Checkpoint 1

• Q1 — B: The three tenets of cell theory are: all living organisms are composed of cells, the cell is the basic unit of structure and organisation, and all cells come from pre-existing cells. Option A incorrectly specifies mitosis (prokaryotes don't undergo mitosis). Options C and D describe features of cells, not the cell theory itself.
• Q2 — B: The defining difference is the presence/absence of membrane-bound organelles. Option A is incorrect because prokaryotes do have DNA (in nucleoid region). Option C is incorrect because prokaryotes have 70S ribosomes. Option D is incorrect because some eukaryotes are unicellular.
• Q3 — B: The nuclear envelope separates transcription (in nucleus) from translation (in cytoplasm). This separation allows for mRNA processing (splicing, capping, polyadenylation) before translation begins — a key feature of eukaryotic gene expression. Options A, C, and D describe incorrect functions.
• Q4 — B: Mitochondria are the site of aerobic respiration and ATP production through oxidative phosphorylation. Chloroplasts produce ATP via photophosphorylation but are specific to photosynthetic organisms. Golgi modifies proteins; RER synthesises proteins.
• Q5 — B: Plasmolysis occurs when a plant cell loses water in a hypertonic solution, causing the plasma membrane to pull away from the cell wall. Turgid (A) is the opposite condition. Lysed (C) describes animal cells in hypotonic solutions. Hypotonic (D) describes a solution, not a cell condition.
• Q6 — B: While the phospholipid bilayer forms the basic barrier, membrane proteins regulate what enters and leaves through channels, carriers, pumps, and receptors. This is the primary mechanism of selective permeability.
• Q7 — B: Facilitated diffusion uses specific membrane proteins (channels or carriers) but does not require ATP and moves substances down their concentration gradient. Option A describes active transport. Options C and D are incorrect.
• Q8 — B: Active transport uses ATP to move substances against their concentration gradient. Osmosis, simple diffusion, and facilitated diffusion are all passive processes that do not require ATP.
• Q9 — A: The amphipathic nature of phospholipids allows them to spontaneously form bilayers in aqueous environments, with hydrophilic heads facing the water and hydrophobic tails shielded in the membrane interior. This creates the basic structure of all cell membranes.
• Q10 — B: In a hypertonic solution, the external solute concentration is higher than inside the cell, so water moves out by osmosis. Animal cells lack cell walls and therefore shrink (crenate). Plant cells would become plasmolysed.

SA1: The statement "all cells come from pre-existing cells" (Virchow, 1858) disproves spontaneous generation — the idea that living organisms could arise from non-living matter. This principle ensures biological continuity: genetic material is passed from parent to daughter cells. During binary fission in prokaryotes, the circular DNA replicates and the cell elongates. The two DNA copies attach to the plasma membrane at opposite poles. The cell membrane and wall grow inward, dividing the cytoplasm into two daughter cells. Each daughter cell receives a complete copy of the parent genome, demonstrating that new cells only arise from pre-existing cells through division.

SA2: Both mitochondria and chloroplasts are double-membraned organelles involved in energy transformations. Mitochondria have a smooth outer membrane and highly folded inner membrane (cristae) to maximise surface area for aerobic respiration. They convert chemical energy in glucose to ATP. Chloroplasts have an outer membrane, inner membrane, and internal thylakoid membranes arranged in stacks (grana) surrounded by stroma. They convert light energy to chemical energy in glucose through photosynthesis. Both contain their own DNA and ribosomes (evidence of endosymbiotic origin). The key difference is energy direction: mitochondria release energy from organic compounds; chloroplasts capture light energy and store it in organic compounds.

SA3: The red blood cell will swell and eventually burst (lyse). Distilled water is a hypotonic solution — it has a higher water potential (less negative) than the cytoplasm of the red blood cell. Water moves by osmosis from an area of higher water potential (distilled water) to an area of lower water potential (cell cytoplasm) across the selectively permeable plasma membrane. As water enters, the cell volume increases. Unlike plant cells, animal cells lack a rigid cell wall to resist expansion. The membrane stretches until it ruptures, releasing cell contents.

SA4: The plasma membrane is selectively permeable because it allows some substances to pass while blocking others. This selectivity is achieved through: (1) The phospholipid bilayer structure — small, nonpolar molecules (O₂, CO₂) can diffuse through the hydrophobic interior, while ions and large polar molecules cannot; (2) Membrane proteins — channels form pores for specific ions, carrier proteins bind and transport specific molecules, and pumps use ATP to actively transport substances against gradients. Together, these features ensure only appropriate substances enter or leave the cell.