2.1- Cell structure

Question 1

A stomach ulcer is caused by damage to the cells of the stomach lining. People with
stomach ulcers often have the bacterium Helicobacter pylori in their stomachs.
A group of scientists was interested in trying to determine how infection by H. pylori
results in the formation of stomach ulcers.
The scientists grew different strains of H. pylori in liquid culture.
The table below shows the substances released by each of these strains.

The scientists centrifuged the cultures of each strain to obtain cell-free liquids. They
added each liquid to a culture of human cells. They then recorded the amount of damage
to the human cells.
Their results are shown below. The error bars show ± 1 standard deviation.

Describe and explain how centrifuging the culture allowed the scientists to obtain a
cell-free liquid. (3)

Answer 1

) 1. Large / dense / heavy cells;
2. Form pellet / move to bottom of tube (when centrifuged);
3. Liquid / supernatant can be removed.
Must refer to whole cells.

Question 2

A stomach ulcer is caused by damage to the cells of the stomach lining. People with
stomach ulcers often have the bacterium Helicobacter pylori in their stomachs.
A group of scientists was interested in trying to determine how infection by H. pylori
results in the formation of stomach ulcers.
The scientists grew different strains of H. pylori in liquid culture.
The table below shows the substances released by each of these strains.

The scientists centrifuged the cultures of each strain to obtain cell-free liquids. They
added each liquid to a culture of human cells. They then recorded the amount of damage
to the human cells.
Their results are shown below. The error bars show ± 1 standard deviation.

role of lysosomes

Answer 2

Break down cells / cell parts / toxins.

Idea of ‘break down / digestion’ needed, not just damage

Question 3

A stomach ulcer is caused by damage to the cells of the stomach lining. People with
stomach ulcers often have the bacterium Helicobacter pylori in their stomachs.
A group of scientists was interested in trying to determine how infection by H. pylori
results in the formation of stomach ulcers.
The scientists grew different strains of H. pylori in liquid culture.
The table below shows the substances released by each of these strains.

The scientists centrifuged the cultures of each strain to obtain cell-free liquids. They
added each liquid to a culture of human cells. They then recorded the amount of damage
to the human cells.
Their results are shown below. The error bars show ± 1 standard deviation.

) H. pylori cells produce an enzyme that neutralises acid.
Suggest one advantage to the H. pylori of producing this enzyme. (2)

Answer 3

1. To stop / reduce them being damaged / destroyed / killed;
   Reject (to stop) bacteria being denatured.
2. By stomach acid.

Question 4

A stomach ulcer is caused by damage to the cells of the stomach lining. People with
stomach ulcers often have the bacterium Helicobacter pylori in their stomachs.
A group of scientists was interested in trying to determine how infection by H. pylori
results in the formation of stomach ulcers.
The scientists grew different strains of H. pylori in liquid culture.
The table below shows the substances released by each of these strains.

The scientists centrifuged the cultures of each strain to obtain cell-free liquids. They
added each liquid to a culture of human cells. They then recorded the amount of damage
to the human cells.
Their results are shown below. The error bars show ± 1 standard deviation.

) What do these data suggest about the damage caused to human cells by the toxin
and by the enzyme that neutralises acid?
Explain your answer. (3)

Answer 4

1. More cell damage when both present / A;
2. Some cell damage when either there on their own / some cell damage in
   B and C;
   MP1 and MP2 − figures given from the graph are insufficient.
3. Standard deviation does not overlap for A with B and C so
   difference is real;
   MP3 and MP4 both aspects needed to gain mark.
4. Standard deviations do overlap between B and C so no real
   difference

Question 5

The scientists carried out a further investigation. They treated the liquid from strain
A with a protein-digesting enzyme before adding it to a culture of human cells. No
cell damage was recorded.
Suggest why there was no damage to the cells. (3)

Answer 5

1. Enzyme (a protein) is broken down (so no enzyme activity);
   Accept hydrolyse / digested for ‘broken down’.
2. No toxin (as a result of protein-digesting enzyme activity);
   Must be in the correct context.
3. (So) toxin is protein.
   This must be stated, not inferred from use of
   ‘protein−digesting enzyme’.

Question 6

Q3.(a) Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue. (3)

Answer 6

1. How to break open cells and remove debris;
2. Solution is cold / isotonic / buffered;
3. Second pellet is chloroplast.

Question 7

Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue. (3)

Answer 7

1. How to break open cells and remove debris;
2. Solution is cold / isotonic / buffered;
3. Second pellet is chloroplast.

Question 8

The figure below shows a photograph of a chloroplast taken with an electron
microscope.

Name the parts of the chloroplast labelled A and B. (2)

Answer 8

1. A stroma;

2. B granum.

Question 9

Name two structures in a eukaryotic cell that cannot be identified using an optical
microscope. (1)

Answer 9

Mitochondrion / ribosome / endoplasmic reticulum / lysosome / cell-surface
membrane.

