Xylem and Phloem

Question 1

What are the reasons for multicellular plants needing transport systems?

Answer 1

-Underground part of the plant requires oxygen and glucose to them as they don’t photosynthesise
-mineral ions need to be transported to make necessary proteins and enzymes
-they have a small surface area: volume ratio
-have a high metabolic rate
-direct diffusion is too slow to meet their metabolic needs

Question 2

What are the main functions of xylem?

Answer 2

-to transport water and mineral ions in solution UP the plant from the roots to the leaves
-provide support

Question 3

What is the main function of phloem?

Answer 3

-to transport sugars and assimilates UP and DOWN the plant

Question 4

What is a vascular system?

Answer 4

made up of xylem and phloem, they are found throughout the plant and transport materials to all parts

Question 5

Describe the vascular system in a cross section of a root. Explain how the xylem’s positioning helps provide support.

Answer 5

-root hair
-epidermis
-cortex
-endodermis
-pericycle
-xylem
-phloem

Xylem is in the centre surrounded by phloem to provide support to the root as it pushes through the soil

Question 6

Describe the vascular system in a cross section of a stem? Explain how the xylem’s positioning helps provide support.

Answer 6

-epidermis
-phloem
-xylem
-cambium
-cortex
-pith

Xylem and phloem are near the outsides to provide a sort of ‘scaffolding’ which reduces bending

Question 7

Describe the vascular system in a cross section of a leaf? Explain how the xylem’s positioning helps provide support.

Answer 7

-upper epidermis
-lower epidermis
-xylem
-phloem
-cambium

Xylem and phloem make up a network of veins which support thin leaves

Question 8

Explain how xylem is adapted for its function.

Answer 8

1) Xylem vessels are very long, tube-like structures formed from vessel elements joined end to end
2) There are no end walls, making a continuous tube allowing water to pass up through the middle easily
3) The cells are dead, so have no cytoplasm, reducing resistance and friction to flow of water
4) Walls are thickened with lignin, which supports the xylem vessels and stops them from collapsing
5) Amount of lignin increases as cell gets older
6) Water and ions move through the vessels through small pits where there is no lignin

Question 9

Explain the roles of lignin in supporting the xylem vessels

Answer 9

-It is very hard and rigid, so strengthens the walls, helping to prevent collapsing when under water tension
-It is waterproof so helps prevent lateral loss of water
-it improves adhesion of water molecules so increases capillary action
-comes in 3 different patterns annular, spiral, and reticulate = different patterns allow flexibility to prevent the stem from just snapping and breaking. The patterns also create pits (gaps where no lignin) which allows for water to move between the different xylem vessels in the case of an air bubble or blockage

Question 10

In the phloem, what are sieve tube elements?

Answer 10

-living cells which form the tube for transporting solutes through the plant
-they are joined end to end to form sieve tubes
-the sieve parts are the end walls which have lots of holes to allow solutes to pass through
-they have no nucleus, thin layer of cytoplasm and few organelles

Question 11

What are companion cells?

Answer 11

-cells which carry out the living functions for both themselves and their own sieve tube element