Phloem Translocation

Question 1

Define phloem translocation

Answer 1

The transport of products of photosynthesis carried out by phloem tissue

Question 2

What are the cells specialized for translocation?

Answer 2

In angiosperms, the sieve-tube elements of the phloem are specialized for cells for translocation

Question 3

How does phloem sap differ from xylem sap?

Answer 3

The aqueous solution that flows through sieve tubes differs from xylem sap mainly because it contains sucrose (30% by weight) and it may also contain amino acids, hormones, and minerals

Question 4

What is a sugar source?

Answer 4

It is an organ that is a net producer of sugar by photosynthesis or starch breakdown

Question 5

Give examples for sources

Answer 5

Plant leaves

Question 6

Give ex for sinks

Answer 6

Growing roots
Stems
Buds
Fruits

Question 7

Which organs may be a sink or source depending on location?

Answer 7

Tubers and bulbs

Question 8

Describe the basic characteristics of phloem transport

Answer 8

The transport of products of photosynthesis carried out by phloem tissue and is known as phloem translocation

In angiosperms, the sieve-tube elements of the phloem are specialized for cells for translocation

Phloem sap, the aqueous solution that flows through sieve tubes differs from xylem sap mainly because it contains sucrose (30% by weight) and it may also contain amino acids, hormones, and minerals

Phloem sap moves from sites of sugar production to sites of sugar use or storage. So, it takes place from sugar source to sugar sink

Sugar source is an organ that is a net producer of sugar by photosynthesis or starch breakdown

Plant leaves –> sources
Growing roots, stems, buds, fruits –> sinks

Storage organs such as tubers and bulbs may be a source or sink, depending on its function

Question 9

What are the 4 main steps of phloem translocation of angiosperms

Answer 9

1. Loading of sugar into sieve tube reduces water potential inside the sieve tube elements at the source.
2. This causes the sieve tube to take up water from xylem by osmosis
3. This uptake of water generates a positive pressure that forces sap to flow along the tube
4. This pressure is reduced by unloading sugar and consequent water loss from phloem to xylem at the sink

Question 10

What is the hypothesis that explains phloem translocation of angiosperms

Answer 10

Pressure flow hypothesis

Question 11

What’s the rate phloem sap moves from source to sink?

Answer 11

1m/hr

Question 12

What is pressure flow?

Answer 12

Phloem sap moves from source to sink at a rate of about 1m/hr by bulk flow driven by positive pressure known as pressure flow

Question 13

Where do sinks usually receive sugar?

Answer 13

From the nearest sugar sources

Question 14

What does the direction of phloem sap transport in each sieve tube depend on?

Answer 14

The locations of sugar source and sugar sink connected by that tube

Question 15

How does sugar get loaded into sieve tube elements?

Answer 15

in some species, from mesophyll cells to sieve tube elements via symplast, passing through plasmodesmata

Question 16

What kind of transport moves sugar into the phloem?

Answer 16

Active transport

Sucrose is more concentrated in sieve tube elements and companion cells than mesophyll cells

Question 17

Where is sucrose unloaded?

Answer 17

At the sink, at the end of sieve tube

Question 18

Why is the concentration of free sugar in sink always lower than in sieve tube?

Answer 18

Because unloaded sugar is
1. consumed during growth and metabolism of cells in sinks or
2. converted to insoluble polymers like starch

Question 19

What mode of transport do sugar molecules and water molecules use from phloem to sink?

Answer 19

diffusion and osmosis