Plant Physiology - Chapter 29

Plant Physiology

Core Concepts

  • Plants face a major challenge due to the high rate of water loss from their photosynthetic surfaces.
  • Leaves possess a waxy cuticle and stomata, enabling plants to acquire carbon dioxide while minimizing water loss.
  • Xylem facilitates water transport from the soil, allowing leaves to open their stomata without desiccation.
  • Phloem is responsible for carbohydrate transport throughout the plant, supporting growth and respiration.
  • Roots actively expend energy to extract nutrients from the soil and establish symbiotic relationships with bacteria and fungi, enhancing nutrient availability.

Photosynthesis on Land - Avoiding Desiccation

  • Bryophytes have evolved to withstand intermittent drying. Vascular plants, on the other hand, extract water from the soil and reduce water loss from their leaves.

Desiccation Tolerance in Bryophytes

  • Most of their surfaces are permeable to water.
  • They exhibit a high surface area-to-volume ratio, relying on diffusion and osmosis for water and nutrient distribution.
  • Bryophytes dry out as the environment desiccates, ceasing photosynthesis.
  • Many bryophytes exhibit desiccation tolerance, like resurrection mosses.

Vascular Plants - Active Hydration Control

  • Vascular plants actively regulate their hydration, even in dry conditions.
  • Water is transported through the plant via bulk flow.
  • Roots enable access to water in the soil.
  • Vascular plants can grow tall and maintain photosynthesis during dry periods due to their access to soil water.

Anatomy of a Vascular Plant

  • Composed of 4 organs and 3 tissues.
  • Parenchyma cells are a cell type found in ground tissue.

The Leaf

  • The leaf is the main site for photosynthesis, utilizing sunlight and CO2 with air spaces between cells.
  • There is a tradeoff between maximizing surface area for photosynthesis and minimizing heat gain. The large surface area increases the risk of desiccation.
  • The leaf contains mesophyll cells, veins, epidermis, and stomata

CO2 Uptake and Water Loss

  • CO2 enters leaves through stomata via diffusion.

Transpiration

  • Water is lost as CO2 diffuses into the leaves. Transpiration is the evaporative water loss from leaves.

Leaf Cuticle and Stomata

  • Epidermal cells secrete a waxy cuticle.
  • The cuticle limits water loss but also restricts CO2 diffusion into the leaf.
  • Stomata, small pores in the epidermis, bypass the limitations of the cuticle.

Stomata Opening and Closing

  • Guard cells regulate the opening and closing of stomata.
  • Uptake of solutes by guard cells leads to water influx via osmosis, causing the guard cells to swell and open the stoma.
  • Release of solutes causes water to flow out of the guard cells, closing the stoma.

Factors Influencing Guard Cell Function

  • The ability of guard cells to change their ion concentration enables them to open and close a stoma.

Photosynthesis Review

  • Oxidation of water produces O2 as a byproduct.
  • Reduction of CO2 forms carbohydrates.
  • Photosynthetic electron transport chain generates ATP and NADPH.
  • Calvin cycle fixes carbon.
  • Overall reaction: Energy+6CO<em>2+12H</em>2O→C<em>6H</em>12O<em>6+6O</em>2+6H2OEnergy + 6CO<em>2 + 12H</em>2O \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2 + 6H_2O

Calvin Cycle

  • The Calvin Cycle converts low energy CO2 to high energy carbohydrates in the C3 pathway.

CAM and C4 Photosynthetic Pathways

  • Some plants have evolved alternative photosynthetic pathways as adaptations to specific environmental conditions. The CAM and C4 pathways are modifications to the C3 pathway.
  • The CAM pathway separates transpiration and photosynthesis temporally and is common in dry habitats where water loss is a serious problem.
  • The C4 pathway separates transpiration and photosynthesis spatially and is common in plants in high light intensity where chloroplasts make O2 at a faster rate than CO2 becomes available. This imbalance leads to photorespiration (bad); the C4 pathway reduces photorespiration.

Water Conservation Through CAM

  • Photosynthesis and water loss often occur simultaneously.
  • Crassulacean acid metabolism (CAM) balances water loss and CO2 accumulation.
  • Stomata open at night to store CO2 and close during the day to conserve water.

Drawbacks of CAM

  • The rate of photosynthetic carbohydrate production tends to be low.
  • ATP is required to drive the uptake of organic acids into the vacuole.
  • The storage capacity of the 4-carbon acid in the vacuole is limited.

Photorespiration

  • Photorespiration occurs when oxygen concentrations in the leaf are high relative to CO2 levels.
  • Some plants have evolved mechanisms to reduce the energy and carbon losses associated with photorespiration.

CAM vs C4 Plants

  • Both CAM and C4 plants produce 4-carbon organic acids as an entry point for photosynthesis.
  • In CAM plants, CO2 capture and the Calvin cycle occur at different times, while in C4 plants, they occur in different cells.

C4 Plants: Concentrating CO2

  • The C4 cycle operates faster than the Calvin cycle.
  • CO2 concentration within bundle-sheath cells builds up, reaching levels five times higher than in the surrounding air.

