model is called "fluid" because the phospholipids are not fixed: they slide past each other, so the membrane can move and bend. It is called a "mosaic" because many proteins 蛋白质 are dotted through it, like tiles in a picture.

What floats in the membrane

The fluid mosaic model: proteins, cholesterol and carbohydrate chains sit in a fluid phospholipid bilayer

So the membrane molecules together give the membrane its stability 稳定性, its fluidity, its permeability 通透性 (control over what gets through), its transport jobs, its signalling jobs, and its cell recognition.

The membrane is partially permeable 半透膜: it lets some substances through easily but blocks others.

Cells talk to each other by cell signalling 细胞信号传递. The main stages are:

1. a cell secretes a signal chemical called a ligand 配体 (for example a hormone 激素).
2. the ligand is carried (often in the blood) to a target cell 靶细胞.
3. the ligand binds to a specific receptor 受体 on the target cell's surface membrane. The shape of the receptor matches that ligand only. Binding then triggers a particular response inside the target cell.

The ligand fits one receptor shape only, so only cells with that receptor respond to the signal

There are six processes. Some are passive 被动 (they need no energy 能量), and some are active (they use energy from ATP).

Simple diffusion

Diffusion 扩散 is the net movement of particles from where they are at a high concentration 浓度 to where they are at a low concentration, until they are spread evenly. Simple diffusion 简单扩散 is when particles pass straight through the bilayer, down the concentration gradient 浓度梯度. Only small or non-polar molecules can do this — such as oxygen 氧气 and carbon dioxide 二氧化碳. It is passive.

Facilitated diffusion

Charged ions 离子 and large polar molecules (such as glucose 葡萄糖) cannot cross the oily bilayer by themselves. In facilitated diffusion 易化扩散 they cross through channel proteins or carrier proteins, still moving down the concentration gradient. It is also passive.

Osmosis

Osmosis 渗透 is the diffusion of water across a partially permeable membrane, from a higher water potential 水势 to a lower water potential. It is passive.

A potato osmometer: the sugar solution rises up the tube as water enters the potato by osmosis

Water crosses to the lower water potential; the solute is too big to cross the partially permeable membrane

Active transport

Active transport 主动运输 moves a substance against its concentration gradient — from low to high concentration. This needs carrier proteins and energy from ATP.

Diffusion and facilitated diffusion are passive (down the gradient); active transport goes against it and needs ATP

Endocytosis and exocytosis

These move large amounts of material in bulk, using ATP.

• in endocytosis 胞吞作用, the membrane folds inwards around material and pinches off a vesicle 囊泡 to bring it into the cell.
• in exocytosis 胞吐作用, a vesicle fuses with the membrane and releases its contents outside the cell.

Endocytosis brings material in by forming a vesicle; exocytosis fuses a vesicle to release its contents

A cell takes in and removes substances across its surface. As an object gets bigger, its volume 体积 grows faster than its surface area 表面积. So the surface area to volume ratio gets smaller as size increases.

For a cube of side $L$:

A large $L$ gives a small ratio. This is why small cells (and thin, flat shapes) exchange materials quickly, while large cells cannot rely on diffusion alone.

As a cube (or cell) grows, its surface area : volume ratio gets smaller

Worked example. Compare the surface area : volume ratio of a cube-shaped cell of side $4$ with one of side $10$.

For side $4$: surface area $= 6 \times 4^2 = 96$ and volume $= 4^3 = 64$, so the ratio is $96 : 64 = 1.5 : 1$. For side $10$: surface area $= 6 \times 10^2 = 600$ and volume $= 10^3 = 1000$, so the ratio is $600 : 1000 = 0.6 : 1$. The larger cell has the smaller ratio, so it exchanges materials across its surface more slowly for its size — which is why large organisms need specialised exchange surfaces such as lungs and gills.

You can show this with agar 琼脂 blocks of different sizes soaked in dye or acid: the smallest block, with the largest surface area to volume ratio, changes colour all the way through fastest. Diffusion across non-living materials can also be studied with dialysis tubing 透析袋 (Visking tubing).

Water potential measures how likely water is to leave a solution. Pure water has the highest water potential. Adding a solute 溶质 (a dissolved substance) lowers it. Water always moves by osmosis from a higher to a lower water potential.

To estimate the water potential of plant tissue, you place pieces in sucrose solutions of different water potentials. The solution that causes no change in mass or length has about the same water potential as the tissue.

Effect on plant cells

• in a solution of higher water potential (for example distilled water 蒸馏水), water enters the cell. The cell swells and becomes turgid 膨胀, but the strong cell wall stops it bursting.
• in a solution of lower water potential, water leaves. The cell contents shrink and the membrane pulls away from the cell wall — this is plasmolysis 质壁分离.

Effect on animal cells

Animal cells have no cell wall to protect them.

• in a solution of higher water potential, water enters and the cell may burst. In a red blood cell this bursting is called haemolysis 溶血.
• in a solution of lower water potential, water leaves and the cell shrinks.

A plant cell becomes turgid or plasmolysed; an animal cell may burst (haemolysis) or shrink

• Describe the membrane as a fluid mosaic: a phospholipid bilayer with proteins, cholesterol and glycoproteins.
• Sort each process: diffusion, facilitated diffusion and osmosis are passive (down a gradient); active transport and endo/exocytosis need ATP and can go against it.
• For osmosis always use water potential ($\Psi$): water moves from high (less negative) to low (more negative) $\Psi$; pure water is $0$, the highest.
• State the outcome by cell type: plant cell turgid/plasmolysed; animal cell lyses/crenates — link it to the water-potential gradient.