The Leaves And Transpiration (T2)

Question 0

In a sentence, what is transpiration?

Answer 0

Transpiration is the loss of water from the leaves of a plant

Question 1

Where is the main site of water absorption on plants and how does this work?

Answer 1

• on the growing tips of the roots are thousands of tiny root hairs which provide the main sites for water absorption
• each root hair is a single specialised cell extended from the epidermis of the root
• the root hair cell penetrates between the soil particles, reaching the soil water
• the soil water has various mineral ions dissolved in it, however not as high a concentration as the inside of the cell
• as the soil water has a higher water potential, it enters the root hair cells by osmosis
• this dilutes the contents of the cell, increasing its water potential
• water moves into the outer tissues of the root…

Question 2

How does transpiration work?

Answer 2

• the epidermis of a leaf is covered in a waxy cuticle so most water passes out of the leaves as water vapour via the stomata
• the water leaves the mesophyll layer cells of the leaf and evaporates into the air spaces between the spongy mesophyll cells before diffusing out the stomata

Question 3

What is the transpiration stream?

Answer 3

• transpiration ultimately means the loss of water as water vapour via the stomata in the leaf cells
• this loss of water creates a water potential gradient which draws water from the surrounding mesophyll cells by osmosis
• this causes a continuous flow of water to be pulled up the xylem from the roots and stem
• this system is know as the transpiration stream

Question 4

What are four functions of the transpiration stream?

Answer 4

• supplies water to the leaf cells for photosynthesis
• transports mineral ions dissolved in the water
• provides turgidity to the plant cells using the water it supplies
• allows evaporation from the leaf surface, cooling the leaf

Question 5

What are the xylem in a plant made of?

Answer 5

Dead cells, arranged in an end to end format, forming a continuous vessel.

Question 6

When mature, what substance do xylem vessels contain?

Answer 6

Cytoplasm?

Question 7

What is lignin?

Answer 7

A woody substance in the walls of the xylem.

Question 8

How do xylem vessels come about?

Answer 8

Xylem begin life as living cells. As the cells develop they become elongated and gradually the original cellulose cell walls become impregnated with lignin. Lignin is made by cytoplasm and impregnable to water. The cells die, forming the resulting hollow tubes.

Question 9

What does lignification mean?

Answer 9

It is the result of the lignin in the walls of the xylem, making the tubes strong and enabling them to carry water up the tall plants without collapsing.

Question 10

Together, what are the xylem and phloem known as?

Answer 10

Vascular bundles

Question 11

What is the function of the xylem?

Answer 11

To transport water and dilutes from the roots of a plant to the leaves.

Question 12

What is the function of the phloem?

Answer 12

To transport sugar for energy and amino acids for making proteins to the young leaves and other developing parts of a plant.

Question 13

Phloem are made up of living cells. Do these cells contain a nucleus?

Answer 13

No.

Question 14

Explain the make up of the phloem.

Answer 14

The phloem are made of living cells which do not contain a nucleus. The tubes are formed end to end but also contain cellulose cell walls and cytoplasm. The end of each cell is made up of a crossed wall of cellulose with holes called a sieve plate. The cytoplasm extends through these holes and links with the next cell. This creates a column of cells forming a long sieve tube.

Question 15

In the phloem, as well as sugar for energy, what else may sugar be transported for, and to where?

Answer 15

Sugar may also be taken to the roots and converted into starch for storage.

Question 16

How are the phloem cells controlled?

Answer 16

By another type of cell that lies alongside the sieve tubes, called a companion cell.

Question 17

What are the four main factors affecting the rate of transpiration?

Answer 17

• light
• temperature
• wind
• humidity

Question 18

How does light affect the rate of transpiration?

Answer 18

• transpiration increases in bright light

- stomata open wider to allow more carbon dioxide into the leaf for photosynthesis

Question 19

How does temperature affect the rate of transpiration?

