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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- explain that stomata respond to changes in environmental conditions by opening and closing and that regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration
- explain that stomata have daily rhythms of opening and closing
- describe the structure and function of guard cells and explain the mechanism by which they open and close stomata
- describe the role of abscisic acid in the closure of stomata during times of water stress, including the role of calcium ions 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
- › In homeostasis questions, be specific about the exact site of hormone synthesis versus the site of release into the circulatory system.
- › Specify that podocytes are found in the squamous epithelium of the visceral layer of the Bowman's capsule.
- › Remember that the stoma is merely a pore; the physiological mechanism (receptors, ion movement) occurs in the guard cells.
- › Remember that an increase in blood water potential leads to a decrease in ADH secretion from the posterior pituitary gland.
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.
Stomatal opening is triggered by light which causes the active transport of K+ ions into the guard cells, lowering their water potential. Water then enters the guard cells by osmosis, increasing turgor pressure. The differential thickening of cell walls causes the guard cells to bow outwards, opening the stomatal pore.
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, which acts as a second messenger. Ca²⁺ activates signaling pathways, including ion channel regulation, that cause the efflux of potassium (K⁺), chloride (Cl⁻) and malate ions. This reduces the osmotic potential of the guard cells, leading to water loss and stomatal closure.
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.
More 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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