1.11 End of Chapter Questions
How to interpret Cambridge Marking Schemes
Key Symbols and Meanings
- A or Accept
- Meaning: Indicates an alternative acceptable answer.
- Example: For the question “Name a process that releases energy in cells,” the answer “respiration” or “aerobic respiration” would both be accepted.
- R or Reject
- Meaning: Highlights answers that are incorrect or unacceptable.
- Example: If the question asks for “a type of asexual reproduction,” the answer “fertilization” would be rejected.
- Bold Semi-Colon (;)
- Meaning: Each semi-colon represents 1 mark for a distinct point.
- Example:
- “Oxygen diffuses into the cell; carbon dioxide diffuses out.” (2 marks: 1 mark for each point).
- Slash ( / )
- Meaning: Denotes alternative answers that are worth the same mark.
- Example: “Lungs/trachea” means either answer is correct.
- Brackets ( )
- Meaning: Text in brackets is not required to earn the mark but does not invalidate the answer if included.
- Example: “Mitosis (in somatic cells)” – “Mitosis” alone earns the mark.
- Underlining
- Meaning: Emphasizes essential words or phrases that must be included to earn the mark.
- Example: If the mark scheme underlines “diffusion,” the answer must explicitly use the word “diffusion” to be awarded marks.
Additional Notes
AW (Alternative Wording)
- Meaning: Acceptable if the essential meaning is conveyed, even if the wording differs slightly from the mark scheme.
- Example: For “Describe the role of the cell membrane,” the mark scheme may accept “controls what enters and exits the cell” instead of “regulates the movement of substances.”
AVP (Additional Valid Point)
- Meaning: Awards marks for relevant additional points made by students that are not explicitly listed in the mark scheme.
- Example: In a question about photosynthesis, if the student mentions “oxygen is a by-product” and it’s not explicitly listed, they may receive an extra mark if labeled as AVP.
ORA (Or Reverse Argument)
- Meaning: Accepts an opposite expression of the same idea.
- Example:
- Question: “What happens to enzyme activity as temperature increases?”
- “Activity increases between 20°C and 40°C; ORA: activity decreases above 40°C due to denaturation.”
Max
- Meaning: Indicates the maximum number of marks that can be awarded for a question or section.
- Example: If a 4-mark question asks for functions of the liver, even if the student lists 6 correct functions, only 4 marks will be awarded.
Questions
1. Which of the following cell structures is visible using a light microscope?
A) Mitochondrion
B) Ribosome
C) Rough ER
D) Smooth ER
Correct Answer: A) Mitochondrion
Notes:
- Ribosomes and ER structures are visible with an electron microscope.
- Mitochondria can be seen with a light microscope, though they are at the edge of resolution (~200 nm).
- Ribosomes, rough ER, and smooth ER are too small to be resolved by a light microscope as they are below 200 nm in size.
2. Why do electron microscopes achieve higher resolution compared to light microscopes?
A) Electrons are negatively charged.
B) Electrons can be focused with electromagnets.
C) Electrons have very short wavelengths.
D) Electrons travel at the speed of light.
C) Electrons have very short wavelengths
Study Notes:
- A and D are unrelated to resolution.
- Shorter wavelengths allow electron microscopes to resolve structures as small as 0.1 nm.
- While B is true (electrons are focused with electromagnets), it does not directly explain higher resolution.
3. Which structure is found in animal cells but not in plant cells?
A) Cell surface membrane
B) Centriole
C) Chloroplast
D) Golgi body
Answer: B) Centriole
Study Notes:
- Centriole is involved in cell division and is unique to animal cells.
- Both A and D are found in all eukaryotic cells.
- C is specific to plant cells.
4. Fill in the table below to compare light microscopes and electron microscopes. Certain sections have already been completed for your reference.
| Feature | Light microscope | Electron microscope |
| source of radiation | ||
| wavelength of radiation used | ||
| maximum resolution | ||
| lenses | ||
| specimen | ||
| stains | ||
| image |
[8]
| Feature | Light microscope | Electron microscope |
| source of radiation | light; | electrons; |
| wavelength of radiation used | 400–700nm; | about 0.005nm |
| maximum resolution | 200nm; | 0.5nm in practice |
| lenses | glass | electromagnets; |
| specimen | living or non living or dead; | non-living or dead |
| stains | coloured dyes | heavy metals; |
| image | coloured | black and white; |
Award 1 mark for each correct entry in the table. [8]
Study Notes:
- Electron microscopes provide greater detail but require dead specimens.
- Light microscopes are versatile for live specimens but have lower resolution.
5. Identify ten structures that can be seen in an electron micrograph of an animal cell but are not present in bacterial cells.
