Cell Membrane & Transport Across Membrane

Question 1

what are the five functions of biological membranes in cells?

Answer 1

definition of cell’s boundaries: separates interior of cell from surrounding environment, selectively permeable phospholipid bilayer allows only desirable substances entry
organisation and localisation: molecules or structures with specific functions embedded in membranes or localised within organelles (eg. hydrolytic enzymes in lysosomes), to organise and compartmentalise functions within cells
regulation of cell’s contents: proteins and other components help regulate transport of substances (to take up useful stuff, remove metabolic waste products, confine chemicals)
signal transduction: specific protein receptors on plasma membrane detect specific signals and triggering specific responses
cell-to-cell communication: membrane proteins for adhesion and communication btw adjacent cells

Question 2

what is the fluid mosaic model of the cell membrane?

Answer 2

the membrane is viewed as:
a mosaic or collage of proteins randomly distributed in or loosely attached to
a fluid phospholipid bilayer which is free to move about laterally

Question 3

what are the characteristics of the fluid mosaic model?

Answer 3

the fluid layer is asymmetrical, and may differ in composition and arrangement of proteins and lipids (phospholipids, glycolipids, cholesterol)
it is fluid or mobile, with possible lateral movement
unit membrane is a dynamic structure, with some embedded proteins moving freely due to weak hydrophobic interactions or fixed in positions by microfilaments on cytoplasmic face
membranes are amphipathic, made up of hydrophilic phosphate head and hydrophobic hydrocarbon tails (the hydrophobic core)

Question 4

how did they prove the mosaic structure of the phospholipid bilayer?

Answer 4

use of electron microscopy and freeze fracturing with a knife, and the fracture plane follows the hydrophobic interior of a membrane, splitting the bilayer while membrane proteins go wholly with one layer

Question 5

what is the structure of a phospholipid, and what role does it play in the cell membrane?

Answer 5

made of: a glycerol backbone with three hydroxyl (-OH) groups, two fatty acid chains, a negatively charged phosphate group, additional small charged molecules which may be linked to the phosphate group. amphipathic, since hydrophobic fatty acids as tail, hydrophilic head.
two layers of phospholipids form the phospholipid bilayer, so membrane has hydrophilic border and hydrophobic core (effective hydrophobic barrier against polar and charged solutes, around 8nm thick which is thicker than glycerol head)

Question 6

what is membrane fluidity, and what enables it?

Answer 6

the ability of phospholipid molecules to move within membrane
membrane’s phospholipid molecules are held together primarily by hydrophobic interactions between the hydrophobic fatty acid tails (weak interactions), so molecules can move freely and laterally (in rare cases, they can flip-flop)

Question 7

how does temperature affect membrane fluidity, and why?

Answer 7

directly related
at low temperatures, low KE and tighter packing of hydrocarbon chains, more hydrophobic interactions between phospholipid molecules, restricting their motion, semisolid
at high temperatures, KE and motion of hydrocarbon chains increases, more lateral movements of individual molecules to overcome hydrophobic interactions and increased space between molecules, fluid state

Question 8

how does the length of phospholipid fatty acid chains affect membrane fluidity, and why?

Answer 8

inversely related

the longer the hydrocarbon chains, the more hydrophobic interactions between chains, so the higher the melting point

Question 9

how does the degree of unsaturation affect membrane fluidity, and why?

Answer 9

directly related
as degree of unsaturation increases, there are less saturated lipids with long, straight hydrocarbon chains which allow for close packing and enhance membrane solidification
unsaturated lipids with kinks prevent the chains from packing closely together, enhancing membrane fluidity

Question 10

how does cholesterol affect membrane stability, membrane fluidity, and membrane permeability?

