Phloem Loading
Compare and contrast symplastic and apoplastic loading.
They are both methods of phloem loading, and both involve a source cell and the sieve tube. However, apoplastic loading is active transport and is done through the cell wall, whereas symplastic loading is passive and requires plasmodesmata
What is the tissue that carries food from a plant’s leaves to its cells?
Phloem
Differentiate between a source and sink.
Source is where organic compounds are synthesized; sink is where the compounds are unloaded.
A xylem has _____ transport while a phloem has _____ transport.
Unidirectional; bidirectional
The unique protruding mouthpiece of an aphid is called a _______ .
stylet
Outline apoplastic loading including all the plant tissue involved.
1. H+ actively transported out of phloem.
2. Proton gradient created.
3. Protons are transported back to the companion cell via cotransport with sucrose (passive transport).
4. results in a buildup of sucrose within the phloem sieve tube for transport from the source to the sink.
Which cells are alive but lack organelles?
Sieve tube cells
What does the Mass Flow theory explain?
Translocation
Materials move around in the phloem via this process.
Active translocation
When measuring rates of phloem transport, it is common practice to use a special CO2 with this quality.
radioactive
What direction are H+ ions actively transported?
From the phloem to the companion cell.
Which organelle does a sieve tube element cell lack.
Nucleus
Define hydrostatic pressure.
Pressure exerted by water against the phloem cell walls.
Define stele.
Central part of the root or stem containing the tissues derived from the procambium.
True or false? The diameter of a sieve tube can affect the rate of phloem transport.
True
What is the role of water in phloem loading/transport?
Water pressure gradient created by increased sucrose concentration in the sieve tube moves the sugars from the source cells into the sink.
Name two things sap is made up of?
Amino acids, sugars, hormones
What is another name for the Mass Flow theory? Why is this name also applicable?
Pressure-Flow Mechanism; the sap flows down hydrostatic pressure gradients.
Distinguish between the shapes of the vessels in monocotyledon and dicotyledon roots.
Monocotyledon: vessels in a radiating circle
Dicotyledon: xylem vessels forming an X shape
The rate of phloem transport is principally determined by the concentration of this dissolved substance.
sugars, sucrose, carbohydrates, etc.
What process is responsible for maintaining the sucrose concentration gradient?
The conversion of sucrose to a larger oligosaccharide.
Why are there so many mitochondria in a companion cell?
To supply energy for active transport of sucrose.
Give three examples of sinks in a plant.
Roots, leaves, seeds, or fruits.
Distinguish between the shapes of the vessels in monocotyledon and dicotyledon stems
Monocotyledon: vascular bundles are scattered
Dicotyledon: vascular bundles arranged in circle
If a stylet is severed during a stylectomy, sap will continue flowing out of the phloem because of this type of pressure.
hydrostatic