Summary Cell Membranes and Transport

Cell Membranes & Transport
 The cell membrane is the boundary between the cell cytoplasm and the environment

Function of cells within membranes
 Controls the entry and exit of molecules in organelles such as mitochondria and
chloroplasts
 Isolates organelles so that specific metabolic reactions can occur within them
 Provide an internal support system e.g. ER
 Isolate enzymes that might damage the cell e.g. lysosomes
 Provide surfaces o which processes/reactions can occur e.g. protein synthesis using
ribosomes and ER
Structure of cell surface membrane
 Phospholipid in membrane form a bilayer which has:
- One layer of phospholipids with its hydrophilic heads pointing towards the
water in the cytoplasm and the other layer pointing outwards the watery
environment which surrounds all cells
- The hydrophilic tails of both layers point inwards to form a hydrophobic core.
The more unsaturated these tails are the more fluid the membrane  lipid
soluble material can move through the membrane by the phospholipid
bilayer
 Proteins of cell surface membranes
- Arranged more randomly
- Associated with phospholipid bilayer in 2 ways:
1. Extrinsic (peripheral) proteins
 Occur on both surfaces of the bilayer but never extend
completely across it
 They may act to give mechanical support to the membrane or
may have other functions such as cell signalling
2. Intrinsic (integral) proteins
 Embedded in the bilayer from one side to the other
 Some are channel proteins to form open, water-filled tubes to
allow water soluble ions to diffuse across the membrane
e.g. aquaporins specifically allow water to pass through
 Some are carrier proteins that bind molecules such as glucose
and amino acids then change their shape in order to move
these molecules
 Cholesterol molecules of cell surface membranes
- Occur within phospholipid bilayer
- Have hydrophilic heads and hydrophobic tails
- Add strength and stability to animal cells
- Hydrophobic tails have the important role of preventing the passage of
dissolved ions and polar molecules across the bilayer
- Regulates fluidity of the cell  prevents close packing when temperature
decreases and prevents excess movement when temperature increases
Functions of membrane proteins

, a) Transport

A protein that spans the membrane may
provide a hydrophilic channel across the
membrane that is selective for a particular
solute



b) Enzyme activity
A protein built into the membrane may be an enzyme with
its active sites exposed to substances in the adjacent
solution. In some cases, several enzymes in a membrane are
organised as a team that carries out sequential steps in a
metabolic pathway
c) Signal transduction



A membrane protein (receptor) may have a binding site with a
specific shape that fits the enzyme of the chemical messenger,
such as a hormone. The external messenger (signalling
molecule) may cause a shape change in the protein that relays
the message to the inside of the cell, usually by binding to a
cytoplasmic protein.


d) Cell-cell recognition

Some glycoproteins serve as
identification tags that are specifically
recognised by membrane proteins of
other cells



Glycolipids
 Made up of a carbohydrate covalently bonded with a phospholipid
 The carbohydrate protein extends from the phospholipid bilayer into the watery
environment outside the cell where it acts as a recognition site for specific chemicals
e.g. the human ABO blood system operates as a result of glycolipids on the cell
surface membrane
 Helps cells to attach to one another to form tissues