4.1- STRUCTURE OF THE CELL-SURFACE MEMBRANE

Question 1

what is the term cell-surface membrane specifically given to?

Answer 1

plasma membrane that surrounds cells + forms boundary between cell cytoplasm + environment

Question 2

What does the cell-surface membrane allow?

Answer 2

different conditions to be established inside + outside of cell

Question 3

What is the function of the cell-surface membrane?

Answer 3

controls movement of substances in and out of cell

Question 4

What do phospholipids form?

Answer 4

a bilayer

Question 5

Where does the hydrophilic heads of both phospholipid layers point to?

Answer 5

outside of cell-surface membrane

Question 6

Why does the hydrophilic heads point to outside of the cell surface membrane?

Answer 6

attracted by water on both sides

Question 7

Where does the hydrophobic tails of both phospholipid layers point?

Answer 7

into the centre of the cell membrane

Question 8

Why does the hydrophobic heads point into the centre of the cell membrane?

Answer 8

repelled by water on both sides

Question 9

How do lipid-soluble materials move through the membrane?

Answer 9

via phospholipid portion

Question 10

Functions of phospholipids in membrane? (3)

Answer 10

allow lipid-soluble substances to enter + leave cell

prevent water-soluble substances entering + leaving cell

make membrane flexible + self-sealing

Question 11

Are proteins interspersed throughout the cell surface membrane?

Answer 11

yes

Question 12

One way in which proteins embedded in phospholipid bilayer?

Answer 12

proteins occur in surface of bilayer + never extend completely across it

Question 13

What do proteins that occur on the surface of the bilayer and not extend completely across it do?

Answer 13

give mechanical support to membrane or in conjunction with glycolipids, as cell receptors for molecules i.e. hormones

Question 14

Another way in which proteins embedded into phospholipid bilayer?

Answer 14

proteins completely span phospholipid bilayer from one side to other

Question 15

examples of proteins that completely span phospholipid bilayer from one side to other?

Answer 15

protein channels

carrier proteins

Question 16

What do protein channels do?

Answer 16

form water-filled tubes to allow water-soluble ions to diffuse across membrane

Question 17

What do carrier proteins do?

Answer 17

bind to ions or molecules like glucose and amino acids, then change shape to move these molecules across membrane

Question 18

Function of proteins in membrane? (6)

Answer 18

provide structural support

act as channels transporting water-soluble substances across membrane

allow active transport across membrane through carrier proteins

form cell-surface receptors for identifying cells

help cells adhere together

act as receptors, i.e. for hormones

Question 19

Where do cholesterol molecules occur within?

Answer 19

phospholipid bilayer of cell-surface membrane

Question 20

What do the cholesterol molecules do?

Answer 20

add strength to membranes

Question 21

Are cholesterol molecules hydrophobic or hydrophilic?q

Answer 21

very hydrophobic

Question 22

As cholesterol molecules are very hydrophobic what important role do they play?

Answer 22

preventing loss of water + dissolved ions from cell

Question 23

What other important role does cholesterol molecules carry out regarding fatty acid tails?

Answer 23

pull together fatty acid tails of phospholipid molecules, limiting their movement and that of other molecules but without making membrane as a whole too rigid

Question 24

Function of cholesterol in membrane? (3)

Answer 24

reduce lateral movement of other molecules including phospholipids

make membrane less fluid at high temperatures

prevent leakage of water + dissolved ions from cell

Question 25

What are glycolipids made up of?

Answer 25

a carbohydrate covalently bonded with a lipid

Question 26

Where does the carbohydrate portion of the glycolipid extend from into?

Answer 26

from phospholipid bilayer into the watery environment outside cell where it acts as a cell-surface receptor for specific chemicals

Question 27

An example where glycolipids act as a receptor for specific chemicals?

Answer 27

human ABO blood system operates as a result of glycolipids on the cell-surface membrane

Question 28

Functions of glycolipids in the membrane? (3)

Answer 28

act as recognition sites

help maintain stability of membrane

help cells to attach to one another and so form tissues

Question 29

What are glycoproteins made up of?

Answer 29

carbohydrate chain attached to many extrinsic proteins on the outer surface of the cell membrane

Question 30

What do the glycoproteins act as?

Answer 30

cell-surface receptors

Question 31

What do glycoproteins act as receptor for more specifically?

Answer 31

hormones and neurotransmitters

Question 32

Function of glycoproteins in the membrane? (3)

Answer 32

act as recognition site

help cells attach to one another and so form tissues

allow cells to recognise one another, e.g. lymphocytes can recognise an organism’s own cells

Question 33

Why do most molecules not freely diffuse across the cell-surface membrane? (4)

Answer 33

not soluble in lipids

too large

of same charge as charge on protein channels

electrically charged

Question 34

Why are molecules that are not soluble in lipids not able to diffuse across the cell-surface membrane?

Answer 34

they cannot pass through the phospholipid bilayer

Question 35

Why are large molecules not able to diffuse across the cell-surface membrane?

Answer 35

cannot pass through the channels in the membrane

Question 36

Why are molecules that have the same charge as the protein channels unable to diffuse across the cell-surface membrane?

Answer 36

they’re repelled

Question 37

Why are molecules that are electrically charged unable to diffuse across the cell-surface membrane?

Answer 37

have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer

Question 38

What is the arrangement of all the various molecules combined into the structure of the cell-surface membrane known as?

Answer 38

fluid-mosaic model

Question 39

Why is the model known as ‘fluid’ in fluid-mosaic?

Answer 39

individual phospholipid molecules can move relative to one another

Question 40

What does the fluidity of the phospholipid molecules give?

Answer 40

give membrane a flexible structure that’s constantly changing in shape

Question 41

Why is the model known as ‘mosaic’ in fluid-mosaic?

Answer 41

as proteins that are embedded in phospholipid bilayer vary in shape, size and pattern in same way as stones/tiles in mosaic