1.3 membrane structure

• invented in 1930 • proposed that the membrane structure was like a sandwich two layers of protein sandwiching a bilayer of phospholipids

• chemical analysis showed that the membrane was composed of phospholipids and protein • electron micrograph images showed the membranes as two dark lines separated by a lighter area • because membranes are semipermeable the proteins could be acting as a barrier to some substances

• electron microscopes showed globular proteins were in the centre of the phospholipid layer • analysis of the protein showed that some were hydrophobic • some proteins extended all the way through and/or within the phospholipid layer due to the hydrophobic properties • an experiment was done were fluorescent markers were attached to antibodies which binded to membrane proteins of mouse cell (red) and human cells (green), which were then fused together. after 40 minutes both green and red markers were mixed through the membrane of the fused cell • this proved that membrane proteins are free to move within the membrane rather being fixed in a peripheral layer

-created by Singer Nicolson proposes that: -all cells have a cell membrane -cell membrane is made of a double layer of phospholipids with proteins and cholesterol embedded in it. -the cell membrane is semipermeable

• allows some molecules through but not all molecules. this is useful as the cell membrane controls what goes in and out of the cells

phospholipid molecules are composed of a polar (charged) phospate head which is hydrophilic and two non-polar fatty tails which are hydrophobic. because the head is hydrophilic (attracted to water) and the tails hydrophobic (repelled by water) the head is positioned on the outside of the membrane and the tails on the inside. the head behaves as a solid and the tails as a liquid so the over overall effect is that the cell membrane is ‘fluid’ the attraction of the tails to each other cause the membrane to be stable and flexible.

part of the molecule (the head) is attracted to water and part (the tails) are repelled by water.

• a component of animal cells (about 20%) • embedded in the phospholipids • have hydrophilic and hydrophobic ends • reduce fluidity and permeability of the membrane to ions e.g. sodium and Hydrogen ions

-embedded in the phospholipids

• transmembrane • spans the entire way through the cell membrane • embedded in the bilayer

-they have individual effects that contribute to the semi-permeable nature of the membrane

• attached to the outer surface of the cell and are involved in cell to cell communications

-act as pumps- transport

• sugar units attached to the outer surface of the membrane which are used for cell recognition by the immune system and working as hormone receptors

T.R.A.C.I.E • Transport • Receptors • Anchorage • Cell recognition • Intercellular joinings • Enzymatic activity

• channel proteins which use facilitated diffusion - protein pumps which use active transport

• used for hormones e. g. insulin, glucagon

cytoskeleton and cell wall attachments

immune system proteins

cells attached to eachother e.g. plasmodesmata

the movement of ions back and forth across the membrane

structure position in the membrane function

part of them is hydrophobic and part is. hydrophilic • hydrophobic fatty acid tails • hydrophilic head