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Membrane transport

Topic 1
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4

Biology SL

Osmosis
➢ The passive transport of water molecules through a semipermeable membrane
○

A partially/semi/selective permeable membrane is one that allows certain
substances to pass through

➢ Water moves as a result of a difference between solute concentrations on either
side of a semi-permeable membrane
○

Solute: substances dissolved in, for example, water

➢ If a solution on one side has a high concentration of solutes, it is called hypertonic
or hyperosmotic
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Water will move towards it

➢ If a solution has a low concentration of solutes, then it is called hypotonic or

hypo-osmotic
○

Water will move away from it

➢ If both sides are of equal concentrations, the solution is isotonic
○

Water will stop moving, as the concentrations in both sides have reached an

equilibrium
Facilitated diffusion
➢ Possible due to some proteins
➢ Even though the proteins change shape in order to transport the substance, it
doesn’t require energy
➢ Specific substances can pass through specific proteins
➢ Molecules move until the concentrations are the same on both sides
...
A specific protein binds to 3 intracellular Na+
2
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ADP is created
■

ATP - 1 phosphate = ADP => phosphorylation

3
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ADP is released leaving a phosphate group attached
4
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4

Biology SL

5
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The restoration of its shape causes the release of K+ in the interior of the
cell
➢ Liver cells use active transport ot accumulate glucose molecules from blood
plasma, even though the liver has a higher glucose concentration than blood
Endocytosis
➢ Allows larger molecules to enter the cells
➢ Carbohydrates activate the process of endocytosis when they recognize a substance
which is needed or is harmful, in the case of white blood cells, for the cell/organism
➢ Process:
1
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Material goes in this portion, which later pinches off
3
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A vesicle comes from the golgi apparatus with a modified substance
2
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Its content is released to outside of the cell
➢ Examples:
○

Pancreatic cells produce insulin, which is transferred to the bloodstream

○

Neurotransmitters are released at synapses in the nervous system

➢ Endo- and exocytosis are also used for transportation of substances that could
enter or exit the cell in other ways
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When those substances are in big amounts

➢ They are possible, because of the hydrophobic and hydrophilic properties of the
phospholipids and the presence of water

3



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