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• Homeostasis


  A process by which a biological system maintains stability while adjusting to its changing environment
• Properties of Homeostatic Mechanisms

  • The mechanism should be triggered by some change in the environment/stimulus—either internal environment or external environment
  • The system should have a receptor or sensor, an integrator, and an effector. Sometimes these parts may be molecules or chemical processes
  • The mechanism is negative feedback if it corrects the initial change. In other words, the response is in the opposite direction of the initial change in the environment
  • The mechanism is positive feedback if it makes the initial change more intense. In other words, the response is in the same direction as the initial change in the environment
• Feedback Mechanisms in Plants


  1. During the day there is enough sunlight for photosynthesis. Photosynthesis in the guard cells causes them to accumulate potassium ions and water (by osmosis). The guard cells swell and open the stoma. As a result, gases can flow into and out of the leaf

  2. Conversely, at night photosynthesis stops. The guard cells remove potassium and water (by osmosis). The guard cells shrink and close the stoma. Subsequently, no gases can move into or out of the leaf
• Regulation in Plants


  • Regulation and coordination systems in plants are simpler than in animals
  • Homeostatic regulation of plants seeks to: 1) Maintain an adequate uptake of water and nutrients from soil into leaves, 2) Control stomatal opening so that water loss is minimized and carbon dioxide is maximized
• What plants must maintain


  • Water balance
  • Oxygen balance
  • Carbon dioxide balance
  • Temperature balance
  • Nutrient balance
• The plant structure includes the blade, midrib, vein, leaf, petiole, stem, node, internode, apical bud, axillary bud, flower, fruit, root, epidermis, cortex, vascular tissue (xylem and phloem)
• Leaf ultrastructure


  • Cuticle
  • Upper epidermis
  • Palisade mesophyll
  • Spongy mesophyll
  • Lower epidermis
  • Xylem (transports water and minerals from roots to leaves)
  • Phloem (transports food from leaves to rest of plant)
  • Chloroplasts (within mesophyll cells)
  • Stoma and guard cells
• Stem ultrastructure


  • Epidermis
  • Cortex
  • Pith
  • Vascular bundles containing xylem and phloem
  • Cambium
• Root ultrastructure


  • Epidermis
  • Cortex
  • Endodermis
  • Pericycle
  • Vascular tissue (xylem and phloem)
  • Root hairs
• Root hairs


  The primary function is water and nutrient acquisition by providing a large surface area for active uptake. They also anchor the plant, interact with soil microorganisms, and secrete acids to solubilize minerals.
• High temperatures


  Causes stomata to open, leading to increased transpiration and evaporative cooling. This helps regulate temperature but risks dehydration.
• Cold temperatures


  Induce cold acclimation, which decreases growth rate but increases freezing tolerance. This involves changes at the cellular and whole-plant level to deal with dehydration, ice crystal formation, biomolecule instability, and disruption to photosynthesis.
• Effects of heat stress vs cold stress


  • Heat stress: Change in phenology, increase in oxidative stress, reduced photosynthesis, inhibited seed germination, reduced biomass
  • Cold stress: Dehydration, altered metabolism, increased oxidative stress, membrane instability, protein disintegration, chlorophyll degradation
• Molecular level effects of cold


  • Decrease membrane stability
  • Metabolism retarded
  • Increase in oxidative stress (ROS production)
  • Reduction in antioxidant enzyme activities
  • Higher electrolyte leakage
  • Ion leakage or altered homeostasis
  • Protein disintegration
  • Chlorophyll degradation
  • Protoplast volume shrinkage
  • Leaves chlorosis and wilting
  • Reduced grain growth and yield
  • Inhibition of seed germination
• Physiological responses to cold exposure


  • Cellular dehydration and formation of intracellular ice crystals
  • Reduced root branching and root surface area
  • Reduced water and nutrient uptake
  • Changes in sensing and signaling
  • Changes in expression of cold-stress related genes
• Heat stress


  Physiological and cellular perturbations
• Cold stress


  Physiological and cellular perturbations
• Change in phenology


  • Increase in oxidative stress (ROS production)
  • Reduction in antioxidant enzyme activities
  • Reduced grain growth and yield
  • Decreased photosynthesis
  • Reduced stomatal conductance and CO₂ fixation
  • Damaged photosynthetic pigments
  • Inhibition of seed germination
  • Poor cell enlargement
  • Loss turgor
  • Reduction in biomass
  • Reduction in carbohydrate metabolism
  • Reduction in production of secondary metabolites
  • Dehydration
• Changes in sensing and signaling


  • Changes in expression of heat-stress related genes
  • Changes in positive/negative regulator gene expression
• Regulation of Extracellular Fluid


  • The composition of extracellular fluids is not precisely regulated in plants
  • Plants are fairly tolerant of changes in the solute concentration of the extracellular fluid providing the solute concentration is hypotonic to the solute concentration inside their cells
  • If the solute concentration of the extracellular fluid is hypertonic to the solute concentration of cytoplasm, water diffuses out of the cytoplasm, resulting in plasmolysis (shrinkage of the cytoplasm) and, potentially cell death
• Gaseous Exchange


  The rate of movement of water, carbon dioxide and oxygen between atmosphere and internal spaces is regulated by the degree of opening of stomata
• Stomata


  • Stomatal pores in the epidermis are bounded by two highly specialised guard cells
  • Guard cells have three structural features which explain their function: They are joined at their ends in pairs, their cell walls are thicker on the side nearest to the stomatal pore, and bands of inelastic cellulose fibres run around each cell
• Regulating stomata


  1. Stomatal movement is the result of changes in the turgor of the guard cells

  2. If water flows into the guard cells by osmosis, their turgor increases and they expand

  3. The relatively inelastic inner wall makes them bend and draw away from each other, opening the pore
• Stomatal opening


  1. Potassium ions move into the vacuoles

  2. Water moves into the vacuoles, following potassium ions

  3. The guard cells expand

  4. The stoma opens
• Stomatal closing


  1. Potassium ions move out of the vacuole and out of the cells

  2. Water moves out of the vacuoles, following potassium ions

  3. The guard cells shrink in size

  4. The stoma closes
• Stomatal opening & closing


  1. Hydrogen must go out, allowing entry of potassium and water for stomatal opening

  2. Removal of potassium and water leads to stomatal closure
• Stomata effectively open in response to blue light, especially under strong red light
• Abundant water


  Stomata open, photosynthesis occurs
• Water shortage


  Stomata close, photosynthesis reduced
• Photosynthesis


  • Plants' ability to manufacture their own food
  • Process of producing carbohydrates from carbon dioxide and water using sunlight energy
• Photosynthesis formula: 6CO2 + 6H2O + Energy => C6H12O6 + 6O2
• Photosynthesis, respiration, leaf water exchange, and translocation of sugar (Photosynthate) in a plant


  1. Light energy powers photosynthesis and respiration

  2. Sugars produced are stored as starch or transported as photosynthates

  3. Water and minerals enter through root hairs
• Mesophyll Cells


  • Sandwiched between the leaf's upper and lower epidermis
  • Contain numerous chloroplasts where photosynthesis takes place
• Chloroplast Structure


  • Outer and inner membranes
  • Thylakoid system with grana
  • Stroma
• Chlorophyll is the pigment found in chloroplasts that is responsible for trapping light energy from the sun
• If any of the ingredients for photosynthesis - light, water and carbon dioxide - is lacking, photosynthesis stops
• Photosynthesis


  Produces food, releases oxygen, uses carbon dioxide, needs light and water
• Respiration


  Releases energy, produces carbon dioxide, uses oxygen, occurs in the dark as well as light