Chapter 7 - Membrane Structure and Function

Question 1

what are state ingredients of membranes?

Answer 1

lipids and proteins - carbohydrates also important

lipids are the most abundant in the plasma membrane

Question 2

what does amphipathic mean? what demonstrates this characteristic?

Answer 2

phospholipids form membranes - they are an amphipathic molecules meaning it has both a hydrophilic region and hydrophobic region

membrane proteins are also amphipathic - embed themselves in phospholipid bilayer

Question 3

which part of the phospholipid faces water? inside?

Answer 3

hydrophobic tail faces inwards while hydrophilic heads face water

Question 4

what is the fluid mosaic model?

Answer 4

the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

Question 5

where are lipids made?

where are membrane phospholipids made and the the assembly of the membrane take place?

Answer 5

smooth ER

membrane phospholipids and assembly of membrane + protein occurs in rough ER and golgi apparatus

Question 6

phospholipid movement in the plasma membrane?

Answer 6

Most drift laterally (weak hydrophobic interaction)
Rarely does a lipid flip b/c it requires energy like ATP

proteins don’t usually move b/c larger but some can

Question 7

which experiment showed that the plasma membrane isn’t static?

Answer 7

took two cells and labeled them with different colors and after an hour the proteins become equally distributed – this experiment shows that membrane isn’t static

Question 8

phospholipid phase in relation to temperature

Answer 8

As temperature cools, membranes go from fluid to solid state b/c phospholipids pack together

Question 9

what allows a membrane to stay fluid at lower temperatures?

Answer 9

membrane remains fluid to a lower temperature if it’s rich in phospholipids with unsaturated hydrocarbon tails - kinks in the tail b/c of double bond means they can’t pack closely together

different environments have different temperatures so you need a balance of sat/unsat to maintain fluidity

Question 10

effects of cholesterol on the membrane

Answer 10

it’s a fluidity buffer

at high temperatures (body temp): cholesterol makes the membrane less fluid by restraining the phospholipid movement - gives structure to cell membrane

but at lower temperatures cholesterol also hinders the close packing of phospholipids and lowers the temperature required for the membrane to solidify

Question 11

what determines most of the function of the plasma membrane?

Answer 11

the proteins

Question 12

what are the two types membrane proteins?

Answer 12

1) integral proteins

2) peripheral proteins

Question 13

what is an integral protein?

Answer 13

Integral protein is able to penetrate the hydrophobic interior of the lipid bilayer

the majority of integral proteins of transmembrane proteins - some only extend partway into the hydrophobic interior

Question 14

what are trans-membrane proteins?

Answer 14

a type of integral proteins that span the membrane: a portion is inside and the other part is sticking outside

the hydrophobic regions of an integral protein consist of one or more stretches of non polar amino acids usually coiled into alpha helices

the hydrophilic regions are exposed to the aqueous solutions on either side of the membrane (extracellular and cytoplasmic side) - some proteins also have hydrophilic channels that allow passage through the membrane of hydrophilic substances

Question 15

what are peripheral proteins?

Answer 15

peripheral proteins are not embedded in the lipid bilayer at all - they are appendages loosely bound to the surface of the membrane (often to exposed parts of integral proteins) or phospholipid head

Question 16

how do membrane proteins attach?

Answer 16

on cytoplasmic side, some membrane proteins are held in place by attachment to the cytoskeleton

on extracellular side, they are attached to fibers of the ECM

Question 17

what are the 6 major functions of membrane proteins?

Answer 17

1) Transport: if they’re too large or hydrophilic, proteins
help these molecules get across w/ channels

2) Enzyme activity: enzymes are embedded in the membrane which is useful because spatially you can keep chemical reactions close together
3) Signal transduction: may have a binding site with a specific shape that fits the shape of the chemical messenger (receptor) or the external messenger(signaling molecule) may cause protein shape to change to allow message to be relayed to the inside of the call
4) Cell-cell recognition: glycoproteins serve as ID tags
5) Intercellular joining: tight or gap junctions can form
6) Cytoskeleton and ECM attachment

Question 18

how do cells recognize other cells?

Answer 18

cell recognize other cells by binding to molecules, that often contain carbohydrates, on the extracellular surface of the plasma membrane

Question 19

membrane carbohydrates

Answer 19

usually short, branched, chains of fewer than 15 sugar units

some are covalently bonded to lipids forming glycolipids

most are covalently bonded to proteins which are glycoproteins

Question 20

membrane sidedness and synthesis

Answer 20

o Distinct inside and outside faces
o The asymmetrical distribution of proteins, lipids, and associated carbohydrates is determined when the membrane is built by the ER and golgi

Question 21

what are the components that make a cell’s membrane selectively permeable?

Answer 21

1) hydrophobic interior: polar molecules and ions can’t pass

2) specific transport proteins

Question 22

what are transport proteins? what do they do?

Answer 22

transmembrane proteins that help hydrophilic substances pass through the lipid bilayer

channel proteins are a type of transport protein that have hydrophilic channels for certain molecules or atomic ions to use as a tunnel through the membrane

Question 23

what are aquaporins?

Answer 23

channel proteins that facilitate the diffusion of water through the membrane

Question 24

what are some types of transport proteins?

Answer 24

channel proteins

carrier proteins

Question 25

what are carrier proteins

Answer 25

they are transport proteins that hold onto their passengers and change shape in a way that shuttles them across the membrane

Question 26

what is diffusion?

Answer 26

the movement of particles of any substance so that they speak out into the available space

spontaneous process, no work must be done to make it happen = no input of energy

Question 27

what is the rule of diffusion?

Answer 27

in the absence of other forces, a substance will diffuse from where it is more concentrate to where it is less concentrate

any substance will diffuse down its OWN concentration gradient!

