Food Making and Growth in Plants

4.1 The Leaf

• Overview: The leaf is a key organ in flowering plants responsible for photosynthesis.
• Four Main Organs of a Flowering Plant:
  • Flowers (reproductive organs)
  • Leaves (photosynthesis)
  • Stem (support and transport)
  • Roots (anchoring and absorption)

Adaptations of a Leaf for Photosynthesis

• Leaf Structure Adaptations:
  • Flat and Wide Shape: Increases surface area for light collection and reduces gas diffusion distances.
  • Veins: Transport water from the soil to the cells.
  • Waxy Cuticle: Waterproof layer to prevent water loss.
  • Palisade Mesophyll: Primary photosynthetic tissue with closely packed cells near the surface for maximum light absorption; contains many chloroplasts that move within the cells based on light levels.
  • Spongy Mesophyll: Fewer cells and chloroplasts but numerous air spaces for gas exchange; facilitates carbon dioxide intake for photosynthesis and oxygen removal.
  • Lower Epidermis: Contains stomata for gas exchange and water vapor release, controlled by guard cells.
  • Vascular Bundles:
    • Xylem: Dead tissue that transports water and minerals from roots to leaves.
    • Phloem: Living tissue that transports photosynthetic products from leaves to other plant parts.
  • Chloroplasts: High surface area due to stacked membranes and chlorophyll.

Activity 4.2: Investigating Leaf Structure

• Examine a fresh leaf to identify the midrib and smaller veins.
• Calculate the surface area of the leaf using graph paper.
• Estimate the total leaf surface area of the entire plant.
• Use a microscope to observe a cross-section of a prepared leaf slide, identifying different tissues.
• Create a plan of the section and detailed drawings, labeling the different tissues and explaining their functions.

Summary

• Leaf internal structures are adapted for photosynthesis.
• Key tissues: waxy cuticle, epidermis, palisade mesophyll, spongy mesophyll, vascular bundles, stomata, and guard cells.
• Each tissue has a specific function (e.g., waxy cuticle prevents water loss, palisade mesophyll maximizes photosynthesis, stomata facilitate gas exchange).
• Light microscopy can be used to study leaf internal structures.

4.2 Photosynthesis

• Definition: Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.

• Equation: $carbon \ dioxide + water \xrightarrow[chlorophyll]{light \ energy} glucose + oxygen$

  • Chemical Equation: $6CO<em>2 + 6H</em>2O \xrightarrow[chlorophyll]{light \ energy} C<em>6H</em>{12}O<em>6 + 6O</em>2$

• Chlorophyll: A green pigment in chloroplasts that absorbs light energy.

• Importance of Photosynthesis: Provides food and oxygen, forming the basis of life on Earth.

Glucose Usage and Storage

• Immediate Use: Some glucose is used for respiration to provide energy for cell functions, growth, and reproduction.
• Conversion and Storage:
  • Glucose is converted to starch for storage because starch is insoluble and does not affect water balance.
  • Glucose is converted into fructose and sucrose for transport.
  • Glucose is converted into cellulose to form new cell walls.
  • Glucose, along with nitrates and other nutrients, is used to produce amino acids, which are then converted into proteins.
  • Glucose is converted into fats and oils (lipids) for storage in seeds and cell membranes.
  • Glucose is used to synthesize important large molecules, such as chlorophyll.

Factors Necessary for Photosynthesis

• Carbon Dioxide: Obtained from the air.
• Water: Absorbed from the soil.
• Light Energy: Captured by chlorophyll.
• Chlorophyll: Captures light energy.

Light-Dependent and Light-Independent Reactions

• Light-Dependent Reactions: Require light energy to split water molecules into hydrogen and oxygen, producing ATP.
• Light-Independent Reactions (Calvin Cycle): Use hydrogen and ATP produced in the light-dependent reactions to convert carbon dioxide into glucose.

Demonstrating the Need for Light

• Destarching: Depriving a plant of light for 2-3 days.
• Covering Leaves: Covering part of a leaf with black paper or foil to prevent light from reaching the covered area.
• Testing for Starch: Testing covered and uncovered leaves for the presence of starch using iodine solution.

Demonstrating Oxygen Production in Water Plants

• Collecting Gas: Collect gas produced by a water plant in light and dark conditions.
• Testing Gas: Test collected gas with a glowing splint to show the presence of oxygen.
• Sodium hydrogen carbonate: It may be added to the water to produce more carbon dioxide.

