Water and Sugar Transport in Plants

Suppose a cell had a water potential of -2 MPa, which was due to -4 MPa of solute potential and ______ MPa of pressure potential. If the cell wall were to be torn open, then the water potential of the cell would change to ____________ MPa, because the ___________ potential would change to ___________ MPa.

+2; zero; pressure; zero

Two aphids insert their stiff, sharp stylets into phloem tubes so they can suck out the sap. They then use the components of the sap for their nutrition. One of the aphids (A) inserts its stylet into a phloem tube in a large, mature leaf; the other (B) inserts the stylet into a phloem tube of the young, growing tissue near the shoot's growing tip (apical meristem). Which aphid is getting a higher concentration of sucrose from the phloem? (

Aphid A

Xylem: a. Carries sugars b. Carries water and ions c. Is only found in bryophytes d. Is only found in gymnosperms e. Both a and b

Carries water and ions

Which of the following allows water to pull up on other water molecules through the xylem tissue of plants? a. Surface tension b. Cohesion c. Adhesion d. Root pressure e. Gravity

Cohesion

Identify and describe the basic components of the water potential of a plant cell including: the influence of solutes and the influence of the cell wall

Differences in water potential determine the direction that water moves. Water always flows from areas of higher water potential to area of lower water potential. 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 a low solute potential compared with pure water. Water moves down its potential gradient toward the region of higher solute concentration.

Describe an example of when a sink cell might convert to a source cell

During the growing season, roots also act as sinks

Which of the following best describes the movement of sucrose from the cytoplasm of a photosynthetic cell, into the extracellular space? a. Simple (direct) diffusion through the cell's plasma membrane b. Active transport from low concentration (intracellular) to high concentration (extracellular) c. Facilitated diffusion via a sucrose transport protein d. Secondary active transport via a proton-sucrose symporter

Facilitated diffusion via a sucrose transport protein

Which statement best describes the movement of sucrose through the phloem?

From mature leaves to roots and seeds

Describe the process of transpiration in the leaves of plants. How does the activity of photosynthetic cells influence the solute potentials and rates of transpiration in the leaves?

Loss of water via evaporation from the aerial parts of a plant is called transpiration. Transpiration occurs whenever two conditions are met: (1) Stomata are open, and (2) the air surrounding leaves is drier than the air inside the leaves. The first condition is usually met during the day, when photosynthesis takes place. The second condition occurs whenever atmospheric humidity is less than 100 percent

Which of the following might increase the water potentials in the cells of a root, making them more positive. a. Photosynthesis b. Transpiration c. Lots of rain making the soil very moist d. Lots of salt (i.e. like what occurred following Hurricane Katrina)

Lots of rain making the soil very moist

Which of the following steps of phloem loading requires ATP? a. Movement of sucrose from sieve tube elements into companion cells b. Movement of sucrose from companion cells into sieve tube elements c. Movement of hydrogen from intracellular to extracellular spaces d. Movement of sucrose from extracellular to intracellular spaces e. Movement of sucrose from a cell's cytoplasm into its central vacuole

Movement of hydrogen from intracellular to extracellular spaces

Describe root pressure

Movement of ions and water into the root xylem is responsible for the process known as root pressure.

Which of the following steps of phloem loading occurs passively, through plasmodesmata? a. Movement of sucrose from sieve tube elements into companion cells b. Movement of sucrose from companion cells into sieve tube elements c. Movement of hydrogen from intracellular to extracellular spaces d. Movement of sucrose from extracellular spaces into companion cells e. Movement of sucrose from the a cell's cytoplasm into its central vacuole

Movement of sucrose from companion cells into sieve tube elements

Describe capillary action

Movement of water up a narrow tub. The movement occurs in response to three factors: (1) adhesion, (2) cohesion, and (3) surface tension.

Describe the difference between active and passive transport

Passive transport occurs when ions or molecules move across a plasma membrane by diffusion -- that is, along their electrochemical gradient. Active transport occurs when ions or molecules move across a plasma membrane against their electrochemical gradient.

Describe the composition of: xylem sap and phloem sap

Phloem sap is often dominated by sucrose and can contain small amounts of minerals, amino acids, mRNAs, hormones and other compounds as well.

