Unit 2 Endosymbiotic theory Tap / Space to flip
Unit 2 Mitochondria & chloroplasts evolved from once-free-living prokaryotes. Evidence: own circular DNA, 70S ribosomes, double membrane, binary fission.
Unit 2 · 10–13% of exam
Cell Structure and Function
Organelles, membrane transport, tonicity, the fluid mosaic, and endosymbiotic theory. Includes water potential — see the dedicated priority deep dive.
Must-know content
- Prokaryote vs. eukaryote: prokaryotes lack membrane-bound organelles and a true nucleus; have a nucleoid, 70S ribosomes, and a cell wall (peptidoglycan in bacteria).
- Organelle functions:
- Nucleus — DNA storage, transcription
- Ribosome — protein synthesis (free in cytosol or on rough ER)
- Rough ER — synthesizes membrane / secreted proteins
- Smooth ER — lipid synthesis, detoxification, Ca²⁺ storage
- Golgi — modifies, sorts, and packages proteins
- Mitochondrion — cellular respiration (double membrane; matrix has its own DNA & ribosomes)
- Chloroplast — photosynthesis (double membrane + thylakoid; own DNA)
- Lysosome — hydrolytic digestion (low pH)
- Vacuole — storage; central vacuole drives plant turgor
- Peroxisome — breaks down fatty acids and H₂O₂
- Endosymbiotic theory: mitochondria & chloroplasts evolved from engulfed prokaryotes. Evidence: own circular DNA, 70S ribosomes, double membrane, binary fission.
- SA:V ratio: volume scales faster than surface area as cells grow, limiting exchange. Folding (microvilli, cristae, thylakoids) increases SA.
- Membrane structure: fluid mosaic — phospholipid bilayer + cholesterol (buffers fluidity) + integral and peripheral proteins + glycoproteins/glycolipids (cell ID).
- Transport:
- Passive — diffusion, facilitated diffusion (channels/carriers), osmosis. No ATP.
- Active — pumps (e.g., Na⁺/K⁺ ATPase). Uses ATP, against gradient.
- Bulk — endocytosis (phagocytosis, pinocytosis, receptor-mediated), exocytosis.
- Tonicity:
- Hypotonic → water enters → animal cells lyse, plant cells turgid.
- Hypertonic → water leaves → animal cells crenate, plant cells plasmolyze.
- Isotonic → no net flow.
Example questions
MCQ A protein destined for secretion would travel through which order of structures? (A) Nucleus → smooth ER → Golgi → vesicle (B) Free ribosome → cytoplasm → vesicle (C) Rough ER → vesicle → Golgi → vesicle → plasma membrane (D) Golgi → rough ER → vesicle
Answer: C. Secreted proteins are translated by ribosomes bound to the rough ER, packaged into transport vesicles, modified in the Golgi, and exocytosed via vesicles fusing with the plasma membrane.
FRQ Explain how the structure of mitochondrial inner membranes (cristae) supports their function.
Answer: Cristae are infoldings of the inner mitochondrial membrane that increase the membrane's total surface area. This expands the area available for the embedded proteins of the electron transport chain and ATP synthase. More surface area means more sites for oxidative phosphorylation per mitochondrion, which raises the maximum ATP-synthesis rate the cell can sustain.
MCQ A plant cell placed in a 0.6 M sucrose solution becomes plasmolyzed. This indicates the solution is: (A) Hypotonic (B) Isotonic (C) Hypertonic (D) Cannot determine
Answer: C. Water moved out of the cell, so the external solute concentration must be higher — the solution is hypertonic. Plasmolysis is the visible separation of the membrane from the wall.
Drill flashcards
Unit 2 Fluid mosaic model Tap / Space to flip
Unit 2 The plasma membrane is a fluid bilayer of phospholipids studded with proteins, cholesterol, and glycolipids/glycoproteins.
Unit 2 Aquaporin Tap / Space to flip
Unit 2 Channel protein that selectively conducts water across the membrane.
Unit 2 Hypotonic solution Tap / Space to flip
Unit 2 Lower solute outside the cell — water enters. Animal cells lyse; plant cells become turgid.
Unit 2 Hypertonic solution Tap / Space to flip
Unit 2 Higher solute outside the cell — water leaves. Animal cells crenate; plant cells plasmolyze.
Unit 2 Cell wall (plants) Tap / Space to flip
Unit 2 Rigid cellulose layer outside the membrane; provides structural support and limits over-expansion under turgor.
Unit 2 Cristae Tap / Space to flip
Unit 2 Folded inner mitochondrial membrane. Increases surface area for the electron transport chain & ATP synthase.
Unit 2 Tight junction Tap / Space to flip
Unit 2 Animal cell-cell junction that seals the space between cells, preventing leakage of extracellular fluid.