Water in Plants

Chapter 9

Outline Molecular Movement Water and Its Movement Through the Plant Regulation of Transpiration Transport of Food Substances (Organic Solutes) in

Solution Mineral Requirements for Growth

Molecular Movement Diffusion - movement of molecules from region of higher

concentration to region of lower concentration

• Molecules move along concentration gradient

• State of Equilibrium - molecules distributed throughout available space

• Rate of diffusion depends on pressure, temperature and density of medium

Molecular Movement Solvent - liquid in which substances dissolve Semipermeable Membranes - membranes in

which different substances diffuse at different rates• All plant cell membranes

Osmosis - diffusion of H2O through semipermeable membrane from region where H2O more concentrated to region where less concentrated

Osmotic Pressure - pressure required to prevent osmosis• Osmotic potential balanced by resistance of cell wall

• H2O potential of cell = osmotic pressure + pressure potential

– Pressure Potential (Turgor Pressure) - pressure that develops against walls as result of H2O entering cell

– Turgid Cell - firm cell due to H2O gained by osmosis

– H2O moves from cell with higher H2O potential to cell with lower H2O potential

Turgid cell

Molecular MovementOsmosis

Osmosis main way H2O enters plants from environment

Molecular MovementOsmosis

Molecular Movement Plasmolysis - loss of H2O through osmosis• Accompanied by shrinkage of protoplasm away from

cell wall

Normal cells versus plasmolyzed cells

Molecular Movement Imbibition• Large molecules (i.e.,

cellulose and starch) develop electrical charges when wet, and attract H2O molecules

• H2O molecules adhere to large molecules

• Results in swelling of tissues• Imbibition 1st step in

germination of seed Seeds before and after imbibition

Molecular Movement Active Transport - process used to absorb and

retain solutes against diffusion, or electrical, gradient by expenditure of energy• Involves proton pump (enzyme complex in plasma

membrane energized by ATP molecules)− Transport proteins - facilitate transfer of

solutes to outside and to inside of cell

Water and Its Movement Through the Plant

Transpiration - H2O vapor loss from internal leaf atmosphere• >90% of H2O entering plant transpired

H2O needed for:

• Cell activities• Cell turgor• Evaporation for cooling– If more H2O lost then taken

in, stomata close

Water and Its Movement Through the Plant

Cohesion-Tension Theory - transpiration generates tension to pull H2O columns through plants from roots to leaves• H2O columns created when H2O molecules

adhere to tracheids and vessels of xylem and cohere to each other

Water and Its Movement Through the Plant

Cohesion-Tension Theory• When H2O evaporates from mesophyll cells, they

develop lower H2O potential than adjacent cells

• H2O moves into mesophyll cells from adjacent cells with higher H2O potential

• Process continued until veins reached• Creates tension on H2O columns, drawing H2O all way

through entire span of xylem cells• H2O continues to enter root by osmosis

Regulation of Transpiration Stomatal apparatus regulates transpiration and

gas exchange• Stomatal Apparatus - 2 guard cells + stoma

(opening)• Transpiration rates influenced by humidity, light,

temperature, and CO2 concentration

Regulation of Transpiration When photosynthesis

occurs, stomata open • Guard cells use energy to

acquire K+ from adjacent epidermal cells

• Causes lower H2O potential in guard cells

• H2O enters guard cells via osmosis

• Guard cells become turgid and stomata opens

Regulation of Transpiration When photosynthesis

does not occur, stomata close• K+ leave guard cells• H2O follows

• Guard cells become less turgid and stomata close

Regulation of Transpiration Stomata of most plants open during day/closed at night H2O conservation in some plants:• Stomata open only at night - Desert plants

− Conserves H2O, but makes CO2 inaccessible during day

Undergo CAM Photosynthesis» CO2 converted to organic acids and stored in

vacuoles at night» Organic acids converted to CO2 during day

• Stomata recessed below surface of leaf or in chambers

− Desert plants, pines

Regulation of Transpiration Guttation - loss of liquid H2O • If cool night follows warm, humid day, H2O droplets

produced through hydathodes at tips of veins• In absence of transpiration at night, pressure in xylem

elements forces H2O out of hydathodes

Guttation in barley plants

Transport of Organic Solutes in Solution

Important function of H2O is translocation of food substances in solution by phloem

Pressure-Flow Hypothesis - organic solutes flow from source, where H2O enters by osmosis, to sinks, where food utilized and H2O exits• Organic solutes move along concentration gradients

between sources and sinks

Transport of Organic Solutes in Solution Specifics of Pressure-Flow Hypothesis:• Phloem Loading - sugar enters by active transport

into sieve tubes• H2O potential of sieve tubes decreases and H2O

enters by osmosis• Turgor pressure develops and drives fluid through

sieve tubes toward sinks• Food substances actively removed at sink and H2O

exits sieve tubes, lowering pressure in sieve tubes• Mass flow occurs from higher pressure at source to

lower pressure at sink• H2O diffuses back into xylem

Mineral Requirements for Growth

Essential Elements - essential building blocks for compounds synthesized by plants

Mineral Requirements for Growth

Macronutrients - used by plants in greater amounts• N, K, Ca, P, Mg, and S

Micronutrients - needed by plants in very small amounts• Fe, Na, Cl, Cu, Mn, Co, Zn, Mo, and B

When any required element deficient in soil, plants exhibit characteristic symptoms

Review Molecular Movement Water and Its Movement Through the Plant Regulation of Transpiration Transport of Food Substances (Organic Solutes) in

Solution Mineral Requirements for Growth