cell membranes and exchange of material hbs 3a. exchange of material occurs at the cell membrane...
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Cell membranes and exchange of material
HBS 3A
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Exchange of material
Occurs at the cell membrane
Involves the intake of essential materials eg nutrients and oxygen and the removal of unwanted materials eg wastes
The processes involved include diffusion, osmosis and active transport
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Structure of the cell membrane
Fluid mosaic model
Describe the roles of:• PhospholipidsWaterproof barrier, acts as a bubble, main
constituent
• Proteins Act as carriers, receptor sites, antigens,
channels
• CarbohydratesAct as receptor sites, antigens
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Exchange of material
Small particles eg water, ions can pass through the gaps between the phospholipids
Larger molecules pass through channels in the proteins or are carried by special proteins
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Movement of particles - diffusion
Occurs without energy
Particles will move from higher concentrations to lower concentrations
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Movement through membranes
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Diffusion through membranesMembranes are differentially permeablePermeable substances (eg small particles) move throughImpermeable substances (eg large particles) can’t move throughMovement is from areas of high concentration to areas of low
concentrationEnergy is not needed
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Facilitated diffusion
Allows larger or less permeable substances to cross
Involves channel proteins
Energy is not needed
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OsmosisMovement of waterThis occurs from areas of high water concentration (low
concentration of solute) to areas of low water concentration (high concentration of solute)
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Concentration gradient
This refers to the difference in concentration across membranes
The steeper the gradient,
the greater the movement
Osmotic pressure refers to the pull of water due to concentration gradients
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Active transport
Movement against a concentration gradient.Energy is needed
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Ion pumps
Involves special proteins that act as pumps
Allows cells to maintain ion concentrations that are very different from the external environment
Energy is used
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EndocytosisMovement due to folding of membrane and bringing in the
trapped substances Energy is needed Phagocytosis – solid substances trappedPinocytosis – liquids are trapped
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Exocytosis
Movement due to vacuoles moving to membrane and attaching to release contents
Energy is needed