membrane structure and function

Question 1

cellular membranes

Answer 1

• fluid mosaic of lipids and proteins
• phospholipids are amphipathic
• proteins

Question 2

What does the fluid mosaic model state?

Answer 2

stated that a membrane is a fluid structure with a mosaic of various proteins embedded in or attached to a bilayer of phospholipids

• proteins are not randomly distributed in the membrane, they often form groups that carry out common functions

Question 3

Proteins are amphipathic
how are they arranged in the membrane?

Answer 3

• hydrophilic regions are oriented outwards towards the extracellular fluid or towards the cytosol inside and outside the membrane
• hydrophobic regions are embedded in the bilayer

Question 4

Davson-Danielli Sandwich model

Answer 4

1935
davson and danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

-> later found problems with this model
- with the generalisation that all membranes are the same
- with the placement of proteins, which are amphipathic

Question 5

Mosaic model

Answer 5

1972
Singer and Nicolson
- membrane proteins are dispersed and individually inserted into the bilayer
- hydrophillic sticking out, hydrophobic inside the bilayer

-> maximises the contact of hydrophilic regions of proteins and phospholipids with water in the cytosol and extracellular fluid and provides the hydrophobic parts with non-aqueous solution

Question 6

Freeze- Fracture Studies

Answer 6

• specialised preparation technique that splits a membrane along the middle of the phospholipid bilayer
• studies of the plasma membrane supported the fluid mosaic model

1. cell is frozen and fractured with a knife
2. fracture plane often follows the hydrophobic interior, splitting the phospholipid bilayer into 2 separated layers
3. membrane proteins go entirely with one of the layers

Question 7

What is the fluidity of membranes?

Answer 7

• membranes are held together mainly by weak hydrophobic interactions
• membranes are not static sheets -> most lipids and some proteins drift laterally
• rarely does a molecule flip-flop transversely across the membrane
• phospholipids move much much faster laterally than proteins, because proteins are much larger than lipids
• proteins move in a highly directed manner
  – possibly driven along cytoskeletal fibres by motor proteins connected to cytoplasmatic region of proteins
• some proteins are immobile held in position via their interaction with cytoskeleton or ECM

Question 8

What happens to the membrane when the temp cools?

Answer 8

membranes switch from a fluid state to a solid state
- phospholipids settle into a closely packed arrangement

• temperature at which the membrane solidifies depends on the type of lipids

-> saturated fatty acids are less fluid than the unsaturated fatty acids

the type of hydrocarbon tails in phospholipids affect the fluidity of the membrane

Question 9

cholesterol in membranes

Answer 9

steroid cholesterol is wedged between the phospholipid molecules
- warm temp: eg 37°
– holds back movement of phospholipids making the membrane less fluid

• cold temp:
  – maintains fluidity by preventing tight packaging

-> cholesterol works as a temperature fluidity buffer by having different effects on the fluidity at different temps

plants use different but related steroid lipids to buffer membrane fluidity

Question 10

Why is it important that membranes are fluid?

Answer 10

fluidity affects the permeability and movement of transport proteins

• if too fluid: cant support protein functions
• organisms living in extreme temps have adaptive differences in membrane lipid composition

Question 11

What are the types of membrane proteins?

Answer 11

Proteins determine most of the membrane’s specific functions

• integral protein: penetrate the hydrophobic core
  – transmembrane proteins: integral proteins that span the membrane
  – hydrophobic regions of integral proteins consists of non polar amino acids, coiled into alpha helices
  hydrophilic region is exposed to aqueous solution
• peripheral protein: bound to the surface of the membrane
  – interact with polar surfaces of the membrane or with proteins embedded in the membrane

Question 12

What are the functions of the membrane proteins?

Answer 12

• transport
• enzymatic activity
• signal transduction
• cell-cell recognition
• intercellular joining
• attachment to cytoskeleton and ECM

Question 13

HIV and cell surface proteins

Answer 13

HIV virus binds to cell-surface protein CD4 and co-receptor CCR5
-> lacking CCR5 are immune to HIV infection
-> drugs masking CCR5 block HIV entrance in nonimmune individuals

Question 14

how do cells recognise each other?

Answer 14

by binding to surface molecules, often carbohydrates on plasma membrane

• membrane carbohydrates may be covalently bonded to
  – lipids to form glycolipids
  – proteins to form glycoproteins (more common)
• carbohydrates vary among species, individuals and cell types of an individual

Question 15

What is the process of synthesis and sidedness of membranes?

Answer 15

1. synthesis of membrane proteins and lipids in ER
2. carbohydrate modification and glycolipid production in Golgi
3. Transport to plasma membrane in vesicles
4. fusion with plasma membrane

Question 16

Permeability of the lipid bilayer

Answer 16

• nonpolar molecules are hydrophobic -> can dissolve in the bilayer and pass through
  – hydrocarbons, CO2, O2
• polar molecules are hydrophilic -> cant cross easily
  – ions, even water
  – transport proteins needed

Question 17

What kind of transport proteins exist?

