cell physiology and membranes

  • Created by: rachael6
  • Created on: 07-01-21 15:17

membrane structure and function

membranes essential part of cells - control entry & exit of substances.

also surround organelles in cells.

in different organelles, membranes have varying functions & may be arranged differently or have varying constituents.

membranes contain 4 main components

  • phospholipids
  • proteins
  • cholesterol
  • glycocalyx
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phospholipid bilayer is basic structure of cell surface membrane - gives it much of selective permeability.

in aqueous environment, phospholipids arranged in bilayers so hydrophobic tails shielded from polar fluid.

membrane made of hydrophilic heads like mix with water but not lipid and 2 hydrophobic tails that mix with lipid not water.

heads always on outside and tails inside of membrane.

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protein molecules

scattered across membrane, floating in bilayer

prodive stability & support as help anchor phospholipid molecules

act as enzymes

many advantages to having enzymes in membrane, kept in ideal place in terms of substrate availablilty & pH & need replaced less often

act as adhesion sites - areas where adjacent cells held together 

can be involved in cell-cell recognition & act as antigens & receptors.  can attach peripherally to bilayer (extrinsic) or integrally embedded in 1 of 2 layers (intrinsic).  some intrinsic proteins (transmembrane) extend across bilayer

can be important in transporting substances across bilayer.  aid transport by acting as channel & carrier proteins.  eg of importance of transmembrane proteins in formation of cellulose cell wall - glucose needed, cant diffuse across bilayer so needs transmembrane proteins to create hydrophilic channels to allow glucose to pass across for formation of cellulose

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cholesterol molecules

lie between phospholipid tails

act as temp stability buffer & important in membrane fluidity 

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extending from outer phospholipid layer of membrane is glycocalyx

contains polysaccharides that bound to membrane proteins (glycoproteins) or to phospholipids (glycolipids)

carbohydrate component always on outside of phospholipid bilayer

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cell recognition and cell receptors

polysaccharides that bind to glycoproteins or glycolipids always found on outside of cell surface membrane & always peripheral to protein or phospholipid 

glycoproteins & glycolipids involved in cell to cell recognition, important in allowing cells of similar types to recognise each other & join to form tissues

some glycoproteins act as antigens.  can also act as receptor sites.  receptor sites provide sites on cell surface membrane that particular molecules fit.  can only occur because receptor sites & specific molecule/substrate concerned are complementary.  receptor sites important in hormone action & in passage of neurotransmitters between neurones.

as glycoproteins & glycolipids form hydrogen bonds with water molecules outside membrane, help stablise membrane 

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fluid mosaic model and membrane fluidity

structure of cell surface membrane described as fluid mosaic model.  phospholipid molecules 'flow' sideways although always keeping bilayer arranged, with protein molecules 'floating' between phospholipid molecules like mosaic tiles

number factors influence fluidity of membrane:

  • more phospholipids with unsaturated hydrocarbon chains, more fluid membrane.  'kinks' in unsaturated tails prevent them being packed together so more movement possible
  • longer hydrocarbon chains decrease fluidity since attractive forces among tails stronger
  • more fluid at higher temp & less as low temp as bilayer 'freezes' into gel state
  • cholesterol acts as temp stability buffer.  at high temp, cholesterol provides additional binding forces & decreases fluidity.  at low temp, cholesterol keeps membrane in fluid state by preventing phospholipids from packing too close & 'feezing'
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passages of substances across membrane

cell surface membrane acts as barrier between cytoplasm & extra cellular fluid, though exchange of substances takes place across it.  route taken & mode of transport depends on number of factors:

  • size of molecule - v. small slip between phospholipid molecules, larger particles only get in or out by cytosis.
  • polarity of substance - non polar (& lipid soluble) molecules move through bilayer; polar substances move through transmembrane proteins 
  • concentration of solution either side of membrane - substance move from high to low concentration by diffusion; if movement against gradient active transport used.  greater gradient, faster diffusion
  • temp - diffusion normally takes place quicker at high temp as more kinetic energy
  • thickness of exchange surface - membranes generally v. thin & therefore ideal or rapid diffusion 
  • surface area - greater, faster diffusion.  many cells where diffusion important, cell surface membrane extended to increase area where diffusion take place eg presence of microvilli

water exception even though polar, v. small so pass through

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transports non polar molecules like lipid soluble substances eg vitamin a and d.  steroid hormones, respiratory gases & water & urea because so small