Answer 19

• transpiration is faster in higher temperatures

- evaporation and diffusion are faster at higher temperatures

Question 20

How does wind affect the rate of transpiration?

Answer 20

• transpiration is faster in windy conditions

- water vapour is removed quickly by air movements speeding up diffusion of more water vapour out of the leaf

Question 21

How does humidity affect the rate of transpiration?

Answer 21

• transpiration is slower in humid conditions

- diffusion out of the leaf is slowed down if the leaf is already surrounded by moist air

Question 22

Explain what happens to the stomata when it is light…

Answer 22

Water enters the guard cells by osmosis from the surrounding epidermis cells. This causes the guard cells to become turgid, swell and change shape. The guard cells bend outwards and open the stomata.

Question 23

What happens to the stomata in the dark?

Answer 23

The guard cells lose water and become flaccid. The stomata close.

Question 24

Why do the stomata close in the dark?

Answer 24

In the dark, leaves cannot photosynthesise and there is no need for leaves to lose water vapour to keep cool.

Question 25

What causes water to enter the guard cells in a leaf when it is light?

Answer 25

The guard cells use energy to accumulate solutes in their vacuoles and water enters them by osmosis.

Question 26

What are the stomata in a leaf surrounded by?

Answer 26

Guard cells

Question 27

Are there more stomata on the lower or upper surface of a leaf and why?

Answer 27

There are more stomata on the lower surface of a leaf, otherwise the leaf would lose too much water due to exposure to direct sunlight. There is also less air movement on the lower surface of a leaf, reducing water loss around the stomata.

Question 28

Name the different structures within a leaf cell…

Answer 28

• cuticle
• upper epidermis
• palisade mesophyll
• spongy mesophyll
• xylem
• phloem
• lower epidermis
• guard cells
• stomata

Question 29

Describe the cells of the upper and lower epidermis of a leaf…

Answer 29

• the cells of the upper and lower epidermis are covered by a thin layer of waxy material called the cuticle
• they posses few chloroplasts
• the cuticle reduces water loss from evaporation and acts as a barrier preventing the entry of disease causing microorganisms like bacteria and fungi

Question 30

Describe the stomata and where they are on a leaf…

Answer 30

• the lower epidermis of a leaf has many pores called stomata
• the upper epidermis has few or no stomata
• stomata enable carbon dioxide to diffuse into a leaf so photosynthesis can take place
• stomata also allow oxygen and water vapour to diffuse out of the leaf
• specialised guard cells form the gap which is the stoma
• these can alter their shape to open or close the stoma, to enable gas exchange

Question 31

What is the mesophyll layer in a leaf cell?

Answer 31

• in the middle of the leaf are two layers of photosynthetic cells, collectively called the mesophyll layer
• the first layer is the palisade mesophyll layer, made up of elongated cells containing hundreds of chloroplasts
• this is the main site of photosynthesis
• palisade cells are close to the surface of the leaf so they have a good light source
• the upper epidermis is also quite transparent so light can pass through easily
• under the palisade layer is the spongy mesophyll layer of rounded, loosely packed cells with air spaces between them
• these cells also photosynthesise but with fewer chloroplasts
• they form the main gas exchange surface of the leaf, absorbing carbon dioxide and releasing water, oxygen and water vapour
• the air spaces allow gaseous exchange between the mesophyll layer and external environment

Question 32

Explain in details the role of the xylem vessels…

Answer 32

• water and mineral ions are supplied to the leaf by the xylem vessels
• these form a continuous transport system through the plant
• water and mineral ions are absorbed by the roots, pass through the stem and through the veins in the leaves
• in the leaves, water leaves the xylem vessels and nourishes the mesophyll cells

Question 33

Explain the role of the phloem vessels in detail…

Answer 33

• the products of photosynthesis (sugars) are transported from the mesophyll cells by the phloem vessels
• the phloem vessels reach all remaining parts of the plant so the tissues and organs which can’t photosynthesise receive nourishment
• the veins in the leaf contain xylem and phloem vessels and form a network of branches