Leaf structure, stomata and guard cells, xylem and phloem, transpiration and translocation, and the factors that change the rate of water loss.
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1.Name the tissues found in a leaf.
Upper and lower epidermis, palisade mesophyll, spongy mesophyll, xylem and phloem, and guard cells around the stomata.
2.What is the function of the upper epidermis?
It is thin and transparent so light can pass through to the palisade layer below, and it is covered by a waxy cuticle.
3.What is the function of the waxy cuticle?
It reduces water loss from the surface of the leaf by evaporation.
4.Why is the palisade mesophyll at the top of the leaf?
Its cells are packed with chloroplasts, so putting them nearest the light absorbs the most for photosynthesis.
5.What is the function of the spongy mesophyll?
It contains air spaces that increase the surface area for gases to diffuse in and out.
6.What are stomata?
Tiny pores in the leaf surface, mostly on the underside, that let carbon dioxide in and oxygen and water vapour out.
7.Which cells control the stomata?
Guard cells, one either side of each pore.
8.Why are most stomata on the lower surface of the leaf?
The underside is shaded and cooler, so less water is lost by evaporation.
9.What happens to guard cells when water is plentiful?
They take in water and become turgid. Their uneven walls make them curve apart, opening the stoma.
10.What happens to guard cells when water is short?
They lose water and become flaccid, closing the stoma and reducing further water loss.
11.What does xylem transport, and in which direction?
Water and dissolved mineral ions, upwards only, from the roots to the stem and leaves.
12.How is xylem adapted to its function?
Hollow dead cells joined end to end with no end walls, forming a continuous tube, strengthened by a substance called lignin.
13.What is transpiration?
The loss of water vapour from the leaves, mainly through the stomata.
14.What is the transpiration stream?
The continuous movement of water from the roots, up through the xylem, and out of the leaves as vapour.
15.Name four factors that increase the rate of transpiration.
Higher temperature, more air movement, higher light intensity, and lower humidity.
16.Why does higher temperature increase transpiration?
Water molecules have more energy, so they evaporate and diffuse away faster.
17.Why does light intensity affect transpiration?
Stomata open in the light so the plant can take in carbon dioxide for photosynthesis, and open stomata let more water out.
18.Why does high humidity reduce transpiration?
There is already a lot of water vapour in the air outside the leaf, so the concentration gradient is smaller and less water diffuses out.
19.Why does air movement increase transpiration?
Wind blows away the water vapour gathered outside the stomata, keeping the concentration gradient steep.
20.What does phloem transport?
Dissolved sugars made in the leaves, to where they are needed for respiration or storage.
21.What is translocation?
The movement of dissolved food substances through the phloem, from the leaves to the rest of the plant.
22.In which direction does phloem transport?
Both up and down the plant, unlike xylem which only goes up.
23.How is phloem adapted to its function?
Living cells joined end to end with sieve plates between them so the contents can flow through, with companion cells alongside supplying energy.
24.What piece of apparatus is used to investigate transpiration?
A potometer.
25.What does a potometer actually measure?
The rate at which water is taken up by the plant, which is used as an estimate of the rate of transpiration.
26.How are root hair cells adapted?
A long hair-like extension gives a very large surface area for absorbing water and mineral ions from the soil.
27.What is meristem tissue?
Tissue at the growing tips of roots and shoots that can differentiate into any type of plant cell.
28.Why do plants need mineral ions such as nitrates?
Nitrate ions are used to make amino acids, which are built into proteins for growth.
29.Why are leaves thin and flat?
A flat shape gives a large surface area for absorbing light, and being thin means a short diffusion path for gases.
30.Where does most gas exchange in a leaf take place?
Through the stomata, into and out of the air spaces in the spongy mesophyll.
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