BIOL220 Transport: Water Potential

Solution (definition)

A liquid containing one or more dissolved solids or gases in a homogeneous mixture.

Water Movement Occurs in Response to Three Forces

1. Adhesion 2. Cohesion 3. Surface Tension

The Two Conditions of Transpiration

(1) Stomata are open. (2) The air surrounding leaves is drier than the air inside leaves.

The transmission of pulling force from the leaf surface to the root is possible for two reasons

(1) Xylem forms a continuous network from roots to leaves, so there are continuous columns of water throughout the plant. (2) All of the water molecules are bonded to each other through hydrogen bonds (cohesion).

Meniscus (definition, other)

*Definition:* (plural: menisci) The concave boundary layer formed at most air-water interfaces due to adhesion and surface tension. *Other:* The effects of adhesion, cohesion, and gravity are responsible for the formation of a concave surface boundary called a meniscus. A meniscus forms at most air-water interfaces—including those found in narrow tubes, such as xylem. Menisci form because adhesion and cohesion pull water molecules up along the sides of the tube, while gravity pulls the water surface down in the middle.

Lignin (definition)

*Definition:* A complex polymer built from six-carbon rings and found in the secondary cell walls of some plants; it is exceptionally stiff and strong. Most abundant in woody plant parts.

Solute Potential (ψs) (definition, other)

*Definition:* A component of the potential energy of water caused by a difference in solute concentrations at two locations. Can be zero (pure water) or negative. *Other:* - Solute potentials (ψs) are always negative because they are measured relative to the solute potential of pure water. Solutions with high concentrations of solutes have low solute potentials. - As the [solute] increases, solute potential decreases.

Pressure Potential (ψP) (definition, other)

*Definition:* A component of the potential energy of water caused by physical pressures on a solution. It can be positive or negative. *Other:* - Refers to any kind of physical pressure on water. Inside a cell, the pressure potential consists of turgor pressure and, in the opposite direction, wall pressure. - Pressure potential can be either positive or negative. When pressure is negative, it is called tension. Tension is what draws liquid up through a drinking straw. - When osmosis and pressure affect a cell at the same time, the combined effect of these two forces determines the net direction of water flow.

Water Potential Gradient (definition, other)

*Definition:* A difference in water potential in one region compared with that in another region. Determines the direction that water moves, always from regions of higher water potential to regions of lower water potential. *Other:* - In most cases, water potential is highest in soil, lower in roots, lower yet in leaves, and lowest in the atmosphere. - Water-potential gradient causes water to move up through the plant. To move up a plant, water moves down the water-potential gradient that exists between the soil, its tissues, and the atmosphere.

Root Hairs (definition, other)

*Definition:* A long, thin outgrowth of the epidermal cells of plant roots, providing increased surface area for absorption of water and nutrients. *Other:* Some epidermal cells produce root hairs, which greatly increase the total surface area of the root.

Isotonic (definition, other)

*Definition:* Comparative term designating a solution that, if inside a cell or vesicle, results in no net uptake or loss of water and thus no effect on the volume of the membrane-bound structure. This solution has the same solute concentration as the solution on the other side of the membrane. *Other:* When two regions separated by a membrane are isotonic with each other, water moves across the membrane in equal amounts in both directions. As a result, there is no net movement of water into or out of the cell.

Hypotonic (definition, other)

*Definition:* Comparative term designating a solution that, if outside a cell or vesicle, results in the uptake of water and swelling or even bursting of the membrane-bound structure. This solution has a lower solute concentration than the solution on the other side of the membrane. Used when the solute is unable to pass through the membrane. *Other:* - The surrounding solution has a lower solute concentration than the cell. There will be a net movement of water into the cell by osmosis. - When an animal cell is placed in a hypotonic solution and water enters the cell via osmosis, the volume of the cell increases and the cell may even burst. This does not happen to plant cells. 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.

Osmosis (definition)

*Definition:* Diffusion of water across a selectively permeable membrane from a region of low solute concentration (high water concentration) to a region of high solute concentration (low water concentration). For osmosis to occur, the solute would not be able to pass through the membrane. *Other:* - Solutes tend to diffuse from regions of high concentration to regions of lower concentration. Also recall that water tends to move across membranes from regions of low solute concentration to regions of higher solute concentration, by osmosis. - Water can also move from a wet region to a dry region. -

Guttation (definition, other)

*Definition:* Excretion of water droplets from plant leaves; visible in the early morning. Caused by root pressure. *Other:* In certain low-growing plants, such as strawberries, enough water can move to force water droplets out of the leaves, a phenomenon known as guttation.

