Overview
Plant biology is a large share of olympiad-level exams, and many students underprepare for it because most high-school courses focus on animals. It is one of the best places to gain points with steady study.
Focus on transport (xylem and phloem), hormones, and how plants respond to light and water stress. Connect everything back to water potential.
Core concepts
8.1Plant structure
Three tissue systems: dermal, vascular (xylem and phloem), and ground. Primary growth happens at apical meristems. Secondary growth (wood and bark) comes from vascular cambium and cork cambium.
Monocots and eudicots differ in seed leaves, venation, vascular bundle arrangement, and floral part number. Use these differences as a checklist when a question asks you to identify a stem or root cross-section.
Key terms: meristem · vascular cambium · Casparian strip · stomata · monocot vs. eudicot
8.2Water and sugar transport
Water potential Ψ = Ψs + Ψp. Water moves from higher to lower Ψ. Transpiration from leaves creates tension that pulls water up the xylem, and cohesion between water molecules keeps the column intact (the cohesion–tension mechanism).
Phloem moves sugars from sources to sinks by pressure flow. Loading sugar at the source lowers Ψ, water enters, and pressure rises. Unloading at the sink reverses this. Guard cells open stomata when K⁺ uptake draws water in. ABA closes stomata during drought.
Key terms: water potential · cohesion–tension · pressure flow · source–sink · guard cells
8.3Plant hormones and responses
Auxin: cell elongation, apical dominance, phototropism (it moves to the shaded side of the shoot). Cytokinins: cell division and delaying senescence. Gibberellins: stem elongation and germination. ABA: dormancy and stomatal closure. Ethylene: fruit ripening and the triple response.
Phytochrome detects red and far-red light. It controls germination in some seeds and flowering time. Photoperiodism actually measures the length of the uninterrupted night.
Key terms: auxin · ABA · phytochrome · photoperiodism · gravitropism
8.4Plant reproduction
Plants alternate between haploid gametophyte and diploid sporophyte generations. In angiosperms, double fertilization produces a diploid zygote and a triploid endosperm.
Know the parts of a flower and the pathway from pollination to seed and fruit.
Key terms: alternation of generations · double fertilization · endosperm · gametophyte · pollination
Practice
Review the concepts above, then complete the practice set. Missed a question? Read the explanation and try a similar problem.
Common mistakes
| Mistake | Instead |
|---|---|
| Assuming roots actively pump water up the stem. | Most water moves passively by transpiration pull. Root pressure contributes little in tall plants. |
| Thinking auxin makes cells on the lit side grow faster. | Auxin accumulates on the shaded side. Those cells elongate more, so the shoot bends toward the light. |
| Thinking photoperiodism measures day length. | Plants measure uninterrupted darkness. A brief flash of light during the night can reset the response. |
Recommended resources
- OpenStax Biology 2e, Chapters 30–32 Textbook (free)
- Botany: An Introduction to Plant Biology (Mauseth) Textbook (listed by USABO)
- HHMI BioInteractive Videos
Further reading
- Campbell Biology, unit on plant form and function.
- Raven Biology of Plants (Evert & Eichhorn).