Plant biology: study guide

Key ideas

Plant biology questions mix structure and function with simple measurements. Xylem and phloem move materials, stomata control gas exchange and water loss, and growth is measured by dry mass.

• Xylem carries water and minerals up from the roots, and phloem carries sugars from sources (leaves) to sinks (roots, fruits). Transpiration pulls water up the xylem.
• Guard cells open and close the stomata. Open stomata allow CO₂ in and water vapor out, so the plant balances photosynthesis against water loss.
• Water lost = rate × area × time. Stomatal index = stomata ÷ (stomata + epidermal cells) × 100, which corrects for cell size.
• Relative growth rate RGR = (ln W₂ − ln W₁) ÷ t is the growth per unit of existing mass. The rate of photosynthesis can be measured as oxygen released per unit time.

Key equations

• water lost = rate × area × time
• stomatal index = stomata ÷ (stomata + epidermal cells) × 100
• RGR = (ln W₂ − ln W₁) ÷ t
• rate = volume ÷ time

Worked example

A leaf of 40 cm² loses 1.5 mg of water per cm² per hour, so in 6 h it loses 1.5 × 40 × 6 = 360 mg. A field with 20 stomata and 80 epidermal cells has an index of 20 ÷ 100 × 100 = 20 %. A seedling whose dry mass rises from 2 g to 6 g in 10 days has RGR = ln 3 ÷ 10 = 0.11 g per g per day.

Common mistakes

• Saying phloem carries water up. Water moves up in the xylem; phloem moves sugars in both directions.
• Using the count of stomata alone as the stomatal index without relating it to epidermal cells.
• Calculating RGR as simple growth per day instead of using the logarithms of the masses.

Practice

Test yourself with practice questions on this topic: Plant biology