Transport across membranes Flashcards
Roles of membranes?
- separating cell contents from outside world
- separating cell components from cytoplasm
- cell recognition and signalling
- holding components for some metabolic pathways
- regulating transport in/out
Components of membrane: Phospholipids?
- align as bilayer: hydrophilic heads attracted to water (cytoplasm & tissue fluid outside cell) & hydrophobic tails being repelled by water
- allows lipid-soluble molecules across but not water-soluble
- make membrane flexible & self-sealing
Components of membrane: Extrinsic proteins
- on either surface of bilayer
- provide mechanical support
- act as cell receptors for molecules e.g. hormones (like glycolipids)
Components of membrane: Intrinsic proteins
- extend across both layers of bilayer
1. channel proteins (water filled tubes so - allow water-soluble ions to dissolve & diffuse across) - form pores - open to to allow specific solutes e.g. ions Na+/Cl-
2. carrier proteins (bind to ions/glucose/amino acids -> change shape to move molecules across) - bind to specific solutes -> undergo conformational change -> transfer bound molecule from 1 side of membrane to other
Components of membrane: Cholesterol?
- restricts lateral movement of other molecules in membrane/makes stronger & more rigid - makes membrane less fluid at high temps so - (prevents gaps between phospholipids too large so - water & dissolved ions don’t leak out)
- very hydrophobic - prevents loss of water & dissolved ions from cell
Components of membrane: Glycoproteins & Glycolipids?
- acts as cell surface receptors for binding of molecules/cell-recognition
- help cells attach to one another to form tissues
- provide stability
Main functions of plasma membranes?
- forms boundary between 2 environments (aka compartmentalisation):
- allows diff conditions on either side of membranes e.g. pH/enzymes etc. so - diff reaction can occur in diff places - selectively permeable barrier so - controls movement of substances in & out
Selectively permeable?
- (due to thick hydrophobic portion of membrane) only certain molecules can diffuse through by simple diffusion across membrane…
- allows lipid-soluble (non-polar) substances & small molecules (e.g. water/O2/CO2) to pass thru
- prevents most water-soluble (polar)
Fluid mosaic model?
- Fluid - because the phospholipids & proteins can move freely
- Mosaic - because the embedded proteins randomly dotted throughout the membrane vary in shape, size and pattern just like a mosaic pattern (mix of diff components)
2 types of transport across membranes?
- Passive transport - doesn’t require any external energy
- Active transport - requires energy in form of ATP (immediate source for cells)
Diffusion definition & types?
The net movement of particles from a region of high concentration to a region of lower concentration until evenly distributed (DOWN a concentration gradient)
- passive
2 types:
- simple diffusion
- facilitated diffusion
Simple diffusion?
- particles simple diffuse across bilayer (thru gaps)
- lipid soluble molecules
- small molecules
Facilitated diffusion - protein channels?
- water-filled, hydrophilic pore/tube in centre - enables water-soluble ions to pass thru
- channels r selective - only open in presence of certain ions when they bind to protein (on binding site)
Facilitated diffusion - Carrier proteins?
- allow larger, water-soluble molecules to pass thru e.g. glucose/amino acids
- bind w a molecule complementary in shape to protein - changes shape in a way that releases molecule to other side
Factors affecting rate of diffusion?
*Temperature
*Concentration gradient
*Surface area
*Diffusion distance
*Size of molecule
*Stirring / moving of molecules
Active transport definition?
The movement of particles from a region of lower concentration to a region of higher concentration (AGAINST its concentration gradient) using ATP and carrier proteins.
- carrier proteins acts as pumps to move substances across
- selective - only certain molecules can bind to carrier proteins to be pumped
Active transport steps?
- molecule/ion to be transported binds to specific carrier proteins on one side of the membrane (at binding site)
- On the inside of the membrane - ATP binds to the carrier protein - causing it to hydrolyse into ADP + Pi (releases small energy)
- This causes carrier protein to change shape and open to the opposite side of the membrane (releasing the molecule/ion to the opp side).
- The inorganic phosphate is released from the carrier protein causing it to revert back to its original shape
- The process can then be repeated
Things that affect fluidity of membranes?
- the more unsaturated phospholipids present (more C=Cs) - the less tightly they pack tgt so - more fluid
- less cholesterol
- as temp increases - fluidity increases as more K.E.
Osmosis definition?
The passive movement (diffusion) of water from a region of higher water potential to a region of lower water potential, through a selectively permeable membrane.
Water potential?
- measure of how free water molecules r to move (potential to do osmosis)/pressure created by water molecules
- measured in units of pressure e.g. kPa
- pure water = 0 (all molecules r free moving)
- addition of solutes reduces water potential (to -ve value)
- water will move from potential closer to 0 to more -ve potential
Isotonic solution?
water potential is same in solution & in cell within solution
- animal cells: no net movement of water
- plant cells: no net movement of water aka incipient plasmolysis
Hypotonic solution?
when water potential of solution is more positive (closer to 0) than cell (conc. of solutes outside cell lower than inside)
- animal cells: water enters cell - may burst aka lysis
- plant cells: vacuoles fill with water - turgor pressure (chloroplasts next to cell wall)
Hypertonic solution?
when water potential of solution is more negative than cell (conc. solutes higher outside cell)
- animal cells: water leaves cell - shrivels aka crenation
- plant cells: vacuoles lose water, cytoplasm shrinks aka plasmolysis (chloroplasts seen in centre of cell)
Water potential equation?
water potential = pressure potential + solute potential
- solute potential (aka osmotic potential): as increase solute conc. water potential for water molecules to undergo osmosis decreases (more -ve)
- pressure potential: (when solution enclosed by cell wall e.g. plants/fungi) water moving into cell wall causes vacuole to swell & exert pressure back on cell wall (creating positive force on wall) - limiting further water uptake (so +ve value)