Answer 17

• channel proteins: have a hydrophilic channel that certain molecules or ions can use
• aquaporins: special transport proteins for water
  – up to 3mio molecules per second can pass through one
• carrier proteins: bind to molecules and change shape to shuttle them across the membrane
  – most are extremely specific

-> allow passage of hydrophillic substances across the membrane

Question 18

movement across the membrane

Answer 18

• small particles use
  – passive transport : diffusion, osmosis, facilitated diffusion
  – active transport
• bulk transport use
  – endocytosis: phagocytosis, pinocytosis, receptor mediated endocytosis
  – exocytosis

Question 19

What is passive transport

Answer 19

• no need of energy
• down a concentration gradient

Question 20

what is active transport

Answer 20

requires ATP
- against the concentration gradient - produces a gradient

Question 21

what is diffusion

Answer 21

tendency of molecules to spread out evenly into the available space

although each molecule moves randomly, diffusion of a propulation of molecules may exhibit a net movement in one direction

• substances diffuse down the concentration gradient
  each substance moves down its own conc gradient, unaffected by the others conc
• until a dynamic equilibrium is achieved

Question 22

what is osmosis

Answer 22

• diffusion of free water across a selectively permeable membrane
• move to the higher solute concentration –> osmosis is affected by the conc gradient of dissolved substances
• water keeps moving until the solute concentration is equal on both sides

Question 23

What is tonicity

Answer 23

tha ability of a surrounding solution to cause a cell to gain or lose water

• depends on the conc of solutes that cannot penetrate the membrane
• has a great impact on cells without walls

Question 24

isotonic

Answer 24

solute conc is the same as inside the cell
- no net water movement
- volume of the cell is stable

Question 25

hypertonic

Answer 25

solute conc is greater than inside the cell
- cell loses water, net diffusion of water, cell structure is shriveled

Question 26

hypotonic

Answer 26

solute conc is less than inside the cell
- cell gains water , cell structure will burst

Question 27

what is osmoregulation

Answer 27

control of water balance

necessary adaptation for life
eg protist paramecium has a conctractile vacuole that acts as a pump
-> it is hypertonic to its pond water environment (pond water is hypotonic)

Question 28

cell wall and water balance

Answer 28

• cell wall helps maintain water balance
• plant cell in hypotonic solution swells until the wall oposes its uptake -> cell is turgid
• isotonic -> no net movement of water -> cell is flaccid
• hypertonic -> cell shriveles and membrane pulls away from cell wall -> plasmolysis

Question 29

facilitated diffusion

Answer 29

• hydrophilic and larger molecules and ions
• transport proteins speed the movement of molecules across the plasma membrane
• very specific
• no energy spent
• movement is down the conc gradient
• channel proteins : allow a specific molecule or ion to cross
  – aquaporins and ion channels (gated channels -> open / close in response to stimulus)
• carrier proteins: undergo a conformation change

Question 30

active transport

Answer 30

by carrier proteins
- energy changes the shape of the carrier proteins
- allows cells to maintain conc gradients that differ from their surroundings

• sodium/potassium pump
  – creates and maintains the opposite conc gradients of Na and K across the membrane
  – Na stimulated the phosphorylation of ATP -> causes conformational changes, Na is released outside

Question 31

ion pumps

Answer 31

• helps with membrane potential = voltage difference across the membrane
• voltage created by differences in distribution of ions
• cytoplasm of cell is negatively charged compared to the outside
  -> passive transport of cations into the cell
  -> passive transport of anions out of the cell

Question 32

electrochemical gradient

Answer 32

• combination of two forces driving the diffusion of an ion across the membrane:
  – chemical force : conc gradient
  – electrical force: effect of membrane potential on ion movement

-> an ion moves down its electrochemical gradient

Question 33

electrogenic pump

Answer 33

= transport protein that generates voltage across a membrane, storing energy that can be used for cellular work

• animals: sodium potassium pump
• plants, fungi, bacteria: proton pump -> actively transports H+ out of the cell

Question 34

Cotransport

Answer 34

• when active transport of a solute indirectly drives transport of another solute
• spends energy
• driven by conc gradient
• proton pump maintains higher H+ conc outside the cell (ATP used)
  -> H+ gradient stored as potential energy

-> sucrose H+ cotransporter drives the movement of sucrose by bringing H+ down its conc gradient

–> downhill diffusion of solute is coupled to uphill transport of second substance against its conc gradient

Question 35

Exocytosis

Answer 35

• release of substance from cell
• transport vesicle migrate to the membrane and fuse with it
• fusion with membrane causes release

Question 36

endocytosis

Answer 36

• cell takes in macromolecules by forming new vesicles from plasma membrane
• phagocytosis
• pinocytosis
• receptor mediated endocytosis

Question 37

phagocytosis

Answer 37

cellular eating

a cell engulfs a particle in a vacuole which fuses with a lysosome to digest the particle

• membrane receptors identify the particle and pseudopodia extend around creating a vacuole
• vacuole fuses with lysosome to digest

Question 38

pinocytosis

Answer 38

cellular drinking

molecules are taken up when extracellular fluid is gulped into tiny vesicles

• non specific in the substances it transports

Question 39

receptor mediated endocytosis

Answer 39

binding of ligands to receptors triggers vesicle formation

• specific endocytosis
• receptor selects targeted substances
• ligand= any molecule that binds specifically to a receptor site of another molecule
• endocytosis pulls only these substances into the cell
• receptors are recycled to the membrane by the same vesicle