Movement into and out of cells
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?
Simple diffusion
- 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.
Which molecules cross the membrane by simple diffusion through the bilayer?
Only small or non-polar molecules can pass straight through the oily bilayer, down the gradient.
Facilitated diffusion and osmosis
- 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 — also passive.
Facilitated diffusion differs from simple diffusion because it:
Ions and large polar molecules cross via proteins, but no energy is used and movement is still down the gradient.
Osmosis is the movement of water from:
Water moves by osmosis down a water-potential gradient (higher → lower) across a partially permeable membrane.
Active transport and bulk transport
- Active transport moves a substance against its gradient (low → high), using carrier proteins and ATP.
- Bulk transport also needs ATP:
- endocytosis — the membrane folds in around material and pinches off a vesicle to bring it in.
- exocytosis — a vesicle fuses with the membrane to release contents out.
Active transport is different from diffusion because it:
Active transport pumps a substance from low to high concentration, requiring carrier proteins and ATP.
In exocytosis, a cell:
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