Stomata

One pore is a stoma. Carbon dioxide enters and oxygen leaves through stomata. Water vapour escapes through them too, in transpiration. Each pore sits between two bean-shaped guard cells, which have chloroplasts. The syllabus explains their movement by the potassium ion exchange theory:

• In light, potassium ions (K⁺) move into the guard cells.
• The cell sap gets stronger, so water enters the guard cells by osmosis.
• The guard cells swell and bend apart, and the pore opens.
• In the dark, potassium ions leave, water follows, and the pore closes.

A small example

Clip dry blue cobalt chloride paper to both sides of a dorsiventral leaf, the usual broad leaf with distinct upper and lower surfaces. The paper on the lower side turns pink first. That side has more stomata, so it loses more water vapour.

Where marks go

The 2024 ICSE Biology paper asked which ions help stomata open. The answer was potassium ions. CISCE's report on the 2024 ICSE Biology paper names "magnesium" as the wrong answer given. The ICSE 2027 specimen paper turns it round. It offers "cobalt ions help in the opening of stomata" as a false statement to reject.

In the 2025 paper, the part that controls opening and closing was guard cells. CISCE's report on that paper says "bean shaped cells" alone was given in place of guard cells. The specimen paper also asks for a labelled stomatal apparatus with its epidermal cells, for 3 marks.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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