What are the key differences between plant cells and animal cells? — Plant cells have: Cell wall (cellulose), large central vacuole (for turgor), chloroplasts (photosynthesis), plasmodesmata (cytoplasmic connections). Animal cells have: centrioles (cell division), lysosomes, no cell wall, no chloroplasts, only small/absent vacuoles.

What are the three main organs of a plant? — Roots: Anchor the plant, absorb water and minerals, often store food. Stem: Supports the plant, transports materials between roots and leaves, may photosynthesize. Leaves: Primary site of photosynthesis and gas exchange.

What is the structure of a leaf adapted for photosynthesis? — Upper epidermis: Transparent, waxy cuticle (reduces water loss). Palisade mesophyll: Tightly packed cells rich in chloroplasts — main photosynthetic layer. Spongy mesophyll: Loosely packed with air spaces for gas exchange. Lower epidermis: Contains stomata (gas exchange). Vascular bundle (xylem and phloem).

What are stomata and how do they work? — Pores in the leaf epidermis, mainly on the underside. Each pore is flanked by two guard cells. When guard cells take up water (turgid) → pore opens. When guard cells lose water (flaccid) → pore closes. Open in daylight (CO₂ in, O₂ and H₂O out); close at night and when water-stressed.

What is the xylem and what does it transport? — A tissue of dead, hollow, lignified cells forming continuous tubes from roots to leaves. Transports water and dissolved mineral ions (unidirectional — upward). Lignin strengthens walls and makes them waterproof — provides structural support.

What is the phloem and what does it transport? — A tissue of living sieve tube cells and companion cells. Transports sucrose (sugars) and amino acids from sources (leaves) to sinks (roots, fruits, growing tips). Movement is bidirectional (up or down) — called translocation.

What is the transpiration stream? — The continuous movement of water through a plant: Roots absorb water → xylem carries water up the stem → leaves lose water vapour through stomata (transpiration) → this creates a tension that pulls more water up. Driven by evaporation from leaves.

What are the three pathways water takes to enter the xylem from soil? — Apoplast pathway: Through cell walls (not crossing membranes). Symplast pathway: Through cytoplasm and plasmodesmata. Vacuolar pathway: Through vacuoles. The Casparian strip (waterproof band in endodermis) forces water to cross the cell membrane (symplast) before entering the xylem — allowing selective mineral uptake.

What factors increase transpiration rate? — Higher temperature (more evaporation). Lower humidity (steeper water potential gradient). Increased air movement/wind (removes water vapour). Higher light intensity (stomata open wider). Larger surface area of leaves.

How is water transported up tall trees against gravity? — Three mechanisms: (1) Root pressure (active uptake of minerals creates osmotic pressure). (2) Capillarity (adhesion to xylem walls). (3) Cohesion-tension theory (cohesion of water molecules + transpiration pull — the main mechanism). Water molecules stick together (hydrogen bonds) and are pulled up as a continuous column.

What is the Source-Sink model of phloem transport? — Source: Where sucrose is loaded into phloem (e.g., mature leaves). Sink: Where sucrose is unloaded and used (e.g., roots, fruits, growing tips). High pressure at source, low pressure at sink → mass flow of solution from source to sink (mass flow hypothesis).

What are auxins and what do they do? — Plant hormones (mainly indole-3-acetic acid, IAA) produced in growing tips. Control cell elongation. Cause phototropism (stems grow toward light — auxin accumulates on dark side, causing cells there to elongate → stem bends toward light). Higher concentrations inhibit root growth.

What is phototropism and how do auxins cause it? — Growth of a plant toward or away from light. Auxin moves from the illuminated side to the shaded side of the shoot tip. Shaded side: higher auxin → more cell elongation → stem curves toward light. Shoots are positively phototropic; roots are negatively phototropic.

What is gravitropism (geotropism)? — Growth response to gravity. Roots: Positively gravitropic — grow downward (high auxin on lower side inhibits root growth on that side → roots curve down). Shoots: Negatively gravitropic — grow upward. Ensures roots find water/minerals and shoots reach light.

What are gibberellins and what are their effects? — Plant hormones produced in young leaves and seeds. Effects: stem elongation (dwarf plant mutants lack gibberellins), seed germination (stimulate amylase production to digest starch), fruit development (seedless fruit production), breaking bud dormancy.

What is abscisic acid (ABA) and when is it produced? — A plant hormone produced under stress conditions (drought, cold, mechanical damage). Effects: closes stomata (guard cells lose water — reduces water loss), promotes seed dormancy, inhibits germination, promotes leaf abscission (leaf drop in autumn). Counter-acts gibberellins.

What is ethylene (ethene) and what does it do in plants? — A gaseous plant hormone. Controls: fruit ripening (ethylene from one ripening fruit speeds ripening of nearby fruits — "one bad apple" effect), leaf, flower, and fruit abscission, seed germination, response to wounding (stimulates defense responses). Used commercially to ripen bananas during shipping.

What is pollination and the difference between self and cross-pollination? — The transfer of pollen from an anther to a stigma. Self-pollination: Pollen transferred to the stigma of the same plant — reduces genetic diversity. Cross-pollination: Pollen transferred to a different plant of the same species — increases genetic diversity. Agents: wind, insects, water, birds.

What happens during fertilization in flowering plants? — After pollination, pollen grain germinates on the stigma → pollen tube grows down the style to the ovule. Two sperm nuclei travel down: one fertilizes the egg cell → zygote (embryo). The second fertilizes the polar nuclei → endosperm (food store). This is double fertilization — unique to flowering plants.

What is seed dispersal and what are the main methods? — The spreading of seeds away from the parent plant to reduce competition. Methods: Wind (dandelion, maple — papery wings/parachutes). Animals (burrs attach to fur; fleshy fruits eaten, seeds excreted). Water (coconuts float). Explosive mechanisms (peas, squirting cucumber — pods burst under tension).

What conditions are needed for seed germination? — Water (activates enzymes, hydrates cells). Oxygen (aerobic respiration for energy). Suitable temperature (enzyme activity — usually 5–25°C depending on species). Light (some seeds; phytochrome system). NOT in the seed: no requirement for soil or light for most seeds.

What are methods of asexual reproduction in plants? — Runners/stolons (strawberries — horizontal stems produce new plants). Rhizomes (ginger — underground stems). Bulbs (onions, tulips). Tubers (potatoes — modified stems). Cuttings (artificial — human-induced). Grafting (joining two plants). All produce genetically identical offspring (clones).

What is alternation of generations in plants? — Plants alternate between two multicellular forms: Sporophyte (diploid, 2n) — produces spores by meiosis. Spores grow into: Gametophyte (haploid, n) — produces gametes by mitosis. Gametes fuse → zygote → new sporophyte. In flowering plants, the gametophyte is tiny (pollen grain/embryo sac); sporophyte dominates.

What is vegetative propagation and its advantages? — A form of asexual reproduction in plants producing genetically identical offspring (clones) without seeds. Advantages: Faster than sexual reproduction, preserves desirable traits (e.g., fruit quality), no fertilization needed. Disadvantages: No genetic variation, susceptible to same diseases, cannot adapt to environmental changes.

What is tropism and name five types? — A growth response of a plant to an external directional stimulus. (1) Phototropism: Response to light. (2) Gravitropism: Response to gravity. (3) Thigmotropism: Response to touch (e.g., climbing plants). (4) Hydrotropism: Response to water. (5) Chemotropism: Response to chemicals (e.g., pollen tube toward ovule).