Endodermis (definition, other)

*Definition:* In plant roots, a cylindrical layer of cells that separates the cortex from the vascular tissue and location of the Casparian strip. *Other:* Is a cylindrical layer of cells that forms a boundary between the cortex and the vascular tissue. The function of the endodermis is to control ion uptake and prevent ion leakage from the vascular tissue.

Pericycle (definition, other)

*Definition:* In plant roots, a layer of cells just inside the endodermis that give rise to lateral roots. *Other:* - Is a layer of cells located just interior to the endodermis that forms the outer boundary of the vascular tissue. The pericycle can produce lateral roots. - Is a meristematic tissue

Casparian Strip (definition, other)

*Definition:* In plant roots, a waxy layer containing suberin, a water-repellent substance that prevents movement of water through the walls of endodermal cells, thus blocking the apoplastic pathway of water and ion movement into the vascular tissue. *Other:* - Endodermal cells are tightly packed and secrete a narrow band of wax called the Casparian Strip. This layer is composed primarily of a compound called suberin, which forms a waterproof barrier where endodermal cells contact each other. - The Casparian strip blocks the apoplastic route by preventing water from moving through the walls of endodermal cells and into the vascular tissue. It does not affect water and ions that move through the symplastic route. - The Casparian strip is important because it means that for water and solutes to reach vascular tissue, they have to move through the cytoplasm of an endodermal cell. Endodermal cells, in turn, act as gatekeepers by regulating what enters the xylem. - By forcing water and ions to cross at least two membranes on their way from the soil to the xylem, one entering the symplast outside the Casparian strip and one leaving the symplast inside the Casparian strip, plants can use specific channel and carrier proteins to control what moves to the shoots. - Endodermal cells allow ions such as potassium (K+) that are needed by the plant to pass through to the vascular tissue. In contrast, these cells can limit the passage of ions such as sodium (Na+) or heavy metals that are not needed or may be harmful.

Cortex (definition, other)

*Definition:* In plants, a layer of ground tissue found outside the vascular bundles of roots and outside the pith of a stem. *Other:* The cortex consists of ground tissue and stores carbohydrates.

Apoplast (definition, other)

*Definition:* In plants, the region outside plasma membranes consisting of the porous cell walls and the intervening extracellular air space. *Other:* The apoplast consists of cell walls, which are porous, and the spaces that exist between cells.

Symplast (definition, other)

*Definition:* In plants, the space inside the plasma membrane. The symplasts of adjacent cells are often connected through plasmodesmata. *Other:* The symplast consists of the cytosol and the continuous connections through cells via plasmodesmata.

Vascular Tissue (definition, other)

*Definition:* In plants, tissue that transports water, nutrients, and sugars. Made up of the complex tissues xylem and phloem, each of which contains several cell types. Also called vascular tissue system. *Other:* Conducting cells of the vascular tissue transport water and nutrients between roots and shoots and are located in the center of roots in buttercups and other eudicots.

Flaccid (definition, other)

*Definition:* Limp as a result of low internal (turgor) pressure (e.g., a wilted plant leaf ). *Other:* Flaccid means it has no turgor pressure and therefore has a pressure potential of 0 MPa.

Transpiration (definition)

*Definition:* Loss of water vapor from aboveground plant parts. Occurs primarily through stomata. *Other:* Water transport from roots to shoots also occurs because of differences in water potential and is ultimately driven by evaporation of water from the stomata of leaves.

Root Pressure (definition, other)

*Definition:* Positive pressure of xylem sap in the vascular tissue of roots. Generated during the night as a result of the accumulation of ions from the soil and subsequent osmotic movement of water into the xylem. *Other:* - Movement of ions and water into the root xylem is responsible for the process known as root pressure. - The Casparian strip in endodermal cells is essential for root pressure to develop. Without an apoplastic barrier between the xylem and the environment, ions and water would simply leak out of roots. - Toots often continue to accumulate ions at night that their epidermal cells acquired from the soil as nutrients, and these nutrients are actively pumped into the xylem. The influx of ions lowers the water potential of xylem below the water potential in the surrounding cells. As a result, water from nearby cells enters xylem. - As water flows into xylem from other root cells in response to the solute gradient, a positive pressure is generated at night that forces fluid up the xylem. More water moves up xylem and into leaves than is being transpired from the leaves. - The force of root pressure is not enough to overcome the force of gravity on the water inside xylem.

