▶️ Answer/Explanation
(a)
Light intensity (controlled by changing the distance of the light source from the plant)
(b)
- Temperature of the water — because temperature affects enzyme activity involved in photosynthesis.
- Carbon dioxide concentration — as it’s a reactant in photosynthesis and will affect the rate.
(c)
Set up the same experiment without any light (in complete darkness) to show that light is necessary for oxygen production/photosynthesis.
(d)
- The bubbles are oxygen, which is a product of photosynthesis.
- In the presence of light, the plant carries out photosynthesis, converting carbon dioxide and water into glucose, with oxygen released as a byproduct.
- The equation for photosynthesis:
6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} \xrightarrow{\text{light}} \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2
Markscheme:
(a) To check there is no carbon dioxide (left in the air)/show any carbon dioxide present;
(b) Soil releases \(\mathrm{CO}_2\) from microorganisms/decomposers/bacteria/fungi
OR
Respiration by microorganisms may affect the result;
(c) Using the same apparatus without a plant
OR
Cover the whole plant with a plastic bag;
(d) No change/limewater stays clear
OR
Because plant takes in carbon dioxide by photosynthesis;
▶️ Answer/Explanation
a.
- The genetic code is made up of triplets of nucleotides called codons.
- DNA contains the bases: adenine (A), thymine (T), cytosine (C), and guanine (G); in RNA, uracil (U) replaces thymine.
- Each codon codes for one specific amino acid.
- Some codons are start or stop codons, signaling the beginning or end of translation.
- The sequence of bases in DNA is transcribed into mRNA via complementary base pairing.
- The mRNA is translated by ribosomes into a polypeptide chain (sequence of amino acids).
- Each gene typically codes for one polypeptide.
- Proteins are formed when one or more polypeptides fold and sometimes join together.
b.
- Channel proteins allow passive transport (diffusion/osmosis) of molecules.
- Large or polar molecules cannot pass freely through the lipid bilayer.
- Facilitated diffusion uses proteins to move molecules down their concentration gradient—no energy (ATP) required.
- Aquaporins are specific channel proteins that enable water movement (osmosis).
- Transport proteins are often specific to the molecule or ion they move.
- Active transport uses carrier proteins or pumps to move substances against their concentration gradient, using ATP.
- The sodium-potassium pump is an example of an active transport mechanism.
c.
- ATP (adenosine triphosphate) is the immediate source of energy for cellular processes.
- ATP is produced through cellular respiration, which breaks down organic compounds, mainly glucose.
- Glycolysis (in the cytoplasm) breaks glucose into pyruvate, producing a small amount of ATP.
- In the presence of oxygen, aerobic respiration occurs in the mitochondria:
- Pyruvate is broken down into carbon dioxide and water.
- A large yield of ATP is generated (up to ~38 ATP per glucose).
- In the absence of oxygen, anaerobic respiration occurs, producing far less ATP.
- ATP is synthesized when ADP (adenosine diphosphate) combines with inorganic phosphate (Pi).
Markscheme:
a.
- The genetic code is based on sets of three nucleotides/triplets of bases called codons.
- Bases include adenine, guanine, cytosine, and thymine in DNA / adenine, guanine, cytosine, and uracil in RNA.
- Each codon codes for one amino acid.
- Some codons are (start or) stop codons.
- DNA is transcribed into mRNA by base-pair matching/complementary base pairing.
- mRNA is translated into a sequence of amino acids/polypeptide.
- Each gene codes for a polypeptide.
- Polypeptides may be joined/modified to form proteins.
b.
- Channel proteins allow diffusion/osmosis/passive transport.
- Large/polar molecules cannot cross the (hydrophobic) membrane freely.
- Facilitated diffusion involves moving molecules through proteins down their concentration gradient/without requiring ATP.
- Aquaporins (specific integral membrane proteins) facilitate the movement of water molecules/osmosis.
- Some proteins (for facilitated diffusion) are specific to molecules/ions.
- Active transport involves moving molecules through proteins against their concentration gradient/requiring ATP.
- (Some) proteins in the membrane are pumps / pumps perform active transport / sodium-potassium pump.
c.
- ATP is a form of energy currency/immediately available for use.
- ATP is generated in cells by cell respiration (from organic compounds).