Answer 10

stability: increases, fluidity: dual effects, permeability: decreases
[stability = fluidity + permeability]

cholesterol molecules are intercalated into lipid monolayers, so its rigid steroid ring interferes with the motions of the hydrocarbon chains of phospholipids, enhancing mechanical stability

at high temperatures, cholesterol restrains phospholipid movements by interfering with hydrocarbon chain motion, decreasing fluidity
at low temperatures, cholesterol prevents hydrocarbon chains from packing closely, increasing fluidity

cholesterol fills in spaces between hydrocarbon chains, plugging transient gaps and decreasing membrane permeability

Question 11

compare the location, structure, solubility, and how they can be released from membranes for integral (intrinsic) and peripheral (extrinsic) proteins

Answer 11

location: integrals are deeply embedded (unilateral or transmembrane), peripheral are loosely bound (on both cytoplasmic and exterior sides)
structure: integral are amphipathic, held in place by extensive hydrophobic interactions with hydrocarbon tails of phospholipids. peripheral are rich in hydrophilic aa to interact with surrounding water and polar surface of bilayer

solubility in aqueous media: integral usually insoluble, peripheral usually soluble

integral released through detergents or non-polar solvents, peripheral released by mild treatment (eg. adjustment of ionic strength or pH)

Question 12

state the six different functions of membrane proteins

Answer 12

anchorage, transport, enzymatic activity, signal transduction, cell-to-cell recognition, intercellular joining

Question 13

how do membrane proteins function in anchorage?

Answer 13

they attach the cell membrane to other substances, stabilising position of cell membrane and maintaining cell shape
on inner cytoplasmic side, bound to microfilaments of cytoskeleton
on exterior side, attach the cell to fibres of extracellular matrix

Question 14

how do membrane proteins function in transport? compare carrier and channel proteins

Answer 14

carrier proteins: bind solutes to its binding site and transport them across the membrane, involving conformational change of protein, may require energy in the form of ATP
channel proteins: some integral proteins have a water-filled central pore that acts as a HYDROPHILIC PORE, passageway that permits movement of hydrophilic, polar, charged particles down conc gradient across cell membrane
- leak channels permit the movement of substances at all time
- gated channels can open and close to regulate ion passage (eg. voltage-gated Na+ or K+ channels

Question 15

how do membrane proteins function in enzymatic activity and signal transduction?

Answer 15

enzymes embedded in the membrane catalyse reactions in extracellular fluid or within the cytosol (depends on active site location)
proteins with very specific 3D conformations are ideal as receptor molecules for chemical signalling between cells (works by binding a ligand to receptor protein, triggering changes in cell). type and number of receptor proteins differ

Question 16

how do membrane proteins function in cell-to-cell recognition and intercellular joining?

Answer 16

recognition proteins are usually glycoproteins, many possible shapes to the carbohydrate side chains, so each cell can have its own specific markers, allowing cells to recognise eo

membrane proteins of adjacent cells may adhere together in various kinds of intercellular junctions (gap or tight junctions)

Question 17

what are the structural features of membrane carbohydrates, how are they bonded, and what can they form?

Answer 17

short, branched chains of fewer than 15 sugar units, covalently bonded to polar ends of phospholipid molecules in outer lipid layer (glycolipids) or covalently bonded to membrane proteins (glycoproteins)

Question 18

what is the function of carbohydrates in the cell membrane?

Answer 18

maintaining the orientation of glycoproteins and glycolipids within membrane, since carbohydrates are highly hydrophilic and must be kept in contact with external aqueous environment

recognition components, involved in: 
sorting of cells into tissues
binding extracellular signal molecules in antibody-antigen reactors
intercellular adhesion to form tissues
cell-to-cell recognition

Question 19

what is transport across membranes important?

Answer 19

maintain suitable pH and ionic concentration for enzyme activity
obtain food supplies for energy and raw materials
excrete toxic substances
secrete useful substances
generate ionic gradients essential for nervous and muscular activity

Question 20

compare the main differences between passive and active processes for transport across membranes (concentration gradient, energy requirements)

Answer 20

passive processes occur down / along conc gradient, active processes occur up / against a conc gradient
passive processes do not require cellular energy expenditure, active processes require cellular energy expenditure in the form of ATP

Question 21

define diffusion

Answer 21

diffusion is the net movement of molecules from an area of higher concentration to a region of lower concentration, down a concentration gradient

(energy is involved in the form of intrinsic thermal energy, and diffusion continues until dynamic equilibrium is reached)

Question 22

compare simple and facilitated diffusion

Answer 22

simple diffusion is for molecules with a small molecular weight and/or are hydrophobic and non-polar and uncharged, and can cross the bilayer(‘s hydrophobic core) directly
facilitated diffusion is for molecules with a large molecular weight and/or are hydrophilic and polar and charged, and need transport proteins to enhance / increase the rate of transport of the substance across the membrane

Question 23

what are the factors that affect the rate of diffusion?