Activity questions

• Question 1: What did you observe during the four hours?
• Question 2: What happens to the glowing splint when plunged into the test tube?
• Question 3: What is the identity of the gas?
• Question 4: What conclusion can you make from this activity?

Need for Carbon Dioxide

• Carbon dioxide from the air is essential for synthesizing sugars during photosynthesis.
• Potassium hydroxide is used to absorb carbon dioxide from the air surrounding a leaf or a plant.
• Increasing carbon dioxide levels in high-intensity light increases the rate of photosynthesis.

Need for Water

• Water is the source of hydrogen needed to produce carbohydrates in photosynthesis.
• Heavy water containing the $^{18}O$ isotope of oxygen can be used to trace the role of water in photosynthesis, demonstrating that oxygen gas produced during photosynthesis comes from splitting water molecules (photolysis).

Need for Chlorophyll

• Variegated leaves with chlorophyll-containing and chlorophyll-free regions are used to demonstrate that chlorophyll is essential for photosynthesis.
• Regions of the leaf containing chlorophyll will test positive for starch after exposure to light, while chlorophyll-free regions will not.

Importance of Photosynthesis

• Energy Source: Converts solar energy into chemical energy available to life.
• Biomass Production: Produces approximately $35 × 10^{15}$ kg of new biological material annually.
• Oxygen Production: Produces oxygen as a waste product, essential for respiration.
• Carbon Dioxide Balance: Removes carbon dioxide from the atmosphere, helping to balance oxygen and carbon dioxide levels.

Photosynthesis in Water Bodies

• A significant portion of global photosynthesis occurs in water bodies, carried out by water weeds, algae, and phytoplankton.
• Phytoplankton produce over half of Earth's biomass.
• Ways are being sought to use algae as a source of human food and fuels.

Deforestation and Global Warming

• Deforestation reduces the amount of carbon dioxide removed from the atmosphere, leading to a build-up of carbon dioxide and global warming.
• Ethiopia has been actively replanting forests to combat deforestation.

Summary

• Photosynthesis equation: $6CO<em>2 + 6H</em>2O \xrightarrow[chlorophyll]{light \ energy} C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Glucose can be converted to insoluble starch for storage.
• Glucose is used in respiration to provide energy.
• Leaves are adapted for maximum photosynthesis.
• Carbon dioxide, water, chlorophyll, and light are essential for photosynthesis and can be demonstrated experimentally.
• Light-dependent reactions require light, while light-independent reactions do not depend directly on light.
• Photosynthesis balances oxygen and carbon dioxide in the atmosphere.
• Photosynthesis is the source of all new plant biomass, crucial in agriculture.
• Deforestation leads to carbon dioxide build-up and global warming.

4.3 Transport

Overview

• Osmosis: Plays a vital role in water uptake and movement within plants.
• Turgor: Rigidity and firmness maintained by water movement into cells via osmosis.

Water Uptake by Roots

• Concentration Gradient: Water moves from the soil (high water concentration) into the plant root cells (lower water concentration).
• Root Hairs: Increase the surface area for osmosis.
• Water Movement: Water moves from root hair cells to neighboring cells by osmosis until it reaches the xylem.

Active Transport

• Mineral Ions: Plants use active transport to absorb mineral ions from dilute soil solutions, requiring energy from respiration.

Transport Systems

• Phloem: Living tissue that transports organic materials (nutrients from photosynthesis) from leaves to the rest of the plant.
• Xylem: Dead tissue that carries water and mineral ions from the soil around the plant.

Key Differences

• Phloem: Transports nutrients, living tissue, bidirectional transport, requires energy.
• Xylem: Transports water and minerals, dead tissue, unidirectional transport (upward from roots), passive process (no energy required).

Importance of Transport

• Food Transport: Sugars produced in leaves are transported to all parts of the plant for respiration and growth.
• Storage: Sugars are converted to starch, which is osmotically inert, for storage in root tubers, stems, leaves, fruits, and seeds.
• Water and Mineral Transport: Minerals are needed for protein production; water is needed for photosynthesis and maintaining turgor pressure.

The Transpiration Stream

• Process: Water is taken into a plant through the roots and moves to the xylem tissue via osmosis.
• Transpiration: Water vapor loss from the leaf surface via stomata.

Factors Moving Water Upwards

• Root Pressure: Pushes water up from the bottom.
• Adhesive Forces: Between water and xylem walls support the water column.
• Cohesive Forces: Between water molecules pull them upwards as molecules evaporate from the leaf surface.
• Evaporation: Constant evaporation of water from leaves is the main pull.