Structures in Animal Cells:
- Nucleus
- Smooth ER
- Rough ER
- 80S Ribosomes (larger than 70S in bacteria)
- Linear DNA
- Chromatin/Chromosomes
- Lysosomes
- Golgi Apparatus
- Mitochondria
- Centrioles/Centrosomes
Study Notes:
These structures are specific to eukaryotes. Prokaryotes (bacteria) lack membrane-bound organelles.
6. When asked to differentiate between two items, it’s often because they share some characteristics and could potentially be mistaken for one another. In your response, it can be beneficial to highlight similarities in addition to differences, especially when applicable. For organelles, keep in mind that distinctions may involve both their structure and function. [10]
Distinguish between the following pairs of terms:
a) magnification and resolution [3]
- magnification is the number of times larger an image is compared with the real size of the object; AW
- resolution is the ability to distinguish between two separate points / the greater the resolution, the greater the detail that can be seen; AW
- a statement linking the terms, such as both terms used with reference to microscopy;
b) light microscope and electron microscope [2]
- light microscope uses light as a source of radiation;
- electron microscope uses electrons as a source of radiation;
c) nucleus and nucleolus [4]
- both organelles / both found in eukaryotic cells;
- nucleolus is located inside nucleus;
- nucleus controls cell activity;
- nucleolus makes ribosomes;
- AVP;; e.g. nucleus surrounded by envelope, no membrane round nucleolus
d) chromatin and chromosome [3]
- chromatin and chromosomes both contain DNA (and protein / histones) / both found in nucleus;
- chromatin is the loosely coiled form of chromosomes;
- chromatin is the form that exists between cell / nuclear divisions;
- chromosomes are formed just before / during, cell / nuclear division;
e) membrane and envelope [3]
- an envelope consists of two membranes (one just inside / outside the other);
- a membrane is a thin (partially permeable) barrier found around cells and some organelles;
- example of at least one organelle surrounded by an envelope is given;
- membranes found in / around all cells, envelopes only in eukaryotes;
f) smooth ER and rough ER [4]
- both consist of flattened membrane-bound sacs;
- both found spreading through cytoplasm of eukaryotic cells;
- smooth ER lacks ribosomes, rough ER has ribosomes on surface;
- one function of smooth ER given, e.g. makes lipids / steroids;
- rough ER transports proteins made by ribosomes on its surface;
g) prokaryote and eukaryote [4]
- prokaryotes have no nucleus, eukaryotes have nucleus;
- prokaryotes are smaller / simpler;
- prokaryotes have few organelles, eukaryotes have many organelles, some membranebound / compartmentalisation / more division of labour;
- one other important difference given / eukaryotes evolved from prokaryotes;
[Total: 23]
7. List:
a) three organelles each lacking a boundary membrane
b) three organelles each bounded by a single membrane
c) three organelles each bounded by two membranes (an envelope)
- In each case, one mark for any three appropriate organelles.
- a e.g. nucleolus, ribosome, centrioles, centrosome, microtubule;;;
- b e.g. lysosomes, rough ER, smooth ER, Golgi body;;;
- c nucleus, mitochondrion, chloroplast;;;
8. Identify each cell structure or organelle from its description below. [9]
a) Manufactures lysosomes:
b) Manufactures ribosomes:
c) Site of protein synthesis:
d) Buds off vesicles forming Golgi body:
e) Transports newly made proteins:
f) Makes ATP:
g) Controls cell activity (contains DNA):
h) Carries out photosynthesis:
i) Growth point for spindle microtubules:
j) Contains chromatin:
k) Thin, partially permeable barrier:
l) ~25 nm organelle:
a) Manufactures lysosomes: Golgi Apparatus
b) Manufactures ribosomes: Nucleolus
c) Site of protein synthesis: Ribosome
d) Buds off vesicles forming Golgi body: Rough ER
e) Transports newly made proteins: Rough ER
f) Makes ATP: Mitochondria
g) Controls cell activity (contains DNA): Nucleus
h) Carries out photosynthesis: Chloroplast
i) Growth point for spindle microtubules: Centrosome
j) Contains chromatin: Nucleus
k) Thin, partially permeable barrier: Membrane
l) ~25 nm organelle: Ribosome
[12]
9. The electron micrograph below shows part of a secretory cell from the pancreas. The secretory vesicles are Golgi vesicles and appear as dark round structures. The magnification is ×8000.
a) Complete the table. Use a ruler to help you find the actual sizes of the structures.
Give your answers in micrometres.
| Structure | Observed diameter (measured with ruler) | Actual size |
| maximum diameter of a Golgi vesicle | ||
| maximum diameter of nucleus | ||
| maximum length of the labelled mitochondrion |
[9]
- 1 mark for each accurately measured ‘observed size’ (to within ±2 mm) and 1 mark
- for each accurately calculated ‘actual size’;;;;;;
- 1 mark for applying the formula;
- 1 mark for measuring in mm and converting mm to µm for each calculation;
- 1 mark for rounding up actual size to no more than one decimal place;
b) Make a fully labelled drawing of representative parts of the cell. You do not have to draw everything, but enough to show the structures of the main organelles.
