Form 2 Biology Online Classes for JCE Exam Success

Success Criteria

Describe the structure and functions of plants.
Discuss the functions of leaves, stems, and roots.
Identify the internal structure of flowering plants (leaves, stems, and roots).
Explain the functions of parts of a leaf (cuticle, epidermis, stomates, mesophyll cells, veins).
Identify the distribution of stomates on the upper and lower sides of a leaf.
Describe the process of photosynthesis.
Discuss the conditions and raw materials necessary for photosynthesis (light, carbon dioxide, and chlorophyll).

External Leaf Structure

Petiole: The narrow stalk attaching the leaf to the stem.
Lamina: The photosynthetic part of the leaf, thin and flat for maximum light exposure.
Midrib: The thick central vein, containing vascular bundles for support and transport of water and nutrients.
Veins: Smaller bundles extending from the midrib, providing water supply and structural support.

Leaf Structure in Relation to Photosynthesis

A leaf consists of a green lamina made from thin-walled cells supported by a network of veins.

Internal Leaf Structure

Cuticle:
- Outermost layer, waxy, transparent, and waterproof.
- Minimizes water loss and protects inner tissues.
Epidermis:
- One-cell-thick layer covering the leaf.
- Protects against damage and disease; secretes the cuticle.
Stomata:
- Small pores on the epidermis allowing gas exchange (CO₂ in, O₂ out).
- Aid in transpiration.
Guard Cells:
- Control stomata opening and closing.
- Bean-shaped with chloroplasts; thicker inner walls.
Differences between Guard Cells and Epidermal Cells:
- Guard cells perform photosynthesis; epidermal cells do not.
- Epidermal cells are transparent; guard cells are not.
- Shape differences: guard cells are bean-shaped.
Adaptations of Guard Cells:
- Thicker walls to facilitate opening.
- Chloroplasts produce sugars, increasing osmotic pressure.
Palisade Mesophyll:
- Main site of photosynthesis with closely packed cells rich in chloroplasts.
- Located just below the epidermis to maximize sunlight absorption.
Spongy Mesophyll:
- Irregular cells with airspaces for gas circulation.
- Fewer chloroplasts than palisade mesophyll.

Photosynthesis

Definition: The process by which plants convert light energy into chemical energy.
Raw Materials: Light, carbon dioxide, and chlorophyll are essential for photosynthesis.

Conditions Necessary for Photosynthesis:

Sufficient light
Availability of carbon dioxide
Presence of chlorophyll in the leaf

Vascular Bundle/Tissue

Found in the midrib and leaf veins.
Composed of xylem and phloem tissues:
- Xylem: Conducts water and dissolved minerals from roots to other parts of the plant.
- Phloem: Translocates manufactured food from photosynthetic areas to other plant parts.

Chloroplast

Organelle where photosynthesis occurs.
Oval-shaped and double-membrane bound.
Contains membranes called lamellae suspended in a fluid-filled matrix.

Leaf Veins and Midrib

Contain vascular bundles (xylem and phloem) for substance transport.
Phloem carries materials from leaves, while xylem brings water to leaves.

Modified Leaves and Their Functions

Leaf Tendrils: Support and climbing.
Spines: Protect against herbivores (e.g., cactus spines).
Pitcher Leaves: Trap insects for nutrients.

Leaf Variations

Leaf Tips: Can be pointed or rounded.

Leaf Arrangement on the Stem

Venation:
- Network or Arcuate Veins: Dicotyledonous plants.
- Parallel Veins: Monocotyledonous plants.

Compound Leaves

A leaf with multiple leaflets attached to a petiole.
Types:
1. Pinnate
2. Bipinnate
3. Trifoliate
4. Digitate

Functions of Leaves

Sites for photosynthesis.
Storage of water and food in succulent leaves.
Reproductive organs in some plants.

