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OCR AS Biology Module 1 unit 1
The phospholipid Bilayer and
Crossing membranes…read more

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Phospholipids Bi-layer
Fluid mosaic model refers to the model of cell membranes structure. The lipid molecules gives fluidity and the
proteins give the membranes a mosaic appearance.…read more

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Key definition; The phospholipid bi-layer is the basic structural component of plasma membranes. It
consists of two layers of phospholipid layers. Proteins are embedded in this layer.
The role of membranes
·Separating cell contents from outside environment
·Separating cell components from cytoplasm
·Cell recognition and signalling
·Holding the components of some metabolic pathways in place
·Regulating transport materials into and out of cells.
The phosphate head is hydrophilic- water loving
The two fatty acid tails are hydrophobic- water hating
Cholesterol gives the membranes mechanical stability. The steroid
molecule fits between the fatty acid tails and makes the barrier more
Glycolipids and Glycoproteins; act as receptor sites e.g. for hormones.
They are also involved in cell signalling that they are self in the
immune response. Glycoproteins can also bind cells together in
tissues. They are also important for allowing drugs to bind.
Muscle cell membranes contain a large number of channels to allow rapid uptake of glucose to provide energy
for muscle contraction
·The plasma membranes of white blood cells contain special proteins that enable the cells to recognise
foreign cells and organelles.
· Internal membranes of chloroplast contain chlorophyll
·Plasma membranes in a growing shoot contain receptors to detect molecules regulated growth.…read more

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Simple Diffusion
Key Definition; Diffusion is the movement of
molecules from a region of high concentration to
a region of lower concentration
The rate of diffusion can be effected by;
Temperature; Increased temperature gives molecules more kinetic energy, more random
movement occurs and so the rate of diffusion increases.
Concentration gradient; having more molecules on one side of the membrane increases
concentration gradient and so increases rate of diffusion
Surface area; greater membrane surface area = faster rate of diffusion e.g. biconcave red blood
cells, folds in epithelial cells, Alveoli in the lungs
Distance/thickness; diffusion is slowed down by thick membranes
Size of molecule; smaller molecules or ions diffuse more quickly than large ones.
Small molecules such as carbon dioxide and oxygen molecules are small enough to diffuse
through the phospholipids bi-layer down the concentration gradient. Steroid hormones are lipid
based and so can diffuse through membranes into cells.…read more

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Facilitated Diffusion- a passive process
Carrier Proteins
Channel Proteins These are shaped so
These form pores in that a specific
the membrane that molecule e.g.
are shaped to allow glucose can fit into
only one type of them. When the
ion through. Many specific molecules
are also "gated" fits the protein
which means they changes shape to
can be open or allow the molecule
closed to pass to the other
side of the
Substance moved by
Simple Diffusion Gases like oxygen and Carbon dioxide.
Lipid based molecules such as steroid
Facilitated diffusion using channel Ions sodium ions and calcium ions
Facilitated diffusions using carrier Larger molecules like glucose and
proteins amino acids…read more

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Osmosis is the movement of water from a
low water potential to a high water
potential across a partially permeable
When a plant A plant Cell in the plasma
cell is placed in cell with concentrated cell wall pulls
pure water the the same solution away from
plasma wall concentra becomes the cell wall.
pushes against tion flaccid. As The cell is
the cell wall outside as water plasmolysed
and prevents inside leaves...
the cell from
bursting it is
Pure water Dilute solution Concentrated Cells are Cells are
Lower water solution haemolysed crenated
Highest water
potential Very low water
potential 0kPA
-10kPA potential
Small amount -500kPA
No solute of solute Large amount of
dissolved dissolved solute dissolved…read more

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