CELL BIOLOGY
State two differences between prokaryotic and eukaryotic cells.
Prokaryotic cells do not have a nucleus, membrane-bound organelles or linear chromosomes. Eukaryotic cells have a nucleus, membrane-bound organelles and linear chromosomes.
What is a chromosome made of?
DNA and proteins.
Which green pigment absorbs light energy for photosynthesis?
Chlorophyll
What is the active site?
The region of an enzyme where the substrate binds.
Define diffusion and osmosis
Diffusion is the net movement of particles from a region of higher concentration to lower concentration. Osmosis is the net movement of water molecules through a partially permeable membrane from a region of higher water potential to lower water potential.
A student observes a cell under a light microscope. The actual diameter of the cell is 25 µm, while its image diameter is 50 mm. Calculate the magnification and show your working.
Magnification = image size ÷ actual size. Convert 50 mm to 50,000 µm. Magnification = 50,000 ÷ 25 = ×2000.
State one difference between mitosis and meiosis.
Mitosis produces two genetically identical daughter cells; meiosis produces four genetically different haploid cells.
State the word equation for photosynthesis and name the organelle where it occurs.
Carbon dioxide + water → glucose + oxygen. Photosynthesis occurs in chloroplasts and requires light energy absorbed by chlorophyll.
Why does a high temperature reduce enzyme activity after the optimum temperature?
The enzyme denatures; its active site changes shape so the substrate no longer fits properly.
What happens to an animal cell placed in a very dilute solution?
Water enters by osmosis; the cell swells and may burst.
Explain how the structures of a red blood cell and a palisade mesophyll cell are adapted to their functions.
A red blood cell has a biconcave shape that increases surface area for gas exchange, contains haemoglobin to transport oxygen, and lacks a nucleus to provide more space for haemoglobin. A palisade mesophyll cell contains many chloroplasts to absorb light for photosynthesis and is located near the upper surface of the leaf where light intensity is high.
Explain how crossing over and independent assortment during meiosis increase genetic variation in gametes.
During prophase I, homologous chromosomes pair and exchange corresponding sections of DNA through crossing over, creating new allele combinations. During metaphase I, homologous chromosome pairs align independently of other pairs. Their random orientation produces different combinations of maternal and paternal chromosomes in the gametes.
Why does photosynthesis slow down when carbon dioxide concentration is low?
Carbon dioxide becomes a limiting factor, so the rate of photosynthesis decreases.
why enzymes are described as biological catalysts.
An enzyme is a protein that acts as a biological catalyst, increasing the rate of a chemical reaction of metabolic reaction without being used up in the process.
A plant cell is placed in a concentrated sugar solution. Explain why it becomes plasmolysed.
Water leaves the cell by osmosis, causing the cytoplasm and cell membrane to pull away from the cell wall.
Explain why a root hair cell is adapted for absorbing water and mineral ions how diffusion occur here.
The root hair cell has a long, hair-like extension that increases its surface area for absorption of water and mineral ions from the soil.
Water enters by osmosis through the partially permeable cell membrane, from a higher water potential in the soil to a lower water potential inside the root hair cell.
Mineral ions enter mainly by active transport against their concentration gradient, using energy released by respiration. Some ions may also enter by diffusion if their concentration gradient allows it.
A human body cell contains 46 chromosomes. Explain the chromosome number and genetic similarity of daughter cells produced by mitosis and cells produced by meiosis.
Mitosis produces two daughter cells, each with 46 chromosomes, genetically identical to the parent cell and usually to each other. Meiosis produces four haploid cells, each with 23 chromosomes, that are genetically different because of crossing over and independent assortment.
Describe a test for starch in a leaf.
Boil the leaf in water, heat it in ethanol in a hot-water bath to remove chlorophyll, rinse it, then add iodine solution. A blue-black colour indicates starch.
A student investigates the effect of temperature on enzyme activity. The reaction rate increases from 10°C to 37°C but decreases sharply at 60°C. Explain the results.
At lower temperatures, increasing temperature gives enzyme and substrate molecules more kinetic energy, increasing the frequency of successful collisions. At 37°C, the enzyme reaches its optimum temperature. At 60°C, the enzyme is likely denatured, changing the active site's shape so the substrate can no longer bind effectively.
A potato cylinder has a mass of 5.0 g before being placed in a sucrose solution. After 30 minutes, its mass is 4.2 g. Calculate the percentage change in mass.
Percentage change = (change in mass ÷ initial mass) × 100. Change = 4.2 − 5.0 = −0.8 g. Percentage change = (−0.8 ÷ 5.0) × 100 = −16%. The mass decreased by 16%.
A bacterial cell and a human cell both contain DNA. Explain three differences in how their genetic material is organised and describe one other structural difference between them.
Bacterial DNA is usually a single circular DNA molecule, whereas human nuclear DNA consists of multiple linear chromosomes.
Bacterial DNA is located in the nucleoid region without a nuclear membrane, whereas human nuclear DNA is enclosed within a nucleus. Bacteria may also contain plasmids, which are small circular DNA molecules. Human cells have membrane-bound organelles such as mitochondria, while bacteria lack membrane-bound organelles.
How many chromosomes are present in a normal human body cell?
46 chromosomes (23 pairs).
Explain the roles of light intensity, carbon dioxide concentration and temperature as limiting factors in photosynthesis.
At low light intensity, insufficient light energy limits photosynthesis. At low carbon dioxide concentration, there is insufficient substrate for carbon fixation. At temperatures below the optimum, enzyme-controlled reactions proceed slowly. Increasing one factor increases the rate only until another factor becomes limiting. At very high temperatures, photosynthetic enzymes may denature, reducing the rate.
Explain how an enzyme's active site enables it to catalyse a specific reaction.
The active site has a specific shape complementary to its substrate. The substrate binds to the active site to form an enzyme-substrate complex. The reaction occurs, products are released, and the enzyme remains unchanged and can be reused.
Explain how the small intestine is adapted for the absorption of digested nutrients into the blood.
The small intestine has numerous villi and microvilli, providing a large surface area. The epithelium is one cell thick, giving a short diffusion distance. Each villus has a rich capillary network to maintain concentration gradients and transport absorbed nutrients away. Epithelial cells also contain mitochondria to provide energy for active transport.