passive process - no ATP but relies on kinetic energy of molecules

movement between phospholipid molecules in bilayer

random & result of random movements as particles bump into membranes

doesnt involve carrier or channel proteins

involves movement down concentration gradient until concentrations equal

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facilitated diffusion

involves intrinsic carrier & channel proteins

transports polar molecules & larger molecules 

polar molecules - diffuse through channel proteins down concentration gradient:  channel proteins span membrane & create hydrophilic channel that allows polar molecules to bypass hydrophobic center of bilayer.  channels may be permanently open or gated which helps regulate flow

larger polar molecules - use carrier proteins which usually carry 1 type of molecule:  carrier proteins have specific receptor/binding sites to which complimentary molecule initially binds - as protein changes shape molecule released on other side of membrane.  often refer to intrinsic proteins as protein pores

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active transport

transports larger molecules against concentration gradient

requires ATP from cellular respiration.  energy used to change protein shape & push ion/molecule into or out of cell

carrier proteins needed to do actual tranpsorting - often referred to as pumps.  each carrier protein specific to 1 type of ion/molecule 

cells which carry out active transport usually have large number of mitochondria 

oxygen supplied as ATP needed produced by respiration & repsiration needs oxygen.  oxygen availability affect rate of as active uptake energy requiring process.  ATP provided from repiration when more oxygen available allowing membrane proteins/carriers to operate at faster rate therefore increasing rate of active uptake

cyanide inhibitor of respiration.  inhibit/decrease rate as respiration not occur so no ATP.  temp affects rate.  respiration enzyme controlled reaction.  enzyme activity increase in increased temp due to increased kinetic energy

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difference between channel and carrier proteins

channels - span membrane & work by creating hydrophilic channels that allow polar molecules to pass by hydrophobic center of bilayer.  channels have specific shape & allow only 1 type ion through.  may be permanently open or have opening controlled (gated channels), this regulates flow.  some water may diffuse through bilayer (since small) but most diffuses through specific channel proteins calls aquaporins

carriers - may selectively transport larger sized molecules like glucose.  molecules bind to site on protein (receptor site) which changes shape to bring molecule through membrane 

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net movement of water from higher water potential (dilute solution, less negative) to lower water potential (more concentrated, more negative) across selectively permeable membrane

water can move through selectively permeable phospholipid membrane in osmosis but most if it moved through special channel proteins called aquaporins

better definition: net movement of water potential through selectively permeable membrane from a solution of less negative water potential to a solution of more negative water potential 

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hypertonic and hypotonic solutions

hypertonic solution - more concentrated (more solute, less water) than cell's contents 

hypotonic solution - less concentrated (less solute, more water) than cell's contents 

isotonic solution - same solute concentration as cells contents 

more dilute, less negative, higher water potential, hypotonic, low solute, high water vice versa

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concentration gradient

refers to concentration gradient either side of membrane 

normally substance move from high to low concentration, movement down a gradient 

during osmosis water molecules keeping moving to try & make concentration on both sides of membrane equal

difference between osmosis & simple diffusion - osmosis is movement of water only while diffusion is movement of any ion/molecules of liquid or gas.  osmosis through selectively permeable membrane while diffusion through any fluid medium

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osmosis and water potential

water potential of solution may be regarded as its tendency to take in water by osmosis from pure water across selectively permeable membrane 

measured in kilopascals (kPa)

pure water has water potential of 0 - unable to take in any more water by osmosis. 0 because all water molecules 'free' - not forming associations with other molecules

water potential is indication of free energy of water molecules.  in solutions some of water molecules not 'free' as form hydration shells around solutes.  presence of solutes also reduces ability of water molecules to diffuse throughout solution

solution always have negative water potential - always have some water in hydration shells

more concentrated solution, more negative water potential as more bound up in hydration shells & not free.  also increasing more likely to take in water by osmosis.  solution with water potential of -600 kPa take in less water than solution of -1000 kPa

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solute potential

potential of solution to take in water.  this potential may or may not be same as water potential.  potential relates to solute concentration only but tendency (water potential) affected by other factors like space available in cell.

eg, turgid cell still have potential to take in water, as still more concentrated then pure water, but because turgid may be unable to take in water, as no space.

