🧪 General Principles🟢 BasicCell Membrane

Membrane Transport

The ways substances cross the cell membrane, either downhill without energy or uphill using energy, which lets a cell control what enters and leaves.

Key points

Simple diffusion lets small or fat-soluble molecules, such as oxygen and carbon dioxide, cross the lipid membrane directly, moving down their concentration gradient without energy.
Facilitated diffusion uses carrier or channel proteins to move substances such as glucose and ions down their gradient; it needs no energy, and the carrier-mediated type can be saturated.
Primary active transport uses energy from ATP to pump substances uphill, against their gradient, and the sodium-potassium pump is the classic example.
The sodium-potassium pump moves three sodium ions out and two potassium ions in for each ATP used, setting up the gradients the cell uses for many other tasks.
Secondary active transport uses the gradient made by a pump to drag another substance uphill, either in the same direction (symport) or the opposite direction (antiport).
Water crosses by osmosis, moving towards the solution with the higher effective osmotic concentration, often through water channels called aquaporins.

Memory trick

Downhill is free (diffusion and facilitated diffusion), uphill costs ATP (primary active); borrow a pump's gradient and you get a second-hand, secondary active ride.

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🧪 General Principles🟢 BasicCell Membrane

Membrane Transport

The ways substances cross the cell membrane, either downhill without energy or uphill using energy, which lets a cell control what enters and leaves.

Key points

Simple diffusion lets small or fat-soluble molecules, such as oxygen and carbon dioxide, cross the lipid membrane directly, moving down their concentration gradient without energy.
Facilitated diffusion uses carrier or channel proteins to move substances such as glucose and ions down their gradient; it needs no energy, and the carrier-mediated type can be saturated.
Primary active transport uses energy from ATP to pump substances uphill, against their gradient, and the sodium-potassium pump is the classic example.
The sodium-potassium pump moves three sodium ions out and two potassium ions in for each ATP used, setting up the gradients the cell uses for many other tasks.
Secondary active transport uses the gradient made by a pump to drag another substance uphill, either in the same direction (symport) or the opposite direction (antiport).
Water crosses by osmosis, moving towards the solution with the higher effective osmotic concentration, often through water channels called aquaporins.

Memory trick

Downhill is free (diffusion and facilitated diffusion), uphill costs ATP (primary active); borrow a pump's gradient and you get a second-hand, secondary active ride.

All of Physiology More General Principles