The Fluid Mosaic Model of Membranes

4.1 Fluid Mosaic Membranes YOUR NOTES⬇

CONTENTS

4.1.1 The Fluid Mosaic Model

4.1.2 Components of Cell Surface Membranes

4.1.3 The Cell Surface Membrane

4.1.4 Cell Signalling

4.1.1 THE FLUID MOSAIC MODEL

The Fluid Mosaic Model of Membranes

Membranes are vital structures found in all cells

The cell surface membrane creates an enclosed space separating the internal cellenvironment from the external environment, and intracellular membranes formcompartments within the cell such as the nucleus, mitochondria and RER

Membranes do not only separate different areas but also control the exchange of materialacross them, as well as acting as an interface for communication

Membranes are partially permeable

Substances can cross membranes by diffusion, osmosis and active transport

Cellular membranes are formed from a bilayer of phospholipids which is roughly 7nm wideand therefore just visible under an electron microscope at very high magnifications

The fluid mosaic model of the membrane was first outlined in 1972 and it explains howbiological molecules are arranged to form cell membranes

The fluid mosaic model also helps to explain:Passive and active movement between cells and their surroundings

Cell-to-cell interactions

Cell signalling

PhospholipidsPhospholipids structurally contain two distinct regions: a polar head and two nonpolar tails

The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water

The fatty acid tail of a phospholipid is nonpolar (hydrophobic) and therefore insoluble inwater

If phospholipids are spread over the surface of water they form a single layer with thehydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking upaway from the water

This is called a phospholipid monolayer

Dr. Asher Rana www.chemistryonlinetuition.com [email protected]

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A phospholipid monolayer

If phospholipids are mixed/shaken with water they form spheres with the hydrophilicphosphate heads facing out towards the water and the hydrophobic fatty acid tails facing intowards each other

This is called a micelle

A micelle

Alternatively, two-layered structures may form in sheets

These are called phospholipid bilayers – this is the basic structure of the cell membrane




A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobictails facing inwards and hydrophilic heads outwards

Phospholipid bilayers can form compartments – the bilayer forming the cell surfacemembrane establishing the boundary of each cell

Internally, membrane-bound compartments formed from phospholipid bilayers provide thebasic structure of organelles, allowing for specialisation of process within the cell

An example of a membrane-bound organelle is the lysosome (found in animal cells), eachcontaining many hydrolytic enzymes that can break down many different kinds ofbiomoleculeThese enzymes need to be kept compartmentalised otherwise they would breakdown most ofthe cellular components




Membranes formed from phospholipid bilayers help to compartmentalise differentregions of the cell




Structure of membranesThe phospholipid bilayers that make up cell membranes also contain proteins

The proteins can either be intrinsic (or integral) or extrinsic (peripheral)

Intrinsic proteins are embedded in the membrane with their arrangement determinedby their hydrophilic and hydrophobic regions

Extrinsic proteins are found on the outer or inner surface of the membrane

The fluid mosaic model describes cell membranes as ‘fluid’ because:The phospholipids and proteins can move around via diffusion

The phospholipids mainly move sideways, within their own layers

The many different types of proteins interspersed throughout the bilayer move aboutwithin it (a bit like icebergs in the sea) although some may be fixed in position

The fluid mosaic model describes cell membranes as ‘mosaics’ because:The scattered pattern produced by the proteins within the phospholipid bilayer lookssomewhat like a mosaic when viewed from above

The distribution of the proteins within the membrane gives a mosaic appearance and thestructure of proteins determines their position in the membrane




Exam Tip

You must know how to draw and label the fluid mosaic model, as well as ensure that you can

describe why the membrane is called the fluid mosaic model.

An example of the diagram you could draw


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4.1.2 COMPONENTS OF CELL SURFACE MEMBRANES

Phospholipids, Cholesterol, Glycolipids, Proteins & Glycoproteins

The cell membranes of all organisms generally have a similar structure

Cell membranes contain several different types of molecules:Three types of lipid:

Phospholipids

Cholesterol

Glycolipids (also containing carbohydrates)

Two types of proteins:Glycoproteins (also containing carbohydrates)

Other proteins (eg. transport proteins)

Phospholipids:Form a bilayer (two layers of phospholipid molecules)

Hydrophobic tails (fatty acid chains) point in towards the membrane interior

Hydrophilic heads (phosphate groups) point out towards the membrane surface

Individual phospholipid molecules can move around within their own monolayers bydiffusion

Cholesterol:Cholesterol molecules also have hydrophobic tails and hydrophilic heads

Fit between phospholipid molecules and orientated the same way (head out, tailin)

Are absent in prokaryotes membranes

Glycolipids:These are lipids with carbohydrate chains attached

These carbohydrate chains project out into whatever fluid is surrounding the cell (theyare found on the outer phospholipid monolayer)

Glycoproteins:These are proteins with carbohydrate chains attached

These carbohydrate chains also project out into whatever fluid is surrounding the cell(they are found on the outer phospholipid monolayer)




Proteins:The proteins embedded within the membrane are known as intrinsic proteins (orintegral proteins)

They can be located in the inner or outer phospholipid monolayer

Most commonly, they span the entire membrane – these are known astransmembrane proteins

Transport proteins are an example of transmembrane proteins as they cross thewhole membrane

Proteins can also be found on the inner or outer surface of the membrane, these areknown as extrinsic proteins (or peripheral proteins)

Exam Tip

Make sure you can draw and label all the above structures on a diagram of the fluid mosaic

model of cell membranes.

