Ch 2 · 14.2 Homeostasis in plants · AS & A Level Biology 9700
Topic 2 of 2 in Homeostasis — AS & A Level Biology 9700 — 48 lessons.
Introduction
Plants cannot move away from unfavourable conditions, so they control their water content from the inside, mainly by opening and closing their stomata. This lesson covers how guard cells work, the role of abscisic acid during drought, and how stomata balance the need for carbon dioxide against water loss through the day. (Syllabus section 14.2.)
Key ideas — Stomatal closure and the guard cell mechanism
Stomata are pores in the leaf epidermis, each surrounded by a pair of guard cells, that allow gas exchange for photosynthesis but also permit water loss by transpiration.
🧑🏫 Teacher's explanation
Think of a plant like a person trying to breathe while also trying not to get dehydrated. To perform photosynthesis, the plant needs to open its 'windows' (stomata) to let carbon dioxide in. However, every time a window is open, water vapor escapes (transpiration). The plant must constantly balance this trade-off.
📖 Definitions
🔍 Worked example — Easy
A guard cell has an internal solute concentration that results in a water potential of Psi_cell = -0.5 MPa. The surrounding epidermal cells have a water potential of Psi_ext = -0.2 MPa. Calculate the direction of water movement.
🔍 Worked example — Standard
Describe the mechanism by which guard cells cause the stomatal pore to open.
🔍 Worked example — Exam-style
Explain how the hormone abscisic acid (ABA) facilitates stomatal closure during water stress.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain how the plant regulates the opening of its stomata to balance gas exchange and water loss. [3] (b) Describe the effect of abscisic acid (ABA) on the guard cells when the plant experiences water stress. [3] [6 marks]
Key ideas — Abscisic acid (ABA) and drought signalling
Abscisic acid (ABA) is a plant hormone synthesised mainly in the roots and leaves in response to water shortage, and it is the principal signal that brings about stomatal closure during drought.
🧑🏫 Teacher's explanation
Think of a plant like a high-stakes business. To make food (glucose), it needs to keep its doors (stomata) open to let in carbon dioxide. However, if the environment gets too dry, opening those doors causes the plant to lose all its precious water through evaporation (transpiration). This is a dangerous trade-off.
📖 Definitions
🔍 Worked example — Easy
A plant is experiencing water stress. Describe the movement of ions in the guard cells that leads to stomatal closure.
🔍 Worked example — Standard
Explain why the loss of solutes from the guard cell leads to the closure of the stoma.
🔍 Worked example — Exam-style
A student observes a plant under drought conditions. Calculate the percentage change in stomatal aperture if the aperture decreases from 50 mum to 10 mum.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the role of abscisic acid (ABA) in the response of the plant to water stress. [2] (b) Explain how the movement of ions in the guard cells leads to the closure of the stomata. [3] [5 marks]
Key ideas — Why stomata open and close: a balance
A leaf faces a dilemma. It needs carbon dioxide for photosynthesis, which can only enter by diffusion through open stomata; but every open stoma also lets water vapour diffuse out by transpiration. Regulating the size of the stomatal aperture balances the two:
🧑🏫 Teacher's explanation
Think of a leaf like a high-performance greenhouse. To make food (glucose), the greenhouse needs to keep its windows (stomata) open to let in fresh air (CO₂). However, if the windows stay open too long while the sun is beating down, the humidity inside drops and the plants 'sweat' too much water through transpiration, eventually causing the plant to wilt.
📖 Definitions
🔍 Worked example — Easy
Describe the effect of light intensity on the stomatal aperture.
🔍 Worked example — Standard
Explain the mechanism by which guard cells cause the stomatal aperture to increase.
🔍 Worked example — Exam-style
A plant is experiencing water stress due to drought. Explain the role of abscisic acid (ABA) and calcium ions in closing the stomata.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the hormone produced by the plant in response to this water stress. [1] (b) Explain how this hormone leads to the closure of the stomata. [3] [4 marks]
Key ideas — Daily rhythms
Stomata show a daily (circadian) rhythm: they open in the morning and close in the evening. The rhythm continues for some time even if a plant is kept in constant light or constant dark, showing it is controlled by an internal "clock" in the guard cells, not only by the light itself. Opening just before dawn lets photosynthesis start as soon as light is available; many plants also partly close their stomata in the hottest part of the day to reduce water loss.
🧑🏫 Teacher's explanation
Imagine a busy shop with a front door. To make money, the shop needs customers to walk in (this is like CO_2 entering the leaf for photosynthesis). However, if the shop leaves the door open all day during a heatwave, it loses all its air conditioning (this is like water escaping via transpiration). The shopkeeper must balance these two needs.
📖 Definitions
🔍 Worked example — Easy
A plant's stomatal aperture (width) is 40 μ m at noon and 10 μ m at midnight. Calculate the percentage decrease in aperture from noon to midnight.
🔍 Worked example — Standard
Explain why stomata typically open in the early morning before light intensity reaches its peak.
🔍 Worked example — Exam-style
Describe the mechanism of stomatal closure during water stress, specifically the role of Abscisic Acid (ABA) and calcium ions.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the hormone produced in response to water stress that triggers stomatal closure. [1] (b) Describe how the movement of ions and water within the guard cells leads to the closure of the stomata. [3] [4 marks]
Worked example
Question. A plant was subjected to a period of drought. Describe and explain how abscisic acid brought about a reduction in water loss from the plant during this period. [6]
🔍 Worked example — Easy
A plant's stomatal aperture is 50 μ m in the morning and 10 μ m during a drought. Calculate the percentage decrease in the aperture.
🔍 Worked example — Standard
Explain why stomata typically open during the day and close at night.
🔍 Worked example — Exam-style
A plant is experiencing water stress. Describe the role of calcium ions in the mechanism of stomatal closure. [3]
⚠️ Common errors
Common mistakes
Practice
Answers