Tissues and Organs in Plants

Tissues and Organs in Plants

• Definition of Tissue: A group of similar specialised cells that work together to perform a specific function.

  • Examples of tissues in plants:

  • Epidermal Tissue: Covers plant surface and protects them.

  • Palisade Mesophyll: Contains numerous chloroplasts for photosynthesis.

  • Spongy Mesophyll: Contains large air spaces and large surface area to increase diffusion of gases.

  • Xylem: Carries water and dissolved mineral ions from roots upwards.

  • Phloem: Transports glucose from leaves around the plant.

  • Meristem: Found in growing tips of roots and shoots, capable of differentiation.

Organs in Plants

• Definition of Organ: A group of tissues that work together.

• Organs collaborate to form organ systems (e.g., stems, roots, leaves).

  • Leaf: An organ made up of several tissues:

  1. Epidermal Tissues: Covered with a waxy cuticle to reduce water loss.

  2. Upper Epidermis: Transparent to allow light to pass through.

  3. Palisade Mesophyll: Contains many chloroplasts where photosynthesis takes place.

  4. Xylem and Phloem: Form a network of vascular bundles that deliver nutrients and water and take away glucose, also providing structural support.

  5. Lower Epidermis: Contains many stomata which allow gases to diffuse in and out of the leaf.

  6. Guard Cells: Control the opening and closing of stomata.

Transport Systems in Plants

• Transpiration: Movement of water through leaves.

  • Transport of Water and Minerals:

  • Xylem vessels carry water and mineral ions from roots to the rest of the plant.

    • Composed of dead cells with no end walls between them, creating a continuous tube.

    • Strengthened by lignin to withstand high water pressures.

    • This upward movement of water is known as transpiration.

  • Phloem: Transports glucose from leaves to other parts of the plant.

    • Composed of living cells which require energy to function.

    • Facilitated by small pores in the end walls allowing cell sap to flow.

    • Translocation: Movement of glucose and other sugars throughout the plant.

    • Transport occurs in both directions.

Evaporation and Transpiration

• Transpiration Mechanism:

  • Caused by evaporation and diffusion of water from a plant’s surface (mostly leaves).

  • Evaporation creates a slight shortage of water in the leaf, leading to more water being drawn from the rest of the plant through xylem cells to replace it, creating a constant transpiration stream.

  • Stages of Transpiration:

  1. Water enters root hair cells by osmosis due to hypertonic conditions.

  2. Water moves from cell to cell through the root by osmosis along the concentration gradient.

  3. Water enters the xylem vessels at the center of the root.

  4. This is followed by the evaporation of water through stomata, assisted by diffusion.

Factors Affecting Transpiration

• Transpiration rate is influenced by four main factors:

  1. Light Intensity:

  • Higher light intensity increases the rate of transpiration by opening stomata, allowing more water to leave.

  • Nighttime results in a lower rate of transpiration as stomata close.

  1. Temperature:

  • Higher temperatures increase the rate of transpiration as water evaporates and diffuses out more quickly.

  1. Air Flow:

  • Increased air flow enhances transpiration because it maintains a high concentration gradient for diffusion by moving away water vapour surrounding the leaf.

  • Low air flow results in a lower transpiration rate.

  1. Humidity:

  • Higher humidity reduces transpiration rates because the concentration difference between inside and outside the leaf decreases.

  • Lower humidity increases transpiration rates as the concentration gradient favors evaporation from stomata.

The Stomata

• Guard Cells: Adapted for opening and closing stomata:

  1. Kidney-shaped structure regulates the size of the stomata.

  2. When the plant has adequate water, guard cells become turgid and plump, opening the stomata for gas exchange necessary for photosynthesis.

  3. When water is scarce, guard cells become flaccid, closing the stomata to prevent water loss.

  4. Guard cells have thin outer walls and thickened inner walls to facilitate the opening and closing process.

  5. Sensitive to light; typically, guard cells close at night to conserve water.

  6. Stomata are generally more prevalent on the undersides of leaves to reduce water loss.

Photosynthesis

• Definition: A chemical reaction occurring in plants that converts carbon dioxide and water into glucose and oxygen, occurring in chloroplasts containing chlorophyll.

• Chemical Equation:
  $ext{6CO}<em>2 + ext{6H}</em>2 ext{O} <br>ightarrow ext{C}<em>6 ext{H}</em>{12} ext{O}<em>6 + ext{6O}</em>2$

• Photosynthesis is an endothermic process requiring light energy that is captured by chlorophyll.

• Adaptations for Efficient Photosynthesis:

  • Leaves are wide for a large surface area to absorb more light.

  • Leaves are thin to minimise diffusion distance for gases.

  • Chloroplasts within leaf cells to capture light.

  • Vascular veins ensure water transport via xylem to the leaf and nutrients from phloem.

  • Air spaces facilitate CO₂ access to cells and O₂ diffusion out.