- Aerobic (cell respiration) requires oxygen.
- Anaerobic (cell respiration) does not require oxygen.
- Glycolysis breaks down glucose into pyruvate.
- Glycolysis occurs in the cytoplasm.
- (By glycolysis) a small amount of ATP is released.
- ADP changes into ATP with the addition of a phosphate group/phosphoric acid.
- In mitochondria/aerobic respiration produces a large amount of ATP / \(38 \mathrm{mols}\) (for the cell, per glucose molecule).
- Oxygen/aerobic respiration is required for mitochondrial production of ATP.
- In mitochondria/aerobic respiration, pyruvate is broken down into carbon dioxide and water.
▶️ Answer/Explanation
a.
- Lysosome: Contains digestive enzymes; breaks down waste materials, old organelles, or ingested particles.
- Golgi apparatus: Modifies, processes, and packages proteins and lipids for secretion or use within the cell.
- Free ribosomes: Site of protein synthesis; produces proteins that function in the cytoplasm.
- Plasma membrane: Controls movement of substances into and out of the cell; acts as a selectively permeable barrier.
- Rough endoplasmic reticulum: Synthesizes proteins (via attached ribosomes) and transports them within the cell.
b. Anaerobic vs. Aerobic Cell Respiration in Eukaryotes
| Characteristic | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen | Requires oxygen | Occurs without oxygen |
| Location | Cytoplasm and mitochondria | Cytoplasm only |
| ATP yield | High (up to ~38 ATP per glucose molecule) | Low (2 ATP per glucose) |
| End products | Carbon dioxide and water | Lactic acid (in animals) |
| Efficiency | More efficient (complete glucose breakdown) | Less efficient (partial glucose breakdown) |
c. Mechanism of Ventilation in the Lungs
- Inhalation (inspiration):
- External intercostal muscles contract → rib cage moves up and out.
- Diaphragm contracts and flattens.
- Thoracic cavity volume increases → lung pressure drops below atmospheric pressure.
- Air rushes into lungs due to pressure difference.
- Exhalation (expiration):
- External intercostal muscles and diaphragm relax.
- Internal intercostal muscles may contract to aid forced exhalation.
- Abdominal muscles contract, pushing diaphragm upward.
- Thoracic volume decreases, lung pressure increases.
- Air is forced out of the lungs.
- Purpose of Ventilation:
- Maintains a concentration gradient between alveolar air and blood.
- Facilitates diffusion of oxygen into blood and carbon dioxide out for efficient gas exchange.
Markscheme:
a.
• Lysosome: (from Golgi apparatus) with digestive enzymes / break down food/organelles/cell
• Golgi apparatus: site that processes/modifies/packages and releases proteins
• Free ribosomes: site of synthesis of proteins (released to cytoplasm)
• Plasma membrane: controls entry and exit of materials/substances in cell
• Rough endoplasmic reticulum: synthesis and transport of proteins (both needed)
b.
Award [1] for each contrasting characteristic. Table format is not necessary for the marks.
c.
• Inspiration/inhalation brings air into lungs
• External intercostal muscles contract
• And move rib cage upwards and outwards
• Diaphragm flattens/contracts
• Increasing thoracic volume
• Pressure decreases from atmospheric pressure so air rushes into lungs
• Expiration/exhalation forces air out
• Internal intercostal muscles contract / external intercostal muscles and diaphragm relax
• Abdominal wall muscles contract and push diaphragm upwards
• Decreasing thoracic volume
• Increasing pressure in lungs so air is forced out
• A concentration gradient between air sacs and blood needs to be maintained
▶️ Answer/Explanation
a. Active Transport Across Cell Membranes
- Moves substances against their concentration gradient (from low to high concentration).
- Involves specific protein pumps embedded in the membrane.
- Requires energy in the form of ATP to function.
- Enables the cell to absorb nutrients even when they are in lower concentrations outside the cell.
b. Effects of pH on Enzyme Activity
- Each enzyme has an optimal pH at which it works most efficiently.
- At the optimum pH, the active site maintains its ideal shape for substrate binding.
- Deviation from the optimal pH (either more acidic or basic) leads to:
- Disruption of hydrogen bonding within the enzyme’s structure.
