Transcript Section 5 – Membranes and cytoskeleton

Unit 1
Cell and Molecular
Bioligy
Section 5
Membranes and cytoskeleton
Cytoskeleton
The cytoskeleton is an intracellular framework made up
of three types of protein filament:
1, microtubules – These are hollow tubes made up of
the protein tubulin and are involved in the movement
of components within the cell
2. Intermediate filaments – These are found under the
cell and nuclear membranes and add mechanical
strength.
3. Actin filaments – These are thin and flexible and add
contractile forcxe during cell division.
Functions of the cytoskeleton

There are many functions which include:





Intracellular movement e.g. movement of
chromosomes
Allow cells to change shape
Aid cell crawling
Strengthen and support cells
Link cell together into tissues
Microtubules

Microtubules are used to help cells divide and to form cell
projections such as cilia or flagella.

These grow out of a structure called a centrosome – The
centrosome is typically present on one side of the nucleus
when the cell is not in mitosis and organises the array of
microtubules that radiate from it through the cytoplasm

Microtubules are built from tubulin dimers which are linked
together to form 13 distinct protofilaments into a hollow tube

Microtubules can grow or shrink rapidly from the centrosome
when required ( e.g. during mitosis).

This property of microtubules is known as dynamic instability.
Intermediate Filaments

strengthen the cell mechanically and help prevent
cells from rupturing.

are important in linking cells within a tissue together
by spanning the cytoplasm from one cell to another.

help support the structure of the nucleus by forming
a meshwork called the nuclear lamina just beneath
the inner nuclear membrane
Actin Filaments

These are sometimes called microfilaments
and are most concentrated in the cortex on
the cytosolic (inner) surface of the cell
membrane

Actin filaments generate contractive forces
which allow a cell to divide in two
The plasma membrane
The general structure of cell membranes is described by
the ‘fluid mosaic’ model where a variety of proteins are
closely associated with a lipid bilayer.
Integral transmembrane protein
Glycolipids Help to coat
the cell
membrane
with a layer
of
carbohydrate
Cholesterol
-stiffens
membrane
Integral lipid-linked protein
Phospholipid bilayer
Integral transmembrane protein
Peripheral protein attached protein
Structural features of the
plasma membrane

The lipid bilayer provides the basic structure and serves
as a permeability barrier

The proteins mediate most of the other functions of the
membrane and give different membranes their individual
characteristics

The lipids in the membrane all have hydrophilic head
groups and hydrophobic tails. Lipids move rapidly within
a layer but only rarely flip from one layer to the other
within the bilayer. It is this rapid movement of lipids that
gives the membrane its fluidity
There are a number of lipid types within the
Plasma Membrane:

Phospholipids – these are the most abundant
lipids within membranes in which the
hydrophilic head is linked to the rest of the
molecule through a phosphate group

Sterols – the most important sterol in animal
cell membranes is cholesterol. This lipid
stiffens cell membranes

Glycolipids – these form part of the
carbohydrate coating of cell membranes
Membrane Lipids
The membrane lipids can be described as
amphipathic, having both hydrophilic and
hydrophobic properties.

The hydrophilic head always aligns itself next to a
solution – either the cystolic or non-cystolic surface