Surface Tension (definition, other)

*Definition:* The cohesive force that causes molecules at the surface of a liquid to stick together, thereby resisting deformation of the liquid's surface and minimizing its surface area. *Other:* Surface tension is a force that exists among water molecules at an air-water interface. In the body of a water column, all the water molecules are surrounded by other water molecules and form hydrogen bonds in all directions. Water molecules at the surface, however, can form hydrogen bonds only with the water molecules beside and below them. Because they have fewer neighbors nearby, surface molecules share stronger attractive forces and bind together more tightly. This enhanced attraction results in tension that minimizes the total surface area.

Bulk Flow (definition, other)

*Definition:* The directional mass movement of a fluid due to pressure differences, such as movement of water through plant xylem and phloem, and movement of blood in animals. *Other:* - Because tracheids and vessels are dead at maturity, the water in xylem does not cross plasma membranes. - In xylem, water movement is driven entirely by differences in pressure potential. The water in a column of xylem cells moves by bulk flow—a mass movement of molecules along a pressure gradient. - The pulling force generated at menisci in leaf cell walls lowers the pressure potential of water in leaves. Even though the tension created at each meniscus is relatively small, there are so many menisci in the leaves of an entire plant that the tension created by summing many small pulling forces is remarkable. It creates a water-potential gradient between leaves and roots that is steep enough to overcome the force of gravity and pull water up long distances.

Wall Pressure (definition, other)

*Definition:* The inward pressure exerted by a cell wall against the fluid contents of a living plant cell. *Other:* When an animal cell is placed in a hypotonic solution and water enters the cell via osmosis, the volume of the cell increases and the cell may even burst. This does not happen to plant cells. 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.

Primary Cell Wall (definition, other)

*Definition:* The outermost layer of a plant cell wall, made of cellulose fibers and gelatinous polysaccharides, that defines the shape of the cell and withstands the turgor pressure of the plasma membrane. *Other:* - First cell wall that is laid down - Made of bundles or cellulose called microfibrils - Arrangement of Microfibrils reenforces wall - Microfibrils can be arranged in different patterns - Permeable to water and solutes - monomers bonded by glycosidic linkages

Epidermis (definition, other)

*Definition:* The outermost layer of cells of any multicellular organism. *Other:* - Is a single layer of cells. - Protects the root. - Some epidermal cells produce root hairs, which greatly increase the total surface area of the root.

Turgor Pressure (definition, other)

*Definition:* The outward pressure exerted by the fluid contents of a living plant cell against its cell wall. *Other:* Turgor pressure is important because it counteracts the movement of water into cells by osmosis.

Water Potential (ψ) (definition, other)

*Definition:* The potential energy of water in a certain environment compared with the potential energy of pure water at room temperature and atmospheric pressure. In living organisms, ψ equals the solute potential (ψS) plus the pressure potential (ψP). *Other:* - This net movement of water—either across a membrane or into the surrounding air—occurs as a direct result of differences in water potential from one region to another. - The term water potential is used 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. - Pure water at room temperature and atmospheric pressure has a water potential of 0 MPa. Solutions that consist of water and solutes have water potentials below 0 MPa (so they are negative). - Water potential is symbolized by the Greek letter ψ (psi, pronounced sigh). - Differences in water potential determine the direc- tion that water moves. The net movement of water is always from areas of higher water potential to areas of lower water potential. - Water travels from areas of high water potential to areas of low water potential

Adhesion (definition, other)

*Definition:* The tendency of certain dissimilar molecules to cling together due to attractive forces. *Other:* Adhesion is a molecular attraction among unlike molecules. In this case, water interacts with a solid substrate—such as the glass walls of a capillary tube or the cell walls of tracheids or vessel elements—through hydrogen bonding. Water molecules are pulled upward as they bond to each other and adhere to the side of the tube.

Cohesion (definition, other)

*Definition:* The tendency of certain like molecules (e.g., water molecules) to cling together due to attractive forces. *Other:* Cohesion is a molecular attraction among like molecules, such as the hydrogen bonding that occurs among molecules in water. Because water molecules cohere, the upward pull by adhesion is transmitted to the rest of the water column. The water column rises against the pull of gravity.

