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All 12 Flashcards — Gas exchange in leaves
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Question
What is a stoma?
Answer
A small **pore** in the leaf surface (mostly the underside) through which **gases enter and leave** the leaf.
Question
What do guard cells do?
Answer
The two cells either side of a stoma that change shape to **open or close the pore**, controlling gas exchange and water loss.
Question
On which leaf surface are most stomata found?
Answer
The **lower (under) surface** — this reduces water loss while still allowing gas exchange.
Question
What is the palisade mesophyll, and what does it do?
Answer
A layer of **tall cells packed with chloroplasts** near the upper surface; it carries out **most of the photosynthesis**.
Question
What is the spongy mesophyll, and what do its air spaces do?
Answer
A layer of **loosely-packed cells with large air spaces**; the spaces let **gases diffuse** to and from every cell.
Question
Why is a leaf thin and flat?
Answer
**Thin** = short diffusion distance for gases; **flat and wide** = large surface area for light and gas exchange.
Question
By what process do gases move in and out of a leaf?
Answer
**Diffusion** — from a higher to a lower concentration, with no energy needed.
Question
Trace the path of CO₂ from the air into a chloroplast.
Answer
In through a **stoma** → through the **spongy-mesophyll air spaces** → across the **cell wall and membrane** → into a **chloroplast**.
Question
What does the waxy cuticle do?
Answer
It is a **transparent, waterproof** coating that **reduces water loss** while still letting light through.
Question
What gases enter and leave during photosynthesis in a leaf?
Answer
**CO₂ diffuses in**; **O₂ (and water vapour) diffuse out** — through the stomata.
Question
Why does the spongy mesophyll have air spaces?
Answer
So **CO₂ can reach every cell** and **O₂ can diffuse away** — they are the leaf's internal corridors for gases.
Question
What is the upper epidermis like, and why?
Answer
A single layer of **transparent, tightly-packed cells with no chloroplasts**, so **light passes through** to the palisade cells below.
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Topic 2.6 hub
Gas exchange
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