B2.1.5: Biological Membranes - OCR A Biology A level A* student notes
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OCR A Biology H420 - 2.1.5 - Biological Membranes (Chapter 5) Short Summary Notes
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Biology A Level
Year 1
Module 2: Foundations in Biology
2.1.5: Biological Membranes
Cells have many membranes eg.
● Plasma membrane or ‘cell surface membrane’ - the cell membrane which separates the
cell from its external environment. Ranges in thickness from between 5-10nm.
● Tonoplast
● Outer mitochondrial membrane
● Inner mitochondrial membrane
● Outer chloroplast membrane
● Inner chloroplast membrane
- There are variations in the structure of membranes, mostly related to the proteins
they contain. The chloroplast and mitochondria are each surrounded by 2
membranes: the outer membrane has pores called ‘porins’ that allow small
molecules to pass through easily, the inner membrane contains many proteins
that perform important functions in photosynthesis (chloroplasts) and respiration
(mitochondria)
● Nuclear envelope
● Endoplasmic reticulum
The roles of membranes within cells and at the surface of cells
At the surface of cells:
● Partially permeable barriers (meaning some substances can travel freely through, and
some cannot penetrate) between the cell and its environment, between organelles and
the cytoplasm and as further internal membranes within organelles
- TMT membranes control what passes through them
- Compartmentalisation: formation of separate membrane-bound areas in a cell
● Sites of cell communication (cell signalling) and recognition by other cells
- Contain proteins that act as receptors for chemicals from other cells that can
cause specific changes in cellular activity
, ● Allow electrical signals to pass along them
- Concentration differences create a small electrical potential across the plasma
membrane
- Then, when conditions are right, specialized channels in the plasma membrane
open and allow rapid ion movement into or out of the cell, and this movement
creates an electrical signal
- Therefore nerve impulses can be transmitted
Within cells:
● Act as partially permeable barriers between organelles and the cytoplasm and as further
internal membranes within organelles
- Control which substances enter and leave the organelle
● Sites of chemical reactions
- Compartmentalisation is vital to a cell as metabolism includes many different
and often incompatible reactions
- Containing reactions in separate parts of the cell allows the specific conditions
required for cellular reactions to be maintained eg. chemical gradients
- Protects vital cell components
● Provide attachment sites for enzymes and other molecules involved in metabolism eg.
cristae in mitochondria
● Can form vesicles for transport systems
The fluid mosaic model of membrane structure and the roles of its components
Cell membrane theory
1. In 1925, Gorter and Grendel proposed that the ‘unit membrane’ is formed of a
phospholipid bilayer
2. Scientists also found that proteins were present in membranes, so in 1935 Davson and
Danielli proposed the ‘pauci-molecular theory’ which saw biological membranes as made
up of lipid bilayers that are coated on both sides with thin sheets of protein
3. In the 1950s the development and use of electron microscopes demonstrated that the
Davson-Danielli model was incorrect
4. Early in the 1970s, Singer and Nicholson used techniques eg. freeze-etching to confirm
the lipid bilayer. They also showed that proteins in the membrane are distributed
throughout the bilayer in a mosaic pattern, and that the membrane is fluid and has
, considerable sideways movement of molecules within it. Hence they proposed the
‘Fluid-Mosaic Model for Plasma Membrane structure’, which is the currently accepted
model!
The fluid-mosaic model
This model consists of the phospholipid bilayer with proteins varying in shape, size and position (in the
same way as the tiles of a mosaic) embedded within it. The bilayer is fluid - the phospholipids are
constantly moving because they’re free to move within the layer relative to each other. This gives the
membrane flexibility.
Components of the plasma membrane/fluid-mosaic model
PHOSPHOLIPIDS - the fundamental building blocks of all cell membranes are phospholipids
● Phospholipids are amphipathic molecules, meaning they have both hydrophilic and
hydrophobic parts
Structure of a phospholipid:
● Membranes are formed from a phospholipid bilayer: the hydrophilic phosphate heads of
the phospholipids form both the innermost and outermost surface of the membrane,
meaning that the fatty acid tails of the phospholipids form a hydrophobic core inside the
membrane
- Is the most stable structure for lipids to adopt because the outer surfaces of the
hydrophilic phosphate heads can interact with water - form hydrogen bonds,
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