Plasma membrane

Question 1

what is selective permeability

Answer 1

allows some substances to cross the membrane more easily than others

Question 2

what is an aquaporin?

Answer 2

membrane channel protein
one molecule of this protein enables billions of water molecules to pass through the membrane every second, many more than could cross on their own

Question 3

what is an ion channel

Answer 3

transport protein embedded in plasma membrane

allows potassium ions to pass through the membrane

Question 4

what are membranes made of?

Answer 4

lipids - mainly phospholipids
proteins
carbohydrates

Question 5

what does amphipathic mean

Answer 5

has both a hydrophilic and hydrophobic region

most membrane lipids are amphipathic

Question 6

describe the structure of a phospholipid bilayer

Answer 6

hydrophilic head exposed to water - made from choline and phosphate
hydrophobic tail - sheltered inside - made from gylcerol and fatty acids

Question 7

what is the structure of membrane proteins

Answer 7

membrane proteins are amphipathic
have hydrophilic heads protruding and in contact with water in the cytosol and extracellular fluid
have hydrophobic parts are inside the membrane in a nonaqueous environment

Question 8

what is the fluid membrane model

Answer 8

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

Question 9

how are proteins organised on the plasma membrane

Answer 9

not randomly distributed

groups of associated proteins in patches where they carry out common functions

Question 10

How is the membrane fluid

Answer 10

bonds between molecules is weaker than covalent bonds
phospholipids shift sideways rapidly
proteins may also move

Question 11

how does the temperature affect membrane fluidity

Answer 11

like fats at lower temperatures - phospholipids settle as the temperature decreases
as temp decreases they solidify
phospholipids with unsaturated hydrocarbon tails remain fluid to a lower temperature

Question 12

what is a saturated lipid

Answer 12

all available carbons have a hydrogen attached

saturated with hydrogen

Question 13

what does unsaturated mean

Answer 13

not every available carbon has a hydrogen

Question 14

why do some phospholipids remain fluid in lower temperatures

Answer 14

phospholipids rich in unsaturated hydrocarbon tails remain fluid at lower temperatures
kinks in the tails where double bonds are located mean that hydrocarbons cannot pack together as closely as saturated hydrocarbon tails can

Question 15

how does cholesterol work in membranes

Answer 15

cholesterol is wedged between phospholipids in the plasma membranes of animal cells
has different effects on membrane fluidity at different temperatures
at body temperature cholesterol makes the membrane less fluid by restraining phospholipid movement
at low temperatures hinders solidification by preventing phospholipids from packing
is a fluidity buffer

Question 16

what cells have cholesterol

Answer 16

cholesterol is wedged between the phospholipid molecules in the plasma membrane of ANIMAL cells
compared to animals plants have very low levels of cholesterol but instead have steroid lipids buffer membrane fluidity

Question 17

what are fluidity buffers in animal and plant cells

Answer 17

animals - cholesterol
plants - steroid lipids

buffers - resist changes in membrane fluidity brought about by changes in temperature

Question 18

why do membranes need to be fluid

Answer 18

the fluidity of a membrane affects both permeability and the ability of membrane proteins to move to where their function is needed

Question 19

what evolutionary adaptations occur in membrane lipid composition

Answer 19

variation in lipid composition depending on environment
some animals or plants that live in cold have high proportion of unsaturated hydrocarbon tails so phospholipids don’t solidify and remain fluid
bacteria and archaea that live in extreme temps have unusual lipids that prevent excessive fluidity

Question 20

how are proteins arranged in the membrane

Answer 20

the membrane is a collage of different proteins often clustered together in groups, embedded in the fluid matrix of the bilayer
different types of cells have different sets of proteins
various membranes within a cell have a unique collection of proteins

Question 21

what are the two types of membrane proteins

Answer 21

integral proteins and peripheral proteins

Question 22

what is an integral protein

Answer 22

penetrate the hydrophobic interior of the lipid bilayer
majority are transmembrane proteins which span
others extend only part way into the hydrophobic interior

Question 23

what are transmembrane proteins

Answer 23

proteins that span the membrane

Question 24

what are peripheral proteins

Answer 24

not embedded into the lipid bilayer
loosely bound to the surface of the membrane
often bound to the exposed parts of integral proteins