The hydrophobic tails force water molecules to
reorganise themselves into a cagelike structure
around the lipid molecules. This cagelike structure
creates a hydrophobic force which holds the bilayer
together
Membrane proteins
There are a number of ways in which membrane proteins
associate with the lipid bilayer:1.
Integral membrane proteins – these are directly
attached to the membrane and can only be
separated from the membrane by disrupting the lipid
bilayer with detergents.These membrane proteins
may be :•
Transmembrane – extend through the bilayer with
part of their mass on either side
•
Lipid-linked. These proteins are covalently
attached to lipid groups in the bilayer but are
located entirely outside the bilayer
2. Peripheral membrane proteins – These
are bound indirectly to one or other face
of the membrane only by weak
noncovalent interactions and are easily
released from the membrane by relatively
gentle extraction procedures which leave
the lipid bilayer intact
Cell membranes have a number of different functions
SMALL
HYDROPHOBIC
MOLECULES
Oxygen
Carbon Dioxide
Nitrogen
Benzene
SMALL
UNCHARGED
POLAR
MOLECULES
Water
Glycerol
Ethanol
Fairly
Rapid Diffusion
LARGER
UNCHARGED
POLAR
MOLECULES
Amino acids
Glucose
Nucleotides
Very slow
Diffusion
IONS
Ca+
Na+
K+
Cl
Rapid Diffusion
Lipid bi-layer
1. The plasma membrane is selectively permeable and prevents
the unrestricted movement of molecules into and out of the cell
In general the smaller the molecule and the more soluble it is in
oil (i.e. hydrophobic/non-polar) the more rapid diffusion will be
across the PM
2. Compartmentalisation — membranes are used extensively in
eukaryotic cells to form structures such as the nuclear
envelope, endoplasmic reticulum, Golgi apparatus,
mitochondria and chloroplasts.
3. Localising reactions in the cell — membranes provide the
structural framework for organising many of the reactions in the
cell as a consequence of compartmentalisation.. Many critical
energy-transducing mechanisms such as the light reactions of
photosynthesis and the respiratory electron transport chain are
closely associated with membranes.
4. Transport of solutes — in addition to their selective
permeability, membranes have special transport proteins that
are used to transport solutes specifically across the membrane,
often against a concentration gradient. This will be covered
more fully in the next lesson
5. Signal transduction — receptors on the membrane
surface recognise and respond to different
stimulating molecules, enabling specific responses
to be generated within the cell. This will be covered
more fully in the next lesson
6. Cell-cell recognition — the external surface of the
membrane is important in that it represents the cell’s
biochemical personality’. In multicellular organisms
this feature enables cells to recognise each other as
similar or different~ which is necessary for the
correct association of cells during development. This
will be covered more fully in the next lesson
Cell membrane proteins
There are four main types of membrane
proteins:1. Transporters – these help the passage of
ions, sugars, amino acids, nucleotides and
many other metabolites which cross the lipid
bilayer too slowly by simple diffusion.
2. Enzymes – These catalyse specific
reactions
3. Linkers – these strengthen and support the
cell membrane as part of the membrane
linked cytoskeleton, give each cell its
distinctive shape and are important in
intercellular junctions
4. Receptors – These detect chemical signals
in the cells environment and relay them to
the cell’s interior.
Transporters
Two main classes of membrane transport
proteins can be distinguished

Channel proteins
These form tiny hydrophilic pores in the
membrane, through which solutes can pass
by diffusion. Most channel proteins let
through inorganic ions only and are often
called ion channels.

Carrier proteins
These bind a solute on one side of the
membrane and deliver it to the other through a
change in the configuration (shape) of the
carrier protein. An important example of this
type of protein is the sodium-potassium pump
This pump exchanges three sodium ions from
the inside of the cell for two potassium ions from
the outside of the cell.
This process uses energy in the form of ATP,
enzymes and involves the protein changing
shape
There are six main stages to the sodium-potassium pump – see
diagram
Both channel proteins and carrier proteins allow movement of
molecules in the same direction as the concentration gradient.
This is called passive or facilitated diffusion. Only carrier proteins
can carry out active transport where molecules are moved
against the concentration gradient. Active transport requires
energy
Enzymes

There are a large number of enzyme proteins
associated with the cell membrane. These all speed
up reactions.

Two important groups of enzymes are the kinases
and phosphorylases both of which help in the
sodium – potassium pump

Kinases catalyse the addition of phosphate to
molecules

Phosphorylases catalyse the removal of phosphate
from molecules
Linkers
These are intrinsic transmembrane proteins which have a variety of
functions
a) Strengthening and supporting cell membrane.
These determine the shape of the cell and the mechanical
properties of the PM in particular which is covered in the inner
(cytosolic) surface by a meshwork of fibrous proteins called the
cell cortex
b) Joining cells together into strong layers or tissues. This is known
as cell adhesion.
c) Restricting the movement of membrane proteins by forming
intercellular junctions e.g. gut cells are linked together in a way
that prevents molecules moving between the gaps
Receptors
Glycoproteins (proteins with short chains of sugar
attached to them) are only found on the non
cytosolic side of the membrane where they act with
other molecules to form a sugar coating. This
coating has many uses e.g.
• It protects the cell surface from mechanical damage
• It prevents cells from sticking together or to
surrounding tissues
• Cell-cell recognition
• ‘docking’ sites for extracellular signalling molecules
such as hormones from the cells surroundings.