B2 - Plant Tissues

What is the function of epidermal tissue in leaves?

To protect the leaf surface

Why is the upper epidermis transparent?

So light can pass through to photosynthetic tissues below

What is the waxy cuticle?

A thin oily layer covering the epidermis

What is the role of the waxy cuticle?

To reduce evaporation and prevent the leaf drying out

What are stomata?

Tiny pores in the lower epidermis

What is the role of stomata?

To allow carbon dioxide to enter and oxygen to exit the leaf; also to allow water vapor to leave

What are guard cells?

Cells that surround stomata and control their opening and closing

What is the function of guard cells?

To regulate gas exchange and minimize water loss by opening/closing stomata

What is the palisade mesophyll?

A layer of tightly packed cells beneath the upper epidermis

Why are palisade mesophyll cells good for photosynthesis?

They have many chloroplasts to absorb light efficiently

What is the spongy mesophyll?

A layer below the palisade mesophyll with loosely arranged cells and air spaces

Why are air spaces present in the spongy mesophyll?

To allow diffusion of gases (CO₂ and O₂) between stomata and mesophyll cells

How does CO₂ reach photosynthetic cells in a leaf?

It diffuses through stomata, through the spongy mesophyll air spaces, to palisade cells

How does O₂ exit the leaf?

It diffuses from mesophyll cells through air spaces and out via stomata

What is xylem tissue's role in a leaf?

To transport water and dissolved mineral ions from roots to leaves

Which ions are transported by the xylem to help leaf function?

Minerals like magnesium (for chlorophyll) and nitrates

What is phloem tissue's role in a leaf?

To transport dissolved sugars produced in photosynthesis to other parts of the plant

What is translocation?

The movement of sugars (and other assimilates) through phloem from source to sink

What is meristem tissue?

Plant tissue in growing tips (shoots and roots) containing stem cells

What can meristem cells do?

They can divide and differentiate into other plant tissues

Where is meristem tissue found?

At the tips of roots and shoots

How is the structure of xylem adapted for its function?

It forms hollow tubes, is strengthened by lignin, and has no end walls so water can flow continuously

How is phloem adapted for its function?

It is made of elongated cells with pores in end walls (sieve plates) to allow sap flow

Why must epidermal tissue be thin in leaves?

To reduce the distance that CO₂ must diffuse to reach mesophyll cells

Why is the leaf structure arranged such that palisade cells are near the upper surface?

Because light is more intense at the top, so putting many chloroplasts near the light maximizes photosynthesis

What role do guard cells play when water is scarce?

They close stomata to reduce water loss

When stomata are open, what happens to water vapor?

It evaporates and diffuses out (transpiration)

What is the function of the lower epidermis compared to upper epidermis?

It contains the stomata (for gas exchange) whereas upper mainly allows light through and protects

What is meant by "leaf is a plant organ"?

It consists of multiple different tissues working together (epidermis, mesophyll, vascular tissues etc.)

Why is the spongy mesophyll less tightly packed than palisade mesophyll?

To create air spaces for gas diffusion

Why does phloem transport sugars both directions?

Because sometimes leaves act as sinks (e.g. in spring) and sometimes as sources

How does the arrangement of tissues in a leaf optimize gas exchange and photosynthesis?

Transparent upper epidermis + palisade layer with many chloroplasts + spongy mesophyll air spaces + stomata for gas movement

What limits how thick a leaf can be?

If it's too thick, CO₂ cannot diffuse quickly enough to cells in the middle

Which tissues must a gas pass through to get from the air outside to a mesophyll cell?

Cuticle → upper epidermis → via stomata → spongy mesophyll → mesophyll cell surface

What happens to sugars produced in the leaf?

Some are used immediately (for respiration), others are stored or transported via phloem to other parts of the plant

How is meristem tissue different from other tissues?

It is undifferentiated, capable of division and producing new cells