Movement into and out of cells
Introduced| English | Meaning |
|---|---|
| concentration gradient/ˌkɒnsənˈtreɪʃn ˈɡreɪdɪənt/ | A change in a substance's concentration across a distance or between regions. |
| diffusion/dɪˈfjuːʒn/ | The net movement of particles from higher to lower concentration due to their random motion. |
| facilitated diffusion/fəˈsɪlɪteɪtɪd dɪˈfjuːʒn/ | Passive movement down a concentration or electrochemical gradient through a membrane channel or carrier protein. |
| osmosis/ɒzˈməʊsɪs/ | The net movement of water molecules through a partially permeable membrane from higher to lower water potential. |
| water potential/ˈwɔːtə pəˈtenʃl/ | A measure of the tendency of water to move, determined by its free energy relative to pure water under specified conditions. |
| active transport/ˈæktɪv ˈtrænspɔːt/ | The movement of substances across a cell membrane against their concentration gradient using energy and carrier proteins. |
| endocytosis/ˌendəʊsɪˈtəʊsɪs/ | Taking material into a cell by engulfing it in a membrane vesicle. |
| exocytosis/eɡˌzɒsɪˈtəʊsɪs/ | Releasing material from a cell by fusing a vesicle with the cell membrane. |
Getting across the membrane
- Substances cross the membrane in six ways.
- Some are passive (no energy needed); some are active (use ATP).
- The key question is always: which way is the concentration gradient 浓度梯度?
Simple diffusion 扩散
- Diffusion is the net movement of particles from high to low concentration, down the concentration gradient.
- In simple diffusion, small or non-polar molecules pass straight through the bilayer — e.g. oxygen and carbon dioxide.
- No energy is used: it is passive.
The main ways substances cross the cell membrane
Movement into & out of cells
net movement: high → low
Drag the concentrations. Particles diffuse down their concentration gradient until both sides are equal.
Which molecules cross the membrane by simple diffusion through the bilayer?
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Only small or non-polar molecules can pass straight through the oily bilayer, down the gradient.
Facilitated diffusion 易化扩散 and osmosis 渗透
- Charged ions and large polar molecules (like glucose) cannot cross the oily bilayer alone.
- Facilitated diffusion: they cross through channel or carrier proteins, still moving down the gradient — passive.
- Osmosis: the diffusion of water across a partially permeable membrane, from higher to lower water potential 水势 — also passive.
Match each way of crossing the membrane to its key feature.
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Diffusion and osmosis are passive (down the gradient); active transport pumps against the gradient using ATP.
Facilitated diffusion uses channel or carrier proteins but needs no energy — it is still passive.
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Ions and large polar molecules cross via proteins, but movement is still down the gradient, so no ATP is used.
Osmosis is the movement of water from:
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Water moves by osmosis down a water-potential gradient (higher → lower) across a partially permeable membrane.
Active transport 主动运输 and bulk transport
- Active transport moves a substance against its gradient (low → high), using carrier proteins and ATP.
- Bulk transport also needs ATP:
- endocytosis 胞吞作用 — the membrane folds in around material and pinches off a vesicle to bring it in.
- exocytosis 胞吐作用 — a vesicle fuses with the membrane to release contents out.
Endocytosis brings material in; exocytosis releases it out
Moving a substance against its gradient using carrier proteins and ATP is called ______ transport.
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Active transport pumps a substance from low to high concentration, so it needs carrier proteins and ATP.
In exocytosis, a cell:
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Exocytosis releases material out (vesicle fuses with the membrane); endocytosis brings material in.
You've got it
- passive (no ATP): simple diffusion (through bilayer), facilitated diffusion (proteins), osmosis (water) — all down the gradient
- active (ATP): active transport (against the gradient, carrier proteins)
- endocytosis brings material in (vesicle forms); exocytosis sends it out (vesicle fuses)
- osmosis = water moving from higher to lower water potential