BIOL220 Transport: Phloem

What happens when a plant is cut?

P-Proteins and a carbohydrate Callose come into play. Whehn pressure drops, they precipitate (come out of solution) and clog up the sieve plate, which limits loss of phloem sap. This makes it so only one sieve tube element of the sieve tube is effected, and the cut will be contained.

Facilitated Diffusion (definition, other)

*Definition:* Passive movement (diffusion) of a substance across a membrane with the assistance of transmembrane carrier proteins or channel proteins. *Other:* - Ions and many large molecules diffuse across phospholipid bilayers slowly if at all, even when their movement is favored by a strong electrochemical gradient. To diffuse rapidly, they must avoid direct contact with the phospholipid bilayer by passing through a membrane protein. - Channels and carriers are responsible for facilitated diffusion.

Phloem Sap (loose definition, other)

*Definition:* Sap that consists primarily of sugars, hormones, and mineral elements dissolved in water. It flows from where carbohydrates are produced or stored (sugar source) to where they are used (sugar sinks). *Other:* - Phloem sap can contain small amounts of minerals, amino acids, mRNAs, hormones, and other compounds as well. - The water in phloem sap moves down this pressure gradient, and sugar molecules are carried along by bulk flow. - The phloem sap has a very low solute potential.

Tonoplast (definition, other)

*Definition:* The membrane surrounding a plant vacuole. *Other:* - It contains two types of protein pumps that work together to accumulate sucrose in the vacuole, much like the phloem loading process. - As protons become concentrated in the tonoplast, they have a tendency to diffuse back out of the vacuole. A proton-sucrose cotransporter then uses the proton gradient to move sucrose from the cytosol into the vacuole against its concentration gradient. An important distinction in this case is that the cotransporter is an antiporter, while in source tissues it is a symporter.

Phloem Loading (steps)

(1) Sucrose is transported into the apoplast via a Sucrose Transporter using Fascillitated Diffusion (Passive Transport), sucrose moves down it's concentration gradient. (2) H+ ATPase pumps protons into the Apoplast creating a electrochemical gradient. A Symporter transports sucrose into the Companion Cell against it's concentration gradient while simultaneously transporting a proton with it's concentration into the Companion Cell. (3) Sucrose moves into the Sieve Tube Element via Plasmodesmata, increasing the solute concentration in the Sieve Tube Element thus decreasing the Solute Potential. As solute potential decreases, water potential decreases. Water from the Xylem flows into the sieve-tube element via osmosis (moving from an area of high water potential to low water potential), which increases pressure potential. (4) Sucrose and water move via bulk flow to an area of lower pressure potential.

Companion Cell (definition, other)

*Definition:* In plants, a cell in the phloem that is connected via many plasmodesmata to adjacent sieve- tube elements. Companion cells provide materials to maintain sieve-tube elements and function in the load- ing and unloading of sugars into sieve-tube elements. *Other:* - Phloem consists largely of two cell types, sieve-tube elements and companion cells. - Sieve-tube elements and companion cells are alive at maturity and lack secondary cell walls. - Companion cells, in contrast, have nuclei and a rich assortment of ribosomes, mitochondria, and other organelles. Companion cells are located adjacent to sieve-tube elements and function as their "support staff."

Sieve Plate (definition, other)

*Definition:* In plants, a pore-containing structure at each end of a sieve-tube element in phloem. *Other:* - In most plants, sieve-tube elements lack nuclei and most other organelles. They are connected to one another, end to end, by perforated sieve plates. - The pores, which are simply enlarged plasmodesmata, create a direct connection between the cytoplasms of adjacent cells.

Sieve-Tube Element (definition, other)

*Definition:* In plants, an elongated sugar- conducting cell in phloem that lacks nuclei and has sieve plates at both ends, allowing sap to flow to adjacent cells. *Other:* - Phloem consists largely of two cell types, sieve-tube elements and companion cells. - Sieve-tube elements and companion cells are alive at maturity and lack secondary cell walls. - In most plants, sieve-tube elements lack nuclei and most other organelles. They are connected to one another, end to end, by perforated sieve plates. - The sieve-tube elements in phloem represent a continuous system for transporting sugar throughout the plant body.

Translocation (definition, other)

*Definition:* In plants, the movement of sugars and other organic nutrients through the phloem by bulk flow. *Other:* - Any tissue, site, or location where an element or a molecule is consumed or taken out of circulation (e.g., in plants, a tissue where sugar exits the phloem). - Sugars can be translocated rapidly—typically 50-100 centimeters per hour (cm/hr). - There is a strong correspondence between the physical locations of sources and sinks. - Active transport, secondary active transport, and passive transport are all involved in translocating sugars in plants.