Question 25

what is the structure of an integral protein

Answer 25

hydrophobic region consists of one or more stretches of 20 - 30 non polar amino acids usually coiled into alpha helices hydrophilic parts exposed to aqueous solutions

Question 26

how do membrane proteins attach to the cell

Answer 26

cytoplasmic side - they attach to the cytoskeleton | extracellular - attach to materials outside the cell e.g. extra cellular matrix

Question 27

what six functions are carried out by membrane proteins

Answer 27

``` transport enzymatic activity signal transduction cell to cell recognition intercellular joining attach to cytoskeleton and extra cellular matrix ```

Question 28

how does a membrane protein function as transport

Answer 28

can be a channel protein - provide a hydrophilic channel selective for a particular solute proteins may change shape to shuttle substance across the membrane proteins may act as pumps

Question 29

membrane proteins as enzymes

Answer 29

may be an enzyme with an active site where a reactant binds

Question 30

membrane proteins that act as signal transductors

Answer 30

a membrane protein may act as a receptor for a signal has a binding site that fits the shape of the chemical messenger the messenger may cause the receptor to change shape allowing it to relay a message to the inside of the cell usually binding to a cytoplasmic protein

Question 31

how do membrane proteins function in cell to cell recognition

Answer 31

some glycoproteins serve as identification tags specifically recognised by the membrane proteins of other cells (short lived)

Question 32

how do membrane proteins function in intercellular joining

Answer 32

membrane proteins are involved in various kinds of cell junctions e.g. gap junctions / tight junctions

Question 33

how does attachment to the cytoskeleton and extra cellular matrix benefit the cell

Answer 33

helps maintain cell shape stabilises the location of certain membrane proteins signal transduction - proteins that bind to ECM molecules like fibronectin / integrins can cause changes in the extracellular and intracellular environment

Question 34

integrin

Answer 34

membrane proteins with two subunits - bind to the ECM outside the cell and to proteins attached to microfilaments on the inside used to transmit signals between external environment and inside the name integrin is based on integrate - integrins transmit signals between the ECM and the cytoskeleton and integrate changes occurring outside and inside the cell

Question 35

fibronectin

Answer 35

attaches the ECM to integrin embedded in the plasma membrane

Question 36

why is cell to cell recognition important

Answer 36

sorts cells into tissues and organs in an animal embryo | basis for the rejection of foreign cells by the immune system

Question 37

what types of membrane carbohydrates are there

Answer 37

glycolipids - carbohydrates covalently bonded to lipids glycoproteins - carbohydrates covalently bonded to proteins carbohydrates are usually short chains of fewer than 15 sugar units

Question 38

what molecules are involved in cell to cell recognition

Answer 38

carbohydrates - mainly glycoproteins and glycolipids

Question 39

where are membrane proteins and lipids synthesised

Answer 39

Endoplasmic reticulum

Question 40

process for making membrane glycoproteins and glycolipids

Answer 40

proteins and lipids synthesised in the ER carbohydrates are added to transmembrane proteins making them glycoproteins in the ER in the golgi glycoproteins undergo further modification in the golgi lipids acquire carbs becoming glycolipids transported in vesicles to plasma membrane vesicle fuses with plasma membrane - positions glycoproteins and glycolipids on the outside face of the pm

Question 41

what types of substances can cross the plasma membrane easily

Answer 41

``` non polar and hydrophobic molecules hydrocarbons carbon dioxide oxygen lipids ``` water can pass but slowly - aquaporins allow water to move at a much faster rate

