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• A vesicle is a small sac of the membrane with a droplet of fluid inside
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• The vesicle is formed on the inside of the plasma membrane
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Vesicles taken in by endocytosis contain water and solutes, but also other molecules that cannot
pass through the membrane
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Protein is synthesised by ribosomes in the rER and accumulates inside it
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The vesicles fuse with the Golgi apparatus, which processes the protein into its final form
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Phospholipids are synthesised in the rER
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• Digestive enzymes are released from gland cells by exocytosis
• It can also be used to expel waste products or unwanted materials
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Particles move from an area of higher concentration to an area of lower concentration
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It doesn’t use any type of energy, it is a passive process
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Non-polar particles diffuse easily
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These channels are holes with a very narrow diameter
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The diameter and chemical properties of the channel ensure that only one type of particle passes
through
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Osmosis
• Osmosis is due to differences in the concentration of substances dissolved in water
• Regions with a higher solute concentration have a lower concentration of water molecules free to
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move
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Osmosis can happen in all cells because water molecules are small enough to pass through the
phospholipid bilayer
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Active transport
• Cells take substances even though there is already a higher concentration inside than outside
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• Sometimes cells pump substances outside the cell even though there is a higher concentration
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outside
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Carried by globular proteins in membranes, usually called pump proteins
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• An axon is part of a neurone cell and consist of a tubular membrane with cytoplasm inside
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A nerve impulse involves rapid movements of sodium and then potassium ions across the axon
membrane
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They occur because of concentration gradients built up by active transport inside and outside the
cell and are carried out by a sodium-potassium pump
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• Each time the pump goes round the cycle it uses one ATP
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The interior of the pump is open; 3 sodiums enter the pump and attach to the binding sites,
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Binding of potassium causes release of the phosphate group; this causes the pump to change
shape again so that is again open to the inside of the axon
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The interior of the pump opens to the inside of the axon and the 2 potassium ions are released;
sodium ions can enter and bind the pump again
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Potassium dissolves and becomes bonded to a shell of water molecules which make it to large to
pass through the pore
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After it passed the pore it bonds again with a shell of water molecules
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Voltage across membranes are due to an imbalance of
positive and negative charges across the membrane
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