Pressure-Flow Hypothesis (definition)

*Definition:* The hypothesis that sugar movement through phloem tissue is due to differences in the turgor pressure of phloem sap. *Other:* - Pressure-flow hypothesis, states that events at source tissues and sink tissues create a pressure potential gradient in phloem. - The water in phloem sap moves down this pressure gradient, and sugar molecules are carried along by bulk flow.

Active Transport (definition, other)

*Definition:* The movement of ions or molecules across a membrane in a single direction, often against a gradient. Requires energy (e.g., from hydrolysis of ATP) and assistance of a transport protein (e.g., pump). *Other:* - Occurs when ions or molecules move across a plasma membrane against their electro-chemical gradient. - Cells must expend energy in the form of ATP to move solutes in an energetically unfavorable direction.

In a plant found in bright sunlight with plenty of water available, which cell types (companion cell, sieve tube element, vessel element) have the lowest/highest solute potential and the lowest/highest pressure potential? indicate whether it is near the source or sink. Then explain.

Companion cell or sieve tube element near source has the lowest (most negative) solute potential. Vessel element near source or sink has the highest solute potential. Sieve tube element near source has the highest pressure potential. Vessel element near source (if leaf is source) has the lowest pressure potential. Explination: At the source, sucrose is actively transported into companion cells and then diffuses into sieve tube elements at the source. This high concentration of sucrose leads to a very low solute potential.

Are companion cells smaller or bigger than sieve tubes? Do companion cells have organelles?

Companion cells are smaller, and they have organelles which supply the sieve tube with solutes, proteins, and other nutrients. Sieve Tube Elements don't have organelles.

Tracheid (xylem) (definition)

In vascular plants, a long, thin, water-conducting cell that has pits where its lignin-containing secondary cell wall is absent, allowing water move- ment between adjacent cells.

Vessel Element (xylem) (definition)

In vascular plants, a short, wide, water-conducting cell that has gaps through both the primary and secondary cell walls, allowing unimpeded passage of water between adjacent cells.

Does phloem contain a secondary cell wall? Does it have lignin?

No, and no.

Identify the Source and Sink: In the late summer, a carrot plant grows a large taproot.

Source- carrot leaves Sink- taproot

Identify the Source and Sink: In the early autumn, a deciduous tree's leaves begin to turn yellow.

Source- leaves Sink- roots

Identify the Source and Sink: In the early spring, a deciduous tree makes new leaves on all of its branches.

Source- roots Sink- new leaves.

Identify the Source and Sink: The following summer a carrot plant grows flowering stalks.

Source- taproot Sink- flowering stalk

Where is the lowest and highest pressure potential during translocation?

The lowest pressure potential is at the sieve tube and mesophyll of the sink, and the highest is at the sieve tube and mesophyll of the source.

Are sugars transported through the symplast or apoplast?

The symplast.

T/F: Within one leaf some cells can be sinks and others source cells.

True. Within one leaf, some cells can be sinks and others can be sources. A leaf can possess both sink and source cells.

Source and Sink Occurrence by Time of Year (other)

• *During the growing season:* Mature leaves and stems that are actively photosynthesizing produce sugar in excess of their own needs. These tissues act as sources. Sugar moves from leaves and stems to a variety of sinks, where sugar use is high and production is low. Apical meristems, lateral meristems, developing leaves, flowers, developing seeds and fruits, and storage cells in roots all act as sinks. • *Early in the growing season:* When a plant resumes growth after the winter or the dry season, sugars move from storage areas to growing areas. Storage cells in roots and stems act as sources; developing leaves act as sinks.

Sucrose (definition, other)

*Definition:* A disaccharide formed from glucose and fructose. One of the two main products of photosynthesis. *Other:* The phloem sap that flows through vascular tissue is often dominated by the disaccharide sucrose

Phloem Unloading (steps)

*B. Not during growing season: sugars stored in the roots* (1) As sucrose moves into Companion cells via Plasmodesmata and then into the sink, water moves into the xylem via osmosis. (2) As sucrose moves into Companion Cells, the solute concentration decreases, thus solute potential increases. As solute potential increases, water potential increases — this explains why water moves into the xylem via osmosis. As water leaves the sieve tube element, pressure potential decreases. (3) H+-ATPase pumps protons into the vacuole of the sink, creating an electrochemical gradient. An Antiporter then transports sucrose against it's gradient into the vacuole while simultaneously transporting a proton out of the vacuole with it's gradient. Sucrose is thus stored away for the next growing season. *A. During growing season: sugars used for growth * (1) As sucrose moves into Companion cells via Plasmodesmata and then into the sink, water moves into the xylem via osmosis. (2) As sucrose moves into Companion Cells, the solute concentration decreases, thus solute potential increases. As solute potential increases, water potential increases — this explains why water moves into the xylem via osmosis. As water leaves the sieve tube element, pressure potential decreases. (3) Sucrose moves via passive transport into the sink to be used for growth.

