▶️ Answer/Explanation
(a)
The optic nerve is the structure that carries visual information from the retina to the brain. It exits the back of the eye at a point known as the optic disc.
- At the optic disc, there are no photoreceptors—meaning no rods or cones are present—because it is the location where the axons of the ganglion cells converge and leave the eye to form the optic nerve.
- As a result, this region cannot detect light or visual stimuli, creating what is known as a blind spot in the visual field.
- Since there are no photoreceptors at this location, no photoreceptor density data can be recorded, which is why the graph shows a gap or lack of data at this angle.
(b)
- Mice have higher total photoreceptor density, but:
- The graph does not separate rods from cones.
- Only cones detect colour; rods do not.
- Mice are dichromatic (two cone types); humans are trichromatic (three cone types).
- Therefore, the graph does not provide enough evidence to conclude that mice have better colour vision. In fact, humans likely have better colour vision.
(c)
- Transmit signals from photoreceptors to ganglion cells.
- A cone typically connects to one bipolar cell → high resolution.
- Multiple rods often connect to one bipolar cell → increased sensitivity, reduced detail.
- Help process light intensity and contrast via ON and OFF pathways.
(d)
| Feature | Chemoreceptors | Mechanoreceptors |
|---|---|---|
| Stimulus type | Chemicals | Mechanical forces (pressure, vibration) |
| Function | Detect taste, smell, blood gases | Detect touch, sound, pressure |
| Example | Olfactory receptors (nose), taste buds | Pacinian corpuscles (skin), cochlear hair cells |
Markscheme:
(a) There are no photoreceptors/cones/rods
OR
It is the blind spot;
(b)
a. Mouse has greater photoreceptor density (than human);
b. The graph does not distinguish between types of photoreceptor;
c. Cones detect colours
OR
Rods provide monochromatic/night vision;
d. There is no data about colour vision in mice;
(c)
a. Bipolar cells interconnect/link photoreceptors to ganglion cells
OR
Bipolar cells pass impulses from photoreceptors to ganglion cells;
b. One bipolar cell receives signals from/synapses with one cone cell;
c. One bipolar cell receives signals from/synapses with several rod cells;
(d)
a. Chemoreceptors stimulated by chemicals but mechanoreceptors are stimulated by movement/forces/pressure;
b. Valid example of each;
▶️ Answer/Explanation
(a)
- Stem cells are undifferentiated cells with the capacity to:
- Self-renew (divide repeatedly), and
- Differentiate into one or more specialized cell types.
- Pluripotent stem cells can give rise to any cell type in the body (e.g., embryonic stem cells).
(b)
Step-by-step:
Measure diameter on the image:
Let’s say the apparent cluster diameter on the screen is measured as 2.7 cm.
The scale bar length is measured as 1.1 cm on the screen.
Scale bar actual length:
The bar represents 500 μm in reality.
Use proportional scaling:
\[
\frac{2.7\,\text{cm}}{1.1\,\text{cm}} \times 500\,\mu\text{m} = 1227\,\mu\text{m}
\]Answer:
The maximum diameter of the cluster is approximately 1227 μm.
(c)
- Stargardt’s disease: Replacing damaged retinal cells to restore vision.
- Other examples:
- Leukemia: Replacing bone marrow to regenerate blood cells.
- Parkinson’s disease: Replacing lost dopamine-producing neurons
(d)
Effects on the treated individual:
- The individual may experience improved quality of life, reduced symptoms, or even reversal of the disease condition.
- In some cases, it could extend lifespan or restore lost function (e.g., vision or mobility).
Effects on progeny:
- Hereditary diseases are caused by defective genes, often present in all cells, including gametes.
- If stem cell therapy does not alter the genetic material in germ cells (sperm/egg), the defective gene can still be passed on.
- Therefore, while symptoms may be treated, the genetic risk remains for future generations.
