Odors can be detected without being in immediate physical proximity to the source.
Molecules are constantly in motion.
Analogy: Similar to moving tennis balls in a room.
If the tennis balls in one room could enter another room via a hole in the wall, the number of balls in both rooms would become equal over time.
Molecular movement follows this similar principle.
Molecular Movement
Molecules are in constant motion.
Brownian Movement: Can be observed using a light microscope.
Example: A diluted drop of ink on a slide shows the particles of the ink in constant motion.
Presence of a concentration gradient:
High concentration of molecules in one region and lower concentration in an adjacent region.
Molecules move along a concentration gradient from high to low concentration.
Moving from lower to higher concentration is against the concentration gradient.
Diffusion
Diffusion: Movement of molecules from a region of higher concentration to a region of lower concentration.
State of Equilibrium: Molecules become distributed throughout the available space.
Rate of diffusion influenced by:
Pressure
Temperature
Density of the medium.
Osmosis
Solvent: Liquid in which substances dissolve.
Semipermeable Membranes: Membranes that allow different substances to diffuse at different rates; all plant cell membranes act this way.
Osmosis: Diffusion of a solvent (usually water) through a semipermeable membrane from a region where water is more concentrated to a region where it is less concentrated.
Osmosis can be measured using an osmometer.
Osmotic Pressure and Osmotic Potential
Osmotic Pressure: The pressure required to prevent osmosis.
Osmotic Potential: Balanced by the resistance of the cell wall.
Pressure Potential (Turgor Pressure): The pressure that develops against the walls as a result of water entering the cell.
Turgid Cell: A cell that is firm due to water gained through osmosis.
Water Potential of a Cell = Osmotic Pressure + Pressure Potential.
Pathway of Water Through a Plant
Osmosis is the primary means by which water enters plant cells.
Water moves through:
Cell walls and intercellular spaces of the epidermis and root hairs.
Across the endodermal cells to reach the xylem.
Throughout the plant via the xylem and diffuses out through stomata.
Plasmolysis
Plasmolysis: The loss of water through osmosis, accompanied by shrinkage of protoplasm away from the cell wall.
Comparison of normal cells versus plasmolyzed cells.
Imbibition
Imbibition: The process by which large molecules (like cellulose and starch) develop electrical charges when wet and attract water molecules.
Water molecules adhere to these large molecules, resulting in the swelling of tissues.
Imbibition is the first step in the germination of seeds.
Active Transport
Active Transport: The process used to absorb and retain solutes against a diffusion or electrical gradient, utilizing energy expenditure.
Involves a Proton Pump: An enzyme complex in the plasma membrane energized by ATP molecules.
Transport Proteins: Facilitate the transfer of solutes in and out of the cell.
Some plants can survive in salty environments by accumulating large amounts of organic solutes, which drive osmosis.
Water and Its Movement Through the Plant
Transpiration: The loss of water vapor from the internal leaf atmosphere.
More than 90% of the water entering a plant is transpired.
Demonstration: Covering the soil of a potted plant and then covering the plant with a bell jar shows transpiration.
Previous Ideas on Movement of Water in Plants
Nehemiah Grew: Suggested that cells surrounding xylem produce a pumping action; however, it is not supported since water moves in dead stems.
Marcello Malpighi: Proposed capillary action moves water in plants; the height that water will rise in a narrow tube is inversely proportional to the diameter of the tube but requires air above the water column.
Stephen Hale: Suggested root pressure moves water, but in summer, root pressure is minimal.
The Cohesion-Tension Theory
Cohesion-Tension Theory: Proposes that transpiration generates tension that pulls water columns through plants from roots to leaves.
Water columns are created when water molecules adhere to tracheids and vessels of xylem and cohere to each other.
Specifics of Cohesion-Tension
When water evaporates from mesophyll cells, they develop a lower water potential than adjacent cells.
Water moves into mesophyll cells from adjacent cells with higher water potential, continuing until veins are reached.
