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What is a source in a vascular plant?

A source is a tissue where sugar enters the phloem.

What does it mean for a cell to be flaccid?

A flaccid cell has no turgor pressure and thus a pressure potential of 0.

What is a sink in a vascular plant?

A sink is a tissue where sugar exits the phloem.

As water is absorbed, it moves through the root cortex toward the xylem through three distinct routes. Name and describe each route.

1. The transmembrane route - through aquaporins/across plasma membranes. 2. The apoplastic route - within porous cell walls (must go to cytoplasm before Casparian strip). 3. The symplastic route - via plasmodesmata.

Describe the process of water movement by cohesion-tension.

1. Water vapor diffuses out of the leaf (transpiration). 2. Water inside the leaf evaporates. 3. Water is pulled up from the xylem to fill the space. 4. Water is pulled up through the xylem to follow the water in step 3. 5. Water is pulled out of the root cortex. 6. Water moves from the soil into the root. (The process is like pulling a long, connected string out of the top of the plant - the rest will follow.)

What does it mean for a cell to be turgid?

A cell that is firm and that experiences wall pressure is said to be turgid. The rigid cell wall limits the amount of water that can enter the cell.

What are antiporters?

Antiporters transport solute against a concentration gradient in the direction opposite that of the solute moving down its concentration gradient.(See page 769.)

What is water potential?

Biologists use the term water potential to indicate the potential energy that water has in a particular environment compared with the potential energy of pure water at room temperature and atmospheric pressure. It is symbolized by the Greek letter psi (Ψ). Water potential can be thought of as water's stored energy or its tendency to move to a new position.

What is bulk flow in reference to water movement in a plant?

Bulk flow is a mass movement of molecules along a pressure gradient. In this case, bulk flow is referring to the was water in a column of xylem cells moves.

What is determined by differences in water potential?

Differences in water potential determine the direction that water moves. Water always flows from areas of higher water potential to areas of lower water potential.

What is capillary action?

Capillary action is the movement of water up a narrow tube due to surface tension, adhesion, and cohesion. This method of water movement can only transport water a limited distance.

How can vascular tissues withstand negative pressures as low as -2.0 MPa without collapsing?

Cell walls in tracheids and vessel elements are reinforced with tough lignin molecules that allows them to withstand extremely negative pressure.

How does the pressure-flow hypothesis differ from the cohesion-tension model?

Cohesion-tension relies on transpiration. The pressure-flow hypothesis relies on differences between turgor pressure in the phloem near source tissues and turgor pressure in the phloem near sink tissues to generate the necessary force. This requires and expenditure of ATP. (See diagram on page 768.)

What are companion cells?

Companion cells have nuclei and a rich assortment of ribosomes. mitochondria, and other organelles. These cells act as a "support staff" for sieve-tube elements.

True or False - Fully expanded leaves act as sinks while actively growing leaves and roots act as sources of sugar.

False - fully expanded leaves act as sources of sugar while actively growing leaves and roots act as sinks.

True or False - Pure water will never tend to move in to the cell.

False - it was always tend to move in to the cell.

What is guttation?

Guttation is the process through which water moves through the xylem at night due to root pressure and forces fluid up a plant and in to the leaves where droplets form from stomata because the water is not transpiring as quickly as it's leaking from the leaves. This only occurs in lower leaves because root pressure does not move water very far.

What is the water potential like in moist soil?

In moist soil, the water that fills crevices between soil particles usually contains relatively few solutes and normally is under little pressure. Since water potential is solute potential ( a negative) plus pressure potential, the water potential of moist soil tends to be high relative to the water potential found in the plant's roots, which is higher in solutes (meaning a more negative potential).

Describe the water-potential gradient that causes water to move up through a plant.

In most cases, water potential is highest in soil, lower in roots, lower yet in leaves, and lowest in the atmosphere.

What causes wilting in plants?

Loss of turgor pressure causes plants to wilt because turgor pressure provides structural support.

What unit is used to measure water potential?

Megapascals (MPa) are used to measure water potential.

What is phloem sap composed of?

