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The Fluid Mosaic model

Learning Objectives Success Criteria

Memory Game – try and draw this in as much detail as possible

Cells have many membranes:

plasma membrane

tonoplast

outer mitochondrial membrane

inner mitochondrial membrane

outer chloroplast membrane

nuclear envelope

What are membranes?

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.

Membranes cover the surface of every cell, and also surround most organelles within cells. They have a number offunctions, such as:

a site for biochemical reactions

Membranes are mainly made of phospholipids

phosphate group

glycerol

fatty acid

phosphoester bond

ester bond

hydrophilichead

hydrophobictail

Membranes are flexible and able to break and fuse easily

Neutrophil engulfing anthrax bacteria.

Cover credit: Micrograph by Volker Brinkmann, PLoS Pathogens Vol. 1(3) Nov. 2005.

5 μm

Membranes allow cellular compartments to have different conditions

pH 4.8Contains digestive enzymes, optimum pH 4.5 - 4.8

pH 7.2

lysosome

cytosol

Membrane acts as a barrier

The polar hydrophilic heads are water soluble and the hydrophobic heads are water insoluble

aqueous solution

Hydrophilic (water-loving) head

Hydrophobic (water-hating) tail

Phospholipids form micelles when submerged in water

air

Question: Explain why phospholipids form a bilayer in plasma membranes (4).

• Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutions

• The fatty acid tails are non-polar and will move away from an aqueous environment

• As both tissue fluid and cytoplasm is aqueous • phospholipids form two layers with the hydrophobic tails facing

inward • and phosphate groups outwards interacting with the aqueous

environment

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Membranes: timeline of discovery

When clear electron micrographs of membranes became available, they appeared to show support for Davson–Danielli’s model, showing a three-layered structure.

2nd cell membrane

This was taken to be the phospholipid bilayer (light) surrounded by two layers of protein (dark).

1st cell membrane

intracellular space (blue)

1 light layer = phospholipid bilayer

2 dark layers: protein

Evidence for the Davson–Danielli model

Later, it was discovered that the light layer represented the phospholipid tails and the dark layers represented the phospholipid heads.

2nd cell membrane

1st cell membrane

intracellular space (blue)

1 light layer = phospholipid tails

2 dark layers: phospholipid heads

Evidence for the Davson–Danielli model

By the end of the 1960s, new evidence cast doubts on the viability of the Davson–Danielli model.

The amount and type of membrane proteins vary greatly between different cells.

It was unclear how the proteins in the model would permit the membrane to change shape without bonds being broken.

Membrane proteins are largely hydrophobic and therefore should not be found where the model positioned them: in the aqueous cytoplasm and extracellular environment.

Problems with the Davson–Danielli model

Evidence from freeze-fracturing

E-face: looking up at outer layer of membrane

This revealed a smooth surface with small bumps sticking out. These were later identified as proteins.

In 1966, biologist Daniel Branton used freeze-fracturing to split cell membranes between the two lipid layers, revealing a 3D view of the surface texture.

P-face: looking down on inner layer of membrane

The fluid mosaic model

This model suggested that proteins are found within, not outside, the phospholipid bilayer.

The freeze-fracture images of cell membranes were further evidence against the Davson–Danielli model.

They led to the development of the fluid mosaic model, proposed by Jonathan Singer and Garth Nicholson in 1972.

E-face

P-face protein

What can we say about the plasma membrane?

• Made up of phospholipids, proteins, carbohydrates, cholesterol

• Hydrophilic heads and hydrophobic tails• Double layer. Hydrophobic tails attracted to other tails• Plasma membrane is fluid – always moving• Some proteins span the entire width of the membrane• Some are just on the interior or exterior surface

Phospholipids in membranes

Generally, the smaller and less polar a molecule, the easier and faster it will diffuse across a cell membrane.

Small, non-polar molecules such as oxygen and carbon dioxide rapidly diffuse across a membrane.

The role of phospholipids in membranes is to act as a barrier to most substances, helping control what enters/exits the cell.

Small, polar molecules, such as water and urea, also diffuse across, but much more slowly.

Charged particles (ions) are unlikely to diffuse across a membrane, even if they are very small.

The fluid mosaic model of the plasma membrane:

The proteins can move freely through the lipid bilayer.

The ease with which they do this is dependent on the number of phospholipids with unsaturated fatty acids in the phospholipids.

The membrane contains many types of protein:

glycoprotein

carbohydrate chain

integral proteinextrinsic protein

carrier protein

Glycocalyx: For cell recognition so cells group together to form tissues

Receptor: for recognition by hormones

Enzyme or signalling protein hydrophilic channel

Cholesterol in cell membranesCholesterol is a type of lipid with the molecular formula C27H46O.

Cholesterol is also important in keeping membranes stable at normal body temperature – without it, cells would burst open.

Cholesterol is very important in controlling membrane fluidity. The more cholesterol, the less fluid – and the less permeable – the membrane.

Proteins in membranesProteins typically make up 45% by mass of a cell membrane, but this can vary from 25% to 75% depending on the cell type.

integral protein

Peripheral (or extrinsic) proteins are confined to the inner or outer surface of the membrane.

Integral (or intrinsic, or transmembrane) proteins span the whole width of the membrane.

peripheral proteinMany proteins are glycoproteins –proteins with attached carbohydrate chains.

carbohydrate chain

Integral proteinsMany integral proteins are carrier molecules or channels.

These help transport substances, such as ions, sugars and amino acids, that cannot diffuse across the membrane but are still vital to a cell’s functioning.

Other integral proteins are receptors for hormones and neurotransmitters, or enzymes for catalyzing reactions.

Extrinsic proteinsExtrinsic (or Peripheral) proteins may be free on the membrane surface or bound to an intrinsic (or integral) protein.

Extrinsic proteins on the extracellular side of the membrane act as receptors for hormones or neurotransmitters, or are involved in cell recognition. Many are glycoproteins.

Extrinsic proteins on the cytosolic side of the membrane are involved in cell signalling or chemical reactions. They can dissociate from the membrane and move into the cytoplasm.

Complete the worksheet

• Ensure you are aware of all the functions of the membrane components

• Highlight any structure-function relationships

Functions of membrane components

Question: Label the diagram (11marks)1

2 10

3

4

5 6

8

9

11

Note: label the proteins based on location or structure, e.g. you do not need to identify receptors and enzymes.

1) carbohydrate; 2) glycoprotein; 3)integral protein; 4) extrinsic protein; 5) carrier protein 6) hydrophilic channel; 7) phosphate group; 8) fatty acid; 9) phospholipid; 10) glycocalyx; 11) phospholipid bilayer click to cover answers

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