Use a full page of plain paper and a sharp pencil. [14]
- quality of drawing:
- heading/labels in pen/labels on left side of the drawing.
- Label lines horizontal to each other.
- No colouring of any structures.
- sharp pencil used/lines are consistently thick throughout;
- more than half of available space used;
- clean, continuous lines / not sketchy;
- interpretation of structures accurate;
- representative parts of main organelles drawn, including those below for which label
- marks are awarded;
[5]
labels:
- nucleus;
- nuclear envelope;
- nuclear pore;
- nucleolus;
- rough ER;
- ribosome(s);
- mitochondrion;
- crista or cristae;
- Golgi body;
- Golgi vesicle / secretory vesicle; [max. 9]
c) The mitochondria in pancreatic cells are mostly sausage-shaped in three dimensions.
Explain why some of the mitochondria in the electron micrograph below appear roughly circular. [1]
- mitochondria will appear circular if they are cut, in transverse section / across (the long axis); AW
d) The figure below shows a diagram based on an electron micrograph of a secretory cell from the pancreas. This type of cell is specialised for secreting (exporting) proteins.
Some of the proteins are digestive enzymes of the pancreatic juice. The cell is very active, requiring a lot of energy.
The arrows show the route taken by the protein molecules.
i. Describe briefly what is happening at each of the stages A, B, C and D. [8]
- A protein made on the ribosome is moving into the rough ER;
- B rough ER buds off small vesicles;
- vesicles fuse to form the Golgi body;
- (therefore) protein moves into Golgi body;
- protein may be modified / processed inside Golgi body;
- C Golgi body buds off Golgi vesicles;
- D Golgi vesicles travel to cell surface membrane;
- Golgi vesicle(s) fuses with cell surface membrane;
- protein / enzyme leaves cell;
- exocytosis / secretion;
ii. Name one molecule or structure which leaves the nucleus by route E. [1]
- ribosome / messenger RNA;
iii. Through which structure must the molecule or structure you named in ii pass to get through the nuclear envelope? [1]
- nuclear pore;
iv Name the molecule which leaves the mitochondrion in order to provide energy for this cell. [1]
- ATP;
[Total: 35]
10. One technique used to investigate the activity of cell organelles is called differential centrifugation. In this technique, a tissue is homogenised (ground in a blender), placed in tubes and spun in a centrifuge. This makes organelles sediment (settle) to the bottom of the tubes. The larger the organelles, the faster they sediment.
By repeating the process at faster and faster speeds, the organelles can be separated from each other according to size. Some liver tissue was treated in this way to separate ribosomes, nuclei and mitochondria. The centrifuge was spun at 1000 g, 10 000 g or 100 000 g (‘g ’ is gravitational force).
a) In which of the three sediments – 1000 g, 10 000 g or 100 000 g – would you expect to find the following?
i. ribosomes
ii. nuclei
iii. mitochondria [1]
- Ribosomes: 100,000 g.
- Nuclei: 1,000 g.
- Mitochondria: 10,000 g.
Study notes:
Definition of Differential Centrifugation
- A technique used to separate cell organelles based on their size and density by spinning a homogenized tissue sample at different speeds in a centrifuge.
Process Overview
- Homogenization:
- Tissue is ground to break open cells and release organelles.
- Centrifugation:
- The sample is spun at increasing speeds to cause organelles to sediment (settle) at the bottom of the tube.
- Larger, denser organelles sediment first, while smaller, lighter organelles require higher speeds.
Expected Sediments at Different Speeds
- 1000 g (Low Speed): Nuclei
- Reason:
- Nuclei are the largest and densest organelles in the cell.
- They sediment at the lowest speed (1000 g) due to their size and weight.
- Reason:
- 10,000 g (Medium Speed): Mitochondria
- Reason:
- Mitochondria are smaller and less dense than nuclei but larger than ribosomes.
- They require a higher speed (10,000 g) to sediment.
- Reason:
- 100,000 g (High Speed): Ribosomes
- Reason:
- Ribosomes are the smallest organelles and have a low density.
- They only sediment at very high speeds (100,000 g).
- Reason:
b) Liver tissue contains many lysosomes. Suggest why this makes it difficult to study mitochondria using the differential centrifugation technique. [4]
- lysosomes are, similar in size to / slightly smaller than, mitochondria;
- therefore sediment at same / similar, g force / speed;
- therefore contaminate mitochondrial sample; AW
- therefore cannot be sure whether effects due to mitochondria or lysosomes in any experiments;
[Total: 5]