1 cell membrane (plasma membrane) n the plasma membrane can be thought of as a gatekeeper, allowing...

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Cell membrane (plasma membrane)

The plasma membrane can be thought of as a gatekeeper, allowing only specific substances in or out and passing messages from the external environment.

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The cell membrane must perform several specific functions: Isolate the cell cytoplasm from

the environment. Regulate the exchange of

essential substances between the cytoplasm and the environment.

Communicate with other cells. Identify the cell as belonging to a

particular species and a particular member of that species.

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Fluid mosaic model

A membrane when viewed from above, looks something like a lumpy, constantly shifting mosaic of tiles.

A bilayer of phospholipids forms a viscous fluid for the mosaic, while an assortment of proteins are the tiles, often sliding about within the phospholipid bilayer.

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Structure of the cell membrane

Phospholipid bilayer» the polar end of the phospholipid

interfaces with the watery environment surrounding the membranes

» the nonpolar fatty acids are found in the interior of the bilayer sheet.

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Proteins function as:» receptors, channels, transporters and

markers» some are integral; span entire

membrane - f. ex. transport protein.

» some are peripheral; at the borders - f. ex. receptor proteins for hormones.

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Composition of a typical membrane: ~ 50% lipid

~ 50% protein Regulation of interactions

1. Passage of water. Freely permeable to water.

2. Bulk passage into the cell. Phagocytosis. Big gulps.

3. Selective transport of molecules. Transports only some molecules.

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Regulation of interactions cont.4. Reception of information.

Identify chemical messages.

5. Expression of cell identity. Molecular name tags.

6. Physical connections with other cells.

In forming tissues, make special connections.

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Solutions Solvent

– Water is the solvent, the most common substance

Solute– The molecules dissolved in the water,

f.ex. sugars, amino acids and ions. Solution

– Solvent and solutes mixed together.

Both the solvent and the solutes seek to move from the area of greater concentration to the area of less concentration.

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Transport across membranes

With reference to water and solutes a membrane is called:

permeable non-permeable partially-permeable Semi-permeable differentially permeable Selectively permeable membrane

Partially permeable membrane

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Passive transport - no energy used Diffusion (simple diffusion)

– Diffusion is the random movement of molecules from an area of high concentration to an area of low concentration.

– Water, gasses like O2, CO2 and lipid- soluble molecules like ethyl alcohol and vitamin A easily diffuse across.

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Osmosis (a special kind of diffusion)– Osmosis is the net movement of

water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.

– Osmosis will occur whenever two solutions containing different concentration of water molecules are separated by a partially permeable membrane.

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The movement of water in osmosis can also be seen as the movement of water from a region of low solute concentration to a region of high solute concentration.

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In relation to cells the concentration

of surrounding solutions can be: Hypertonic if the concentration of

solutes in the solution is higher than in the cell.

– Water will move in or out?

Isotonic if the concentration of solutes in the solution is equal to that in the cell.


Hypotonic if the concentration of solutes in the solution is lower than in the cell.


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Facilitated diffusion– Facilitated diffusion is the transport

of molecules across a membrane by a carrier protein in the direction of lowest concentration.

– Its is a boat with no oars, sail or engine - it can only work when the tide is in the right direction.

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Active transport - energy used– Active transport is the transport of a

solute across a membrane to a region of higher concentration by the expenditure of energy.

– It moves molecules against a concentration gradient.

– The energy is provided by ATP or adenosine triphosphate

ATP ADP + energy

ATP ase

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There are three main types– The sodium potassium pump (Na, K)

Most animal cells maintain a higher level of K+ in side the cell and lower Na+ level than on the outside.

The pump transports 3 Na+ out for every 2 K+ it moves in.

– The proton pump (H+) Most likely to be found in the

membranes of mitochondria and chloroplasts.

Energy from metabolism or photosynthesis is used to produce ATP.

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– Coupled channels In one channel a molecule is linked

to another that is moving down a concentration gradient.

The other channel pumps out the carrier molecule and so keeps up the concentration gradient.

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Endocytosis is when materials are surrounded by and taken into membrane lined vesicles.– Phagocytosis - cell eating

big parts or whole cells are taken in.

– Pinocytosis - cell drinking minute vacuoles (drops) are taken in

Exocytosis– The emptying of a membrane lined

vesicle at the surface of the cell.

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The relationship between the nucleus, rough endoplasmic reticulum (rER), Golgi complex and the cell surface

– information about a protein leaves the nucleus through a nuclear pore

– the protein is synthesised in the ribosomes on the rEr

– after travelling through the rEr it is encapsulated in a vesicle

– the vesicle fuses with a Golgi complex where the enzyme is further modified

– at the ends or the cisternae it goes to a secretory vesicle

– which carries it to the cell membrane, – where it fuses with the membrane and the

enzyme is released outside the cell

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Membrane proteins Transport across membranes

– channels, carriers, pumps Antibody recognition sites

– identification tags f.ex. immune cells recognise bacteria and mark them for destruction

– blood groups, tissue groups Hormone binding sites

– act as a triggers that sets of a cellular response when hormones bind to them

Catalysis for biochemical reactions– act as enzymes, particularly on the inside of

the cell membrane Sites of electron carriers

– the proton pump

1 cell membrane (plasma membrane) n the plasma membrane can be thought of as a gatekeeper, allowing...

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