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Summary Cell membrane and transport
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Compiled and Summarised notes all based on the chapter Cell membrane and transport and units falling under it, with diagrams included
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Cell structure_ A level
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AS Bio notes Raida 12DCell membrane and transport:
4.1 The importance of membranes:
All living cells are surrounded by a very thin membrane, the cell surface membrane
This controls the exchange of materials such as nutrients and waste products between
the cell and its environment
Inside cells, regulation of transport across the membranes of organelles is also vital
Important functions include that they enable cells to receive hormone messages
In chloroplasts they contain the light-absorbing pigments needed for photosynthesis
4.2 Structure of membranes
Phospholipids
Phospholipids help to form the membranes that surround cells and organelles
They form a single layer with their heads in the water, because the heads are polar
(hydrophilic)
Their tails project out of the water, because they are non-polar (hydrophobic)
If the phospholipids are mixed with water, they form either:
1. Ball-like structures called Micelles
2. Sheet-like structures called Bilayers
In a micelle, all the hydrophilic heads face outwards into the water
They shield the hydrophobic tails from the water
In the middle of the ball, the tails point in towards each other, creating a hydrophobic
environment in the micelle
In bilayers, the hydrophobic tails are also shielded from the water by the hydrophilic
heads
The phospholipid bilayer is the basic structure of membranes
Bilayer means two layers
The bilayer/membrane is about 7 nm wide
Membranes also contain proteins
The fluid mosaic model of membrane structure:
> fluid mosaic model: the model of membrane structure in which protein molecules are free to
move about in a fluid bilayer of phospholipid molecules
The fluid mosaic membrane is a hypothesis for the membrane structure
It is described as ‘fluid’ because both the phospholipids and the proteins can move about
by diffusion
The phospholipid molecules move sideways in their layers
Some of the protein molecules also move about within the phospholipid bilayer
Others remain fixed to structure inside or outside the cell
The word ‘mosaic’ describes the pattern produced by the scattered protein molecules
when the surface of the membrane is viewed from above
Features of the fluid mosaic model
, The membrane is a double layer (bilayer) of phospholipid molecules
The individual phospholipid molecules move about by diffusion within their layers
The phospholipid tails point inwards, facing each other and forming a non-polar
hydrophobic interior
The phospholipid heads face outwards into the aqueous medium that surrounds the
membranes
Some of the phospholipid tails are saturated and some are unsaturated
Unsaturated tails contain double bonds
The more unsaturated they are, the more fluid the membrane
This is because the unsaturated fatty acid tails are bent and therefore fit together more
loosely
Fluidity is also affected by tail length; the longer the tail, the less fluid the membrane
As temperature decreases, membranes become less fluid
However, in some organisms which cannot regulate their own temperature, such as
bacteria and yeasts, respond by increasing the proportion of unsaturated fatty acids in
their membranes
Proteins may be found in the inner layer, the outer layer or, most commonly, spanning
the whole membrane
In that case they are known and transmembrane proteins
The proteins have hydrophobic (non-polar) and hydrophilic (polar) regions
They stay in the membrane because the hydrophobic regions, made from hydrophobic
amino acids, are next to the hydrophobic fatty acid tails and are repelled by the watery
environment on either side of the membrane
The hydrophilic regions, made from hydrophilic amino acids, are repelled by the
hydrophobic interior of the membrane and therefore face into the aqueous environment
inside or outside the cell, or line hydrophilic pores which pass through the membrane
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