3. Transport across cell membranes

why do cell membranes have a ‘fluid mosaic’ structure

• the bilayer is ‘fluid’ as phospholipids are constantly moving

• cholesterol molecules are present in the bilayer

• proteins are scattered through the bilayer, like tiles in a mosaic

  • these include channel proteins and carrier proteins

  • receptor proteins on the cell-surface membrane that allow the cell to detect chemicals released from other cells

  • some proteins and lipids have a polysaccharide chain attached - glycoproteins/lipids

function of cholesterol in cell membranes

• it gives the membrane stability

• it’s always present and fits between the phospholipids, binding to the hydrophobic tails which causes them to pack more closely together

  • this restricts movement of phospholipids which makes the membrane less fluid and more rigid

• cholesterol helps to maintain the shape of animal cells

use of carrier proteins

• moves large molecules across membranes, down their concentration gradient

• different carrier proteins facilitate the diffusion of different molecules

  • a large molecule attaches to a carrier protein in the membrane

  • the protein changes shape

  • this releases the molecule on the opposite side of the membrane

use of channel proteins

• channel proteins form pores in the membrane for charged particles to diffuse through down their concentration gradient

• different channel proteins facilitate the diffusion of different charged particles

what are the two main differences between active transport and facilitated diffusion

1) active transport usually moves solutes from a low to high concentration

• in facilitated diffusion, they always move from a high to low concentration

2) active transport requires energy - facilitated diffusion doesn’t

• ATP undergoes hydrolysis which releases energy so that the solutes can be transported

what are co-transporters

• they’re a type of carrier protein that bind two molecules at a time

  • the concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient

what factors affect the rate of active transport

• the speed of individual carrier proteins - the faster they work, the faster the rate of active transport

• the number of carrier proteins present - the more proteins there are, the faster the rate of active transport

• the rate of respiration in the cell and the availability of ATP - if respiration is inhibited, active transport can’t take place

co-transport in the Mammalian Ileum

glucose enters the ileum with sodium ions

1) sodium ions are actively transported out of the ileum epithelial cells, into the blood, by the sodium-potassium pump. This creates a concentration gradient - there’s now a higher concentration of sodium ions in the lumen of the ileum than inside the cell

2) this causes sodium ions to diffuse from the lumen of the ileum into the epithelial cells, down their concentration gradient. They do this via the sodium-glucose co-transporter proteins

3) the co-transporter carries glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases.

4) glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel by facilitated diffusion.