Checkpoint 2

• Q1 — B: The external solution has lower solute concentration (hypotonic to the cell). Water moves into the cell by osmosis down its water potential gradient. The cell swells and may burst (lyse) because animal cells have no cell wall to resist expansion.
• Q2 — C: Moving glucose against a concentration gradient (low → high) requires energy and carrier proteins — this is active transport. Osmosis moves water only; facilitated diffusion moves substances down a gradient; exocytosis moves materials out of the cell.
• Q3 — D: The transpiration-cohesion-tension mechanism: water evaporates from leaves (transpiration), creating negative pressure (tension) that pulls the water column up via hydrogen bonding between water molecules (cohesion).
• Q4 — B: The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs to be oxygenated. Arteries are defined by carrying blood away from the heart — not by oxygen content. The pulmonary artery is the key exception to the "arteries = oxygenated" rule.
• Q5 — C: Fick's Law: rate ∝ (SA × concentration gradient × solubility) / membrane thickness. Increasing thickness is in the denominator — it decreases the rate of exchange.
• Q6 — B: SA:V = 24/8 = 3:1. As organisms grow, this ratio decreases (a 4cm cube has SA:V of 1.5:1). Lower SA:V means less surface relative to volume — multicellular organisms need specialised exchange surfaces.
• Q7 — C: CO₂ is only used in Stage 2 (Calvin cycle) for carbon fixation. Stage 1 uses only light and water — it continues producing ATP, NADPH, and O₂. But without CO₂, Stage 2 cannot run and no glucose is produced.
• Q8 — D: Human muscle cells use lactic acid fermentation when oxygen is insufficient. Ethanol + CO₂ is the yeast/plant pathway. CO₂ + water are the products of aerobic respiration, not anaerobic.
• Q9 — C: Photosynthesis and respiration are independent processes occurring in different organelles. A plant cell photosynthesises in chloroplasts and simultaneously respires in mitochondria. During the day, photosynthesis typically produces more O₂ and glucose than respiration consumes.
• Q10 — B: Emphysema destroys alveolar walls, reducing total surface area. By Fick's Law, less surface area → less O₂ absorbed per breath. Insufficient O₂ delivery to muscles forces them into anaerobic respiration → lactic acid buildup → fatigue during mild activity.

SA1: The concentrated salt solution has a lower water potential than the plant cell's cytoplasm. Water moves out of the cell by osmosis, down the water potential gradient, across the selectively permeable cell membrane. The vacuole shrinks as it loses water, and the cell membrane pulls away from the cell wall — a process called plasmolysis. The cell wall, being rigid, prevents the cell from collapsing entirely, but the cell becomes flaccid and loses turgor pressure. If returned to water, the process reverses (deplasmolysis).

SA2: Xylem: water flows upward (unidirectional, root to leaf), driven by transpiration pull — evaporation from leaves creates negative pressure (tension) that pulls water up via cohesion between water molecules. Xylem vessels are dead at maturity and have no end walls, forming a continuous hollow tube that minimises resistance to flow. Veins: blood flows toward the heart, driven by skeletal muscle contractions squeezing the veins and by the suction effect of the heart. Veins have one-way valves that prevent backflow, ensuring blood continues moving toward the heart even against gravity.

SA3: Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (requires light + chlorophyll). Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. These processes are essentially the reverse of each other — the outputs of photosynthesis (glucose and O₂) are the inputs of respiration, and the outputs of respiration (CO₂ and H₂O) are the inputs of photosynthesis. They occur in different organelles: photosynthesis in chloroplasts (thylakoids + stroma); respiration in the cytoplasm (glycolysis) and mitochondria (Krebs cycle + ETC). Both run simultaneously in plant cells during daylight.

SA4: As an organism increases in size, its surface area increases as a square of its dimensions while its volume increases as a cube — meaning the SA:V ratio decreases. A single-celled organism has a high SA:V ratio and every part of its cytoplasm is close to the external surface, so direct diffusion across the membrane is sufficient. A large multicellular animal has a very low SA:V ratio — most cells are far from the external surface and cannot exchange gases or nutrients directly. Three adaptations that solve this problem: (1) specialised exchange surfaces such as alveoli in lungs (folded to maximise SA, thin walls to minimise diffusion distance) and villi/microvilli in the intestine; (2) a circulatory system that transports O₂, nutrients and wastes between exchange surfaces and body cells; (3) a ventilation system (breathing) that maintains steep concentration gradients at exchange surfaces by continuously renewing the air or water in contact with them.