Biology cell membranes: mark-scheme scheme answers

What's the approximate width of the cell membrane

7-10nmph

Explain why facilitated diffusion does not require energy from a cell.

moves down its concentration gradient and particles have their own energy

Explain why glucose cannot pass through a cell membrane by simple diffusion

Too large to fit through the Bilayer of phospholipids

Acting as a barrier

Name roles of the cell membrane

compartmentalisation- create s concentration gradient.

Describe the structure of the plasma membrane

Phospholipid bilayer contains proteins = hydrophobic head

Cholesterol inside bilayer between phospholipids

Glycoproteins and glycolipids sticking out of bilayer

How does the "fluid mosaic model" describe the structure of the plasma membrane?

Proteins scattered randomly between phospholipids

Proteins/phospholipids are free to move

Phospholipid belayer hydrophobic tail = facing inwards & hydrophilic phosphate head group facing out wards (external)

What two molecules is ATP made of

adenine and ribose

State control variables in the betroot experiment

time taken to wash beetroot

Same sa or length of betreet

Explain now carrying out replicates improves betroot investigation

Can calculate the mean percentage _ mean can be calculated _ identify anomalies _ improves repeatability

Outline the role of membranes within cells

Compartmentalisation

Provides attachment sites fit enzymes

Sites for chemical reactions

What's the effect of boiling and ethanol on membranes

.boiling damages cell surface membrane = so no osmosis

Ethanol a diss ones phospholipid belayer = so no osmosis

State how the student could reduce the uncertainty of their data

use more accurate apparatus

Define osmosis

Diffusion of water through a selectively permeable membrane down water potential gradient.

Explain the importance of osmosis in plants

Water enters vacuole.

Pressure against the cell wall ( turgor pressure)

Supports plant

Why is using percentage change in mass useful

Not an plants have same initial mass

Allows comparison

Suggest a practical error that affects osmosis -

Inadequate drying

More mass than other pieces

Specification point: what is the role of membranes with drugs

membrane bound receptors as sites where hormones and drugs can bind to

Specification point: state role of membranes

-Barriers

- chemical reactions

- cell communication

Explain why facilitated diffusion via GLUT proteins requires no metabolic energy ( 2)

Particles have their own kinetic energy

Movement down concentration gradient

Explain why glucose cannot pass through a cell membrane by simple diffusion

- phospholipids act as a barrier

- (glucose) molecules too large

Outline the role of membranes within cells. 3

purpose of / need for , compartments / separation ✓

allow formation of concentration gradients ✓

sites of , chemical reactions

Explain the role of the membrane in the rough endoplasmic reticulum. (2)

- compartmentalisation

- seperate proteins from cytoplasm

Suggest and explain why a low pH might cause the red pigment to leak out of the beetroot cells. (2)

(low pH) denatures / changes tertiary structure of , (membrane) proteins ✓

therefore membrane permeability (to pigment) is increased ✓

Explain why progesterone can move across membranes. (2)

-is hydrophobic or fat soluble

- diffuses through / is not repelled by ,the phospholipid (bilayer) / hydrophobic tails

How does the fluid mosaic model describe the structure of plasma membranes? ( 2)

phospholipid bilayer ✔

hydrophilic head facing outwards / towards external enviro

2 nment AND

hydrophobic tails, facing inwards / away from external environment ✔

Outline how the students could modify their investigation to get a more accurate value for the pH at which the red pigment begins to leak out of the beetroot cells.

(2)

use pH buffer range with narrower intervals ✓

pH (buffers) , close to .... Or between ....

Explain how the structure of phospholipid molecules allows for the formation of plasma membranes. (3)

- phosphate head , is hydrophilic bonds with water ✓

- fatty acid tails are hydrophobic ✓

- heads orientate towards water / tails orientate towards other fatty acids / tails orientate away from water ,

- - a bilayer formed ✓

24. The rough endoplasmic reticulum is where translation of some proteins takes place in a eukaryotic cell.

(3)

phospholipid (molecules form) bilayer ✓

(forming) flattened sacs ✓

covered with ribosomes /

For which macromolecule does a plant need both nitrogen and phosphorus?

DNA

Sodium chloride in solution dissociates into Na+ and Cl−.

Explain how the Casparian strip prevents these ions from reaching the xylem of the plant by the apoplast pathway.

(3)

strip is

1 impervious to, water / solutions (1)

2 forces water / solutions, to pass through cell surface, membrane (1)

3 phospholipid (bilayer), repels ions

The student kept the ethanol solutions at a constant temperature. State two other variables which need to be controlled in this investigation to ensure the data collected are valid. Beetroot experiments

- time taken to wash slices ✓

- use same colorimeter filter / same blank ✓

- pieces taken from same part of beetroot

Explain how carrying out replicates would improve this investigation. (2)

can calculate a mean ✓

allows anomalies to be identified ✓

Describe how you would use the same equipment to test if the thickness of the exchange surface affects the diffusion rate.

(2)

What does reduce uncertainty mean ? Example of how

Increasing accuracy of apparatus =

So use more accurate apparatus

Define osmosis [2]

diffusion of water across a selectively permeable membrane down water potential gradient

Explain the importance of osmosis in plant support.[4]

- water enters vacuole

- pressure against cell wall

- turgor pressure

- turgid cells support plant

Explain why it was necessary to calculate percentage change in mass.

different (starting) masses (of plant pieces) ✓

allows comparison (between plant pieces of different mass) ✓

Examples of practice errors measuring osmosis

- inadequate drying

- so more masses heavier than others

- piece cut from different (part of) potato ✓

(so) cells might have different water potential (at start) ✓

Do cell membranes provide support to cells?

