The Transport of Substances across a Cell Membrane

Passive Transport – process that enables substances to move in and out of cells without an input of energy from the cell.

concentration gradient – a difference in concentration between one side of a membrane and the other.

There are three forms of passive transport:
1. Diffusion
2. Osmosis
3. Facilitated Diffusion

1. Diffusion
Molecules and ions dissolved in the cytoplasm and extracellular fluid are in constant random motion.

Diffusion – is the net movement of ions or molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached.

Factors that affect diffusion: relative concentrations both inside and outside of the cell how readily a molecule or ion can cross the membrane

Factors that affect the rate of diffusion: molecule size – the rate of diffusion decreases with increasing molecular size molecule polarity – polar molecules diffuse slower than non-polar molecules of the same size molecule or ion charge – charged molecules or ions cannot diffuse across a cell membrane
Temperature and pressure – the rate of diffusion increases with increased temperature and pressure

2. Osmosis
The regulation of water entry is of crucial importance to a cell. If too much water enters a cell, it swells. If too much water leaves a cell, it shrinks. This can affect the ability of a cell to function.

Osmosis – the movement of water from an area of higher concentration to an area of lower concentration, across a semi-permeable membrane until equilibrium is reached.

In osmosis, water molecules move because the membrane is impermeable to the solute, and the solute concentrations may differ on either side of the membrane.

The concentration of all solutes in a solution determines its osmotic concentration.
If two solutions have unequal osmotic concentrations, the solution with the higher concentration is hypertonic
The solution with the lower concentration is hypotonic
When two solutions have the same osmotic concentration, they are isotonic (iso = “equal”)

3. Facilitated Diffusion
Facilitated diffusion – the process of diffusion that is mediated by a membrane protein
i) channel proteins
Have a hydrophilic interior that provides an aqueous channel through which polar molecules can pass when the channel is open
Has