I created these notes throughout y12&13 and used them for my A level 2022 exams. I got a grade 9 in GCSE and am awaiting for my a level biology result. These notes were made using the CGP revision guide, AQA Biology textbook and class notes in order to cover everything in the course. I have also ad...
4.1 STRUCTURE OF THE CELL-SURFACE
MEMBRANE
09 October 2020
09:53
The basic structure of all cell membranes is the same.
Acts as a barrier between the cell and its environment, controlling which substances
can enter or leave the cell.
● Cell membrane is partially permeable.
Phospholipids form bilayers.
The hydrophilic heads point towards the outside and inside of the cell, attracted by
water on both sides.
The hydrophobic tails point into the centre of the cell membrane, repelled by the
water on both sides.
The function of the phospholipids are to:
● Allow lipid-soluble substances to enter and leave the cell.
● Prevent water soluble substances from entering and leaving the cell.
● Make the membrane flexible and self-sealing.
Proteins are embedded within the phospholipid bilayer.
● Some proteins occur on the surface to give mechanical support to the
membrane or act as cell receptors for molecules such as hormones.
● Other proteins completely span the phospholipid bilayer, from on side to the
other.
These proteins which go through the membranes are:
● Protein channels which form water filled tubes, allowing water soluble ions to
diffuse across the membrane.
● Carrier proteins which bind to ions or molecules like glucose, then change
shape in order to move these molecules across the membrane via active
transport.
The function of proteins in the membrane are to:
● Provide structural support.
● Act as channels transporting water-soluble substances across the membrane.
● Allow active transport across the membrane through carrier proteins.
● Form cell-surface receptors for identifying cells.
● Help cells adhere together.
● Act as receptors.
, Cholesterol occurs within the phospholipid bilayer.
● Add stability to the membranes by making them less flexible.
Cholesterol molecules are hydrophobic, and so prevent loss of water and dissolved
ions from the cell.
They pull together the fatty acid tails, limiting their movement and that of other
molecules.
● Without making the membrane too rigid as a whole.
The functions of cholesterol are to:
● Reduce lateral movement of other molecules, including phospholipids.
● Make the membrane less fluid at high temperatures.
● Prevent leakage of water and dissolved ions from the cells.
Some cells may have more cholesterol than others because they aren't supported by
other cells.
● Cholesterol helps maintain the cells shape.
Glycolipids are made up of a carbohydrate covalently bonded with a lipid.
The carbohydrate portion extends from the phospholipid bilayer into the watery
environment outside the cell.
● Acts as a cell surface receptor for specific chemicals.
The functions of glycolipids are to:
● Act as recognition sites.
● Help maintain the stability of the membrane.
● Help cells to attach to one another and so form tissues.
Glycoproteins are carbohydrate chains attached to many extrinsic proteins on the
outer surface.
Act as cell surface receptors.
The function of glycoproteins in the membrane are to:
● Act as recognition sites.
● Help cells to attach from one another to form tissues.
● Allow cells to recognise one another, i.e. Lymphocytes can recognise an
organisms own cells.
The cell surface membrane controls the movement of substances in and out the cell.
Most molecules don't freely diffuse across it because many are:
● Not soluble in lipids, and so cannot pass through the phospholipid bilayer.
● Too large to pass through the channels in the membrane.
● Same charge as the charge on the protein channel, and so are repelled.
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