Homeostasis in plants
Cambridge A-Level Biology (9700) · Unit 14: Homeostasis · 7 flashcards
Homeostasis in plants is topic 14.2 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 14 — Homeostasis , alongside Homeostasis in mammals.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 7 key concepts — covering the precise wording mark schemes reward.
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Explain that stomata open and close in response to changing environmental conditions, and that controlling the size of the stomatal aperture balances taking in CO₂ by diffusion against keeping water loss by transpiration to a minimum.
- Explain that stomata open and close following a daily rhythm.
- Describe how guard cells are structured and what they do, and explain the mechanism they use for opening and closing stomata.
- Describe how abscisic acid causes stomata to close during water stress, including how calcium ions act as a second messenger.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Homeostasis in plants
- › Remember that the stoma is merely a pore; the physiological mechanism (receptors, ion movement) occurs in the guard cells.
Explain how the regulation of stomatal aperture achieves a balance between carbon dioxide uptake and water loss in plants.
Stomata opening allows CO₂ diffusion for photosynthesis but also leads to water loss via transpiration. Stomatal aperture is regulated by guard cells to balance the need for CO₂ uptake with the need to minimize water loss, optimizing photosynthesis while preventing excessive dehydration. Environmental factors such as light intensity and water availability influence this balance.
Describe the daily rhythms of stomatal opening and closing. What is the primary driver?
Stomata typically open during the day to allow for photosynthesis and close at night to conserve water. The primary driver of this daily rhythm is light availability, though it can be affected by water availability, CO₂ concentrations, and internal circadian rhythms.
Describe the structure of guard cells and relate this structure to their function in controlling stomatal aperture.
Guard cells are kidney-bean shaped cells surrounding the stomatal pore. They have unevenly thickened cell walls (thicker on the side facing the pore) and contain chloroplasts. When guard cells become turgid, the thinner outer walls stretch more than the thicker inner walls, causing the cells to bow outwards and open the stoma.
Outline the mechanism by which guard cells open stomata.
In light, ATP-powered proton pumps in the guard cell membrane actively transport H⁺ out. The resulting electrochemical gradient opens K⁺ channels, and K⁺ diffuses into the guard cells, lowering their water potential. Water enters by osmosis and turgor rises. The thicker inner walls stretch less than the outer walls, so the guard cells bow outwards and the stoma opens.
Describe the role of abscisic acid (ABA) in the closure of stomata during times of water stress.
During water stress, ABA is produced in roots and transported to the leaves. ABA binds to receptors on guard cell membranes, leading to the influx of calcium ions (Ca²⁺) which act as a second messenger. Ca²⁺ initiates a cascade of events leading to the efflux of K⁺ ions from guard cells, decreasing turgor pressure and closing the stomata.
Explain the role of calcium ions (Ca²⁺) as a second messenger in ABA-mediated stomatal closure.
ABA binding triggers Ca²⁺ influx into the guard cells, where Ca²⁺ acts as a second messenger. It opens channels that let K⁺, Cl⁻ and malate ions leave the guard cells. With fewer solutes inside, the water potential of the guard cells rises, water leaves by osmosis, the cells lose turgor and the stoma closes.
What environmental factors, besides water availability, can influence stomatal aperture?
Besides water availability, environmental factors such as light intensity, carbon dioxide concentration, and temperature can also influence stomatal aperture. High light intensity often promotes opening, while high CO₂ concentrations can trigger closing.
Review the material
Read full revision notes on Homeostasis in plants — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 14 — Homeostasis
Homeostasis in plants sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
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