Question 42

what types of substances cannot cross the plasma membrane without assistance

Answer 42

``` polar and hydrophilic molecules macromolecules ions amino acids water glucose and other sugars charged atoms or molecules ```

Question 43

what are the different types of transport proteins

Answer 43

channel proteins - hydrophilic channel e.g. aquaporins carrier proteins - hold on to passengers and change shape e.g. carrier protein in the plasma membrane of red blood cells transports glucose

Question 44

diffusion

Answer 44

the movement of particles of any substance so that they spread out into the available space needs no input of energy - is a spontaneous process called passive transport - cell does not have to expend energy

Question 45

concentration gradient

Answer 45

the region along which the density of a chemical substance increases or decreases a substance will move up or down its concentration gradient in diffusion a substance will move down its concentration gradient - no energy required

Question 46

in diffusion - what is dynamic equilibrium

Answer 46

substances will move down their concentration gradient until there is dynamic equilibrium - the solute molecules continue to cross the membrane but at roughly equal rates in both directions

Question 47

in diffusion - what happens when there are two solutes

Answer 47

each substance diffuses down its own concentration gradient - unaffected by the concentration gradient of other substances

Question 48

how does oxygen enter the cell

Answer 48

oxygen diffuses across the cell membrane during cellular respiration dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the oxygen as it enters, diffusion into the cell will continue

Question 49

what is passive transport

Answer 49

requires no energy | diffusion is passive transport because the concentration gradient (potential energy) drives the process

Question 50

what is osmosis

Answer 50

the diffusion of free water across a selectively permeable membrane water diffuses across a membrane from the region of high free water concentration (lower solute) to low free water concentration water follows solute until the solute concentrations on both sides of the membrane was equal

Question 51

tonicity

Answer 51

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

Question 52

how does tonicity work

Answer 52

water follows solute | if there is more solutes outside the cell - water will leave the cell and vice versa

Question 53

isotonic

Answer 53

the concentration of a solution is the same inside and outside the cell - equal no net movement of water - water diffuses across the membrane but at the same rate in both directions the volume of an animal cell is stable

Question 54

hypertonic

Answer 54

more solutes in the solution outside the cell water follows solute - water will leave the cell the cell will lose water, shrivel and probably die

Question 55

hypotonic

Answer 55

less solute in the water surrounding the cell / more solute inside the cell water follows the solute water will enter the cell faster than it leaves cell will swell and lyse

Question 56

osmoregulation

Answer 56

in hypertonic or hypotonic environments organisms that lack rigid cell walls must have adaptations to survive the control of solute concentration and water balance e.g. paramecium - contractile vaccuole which acts to pump out water

Question 57

turgor pressure

Answer 57

in cells with cell walls cell walls help the cell maintain water balance cell swells when water enters by osmosis cell expands but when it reaches a certain level the cell wall will not let it expand any more - exerts turgor pressure - to oppose any further water uptake the cell is turgid

Question 58

turgid

Answer 58

in plant cells that have a cell wall cell is full water so that the cell wall exerts turgor pressure turgid - very firm plants that are not woody rely for structural support by cells kept turgid by surrounding in hypotonic solution - net tendency for water to enter the cell

Question 59

why do plants need to be in a hypotonic solution

Answer 59

more solute inside the cell water follows solute and enters the cell cell is turgid

Question 60

what happens when a plant is in an isotonic environment

Answer 60

no movement of water into the cell - less turgid / becomes flaccid

Question 61

what happens to a plant cell - with a cell wall - in a hypertonic environment

Answer 61

more solute outside the cell water follows solute and moves out of cell - lose water to the environment and shrinks or shrivels as the plant cell shrivels its plasma membrane pulls away from the cell wall - plasmolysis - causes plant to wilt

Question 62

plasmolysis

Answer 62

if a plant is in a hypertonic solution water will flow out of the cell and the cell will shrivel as the plant shrivels the plasma membrane pulls away from the cell wall. This is called plasmolysis plasmolysis causes the plant to wilt plant cells / bacteria / fungi also plasmolyse in hypertonic environments

Question 63

facilitated transfusion

Answer 63

many molecules that can't cross because of the lipid bilayer diffuse passively with the help of transport proteins that span the membrane e.g. acqaporins

Question 64

how does water enter cells

Answer 64

diffuse through aquaporins - water channel proteins facilitate massive levels of diffusion of water in plant and animal cells cells that need more water have more aquaporins

Question 65

ion channels

Answer 65

