membranes
what are the roles of the plasma membrane?
1. selectively permeable barrier
2. keeps cell contents together
3. cell to cell communication
4. allows creation of concentration gradients
5. site of various chemical reactions
what is the role of internal membranes?
1. compartmentalisation
2. allows creation of concentration gradients
3. site of various chemical reactions
plasma membrane (outside of cell)
mainly made up of phospholipids - hydrophilic head and hydrophobic tail
they form bilayers
these structures allow the tails to be away from water
the cell membrane is a phospholipid bilayer
membrane is about 7nm wide
phospholipid bilayer structure
• phosphate head (hydrophilic)
|
_ glycerol
'' 2 ester bonds
|| 2 fatty acid tails
why phospholipids form a bilayer in plasma membranes?
phosphate heads are hydrophilic so face outwards fatty acid tails face inwards because they're hydrophobic therefore they form a bilayer
fluid mosaic model
fluid: all the phospholipids and proteins are free to move
mosaic: irregular scattered pattern of proteins
components of the plasma membrane
phospholipid bilayer - selectively permeable barrier
cholesterol - binds the phospholipids together to regulate fluidity
glycoproteins - act as receptors and allow cell to cell communication
integral proteins - act as channels allowing larger or charged molecules in or out of the membrane
peripheral proteins - control cell shape, mobility or enzyme action
factors affecting membrane permeability
temperature - an increased temp increase
solvents - increase
unsaturated fatty acids - increase
how can polar and non-polar molecules through the membrane?
polar/charged molecules: cannot pass directly through the bilayer bc they are repelled by the hydrophobic fatty acid tails (glucose- too big, animo acids. Na+, Cl-)
^these molecules pass through protein channels instead (diffusion through these channels is facilitated diffusion)
non-polar molecules: are lipid soluble so can pass directly through the bilayer (oestrogen, testosterone)
saturated fatty acids
•no c=c bonds
•no kinks
•phospholipids packed tightly -less fluid
•no gaps
•reduces permeability as the phospholipids don't move around as much
unsaturated fatty acids
•contains c=c
•kinks
•pack less tightly together -more fluid
•creates gaps
•increases permeability as the phospholipids can move around more
lower temp on permeability
•less kinetic energy
•less fluid
•less permeability
higher temp on permeability
•more kinetic energy
•more fluid
•phospholipids move more
•creates gaps
•more permeability
effect of temp on permeability (beetroot)
membrane becomes more fluid,
more pigment is released as temp increases and phospholipid bilayer becomes progressively more dirupted
increase in KE make fatty acid tails less rigid
increase permeability
effect of solvents on permeability
increase fluidity and lipids dissolve
diffusion
the net movement of molecules from a high conc to a low conc through a partially permeable membrane
types of molecules that pass directly through bilayer
h2o, o2, co2, lipid soluble molecules
what affect rate of diffusion?
1. steepness of ghe conc gradient
2. temp
3. surface area
4. type of molecule
facilitated diffusion
transport of substances across a membrane by a trans-membrane protein molecule
passive - no metabolic energy required (ATP)
down a concentration gradient
molecules pass through protein channels by FD: glucose, amino acids, Na+, Cl-
channel + carrier proteins
channel: allows charged substances to diffuse across membranes
carrier: have a binding site for a specific solute
active transport
pumping of substances acriss a membrane by a trans-membrane protein pump molecule
active - requires metabolic energy (ATP)
against the concentration gradient
endocytosis and exocytosis
active
protease, insulin, food
endo- enter cell
exo- exit cell
cytoskeleton is needed
energy is needed
water potential
tendency of water molecules to diffuse from one place to another
kPa
osmosis
the net movement of water from a high water potential to a low water potential across a partially permeable membrane
hypotonic
a region of
•higher water potential
•lower solute conc
•water moves into cell
hypertonic
a region of
•lower water potential
•higher solute conc
•water move out of cell
isotonic
a region where there are equal water potentials on either side of a membrane
cells placed in distilled water
cells swell and burst
cells placed in concentrated salt solution
cells shrink and shrivel
plants cell placed in distilled water
cell stiffens but generally retains shape and becomes turgid
plant cell placed in concentrated salt solution
cell body shrinkd and pulls away from cell wall, becomes flaccid/plasmolysed
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