Transport across cell-surface membranes

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  • Created by: LilyIM
  • Created on: 07-05-16 13:46

Cell-surface membranes

The basic structure of all cell surface membranes are the same. They are called Plasma membranes.

Surface membrane structure;


The 'head' has a charge because of the phosphate group making it hydrophilic. The fatty acid tail is hydrophobic. They form a bi-layer.

Protein molecules

They float in the bilayer. Some are extrinsic which float in one half of the bilayer. Intrinsic pass through the whole bilayer. Proteins can function as:

Channels, carriers, antigens, receptors, enzymes.


Found almong phospholipids. They stabilise the membrane by restricting movement of other molecules.

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More cell-surface structure

Proteins and phosholipids can have short carbohydrates chains to them.They are called Glycoproteins and Glycolipids.


Provide stabilty, allow cell to recognise each other, and also helps cells form tissues.


Instead of providing stabilty they allow cells to recognise each other.

It is FLUID MOSAIC because....

The phospholipids move around making the bi-layer fluid.

The proteins are embedded in various shape, size and patterns making them look 'mosaic' 

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Movement across membranes (Diffusion)

Diffusion is the movement of molecules from a area of high concentration to a area of low concentration down a concentration gradient.

It is passive as it does not require additional energy.

E.g oxygen is small and lipid-soluble so can diffuse right in eh!

Why some molecules can't diffuse

Not soluble in lipids

Too large

Same charge as the proteins channels and are repelled

Polar and are repelled by hydrophobic tails 

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Facilitated diffusion

Facilitated means 'helped'.

Proteins 'help' these two helping proteins are;

Channel proteins, they have hydrophilic channels molecules can go through. Water soluble molecules can pass through them.

Carrier proteins; the protein changes shape when a molecule binds. and releases it to the other side of the membrane. 

It is passive since no additional energy is used. Only kinetic movement.

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Water is small, and even though it has charges can slip through the bilayer by osmosis.

The more water molecules in a solution, the higher the water potential. Water has a water potential of 0 the highest. If the solution has solute in it the more solute it has the more negative the water potential will be.

Osmosis is where water diffuses from areas with high water potential to areas of low water potential through a selectively permeable membrane down a con gradient.

Passive since it doesn't require additional energy.

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Active transport

Cells take up substances against their concentration gradient. This is active trnasport. Uses a facilitator like facilitated diffuses but also uses ATP because the substance is going against the concentration gradient.

The ATP is from the hydrolysis of ATP. 


This is direct active transport

Carrier protein binds with molecule on it's receptor site. On the inside of the cells ATP binds to the protein causing it to split in ADP and  a phosphate molecule. Protein changes shape. Molecule is released to the other side. Phosphate is released causing it to revert to it's orginal shape. ADP is reverting to ATP in respiration.  

When two molecules need to moved at one time. A example of this is using a sodium potassium pump in co-transport. 

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Co-transport is where two molecules need to be transported at one time. It is active. 

A example is the sodium-potassium pump which allows absorption through the iluem in mammels. 

Sodium potassium pump

1. Sodium ions are actively pumped into blood by sodium-potassium pump.

2.Sodium concentration in lumen of Iluem now higher then inside the cell.  

3. Sodium ions diffuse down con gradient into cell through co-transport carrier protein, for this to happen glucose must also attach, regardless of glucose con in the cell it diffuses in.

4.Glucose concentration is now higher in cell so diffuses down con into blood by a carrier protein in the process of facilitated diffusion.

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Cell adaption for rapid transport

You can increase transport by increasing a cell membrane e.g Microvilli

1.Folds of plasma membrane increases surface area.

2.For larger surface area for attachment of transport proteins. (Carrier protein).

You can also increase channel and carrier proteins across the membrane. 

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Osmosis in animal and plant cells


When water enters the cell and water potential is high:  it swells and bursts. (RBC) Contents such as haemoglobin are released and C-S-M is like everywhere. 

No change in water potential in/out:  The cell is normal

Water leaves so the cell has a low water potential:  The cells shrinks. Hamoglobin is more concentrated so the cell is darker.


Enters: Turgid. protoplast is pushed against the cell wall. 

Same: Incipient Plasmolysis. Protoplast is beginning to be pulled away from the wall.

Leaves: Plasmolysed. Protoplast totally comes away from the cell wall. 

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