Question 10

Q4.Starch and cellulose are two important plant polysaccharides.
The following diagram shows part of a starch molecule and part of a cellulose molecule.

(a) Explain the difference in the structure of the starch molecule and the cellulose
molecule shown in the diagram above (2)

Answer 10

1. Starch formed from α-glucose but cellulose formed from β-glucose;
2. Position of hydrogen and hydroxyl groups on carbon atom 1 inverted.

Question 11

Starch molecules and cellulose molecules have different functions in plant cells.
Each molecule is adapted for its function.
Explain one way in which starch molecules are adapted for their function in plant
cells. (2)

Answer 11

1. Insoluble;
2. Don’t affect water potential;
   OR
3. Helical;
   Accept form spirals
4. Compact;
   OR
5. Large molecule;
6. Cannot leave cell

Question 12

Explain how cellulose molecules are adapted for their function in plant ce
lls (3)

Answer 12

1. Long and straight chains;
2. Become linked together by many hydrogen bonds to form fibrils;
3. Provide strength (to cell wall).

Question 13

Silkworms secrete silk fibres, which are harvested and used to manufacture silk fabric.
Scientists have produced genetically modified (GM) silkworms that contain a gene from a
spider.
The GM silkworms secrete fibres made of spider web protein (spider silk), which i
s stronger than normal silk fibre protein.
The method the scientists used is shown in the figure below.

Suggest why the plasmids were injected into the eggs of silkworms, rather than into
the silkworms.
(2)

Answer 13

1. (If injected into egg), gene gets into all / most of cells of silkworm;
2. So gets into cells that make silk.

Question 14

5b - Suggest why the scientists used a marker gene and why they used the EGFP gene.

Answer 14

1. Not all eggs will successfully take up the plasmid;

2. Silkworms that have taken up gene will glow.

Question 15

5c- The scientists ensured the spider gene was expressed only in cells within the silk
glands.
(c) What would the scientists have inserted into the plasmid along with the spider gene
to ensure that the spider gene was only expressed in the silk glands of the
silkworms?

Answer 15

Promoter (region / gene).

Question 16

5d- Suggest two reasons why it was important that the spider gene was expressed only
in the silk glands of the silkworm

Answer 16

1. So that protein can be harvested;

2. Fibres in other cells might cause harm.

Question 17

Q6.(a) Describe how you could make a temporary mount of a piece of plant tissue to observe
the position of starch grains in the cells when using an optical (light) microscope. (4)

Answer 17

1. Add drop of water to (glass) slide;
2. Obtain thin section (of plant tissue) and place on slide / float on drop of
   water;
3. Stain with / add iodine in potassium iodide.
4. Allow any appropriate method that avoids trapping air
   bubbles
5. Lower cover slip using mounted needle.

Question 18

A transmission electron microscope was used to produce the image in the figure
above.
Explain why. (2)

Answer 18

1. High resolution;

2. Can see internal structure of organelles.

Question 19

Describe how phospholipids are arranged in a plasma membrane. (2)

Answer 19

1. Bilayer;
   Accept double layer
   Accept drawing which shows bilayer
2. Hydrophobic / fatty acid / lipid (tails) to inside;
3. Polar / phosphate group / hydrophilic (head) to outside;
4. & 3. need labels
5. & 3. accept water loving or hating

Question 20

Cells that secrete enzymes contain a lot of rough endoplasmic reticulum (RER) and
a large Golgi apparatus.
(i) Describe how the RER is involved in the production of enzymes. (2)

Answer 20

1. (Rough endoplasmic reticulum has) ribosomes;
   accept “contains / stores”
2. To make protein (which an enzyme is);
   Accept amino acids joined together / (poly)peptide
   Reject makes amino acids
   Ignore glycoprotein

Question 21

Describe how the Golgi apparatus is involved in the secretion of enzymes. (1)

Answer 21

) (Golgi apparatus) modifies (protein)
OR
packages / put into (Golgi) vesicles
OR
transport to cell surface / vacuole;
Accept protein has sugar added
Reject protein synthesis
Accept lysosome formation

Question 22

Q2.​ ​ ​ (a) The structure of a cholera bacterium is different from the structure of an epithelial cell from
the small intestine. Describe how the structure of a cholera bacterium is different. [5]

Answer 22

1 Cholera bacterium is prokaryote;
2 Does not have a nucleus/nuclear envelope/ has DNA free in cytoplasm/has loop of DNA;
3 & 4 Any two from
No membrane-bound organelles / no mitochondria / no golgi / no endoplasmic reticulum / no microvilli etc;
5 Small ribosomes only;
6 & 7 Any two from
Capsule / flagellum / plasmid / cell wall / etc; [5 max]
(Total 5)

Question 23

The epithelial cells of the small intestine have large numbers of organelle A.
Explain how this is an adaptation for the function of these cells. [3]

Answer 23

(d) provide/produce energy or ATP (reject create energy);
high respiration (rate) (accept lots);
for active uptake / transport (accept description);
absorption of digested food/substances/products/correctly named product; [3 max]

Question 24

​A scientist examined the structure of mustard plant leaves. He viewed temporary mounts of
leaf tissues with an optical microscope. The figure below shows a drawing of typical results.