channel proteins that transport ions | many ion channels are gated channels

Question 66

gated channel

Answer 66

gated channels are ion channels that open and close in response to a stimulus e.g. electrical in nerve cells other gated channels open or close when a specific substance other than the one to be transported binds to the channel

Question 67

ion channels and carrier proteins are forms of passive transport - why

Answer 67

ion channels and carrier proteins involve facilitated diffusion of a substance down the concentration gradient - no energy is required

Question 68

what is the difference between passive and active transport across a plasma membrane

Answer 68

passive transport relies on substances moving down their concentration gradient - no energy is required active transport moves solutes against their concentration gradient from a side where they are less concentrated to a side where they are less concentrated active transport needs energy

Question 69

what types of proteins are required for active transport

Answer 69

active transport is carried out by carrier proteins (not channel proteins) they are transporting solutes against their concentration gradients

Question 70

give an example of active transport

Answer 70

sodium potassium pump - the cell has higher concentrations of potassium than its surroundings and a much lower concentration of sodium ions the sodium potassium pump exchanges sodium for potassium across the plasma membrane of animal cells

Question 71

where does the cell get the energy required for active transport

Answer 71

ATP hydrolysis e.g. ATP phosphate group transferred to the transport protein and changes its shape in a manner that translocates a solute bound to the protein across the membrane

Question 72

electrochemical gradients

Answer 72

there is a difference in charge across membranes cytoplasmic side is negative / extracellular side is positive. because the inside of the cell is negative and the outside is positive it effects ion movement an ion diffuses down its electrochemical gradient (as well as concentration gradient)

Question 73

membrane potential

Answer 73

charge - inside the cell is negative and outside is positive the voltage across the membrane is called the membrane potential

Question 74

electrogenic pump

Answer 74

ions usually diffuse down their concentration and electrochemical gradient however sometimes the two are at odds - electrical forces due to membrane potential oppose simple diffusion - active transport is necessary a transport protein that generates a voltage across a membrane is called an electrogenic ump

Question 75

give examples of an electrogenic pump

Answer 75

in animal cells - sodium potassium pump - moves three sodium out of the cell for every 2 potassium pumped in (against their concentration gradients) in plants, bacteria and fungi - proton pump - actively transports protons (hydrogen ions) out of the cell creating a positive charge

Question 76

proton pump

Answer 76

a proton pump transports protons in the form of hydrogen ions out of the cell. This causes a charge /voltage which can be stored as energy that the cell can use most proton pumps are powered by ATP hydrolysis (remember proton pump in cellular respiration)

Question 77

cotransport

Answer 77

a transport protein (a cotransporter) can couple the downhill diffusion of a solute to the uphill transport of a second substance against its own concentration gradient e.g. H+/sucrose cotransporter in plants

Question 78

exocytosis

Answer 78

the process by which the cell secretes certain molecules by the fusion of vesicles with the plasma membrane a transport vesicle that has budded from the golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane - the two membranes fuse and the contents of the vesicle spill out of the cell and the vesicle membrane becomes part of the plasma membrane

Question 79

give examples of cells that use exocytosis to export products

Answer 79

pancreas - insulin is secreted by exocytosis nerve cells use exocytosis to release neurotransmitters proteins needed to make plant cell walls are delivered through exocytosis

Question 80

endocytosis

Answer 80

cell takes in molecules and particles by forming new vesicles from the plasma membrane three types - phagocytosis / pinocytosis / receptor mediated endocytosis

Question 81

phagocytosis

Answer 81

the cell engulfs a particle by extending pseudopodia around it and packaging it within a sac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes e.g. amoeba engulfing a algal cell

Question 82

pinocytosis

Answer 82

cell continuously gulps droplets of extracellular fluid into tiny vesicles forming infoldings of the plasma membrane which pinch off.

Question 83

receptor mediated endocytosis

Answer 83

a specialised type of pinocytosis (endocytosis by pinching in) that enables the cell to acquire bulk quantities of specific substances even though those substances may not be very concentrated in the extracellular fluid specific solutes bind to receptor proteins / the receptor proteins cluster in coated pits / each coated pit forms a vesicle e.g. cholesterol

Question 84

what effect does exocytosis and endocytosis have on the plasma membrane

Answer 84

they provide mechanisms for rejuvenating and remodelling the plasma membrane exocytosis adds new membrane / exocytosis subtracts the amount of plasma membrane in a nongrowing cell remains fairly constant the addition of membrane by one process appears to be offset by the other