phloem transport

Question 1

What is autotrophic growth and how is it fueled?

Answer 1

Autotrophic growth occurs after germination.
It is fueled by photosynthesis, where plants produce their own food.
Sugars are transported throughout the plant to support growth and development.

Question 2

What are source and sink organs in plants?

Answer 2

Source organs: Photosynthetic leaves that produce sugars (photoassimilates) and export them to sinks.
Sink organs: Non-photosynthetic organs that import sugars for growth and development. Examples include:
Young leaves, Seeds, Fruits, Flowers, Roots, Tubers, Meristems

Question 3

What substances are transported through the phloem?

Answer 3

Carbohydrates (mainly sucrose)
Hormones
Amino acids
Some inorganic ions
RNAs and proteins
Secondary metabolites

Question 4

What are the two main types of phloem cells and their functions?

Answer 4

1️⃣ Sieve elements (SE):

Conduct sugars over long distances.
Living but lack most organelles.
Non-metabolically active.
2️⃣ Companion cells (CC):

Load sugars into sieve elements from parenchyma cells.
Perform metabolic functions for sieve elements.
Require ATP for active transport.

Question 5

How do sieve elements form sieve tubes?

Answer 5

Sieve elements (SE) are connected end-to-end to form sieve tubes.
Sieve plates (pores) in the walls between cells allow sap to flow.
Pore diameter: 1-5μm.
Transport is facilitated by these continuous tube-like structures.

Question 6

How is sucrose loaded into the phloem via the apoplastic route?

Answer 6

1️⃣ Sucrose leaves mesophyll cells and enters the apoplast (space between cell walls).
2️⃣ Sucrose-H+ symporter on the companion cell takes up sucrose using energy (ATP).
3️⃣ Sucrose enters the sieve elements for transport.

No plasmodesmata are involved in this route.

Question 7

How is sucrose loaded into the phloem via the symplastic route?

Answer 7

1️⃣ Sucrose moves through plasmodesmata (cytoplasmic connections) from mesophyll cells to companion cells and sieve elements.
2️⃣ Sucrose may be converted to oligosaccharides (larger sugars) in the companion cells.
3️⃣ These larger sugars are “trapped” in the phloem since they can’t diffuse back to mesophyll cells.

Question 8

How does the pressure flow model explain sugar movement in the phloem?

Answer 8

1️⃣ At the source:
Sucrose is loaded into sieve elements, increasing solute concentration and lowering water potential (Ψ).
Water enters from the xylem, increasing pressure (Ψp) at the source.
2️⃣ At the sink:
Sucrose is unloaded, increasing water potential (Ψ) in sieve elements.
Water returns to the xylem, lowering pressure (Ψp) at the sink.
3️⃣ Result: High pressure at source, low pressure at sink drives the flow of phloem sap.

Question 9

How is sucrose unloaded from the phloem in sink organs?

Answer 9

1️⃣ Symplastic route: Sucrose moves through plasmodesmata directly into sink cells.
2️⃣ Apoplastic route:
Sucrose exits sieve elements into the apoplast via efflux proteins.
It is taken up by sink cells via sucrose transporters.
3️⃣ Enzymatic cleavage:
Sucrose is cleaved by acid invertase into glucose and fructose.
Monosaccharides are then taken up into sink cells via monosaccharide transporters.

Question 10

How are sieve elements and companion cells connected, and why is this important?

Answer 10

Plasmodesmata: Cytoplasmic connections allow the exchange of solutes between sieve elements and companion cells.
Companion cells provide metabolic support to sieve elements (which lack organelles).
Solutes can move symplastically from companion cells to sieve element