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Chapter 2.3 Transport across membranes
All membranes act as barriers controlling what passes through them and allowing the fluids either
side of them to have different compositions. Many chemical processes take place on membrane
The cell surface membrane must be flexible to allow the cell to change shape very slightly as its
water content changes, or quite dramatically when a white blood cell engulfs a bacterium.
Chemical secretions made by the cell are packaged into membrane bags known as vesicles, so some
membranes must be capable of breaking and fusing together rapidly.
The membrane is made up of lipids and proteins arranged in a very specific way.
A monolayer may form at a surface between air and water. This is rare in living cells as there are
water-based solutions on either side of the membranes.
Water is on each side of the phospholipid molecule, so a bilayer is formed with the hydrophilic
heads pointing into the water while the hydrophobic tails are protected in the middle.
The lipids in the membrane are polar lipids. These are lipids molecules with one end joined to a
polar group. Many of the polar lipids in the membrane are phospholipids, with a phosphate group
forming the polar part of the molecule.
The fatty acid chains of a phospholipid are neutral and insoluble in water. In contrast, the phosphate
head carries a negative charge and is soluble in water.
When phospholipids come into contact with water, the two parts of the molecule behave differently
The polar phosphate part is hydrophilic (water-loving) and dissolves in water
The lipid tails are hydrophobic (water-hating) and insoluble in water.
If the water molecules are tightly packed in water they form either: -
A monolayer with the hydrophilic heads in water and the hydrophobic lipid tails in the air.
Micelles where all the hydrophilic heads point outwards and all the hydrophobic tails are
The proportion of phospholipids containing unsaturated fatty acids in the bilayer affects how freely
the moving proteins float about in the membrane.
Membrane proteins help substances across the membrane. The proteins form pores or channels
(some permanent and some temporary) which allow specific molecules to move through the pores.
Proteins may act as specific receptor molecules e.g. making cells sensitive to a particular hormone.
Membrane structure can be investigated by looking at the effect of alcohol concentration or
temperature on the permeability of the membrane.
Small molecules pass through the membrane by diffusion.
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Other molecules move by diffusion through protein-lined pores. This is a form of passive transport.
Substances with a strong positive or negative charge and large molecules can't pass through cell
membranes by single diffusion. They are passed through facilitated diffusion. This is when proteins
allow specific substances to move through down a concentration gradient. This is through channel
proteins which form pores through the membrane. Each type of channel protein allows one
particular type of molecule through.…read more
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Surface area × concentration gradient
Rate of diffusion Thickness of exchange membrane of barriers.
Gaseous exchange happens in the alveoli.
An alveolus is made up of a single layer of flattened epithelial cells. The capillaries close to the
alveoli have a wall which is one cell thick. Between the two is a layer of elastic connective tissue,
holding everything together. The elastic tissue helps force air out of the lungs.…read more