B4 Flashcards
role of plasma membrane as cell surface membrane
- barrier between cytoplasm and environment
- allows different conditions inside/ outside cell—-> COMPARTMENTALISATION
- controls movement of substances inside/outside cell
phospholipid bilayer function
- allow lipid-soluble substances to enter/leave
- prevent water soluble substances entering/leaving
- make the membrane flexible and self sealing
allows:
- small
- non polar
- lipid soluble
parts of cell surface membrane
*phospholipid bilayer
- allow lipid-soluble substances to enter/leave
- prevent water soluble substances entering/leaving
- make the membrane flexible and self sealing
allows:
- small
- non polar
- lipid soluble
*proteins
peripheral:
- surface of bilayer
- mechanical support
- glycoproteins + glycolipids = cell receptors
integral:
- protein channels: water filled tubes that allow water soluble ions to diffuse across membrane
- carrier proteins: bind to ions/molecules and change shape to move the protein across the membrane
*cholesterol
- within phospholipid bilayer
- adds strength and stability to cell membrane
- very hydrophobic, preventing ion and water loss from cell
- pulls together fatty acid tails, limiting lateral movement of molecules
- makes membrane less fluid at higher temperaturs
- prevents leakage of water and dissolved ions from cell
*glycolipids
- when a carbohydrate covalently bonds to a lipid
- the carbo portion extends into the environent, where it acts as a CELL SURFACE RECEPTOR
- RECOGNITION SITES
- MAINTAINS MEMBRANE STABILITY
- HELPS CELL ADHESION TO FORM TISSUES
*glycoproteins
- carbohydrate chains attached to peripheral proteins on outer membrane surface
- act as CELL SURFACE RECEPTORS, more specifically for HORMONES and NEUROTRANSMITTERS.
- RECOGNITION SITES
- HELPS CELL ADHESION
- CELL-CELL SIGNALLING e.g. lymphocytes recognising an organisms own cells
why do most molecules not freely diffuse across the cell membrane
not lipid soluble
too large
of the same charge as the protein channels so are repelled
electrically charged (polar) so cannot pass through the non polar hydrophobic tails in the bilayer
fluid mosaic model
fluid: lipids and membrane proteins move laterally throughout membrane. the individual phospholipid molecules can move relative to each other. this gives the membrane a flexible structure that is constantly changing shape
mosaic: membrane is made of many different components. the proteins embedded vary in shape, size and pattern
functions of membranes within cells
- control the entry/ exit of materials in discreet organelles e.g. mitochondria, chloroplasts
- separates organelles from cytoplasm so that specific metabolic reactions can take place within them
- provide an internal transport system e.g. ER
- isolate enzymes that may damage the cell e.g. lysozymes
- provide surface area on which reactions can occur e.g. protein synthesis on ribosomes on RER
to increase the movement of substances across the cell membrane:
*folded membrane
–> microvilli
INCREASE SURFACE AREA
*increase density of protein carriers/channels
–> MORE diffusion/ active transport
(CANNOT MAKE MEMBRANE THINNER)
*more mitochondria
–> provide more ATP to release energy for active transport
*increase concentration gradient
diffusion
the net movement of molecules/ ions from a region where they are more highly concentrated to one where their concentration is lower UNTIL EVENLY DISTRIBUTED
protein channels
water filled, hydrophobic channels
allow specific water soluble ions to pass through
selectively permeable to ions with opposite charges (if same - repelled)
remain closed if ion isnt present to control entry/exit
- ions bind to protein channel, changing its shape
open on one side, closed on the other
carrier proteins
when a molecule specific to the protein is present, binds to protein
- the protein changes shape, releasing the molecule to the inside of the membrane
- no external energy is needed
- molecules move high–> low concentration
osmosis
the passage of water from a region where it has a higher water potential to one where it has a lower water potential through a SELECTIVELY PERMEABLE MEMBRANE.
active transport
the movement of molecules or ions into or out of a cell from a region of low to high concentration using ATP and CARRIER PROTEINS.