This creates tension on water columns, drawing water through the entire span of xylem cells.
Water continues entering the root by osmosis and is forced by Casparian Strips to enter the endodermal cells leading to the xylem.
Regulation of Transpiration
Stomatal Apparatus: Regulates transpiration and gas exchange.
Composed of 2 guard cells and stoma (opening).
Factors influencing transpiration rates:
Humidity
Light
Temperature
Carbon dioxide concentration.
When Stomata Open
Stomata open when photosynthesis occurs:
Guard cells expend energy to acquire potassium ions from adjacent epidermal cells.
This process causes a lower water potential in guard cells.
Water enters guard cells by osmosis, making them turgid, which opens the stomata.
When Stomata Close
Stomata close when photosynthesis does not occur:
Potassium ions leave guard cells, causing water to leave as well.
Guard cells become less turgid, resulting in stomata closing.
Stomata and Water Conservation
Most plants have open stomata during the day and closed at night.
Some plants adapt to conserve water by:
Opening stomata only at night (e.g., desert plants).
This conserves water but makes carbon dioxide inaccessible during the day.
Undergo CAM Photosynthesis: Carbon dioxide is converted to organic acids at night and stored in vacuoles. During the day, organic acids are converted back to carbon dioxide.
Stomata may be recessed below the leaf surface or in specialized chambers (seen in desert plants and pines).
Guttation
Guttation: The loss of liquid water.
Occurs if a cool night follows a warm, humid day, producing droplets through hydathodes at the tips of veins.
In the absence of transpiration at night, pressure in xylem elements forces water out of hydathodes.
Transport of Food (Organic Solutes) in Solution
An important function of water is the translocation of food substances in solution by phloem.
Pressure-Flow Hypothesis: Organic solutes flow from a source (where water enters by osmosis) to sinks (where food is utilized and water exits).
Organic solutes move along concentration gradients between sources and sinks.
Specifics of Pressure-Flow Hypothesis
Phloem Loading: Sugar enters by active transport into sieve tubes.
The water potential of sieve tubes decreases, leading to water entering by osmosis.
Turgor pressure develops, driving fluid through sieve tubes towards sinks.
Food substances are actively removed at the sink, and water exits sieve tubes, lowering pressure in the sieve tubes.
Mass flow occurs from higher pressure at the source to lower pressure at the sink.
Water diffuses back into the xylem.
Mineral Requirements for Growth
Essential Elements: Vital as building blocks for compounds synthesized by plants.
Macronutrients and Micronutrients
Macronutrients: Used by plants in larger amounts, including:
Nitrogen,
Potassium,
Calcium,
Phosphorus,
Magnesium,
Sulfur.
Micronutrients: Needed in very small amounts, including:
Iron,
Sodium,
Chlorine,
Copper,
Manganese,
Cobalt,
Zinc,
Molybdenum,
Boron.
Deficiencies in any required element result in characteristic symptoms in plants.
Symptoms of Some Deficiencies
Potassium Deficiency: Yellowing of leaves beginning at the margins and continuing to the center; lower leaves may be mottled and often brown at the tips.
Phosphorus Deficiency: Plant stunted with darker green leaves; lower leaves often purplish between veins.
Leaves with Calcium and Nitrogen Deficiency Symptoms
Calcium Deficiency: Terminal bud often dead; young leaves may appear hooked at tips; tips and margins of leaves may be withered; roots dead or dying.
Nitrogen Deficiency: Uniform loss of color in leaves, starting first with the oldest leaves.
Leaves with Sulfur, Magnesium, and Iron Deficiency Symptoms
Sulfur Deficiency: Leaves pale green with dead spots; veins are lighter in color than the rest of the leaf.
Magnesium Deficiency: Veins of leaves remain green while the area between them turns yellow, with sudden appearances of dead spots; leaf margins may curl.
Iron Deficiency: Larger veins stay green while the rest of the leaf yellows, predominantly affecting younger leaves.