- Altered shape of the active site, reducing enzyme-substrate binding.
- Extreme pH values can cause denaturation, making the enzyme non-functional.
c. Anaerobic Respiration: Yeast vs Humans
| Organism | End Product(s) | Notes |
|---|---|---|
| Yeast | Ethanol + Carbon Dioxide | Used in baking and alcohol fermentation |
| Humans | Lactic Acid | Occurs during vigorous exercise |
- In both cases, glucose is partially broken down without oxygen.
- Pyruvate is the common intermediate:
- In yeast, pyruvate → ethanol + CO₂
- In humans, pyruvate → lactic acid
Markscheme:
a.
Transport against a concentration gradient / from low to high concentration;
Through protein pumps;
Uses energy/ATP;
b.
Enzymes have a pH optimum;
Active site works best at this \(\mathrm{pH}\);
Activity decreases above and below the optimum;
By interfering with \(\mathrm{H}\)-bonding/active site structure;
Denaturing by extremes of \(\mathrm{pH}\) so enzyme activity/reaction stops;
c.
Yeast: pyruvate to ethanol and carbon dioxide;
Humans: pyruvate to lactic acid;
Award [1 max] if products are appropriately linked to organisms without the mention of pyruvate.
▶️ Answer/Explanation
a.
| Process | Description |
|---|---|
| Ventilation | Movement of air into and out of the lungs (inhalation and exhalation); involves respiratory muscles. |
| Gas Exchange | Diffusion of oxygen and carbon dioxide between: alveoli and capillary blood / capillaries and body cells. |
| Cell Respiration | The release of energy (ATP) from organic molecules (e.g., glucose), mainly in the mitochondria. |
b.
- Begins with glycolysis in the cytoplasm:
- Glucose is partially oxidized.
- Forms 2 pyruvate molecules.
- Produces a small amount of ATP.
- Pyruvate enters the mitochondria where it is further broken down.
- Requires oxygen.
- Produces carbon dioxide and water.
- Results in a large ATP yield (approximately 36–38 ATP per glucose).
c.
- Temperature:
- Higher temperature increases kinetic energy → more frequent collisions.
- Optimum temperature gives max activity.
- Too high temperature denatures enzymes (loss of active site structure).
- pH:
- Each enzyme has an optimal pH.
- Deviations from optimum can reduce activity.
- Extreme pH can denature enzymes by altering 3D structure (tertiary structure).
- Substrate Concentration:
- More substrate → increased reaction rate (more collisions).
- Plateaus when all active sites are occupied (enzyme saturation).
Markscheme:
Answer to part (a):
– Ventilation is moving air into and out of the lungs (inhalation and exhalation);
– Involves (respiratory) muscle activity;
– Gas exchange involves the movement of carbon dioxide and oxygen;
– Occurs between alveoli and blood (in capillaries) / between blood (in capillaries) and cells;
– Cell respiration is the release of energy from organic molecules/glucose;
– (Aerobic) cell respiration occurs in mitochondria;
To award [4 max], responses must address ventilation, gas exchange, and cell respiration.
Answer to part (b):
– During glycolysis, glucose is partially oxidized in the cytoplasm;
– (Small amount/yield of) ATP is produced;
– (Two) pyruvate molecules are formed by glycolysis;
– Pyruvate is absorbed into/broken down in the mitochondrion;
– Requires oxygen;
– Carbon dioxide is produced;
– Water is produced;
– Large amount/yield of energy/ATP molecules (per glucose molecule);
Answer to part (c):
– Collisions between enzyme/active site and substrate;
– Enzyme activity increases as temperature rises;
– More frequent collisions at higher temperatures;
– Each enzyme has an optimum temperature / enzymes have optimal temperatures;
– High temperatures (above optimum) denature enzymes;
– Each enzyme has an optimum pH / enzymes have optimal pHs;
– Increase or decrease from optimum \(\mathrm{pH}\) decreases rate of reaction/activity;
– Extreme \(\mathrm{pH}\) alters/denatures the tertiary/3D protein/enzyme structure;
– Increasing substrate concentration increases the rate of reaction;
– Higher substrate concentration increases chance of collision;
– Until plateau (when all active sites are busy);
Accept clearly annotated graph.