Topic 5: Nutrition in Plants and Animals

1. Introduction to Nutrition

Nutrition is the process by which living organisms acquire and process food nutrients for energy, growth, tissue repair, and defense against disease.

Nutritional modes are broadly categorized into:

• Autotrophic Nutrition: Organisms manufacture their own organic food from simple inorganic raw materials (water and carbon dioxide) using external energy sources.
  • Photoautotrophy: Uses light energy (e.g., green plants, algae, cyanobacteria).
  • Chemoautotrophy: Uses chemical energy from oxidation reactions (e.g., nitrifying bacteria).
• Heterotrophic Nutrition: Organisms feed on complex organic food manufactured by other organisms.
  • Holozoic: Ingesting solid organic food, digesting it, and absorbing soluble products (e.g., humans, amoeba).
  • Saprophytic (Saprotrophic): Feeding on dead and decaying organic matter by secreting extracellular enzymes (e.g., fungi, mucor, bacteria).
  • Parasitic: Obtaining nutrients from a living host, causing it harm (e.g., tapeworms, dodder plant).

2. Photosynthesis in Plants

Photosynthesis is the process by which green plants manufacture glucose from carbon dioxide and water using light energy trapped by chlorophyll, releasing oxygen as a byproduct.

$$\text{6CO}_2 + \text{6H}_2\text{O} \xrightarrow[\text{Chlorophyll}]{\text{Light Energy}} \text{C}6\text{H}{12}\text{O}_6 + \text{6O}_2$$

Stages of Photosynthesis

1. Light-Dependent Stage (Photochemical Stage):
   • Occurs in the grana of chloroplasts.
   • Chlorophyll molecules absorb sunlight energy.
   • Part of this energy splits water molecules into hydrogen ions and oxygen gas (photolysis): $$\text{2H}_2\text{O} \xrightarrow{\text{Light Energy}} \text{4H}^+ + \text{4e}^- + \text{O}_2$$
   • Oxygen is released as a waste product, while ATP and reduced NADP are formed for the next stage.
2. Light-Independent Stage (Dark Reaction / Carbon Fixation):
   • Occurs in the stroma of chloroplasts.
   • Does not directly require light, but depends on products from the light stage (ATP and NADPH).
   • Carbon dioxide is combined with hydrogen ions to form glucose molecules through a series of enzyme-controlled steps (the Calvin Cycle).

External Factors Affecting Photosynthesis (Limiting Factors)

• Light Intensity: Rate increases with light intensity until a saturation point where another factor becomes limiting.
• Carbon Dioxide Concentration: Higher $\text{CO}_2$ concentration increases photosynthetic rate up to a plateau.
• Temperature: Photosynthesis is catalyzed by enzymes; rates double for every $10^\circ\text{C}$ rise up to an optimum ($30^\circ\text{C} - 35^\circ\text{C}$), after which enzymes denature.

3. Dentition in Mammals

Mammals are heterodonts (possessing different types of teeth specialized for specific functions) and diphyodonts (having two sets of teeth: milk/deciduous teeth and permanent teeth).

Types of Teeth and Their Adaptations

• Incisors: Chisel-shaped with sharp edges; used for biting and cutting food.
• Canines: Pointed and sharp; used for tearing flesh and gripping prey (prominent in carnivores; reduced or absent in herbivores as a diastema).
• Premolars: Broad crowns with cusps; used for grinding and crushing food.
• Molars: Larger flat surfaces with multiple cusps at the back of the jaw; used for thorough chewing and grinding.

Dental Formulas

A dental formula shows the number of each type of tooth in one half of the upper and lower jaws. Total teeth = Sum of formula $\times 2$.

$$\text{Dental Formula Representation: } i , \frac{a}{b}, , c , \frac{c}{d}, , pm , \frac{e}{f}, , m , \frac{g}{h}$$

• Human Adult (32 teeth): $i , \frac{2}{2}, , c , \frac{1}{1}, , pm , \frac{2}{2}, , m , \frac{3}{3}$
• Herbivore - Rabbit/Sheep (e.g., Rabbit - 28 teeth): $i , \frac{2}{1}, , c , \frac{0}{0}, , pm , \frac{3}{2}, , m , \frac{3}{3}$
• Carnivore - Dog (42 teeth): $i , \frac{3}{3}, , c , \frac{1}{1}, , pm , \frac{4}{4}, , m , \frac{2}{3}$

4. Mammalian Digestive System

Digestion involves the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream.

Major Regions and Secretions

• Mouth / Buccal Cavity:
  • Teeth mechanically break down food (mastication) to increase surface area.
  • Salivary glands secrete saliva containing salivary amylase (ptyalin), which converts cooked starch into maltose at an alkaline pH ($\text{pH } 6.8 - 7.2$).
• Stomach:
  • Muscular walls churn food into a semi-liquid mixture called chyme.
  • Gastric glands secrete gastric juice containing:
    • Hydrochloric Acid ($\text{HCl}$): Creates an acidic medium ($\text{pH } 1.5 - 2.0$), kills ingested bacteria, and activates pepsinogen into active pepsin.
    • Pepsin: Breaks down proteins into polypeptides.
    • Rennin: Coagulates milk protein (caseinogen into insoluble casein) in infants.
• Duodenum & Small Intestine (Ileum):
  • Receives bile from the gall bladder (emulsifies fats into tiny droplets) and pancreatic juice containing trypsin, pancreatic amylase, and lipase.
  • Intestinal juice (succus entericus) contains enzymes like maltase, sucrase, lactase, and peptidase to complete digestion of carbohydrates, lipids, and proteins.
  • Absorption: The inner lining of the ileum features numerous finger-like projections called villi and microvilli, providing an enormous surface area for the absorption of glucose, amino acids, fatty acids, and glycerol into blood capillaries and lacteals.