▶️ Answer/Explanation
(a)
The student should circle one pair of the inward-pointing horizontal sections opposite each other on the DNA diagram. These represent the complementary base pairs (A-T or C-G).
(b) 21%
Explanation:
According to Chargaff’s rules in DNA:
- Adenine (A) pairs with Thymine (T), so if T is 29%, then A must also be 29%.
- This accounts for 58% of the bases (29% A + 29% T).
- The remaining 42% must be equally divided between Cytosine (C) and Guanine (G) since they pair together.
- Therefore, G = 42% ÷ 2 = 21%.
(c)(i) gene
A gene is a segment of DNA that contains the instructions for making a specific protein.
(c)(ii)
Explanation of protein synthesis:
- Transcription: In the nucleus, the DNA code for a specific gene is copied into messenger RNA (mRNA). This process is called transcription.
- mRNA processing: The mRNA molecule is modified and then leaves the nucleus through nuclear pores.
- Translation: The mRNA attaches to a ribosome in the cytoplasm. Transfer RNA (tRNA) molecules bring specific amino acids to the ribosome based on the mRNA code.
- Peptide bond formation: The ribosome joins the amino acids together with peptide bonds, forming a polypeptide chain.
- Protein folding: The polypeptide chain folds into its specific three-dimensional shape to become a functional protein.
- Energy requirement: The entire process requires energy in the form of ATP.
(c)(iii) Two types of cell membrane proteins:
- Carrier proteins: These help transport molecules across the cell membrane, often against concentration gradients (active transport).
- Receptor proteins: These detect and respond to chemical signals from outside the cell, triggering specific cellular responses.
- Other possible answers: Channel proteins, enzyme proteins, cell recognition proteins, or structural proteins.
▶️ Answer/Explanation
(a)(i) -44%
Explanation: From the graph, in 1993 there were 180 resistant infections per 1000 people, and in 1995 there were 100. The percentage change is calculated as: ((100-180)/180) × 100 = -44.44%, which rounds to -44% to two significant figures. The negative sign indicates a decrease in resistant infections.
(a)(ii)
Explanation: The data shows several key patterns: First, erythromycin usage peaked in 1989 at 2.8 doses per 1000 people. Second, resistant infections weren’t recorded until 1991. Third, resistant infections peaked in 1993 at 180 per 1000 people, four years after the peak in antibiotic usage. Finally, both antibiotic usage and resistant infections showed a declining trend after their respective peaks.
(a)(iii)
Explanation: The decrease in resistant infections from 1993-1995 could be due to: 1) Reduced erythromycin usage leading to less selection pressure for resistant strains, 2) Development of new antibiotics or alternative treatments, 3) Increased public awareness about proper antibiotic use, 4) Improved hygiene and infection control measures in healthcare settings, or 5) Implementation of policies restricting unnecessary antibiotic prescriptions.
(a)(iv)
Explanation: Bacteria develop resistance through several mechanisms: First, random mutations occur in bacterial DNA that may confer resistance. Second, bacteria can acquire resistance genes through horizontal gene transfer via plasmids. Third, when exposed to antibiotics, resistant bacteria survive while non-resistant ones die. Fourth, the resistant bacteria reproduce, passing on their resistance genes. Finally, over time, the resistant strains become dominant in the population through natural selection.
(b)(i)
Explanation: Two key features of all prokaryotes are: 1) They lack a membrane-bound nucleus – their DNA is free in the cytoplasm. 2) They have a cell wall made of peptidoglycan that maintains cell shape and provides protection.
(b)(ii) Movement.
Explanation: The flagellum is a whip-like appendage that rotates to propel the bacterium through liquid environments. This movement helps bacteria find nutrients, avoid harmful substances, or reach favorable environments. The rotation can be clockwise or counterclockwise, allowing for directional movement.
▶️ Answer/Explanation
(a)(i)
The food web should show:
- aquatic plants → midges → stoneflies → salmon → herons
- aquatic plants → mayflies → salmon → herons
- aquatic plants → freshwater shrimps → salmon → herons
Explanation: A food web illustrates all the feeding relationships in an ecosystem. The arrows should point from the food source to the consumer. In this case, aquatic plants are the producers at the base, with multiple pathways leading up to the top predator (herons).
(a)(ii)
| Name of the trophic level | Organism in Fig. 4.1 |
|---|---|
| producer | aquatic plants |
| primary consumer | mayflies / midges / freshwater shrimps |
| secondary consumer | salmon / stoneflies |
| tertiary consumer | salmon / herons |
| quaternary consumer | herons |
Explanation: Trophic levels represent the position an organism occupies in a food chain. Producers (aquatic plants) are at the base, followed by primary consumers (herbivores), then secondary consumers (carnivores that eat herbivores), and so on up to the top predators.
