Plant Power

Plant Power

A leaf looks flat and simple. Inside it is a stack of specialised layers, each doing one job, and the stack is not in that order by accident. Two words carry the topic. A **tissue** is a group of similar cells doing one job. An **organ** is several tissues working together to do something none of them could do alone, and a leaf is an organ. Get each tissue matched to its job, and to its position, and you own one of Paper 1's most reliable topics.

The tissues of a leaf

Six tissues appear in this topic. Learn each one as a **structure and a job together**, because the structure is usually how the exam identifies it:

Which tissue is this?

Label the leaf

Drag each label onto the right tissue in this leaf cross-section.

The upside-down leaf

Imagine a leaf held upside down in bright light for several days, with everything else unchanged. Which tissue is now worst placed for its job, and why?

• Palisade mesophyll, because it is now the layer furthest from the light it needs for photosynthesis
• Xylem, because water would now have to travel downwards
• Spongy mesophyll, because its air spaces would fill up
• The upper epidermis, because the cuticle would face downwards

The trade-off at the stomata

The lower epidermis is dotted with pores called **stomata** (one is a **stoma**), each held between two **guard cells**. Open stomata let **carbon dioxide in** for photosynthesis. They also let **water vapour out**, and the plant has no way of having one without the other. So the guard cells manage a permanent compromise: **open in the light**, when photosynthesis is possible and worth the water; **closed in the dark or when water is short**, accepting that photosynthesis stops. Almost every question about stomata is really about that trade-off.

Find the stoma

Tap the stoma: the pore held between the two guard cells in the lower epidermis.

Closed for the drought

During a drought a plant keeps its stomata closed through the middle of the day. It saves water. What does it lose by doing so?

• Carbon dioxide can no longer diffuse in, so photosynthesis slows or stops and growth suffers
• It can no longer respire, because no oxygen can enter
• It loses water faster, because the guard cells swell
• Nothing: closing the stomata has no cost at all

Two flows

The leaf drives two transport streams, in two different tissues, and they are not variations on one idea. Muddling them is the commonest error in this topic:

Up in spring, down in summer

In summer, sugar made in the leaves travels down to be stored in the roots. In spring, sugar stored in the roots travels up to feed new buds. Which tissue is doing this, and what does it show?

• The phloem, and it shows that translocation genuinely runs in both directions depending on where the sugar is needed
• The xylem, since it is the tissue that carries substances up the plant
• Xylem going up in spring and phloem going down in summer
• The meristem, because the new buds are growing

Speed it up

Select ALL THREE changes that would INCREASE a plant's rate of transpiration.

• Moving the plant from still air into a breeze
• Raising the temperature of the room
• Turning on a bright lamp above the plant
• Sealing the plant inside a bag so the air becomes humid
• Moving the plant into a cold room
• Putting the plant in darkness

The potometer

A **potometer** measures how fast a shoot takes up water, by timing an air bubble moving along a scale. Be precise about what it measures. It records **water uptake**, and most of the water taken up is lost by transpiration, so uptake is used as an **estimate** of the transpiration rate. A small amount is used inside the plant, so the two are close rather than identical. To investigate one factor you have to **control the others**: temperature, light intensity, air movement and humidity. Change the wind speed while the lamp also gets brighter and the result tells you nothing about either. The rate is then **distance moved ÷ time taken**.

Potometer maths

In a potometer the air bubble moves 60 mm along the scale in 5 minutes. Calculate the rate of water uptake, in mm per minute.

Leaf summary

A leaf is an organ built from tissues. _____ mesophyll sits near the top, packed with chloroplasts, while spongy mesophyll below has air spaces for gas exchange. Water and minerals travel up the _____, which is made of dead cells strengthened with lignin, in a flow called transpiration. Sugars travel through the phloem in _____ directions, a flow called translocation. Stomata open to let carbon dioxide in, at the cost of losing _____.