Page 3 - Microsoft Word - 4 Cell Membranes and transport.docx
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Function of transmembrane proteins a) If the signalling molecules are hydrophobic
• act as gateways and can transform, helping in (e.g., steroid hormones such as oestrogen)
facilitated diffusion and active transport • they can diffuse directly across the cell membrane
and bind to receptors in the cytoplasm or nucleus.
b) If the signalling molecule is water-soluble
1) signal arrives at protein receptor in cell membrane
2) the receptor’s shape is complementary to the ligand
3) the signal brings about a change in the
receptor’s shape
4) changing the shape of the receptor allows it to
interact with the next component of the pathway so
the message gets transmitted
5) binding triggers/stimulates reactions within the cell
6) cell signalling results in a response which may
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be intracellular or extracellular
Channel proteins
• do not require energy
• transport substances through membrane passively,
along their concentration gradient
• used for both active transport and facilitated diffusion
Carrier proteins
• require energy
• go against the concentration gradient Image: https://croteaubio.wordpress.com/
• take substances from outside and pumps it inside or
vice versa 4.2 Movement of substances
• used for active transport into and out of cells
Cell surface receptors a) Diffusion
• present in membranes and binds with particular > Net movement of molecules or ions from a region of
substances higher concentration to a region of lower
• used for signalling, endocytosis, cell adhesion, concentration down a gradient, as the result of the
cell markers random movement of particles.
• passive process
Cell surface antigen • molecules tend to reach an equilibrium situation
• acts as cell identifying markers
• each type of cell has its own antigen Factors affecting diffusion
• as steepness of gradient increases,
• this enables cells to recognise other cells and diffusion increases
behave in an organised way
• as temperature increases, diffusion increases
Cell signalling • as surface area increases, diffusion increases
• as distance increases, diffusion decreases
• cells detect signals with cell receptors, i.e.,
glycoproteins and glycolipids, present on their • smaller and non-polar molecules like fats diffuse
membrane much easily across the cell surface membrane as
• the signalling molecule binds to the receptor as their they’re soluble in phospholipid tails
shapes are complementary to each other
• this creates a chain of reactions in the cell, leading b) Facilitated diffusion
to a response > Diffusion of a substance through transport proteins in
a cell surface membrane.
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