Unit 1Cell 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.

Phospholipid bilayer

Glycolipids -Help to coat the cell membrane with a layer of carbohydrate

Cholesterol-stiffensmembrane

Integral transmembrane protein

Integral transmembrane protein

Integral lipid-linked 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

OxygenCarbon Dioxide

NitrogenBenzene

Rapid Diffusion

SMALL UNCHARGED

POLAR MOLECULES

WaterGlycerolEthanol

FairlyRapid Diffusion

LARGER UNCHARGED

POLAR MOLECULES

Amino acidsGlucose

Nucleotides

Very slowDiffusion

IONS Ca+Na+K+Cl

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.