- This is the process by which organisms obtain and Assimilate nutrients.
- There are two modes of nutrition; Autotrophism and Heterotrophism.
- This is where living organism manufacture its own complex food substances from simple substances such as carbon (iv) oxide, water, light or chemical energy.
- Where sunlight is used as a source of energy, the process is referred to as photosynthesis.
- Photo means light while synthesis means to make.
- Some none green plants make their own food using energy obtained from certain chemicals through a process called chemosynthesis.
- Organisms that make their own food are referred to as autotrophs.
- This is where organisms take in complex food materials such as carbohydrates, proteins and fats obtained from bodies of plants and animals.
- Organisms that feed on already manufactured foods are called Heterotrophs.
External Structure of a Leaf
A leaf is a flattened organ which is attached to the stem or a branch of a plant.
Parts of a leaf
Lamina: This is the flat surface. It is green in colour and contain the photosynthetic tissue.
Midrib: This is a thick structure running through the middle of the leaf
Veins: They arise from the midrib to forming an extensive network of veins.
Leaf Apex: This is the tip of the leaf and usually it is pointed.
Petiole: It attaches the leaf to the stem or branch.
In some monocotyledonous plants the leaves are attached to the stem by the leaf sheath.
Practical Activity 1: To examine the External Features of a Dicotyledonous and Monocotyledonous leaf
Study Question 1
Internal Structure of a Leaf
- Internal structure of the leaf is composed of the following parts.
- It is a thin waterproof and transparent layer that coats the upper and lower surfaces of the leaf.
- It reduces excess water loss and protects the inner tissue of the plant against mechanical injury.
- It also prevents entry of disease causing micro organisms.
- Since it is transparent, it allows penetration of light for photosynthesis.
- It is a one cell thick tissue on both the upper and lower leaf surfaces.
- It secretes the cuticle and also protects the inner tissues from mechanical damage and prevents entry of pathogens.
- Epidermal cells have no chloroplast except the guard cells.
- Guard cells are special bean shaped cells. They have chloroplast and are able to carry out photosynthesis hence controlling the opening and closing of the stomata.
- Air moves into and out of the leaf through the stomata.
- Palisade layer.
- This is layer of cells located beneath the upper epidermis.
- It is made of cylindrical shaped cells closely packed together. They have numerous chloroplasts containing chlorophyll.
- Their position and arrangement enables them to receive maximum light.
- Spongy Mesophyll Layer.
- This is below the palisade layer. The cells are irregularly shaped and loosely packed creating large air spaces in between them.
- The air spaces allow gases to diffuse in between the cells. They contain fewer chloroplasts as compared to the palisade cells.
- Leaf Veins.
- Each vein is a vascular bundle consisting of xylem and phloem.
- Xylem conducts water and mineral salts from the roots to the leaves while the phloem translocates manufactured food from the leaves to the rest of the plant.
Study Question 2
Adaptations of Leaves to Photosynthesis.
- Broad and flat lamina to increase surface area of Carbon (IV) oxide and sunlight absorption.
- Thin transparent cuticle and upper epidermis; to allow easier penetration of light to photosynthetic cells;
- Thin; for faster diffusion of gases;
- Palisade cells placed next to the upper surface; to trap maximum light for photosynthesis;
- Palisade cells with numerous chloroplasts; to trap maximum amount of light for photosynthesis;
- Large/ intercellular air spaces in the spongy mesophyll layer; for storage of Carbon (IV) oxide for easier gaseous exchange;
- Waxy water proof cuticle; to reduce water loss sand reflect excess light;
- Leaf mosaic/ non-overlapping leaves; for maximum exposure to light;
- Guard cells, modified cells to open and close stomata; to control amount of water loss from the leaf and allows gaseous exchange;
- Leaves have leaf veins; xylem to conduct water to photosynthetic cells, Phloem to translocate products of photosynthesis to other parts of plant;
- They are disc shaped organelles found in the cytoplasm of plant cells.
- Each chloroplast has a double membrane; the inner and outer membrane.
- Chloroplasts are made of layers of membranes called lamellae contained in a fluid matrix called stroma.
- Several lamellae come together to form the granum (grana).
- Granum contains chlorophyll molecules and other photosynthetic pigments.
- The stroma contains enzymes that speed up the rate of photosynthesis.
Practical Activity 2: To Observe Distribution of Stomata
Study Question 3.
The Process of Photosynthesis
- The raw materials for photosynthesis are; water and carbon (IV) oxide. The process however requires the presence of sunlight energy and chlorophyll pigment.
- The products of photosynthesis are glucose and oxygen. The process can be summarized using an equation as shown below.
6H2O + 6CO2 ———-> C6H12O6+ 6O2
Water + Carbon (IV) oxide Glucose + Oxygen.
The above chemical equation translates as:
Six molecules of water plus six molecules of carbon (IV) Oxide produce one molecule of sugar plus six molecules of oxygen
- The process of photosynthesis is however more complex than shown in the above equation and can be divided into two stage; the light and dark stages.
Light stage (Light Dependent Stage)
– Occurs in the grana containing chlorophyll which traps / absorbs sun light energy.
– This Energy is used to split water molecules into hydrogen ion and oxygen gas.
– This process is called photolysis of water and is shown below.
2H2O 4H + O2
(Water) Hydrogen atom Oxygen
– Hydrogen atoms produced here enter into the dark stage.
– Oxygen gas removed through stomata or is used for respiration within the plant;
– Some Light energy is used in Adenosine Triphosphate (ATP) formation; ATP an energy rich compound.
– ATP is later used in the dark stage.
Dark stage. (Light Independent Stage)
– Carbon (IV) oxide combines with hydrogen atoms to form glucose/simple carbohydrate.
– This is called Carbon (IV) Oxide fixation.
Carbon (IV) oxide + Hydrogen Atom Simple Carbohydrate
CO2 + 4H C6H12O6
– This stage takes place in the stroma and proceeds whether light is present or not.
– ATP Energy from light stage is used to provide the required energy in this reaction;
– Simple sugars formed are used for respiration to provide energy or are converted to storable forms e.g lipids, proteins, starch, cellulose, etc.
Study Question 4
Practical Activity 3: To Investigate the Presence of Starch in a Leaf.
Study Question 5
Factors Affecting the Rate of Photosynthesis
- Light Intensity.
Increase in light intensity increase the rate of photosynthesis up to a certain level where it slows down and finally levels off.
Very bright sunshine may damage the plant tissues due to high amount of ultra violet light.
Light quality or light wavelength also affects the rate of photosynthesis.
Red and blue wavelengths of light are required by most plants for photosynthesis.
2. Carbon (IV) oxide concentration
Increase in Carbon (IV) oxide concentration increases the rate of photosynthesis linearly up to a certain level after which it slows down and levels off
Carbon (IV) oxide concentration
Photosynthesis is an enzyme controlled process, therefore increase in temperature increase the rate of photosynthesis up to the optimum temperature.
Increase in temperature beyond the optimum decreases the rate sharply as the enzymes become denatured.