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pressure potential

effect of pressure on solution.  plant cell thats's turgid exert considerable pressure on cell wall, whereas one that's not exert less pressure.  pressure influences ability of cell to take in or lose water by osmosis.  pressure potential usually positive (can be 0)

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water potential = solute potential + pressure potential

3 terms often written as variations of greek letter psi Ψ

water potential = Ψcell

solute potential = Ψs

pressure potential = Ψp

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osmosis in plant cells - turgid

plant cells rely on turgor for support.  herbaceous (non woody) plants almost totally reliant on turgot pressure.  cell wall has important role as strength limits expansion of cell membrane as water enters cell by osmosis.  opposing forces of cell membrane & cell wall create turgor

if plant cell in pure water, concentration gradient exist because there's higher concentration of water molecules in surrounding solution compared to cell - surrounding solution hypotonic 

water molecules diffuse into cytoplasm & vacuole of cell by osmosis as water molecules move down concentration gradient from higher water potential (less negative) where higher concentration in surrounding solution to lower water potential (more negative) where they're in lower concentration in cell through partially permeable membrane 

water enter cytoplasm & vacuole & cause cell to swell - cell gain mass.  cell membrane press against cell wall & cell wall resists & presses back on cell contents & cell become turgid.  cell wall prevents cell bursting & prevent too much water entering  cell

turgor important as provides support to keep plant upright

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osmosis in plant cells - plasmolysed

if cell in concentrated in salt solution concentration gradient exist because there's higher concentration of water molecules in cell compared to surrounding solution

water molecules diffuse out of cytoplasm & vacuole of cell by osmosis as water molecules move down concentration gradient from higher potential (less negative) where in higher concentration in cell to lower water potential (more negative) where in lower concentration in surrounding solution through partially permeable membrane 

water leave cytoplasm & vacuole - cell lose mass

cell shrinks slightly & become flaccid then membrane pulls away from cell wall & cell become plasmolysed

during plasmolysis so much water leaves cells that cell contents shrink & cell membrane gets pulled away from cell wall

wilitng common occurrence in many plants, plasmolysis less likely to happen in healthy plants

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plasmolysis pt2

incipient plasmolysis - when cell membrane just loses contact with cell wall - expect at points where adjacent protoplasts (plant cell minus wall)  joined by plasmodesmata 

in nature plasmolysis doesn't occur very often however following situations make more likely to occur:

  • plants growing in field with too much fertilizer
  • seed from woodland tree being carried to salt marsh & starting to germinate in this environment

starch (energy store in plants) osmotically inert.  this because starch insoluble so water doesn't attach to it so starch does therefore not influence water potential.  advantage to plants as water potential of cell less negative so wont draw water away from other areas

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osmosis in animal cells

pure water

cytoplasm in cell quite concentrated with proteins & other substances which are too big to get through cell membrane, however, water molecules can easily get through

water keep moving to inside of cells as other molecules cant diffuse through membrane pores to allow even balance on both sides.  cell membrane stretch & eventually burst (lysis)

concentrated (hypertonic) solution

if animal cells in hypertonic solution, cells lose water by osmosis, shrink & shrivel up - crennation (cells crennated).   in healthy cells blood & tissue fluid kept at correct water potential to ensure neither lysis or crennation take place

why dissolving more solutes into substance decreases its water potential; solutes dissolves & water molecules cluster around solute molecules forming hydration shells.  this reduces capacity of water molecules to move freely.  means solute potential decreases & water potential decreases

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transporting substances in bulk

some substances can be transported into or out of cell without having to pass through membrane itself.  process important in transporting:

  • large molecules too big for carriers
  • bulk transport of smaller molecules (eg water)

endocytosis is movement of substances into cell & exocytosis is movement of substances out of cell

in endocytosis cell surface membrane infolds around substances entering cell from outside to form membrane bound sac of vesicle, which then pinches off inside of cell surface membrane.  when vesicles taken into cell, fluid nature of cell surface membrane allows it to reform & close gap created by cytosis

2 types of endocyosis:

  • phagocytosis - (cell eating) involves transport of solid material into cell eg engulfing of bacteria by phagocytes
  • pinocytosis - (cell drinking) involves transport of fluid or solutes into cell
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transporting substances in bulk pt2

both involve endocytosis (formation of vesicles at cell surface membrane).  pinocytosis involves uptake of liquid or dissolved material whereas phagocytosis involves uptake of solid materials

exocytosis is movement of substances out of cell.  secretory vesicles (possibly having budded off from golgi apparatus) move to & and fuse with cell surface membrane.  contents of vesicles then released outside cell.  as with endocytosis, creation of gap in cell surface membrane followed by reforming of membrane

exocytosis important in secretion of many proteins from cells, including digestive enzymes & many hormones 

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