unit 1.3 cell membranes and transport

Question 1

what is the function of the cell membrane?

Answer 1

the boundary that separates the living cell from its non - living surroundings
it also controls which substances pass into and out of the cell
controls the uptake of nutrients
allows waste products out of the cell
responsible for secreting substances such as enzymes and glycoproteins

Question 2

what is the structure of the cell membrane?

Answer 2

it is made up of almost entirely phospholipids and proteins

Question 3

phospholipids?

Answer 3

can form bilayers, with one sheet of phospholipid forming over another. Phosphate head = hydrophilic + attracted to other molecules such as water

2 fatty acid tails of the phospholipid = hydrophobic (non polar) + repel water

phospholipid bilayer forms the basis of the membrane structure

phospholipid component allows lipid soluble molecules to enter and leave the cell but prevents water soluble molecules from doing so.

Question 4

Membrane proteins?

Answer 4

arranged randomly in the proteins in contrast to the more regular patterns of phospholipids

Question 5

extrinsic proteins?

Answer 5

occur on the surface of the bilayer or are partly embedded in it.
they provide structural support and also form recognition sites by identifying cells

Question 6

intrinsic proteins?

Answer 6

go right through the phospholipid bilayer ; some act as channels or carriers to facilitate the diffusion of polar molecules across the cell membrane

others form pumps and carry out active transport against a conc gradient

Question 7

who came up with the fluid mosaic model?

Answer 7

singer and nicholson proposed a model to describe the arrangement of phospholipids and proteins in cell membranes in 1972

Question 8

why is it called the fluid mosaic model?

Answer 8

the phospholipids are fluid as each molecule can move relative to each other

the proteins form a mosaic pattern within the phospholipid bilayer - as they’re different sizes and shapes and form different patterns?

Question 9

phospholipid bilayer?

Answer 9

forms the basis of the cell membrane and allows transport of small non polar molecules into and out of the cell by simple diffusion

Question 10

cholestrol?

Answer 10

found in animal cells
fit between the phospholipid molecules, increasing the rigidity and stability of the membrane

Question 11

glycoproteins?

Answer 11

proteins combined with polysaccharide
stick out of some membranes

Question 12

glycolipids?

Answer 12

lipids combined with polysaccharide and they = found in outer layer of the membrane and are involved in cell to cell recognition

Question 13

The membrane as a barrier

Answer 13

cell surface membrane = selectively permeable to water and some solutes

non polar molecules can freely pass through the membrane by simple diffusion

polar molecules cannot diffuse across the non - polar centre of the phospholipid bilayer as they are insoluble in lipid.

Question 14

what factors affect cell membrane permeability?

Answer 14

increasing temperature - stable up to a temp of 40 degrees and anything above - becomes unstable,
increasing ethanol conc - ethanol dissolves phospholipids . greater the conc - more permeable membranes become
increasing nacl conc - sodiumions attach to the oxygen atoms on the phosphate heads of the phospholipid bilayer. reduces mobility of the phospholipid molecules so less belatin = released
as nacl increases - permeability decreases
increasing detergent - reduces surface tension of phospholipids and disperse the membrane.

as conc increases , permeability = increases

Question 15

definition of diffusion?

Answer 15

the movement of molecules or ions from a region of a high concentration to a region of a lower concentration down a concentration gradient
passive process

Question 16

factors that affect the rate of diffusion?

Answer 16

concentration gradient - greater the conc gradient - greater the rate of diffusion, steeper the triangle - faster diffusion will occur

distance of travel - shorter - grater the rate of diffusion

surface area of the membrane - larger , the greater the rate of diffusion

thickness of the membrane -thinner the membrane -greater the rate of diffusion

increase in temp - increased temp increases kinetic energy and therefore increases the rate of diffusion

particle size - small particles diffuse faster than larger molecules

Question 17

what is simple diffusion?

Answer 17

the diffusion of non polar molecules such as oxygen and carbon dioxide that occurs across the phospholipid bilayer

Question 18

what is faciliated diffusion?

Answer 18

where polar molecules cannot cross the phospholipid bilayer and therefore must use an intrinsic protein to facilitate transport across the membrane.
for example glucose - insoublein lipid

Question 19

how is simple diffusion represented on a graph?

