Ch 9.2

Question 1

What are some examples of sources?

Answer 1

• mature leaves and green stems
• tubers and bulbs at the start of the growing season (spring)
• storage in seeds when they begin germinating

Question 2

What are some examples of sinks?

Answer 2

• roots
• developing leaves
• flowers, fruit, seeds
• storage roots, tubers and bulbs

Question 3

What does the phloem transport?

Answer 3

-organic compounds (ie. sugars) from sources to sinks

Question 4

Function of phloem

Answer 4

• loading of carbohydrates (sugars such as sucrose) at the source
• transport of carbohydrates
• unloading of carbohydrates at the sink

Question 5

Structure of phloem

Answer 5

-phloem is made up of sieve tubes

Question 6

Structure of sieve tubes

Answer 6

• alive, unlike xylem, because they perform active transport, which requires a live membrane
• sieve tubes (which make up phloem) are made of sieve tube cells

Question 7

Structure of sieve tube cells

Answer 7

• alive, but do not have the organelles needed to keep themselves alive on their own (they have reduced cytoplasm, nucleus and organelles to increase space for transport of nutrients)
• companion cells are located next to sieve tube cells and provide the energy, proteins, enzymes and genetic information needed by the sieve tube cells
• sieve tube cells are connected to each other to make the long phloem tube
• the ends of the sieve tube cells have sieve plates; sieve plates are perforated ends that allow carbohydrates to travel easily from one cell to another.

Question 8

Structure of companion cells

Answer 8

• lots of mitochondria to provide energy for active transport done by phloem
• located next to sieve tube cell
• provide energy, proteins, enzymes and genetic information needed by the sieve tube cells

Question 9

Translocation

Answer 9

-movement of organic solutes in a plant

Question 10

How does translocation in plants work?

Answer 10

1. Sugars are made at the leaf (source) by photosynthesis
2. The sugar (in the form of sucrose) is actively transported from the leaf into the phloem
   - energy required for active transport is provided by companion cells
3. High sucrose concentrations in the phloem creates a concentration gradient for water between the phloem and the xylem that is beside it
4. Water moves by osmosis from the xylem into the phloem.
5. When the water moves into the phloem, it increases the pressure within the phloem near the source
   - water molecules are held together by hydrogen bonds, keeping them in a particular arrangement and they cannot be compressed
   - because they cannot be compressed, this creates pressure
6. Fluid wants to move because the pressure is too high at the source; it moves towards the sink where the pressure is low
7. At the sink, sucrose is actively transported out of the phloem, into the sink (ie. roots)
8. Since the concentration of sucrose drops, the water gradient switches and water moves by osmosis back into the xylem
9. Thus, there is lower pressure in the sink; the cycle continues as fluid in the phloem moves from high pressure (source) to low pressure (sink)

Question 11

Compare & contrast xylem/phloem

Answer 11

• xylem is made of dead cells; phloem is made up of alive cells
• xylem walls are thick; phloem walls are thin
• xylem walls are made of lignin (rigid); phloem walls are made of cellulose
• xylem does not have cytoplasm; phloem has cytoplasm lining
• xylem transports water/minerals to leaves; phloem transports food to sinks (growing parts/storage organs)
• xylem’s direction of flow is unidirectional (upwards); phloem’s direction of flow is bidirectional (up and down)
• xylem tissue also has fibres; phloem tissue also has companion cells