No

What is isotonic?

the movement of water into the potato cells is equal to the movement of water out of the potato cells

describes the role of cholesterol in cell surface membranes in the human body?

Cholesterol binds to phospholipid fatty-acid tails, increasing the packing of the membrane, therefore reducing the fluidity of the membrane.

Which of the following is not a role of an intracellular membrane?

Cell signalling

Suggest why some cells did not stain blue at 20 °C.

- dead cells

Describe the functions of glycoproteins in the cell surface membrane. [6]

1 acting as) antigens ;

2 CELL recognition, (of cells) as, self /

non-self

3 CELL SIGNALLING described ; cells responds to chemicals of another cell

4 receptor / binding site, for, hormone

5 ref. to receptor / binding site / trigger, on transport proteins / AW ;

6 cell adhesion / to hold cells together TO FORM TISSUE;

7 attach to water molecules (to stabilise membrane / cell -

State one function for each of the components cholesterol glyco proteins and phospholipids

- stabilises lateral movement

- allow communication across membrane

- act as a barrier or acts as selectivity permeable

Explain what is meant by the term cell signalling. [2]

communication between cells

cell, recognition / identification ;

cells work together; to trigger, response

explain how a glycoprotein can act as a receptor

(receptor) specific shape

complementary to (shape of), trigger ;

(trigger binds / attaches to receptor ;

Small non-polar substances enter cells in different ways to large or polar substances.

Outline the ways in which substances, other than water, can enter a cell through the plasma (cell surface membrane) [5]

small, non-polar substances

diffuse (through membrane / phospholipid bilayer) ;

large substances

(using), transport / carrier, proteins ;

endocytosis / phagocytosis / described ;

polar substances

through, pore / channel, proteins ; (

(using), transport / carrier, proteins ;

Suggest three ways in which the student could have improved her investigation.

- more samples at each temperature ;

same / fixed, volume of water ;

all samples same, size / surface area ; ref to further cutting to increase surface area ;

pieces, rinsed / blotted, after cutting ;

more (intermediate) temperatures ;

same beetroot used / same part of beetroot used

Describe the structure of a plasma (cell surface) membrane. [5] or [3]

phospholipid bilayer containing proteins ;

head / hydrophilic region, facing outwards

OR

tail / hydrophobic region, facing inwards ;

ref to intrinsic and extrinsic (glyco)proteins /

described ;

idea of: glycoproteins / glycolipids, sticking out (of

bilayer / membrane);

cholesterol, inside bilayer / between phospholipids ;

State two functions of membranes WITHIN cells.

forms, vesicles / (named) organelle(s) ;

separate (contents of) organelles from cytoplasm /

compartmentalisation ;

site of (named), processes / reactions ;

provides surface for attachment (of enzymes / ribosomes) ;

control what substances, enter / leave, organelles ; or

Describe the arrangement and functions of two named components of a cell surface membrane.

A1 phospholipids form bilayer /described

F1 provide barrier to, large molecules e.g glucose

A2 proteins form, pores carriers

extrinsic / intrinsic / transmembrane / described,

proteins ;

F2 for (active) transport

A3 cholesterol molecules fit, within bilayer

F3 regulates fluidity ;

A4 glycoproteins / glycolipids , on surface

F4 cell signalling

Which component of a cell membrane becomes more fluid as temperature increases?

phospholipid bilayer

State what is meant by cell signalling. [2]

- communication between cells

(molecule released by one cell, attaches to another cell)

Explain how cell surface membranes contribute to the process of cell signalling [4] or [6]

release of signal molecule by, exocytosis

idea that: (proteins) glycoproteins / glycolipids, act as receptors OR;

idea that: receptor / signal, is specific;

idea that: shape of receptor and signal are complementary ;

idea that: attachment of signal molecule causes change (inside cell / on cell surface) ;

cell surface membrane allows entry of some signal molecules ;

Outline how the vesicles are moved from one organelle to another. [2]

cytoskeleton

provide, pathways for movement

(vesicle) moves along, microfilaments / microtubule ;

Cells in the pancreas secrete proteins such as the enzymes pancreatic amylase and protease. Describe how these extracellular enzymes are secreted from the cells.

-exocytosis

-vesicle fuses with cell surface membrane

-releasing its contents to the exterior

State two examples of active transport in cells.

For each example, you should name the substance that is transported and the cell

involved.

mineral) ions / salts / named e.g, (into) root hair (cell) ; hydrogen ions (out of) companion cells ;

(mineral) ions / salts / named e.g, (across) endodermis ; sucrose out of sieve tube at sink ;

Name the process by which water leaves a ce

Osmosis

describe the routes that water molecules take through the cell surface membrane

-fit between phospholipids

-via protein channels

.

Suggest how

receptor found only on, correct / target, (named) organelle ; idea that: address protein provides a way of,

labelling / identifying / recognising, the vesicle ; protein / COPI / COPII, has a specific shape ;

(shape of) receptor and (address) protein are compleme