Cell Membranes

Biology AS Edexcel

Cell Membranes:

Structure, Fluid Mosaic Model, Diffusion, Facilitated Diffusion, Osmosis, Active Transport

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Structure of Cell membranes

The membranes is made up mainly of two types of molecules, Lipids and Proteins. They are arranged in a very specific shape.

The phospholipid bilayer has:

  • The lipids in the membrane are of a particular type called polar lipids. These are lipid molecules with one end joined to a polar group.
  • Many of the polar lipids in the membrane are phospolipids, with a phosphate group forming the polar part of the molecules.
  • The fatty acid chains of a phospholipid are neutral and insolule in water.
  • The polar phosphate part is hydrophilic (water-loving) and dissolves readily in water.
  • The lipid taile are hydrophobic (water-hating) and insoluble in water, they form a monolayer, with hydrophilic heads in water and the hydrophobic lipid tails in air.

Water on each side of the phospholipid molecules form a bilayer, with hydrophilic heads and in the water hydrophobic tails.

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Fluid Mosaic Model

It is called a fluid mosaic model because of the hydrophobic lipids and membrane proteins that move sideways throughout the membrane. This is not a 'solid' but like a 'fluid'.

Mosaic because it's made up of many parts such as proteins, glycoproteins, phospholipids and carbohydrate chains attached to glycoprotein.

It folds into a bi-layer because the tails are hydrophobic and it folds into a bi-layer as the heads are hydrophilic and they are polar so they're surronded by water.

Proteins float and move around in between phospholipids, the proteins form a pattern in between phospholipids.

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Diffusion is the net movement pf the molecules in a liquid or gas form an area of highly concentrated to an area of low concentration.

The move down a concentration gradient, the movement is dependent on the tempreture, if you have no. of molecules due to energy they will spread until reaching a uniform distribution.

When they reach the uniform distribution the movement no longer changes in concentration because equal numbers of molecules move in every direction.

For molecules like oxygen, carbon dioxide, move by diffusion through protein-lined pores, as a form of passive transport, whereas the larger molecules cannot move across the membrane by simple diffusion

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Facilitated Diffusion

Facilitated diffusion has proteins in the membrane which allow specific substances to move passively down the concentration gradient.

They also have channel proteins which form pores through the membrane.

They move form a high to low concentration, the molecules which needed to transport travel across membrane in protein carriers of match shape, the protein carriers changes shape and the molecules passes into the cell.

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Osmosis is the net movement of free water molecules, they move across a membrane down their concentration gradient.

Moves from a region with lower solute concentration to a solution with a higher solute concentration.

This movement is through a partially permeable membrane.

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Active Transport

Active transport has a carrier protein or intrinsic protein which moves from a low concentration of molecules to a high concentration.

The protein carrier changes shape to allow glucose (molecules or ions) into the cell. This needs energy from adenosine triphosphate.

The molecules are carried across the membrane in protein carriers of matching shape the ATP breaks down to ADP and energy.

Lastly the molecules have moved from a low concentration to a high concentration to make it possible for cells to move substances against their concentration gradient:

  • all kinds of molecules possible,
  • through carrier proteins
  • active- needs energy from the breakdown of ATP
  • up a concentration gradient
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