Movement into and out of cells · ⁨物质进出细胞⁩

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

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.

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

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