Describe the general flow of fluids as they move through: xylem and phloem

Plants use xylem to pull fluids from roots to shoots, and phloem to push fluids from sources (sites of loading) to sinks (sites of unloading).

What creates the "flow" of fluid through phloem tissue?

Pressure differences between sources and sinks

Which cell type transports the bulk of the photosynthetic products long distances?

Sieve tube members/elements

Describe the pressure-flow hypothesis

States that events at source tissues and sink tissues create a pressure potential gradient in phloem. Turgor pressure in the phloem near sucrose tissues and turgor pressure in the phloem near sink tissues generate the necessary force to move phloem sap

Describe the four steps of phloem loading

Step 1: Passive transport moves sucrose from inside photosynthetic cell to extracellular space. Step 2: Proton pumps drive loading of extracellular sucrose into the companion cells. Step 3: Sucrose diffuses down its concentration gradient into sieve tube members. Step 4: Water flows into sieve tube members via osmosis, drives turgor pressure higher (+).

Identify and describe each of the following, with regards to its role in fluid transport: plasmodesmata, endodermal cells and the Caspian strip, cell walls and plasma membranes.

The Caspian strip blocks the apoplectic route by preventing water from moving through the walls on endodermal cells and proceeding to the vascular tissue. The Caspian strip is important because it means that for water and solute to reach vascular tissue, they have to move into the cytoplasm of an endodermal cell. Endodermal cells, in turn, act as filters.

As a plant cell swells with water and becomes turgid (stiff with water), the number of water molecules successfully entering the cell will decrease. Why?

The cell wall resists (pushes back against) the increasing number of water molecules in the cell.

Identify and describe the anatomy of the three major tissue types of the plant

The epidermis is a single layer of cells. The cortex consists of ground tissue and store carbohydrates. The endodermis is a cylindrical layer of cells that forms a boundary between the cortex and the vascular tissue.

Describe cohesion-tension theory

The leading theory to explain how water moves through xylem is the cohesion-tension theory. In science, we say something is a "theory" when a lot of scientific evidence in favor of it and generally accepted as correct by the scientific community. The theory states that water is pulled up into the stem and leaves along its water potential gradient. This movement is driven by photosynthesis and transpiration in the leaves

Pressure-flow theory describes the movement of fluid:

Through sieve tube members

Define and describe the function of: translocation, a source cell, and a sink cell

Translocation is the movement of sugars by bulk flow throughout a plant--specifically, from sources to sinks. In vascular plants, a source is a tissue where sugar enters the phloem; a sink is a tissue where sugar exits the phloem.

________________ tissue is responsible for transporting _____________ and _____________ absorbed by the root system, up to into the stem system. To gain access to this tissue, absorbed fluids must first move through the ________________ and ____________tissues (non-vascular) of the root system.

Vascular; water and minerals (or ions); epidermal and ground

Describe the two types of phloem unloading

Version 1: Phloem unloading into growing leaves of sugar beets; passive transport across membrane then use ATP within cell indirectly. Version 2: Phloem unloading into roots of sugar beets; active transport across tonoplast, requiring direct use of ATP

Describe the differences between the movement of water into and out of plant cells vs. animal cells.

When an animal cell is placed in a hypotonic solution and water enters the cell via osmosis, the volume of the cell increases until the cell hurts. This does not happen to plant cells, however. If a plant cell swells in response to incoming water, its plasma membrane pushes against the relatively rigid cell wall. The cell wall resists expansion of the cell volume by pushing back.

This figure shows unloading of phloem. Why is phloem unloading passive in the young leaf cell, but active in the roots cells of this sugar beet plant?

Young leaves immediately use unloaded sucrose, creating a concentration gradient. Sucrose builds up in storage in roots.

