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
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
