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 mechanical support

Question 3

What is the main function of phloem?

Answer 3

to transport sugars and assimilates UP and DOWN the plant

Question 4

describe the 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

• long, tube-like structures formed from vessel elements joined end to end
• no end walls, making a continuous tube allowing water to pass up through the middle easily
• cells are dead, so have no cytoplasm, reducing resistance and friction to flow of water
• Walls are thickened with lignin, which supports the xylem vessels and stops them from collapsing
• lignin increases as cell gets older
• 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

• hard and rigid, so strengthens the walls, helping to prevent collapsing when under water tension
• waterproof so helps prevent lateral loss of water
• 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

what are sieve tube elements?

Answer 10

living cells which form the tube for transporting solutes through the plant
- joined end to end to form sieve tubes
- sieve parts are the end walls which have lots of holes to allow solutes to pass through
- 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

Question 12

what are two pieces of evidence for translocation occurring in the phloem? (2 marks)

Answer 12

• the rate of movement of sugars in the phloem is many times faster than could be achieved by diffusion alone
• an insect such as an aphid feeds by inserting its mouth into the phloem

Question 13

when bark is removed from a tree, the phloem is also removed. if a complete ring of bark is removed, the tree trunk swells above the cut. explain why (2 marks)

Answer 13

• due to accumulation of organic solutes as they can no longer move through after the cut
• water potential decreases so water moves into the cells

Question 14

state one function of the xylem tissue (1 mark)

Answer 14

• transport of water from roots to leaves of a plant

Question 15

state the function of lignin and explain why it is important that the xylem vessel becomes lignified in this way. (3 marks)

Answer 15

• provides mechanical strength to prevent xylem tubes collapsing
• resistance against pathogens makes cell walls impermeable
• enables transport of solutes via tracheids

Question 16

explain the function of lignin pits? (2 marks)

Answer 16

• allows water and nutrients to move between vessels
• bypass blockage
• supplies water to other cells

Question 17

describe and explain how water is moved up the xylem from the roots to leaves (5 marks)

Answer 17

• water moves up the xylem via the transpiration stream
• water molecules leave the spongy mesophyll cells and into the air spaces via osmosis down a water potential gradient
• then they move into adjacent cells via osmosis
• adhesive and cohesive forces between water molecules force it up the plant
• capillary action
• mass flow
• high pressure at the bottom

Question 18

name the sugar molecule most commonly translocated (1 mark)

Answer 18

sucrose

Question 19

state 2 adaptations of sieve tubes which enable mass flow to occur (2 marks)

Answer 19

• elongated
• no nucleus