2.5 Biological Membranes YOUR NOTES 2.5.1 THE CELL SURFACE MEMBRANE Roles of Membranes Membranes are vital structures found in all cells The cell surface membrane creates an enclosed sp ace separating the internal cell environment from the external environment Intracellular membranes (internal memb...
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YOUR NOTES
2.5 Biological Membranes ⬇
CONTENTS
2.5.1 The Cell Surface Membrane
2.5.2 Membrane Structure & Permeability
2.5.3 Diffusion & Facilitated Diffusion
2.5.4 Practical: Investigating the Rate of Diffusion
2.5.5 Active Transport
2.5.6 Endocytosis & Exocytosis
2.5.7 Osmosis
2.5.8 Osmosis in Animal & Plant Cells
2.5.9 Practical: Investigating Water Potential
2.5.1 THE CELL SURFACE MEMBRANE
Roles of Membranes
Membranes are vital structures found in all cells
The cell surface membrane creates an enclosed space separating the internal cell
environment from the external environment
Intracellular membranes (internal membranes) form compartments within the cell, such
as organelles (including the nucleus, mitochondria and RER) and vacuoles
Membranes not only separate different areas but also control the exchange of materials
passing through them; they are partially permeable
Membranes form partially permeable barriers between the cell and its environment,
between cytoplasm and organelles and also within organelles
Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active
transport
Membranes play a role in cell signalling by acting as an interface for communication
between cells
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YOUR NOTES
2.5 Biological Membranes ⬇
Exam Tip
An example of a membrane-bound organelle is the lysosome (found in animal cells), each
containing many hydrolytic enzymes that can break down many different kinds of
biomolecule. These enzymes need to be kept compartmentalised otherwise they would
breakdown most of the cellular components
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YOUR NOTES
2.5 Biological Membranes ⬇
The Fluid Mosaic Model of Membranes
The fluid mosaic model of membranes was first outlined in 1972 and it explains how
biological molecules are arranged to form cell membranes
The fluid mosaic model also helps to explain:
Passive and active movement between cells and their surroundings
Cell-to-cell interactions
Cell signalling
The fluid mosaic model describes cell membranes as ‘fluid’ because:
The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about
within it (a bit like icebergs in the sea) although some may be fixed in position
The fluid mosaic model describes cell membranes as ‘mosaics’ because:
The scattered pattern produced by the proteins within the phospholipid bilayer
looks somewhat like a mosaic when viewed from above
The fluid mosaic model of membranes includes four main components:
Phospholipids
Cholesterol
Glycoproteins and glycolipids
Transport proteins
Phospholipids
Phospholipids form the basic structure of the membrane (the phospholipid bilayer)
The tails form a hydrophobic core comprising the innermost part of both the outer and inner
layer of the membrane
Phospholipids bilayers act as a barrier to most water-soluble substances (the non-polar
fatty acid tails prevent polar molecules or ions from passing across the membrane)
This ensures water-soluble molecules such as sugars, amino acids and proteins
cannot leak out of the cell and unwanted water-soluble molecules cannot get in
Phospholipids can be chemically modified to act as signalling molecules by:
Moving within the bilayer to activate other molecules (eg. enzymes)
Being hydrolysed, which releases smaller water-soluble molecules that bind to specific
receptors in the cytoplasm
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