Anatomically, phloem is made of two cell types. These two cells types are ___________ __________ and _________ _____________ ___________. Both cells types are alive at maturity. The cytoplasms of these two cells are linked via __________________, which are open protein tunnels the run through the plasma membranes of the cells.

companion cells and sieve tube members; plasmodesmata

In plants, movement of water into and out of a cells is determined by

differences in solute concentrations and pressure generated by the rigid cell wall pushing back against expanding cell

An absorbed mineral has the potential of moving through the _____________ tissue(s) of the root system of plants without actually moving into or out of a cell. This is due the porous structure of the cell walls

epidermal and ground

If a cell's water potential is +0.5 MPa, and the cell is place in a solution with a water potential of -0.5 MPa, then water would likely ___________ (enter/leave) the cell.

leave

Plants lose water to the environment from the surface of the __________ (anatomical region) by the process of _____________. This water loss accounts for approximately _________ % of the total water loss to the plant. This water loss occurs through _________ (specific structure on underside of leaves). These structures open to allow for exchange of __________ during photosynthesis. Water loss combined with production of solutes in the stem system of the plant creates __________ ___________ gradients that ________ water up from the roots of the plant body.

leaves; transpiration. 90%. stomata. gases. solute potential; pull

Companion cells do not conduct fluids, instead their functions are to:____________________________.

load/unload solutes

_____________ plus _________________ in the leaves causes water potentials to be very negative in this location of the plant body. In contrast, absorption of ____________ from the soil causes the water potentials in the ______________ to be more positive. Water potentials move from regions of _____________ _____________ water potentials to regions of _____________ _______________ water potentials (across the whole plant body).

photosynthesis plus transpiration; water; roots; more positive (higher); more negative (lower)

If a cell's volume increases to the point that the ____________ _____________ begins pushing on the cell wall, then that cell is said to have ___________ pressure. In response, the cell wall may resist the push from the swollen cell. The resistance from the cell wall is referred to as __________ pressure. It is measured as the _____________ potential of a cell.

plasma membrane; turgor; physical; pressure

Water ___________ for plant cells represent the likelihood for water to move into a solution (or cell) compared to pure water. In general, negative water potentials __________ water towards, and positive potentials ________ water away from the solution (or cell).

potential; pull; push

The water potential of a plant cell is the sum of the __________ potential and the ___________ potential inside of that cell. The equation that represents this relationship is: __________________

pressure; solute; Yw = Ys + Yp

Movement of fluids through phloem is _____________. This means fluid is ____________ through the phloem vessels rather than being pulled through (such as it is through xylem). High___________ pressure is created when companion cells actively pump in ___________ from the___________ space in the leaves (where photosynthesis is taking place). The sucrose then diffuses into the neighboring _________ ___________ ___________ through plasmodesmata. Water is attracted from the ____________, into the sieve tube members. This increases the overall volume of the sieve tube members, and drives up (more positive) the ____________ ___________. In other locations of the plant, sucrose leaves the __________ . Water then diffuses out of the phloem. The loss of solutes and water in these areas causes the __________ potential of phloem to decrease.

pressurized; pushed; turgor; solutes; extracellular space; sieve tube members; xylem; turgor pressure; phloem;pressure (or water)

The other two mechanisms that influence the movement of bulk fluids through xylem include _____________ _____________ and ____________ _____________.

root pressure and capillary action

Phloem is responsible for conducting ___________ throughout the plant body, in a process known as ___________________. Sucrose is produced via the process of ________________ in the green parts of the plant body. Cells that produce sucrose in excess (net production) are known as ________________. Cells that consume sucrose in excess (net consumption) are known as ______________. This state can change depending on the needs of the plant. For example: A sink cell during the productive summer months will ____________ the sucrose is receives. Then, in the early spring months, that same cell might begin mobilizing the stored sucrose and making it available to young growing parts of the plant. In this case, the sink cell switches over to become a ___________ cell.

sugar (or sucrose); photosynthesis; sources; sinks; store; source

Anatomically, xylem is made of two cell types that conduct (move along) fluids. These two cells types are ___________ and _________ _____________. Both cells types are dead at maturity, so the only cell parts present in this mature tissue are the __________ __________.

tracheids and vessel elements; cell wall

Plant physiologists refer to the loss of water from the leaves of a plant:

transpiration

Which cell type is found in the xylem tissue of plants

vessel elements and tracheid

Transport of water through xylem is driven by ____________ __________ differences, which are calculated in MPa. These pressure differences are usually ________ (sign), which means water molecules are _________ to new locations in the plant, rather than being pushed to those locations.

water potential; negative; pulled