2.5 Biological Membranes

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2.5 Biological Membranes
Plasma membranes are partially permeable as they allow some but not all substances to pass
through them.
Very small molecules diffuse through the plasma membrane
Some substances dissolve in the lipid layer to pass through
Larger substances pass through protein channels or are carried by carrier proteins
Roles of the plasma membrane
Separates the cell's contents from the external environment
Regulates transport of materials into and out of the cell
May contain specific enzymes involved in metabolic pathways
Contains antigens so that the immune system can recognise the cell as being self and not
attack it
May release chemical signals to other cells and contains receptors for cell communication
and signalling (hormone bind to membrane bound receptors)
May be the site of chemical reactions
Roles of membranes within cells include:
The cristae of mitochondria which provides a large surface area for aerobic respiration
The thylakoid of chloroplasts which house chlorophyll and are the site of photosynthesis
The plasma membrane of the epithelial cells of the small intestine which contain digestive
enzymes that breakdown certain sugars
Fluid mosaic model ­ theory of cell membrane structure with proteins embedded in a sea of
Channel proteins ­ allow ions to mass through
Carrier proteins ­ allow specific molecules across the membrane
Glycolipid ­ lipid/phospholipid with a carbohydrate chain
Glycoprotein ­ protein with a carbohydrate chain
Others include: Enzymes, antigens & receptor sites for hormones
Cholesterol ­ regulates fluidity and gives mechanical stability and resists the effect of
temperature changes on the membrane
Glycocalyx ­ the hydrophilic area just outside the cell consisting of carbohydrate chains
attached to both lipids and proteins
Neuron cell membranes

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Protein channels and carriers covering the long axon allow the transport of ions to bring
the conduction of electrical impulses along their length
They have a myelin sheath of flattened cells around them several times to give more
membrane layers and to insulate the electrical impulses
Root hair cell membranes
They have many carrier proteins which
transport nitrate ions from the soil into the
cell as part of the nitrogen cycle.…read more

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Passive processes only use the kinetic energy of the molecules, not ATP.
When molecules move down their concentration gradient they are still moving randomly but
remain evenly dispersed which is called net diffusion. They have reach equilibrium.…read more

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Pure water has the highest water potential of 0kPa
Solute molecules lower the water potential
Water molecules move from a high water potential to a low water potential
When water potential is equal on both sides there will be no net movement of osmosis
Water with solutes has negative water potential values
Active transport
Active transport ­ the movement of substances against their concentration gradient across a cell
membrane requiring ATP
Endocytosis bulk transport of molecules too large to pass through a cell membranes into…read more

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Carrier proteins
ATP allows some carrier proteins to change their conformation to carry the molecule from one
side of a gradient to another
Bulk transport
Pinocytosis ­ cells ingesting liquids
Phagocytosis ­ cells ingesting solid matter (e.g. WBC ingesting a bacteria)
1. A membrane bound vesicle, containing substance to be secreted, is moved towards the
cell surface
2. The vesicle fuses to the cell membrane
3.…read more

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Factors affecting membranes
Increased temperature
Molecules have more kinetic energy and therefore move faster
Membrane fluidity increases as phospholipids have more kinetic energy
Permeability increases
The tertiary structure of the proteins may be compromised both on the plasma membrane
and the cytoskeleton threads beneath the membrane surface which may impede their
functions (e.g.…read more


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