Question 28

what is a concentration gradient?

Answer 28

the region along which the density of a chemical substance increases or decreases

concentration gradient represent potential energy and drive diffusion

Question 29

what is passive transport?

Answer 29

the diffusion of a substance across a biological membrane

passive because the cell doesn’t have to input energy

solute will diffuse down its concentration gradient

(random but directional)

Question 30

what is dynamic equilibrium?

Answer 30

Dynamic equilibrium: no gradient and molecules cross the barrier at the same rate

Question 31

how does water diffuse?

Answer 31

water diffuses across the membrane from the region of higher free water concentration/low solute contraction to that of lower free water concentration/higher solute concentration until the solute concentration on both sides of the membrane are more equal

WATER MOVES TO DILUTE

Question 32

what is osmosis?

Answer 32

diffusion of free water across a selectively permeable membrane

Question 33

isotonic solution

Answer 33

if a cell without a cell wall (like an animal cell) is immersed in an environment that is isotonic to the cell, there will be no net movement of water across the plasma membrane

water diffuses across membrane but at the same rate in both directions

Question 34

hypertonic solution

Answer 34

if a cell is in a solution that is hypertonic to the cell, the water will move out of the cell, shrivel, and probably die

Question 35

hypotonic solution

Answer 35

if a cell is in a solution that is hypotonic to the cell then water will enter the cell faster than it leaves and the cell will swell and burst

Question 36

what is osmoregulation?

Answer 36

the control of solute concentrations and water balance in cells that don’t have rigid cell walls

Ex. The protest paramecium which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Question 37

what does turgid mean? what’s it in reference to?

Answer 37

plant cells that have cell walls

when a plant cell swells as water enter by osmosis, the cell wall will only expand so much before it exerts a back pressure on the cell that opposes further water uptake and will become turgid = firm

Question 38

what is flaccid?

Answer 38

if a plant’s cells and their surrounding are isotonic and there’s no net tendency for water to enter/leave then the cells become flaccid = limp

Question 39

what is plasmolysis?

Answer 39

if a plant cell is in a hypertonic environment the cell will shrivel and its plasma membrane will pull away from the cell wall at multiple places –> causes plant to wilt and can lead to plant death

bacteria and fungi do the same thing

Question 40

what are ion channels?

Answer 40

facilitated diffusion

channel proteins that transport ions - electrochemical gradient

moves down electrochemical gradient - no energy

Question 41

what are gated channels?

Answer 41

ion channels that open or close in response to a stimulus

facilitated diffusion - no energy required

Question 42

what is active transport?

Answer 42

Proteins use energy to move solutes across a membrane against their concentration gradient

from low to high concentration of solute

ATP supplies energy for active transport

Question 43

what kind of proteins participate in active transport?

Answer 43

carrier proteins, no channel proteins

this is because when channel proteins are open they allow solutes to diffuse down their concentration gradients rather than picking them up and transporting them against their gradient

Question 44

what is the sodium-potassium pump?

Answer 44

active transport

exchanges Na+ for K+ across the plasma membrane of the animal cell

• 3 Na+ ions move out of the cell for every 2 K+ ions that go in
• a net of 1 positive charge is transferred from inside to out
• the electrochemical gradient created helps cotransport other molecules (such as glucose and amino acids) into the cell and is used to conduct nerve impulses
• Potassium pump changes shape depending on what it’s bound to
• Outside of cell membrane becomes more positive with every pump

Question 45

what are types of passive transport?

Answer 45

1) diffusion (hydrophobic molecules and small uncharged polar molecules diffuse through lipid bilayer)
2) facilitated diffusion ( hydrophilic substances diffuse through membrane with assistance of transport proteins: channel/carrier down concentration gradient)

Question 46

what are the charges of the extracellular and cytoplasmic sides of the membrane?

Answer 46

cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of uneven distribution of anions/cations

Question 47

what is the membrane potential?

Answer 47

the voltage across a membrane

Question 48

what is the electrochemical gradient?

Answer 48

two forces drive the diffusion of ions across the membrane:

1) a chemical force (ion’s concentration gradient)
2) electrical force (effect of membrane potential on ion
movement)

This determines the transport of ions: important in the transmission of nerve impulses

Question 49

what is cotransport?

Answer 49

a type of active diffusion - a transport protein (cotranporter) can couple the “downhill” diffusion of the solute to the “uphill” transport of a second substance against its own concentration gradient

ex. the carrier protein like sucrose-H+ cotransporter in a plant cell is able to use the diffusion of H+ down its electrochemical gradient into the cell to drive the uptake of sucrose

Question 50

how do larger molecules such as proteins and polysaccharides cross the membrane?

Answer 50

bulk transport!

the cross in bulk, packaged in vesicles = requires energy

Question 51

what is exocytosis?

Answer 51

the cell secretes certain molecules by the fusion of vesicles with the plasma membrane

transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell

ex. insulin

Question 52

what is endocytosis?

Answer 52

the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane

almost like reverse exocytosis

Question 53

what are the three types of endocytosis?

Answer 53

1) phagocytosis
2) pinocytosis
3) receptor-mediated endocytosis (form of pinocytosis)

Question 54

what is a ligand?

Answer 54

a term for any molecule that binds specifically to a receptor site on another molecule

ex. iron, LDL

Question 55

what is phagocytosis?

Answer 55

cell engulfs a particle by extending pseudopodia around it and packaging it into food vacuole

macrophages use phagocytosis

Question 56

what is pinocytosis?

Answer 56

smaller solutes are brought it through pinching of membrane – molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles

Question 57

what is receptor-mediated endocytosis?

Answer 57

binding of ligands to receptors triggers vesicle formation