Cell Membrane

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Phospholipid Bilayer

- Phospholipids consist of a hydrophillic phosphate head and a hydrophobic fatty acid tail

- Phospholipids can form bilayers, with one sheet of phospholipid molecules, opposite another

- The inner layer of the phospholipid bilayer has its hydrophillic heads pointing in, towrads the cell and interacts with the water in the cytoplasm 

- The outer layer of the phospholipid bilayer has its hydrophillic head pointing outwards, interacting with the water surrounding the cell

- The phospholipid bilayer allows lipid-soluble molecules across, but not water-soluble molecules. This is because the phospholipid bilayer is hydrophobic so lipid-soluble molecules move through the membrane more easily than water-souble molecules 

- The phospholipid bilayer acts a barrier to large charged molecules such as glucose and ions. It is freely permebale to non-charged molecules and very small polar molecules. 

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Proteins are scattered throughout the phospholipid bilayer of the membrane. There are two ways in which they are embedded. 

Extrinsic Proteins: They are on either surface of the bilayer. They provide structual support and form recognition sites, for example indentifying cells

Intrinsic Protiens: They extend across both layers of the phospholipid bilayer 

Channel Proteins

- Channel protiens are water filled channels that allow water soluble molecules to pass through as the channels are hydrophillic. They are ususually small but highly selective and only allows specific molecules and ions through

Carrier Protiens

- They change shape to allow molecules to pass through and it releases the molecule on the other side of the membrane. Carrier proteins act as carriers in Active Transport and Faclilated Diffussion for large or charged molecules

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The Fluid Mosaic Model

The fluid-mosaic model is a model of the cell membrane proposed by Singer and Nicolson in 1972. It is called "fluid" because the phospholipids and protiens move around each other. The model is called "mosaic" because the protiens are arranged between the phospholipids and the protiens vary in shape, size and pattern. 

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Diffusion is the movement of molecules from a region of high concentration to a region of low concentartion


- There is a concentration gardient

- Molecules have kinetic energy 

- No ATP energy is required

- No specific protiens are required 

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Rates of Diffusion

Temperature: Increasing temperature increases kinetic energy and therefore increases the rate of diffusion

Concentartion Gradient: Having more moelcules on one side of the moelcule thane the other increases the diffusion rate

Surface Area: Bigger surface area, bigger rate of diffusion 

Thickness of Membarne: the thicker the membrane the slower the rate of diffusion

Diffusion Distance: The shorter the diffusion distance, the greater rate of dffusion

Size of molecules: Smaller molecules diffuse more quickly than larger ones 

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