Antiporter (definition, other)

*Definition:* A carrier protein that allows an ion to diffuse down an electrochemical gradient, using the energy of that process to transport a different substance in the opposite direction against its concentration gradient. *Other:* Antiporters work in a similar way, except that the solute being transported against its concentration gradient moves in the direction opposite that of the solute moving down its concentration gradient.

Symporter (definition, other)

*Definition:* A cotransport protein that allows an ion to diffuse down an electrochemical gradient, using the energy of that process to transport a different substance in the same direction against its concentration gradient. *Other:* Symporters transport solutes against a concentration gradient, using the energy released when a different solute moves in the same direction along its electrochemical gradient.

Proton Pump (H+-ATPase) (definition, other)

*Definition:* A membrane protein that can hydrolyze ATP to power active transport of protons (H+ ions) across a membrane against an electrochemical gradient. Also called H+-ATPase. *Other:* Proteins like these are called proton pumps, or more formally, H+-ATPases. Proton pumps establish a large difference in charge and in hydrogen ion concentration on the two sides of the membrane. The resulting electrochemical gradient favors the entry of protons into the cell.

Cotransporter (definition, other)

*Definition:* A transmembrane protein that facili- tates diffusion of an ion down its previously established electrochemical gradient and uses the energy of that process to transport some other substance, in the same or opposite direction, against its concentration gradient. *Other:* The electrochemical gradients established by pumps are used to transport other molecules or ions by two types of membrane proteins called cotransporters.

Carrier Protein (definition, other)

*Definition:* A transmembrane protein that facilitates diffusion of a small molecule (e.g., glucose) across a membrane by a process involving a reversible change in the shape of the protein. Also called carrier or transporter. *Other:* Carrier proteins undergo a conformational change that transports specific molecules across the lipid bilayer.

Channel Protein (definition, other)

*Definition:* A transmembrane protein that forms a pore in a cell membrane, which may open or close in response to a signal. The structure of most channels allows them to admit just one or a few types of ions or molecules. *Other:* Channel proteins form pores that selectively admit certain ions.

Pump (definition, other)

*Definition:* Any membrane protein that uses energy (e.g., ATP) to change shape and power the active transport of a specific ion or molecule across a membrane in a single direction, often against its gradient. *Other:* - Active transport always involves membrane proteins. Pumps are proteins that change shape when they bind ATP or a phosphate group from ATP. As they move, pumps transport ions or molecules against an electrochemical gradient. - Pumps establish an electrochemical gradient that favors the movement of an ion or molecule back across the plasma membrane. - The electrochemical gradients established by pumps are used to transport other molecules or ions by two types of membrane proteins called cotransporters.

Source (definition, other)

*Definition:* Any tissue, site, or location where a substance is produced or enters circulation (e.g., in plants, the tissue where sugar enters the phloem). *Other:* - Any tissue, site, or location where an element or a molecule is consumed or taken out of circulation (e.g., in plants, a tissue where sugar exits the phloem). - Mature leaves that act as sources send sugar to tissues on the same side of the plant. - Experiments with tall plants also show that leaves on the upper part of the stem send sugar to apical meristems, but leaves on the lower part of the plant send sugar to the roots.

Sink (definition, other)

*Definition:* Any tissue, site, or location where an element or a molecule is consumed or taken out of circulation (e.g., in plants, a tissue where sugar exits the phloem). *Other:* Any tissue, site, or location where an element or a molecule is consumed or taken out of circulation (e.g., in plants, a tissue where sugar exits the phloem).

Passive Transport (definition, other)

*Definition:* Diffusion of a substance across a membrane. When this event occurs with the assistance of membrane proteins, it is called facilitated diffusion. *Other:* - Passive transport occurs when ions or mol- ecules move across a plasma membrane by diffusion, along their electrochemical gradient. - There is no expenditure of energy required for the movement to occur. - Small, uncharged molecules diffuse across phos- pholipid bilayers rapidly. - Ions and many large molecules diffuse across phospholipid bilayers slowly if at all, even when their movement is favored by a strong electrochemical gradient. To diffuse rapidly, they must avoid direct contact with the phospholipid bilayer by passing through a membrane protein.

Vascular Bundle (definition, other)

*Definition:* In a plant stem, a cluster of xylem and phloem strands that run the length of the stem. *Other:* - Vascular bundles, which contain xylem and phloem, run the length of plants, and certain bundles extend into specific branches, leaves, and lateral roots. Phloem sap does not move from one vascular bundle to another—instead, each bundle is independent. - The phloem in the leaves on one side of a plant connects directly with the phloem of branches, stems, and roots on the same side, through a specific set of vascular bundles.