(a) Describe how temporary mounts are made. (2)

Answer 24

1. Thin slice/section;
2. Put on slide in water / solution / stain;
3. Add cover slip;

Question 25

Scientists use optical microscopes and transmission electron microscopes (TEMs) to investigate cell
structure. Explain the advantages and limitations of using a TEM to investigate cell structure. [5]

Answer 25

Advantages:
1 Small objects can be seen;
2 TEM has higher resolution;
3 Wavelength of electrons shorter;
Advantages: allow maximum of 3 marks.
Limitations:
4 Cannot look at living cells;
5 Must be in a vacuum;
6 Must cut section / thin specimen;
7 Preparation may create artefact
8 Does not produce colour image;
Limitations: allow maximum of 3 marks. [5 max]

Question 26

Explain why the solution used was

i) ice-cold (1

Answer 26

Reduce / prevent enzyme activity;

Question 27

Explain why the solution used was isotonic. (2)

Answer 27

Prevents osmosis / no (net) movement of water;
So organelle / named organelle does not burst / shrivel;
Q Allow reference to cell rather than organelle for first mark
point only.
Regard damage as neutral

Question 28

People with mitochondrial disease have mitochondria that do not function properly.
Some people with mitochondrial disease can only exercise for a short time. Explain
why a person with mitochondrial disease can only exercise for a short time. (2)

Answer 28

(Mitochondria) use aerobic respiration;

Mitochondria produce ATP / release energy required for muscles (to contract);
Q Do not accept reference to making / producing energy.

Question 29

Name two structures present in eukaryotic cells that are not present in the cells of
prokaryotes. (2)

Answer 29

two of the following:
nucleus;
OR
nuclear envelope / mitochondria / chloroplasts / sER / rER /
golgi apparatus / 80s ribosomes
linear DNA / chromosomes / lysosomes / vacuole / vescicles /
cellulose cell wall;

Question 30

Describe how you could make a temporary mount of a piece of plant tissue to observe
the position of starch grains in the cells when using an optical (light) microscope. (4)

Answer 30

1. Add drop of water to (glass) slide;
2. Obtain thin section (of plant tissue) and place on slide / float on drop of
   water;
3. Stain with / add iodine in potassium iodide;
4. Allow any appropriate method that avoids trapping air
   bubbles
5. Lower cover slip using mounted needle;

Question 31

Explain why the homogenate was filtered before spinning at low speed in the
centrifuge. (2)

Answer 31

removes debris / intact cells / sand;

which would contaminate sediment A / interfere with the results;

Question 32

What property of cell organelles allows them to be separated in this way? (1)

Answer 32

density / size / mass / weight;

Question 33

Explain why the organelles in sediment C could be seen with a transmission electron
microscope but not with an optical microscope. (2)

Answer 33

an electron microscope has a higher resolution;

electrons with shorter wavelength;

Question 34

Read the following passage.

In a human, there are over 200 different types of cell clearly distinguishable from each
other.
What is more, many of these types include a number of different varieties. White blood
cells,
for example, include lymphocytes and granulocytes.
Although different animal cells have many features in common, each type has adaptations.
5 associated with its function in the organism. As an example, most cells contain the same
organelles, but the number may differ from one type of cell to another. Muscle cells contain
many mitochondria, while enzyme-secreting cells from salivary glands have particularly
large
amounts of rough endoplasmic reticulum.
The number of a particular kind of organelle may change during the life of the cell. An
10 example of this change is provided by cells in the tail of a tadpole. As a tadpole matures
into
a frog, its tail is gradually absorbed until it disappears completely. Absorption is associated
with an increase in the number of lysosomes in the cells of the tail.
Use information from the passage and your own knowledge to answer the following
questions.
(a) Explain the link between.

(i) mitochondria and muscle cells (lines 6 - 7); (3)

Answer 34

Mitochondria site of respiration;
Production of ATP / release of energy;
For contraction;
Do not award credit for making or producing energy.

Question 35

Explain the link between rough endoplasmic reticulum and enzyme-secreting cells from salivary glands
(lines 7 - 8). (2)

Answer 35

Enzymes are proteins;

Proteins synthesised / made on ribosomes;

Question 36

Read the following passage.

