cell membranes and transport Flashcards
components of cell membranes
- phospholipids
- protein (extrinsic, intrinsic)
- glycoproteins
- glycolipids
- cholestorol
structure of phospholipid bilayer
- inner layer = hydrophilic heads pointing towards the cell, touches cell cytoplasm
- outer layer = hydrophilic heads point outwards, touch water surrounding cell
- membrane centre = hydrophobic tails face each other
what does the phospholipid bilayer allow through
- lipid soluble molecules
- small un polar molecules
extrinsic proteins in phospholipid bilyer
- partially span membrane
- structural support
- form recognition sites to identify cells
- receptor sites of hormone attachment
intrinsic proteins in phospholipid bilayer
- span both layers
- carriers transport water soluble substances across
- allow AT of ions by forming channels
carrier or channel
function of glycoproteins and glycolipids in phospholipid bilayer
- hormone receptors
- cell-to-cell recognition
- cell adhesion
- in outer layer
function of cholesterol in phospholipid bilayer
- only in animal cells
- provide rigidity and stability
what is the glycocalyx on phospholipid bilayer
- extracellular surface of glycosylated proteins
- acts as a barrier
- provides protection
what is the fluid mosaic model
- fluid; individual phospholipids can move relative to each other
- mosaic; proteins embedded vary in shape, size, pattern
what is the phospholipid bilayer permeable to
- it is selectively permeable
- lipid soluble substances (Vit A) and dissolved molecules (O2, CO2) dissolve and move directly through
factors that increase the permeability of cell membranes
- temperature
- organic solvents
how does temp increase cell membrane permability
- above 40c
- increase vibrations of phospholipids
- they move further apart
how do organic solvents increase membrane permeability
- they dissolve phospholipids
what is the phospholipid bilayer not permeable to
- water soluble substances (ions, glucose, polar molecules) can’t pass through hydrophobic fatty acid tails
- must use intrinsic proteins
what is ficks law?
diffusion rate =
SA times difference in concentration
divided by
length of diffusion pathway
what is a channel protein?
- pores lined with polar (hydrophilic) groups
- allows charged ions through
- for diffusion and FD
what is a carrier protein?
- allows large, polar molecules through
e.g. water soluble, amino acids - molecule binds and the protein changes shape
- for FD, AT
- intrinsic
what is simple diffusion?
the movement of molecules from a region of high conc to low conc, down a conc gradient
- passive process (respiratory inhibitors have no effect)
- occurs through phospholipid bilayer
what is the diffusion rate increased by?
- higher conc gradient
- thinner membrane / shorter diffusion distance
- larger SA
- more pores in membrane
- smaller molecules
- being non-polar (fat-soluble)
- increased temperature
what is facilitated diffusion
process of diffusion for polar molecules that can’t directly pass through the phospholipid bilayer
- using protein channels / carriers
- eventually reaches a max rate due to limiting number of channels
- not affected by respiratory inhibitors
- passive
what is co-transport
- a type of FD where 2 different substances use the same carrier protein at the same time
stages of co-transport; when absorbing Na+ and glucose into blood and kidney nephron
- 1 glucose molecule, 2 Na+ bind to carrier protein, outside of cell
- protein changes shape. deposits them inside cell
- they diffuse separately through the cell
- reach other side. glucose enters blood by FD
- 2 Na+ carried out epithelial cell by AT. K+ ions move in on same carrier
- Na+ conc is always low in epithelial cell = brings more glucose in
(glucose moves against conc gradient)
what is active transport?
movement of molecules against conc gradient
requires energy from ATP from respiration
role of ATP in AT
ATP activates carrier proteins to move molecule across the cell membrane
- used to transport molecules across
evidence that AT and respiration are linked
- AT relies on ATP
- respiratory inhibitors (cyanide) or a lack of O2, prevents transport as no ATP is available
types of bulk transport
- endocytosis; phagocytosis, pinocytosis
- exocytosis (secretion)
what is endocytosis?
the active process of the cell membrane using extensions and cytoplasm to engulf material by putting it into vesicles
process of phagocytosis
- the uptake of solid material
- engulfed, packed into vesicle
- lysosome fuses with vesicles, enzymes digest the cell
- products absorbed into cytoplasm
process of pinocytosis
- uptake of liquids
- cell membrane folds inwards, surrounding liquid droplet
- engulfs material
- fuses together forming a vesicle
- vesicle enters cell
why does bulk transport require ATP
energy is needed for the cell membrane to move
process of exocytosis
- vesicle from golgi body moves to cell membrane
- vesicle fuses with cell membrane
- empties contents, releasing molecules
what is osmosis
diffusion of water, from a region of high water potential to low water potential across a selectively permeable membrane
what is water potential
the tendency of water molecules to move
- solute potential + pressure potential
- measure in kPa (kiloPascals)
- always negative
- 0 in pure water
- becomes more negative as conc of solution increases
what is solute potential
the osmotic strength of a solution
- always negative (or 0 - pure water)
- when more solute is dissolved, there are fewer free water molecules so their KE reduces
what is pressure potential
the outward pushing force of water in a vacuole against a cell wall
- positive or negative
dilute solution meaning
higher water potential
less solute dissolved
concentrated solution meaning
lower water potential
- water weakly bonded to solute = fewer free to move = lower KE
what are turgid cells (plants)
in hypotonic solutions (fewer solutes / dilute) cells take up water by osmosis
- pressure potential increases as cytoplasm pushes on cell wall
swollen
- provides support = maintains shape
what is incipient plasmolysis
cell has lost enough water for the cell membrane to begin drawing away from the cell wall
- pressure potential is 0
what are plasmolysed cells?
cells in hypertonic (more concentrated) solutions become flaccid, cytoplasm/cell membrane retract from cell wall
water leaves the cell by osmosis
causes plants to wilt
what solution must animal cells be in and why
isotonic
they lack a cell wall
animal cells in a hypotonic solution
they burst
haemolysis occurs
e.g. in distilled water
animal cells in hypertonic solution
they shrink
become crenated
e.g. salt solution
what is tonicity?
the concentration of solutes
hypertonic solution meaning
higher concentration of solute than cell
lower water potential
hypotonic solution meaning
lower concentration of solute than cell
higher water potential
isotonic solution meaning
same concentration of solute
same water potential
no net movement of water between solutions
describe how secretory vesicles are produced and how digestive enzymes are secreted from the cell. (3)
- golgi body packages enzymes into vesicles
- vesicles move to cell membrane and fuse together
- release contents outside cell by exocytosis
how is the concentration gradient of sodium ions maintained while allowing glucose in? (4)
- sodium-potassium pump transports Na+ out of epithelial cell into bloodstream
- by active transport
- reduces conc. of Na+ in cytoplasm of epithelial cell
- high Na+ conc in lumen from dietary sources
- co-transport of glucose with Na+ ions from lumen of small intestine