C4 Plant Photosynthesis

  • C4 plants exhibit high rates of photosynthesis because they minimize photorespiration.
  • C4 plants have a more favorable CO2:H2O exchange ratio than C3 plants.
  • C4 photosynthesis requires more energy because ATP is used to regenerate PEP in the C4 cycle.
  • C4 photosynthesis confers an advantage in hot, sunny environments where photorespiration rates would otherwise be high.

Water Transport

  • This section discusses water transport in plants, presumably via xylem, which is detailed in the following sections.

Xylem Anatomy

  • Xylem cells are elongated.
  • Lignin provides structural support.
  • Water enters and exits xylem conduits through pits.

Xylem Vessels

  • Water flows faster through xylem conduits with a larger radius.
  • Vessel elements are typically larger and longer than tracheids, allowing for higher rates of water transport.

Forces That Pull Water from the Soil

  1. Evaporation from stomata.
  2. Hydrogen bonds.

Risks to Xylem Conduits

  • Collapse due to negative pressure pulling conduit walls inward.
  • Cavitation due to air leaks through pits.
  • Cavitation due to freeze and thaw cycles.

Mutant Xylem Vessels

  • Mutant xylem vessels with twice the diameter will have higher water transport capacity but will be more susceptible to cavitation from freezing.

Carbohydrate Transport

  • This section introduces carbohydrate transport, presumably via phloem.

Phloem

  • Transports sap containing carbohydrates, amino acids, inorganic forms of nitrogen, ions, hormones, protein signals, and RNA.
  • Sieve elements are responsible for sugar transport; they are living cells with reduced cellular components supported by companion cells.

Transport from Source to Sink

  1. Turgor pressure in source phloem is high because water is drawn in by osmosis as sugars are added to the phloem.
  2. Pressure difference between source and sink drives the movement of phloem sap.
  3. Turgor pressure in sink phloem is low because water flows out by osmosis as sugars exit the phloem and are utilized by sink cells.

Root Branching and Root Hairs

  • Roots are responsible for absorbing water and nutrients, except for CO2.
  • Extensive branching and root hairs create a large surface area for contact with the soil.

Plants and Essential Mineral Nutrients

  • Table 29.1 outlines essential nutrients for plant metabolism and structure, including:
    • Nutrients covalently bonded with carbon compounds:
      • Nitrogen (1.5%): Component of amino acids, nucleic acids, nucleotides, and coenzymes.
      • Phosphorus (0.2%): Component of nucleic acids, nucleotides, coenzymes, and phospholipids; key role in reactions involving ATP.
      • Sulfur (0.1%): Component of two amino acids, coenzyme A, and other essential organic compounds.
    • Nutrients that remain in ionic form:
      • Potassium (1.0%): Cofactor for many enzymes, major cation in cell osmotic balance.
      • Calcium (0.5%): Cofactor for enzymes involved in hydrolysis of ATP and phospholipids, second messenger in metabolic regulation, important structural role in cell walls.
      • Magnesium (0.2%): Enzyme cofactor, component of chlorophyll.
      • Chlorine, zinc, sodium (each ≤0.01): Cofactor for enzymes involved in photosynthesis and other reactions.
    • Nutrients involved in redox reactions:
      • Iron, manganese, copper, nickel, molybdenum (each ≤0.01): Component of enzymes that catalyze electron transfer.
    • Nutrients present in cell walls:
      • Silicon, boron (0.1, <0.01): Contribute to the structural integrity of cell walls and defense against herbivores.

Root Selective Nutrient Uptake

  1. Solutes that enter the cytoplasm of a root cell can move toward the xylem through plasmodesmata.
  2. Or they can move in the water-filled spaces of the cell walls.
  3. At the endodermis, the Casparian strip prevents solutes and water from moving in the walls, forcing them to pass through cell membranes

Plant Nutrient Uptake: Energy Requirements

  • Nutrients move by diffusion through water films bound to soil particles.
  • Active uptake of nutrients maintains a concentration gradient between the root and the soil.
  • Active growth of roots and root hairs maintains a high surface area-to-volume ratio for rapid rates of diffusion.
  • Roots release protons into the environment, acidifying the soil and liberating nutrients from soil particles, which the roots then absorb.

Microorganisms Near Plant Roots

  • Microorganisms consume carbohydrates that roots leak into soil when transporting nutrients to a plant

Symbioses Between Plant and Fungi

  • Ectomycorrhizae: Fungal cells surround but do not penetrate root cells. Carbon and nutrients are exchanged through cell membranes.
  • Endomycorrhizae: Fungal cells penetrate root cells, enhancing carbon and nutrient exchange.

Symbioses Between Plants and Bacteria

  • Root nodules formed by nitrogen-fixing bacteria.
  • Nitrogen is important for plant growth, and plants must acquire it in sufficient amounts to grow.
  • Atmospheric nitrogen is in a chemical form that cannot be used by plants.
  • Symbiotic interactions with nitrogen-fixing bacteria and archaea provide nitrogen in a usable form.

Nitrogen Fixation

  • Symbiosis between legumes and rhizobia.
  • Plant supplies carbohydrates to rhizobia.
  • Rhizobia supply fixed nitrogen in usable forms.
  • Phloem samples from mycorrhizae will likely have the highest phosphorus content due to the symbiotic relationship enhancing phosphorus uptake.