Answer 19

just a straight line
directly proportional
respiratory inhibtor will not stop this diffusion

Question 20

what are the 2 types of proteins which facilitate diffusion

Answer 20

channel proteins - consists of pores lined with groups and allows charged ions to pass through (Na+). Each channel protein = specific for one type of ion and can open and close depending on the needs of the cell.

carrier proteins - allows the facilitated diffusion across the membrane of larger polar molecules such as sugars and amino acids. A particular molecule attaches to a carrier protein at its binding site and causes carrier protein to change shape or rotate within the membrane. this releases the molecule on the other side of the membrane.

Question 21

what does facilitated diffusion look like on a graph?

Answer 21

Initial increased rate of diffusion as concentration gradient = steeper. rate of diffusion levels off at higher concentration differences
due to carrier or channel proteins being occupied - limits rate of diffusion.
facilitated diffusion = not affected by respiratory inhibitors

Question 22

what is co transport?

Answer 22

type of diffusion that brings molecules and ions into cells together on the same carrier protein.

Question 23

what is active transport?

Answer 23

movement of substances from an area of low concentration to an area of high concentration against a concentration gradient

Question 24

how is active transport represented on a graph?

Answer 24

initial increased rate of uptake as concentration gradient increases. rate then levels off at higher concentration differences.
active transport = affected by respiratory inhibitors and rate of transport sharply drops after addition of a respiratory inhibitor

Question 25

What is osmosis?

Answer 25

the movement of water from an area of higher water potential to an area of lower water potential through a partially permeable membrane.

Question 26

what has the highest water potential?

Answer 26

water with one of 0

Question 27

what is the equation of the water potential of the cell?

Answer 27

solute potential + pressure potential

Question 28

what is the solute potential?

Answer 28

concentration of dissolved substances inside the cell vacuole

Question 29

what is the pressure potential?

Answer 29

when the outward pressure builds up the cell wall and develops an opposing force

Question 30

how will water move?

Answer 30

from a high water potential - less negative
to a lower water potential - more negative

Question 31

What does it mean when hypotonic?

Answer 31

if water potential of the external solution is higher than the solution inside the cell

water will move into the cell by osmosis. causing the cell to swell. animal cells may burst as they have no cell wall to prevent bursting - called lysis.

plant cells = turgid as cell contents pushes against cell wall.

Question 32

what is it meant by hypertonic?

Answer 32

if water potential of the external solution is hypertonic, water will move out of the cell by osmosis , causing the cell to shrink.

in animal cells - the cell will simply shrink

in plant cells, plamolysis occurs, the vacuole and cytoplasm shrink causing the cell membrane to pull away from the cell wall.

plant cell would be plasmolysed and flaccid.

Question 33

turgid?

Answer 33

cells hold as much wate as possible and further entry of water = prevented as cell wall cannot expand further

Question 34

plasmolysis?

Answer 34

when water leaves plant cell by osmosis, the cytoplasm and vacuole shrink and the cell membrane pulls away from the cell wall

Question 35

fwhat is meant by isotonic?

Answer 35

where the external solution has the same water potential as the cell contents. There will be no net movement by osmosis. Animal cells = isotonic medium naturally. Water potential = equal to the water potential of the external solution at this point.

plane cells - cells become flaccid. This is the point of incipient plasmolysis.

Question 36

data?

Answer 36

when potato cells= fully turgid at top
potential of the cell = 0

incipient plasmolysis = water potential = 0 middle

potato cells = fully plasmolysed
potential = 0
potential of the cell = greater than potential of the solute

Question 37

what is endocytosis?

Answer 37

where the cell membrane engulfs particles or liquid forming a vesicle which enters the cytoplasm

Question 38

what are the 2 types of endocytosis?

Answer 38

phagocytosis - cell eating
pinocytosis - cell drinking (involves entry of liquid into the cell)

Question 39

Exocytosis?

Answer 39

substances leave the cell after being transported through the cytoplasm in transport vesicles to the Golgi body and then to the cell membrane via secretory vesicles.