2.3.2: Plant organ system
Not started yet — this one needs some love.
Root hair cells: large surface area; thin cell wall; no chloroplasts; absorb water by osmosis and mineral ions by active transport.
Xylem: hollow dead cells; lignin strengthens walls; continuous tube from roots to leaves; water travels up in the transpiration stream (driven by evaporation from leaves).
Transpiration: water evaporates through stomata → water potential gradient pulls water up xylem → roots absorb more water by osmosis. Passive — no energy required.
Factors increasing transpiration rate: ↑ temperature, ↑ light intensity, ↑ air movement (wind), ↓ humidity.
Factors decreasing transpiration rate: ↑ humidity, ↓ temperature, darkness.
Guard cells: swell (turgor) in light → stomata open; lose water in dark/drought → stomata close.
Stomata open in light (CO₂ needed for photosynthesis); close in darkness and drought (reduce water loss).
Phloem: living cells; sieve plates (pores in end walls) allow flow; transports dissolved sucrose in both directions (translocation).
Phloem has no detailed mechanism required — just know it transports sugars by translocation.
Common exam mistakes
Transpiration is passive — it does NOT require energy. Do not say energy is needed to move water up the xylem.
Xylem is strengthened by lignin (not cellulose). Xylem does NOT contain mitochondria.
Phloem transports sucrose/dissolved sugars — not just 'food' (too vague) and NOT mineral ions.
When comparing xylem and phloem, must make comparative statements — not just describe each separately.
High humidity REDUCES transpiration (the air is already saturated with water so less evaporates) — students often get this the wrong way round.
Stomata are widest open at midday (highest light intensity → maximum photosynthesis → maximum CO₂ uptake needed).
Phloem transports sugars in any direction (not just up or down); xylem transports water only upward.