Biological Membranes

AS level Biology A OCR spec

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  • Created on: 31-10-16 14:45
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Biological Membranes
The roles of membranes within cells and at the surface of cells, including the roles of membranes
as:
- Partially permeable barriers between the cell and its environment, between organelles and
the cytoplasm and within organelles: Membranes are structures that separate the contents
of the cells from their environment. They also separate different areas within cells
(organelles) ­ this is compartmentalisation.
Compartmentalisation is important as different reactions can be contained in different parts
of the cell and the optimum conditions for those reactions can be reached.
This partially permeable membrane also allows for molecules to pass through the
membrane needed for cells to survive (ex: glucose ­ energy source), and it allows molecules
to pass out of the cell in order to excrete waste materials (ex: excess ions).
- Sites of chemical reactions: membranes provide a surface on which enzymes can be located
allowing chemical reactions to proceed
- Sites of cell communication (cell signalling): membranes contain receptors when chemicals
bind there it elicits a response form the cell ­ this is cell signalling
The fluid mosaic model of membrane structure and the roles of its components:
Membranes have a fluid mosaic model structure; they are made up of a phospholipid bilayer. The
tails of the phospholipids are hydrophobic and form a hydrophobic core by facing each other. The
phospholipid heads are polar which makes them hydrophilic and they face the aqueous
environment.
The phospholipid tails are made of fatty acids these can be either saturated or unsaturated. If they
are unsaturated the membrane will be more fluid, because the fatty acids are bent and can't fit
closely together. Cholesterol is a lipid (like a phospholipid has hydrophilic and hydrophobic ends) it
helps regulate membrane fluidity. Cholesterol molecules are positioned between phospholipids in
the membrane. It works by the hydrophobic end interacting with the tails of the phospholipids, and
the hydrophilic end interacting with the heads of the phospholipids. This helps pull them together,
adding stability to the membrane. The cholesterol also ensures the membrane doesn't get too solid
by stopping the phospholipid molecules from grouping too closely and crystallising.
Membrane proteins play important roles in the various functions of membranes. There are 2 types
in the cell-surface membrane:

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Intrinsic proteins/integral proteins: they are embedded through both layers of a membrane. They
are able to stay in place due to the interaction of their amino acid hydrophobic R-groups with the
hydrophobic core of the membrane. 2 intrinsic proteins are:
Channel proteins: provide a hydrophilic channel allowing the passive movement (down the
concentration gradient) of polar molecules and ions through a membrane.
Carrier proteins: play a role in passive (down the concentration gradient) and active transport
(against the concentration gradient) into cells.…read more

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Because alcohol is less polar (and in some cases not polar at all) it can pass through the
membrane and their presence between the phospholipids disrupts the membrane. When
the membrane is disrupted it becomes more fluid and permeable. This disrupts the cell from
being able to carry out its function.
The movement of molecules across membranes, including:
Diffusion and facilitated diffusion as passive methods:
Passive movement uses energy from the natural motion of particles.…read more

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Carrier proteins are involved too, these change shape when a specific molecule binds to
them.
Rate of facilitated diffusion affected by: temperature, concentration gradient, membrane surface
area and thickness, the number of channel proteins present (more channel proteins = higher rate of
diffusion).
Active transport: Active transport is the movement of particles into and out of a cell from a
region of low concentration to a region of higher concentration.…read more

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The diffusion of water into a solution leads to an in the volume of said solution.…read more

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