Comprehensive and in-depth notes on the biological aspects of this chapter, using specification points as headings to ensure that all required material is included- and no irrelevant content (like many of the textbooks). Created and used by an A-Level Biology student for the NEW SPECIFICATION from ...
a) The roles of membranes within cells and at the surface of cells.
To include the roles of membranes as,
• Partially permeable barriers between the cell and its environment, between organelles
and the cytoplasm and within organelles
• Sites of chemical reactions
• Sites of cell communication (cell signalling).
Membrane Function
Cells and many organelle inside of them are surrounded by a membrane, that have a whole range
of functions.
Membranes at the cell surface (plasma membranes)
Plasma membranes are a barrier between the cell and its environment, controlling which
substances enter and exit the cell.
They're partially permeable - so they let some substances through and some not
Substances can move across the plasma membrane by diffusion, osmosis or active transport.
Plasma membranes also allow the recognition by other cells (i.e. the immune system’s cells) and
cell communication.
Membranes within cells
The membranes around organelle divide the cell into compartments - they act as a barrier between
the organelle and the cytoplasm. This makes them more efficient.
Example: The substances needed for respiration (like enzymes) are kept together inside a
mitochondrion by the mitochondrion's outer membrane
Membranes can form vesicles to transport substances between different areas of the cell
Example: proteins are transported in vesicles from the rough endoplasmic reticulum to the
Golgi apparatus during protein synthesis.
Membranes within cells are also partially permeable so they can control which substances enter or
exit the organelle.
Example: RNA leaves the nucleus via the nuclear membranes. DNA is too large to pass
through the partially permeable membrane so it stays within the nucleus.
You can also get membranes within organelles - these act as barriers between the membrane
content and the rest of the organelle.
Example: Thylakoid membranes within the chloroplasts keep the components needed for
light-dependent reactions of photosynthesis together.
Membranes within the cells can be the site of chemical reactions. The membranes of some
organelles are folded, increasing their surface area and making chemical reactions more efficient.
, Example: The inner membrane of mitochondria contains enzymes needed for respiration.
The membranes of some organelle are folded increasing their surface area, increasing the
number of proteins present and makes respiration more efficient
(b) The fluid mosaic model of membrane structure and the roles of its components.
To include phospholipids, cholesterol, glycolipids, proteins and glycoproteins
AND
the role of membrane-bound receptors as sites where hormones and drugs can bind.
Membrane Structure
The structures of all membranes are basically the same. they're all composed of lipids (mainly the
type called phospholipids), proteins and carbohydrates (usually attached to proteins and lipids).
Fluid Mosaic model
This is the arrangement at which the molecules in the membrane are in. In the model, the
phospholipid molecules form a continuous, double layer (bilayer). The bilayer is described as being
fluid as it is constantly moving.
Protein molecules are scattered through the bilayer, like tiles within a mosaic.
Some proteins have a carbohydrate attached to them, called a glycoproteins. Some lipids have a
carbohydrate attached called a glycolipids.
Cholesterol molecules are also present within the bilayer
Phospholipids - The main build up of the layer. Consisting of lipids in a continuous double
(bilayer) layer.
Proteins- Protein molecules scattered throughout the bilayer.
Glycoproteins - Proteins which have carbohydrates attached to them.
Glycolipids - Lipids which have carbohydrates attached to them.
, Membrane components
Phospholipids:
Phospholipid molecules form a barrier to dissolved substances. Phospholipids have a ‘head’ and a
‘tail’. The head is hydrophilic - it attracts water. And the tail is hydrophobic - it repeals water.
The molecules automatically arrange themselves into a bilayer - the heads face outward on either
side of the membrane.
The centre of the bilayer is hydrophobic so the membrane doesn't allow water-soluble substances
(like ions and polar molecules) to diffuse through it. It acts as a barrier to these dissolved barriers.
Fat soluble substances, e.g. fat-soluble vitamins, dissolve in the bilayer and pass directly through
the membrane.
Cholesterol:
Cholesterol gives the membrane stability. It is a type of lipid that present in all cell membranes
(except bacterial cell membranes). Cholesterol fit between the phospholipids, causing them to pack
more closely together, making them less fluid and more rigid. Cholesterol also has hydrophobic
regions, so its able to create dirtier barriers to polar substances moving through the membrane.
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