Water and the plant cells

Most abundant and best solvent known

water

most water is lost by the plant evaporate from the lae

Transpiration

important mans of dissipating the heat input from sunlight

how does the heat dissipates

due to water molecules escaping into the atmosphere having a higher than average energy which breaks the bonds holding them in liquid

for a typical leaf how much is dissipates

nearly half of the net heat input from sunlight

Stream of water taken up by the roots

an important means of bringing dissolved soil minerals to the roots surface for absorption

structure and properties of water

• consists of an oxygen atom covalently bonded to two hydrogen atoms at a 105 angle

• partial negative charge

• polar molecule

• hydrogen bond

the versatility of water as a solvent

due to the its small size and polar nature

The polarity of water makes it a particularly good solvent for

Ionic substances and sugars

Shells of hydrogen

When the polar ends of water molecules can orient themselves next to charged or partially charged group in macromolecules

Mixtures

Two or more substances mixed together

Types of mixtures

1. Heterogeneous

2. Homogeneous

Heterogeneous mixtures

The properties of the mixtures are different and not consistent

Homogeneous mixtures

The properties of the mixtures are consistent ( salt water)

The extensive hydrogen bonding between water molecules results in

High specific heat and high latent heat of vaporization

Specific heat

The heat energy required to raise the temperature of a substance by a specific amount

Why does water requires a relatively large energy input to raise its temperature

When the temperature of water is raised, the molecules vibrate faster and with greater

amplitude. To allow for this motion, energy must be added to the system to break the hydrogen

bonds between water molecules.

Latent heat of vaporization

The energy needed to separate molecules from the liquid phase and move them into the gas phase at constant temperatures

When does latent heat of vaporization occurs

During transpiration

Transpiration

An important component of temperature regulation in plants

Water molecules at an air-water interface are more strongly attracted to

Neighboring water molecules than to gas phase in contact with the water surface

Surface tension

The energy required to increase the surface area

Cohesion

The mutual attraction between molecules and is caused by the extensive hydrogen bonding in the water

Adhesion

The attraction of water to a solid phase such as a cell wall or glass surface

Capillarity

Cohesion, adhesion, and surface tension give rise to the movement of water along a capillary tube

The upward movement of water in a vertically oriented glass capillary is due to

1. The attraction of water to the polar of the glass tube (adhesion)

2. The surface tension of the water

Cohesion gives water a

High tensile strength

Tensile strength

The maximum force per unit area that a continuous column of water can withstand before breaking

A sealed syringe can be used to create positive and negative pressures in a fluid like water. Pushing on the plunger compresses the fluid, and a positive pressure builds up.

How does water move across plant membranes

1. Diffusion of water molecules across the plasma membrane’s lipid

   bilayer

2. Bulk flow, Aquaporins are integral membrane proteins that form water-selective channels across the membrane. Because water diffuses faster through

   such channels than through a lipid bilayer, aquaporins facilitate water movement into plant cells

Bulk flow

the movement of groups of molecules, most often in response to a pressure gradient.

Example of bulk flow

Garden hose, a river flowing, and rain falling

Water can cross plant membranes by diffusion of individual water molecules through the membrane bilayer,

random thermal agitation

Water molecules in a solution are not static; they are in continuous motion, colliding with one another and exchanging kinetic energy

Diffusion

The net movement of molecules from regions of high concentration to region of low concentration, down a concentration gradient

Thermal motion of molecules leads to diffusion-the gradual mixing of molecules and eventual dissipation of concentration differences.

Pessure-driven bulk flow of water is the predominant mechanism responsible for

long-distance transport of water in the xylem

Selectively permeable

They allow the movement of water and other small uncharged substances across them more readily than the movement of larger solutes

and charged substances

Osmosis

Occurs spontaneously in response to a driving force. The direction and rate of water flow across a membrane are determined not solely by the concentration gradient of water or by the pressure gradient, but by the sum of these two drivingforces.

Chemical potential

the free energy per mole of water

The major factors influencing the water potential in plants ar

Concentration, pressure, and gravity

Osmotic potential

Represents the effect of dissolved solutes on water potential.

The principles of water potentials

1. Transports across cell membranes

2. A measure of the water status of a plant

Water potential of plants under various growing conditions, and sensitivity of various physiological processes to water potential.