Leaf tissues and their functions explained clearly

Organisation • Plant tissues, organs and systems

Flashcards

Test your knowledge with interactive flashcards

How do sieve plates help phloem function?

Click to reveal answer

Sieve plates allow sap to flow between sieve tube elements while maintaining controlled movement along the phloem.

Key concepts

What you'll likely be quizzed about

Epidermal tissue: protection and light transmission

Definition: A single outer cell layer covering the leaf surfaces. Structure: Cells are flat and often transparent with a thin cuticle on the outer surface. Effect on function: Transparency allows light to reach the palisade mesophyll so photosynthesis occurs efficiently. The cuticle reduces uncontrolled water loss by evaporation; a thinner cuticle on the underside supports gas exchange where more stomata are present .

Palisade mesophyll: specialised for photosynthesis

Definition: A layer of elongated, closely packed cells just below the upper epidermis. Structure: Cells contain many chloroplasts and orient vertically to intercept light. Effect on function: High chloroplast density increases light capture and therefore the rate of photosynthesis. Close packing maximises the number of photosynthesising cells per unit leaf area, increasing carbohydrate production .

Spongy mesophyll: diffusion and transpiration

Definition: A layer of rounded cells with large air spaces found beneath the palisade layer. Structure: Cells are loosely arranged, creating continuous air spaces linked to stomata. Effect on function: Large internal air space increases the surface area for gas exchange and permits rapid diffusion of CO2 to palisade cells and O2 away from them. Evaporation from these cells into the air spaces contributes to transpiration, maintaining the transpiration stream in xylem vessels .

Guard cells and stomata: dynamic pore control

Definition: Paired specialised epidermal cells that border each stomatal pore. Structure: Guard cells have unevenly thickened walls and change shape when turgor pressure changes by osmosis. Effect on function: Turgid guard cells curve to open the stoma, allowing CO2 in for photosynthesis and water vapour and O2 out. Flaccid guard cells close the pore to reduce water loss, so guard cell control balances gas exchange with water conservation. Stomatal distribution and density affect overall leaf gas exchange capacity .

Xylem: water transport and structural support

Definition: Vessels formed from dead cells that conduct water and minerals from roots to leaves. Structure: Hollow, continuous tubes with thick walls reinforced by lignin and few or no end walls. Effect on function: Lignified walls prevent collapse under tension created by transpiration pull, enabling an unbroken column of water to move upward. The hollow, end-wall-free structure minimises resistance and allows rapid one-way movement of water to supply mesophyll cells for photosynthesis and to sustain turgor .

Phloem: distribution of sugars

Definition: Living tissue that transports sugars and other solutes around the plant. Structure: Sieve tube elements with perforated sieve plates form channels; companion cells provide metabolic support. Effect on function: Sieve plates allow movement of sap between cells while companion cells supply ATP for active loading and unloading of sugars. Phloem translocation moves sucrose from source tissues (leaves) to sink tissues (roots, growing tips or storage organs), supporting growth and storage in the plant .

Key notes

Important points to keep in mind

Epidermis is transparent and often covered by a cuticle to limit water loss while allowing light through.

Palisade cells are tall, densely packed and chloroplast-rich; form the main photosynthetic layer.

Spongy mesophyll air spaces shorten diffusion pathways for CO2 and O2, increasing exchange speed.

Guard cell turgor controls stomatal aperture by osmosis; opening increases CO2 uptake but raises water loss.

Xylem vessels are dead, lignified tubes that resist tension and allow an unbroken water column.

Phloem is living tissue with sieve plates and companion cells that enable active transport of sugars.

Transpiration rate increases with temperature, wind and light and decreases with higher humidity.

Limiting factors for photosynthesis include light intensity, carbon dioxide concentration, temperature and chlorophyll availability.