(4) Transport across cell membranes

Question 1

what is a plasma membrane

Answer 1

all membranes around and within all cells and organelles that all have the same basic structure

Question 2

what is a cell surface membrane

Answer 2

a membrane that surrounds cells and forms a boundary between the cytoplasm and the environment

Question 3

what are 3 reasons that phospholipids are important components of cell surface membranes

Answer 3

hydrophilic head and hydrophobic tail:

1) allow lipid soluble substances to enter and leave the cell
2) prevent water soluble substances entering and leaving the cell
3) make the membrane flexible and self sealing

Question 4

what is a protein channel

Answer 4

forms water filled hdrophilic tubes to allow specific water soluble ions to diffuse across the membrane. They are selective and so open in the presence of a specific ion. In this case, the ion binds to the protein and they open at one side of the membrane but closes at the other side

Question 5

what are carrier proteins

Answer 5

bind to ions or molecules like glucose and amino acids and then change shape in order to move these molecules across the membrane. The molecule that is specific to the protein binds to the protein and and causes it to change shape so the molecule is released into the membrane

Question 6

what are the 6 functions of protein in the membrane

Answer 6

1) provide structural support
2) act as channels transporting water soluble substances across the membrane
3) allow active transport across the membrane through carrier proteins
4) form cell surface receptors for identifying cells
5) help cells adhere together
6) act as receptors for hormones etc

Question 7

what are the functions of cholesterol in the membrane

Answer 7

1) reduce lateral movement of other molecules including phospholipids
2) make the membrane less fluid at high temperatures
3) prevent leakage of water and dissolved ions from the cell

Question 8

what are the functions of glycolipids

Answer 8

1) act as recognition sites
2) help maintain stability of membrane
3) help cells attach to each other to form tissue

Question 9

what are the functions of glycoproteins

Answer 9

1) act as recognition sites
2) help cells attach to form tissues
3) allow cells to recognise one another

Question 10

why do most molecules not freely diffuse across the cell surface membrane

Answer 10

1) not lipid soluble so can’t pass through phospholipid layer
2) too large to pass through channels in membrane
3) of the same charge as the protein channel so are repelled
4) polar or electrically charged so difficult to pass through non polar hydrophobic tails in phospholipid bilayer

Question 11

what is the fluid mosaic model of the cell surface membrane

Answer 11

they way in which the various molecules are combined into the structure of the cell surface membrane

Question 12

what is the fluid part of the fluid mosaic model

Answer 12

it is fluid as the individual phospholipid molecules can move relative to one another- gives it a flexible structure that is always changing in shape

Question 13

what is the mosaic part of the fluid mosaic model

Answer 13

mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern

Question 14

define diffusion

Answer 14

the net movement of molecules or ions from a region of higher concentration to a region of lower concentration down a concentration gradient (passive)

Question 15

what does passive mean

Answer 15

the energy comes from the natural inbuilt motion of particles rather than from external sources such as ATP

Question 16

what is facilitated diffusion

Answer 16

a passive process that only happens at specific points on the plasma membrane where there are protein channels and carrier proteins and helps charged ions and polar molecules to diffuse

Question 17

deifne osmosis

Answer 17

the passage of water from a region of higher water potential to a region of lower water potential through a selectively permeable membrane

Question 18

when does pure water have a water potential of 0

Answer 18

at 25 c and 100kPa

Question 19

define active transport

Answer 19

the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

Question 20

what is ATP used for in active transport

Answer 20

directly move molecules

individually move molecules using a concentration gradient which has already been set up

Question 21

how is active transport different to passive transport

Answer 21

1) metabolic energy in the form of ATP is needed
2) substances are moved against a concentration gradient
3) carrier protein molecules which act as pumps are involved
4) very selective process

Question 22

what happens in the sodium- potassium pump

Answer 22

sodium ions are actively removed from the cell/ organelle while potassium ions are actively taken in from the surroundings

(essential for the creation of a nerve impulse)

Question 23

what are microvilli

Answer 23

finger like projections of the cell surface membrane about 0.6 um in length

Question 24

what do microvilli do

Answer 24

provide more SA for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport take place

Question 25

define co-transport

Answer 25

the transport of one substance coupled with the transport of another substance across a plasma membrane in the same direction through the same protein carrier