In a human, there are over 200 different types of cell clearly distinguishable from each
other.
What is more, many of these types include a number of different varieties. White blood
cells,
for example, include lymphocytes and granulocytes.
Although different animal cells have many features in common, each type has adaptations.
5 associated with its function in the organism. As an example, most cells contain the same
organelles, but the number may differ from one type of cell to another. Muscle cells contain
many mitochondria, while enzyme-secreting cells from salivary glands have particularly
large
amounts of rough endoplasmic reticulum.
The number of a particular kind of organelle may change during the life of the cell. An
10 example of this change is provided by cells in the tail of a tadpole. As a tadpole matures
into
a frog, its tail is gradually absorbed until it disappears completely. Absorption is associated
with an increase in the number of lysosomes in the cells of the tail.

Use information in the passage to explain how a tadpole’s tail is absorbed as a tadpole
changes into a frog. (2)

Answer 36

Lysosomes produce / contain enzymes;

Which break down / hydrolyse proteins / substances / cells of tail;

Question 37

describe how you would obtain a sample of
undamaged chloroplasts. Use your knowledge of cell fractionation and
ultracentrifugation to answer this question. (6)

Answer 37

1. Chop up (accept any reference to crude breaking up);
2. Cold;
3. Buffer solution;
4. Isotonic / same water potential;
5. Filter and centrifuge filtrate;
6. Centrifuge supernatant;
7. At higher speed;
8. Chloroplasts in (second) pellet;

Question 38

Q7.(a) Describe how bacteria are destroyed by phagocytes. (3)

Answer 38

1. (Phagocyte engulfs) to form vacuole / vesicle / phagosome;

Accept surrounds bacteria with membrane

2. Lysosome empties contents into vacuole / vesicle / phagosome;

Accept joins / fuses

3. (Releasing) enzymes that digest / hydrolyse bacteria;
   Ignore breakdown / destroy / lytic enzymes

Question 39

Give two structures a bacterial cell may have that a white blood cell does not have. (2)

Answer 39

Examples,

1. Cell wall;
2. Capsule / slime layer;
3. Circular DNA;

Reject “circular chromosome”

4. Naked DNA / DNA without histones;
5. Flagellum;
6. Plasmid;
7. Pilus;
8. 70s / smaller ribosomes;
9. Mesosome;

Question 40

Describe and explain how cell fractionation and ultracentrifugation can be used to
isolate mitochondria from a suspension of animal cells. (5)

Answer 40

1. Cell homogenisation to break open cells;
2. Accept suitable method of breaking open cells.
3. Filter to remove (large) debris / whole cells;
4. Reject removes cell walls.
5. Use isotonic solution to prevent damage to mitochondria / organelles;
6. Ignore to prevent damage to cells.
7. Keep cold to prevent / reduce damage by enzymes / use buffer to
   prevent protein / enzyme denaturation;
8. Centrifuge (at lower speed / 1000 g) to separate nuclei / cell fragments /
   heavy organelles;
9. Ignore incorrect numerical values.
10. Re-spin (supernatant / after nuclei / pellet removed) at higher speed to
    get mitochondria in pellet / at bottom.

Question 41

Describe the principles and the limitations of using a transmission electron
microscope to investigate cell structure. (5)

Answer 41

Principles:
1. Electrons pass through / enter (thin) specimen;
2. Denser parts absorb more electrons;
3. (So) denser parts appear darker;
4. Electrons have short wavelength so give high resolution;
Principles:
Allow maximum of 3 marks
Limitations:
5. Cannot look at living material / Must be in a vacuum;
6. Specimen must be (very) thin;
7. Artefacts present;
8. Complex staining method / complex / long preparation time;
9. Image not in 3D / only 2D images produced.
Limitations:
Context of limitation must be clear, not simply explaining how
TEM works
E.g “allows you to see organelles as a thin section is used” is
not a limitation
Allow maximum of 3 marks
Ignore ref to colour

Question 42

The student cut thin sections of tissue to view with an optical microscope.
Explain why it was important that the sections were thin. (2)

Answer 42

1. To allow (more) light through;
   Accept: transparent
2. A single / few layer(s) of cells to be viewed.
   Accept: (thin) for better / easier stain penetration

Question 43

Suggest two ways the student could improve the quality of her scientific drawing of
this gill.
[2 marks]

Answer 43

1. Don’t use shading;
2. Only use single lines/don’t use sketching
   (lines)/ensure lines are
   continuous/connected;
3. Add further labels/annotations;
4. Don’t cross label lines;
5. Add magnification/scale (bar);

Question 44

Q1. the table shows the dimensions of different organelles found in animal cells. Name those organelles in the table that would be visible using good quality light microscope to explain your answer (
3)
um
Lysosome- 0.1 
Mitocho- 2
nucleus- 5
ribosome- 0.02
vesicle- 0.05

Answer 44

mitochondrion
nucleus

resolution of light microscope is not good enough to show objects smaller than 0.2um