Topic 4: Phloem

Question 1

In the vascular bundle, where is the phloem located? (1 point)

Answer 1

1. Between the xylem and the sclerenchyma

Question 2

State the 3 main specialised cells and tubes that make up phloem tissue (3 points)

Answer 2

1. Phloem sieve tubes
2. Companion cells
3. Transfer cells

Question 3

What are the phloem sieve tubes made of? (1 point)

Answer 3

1. Sieve tube elements

Question 4

What do the phloem sieve tubes do? (1 point)

Answer 4

1. Carry dissolved organic solutes

Question 5

What do the companion cells do? (1 point)

Answer 5

1. Supply substances to sieve tubes

Question 6

What are transfer cells? (1 point)

What do transfer cells do? (1 point)

Answer 6

1. They are specialised parenchyma cells

1. Actively transport products of photosynthesis into or out of the sieve tube using ATP

Question 7

Describe the structure of the phloem (10 points)

Answer 7

1. It is living
2. Has phloem sieve tubes made of sieve tube elements
3. Has companion cells
4. Has transfer cells
5. Sieve tube elements have no nucleus
6. Sieve tube elements have a thin peripheral cytoplasm with few organelles
7. Sieve tube elements have a fluid filled lumen
8. Sieve tube elements have sieve plates, so it is a continuous tube
9. The plasmodesmata link the sieve tube element and the companion cell
10. Companion cells have all organelles e.g. nucleus, more ribosomes + mitochondria

Question 8

What is the function of the phloem? (1 point)

Answer 8

1. Translocation of organic solutes

Question 9

Define translocation (3 points)

Answer 9

1. The TRANSPORT of DISSOLVED ORGANIC SOLUTES (e.g. sucrose produced in leaves by photosynthesis, then dissolved in water)
2. by MASS FLOW,
3. from SOURCE TO SINK in PHLOEM

Question 10

In which direction does translocation take place? (1 point)

Answer 10

1. either direction, up or down the plant - from a source to a sink

eg.
from leaves down to roots

from leaves up to developing flowers

from roots up to developing buds

Question 11

What is a source? (1 point)

Answer 11

1. Where products of photosynthesis are produced e.g. leaf (can be roots if starch is hydrolysed)

Question 12

What is a sink? (1 point)

Answer 12

1. Where sugars (e.g. sucrose) are being USED but not produced e.g. buds, roots

Question 13

The ________ _______ need to dissolve in water to be transported, so the phloem contains ________.

Answer 13

organic solutes

water

Question 14

Describe the mechanism of translocation (5 steps)

Answer 14

1. Organic solutes (e.g. sucrose, amino acids) are made in the source and then released
2. Solutes pass from cell to cell until they reach a transfer cell (next to the phloem) and the solutes are loaded into the phloem
3. Solutes are transported to the sink
4. Solutes are unloaded from the phloem into a transfer cell at the sink
5. Solutes pass from cell to cell in the sink to supple cells with solutes

Question 15

What are the 3 {features/adaptations} of transfer cells & how does the feature help with translocation? (3 points)

Answer 15

1. Many small infoldings of cell wall and cell membrane - increases surface area
2. Many plasmodesmata - link cytoplasm with adjacent cells
3. Lots of mitochondria - provide energy (in the form of ATP) for the loading of solutes into the sieve tube by active transport

Question 16

Key Summary: Explain {translocation / how sucrose} are transported in plants (5 points)
(aka the pressure flow mechanism - can replace “sucrose” with solutes or amino acids)

Answer 16

1. Sucrose produced SOURCE moves by ACTIVE TRANSPORT using ATP from transfer cells into PHLOEM sieve tube, so high sucrose concentration in sieve tube at source
2. WATER ENTERS sieve tube from xylem BY OSMOSIS
3. Increased water volume INCREASES HYDROSTATIC PRESSURE
4. Sucrose unloaded from sieve tube at sink, sucrose concentration decreases, so water moves into xylem by osmosis, LOWERING HYDROSTATIC PRESSURE
5. Fluid with dissolved sucrose moves BY MASS FLOW DOWN A PRESSURE GRADIENT from HIGH TO LOW HYDROSTATIC PRESSURE along the sieve tube - source to sink

Question 17

Key Summary: How is phloem tissue specialised for {solute/sucrose/amino acid} transport? (4 points)
(describe 1 feature & give explanation on how it is {useful/specialised} per point)

Answer 17

1. Sieve tubes with SIEVE PLATES WITH PORES – continuous tube which FLUID WITH DISSOLVED SOLUTES CAN FLOW THROUGH
2. Sieve tubes have no nucleus, very little cytoplasm, very few organelles – LESS RESISTANCE to flow of fluid
3. Companion cells with lots of mitochondria and ribosomes linked to sieve tubes by plasmodesmata - METABOLICALLY SUPPORT sieve tube element cells by providing ATP, proteins and nutrients which move through plasmodesmata into sieve tubes
4. Transfer cells have FOLDED WALLS – increase surface area for transfer of solutes - loading solutes into or unloading solutes from sieve tubes