You can use an annotated diagram to state the functions of the above structures.


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4.1.3 THE CELL SURFACE MEMBRANE

Cell Surface Membranes

PhospholipidsForm the basic structure of the membrane (phospholipid bilayer)

The tails form a hydrophobic core comprising the innermost part of both the outer and innerlayer of the membrane

Act as a barrier to most water-soluble substances (the non-polar fatty acid tails preventpolar molecules or ions from passing across the membrane)

This ensures water-soluble molecules such as sugars, amino acids and proteinscannot leak out of the cell and unwanted water-soluble molecules cannot get in

Can be chemically modified to act as signalling molecules by:Moving within the bilayer to activate other molecules (eg. enzymes)

Being hydrolysed which releases smaller water-soluble molecules that bind to specificreceptors in the cytoplasm

CholesterolIncreases the fluidity of the membrane, stopping it from becoming too rigid at lowtemperatures (allowing cells to survive at lower temperatures)

This occurs because cholesterol stops the phospholipid tails packing too closelytogether

Interaction between cholesterol and phospholipid tails also stabilises the cell membraneat higher temperatures by stopping the membrane from becoming too fluid

Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising themand causing phospholipids to pack more closely together

They also contribute to the impermeabilty of the membrane to ions

Increases mechanical strength and stability of membranes (without it membranes wouldbreak down and cells burst)




Glycolipids & glycoproteinsGlycolipids and glycoproteins contain carbohydrate chains that exist on the surface (theperiphery/extrinsically), which enables them to act as receptor molecules

This allows glycolipids and glycoproteins to bind with certain substances at thecell’s surface

There are three main receptor types:signalling receptors for hormones and neurotransmitters

receptors involved in endocytosis

receptors involved in cell adhesion and stabilisation (as the carbohydrate part canform hydrogen bonds with water molecules surrounding the cell

Some act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO bloodgroup antigens are glycolipids and glycoproteins that differ slightly in their carbohydratechains)

ProteinsTransport proteins create hydrophilic channels to allow ions and polar molecules totravel through the membrane. There are two types:

channel (pore) proteins

carrier proteins

Each transport protein is specific to a particular ion or molecule

Transport proteins allow the cell to control which substances enter or leave

Exam Tip

Membranes become less fluid when there is:

• An increased proportion of saturated fatty acid chains as the chains pack together tightly

and therefore there is a high number of intermolecular forces between the chains

• A lower temperature as the molecules have less energy and therefore are not moving as

freely which causes the structure to be more closely packed

Membranes become more fluid when there is:

• An increased proportion of unsaturated fatty acid chains as these chains are bent, which

means the chains are less tightly packed together and there are less intermolecular forces

• At higher temperatures, the molecules have more energy and therefore move more freely,

which increasing membrane fluidity




4.1.4 CELL SIGNALLING

Cell Signalling

Cell signalling is the process by which messages are sent to cells

Cell signalling is very important as it allows multicellular organisms to control /coordinate their bodies and respond to their environments

Cell signalling pathways coordinate the activities of cells, even if they are large distancesapart within the organism

The basic stages of a cell signalling pathway are:A stimulus or signal is received by a receptor

The signal is converted to a ‘message’ that can be passed on – this process is knownas transduction

The ‘message’ is transmitted to a target (effector)

An appropriate response is made

The basic stages of a cell signalling pathway

Transmission of messages in cell signalling pathways requires crossing barriers such as cellsurface membranes

Cell surface membranes are therefore very important in signalling pathways as themembrane controls which molecules (including cell signalling molecules) can movebetween the internal and external environments of the cell

Signalling molecules are usually very small for easy transport across cell membranes

Typically in cell signalling pathways, signalling molecules need to cross or interact with cellmembranes




LigandsSignalling molecules are often called ligands

Ligands are involved in the following stages of a cell signalling pathway:Ligands are secreted from a cell (the sending cell) into the extracellular space

The ligands are then transported through the extracellular space to the target cell

The ligands bind to surface receptors (specific to that ligand) on the target cellThese receptors are formed from glycolipids and glycoproteins

The message carried by the ligand is relayed through a chain of chemicalmessengers inside the cell, triggering a response

The role of ligands in a cell signalling pathway




Exam Question: Easy




Exam Question: Medium

Exam Question: Hard


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The Fluid Mosaic Model of Membranes

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