Answer 23

concentration gradient: the steeper, the faster
diffusion distance: the shorter, the faster
surface area: the larger, the faster
type of structure: presence of transient gaps in cell membrane enhances diffusion
size of molecule: the smaller, the faster
temperature: the higher, the faster

Question 24

define osmosis

Answer 24

osmosis is the net movement of water molecules from a region of less negative water potential to a region of more negative water potential across a partially permeable membrane

Question 25

define water potential

Answer 25

water potential is the measure of the tendency for water to move from one region to another
*always less than or equals to zero (pure water, the presence of solutes will make water potential more negative, and water always moves from a region of less to more negative water potential)

Question 26

what are the factors that affect water potential in plant and animal cells? (provide the formula)

Answer 26

plant cells: solute concentration (negative) plus pressure exerted by cell wall on it contents (pressure potential, positive)
animal cells: water potential is equal to solute potential, since it has no cell wall

Question 27

define solute potential

Answer 27

the measure of the ability of a solute to make the water potential more negative
* always negative, the more solute molecules present, the more negative

Question 28

define pressure potential

Answer 28

pressure potential is the measure of the pressure exerted by cell wall on its contents (NA for animal cells)
if pressure is applied, solution becomes less negative
- pressure potential increases as cell absorbs water and increases in volume
*always positive

Question 29

what is the effect of placing animal cells into hypotonic, isotonic, and hypertonic solutions?

Answer 29

hypotonic (less negative water potential): swells and lyses
isotonic (equivalent water potential): no net movement, no change
hypertonic (more negative water potential): shrivelled

Question 30

what is the effect of placing plant cells into hypertonic, isotonic, and hypotonic solutions?

Answer 30

hypotonic (less negative water potential): turgid
isotonic (equivalent water potential): no net movement, no change
hypertonic (more negative water potential): plasmolysed

Question 31

define active transport

Answer 31

the movement of substances from a region of lower concentration to a region of higher concentration, against the concentration gradient, with energy required provided in the form of ATP

Question 32

what are the three types of active transport carriers?

Answer 32

uniport: only one substance transported across the membrane at a time
symport: two substances transported across the membrane together in the same direction
antiport: two substances transported across the membrane together in opposite directions (eg. sodium-potassium pump, transports 3 Na+ ions out for every 2 K+ ions in)

Question 33

what is primary active transport?

Answer 33

primary refers to direct use of ATP by a carrier protein

Question 34

what is bulk transport of substances in an out of a cell?

Answer 34

endocytosis and exocytosis are active processes that require ATP expenditure for the transport of macromolecules in and out of the cell

Question 35

compare endocytosis and exocytosis

Answer 35

endocytosis is where the cell takes in macromolecules by invagination of plasma membrane, but exocytosis is where the cell secretes macromolecules by fusion of vesicles with the plasma membrane
endocytosis’ vesicles form from localised region of plasma membrane that invaginates and pinches off cytoplasmic membrane, exocytosis’ vesicles bud off the ER or GA and fuse with last a membrane
endocytosis is to incorporate cellular substances, exocytosis is to export secrete products or remove waste materials

Question 36

what are the three types of endocytosis, and how do they work?

Answer 36

phagocytosis: large solid particles (eg. food, bacteria) enclosed by pseudopodia (singular: pseudopodium) that extend outwards from the cell
pintocytosis: droplets of extra cellular fluid incorporated into small vesicles, non-specific receptor-mediated
endocytosis: coated pits form clathrin-coated vesicles when specific molecules bind to receptor proteins on the cell surface, reinforced on cytoplasmic side by the protein clathrin, selective process