active vs passive transport
requires metabolic energy from ATP
moving substances against concentration gradient
carrier proteins which act as ‘pumps’ are involved
selective process
the process of direct active transport of a single ion/ molecule
- carrier proteins span the plasma membrane
- they bind to the molecule to be transported
- on the receptor site of the protein
- on the inside of the cell/organelle, ATP binds to the protein, causing it to split into ADP and Pi, releasing energy
- the protein molecule changes shape and opens on the other side of the membrane
- the molecule is released on that side
- the Pi is released from the protein, causing it to revert to its original shape
–> it recombines with ADP –> ATP in respiration
in the Na K pump…
sodium ions are actively removed from the cell
potassium ions are actively taken in
co-transport of sodium ions and glucose
- sodium ions are actively transported out of the epithelial cells, by the sodium potassium pump, into the blood
- this active transport maintains a higher concentration of Na+ in the lumen of the intestine than inside the epithelial cells
- sodium ions diffuse into the epithelial cells down this concentration gradient through a co-transport carrier protein. they carry an amino acid/ glucose molecule with them
- the glucose/amino acids pass into the blood plasma by facilitated diffusion
both sodium ions and glucose molecules move into the cell, Na moves down conc gradient, gluclose moves against
it is the sodium ion concentration, rather than ATP directly, that powers the movement of glucose and amino acids into the cells.
this makes it an indirect rather than direct form of active transport
role of ATP hydrolase
converts ATP –> ADP + Pi
and releases energy
–> this energy allows ions to be moved against a concentration gradient in active transport.
outline the ways in which substances move passively across membranes (5)
- diffusion
- pore/ channel proteins for facilitated diffusion
- movement from high to low concentration / down conc gradient
- membrane must be permeable to substance diffusing
- oxygen/ other example can diffuse through membranes
- from a region of lower to a region of higher solute concentration
- membranes are nearly always free permeable to water
explain the role of vesicles in transportation of materials within cells
- vesicles are membrane bound packages
- formed by budding off a piece from a membrane
- can carry proteins
- rough ER synthesizes proteins
- proteins enter ER
- transported to golgi apparatus for processing
- transported to specific cellular organelles
- fuse with membrane of organelle so contents of vesicle joins organelle
- transported to plasma membrane
- fuses with plasma membrane
- fuses with plasma membrane, releases contents
- exocytosis
outline, with an example, the process of mitosis
- vesicles carry material to plasma membrane
- vesicle fuses with membrane
- by joining of phospholipid bilayers
- aided by the fluidity of the membrane
- material released/ expelled from cell
- membrane flattens
- e.g. exocytosis of neurotransmitter
- in the presence of calcium neurotransmitter vesicles release their contents into the synapse
describe how the phospholipid bilayer allows diffusion of non-polar, lipid soluble substancesb
- the phospholipid bilayer prevents diffusion of large, polar, lipid-insoluble substances
- carrier proteins allow active transport and facilitated diffusion
- channel proteins allow facilitated diffusion
- the shape/ charge of the carrier/ channel protein determines which substances are able to move through it
explain the arrangement of phospholipids in the cell-surface membrane
- form a phospholipid bilayer
- hydrophobic tails point away from water
- hydrophillic heads point towards as they are attracted to water
- their arrangement is fluid- the phospholipids can move in relation to one another
name and describe five ways substances can move across the cell-surface membrane into a cell
- simple diffusion of small/non-polar molecules down a concentration gradient
- facilitated diffusion down a concentration gradient via protein carrier/ channel
- osmosis of water down a water potential gradient
- active transport against a concentration gradient via protein carrier using ATP
- co-transport of 2 different substances using a co-transport carrier protein
how does the movement of Na+ out of the cell allow the absorbtion of glucose into the cell lining the ileum
- maintains a concentration gradient for Na+ from ileum into cell
- Na+ moves into the cell by facilitated diffusion
–> brings glucose with it
by co-transport
describe how the movement of substances across cell membranes is affected by membrane structure
- phospholipid bilayer allows movement of non-polar, lipid-soluble moleculees
- phospholipid bilayer prevents movement of polar, lipid insoluble substances
- carrier proteins allow active transport
- channel/ carrier proteins allow facilitated diffusion
- shape/ charge of carrier/ channel determines which substances move
- number of channels/ carriers determines how much movement
- membrane surface area determines how much movement
- cholesterol affects fluidity
compare and contrast the process by which water and inorganic ions enter cells
- both move down concentration gradient
- both move through protein channels in the membrane
- ions can move against a concentration gradient by active transport