Factors Affecting Transpiration

• Temperature: Higher temperature increases evaporation.
• Humidity: Water evaporates more rapidly into dry air than humid air.
• Wind: Removes water-vapor-rich air from around the leaf, maintaining a concentration gradient and increasing evaporation.
• Light: Speeds up transpiration because open stomata for plenty of photosynthesis.

Methods to Reduce Water Loss

• Closure of stomata to stop water loss.
• Waxy, waterproof cuticle on the leaf surface.
• Stomata located on the underside of the leaf.
• A thicker cuticle which is found in very hot environments.

Adaptations of Plants to Reduce Water Loss

• Thick, waxy cuticles.
• Hairy leaves that trap moisture.
• Reduced leaves (spikes) to minimize surface area.
• Stomata sunken into pits.
• Rolled leaves to trap moist air.

Transpiration and Agriculture

• Irrigation: Crop plants require adequate water to transpire and grow properly.
• Shelter: Crops in sheltered locations lose less water.
• Crop Selection: Choosing plants suited to local conditions improves yields.

Summary

• Water transport from roots to the rest of the plant happens passively in the xylem.
• Water uptake is through osmosis.
• Adhesive and cohesive forces support water column and water molecule movements.
• The movement of water through the plant is transpiration influenced by factors like temperature and wind.
• Farmers need to irrigate the crops as plants lose water by transpiration.
• Transport of water in plants can be demonstrated using simple experiments.
• Mineral salts are taken in through the roots using active transport.
• Organic materials such as sugars from photosynthesis are moved around the plant in the phloem in an active process.

4.4 Response in Plants

Overview

• Coordination: The ability of living organisms to take in information about their surroundings and react appropriately.
• Plant Hormones (Phytohormones): Chemical messengers that coordinate flowering, cell division, and cell elongation.

Germination of Seeds

• Seed Structure:
  • Food storage tissue (endosperm).
  • Embryo plant (plumule, radicle, cotyledons).
  • Testa (seed coat).
• Types of Seeds:
  • Monocotyledons (one seed leaf).
  • Dicotyledons (two seed leaves).
• Germination Process:
  • Seed absorbs water and swells.
  • Testa bursts, and the radicle emerges.
  • Radicle elongates and pushes the seed out of the ground.
  • The plumule emerges, and the first true leaves are produced.

Epigeal and Hypogeal Germination

• Epigeal Germination: Cotyledons are carried above the soil (e.g., bean, castor oil seeds).
• Hypogeal Germination: Cotyledons remain below the ground (e.g., maize, wheat).
• Examples of grains exhibiting hypogeal germination are wheat, sorghum and millet.

Plant Hormones and Growth

• Auxins (IAA): Promote cell elongation, apical dominance, and root growth.
• Gibberellins: stimulate plant growth and help seeds to break dormancy.
• Cytokinins: Stimulate cell division.
• Ethylene: Causes fruit to ripen.
• Abscisic Acid (ABA): Induces growth inhibition.

Tropic Responses

• Phototropism: The reaction of plants to stimuli that come from one direction.
• Geotropism: Movement in response to the stimulus of gravity.
  • Roots are positively geotropic
  • Shoots are negatively geotropic.
• Hydrotropism: roots respond to water as well and grow accordingly to the stimulus.

Investigating Tropic Responses

• Used a klinostat to rotate the plant so that all the parts receive equal amounts of gravity.
• Showed that growth is needed for growth to take place.
• When that side of plants is illuminated on one side this leads t a build up of IAA, which then leads to growth of that side.
• Upward growth for the root is influenced by the action of IAA, however in the root it in-fact inhibits growth.

Importance of Tropic Responses

• When there is positive hydrotropism roots come in touch with the water/ the much water and mineral salts that possible to the plant.
• The action of responding to light helps the leaves to become well exposed, whihc then maximises the amount of light that is available.

Summary

• Monocot and dicot seeds germinate differently.
• Plants have hormones, including auxins, gibberellins, and ethylene.
• Plant hormones control growth, respond to stimuli, and affect flowering and leaf fall.
• If you remove the leading shoot from a plant you remove the apical dominance – this is the effect of the auxin made in the lead shoot which inhibits the growth of side shoots.
• Sunlight affects/ slows upward growth and causes responses to light.
• They respond to water by hydrotropism
• Plants generate different levels of auxins depending on the stimulus, this the affects the plants growth.