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CIE A Level Biology (9700) exams from 2022 Revision Notes
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14.2 Homeostasis in Plants CONTENTS 14.2.1 Stomata 14.2.2 Guard Cells 14.2.3 Abscisic Acid & Stomatal Closure
14.2.1 STOMATA Stomata Plants carry out homeostasis – just like animals they need to maintain a constant internal environment For example, mesophyll cells in leaves require a constant supply of carbon dioxide for photosynthesis Stomata (specifically the guard cells) control the diffusion of gases in and out of leaves This means stomata control the entry of carbon dioxide into leaves
Response of guard cells & stomata table
Regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration
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CIE A Level Biology (9700) exams from 2022 Revision Notes
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14.2 Homeostasis in Plants Advantages & disadvantages of stomatal opening & closure table
Exam Tip
A stoma is actually the aperture (hole) between two guard cells, but the term is often used to refer to the whole unit (the two guard cells and the hole between them). Don’t forget – stoma (singular) refers to one of these units, whereas stomata (plural) refers to many!
Opening & Closing of Stomata Stomata open and close in a daily rhythm Even when the plant is kept in constant light or constant darkness, the daily rhythm of opening and closing of the stomata continues Opening of stomata during the day: maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen allows the outward diffusion of water vapour in transpiration Closing of stomata at night when photosynthesis cannot occur: reduces the rate of transpiration conserves water
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14.2 Homeostasis in Plants 14.2.2 GUARD CELLS Guard Cells Structure of guard cells Each stoma is surrounded by two guard cells Guard cells have the following features: Thick cell walls facing the air outside the leaf and the stoma Thin cell walls facing adjacent epidermal cells Cellulose microfibrils arranged in bands around the cell Cell walls have no plasmodesmata Cell surface membrane is often folded and contains many channel and carrier proteins Cytoplasm has a high density of chloroplasts and mitochondria Chloroplasts have thylakoids but with few grana (unlike those in mesophyll cell chloroplasts) Mitochondria have many cristae Several small vacuoles rather than one large vacuole
The structure of guard cells
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14.2 Homeostasis in Plants Mechanism to open stomata Guard cells open when they gain water and become turgid Guard cells gain water by osmosis A decrease in water potential in the guard cells is required for water to enter the cells by osmosis In response to light, ATP-powered proton pumps in the guard cell surface membranes actively transport hydrogen (H+) ions out of the guard cell This leaves the inside of the guard cells negatively charged compared to the outside This causes channel proteins in the guard cell surface membranes to open, allowing potassium (K+) ions to move down the electrical gradient and enter the guard cells The potassium (K+) ions also diffuse into the guard cells down a concentration gradient The combination of the electrical gradient and concentration gradient is known as an electrochemical gradient The influx of potassium (K+) ions increases the solute concentration inside the guard cells, lowering the water potential inside the cells Water now enters the guard cells by osmosis through aquaporins in the guard cell surface membranes Most of the water enters the vacuoles, causing them to increase in size This increases the turgor pressure of the guard cells, causing the stoma to open The bands of cellulose microfibrils only allow the guard cells to increase in length (not diameter) The thin outer walls of the guard cells bend more easily than thick inner walls This causes the guard cells to become curved, opening up the stoma
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14.2 Homeostasis in Plants
The mechanism to open stomata
Mechanism to close stomata When certain environmental stimuli are detected (that lead to the closing of the stomata), the proton pumps in the guard cell surface membranes stop actively transporting hydrogen (H+) ions out of the guard cell The potassium (K+) ions leave the guard cells The water potential gradient is now reversed and water leaves the guard cells by osmosis This causes the guard cells to become flaccid, closing the stoma
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14.2 Homeostasis in Plants 14.2.3 ABSCISIC ACID & STOMATAL CLOSURE Abscisic Acid & Stomatal Closure During times of water stress, the hormone abscisic acid (ABA) is produced by plants to stimulate the closing of their stomata Certain environmental conditions can cause water stress, such as very high temperatures or reduced water supplies Guard cells have ABA receptors on their cell surface membranes ABA binds with these receptors, inhibiting the proton pumps and therefore stopping the active transport of hydrogen (H+) ions out of the guard cells ABA also causes calcium (Ca2+) ions to move into the cytoplasm of the guard cells through the cell surface membranes The calcium ions act as second messengers: They cause channel proteins to open that allow negatively charged ions to leave the guard cells This stimulates the opening of further channel proteins that allow potassium (K+) ions to leave the guard cells The calcium ions also stimulate the closing of channel proteins that allow potassium (K+) ions to enter the guard cells This loss of ions increases the water potential of the guard cells Water leaves the guard cells by osmosis The guard cells become flaccid, causing the stomata to close
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14.2 Homeostasis in Plants
Closure of a stoma in response to abscisic acid (ABA)
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