Phloem Loading (loose definition, other)

*Definition:* Transfer of sugars (photosynthetic) from mesophyll cells to sieve tube elements in the leaf. *Other:* - Pressure flow often requires that plants expend energy to set up a water-potential gradient in phloem. To establish a high pressure potential in sieve-tube elements near source cells, large amounts of sugar have to be transported into the phloem sap— enough to raise the solute concentration of sieve-tube elements. - In some cases, loading is active—it requires an expenditure of ATP and some sort of membrane transport system. But when sucrose concentrations in source cells are extremely high, movement of sucrose into sieve-tube elements can also occur via passive diffusion through plasmodesmata. - Phloem unloading at sinks can also be active or passive, depending on the tissue. When sugar is unloaded against its concentration gradient, an expenditure of ATP and a second membrane transport mechanism are required.

Secondary Active Transport (definition)

*Definition:* Transport of an ion or molecule in a defined direction (often against its gradient) made possible by the transport of another ion or molecule being moved along its gradient. Also called cotransport. *Other:* When solutes move through cotransport proteins, secondary active transport occurs.

Phloem Unloading (loose definition, other, example)

*Definition:* the transfer of sugars (photosynthetic) from sieve tube elements to the receiver cells of consumption end (i.e., sink organs) is called as phloem unloading. *Other:* The membrane proteins that are involved in transporting sucrose molecules out of the phloem, as well as the mechanism of movement, vary among different types of sinks within the same plant. Mechanisms for phloem unloading also vary among different species. *Example:* Sugar beets. Two major sinks are found in sugar beets: young, actively growing leaves, and the enlarged roots. In young leaves of sugar beets, sucrose is unloaded along a concentration gradient by simple diffusion. This passive transport occurs because sucrose is rapidly used up inside the cells of young leaves to provide energy for ATP synthesis as well as carbon for the synthesis of cellulose, proteins, nucleic acids, and phospholipids needed by growing cells. In the roots of the same plant, however, an entirely different mechanism is responsible for unloading sucrose. Root cells in this species have a large vacuole that stores sucrose.

Phloem Anatomy

- Phloem consists largely of two cell types, sieve-tube elements and companion cells. - Sieve-tube elements and companion cells are alive at maturity and lack secondary cell walls. - In most plants, sieve-tube elements lack nuclei and most other organelles. They are connected to one another, end to end, by perforated sieve plates - Companion cells have nuclei and a rich assortment of ribosomes, mitochondria, and other organelles. Companion cells are located adjacent to sieve-tube elements and function as their "support staff." - Phloem in primary and secondary vascular tissue is continuous throughout the plant—meaning that there is a direct anatomical connection between the phloem from the tips of shoots to the tips of roots. - The sieve-tube elements in phloem represent a continuous system for transporting sugar throughout the plant body. - Vascular bundles, which contain xylem and phloem, run the length of plants, and certain bundles extend into specific branches, leaves, and lateral roots. Phloem sap does not move from one vascular bundle to another—instead, each bundle is independent.

Pressure Flow Hypothesis: High Turgor Pressure near Sources causes Phloem Sap to Flow to Sinks

- sucrose moves from source cells into companion cells and from there into sieve-tube elements. Because of this phloem loading, the phloem sap in the source tissue has a high concentration of sucrose. - water moves along a water-potential gradient—flowing passively from xylem across the selectively permeable plasma membrane of sieve-tube elements. In response, turgor pressure begins to build in the sieve-tube elements in the source region. - Cells in the sink remove sucrose from the phloem sap by passive or active transport. As a result of this phloem unloading—a loss of solutes—the water potential in sieve-tube elements increases until it is higher than the water potential in adjacent xylem cells. - Water flows across the selectively permeable membranes of sieve-tube elements into xylem along a water-potential gradient. In response, turgor pressure in the sieve-tube elements in the sink drops. - The net result of these events is high turgor pressure in phloem at a source tissue and low turgor pressure at the sink, created by the loading and unloading of sugars, respectively. This difference in pressure drives phloem sap from source to sink via bulk flow. There is a one-way flow of sucrose and a continu- ous loop of water movement. Water returns to the source tissue via the xylem.

What are the to types of membrane proteins that are involved in facilitated diffusion?

1. Channel Proteins 2. Carrier Proteins

Model for Phloem Loading

1. Proton pumps in the membranes of companion cells create a strong electrochemical gradient that favors a flow of protons into companion cells. 2. A symporter in the membranes of companion cells uses the proton gradient to bring sucrose into companion cells from the source cells. 3. Once inside companion cells, sucrose moves into sieve-tube elements via plasmodesmata.

What are the two types of Cotransporters?

1. Symporters 2. Antiporters

Explain how biochemical energy is used to power the movement of sugar from source to sink.

A proton pump requires ATP to establish a proton gradient across the membrane of companion cells. Protons are pumped out of the cell and the energy in this electrochemical gradient is used to cotransport sucrose and H+ into the companion cell. The sucrose can then move through plasmodesmata into the sieve tube members.