Capillary Action (definition, other)

*Definition:* The tendency of water to move up a narrow tube due to adhesion, cohesion, and surface tension. *Other:*

Cohesion-Tension Theory (definition, other, steps)

*Definition:* The theory that water movement upward through plant vascular tissues is due to loss of water from leaves (transpiration), which pulls a cohesive column of water upward. *Other:* - The plant does not expend energy to create the pulling force. The force is generated by energy from the Sun, which drives evaporation from the leaf surface. Water transport is solar powered. - The cohesion-tension theory of water movement states that, because of the hydrogen bonding between water molecules, water is pulled up through xylem in continuous columns. *Steps:* 1. *Water vapor diffuses out of leaf:* Water vapor diffuses through the stoma to the atmosphere. 2. * Water evaporates inside leaf:* As water exits the leaf, the humidity of the spaces inside the leaf drops, causing water to evaporate from the menisci that exist at the air-water interfaces. 3. *Water is pulled out of xylem:* The resulting tension created at the menisci pulls water that surrounds nearby cells, which in turn pulls water out of the xylem. 4. *Water is pulled up xylem:* Tension is transmitted from water in leaf xylem through the stem all the way to root xylem by cohesion (continuous hydrogen bonding). 5. *Water is pulled out of root cortex:* Water moves by osmosis as a result of the active transport of ions into the xylem. 6. *Water is pulled from soil into root:* Water enters the root by osmosis as a result of the active uptake of ions into root cells.

Secondary Cell Wall (definition, other)

*Definition:* The thickened inner layer of a cell wall formed by certain plant cells as they mature and after they have stopped growing; contains lignin in water-conducting cells. Provides support or protection. *Other:* - Lignin is found in secondary cell walls - Lignin is a lot less permeable to water than the primary cell wall - Lignin has a lot of hydrophobic ring structures - Lignin is found in vascular tissue of vessel elements

Wilt (definition, other)

*Definition:* To lose turgor pressure in a plant tissue. *Other:* - If plants keep their stomata open and leaf cells lose water faster than the water is replaced, the positive turgor pressure normally inside living cells drops to 0. If the cells do not regain turgor quickly, they are at risk of dehydration and death. - When an entire tissue loses turgor pressure, it will wilt. - Unless corrected, extensive wilting may lead to the death of the tissue and, eventually, the plant.

Suberin (definition, other)

*Definition:* Waxy substance found in the cell walls of cork tissue and in the Casparian strip of endodermal cells. *Other:* Endodermal cells are tightly packed and secrete a narrow band of wax called the Casparian Strip. This layer is composed primarily of a compound called suberin, which forms a waterproof barrier where endodermal cells contact each other.

Water Potential in Soils (other)

- *In moist soil:* The water that fills crevices between soil particles usually contains relatively few solutes and normally is under little pressure. As a result, its water potential tends to be high relative to the water potential found in a plant's roots, which is higher in solutes. - *Low water potential:* When the water potential in soil drops, water is less likely to move from soil into roots. If soil water potential is low enough, water may even move from roots to the soil. This situation would be deadly to plants. - *Salty Soils:* Soils near ocean coastlines have water potentials as low as -4 MPa or less due to high solute concentrations. This is much lower than the water potential typically found inside plant roots. - *Dry Soils:* When soils dry, water no longer flows freely in the spaces between soil particles. All of the remaining water adheres tightly to soil particles, creating a tension that lowers the water potential of soil water.

Atmospheric Water Potential (other)

- In the atmosphere, water exists as a vapor with no solute potential. The pressure exerted by water vapor in the atmosphere depends on temperature and humidity. The lower the pressure potential, the faster liquid water evaporates into the atmosphere. - When air is dry, few water molecules are present and the pressure they exert is low, increasing the rate of evaporation. - When air is warm, water molecules move farther apart and also exert lower pressure. - Warm, dry air has an extremely low water potential, often approaching -100 MPa. When the weather is cool and rainy or foggy, however, the water potential of the atmosphere may be equal to the water potential inside a leaf. But normally, the water potential of the atmosphere is lower than the water potential inside a leaf, so water in the leaf evaporates quickly.

What is the importance of secondary cell walls?

- The secondary thickenings characteristic of the cell walls in tracheids and vessel elements. The cells in vascular tissue have walls that are reinforced with tough lignin molecules. As a result, Xylem consists of rigid "pipes" that can withstand internal tension without collapsing. - The evolution of lignified secondary cell walls was an important event in the evolution of land plants because it allowed vascular tissue to withstand extreme negative pressures.