(a)(iii)
Energy transfer occurs through the following process:
- Primary consumers (like mayflies) obtain chemical energy from eating aquatic plants
- When secondary consumers (like salmon) eat primary consumers, they gain some of this energy
- Energy is used for respiration, growth, reproduction, and other life processes
- Only about 10% of the energy is transferred between trophic levels
- The remaining energy is lost as heat, in undigested material, or used for metabolism
Explanation: Energy flow through ecosystems is inefficient. Most energy is lost at each transfer, which is why food chains rarely have more than 4-5 trophic levels. The energy that isn’t lost is incorporated into the biomass of the secondary consumers.
(a)(iv)
Overharvesting salmon would likely cause:
- Decrease in heron population (less food available)
- Increase in mayflies, stoneflies, and freshwater shrimps (reduced predation)
- Possible decrease in midges (if stoneflies increase)
- Variable impact on aquatic plants (may increase or stay stable)
Explanation: Removing a key predator like salmon disrupts the entire food web. Prey species may increase dramatically, which can then affect other species they interact with. This demonstrates the interconnectedness of ecosystems.
(b) A decomposer is an organism that breaks down dead organic matter and waste products, returning nutrients to the ecosystem.
Explanation: Decomposers like bacteria and fungi play a crucial role in nutrient cycling. They break down complex organic compounds into simpler forms that plants can reuse, completing the nutrient cycle.
(c) It’s more energy efficient because:
- Energy transfer between trophic levels is only about 10% efficient
- Eating plants directly means accessing more of the original solar energy
- Less energy is lost to respiration, movement, and heat in plant-based diets
- Livestock production requires more land, water, and resources
Explanation: The “10% rule” of energy transfer explains why shorter food chains are more efficient. Each time energy moves up a trophic level, 90% is lost. Therefore, eating producers (plants) gives humans access to more of the original energy captured through photosynthesis.
▶️ Answer/Explanation
(a) Name: mitochondrion; Function: aerobic respiration
Explanation: The structure labelled A is identified as a mitochondrion based on its characteristic appearance in the photomicrograph. Mitochondria are often described as the “powerhouses” of the cell because their primary function is to carry out aerobic respiration. This process involves breaking down glucose molecules in the presence of oxygen to produce ATP (adenosine triphosphate), which serves as the main energy currency for cellular activities. The inner membrane of mitochondria is highly folded into cristae, which increases the surface area for the enzymes involved in respiration.
(b) 1. Length of the cell structure in the image/picture/photomicrograph/diagram; 2. Magnification
Explanation: To calculate the actual size of a structure from a micrograph, you need two key pieces of information. First, you must measure the length of the structure as it appears in the image (usually in millimeters). Second, you need to know the magnification factor of the image, which tells you how many times larger the image is compared to the actual specimen. The actual size can then be calculated using the formula: Actual size = Image size ÷ Magnification. Without either of these values, the calculation cannot be performed.
(c) 0.75 µm
Explanation: To convert millimeters to micrometers, we use the conversion factor that 1 millimeter equals 1000 micrometers. Therefore, we multiply the given value by 1000: 0.00075 mm × 1000 = 0.75 µm. This conversion is important in microscopy as most cellular structures are measured in micrometers, which are more appropriate for their scale. The mitochondrion’s size of 0.75 µm is typical for these organelles, which usually range from 0.5 to 1.0 µm in diameter and up to 10 µm in length.
▶️ Answer/Explanation
Completed sentences with explanations:
1. soluble – Enzymes break down large food molecules into smaller, soluble molecules that can be absorbed through the intestinal wall into the bloodstream. This solubility is crucial for nutrient transport throughout the body.
2. stomach – Pepsin is produced by the stomach’s gastric glands as an inactive precursor called pepsinogen, which is then activated by the acidic environment in the stomach.
3. hydrochloric acid – The stomach lining secretes hydrochloric acid (HCl) which creates the strongly acidic environment (pH 1.5-2) needed for pepsin activation and optimal activity.
4. microorganisms / pathogens – The hydrochloric acid serves a dual purpose: it provides the acidic conditions pepsin needs while also killing most harmful bacteria and other pathogens that might be present in ingested food.
5. pancreas / small intestine – Trypsin is produced by the pancreas (as trypsinogen) and released into the small intestine where it’s activated and works on protein digestion.
6. alkaline – Unlike pepsin, trypsin works best in alkaline conditions (pH 7.5-8.5), which is why it’s active in the small intestine rather than the stomach.
7. bile / bile salts – Bile, produced by the liver and stored in the gallbladder, neutralizes the acidic chyme from the stomach, creating the alkaline environment trypsin needs.
8. emulsification – Bile salts emulsify fats, breaking large fat globules into smaller droplets, increasing the surface area for lipase enzymes to work on during fat digestion.
Key concepts: This question covers the complete digestive process involving enzymes, highlighting how different parts of the digestive system create specific pH conditions optimal for different enzymes. It shows the progression from stomach (acidic) to small intestine (alkaline) environments and how the body produces complementary substances (like bile) to facilitate this transition and enhance digestion.