3.2.3 - transport across cell membranes

The arrangement and any movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model of membrane structure. Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane.

describe the structure of a phospholipid (two main components)

• has a hydrophilic head

• and two hydrophobic tails

how are the phospholipids arranged in the cell-surface membrane

in a phospholipid bilayer

why do the phospholipids arrange themselves like this

• the hydrophilic heads are attracted by the water present on either sides of the membrane —→ so they face outward

• the hydrophobic heads are repelled by the water present on either sides of the membrane —→ so they face inward

diagram of a phospholipid bilayer

function of phospholipids in the cell-surface membrane (i.e. what does it allow to move in / out?)

• allows lipid-soluble substances (e.g. O2, CO2, steroid hormones) to move in + out of the cell

• prevents water soluble substances entering + leaving the cell

why can lipid-soluble substances move through the cell-surface membrane

• they are hydrophobic / non-polar 

• so they dissolve in lipids rather than water

• the core of the membrane = also non-polar (bc of the hydrophobic tails facing inward)

• so molecules can dissolve in the lipid bilayer easily, and can move through directly by simple diffusion (w/o the need for a transport protein)

phospholipids make the membrane f________ and s___- s__________.

flexible and self-sealing

in the phospholipid bilayer, there are proteins interspersed throughout it