Phloem sap that flows through vascular tissue is often dominated by sucrose (table sugar). Phloem sap may also contain small amounts of minerals, amino acids, mRNAs, hormones, and other compounds.

How is water movement by cohesion-tension possible?

The cohesion-tension theory is possible for two reasons: 1. there are continuous columns of water throughout the plant, and 2. all of the water molecules present hydrogen bonds to one another (cohesion). This water transport is solar powered, because energy from the sun evaporates the water on the leaves.

What is pressure potential?

Pressure potential (Ψp) refers to any kind of physical pressure on water.

As a general rule, soil usually has a high water potential. What are some exceptions to this rule?

Salty soils and dry soils are exceptions to this rule. Salty soils have a lower (more negative) solute potential, causing the water potential to be low. Dry soils lose the pressure potential and water sticks to soil particles tightly, meaning water can no longer flow in to the plant and may even be sucked out of the plant.

What is the leading hypothesis to explain long-distance water movement in vascular plants?

The cohesion-tension theory is the leading hypothesis to explain long-distance water movement in vascular plants.

What is the cohesion-tension theory?

The cohesion-tension theory states that water is pulled to the tops of trees along a water-potential gradient, vie forces generated by transpiration at leaf surfaces.

What is the cortex of a plant root?

The cortex is a layer within the epidermis and outside of the endodermis that stores carbohydrates.

What cell types make up phloem?

Sieve-tube elements and companion cells make up phloem.

What are sieve tube elements?

Sieve-tube elements are cells that lack nuclei and most other organelles. They are connected to one another, end to end, by perforated sieve plates. (See diagram on page 767.) The pores between sieve-tube elements create a direct connection between the cytoplasm of adjacent cells in which phloem sap may pass through.

What is solute potential?

Solute potential (Ψs) is the tendency for water to move in response to differences in solute concentrations. The solute potential of a solution is defined by its total solute concentration relative to pure water. If water contains a high concentration of solutes, then it has low solute potential compared with pure water. Water moves down its potential gradient toward the region of higher solute concentration. (Moves from areas of high potential to low potential.)

What are symporters?

Symporters transport solutes against a concentration gradient, using energy released when a different solute moves in the same direction along its electrochemical gradient.(See page 769.)

What is the Casparian strip?

The Casparian strip is a narrow band of wax secreted by endodermal cells that is primarily composed of a compound called suberin, which forms a water-repellent cylinder at the endodermis.

Why is the Caspian strip important? What does this mean for endodermal cells?

The Casparian strip is important because it means that for water and solutes to reach vascular tissue, they have to move in to the cytoplasm of an endodermal cell. This means that endodermal cells act as filters. Also, without the Casparian strip to act as a barrier to create root pressure, water would simply leak out.

What is the pressure-flow hypothesis?

The pressure-flow hypothesis states that events at source tissues and sink tissues create a pressure potential gradient in phloem.

What is the solute potential of pure water?

The solute potential of water is 0 MPa.

What is translocation?

Translocation is the movement of sugars by bulk flow throughout a plant - specifically from sources to sinks.

True or False - Water moves from roots to leaves passively and with no expenditure of ATP.

True

What is turgor pressure?

Turgor pressure is the pressure inside of a cell.

What happens during the process of transpiration in trees?

Water evaporates from the aerial parts of a plant because stomata are open (these open during the day to support gas interactions) and the air surrounding the leaves is drier than the air inside the leaves.

How can the system involving a cell's water potential reach equilibrium?

When a positive turgor pressure plus a cell's negative solute potential equals 0 MPa - the water potential of pure water - the system is at equilibrium. (See diagram on page 757.)

What happens to water potential when selectively permeable membranes are present? No membranes?

When selectively permeable membranes are present, water tends to move by osmosis from areas of high solute potential to areas of low solute potential. When no membranes are present to stop it, water moves from areas of high pressure potential to areas of low pressure potential.

Over time, irrigated soils tend to become salty. Why?

When soils are irrigated, much of the water evaporates, leaving behind solutes that collect in the first few inches below the surface. This makes the soil salty.