Cell membranes

Question 1

describe the structure of a phospholipid

Answer 1

a polar hydrophilic phosphate head attached to who non-polar hydrophobic fatty acid tails.

Question 2

what is the basis go the cell membrane called?

Answer 2

a phospholipid bilayer

Question 3

why can’t hydrophilic substances pass through the membrane?

Answer 3

hydrophilic substances are polar which can’t easily pass through the non polar region of the membrane. However water molecules can pass as they are very small but the process is slow as it is polar.

Question 4

why is the lipid bilayer model also known as the fluid mosaic model?

Answer 4

because the phospholipids are free to move (fluid) and the proteins embedded can vary in size and shape (like tiles in a mosaic)

Question 5

structure of cholesterol?

Answer 5

cholesterol is a lipid with a polar, hydrophilic end which attracts to the phosphate head and a non-polar, hydrophobic end which attracts to the tails

Question 6

what is the function of cholesterol?

Answer 6

to add stability to the membranes and control fluidity. It also helps to reduce the movement of water soluble chemicals across the membrane

Question 7

What are intrinsic proteins?

Answer 7

proteins that are fully embedded in the membrane from one side to the other. Intrinsic proteins have hydrophobic amino acids outside the surface which interact with the hydrophobic tails.

Question 8

What is the function of carrier proteins?

Answer 8

They change their shape and position, to transfer molecules or ions involving passive transport and active transport.

Question 9

what is the function of channel proteins?

Answer 9

provides a channel which allows polar molecules and ions to diffuse involving passive transport

Question 10

what are extrinsic proteins? and what is its role?

Answer 10

proteins found on only one side of the membrane. Some extrinsic proteins play a structural role, act as enzymes and some act as receptors

Question 11

what are glycoproteins and what are the functions?

Answer 11

proteins with an attached carbohydrate. They play a role in cell adhesion and as receptors for cell signalling

Question 12

what are glycolipids and what is the function?

Answer 12

lipids (phospholipids) attached to a carbohydrate. These molecules are also called antigens which can be recognised by the immune system.

Question 13

what is the affect of temperature on the cell membrane?

Answer 13

When temperature increases, the phospholipids will have more kinetic energy. This makes the membrane more fluid and it will begin to lose its structure. The loss of structure increases permeability. Proteins will also be denatured.

Question 14

what is the affect of solvents on the cell membrane?

Answer 14

organic solvents will dissolve the membrane which causes it to becomes more fluid and permeable.

Question 15

what is diffusion?

Answer 15

a passive process where the net movement of particles from an area of a high conc to low conc

Question 16

factors affecting diffusion

Answer 16

-temperature
-concentration difference: the greater the conc gradient, the higher the rate of diffusion as the overall movement will be larger.
-surface area
-membrane thickness

Question 17

what is facilitated diffusion?

Answer 17

diffusion across a membrane through channel proteins and carrier proteins.

Question 18

how does facilitated diffusion occur in the carrier proteins?

Answer 18

carrier proteins have a binding site for specific chemicals. When the chemical binds the tertiary structure of the protein changes which brings the chemical across the membrane where it is now released

Question 19

how does facilitated diffusion occur in protein channels?

Answer 19

The central pore in protein channels are filled with hydrophilic amino acids and contains water allowing hydrophilic substances to pass. Protein channels are also selective and needs a trigger e.g chemical to bind or voltage change.

Question 20

are protein channels selectively permeable?

Answer 20

yes as most protein channels are specific to one molecule or ion.

Question 21

the factors of facilitated diffusion is the same as simple diffusion but facilitated diffusion has one extra factor which is…

Answer 21

facilitated diffusion is also dependent on the number of channel proteins.

Question 22

what is active transport?

Answer 22

the movement of molecules or ions from a region of lower concentration to a region of higher concentration. This requires metabolic energy supplied by ATP and carrier proteins. Energy is needed as the particles are moving against the concentration gradient.

