TRANSPORT ACROSS CELL MEMBRANES
4.1-4.5
4.1. Structure of the cell membrane
All membranes around and within all cells (including those around and within
cell organelles) have the same basic structure and are called plasma
membranes.
The cell-surface membrane is the name specifically given to the plasma
membrane that surrounds cells and forms the boundary between the cell
cytoplasm and the environment.
Functions of the cell membrane:
● keeping all cellular components inside the cell.
● allowing selected molecules to move in and out of the cell.
● allowing a cell to change shape.
● isolating organelles from the rest of the cytoplasm, allowing cellular
processes to occur separately.
● a site for biochemical reactions.
What is the cell-surface membrane?
○ This is the name given specifically to the plasma membrane that
surrounds cells.
○ It forms a boundary between the cell cytoplasm and the surrounding
environment.
○ It allows different conditions to be established inside and outside the
cell.
, ○ It does this by controlling the movement of substances in and out of the
cell.
○ It is made up of different molecules that form its structure.
Phospholipids
What are phospholipids?
● One of the main components of membranes are phospholipids, a type of
lipid made from two fatty acid chain ‘tails’ attached to a phosphate
group ‘head’
● The phosphate group head is polar and hydrophilic (‘water-loving’),
while the fatty acid chains of the tail are non-polar and hydrophobic
(‘water-hating’).
hydrophobic tails
hydrophilic head
● The shape of the structures that phospholipids form is due to their polar
nature, and the way they interact with water.
Phospholipids form a BILAYER. They are important components of cell-surface
membranes for the following reasons:
1. The hydrophilic heads of both phospholipid layers point to the OUTSIDE
of the cell-surface membrane, ATTRACTED by the water on both sides.
, 2. The hydrophobic tails of both phospholipid layers point into the CENTRE
of the cell-surface membrane, REPELLED by the water on both sides.
Lipid-soluble material moves through the membrane via the phospholipid
portion. The FUNCTION of phospholipids in the membrane are to:
● Allow lipid-soluble substances to enter and exit the cell.
● Prevent water-soluble substances entering and leaving the cell.
● Make the membrane flexible and self-sealing.
Proteins
Proteins typically make up 45% by mass of a cell membrane, but this can vary
from 25% to 75% depending on the cell type.
Proteins are interspersed throughout the cell-surface membrane. They are
embedded in the phospholipid bilayer in two ways:
1. Extrinsic proteins - some proteins occur in the SURFACE of the bilayer
and never extend completely across it. They act either to give
mechanical support to the membrane or, in conjunction with glycolipids,
as cell receptors for molecules such as hormones.
2. Intrinsic proteins - other proteins completely span the phospholipid
bilayer from one side to the other. Some are protein channels, which
form water-filled tubes to allow water-soluble ions to diffuse across the
membrane. Others are carrier proteins that bind to ions or molecules like
glucose and amino acids, then change SHAPE in order to move these
molecules around the membrane.
★ Many proteins are glycoproteins –
proteins with attached carbohydrate chains
4.1-4.5
4.1. Structure of the cell membrane
All membranes around and within all cells (including those around and within
cell organelles) have the same basic structure and are called plasma
membranes.
The cell-surface membrane is the name specifically given to the plasma
membrane that surrounds cells and forms the boundary between the cell
cytoplasm and the environment.
Functions of the cell membrane:
● keeping all cellular components inside the cell.
● allowing selected molecules to move in and out of the cell.
● allowing a cell to change shape.
● isolating organelles from the rest of the cytoplasm, allowing cellular
processes to occur separately.
● a site for biochemical reactions.
What is the cell-surface membrane?
○ This is the name given specifically to the plasma membrane that
surrounds cells.
○ It forms a boundary between the cell cytoplasm and the surrounding
environment.
○ It allows different conditions to be established inside and outside the
cell.
, ○ It does this by controlling the movement of substances in and out of the
cell.
○ It is made up of different molecules that form its structure.
Phospholipids
What are phospholipids?
● One of the main components of membranes are phospholipids, a type of
lipid made from two fatty acid chain ‘tails’ attached to a phosphate
group ‘head’
● The phosphate group head is polar and hydrophilic (‘water-loving’),
while the fatty acid chains of the tail are non-polar and hydrophobic
(‘water-hating’).
hydrophobic tails
hydrophilic head
● The shape of the structures that phospholipids form is due to their polar
nature, and the way they interact with water.
Phospholipids form a BILAYER. They are important components of cell-surface
membranes for the following reasons:
1. The hydrophilic heads of both phospholipid layers point to the OUTSIDE
of the cell-surface membrane, ATTRACTED by the water on both sides.
, 2. The hydrophobic tails of both phospholipid layers point into the CENTRE
of the cell-surface membrane, REPELLED by the water on both sides.
Lipid-soluble material moves through the membrane via the phospholipid
portion. The FUNCTION of phospholipids in the membrane are to:
● Allow lipid-soluble substances to enter and exit the cell.
● Prevent water-soluble substances entering and leaving the cell.
● Make the membrane flexible and self-sealing.
Proteins
Proteins typically make up 45% by mass of a cell membrane, but this can vary
from 25% to 75% depending on the cell type.
Proteins are interspersed throughout the cell-surface membrane. They are
embedded in the phospholipid bilayer in two ways:
1. Extrinsic proteins - some proteins occur in the SURFACE of the bilayer
and never extend completely across it. They act either to give
mechanical support to the membrane or, in conjunction with glycolipids,
as cell receptors for molecules such as hormones.
2. Intrinsic proteins - other proteins completely span the phospholipid
bilayer from one side to the other. Some are protein channels, which
form water-filled tubes to allow water-soluble ions to diffuse across the
membrane. Others are carrier proteins that bind to ions or molecules like
glucose and amino acids, then change SHAPE in order to move these
molecules around the membrane.
★ Many proteins are glycoproteins –
proteins with attached carbohydrate chains