  • Guard cells regulate stomatal opening to manage water loss and gas exchange.

Uses of Glucose in Plants

• Glucose produced through photosynthesis has various uses:

  1. Respiration: Provides energy for the plant, converting glucose for cellular processes.

  2. Cellulose Production: Converted from glucose to make cell walls, providing strength and support to the plant.

  3. Protein Synthesis: Glucose combined with nitrate ions forms amino acids, subsequently synthesized into proteins essential for growth and repair.

  4. Storage: Glucose can be converted into lipids for seed storage or stored as starch within roots.

Factors Affecting the Rate of Photosynthesis

• Law of Limiting Factors: A process depending on two or more factors is limited by the factor in shortest supply.

  • Factors affecting photosynthesis include:

  • Light Intensity: Increasing light intensity increases the rate of photosynthesis until a certain point.

  • CO₂ Concentration: Low concentrations reduce rates; increases beyond optimal levels have no effect.

  • Temperature: Increasing temperatures enhance photosynthesis until the enzymes denature.

  • Chlorophyll: Affected by disease or stress, thus potentially limiting photosynthesis.

Creating Ideal Conditions for Farming

• Greenhouse Practices:

  1. Use of greenhouse gases to trap heat ensures temperature does not become limiting.

  2. Deployment of artificial lights to support photosynthesis.

  3. Increasing CO₂ concentrations using paraffin heaters.

  4. Enclosed environment protects plants from pests.

  5. Use of fertilizers to replenish soil minerals.

Rate of Photosynthesis Practical

• Use of Pondweed to Measure Photosynthesis:

  • Based on oxygen release corresponding to the rate of photosynthesis:

  1. A light source, such as a lamp, is positioned at a specific distance from the pondweed.

  2. Leave the pondweed to photosynthesize for a defined time; oxygen bubbles will form indicating photosynthesis activity.

  3. Collect oxygen released in a capillary tube using a syringe, measuring the bubble length against a ruler.

  4. Control Variables: Temperature, time, light distance.

  5. Dependent Variable: Volume of oxygen produced, Independent Variable: Distance from the light source.

Respiration and Metabolism

• Definition of Respiration: The process of transferring energy from glucose, crucial for cell function in both plants and animals; occurs in every cell.

• Type of Reaction: Exothermic, releasing energy to the environment. Functions of energy derived from respiration include:

  • Building larger molecules from smaller subunits.

  • Breaking down large molecules into smaller units.

  • Facilitating active transport.

  • Allowing muscle contraction.

• Cells with the Greatest Number of Mitochondria:

  1. Muscle Cells: High energy demand for contraction.

  2. Liver Cells: Involved in numerous chemical reactions.

  3. Sperm Cells: Require energy for motility and fertilization.

• Metabolism: The sum of all chemical reactions in the body, including:

  • Converting glucose into other compounds (e.g., starch, glycogen, cellulose).

  • Forming lipids from fatty acids and glycerols.

  • Protein synthesis and breakdown of proteins into urea.

Aerobic and Anaerobic Respiration

• Aerobic Respiration:

  • Uses oxygen to yield energy from glucose. It is the most efficient method, occurring continuously in plants and animals, primarily in mitochondria.

  • Equation:
    $ext{Glucose} + ext{Oxygen} <br>ightarrow ext{Carbon Dioxide} + ext{Water} + ext{Energy (ATP)}$

• Anaerobic Respiration:

  • Occurs without oxygen, involving muscle cells during strenuous activity.

  • Equation in Muscle Cells:
    $ext{Glucose} <br>ightarrow ext{Lactic Acid} + ext{Energy}$

  • Less energy is produced; useful in short bursts of intense exercise but leads to lactic acid buildup.

  • In plants and yeast, anaerobic respiration results in ethanol and carbon dioxide, also known as fermentation:

  • Fermentation Equation: $ext{Glucose} ightarrow ext{Ethanol} + ext{Carbon Dioxide}$

    • Applications of Fermentation:

    • Production of CO₂ for rising bread.

    • Alcoholic beverages through the fermentation process.

Exercise and Respiration

• Exercise Impact on Respiration:

  • Increased exercise raises energy demand, subsequently increasing the rate of aerobic respiration.

  • During vigorous activity, if oxygen demand surpasses availability, the body resorts to anaerobic respiration, resulting in lactic acid build-up leading to fatigue.

  • Long exercises impose oxygen debt, requiring extra oxygen post-exercise to metabolize accumulated lactic acid into harmless substances.

  • Oxygen Debt: The additional oxygen required to deactivate lactic acid back to glucose in liver cells.

• Physiological Responses During Exercise:

  • Heart and breathing rates increase due to the higher oxygen demand.

  • Increases in the volume of oxygen absorbed and carbon dioxide expelled.

  • Adaptations in muscle cells, e.g., high mitochondrial content for efficient aerobic challenges.

  • Glycogen reserves are established for energy storage and usage.