Question 23

Describe the process of active transport in a carrier protein?

Answer 23

1) The molecule/ion binds to receptors in the channel of the carrier protein on the outside of the cell.
2) Inside of the cell, ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.
3)Binding of the phosphate molecule to the carrier protein causes the protein to change shape
4)the molecule is released to the other side of the cell
5)the phosphate molecule is released from the carrier protein and recombines with ADP to form ATP causing the carrier protein to return back to its original shape.

Question 24

what are the two points you need to know about active transport?

Answer 24

1) it requires lots of ATP so cells need lots of mitochondria
2)the process is selective, specific substances are transported by specific carrier proteins.

Question 25

what is osmosis?

Answer 25

Osmosis is the NET movement of water molecules from a region of higher water potential to a region of MORE NEGATIVE water potential through a partially permeable membrane.

Question 26

what is water potential?

Answer 26

the pressure exerted by water molecules as they collide with a membrane of container. It is measured in units of pressure pascals (Pa) or kilopascals (kPa). The symbol is called psi and looks like the letter u with a line through

Question 27

pure water has the highest water potential, what is it?

Answer 27

0kPa. The more concentrated the solution, the more negative the water potential.

Question 28

how does hydrostatic pressure occur?

Answer 28

When water is moved into the cell

Question 29

what happens when hydrostatic pressure increases in animal cells?

Answer 29

the cell membrane will not withstand the pressure and will burst, an event called cytolysis.

Question 30

what happens when water moves into a plant cell?

Answer 30

hydrostatic pressure increases which increases the turgor pressure which is the pressure against the cell wall. As the turgor pressure increases it resists the entry of further water and the cell is turgid.

Question 31

what happens when water moves out of a plant cell?

Answer 31

the cell surface membrane is pulled away cell water causing it to be plasmolysed.

Question 32

what is bulk transport?

Answer 32

it is another form of active transport. Larger molecules (enzymes) that are too big to move through the channel or carrier proteins move through bulk transport.

Question 33

describe endocytosis

Answer 33

endocytosis is a form of bulk transport into the cell. Endocytosis for liquids is called pinocytosis and for solids it is called phagocytosis. The process is the same for both:
-the cell membrane invaginates (bends inwards)
-the membrane encircles the material forming a vesicle
-At this stage, what takes place depends on the material in the vesicle

Question 34

describe exocytosis

Answer 34

a form of bulk transport out the cell. Vesicles formed by the Golgi apparatus, move towards and fuse with the surface cell membrane causing them to be released.

Question 35

why is energy in the form of ATP needed for endocytosis and exocytosis?

Answer 35

because of the movement of vesicles along the cytoskeleton

Question 36

Describe how to investigate the effect of temperature on the permeability of cell membranes using beetroot

Answer 36

1) cut 5 equal pieces of beetroot using a cork borer.
2) place each sample in a boiling tube with distilled water
3)put each tube in different water baths with different temperatures of 10 degrees celsius intervals.
4) take out the samples after 5 minutes
5)put the samples in cuvettes and record their absorbencies using a colorimeter with a blue filter

Question 37

what is the function of phospholipids?

Answer 37

provides a barrier to polar/large molecules

Question 38

what is cell signalling?

Answer 38

the communication between cells

Question 39

how do the cell surface membranes contribute to the cell signalling?

Answer 39

cell signal molecule is released by secretion, proteins have receptors and the shape of the signal and the receptor are complementary.

Question 40

what is the purpose for organelles inside cells to have membranes?

Answer 40

-compartmentalisation
-site for attachment
-partially permeable

Question 41

Describe how the structure of a cell membrane allows them to act as the bilayer of the plasma membrane (3 marks)

Answer 41

hydrophilic phosphate head and hydrophobic acid tails. (1) The medium outside cells and inside is aqueous (1) causing fatty acid tails to point inwards and phosphate heads out (1)

Question 42

what is a CISternae?

Answer 42

flattened membrane vesicle