Types of Stems

Erect Stem
Creeping Stem
Climbing Stem
Underground Stem

Types of Underground Stems

Tuber: Swollen tips of underground branches (e.g., Potato).
Rhizome: Fleshy, horizontal stems (e.g., Ginger).
Corm: Condensed structures growing vertically (e.g., Colocasia).
Bulb: Disc-shaped stems with fleshy leaves (e.g., Onion, Garlic).

External Parts of Stems and Their Functions

Common features: leaves, nodes, internodes, terminal buds, and auxiliary buds.
Some stems may have thorns or tendrils for support.

Internal Structure of Stems

Dicotyledonous Stems

Epidermis: Upper protective layer.
Cortex: Stores water and other substances; includes pith (ground tissue).
Vascular bundles arranged in a ring with phloem alternating with xylem.

Monocotyledonous Stems

Vascular bundles arranged in a ring, alternating xylem and phloem.

Similarities between Monocotyledonous and Dicotyledonous Stems

Both protect and conduct water, salts, and food.
Both contain epidermis, cortex, pericycle, and vascular bundles.

Differences and Similarities between Monocotyledonous and Dicotyledonous Stems

Monocotyledonous Stems:

Vascular bundles are many and scattered.
Some have hollow pith or pith is absent.
No cambium layer, so cannot undergo secondary growth; very little cortex.

Dicotyledonous Stems:

Vascular bundles are few and arranged in a concentric ring near the epidermis.
Pith is large and well developed.
Presence of cambium allows for secondary growth; cortex has several layers of cells.

Differences between the Internal Structure of a Root and a Stem

Roots:

Has root hairs.
No cuticle present.
Xylem and phloem are arranged alternately.
In xylem, the small vessels are towards the outside; cortex is the widest tissue.

Stems:

No root hairs.
Cuticle is present.
Xylem and phloem are arranged on the same radii.
In xylem, the smallest vessels are towards the inside; pith is the widest tissue.

Transport Structures of a Flowering Plant

Xylem Vessels: Transport water and mineral salts from the soil to other parts of the plant.
Phloem Vessels: Translocate manufactured food from leaves to other parts of the plant.

Adaptations of Xylem Vessels

Lignified/Thickened Walls: Prevent collapsing.
Narrow Structure: Facilitates capillary action.
No Cross Walls: Allows for a continuous flow of water.
Bordered Pits: Enable lateral movement of water.

Functions of Stems

Support leaves, exposing them to sunlight for photosynthesis.
Store food substances (e.g., starch and sucrose) in their cortex.
Transport substances from roots to leaves and vice versa.
Support flowers, facilitating pollination and seed dispersal.
Thorny stems deter animals from grazing.

Types of Root Systems

Tap Root: Develops from the radicle, consisting of one main branch and sub-branches (e.g., dicot roots).
Adventitious Roots: Develop from other parts of the plant, forming a fibrous root system (e.g., monocot roots).

Modified Roots and Their Functions

Aerial Roots: Absorb moisture from the air.
Prop Roots: Grow from the stem into the soil for support.
Buttress Roots: Provide strong anchorage for tall trees.
Climbing Roots: Help plants attach to solid supports.
Breathing Roots: Absorb gases from the air (e.g., pneumatophores of mangrove trees).

Functions of Roots

Anchor plants firmly in the soil.
Absorb water and mineral salts.
Store food substances.

Photosynthesis

The process by which organisms, mainly plants, produce food from carbon dioxide and water using sunlight energy, often involving chlorophyll.

Importance of Photosynthesis

Regulates carbon dioxide and oxygen levels in the environment.
Enables autotrophs to meet their nutritional needs.
Converts sunlight energy into chemical energy for use by other organisms.

Stages of Photosynthesis

Light Stage:
- Occurs in the grana of chloroplasts.
- Photolysis of Water: Splitting of water molecules into hydrogen ions and oxygen gas.
- Formation of ATP: Energy from sunlight is used to form ATP, which stores energy for the dark stage.
Dark Stage:
- Light-independent reactions occurring in the stroma.
- Combines carbon dioxide with hydrogen ions to form simple carbohydrates.
- Simple sugars can be quickly converted to starch to prevent excessive water loss through guard cell activity.