Evaporation in plants (other)

- While stomata are open, the moist interior of the leaf is exposed to the dry atmosphere. As a result, large quantities of water evaporate from the leaf. If the lost water is not replaced with water absorbed by roots, plant cells will dry out and die. - Evaporation from leaves is a challenge for plants, but it is also beneficial. Because water enters a plant through its roots and exits through its leaves, there is a regular flow of water from roots to shoots. - Minerals that are absorbed by roots are carried to the leaves by this flow. Without this movement of water, shoots would be unable to receive the minerals they need to grow. - Evaporation cools the plant, just like sweating cools your body. Heavy rates of evaporation can lower leaf temperatures by as much as 10−15°C.

What are the three routes water can take through the root cortex to the xylem?

1. Symplastic Route 2. Transmembrane Route 3. Apoplastic Route

Megapascals (MPa) (definition)

A unit of pressure (force per unit area) equivalent to 1 million pascals (Pa).

Pascal (Pa) (definition)

A unit of pressure (force per unit area).

Solute (definition)

Any substance that is dissolved in a liquid.

How and why is the water potential gradient different at night?

At night, water potential will be much higher inside the plant because the stomata are closed, so there is no tension generated from the gradient with the atmosphere.

Hypertonic (definition)

Comparative term designating a solution that, if outside a cell or vesicle, results in the loss of water and shrinkage of the membrane-bound structure. This solution has a greater solute concentration than the solution on the other side of the membrane. Used when the solute is unable to pass through the membrane.

Wet Adapted Plant Species & Low Solute Potentials (other)

Plants that are adapted to wetter sites cannot tolerate low solute potentials in their tissues. When conditions get hot and dry, they have to close their stomata and stop photosynthesis—meaning that their growth will stop or slow down dramatically compared to dry-adapted plants.

Salt Adapted Plant Species (other)

Salt-adapted species often respond to low water potentials in soil by accumulating solutes in their root cells, which lowers their solute potential. These plants have enzymes that increase the concentration of certain organic molecules in the cytoplasm. As a result, They can keep the water potential of their tissues lower than that of salty soils and are able to absorb what little water is available.

Dry Adapted Plant Species (other)

Species that are adapted to dry sites cope by tolerating low solute potentials. As soils dry, their water potential decreases. But if a plant's solute potential also drops, it can maintain a water-potential gradient that continues to bring water into the plant. They are able to keep acquiring water and grow because the solute potentials of their tissues can drop to stay below the soil water potential.

Turgid (definition)

Swollen and firm as a result of high internal pressure (e.g., a plant cell containing enough water for the cytoplasm to press against the cell wall).

What is the function of the Casparian strip? Describe in terms of apoplast and symplast. During which time of day would a plant without a Casparian strip experience the greatest negative effects?

The Casparian strip seals the apoplast, so that all water and solutes entering and leaving the vascular tissue must enter the symplast across a selectively permeable membrane. At night when transpiration is not occurring, the water potential gradient may be directed toward the soil instead of the leaf. The Casparian strip greatly limits the rate of water loss through the roots because all water has to enter the symplast at the endodermal cells to move out of the plant.

Apoplastic Route (loose definition)

The apoplastic route is outside the plasma membranes. The apoplast consists of cell walls, which are porous, and the spaces that exist between cells. Water moving along the apoplastic route must eventually pass through the cytoplasm of endodermal cells before entering xylem.

Symplastic Route (loose definition)

The symplastic route is inside the plasma membranes. The symplast consists of the cytosol and the continuous connections through cells via plasmodesmata.

Transmembrane Route (loose definition)

The transmembrane route is based on flow through aquaporin proteins—water channels located in the plasma membranes of many cells. Some water may also diffuse directly across plasma membranes.

Distinct Tissues of Roots

• *The epidermis:* Is a single layer of cells. In addition to protecting the root, some epidermal cells produce root hairs, which greatly increase the total surface area of the root. • *The cortex:* consists of ground tissue and stores carbohydrates. • *The endodermis* is a cylindrical layer of cells that forms a boundary between the cortex and the vascular tissue. The function of the endodermis is to control ion uptake and prevent ion leakage from the vascular tissue. • *The pericycle:* Is a layer of cells located just interior to the endodermis that forms the outer boundary of the vascular tissue. The pericycle can produce lateral roots. • *Conducting cells of the vascular tissue:* transport water and nutrients between roots and shoots and are located in the center of roots eudicots. Notice that, in these plants, phloem is situated between each of four arms formed by xylem, which is arranged in a cross-shaped pattern.