Understanding Homeostasis

• Homeostasis is a self-regulating process that organisms use to maintain stability while adjusting to their environment, ensuring survival.
• If homeostasis is unsuccessful, it results in a disaster or the death of the organism.

Aspects of Homeostasis

• The three major components of homeostasis are a receptor, a control center, and an effector.
• The receptor gathers information and sends it to the control center.
• The control center processes the information and sends signals to the effector.

Need for Homeostasis in Plants

• Plants need to maintain water, oxygen, carbon dioxide, temperature and nutrient balances.

Feedback Mechanism

• Feedback mechanisms maintain hormone levels within desired limits, triggered by increases or decreases in hormone levels.
• Two types of feedback mechanisms include Positive and Negative feedback.

Negative Feedback

• Negative feedback involves the body sending signals to increase secretions when levels fall below normal, or decrease secretions when levels rise above normal, to restore balance.
• An increase in blood sugar level stimulates insulin secretion to maintain sugar level.
• A fall in blood levels stimulates glucagon secretion which stimulates the breakdown of glycogen to glucose to return sugar levels to normal.

Positive Feedback

• Positive feedback mechanisms amplify changes rather than reversing them, making them rare.
• Oxytocin is an example of this and it is released from the posterior pituitary gland during labor, stimulating contractions that intensify until birth.
• The process stops when the stimulus to pressure receptors ends.

Feedback Mechanism Explanation

• Feedback mechanism accelerates or slows down a process and regulate biochemical pathways.
• Negative feedback involves the accumulation of an end product that inhibits the action of the first enzyme, slowing the reaction.
• Positive feedback involves the end product stimulating an enzyme to increase overall production.

Water Potential

• Water potential is the potential energy of water in a system compared to pure water, when pressure and temperature are constant.
• Water potential can be measured with in a given environment or system, measuring the ability of water to flow freely in that environment.

Solution Types

• Isotonic solution: A solution with the same solute concentration as another solution, resulting in no net movement of water.
• Hypertonic solution: A solution with a higher solute concentration than another solution, causing water particles to move out of the cell.
• Hypotonic solution: A solution with a lower solute concentration than another solution, causing water particles to move into the cell.

Osmotic Potential or Solute Potential

• Osmotic potential is the measure of the potential of water molecules to move from a hypotonic to a hypertonic solution across a semi-permeable membrane.
• The water potential of pure water is zero.
• Solute potential, also called osmotic potential, is negative in plant cells and zero in distilled water.
• Cell cytoplasm has typical solute potential values that range from -0.5 to -1.0 MPa.
• Solutes reduce water potential by consuming the potential energy available in the water.

Osmoregulation

• Osmoregulation is the process by which an organism regulates water and electrolyte balance to maintain homeostasis.
• Fluids inside and surrounding cells consist of water, electrolytes, and nonelectrolytes.
• Electrolytes dissociate into ions when dissolved in water
• Nonelectrolytes do not dissociate into ions in water.

Types of plants based on Osmoregulation

• Hydrophytes
• Mesophytes
• Halophytes
• Xerophytes

Hydrophytes

• Hydrophytes are aquatic plants that either remain totally submerged or partially submerged, living in water-saturated soil.
• Macrophytes are common components of wetlands, where water content is high and oxygen concentration is very low.

Adaptations of Hydrophytes

• Thin and waxy cuticles exist in floating hydrophytes but this facilitates gaseous exchange and prevents excessive transpiration, but fully submerged plants lack these but can possess thin ones.
• Large air cavities are present in spongy and palisade mesophyll cells allowing gaseous exchange, water balance, and floatation.
• Roots are reduced to allow water to be directly up-taken by leaves and provide anchorage.
• Leaves are very thin in submerged hydrophytes which increases surface area to allow the rate of water to increase and encourages mineral diffusion.
• Some hydrophytes have leaves modified to be wider and flattened for flotation.

Mesophytes

• Mesophytes thrive in moderate conditions, neither aquatic nor water-scarce, and possess characteristics of both hydrophytes and xerophytes.
• They require a moderate concentration of water and temperature, with highly developed vascular and mechanical tissues.
• They contain branched roots and a root cap that protects the root tip and promotes geotropic movement.
• Mesophytes may develop perennating organs such as corms, rhizomes, and bulbs for storing food and water.
• Monocot mesophytes have fibrous root systems, while dicot mesophytes have tap root systems, along with abundant root hairs.
• Leaves are comparative thin and large increase the surface area , aiding light absorption and photosynthesis.
• Waxy cuticles encircle the epidermis, preventing water loss.
• Developed and differentiated mesophylls (palisade and spongy parenchyma) aid gaseous exchange.
• Stomata are typically found on the lower leaf surface, minimizing excessive evaporation and stays open unless extreme water loss occurs.
• Well-developed structures allow plants to keep water balance and conduction of water and minerals.
• Xylem facilitates water absorption from the roots.
• Phloem helps in the conduction of organic minerals all around the plants.
• Mesophytes in rainy climates have hydathodes, special organs that exude excessive water as droplets (guttation).

Halophytes

• Halophytes are salt-tolerant plants living in high-salinity environments like saline deserts, mangrove swamps, marshes, and seashores.
• Pneumatophores compensate for the lack of soil aeration in coastal regions, developing negatively geotropic roots.
• Stilt roots offer anchorage in muddy or loose sandy soils such as Rhizophora mucronata
• Adventitious roots provide additional support to the plants, developing from the basal parts of tree trunks.
• Leaves are typically thick, entire, succulent, small, and often glassy in appearance.
• Stems and leaves of halophytes are covered with trichomes to help them adapt to their environment.
• Submerged marine halophytes have thin leaves with poorly developed vascular systems and frequent green epidermis to absorb nutrients directly.
• Some evolved salt glands to excrete excess salt from plant tissues, and these are collectively termed recreto-halophytes.

Xerophytes

• Xerophytes are adapted to dry environments through mechanisms that prevent water loss or store water.
• Succulents have thickened, fleshy parts to retain water, deriving from the Latin word "sucus" for "juice" or "sap."
• Leaf Adaptations
• Very few stomata
• Sunken stomata
• Hairs surrounding stomata
• Needle-shaped or small leaves.
• Thickened waxy cuticle.
• Other xerophytic adaptations that might be present include:
• Waxy leaf coatings
• The ability to drop leaves during dry periods
• The ability to reposition or fold leaves to reduce sunlight absorption
• The development of a dense, hairy leaf covering.
• Deep roots
• Shallow roots

Excretion in Plants

• Plants excrete through different mechanisms, without special organs.
• Waste products of respiration and photosynthesis are repurposed as raw materials for each other.
• Oxygen, a byproduct of photosynthesis, is used in respiration, and carbon dioxide from respiration is used in photosynthesis.
• Gaseous wastes such as oxygen, carbon dioxide, and water vapor are removed through stomata and lenticels.
• Some wastes are stored in leaves and bark, which are shed to eliminate the waste.
• Other wastes (tannins, resins, gum, rubber, essential oils) are rendered harmless and stored as solid bodies.
• Excretion is less problematic in plants because they reuse waste for anabolic processes and photosynthesis, utilizing water and CO2 from respiration.