Markscheme:
(a)
- Undifferentiated / pluripotent
- Ability to divide or differentiate into any type of cell / differentiate along different pathways
(b)
- Correct formula: \( \frac{2.7\, \text{cm} \times 500\, \mu\text{m}}{1.1\, \text{cm}} = X\, \mu\text{m} \)
- Correct answer with unit: \( 1227\, \mu\text{m} \)
(c)
- Treatment of Stargardt’s disease / leukemia / diabetes / heart disease / Parkinson’s disease
(d)
- Could improve quality/length of life of the treated person
- Disease could still be passed on to progeny if defective gene/allele in gametes is not replaced/changed
▶️ Answer/Explanation
(a)
- Stem cells are unspecialized cells that can divide and differentiate into various types of specialized cells. This makes them useful for replacing damaged or diseased tissues in the body.
- They can be sourced from embryos, bone marrow, or umbilical cord blood, making them relatively accessible.
- Stem cells are used to treat conditions like leukemia, where damaged bone marrow can be replaced.
- They also allow scientists to test drugs on lab-grown tissues without harming real patients.
(b)
- Leaf cells contain chloroplasts, which have chlorophyll, the pigment that absorbs light energy.
- This light energy is used during photosynthesis to convert carbon dioxide and water into glucose, a form of chemical energy.
- Blue and red wavelengths of light are absorbed best, while green is mostly reflected.
- During photolysis, light splits water molecules, releasing oxygen and providing electrons for the process.
- The energy captured is stored in the form of glucose, which the plant uses or stores as starch.
(c)
- Platelets help seal wounds by forming blood clots. When a blood vessel is damaged, they release clotting factors that cause fibrinogen to turn into fibrin, creating a mesh that prevents blood loss and blocks pathogen entry.
- Phagocytes are white blood cells that engulf and digest pathogens. They move to infection sites and ingest invaders using enzymes.
- Lymphocytes identify specific antigens on pathogens and produce antibodies. These antibodies neutralize or destroy pathogens. Some lymphocytes become memory cells, giving long-term immunity after infection or vaccination.
Markscheme:
(a) Therapeutic use of stem cells:
a. Unspecialized/undifferentiated stem cells can divide/differentiate along different pathways
b. (Stem cells are accessible as they) come from embryos/bone marrow/umbilical cord blood/adult tissue
c. (Stem cells) can regenerate/repair diseased/damaged tissues in people
d. Valid specific example (e.g., treating leukemia, spinal cord injuries)
e. Drugs can be tested on stem cells (in laboratories to see if they are harmful)
(b) Leaf cells and light energy:
a. Leaf cells contain chloroplasts
b. Light is absorbed by chlorophyll (in chloroplasts)
c. Other pigments absorb different wavelengths
d. Light energy is used in photosynthesis
e. (Light is needed) to combine water and carbon dioxide/fix carbon dioxide
f. Carbon compounds/organic compounds/glucose/starch/carbohydrate are produced
g. Blue and red light is absorbed
h. Perform photolysis OR split water molecules
(c) Blood cell defense mechanisms:
Platelets: [3 max]
a. Damage/cuts to blood vessels causes platelets to be activated
b. The platelets release clotting factors
c. Initiates cascade of reactions OR fibrinogen is converted to fibrin
d. Forms a mesh over the damaged area
e. Prevents pathogens from entering the body
Phagocytes: [3 max]
f. Phagocytes/phagocytic white blood cells in the blood travel to the site of infection
g. (Phagocytes) squeeze between the capillary cells
h. (Phagocytes) engulf/ingest/take in pathogens
i. The pathogen is digested/broken down by/within the phagocyte
Lymphocytes: [3 max]
j. Lymphocytes recognize a particular fragment/antigen of a pathogen
k. (Lymphocytes) release antibodies
l. (Antibodies) provide specific immunity
m. Memory cells provide rapid response giving long-term immunity (to pathogens previously recognized)
n. Antibodies destroy pathogens