Word and Chemical Equations for Photosynthesis

Word Equation: Carbon Dioxide + Water → Glucose + Oxygen
Chemical Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Exchange of Gases in the Leaf

Carbon Dioxide: Enters through stomata; used in photosynthesis.
Oxygen Production: Oxygen is produced during photosynthesis; some is expelled through stomata, and some is used for respiration.
Respiration: Oxygen enters the plant through stomata, especially at night; carbon dioxide is released as a by-product.

Functions of Glucose in Plants

Respiration: Provides energy.
Cell Wall Formation: Converted to cellulose or lignin.
Protein Formation: Combined with nitrogen.
Lipid Formation: Used to create lipids.

Storage of Glucose

Excess glucose forms starch through condensation; starch is stored in leaves, stems, or roots. When needed, starch is converted back to glucose.

Factors Affecting the Rate of Photosynthesis

Sunlight:
- No photosynthesis in the dark; slow in dim light.
- Increases with light intensity up to a maximum point.
Carbon Dioxide:
- Inadequate supply lowers photosynthesis rates.
- Increases with concentration until it levels off.
Temperature:
- Slow at low temperatures; increases with temperature until around 40°C, then slows due to enzyme denaturation.
Chloroplasts:
- More chlorophyll increases photosynthesis.
Water:
- Lack of water leads to wilting; excess water increases photosynthesis to a maximum point.

Experiments on Photosynthesis

Testing for Starch in a Leaf:
- Procedure:
  1. Boil leaf in alcohol to remove chlorophyll.
  2. Rinse in hot water to soften.
  3. Apply iodine solution.
- Results: Blue-black indicates starch presence; brown indicates absence.
Testing Light Necessity:
- Procedure:
  1. Place potted plant in the dark for 24 hours.
  2. Cover parts of the leaf with black foil.
  3. Expose to sunlight for 5 hours.
  4. Test the leaf for starch.
- Results: Blue-black in exposed parts; no change in covered parts confirms light is necessary.
Testing Chlorophyll Necessity:
- Procedure:
  1. Collect a variegated leaf (with and without chlorophyll).
  2. Test both parts for starch.
- Results: Only the chlorophyll-containing part turns blue-black, confirming chlorophyll’s necessity.
Testing Carbon Dioxide Necessity:
- Procedure:
  1. Place a potted plant in the dark for 24 hours.
  2. Set up two flasks, one with sodium bicarbonate (provides CO₂) and the other with sodium hydroxide (absorbs CO₂).
  3. Leave in sunlight for 5 hours.
  4. Test leaves for starch.
- Results: Leaf from the flask with CO₂ turns blue-black; the other does not, confirming CO₂ is necessary.

Testing if Oxygen is Produced During Photosynthesis

Materials:

Aquatic plants
Collecting tube
Water funnel
Glowing splint

Procedure:

Place a funnel over the aquatic plants and set up a collecting tube underneath.
Leave the apparatus in sunlight for about 5 hours, allowing the water level to drop as oxygen is collected.
Carefully remove the collecting tube and introduce a glowing splint near its mouth.

Results:

The glowing splint reignites, indicating the presence of oxygen.

Conclusion:

Oxygen is produced during photosynthesis.

Questions

At what temperature was the rate of oxygen production the highest?

The specific temperature would need to be referenced from the graph, but generally, it would be the peak point on the curve.

How does temperature affect the rate of oxygen production?

As temperature increases, the rate of oxygen production typically increases up to an optimal temperature. Beyond this point, the rate may decrease due to enzyme denaturation and other stress factors.

What is the explanation for the lowest oxygen production at 50°C?

At 50°C, enzymes involved in photosynthesis are likely denatured, leading to reduced metabolic activity and lower oxygen production. High temperatures can disrupt cellular processes and damage plant tissues, negatively impacting photosynthesis.