Introduction to Nutrition
All living organisms require energy to survive, grow, and repair their body tissues. This energy is derived from the food we eat. The process by which we take in food and utilize it for various body functions is called nutrition.
Nutrient: A constituent of food that helps in the normal functions of the body is known as a nutrient.
Major Types of Nutrients
Our body requires five major types of nutrients, which are broadly classified into two categories based on the quantity needed:
| Category | Nutrients | Primary Function |
|---|---|---|
| Macronutrients (Required in large amounts) |
Carbohydrates | Provide immediate energy. |
| Proteins | Building blocks for cell growth and repair. | |
| Fats | Energy storage and insulation. | |
| Micronutrients (Required in small amounts) |
Vitamins | Regulate body processes and immunity. |
| Minerals | Play specific structural and metabolic roles. |
Modes of Nutrition
1. Autotrophic Nutrition (Self-feeders)
Organisms that prepare their own food from simple inorganic substances.
- Phototropic: Use sunlight, water, and $CO_2$ (Photosynthesis). e.g., Green plants.
- Chemotropic: Use chemical energy instead of sunlight. e.g., Purple bacteria, some algae.
2. Heterotrophic Nutrition (Consumers)
Organisms that cannot make their own food and depend on others.
- Holozoic: Consuming whole food and breaking it down inside the body. e.g., Humans, Animals.
- Parasitic: Deriving nutrition from a living host. e.g., Tapeworms, Cuscuta.
- Saprophytic: Feeding on dead and decaying organic matter. e.g., Fungi, Bread mould.
Steps in Holozoic Nutrition
In humans and most animals, holozoic nutrition involves five distinct steps:
- 1. Ingestion: The act of taking food into the body.
- 2. Digestion: Breaking down complex food into simpler, soluble substances via mechanical (teeth) and chemical (enzymes) methods.
- 3. Absorption: Process where digested food passes into the blood or cytoplasm.
- 4. Assimilation: Utilization of absorbed nutrients for energy, growth, and repair.
- 5. Egestion: Elimination of undigested food from the body.
Quick Tip: Remember the order of steps using the acronym I-D-A-A-E (Ingestion, Digestion, Absorption, Assimilation, Egestion).
The Human Alimentary Canal
The human digestive system is a long, continuous muscular tube called the Alimentary Canal, along with associated digestive glands. It measures about 9 meters in an adult.
- Main Organs: Mouth (Buccal Cavity), Oesophagus (Food Pipe), Stomach, Small Intestine, Large Intestine, Rectum, and Anus.
- Associated Glands: Salivary Glands, Liver, and Pancreas.
The Mouth and Teeth
The Teeth: Digestion begins in the mouth where teeth mechanically break down food into smaller pieces. This increases the surface area for enzymes to act upon.
1. Sets of Teeth
- Milk Teeth (Temporary): The first set of 20 teeth that grow during infancy and fall off between ages 6 and 8.
- Permanent Teeth: The second set of 32 teeth that replace milk teeth and last throughout adult life.
2. Types of Teeth
| Tooth Type | Number (Each Jaw) | Function |
|---|---|---|
| Incisors | 4 | Biting and cutting food. |
| Canines | 2 | Tearing and piercing (long in carnivores). |
| Premolars | 4 | Chewing and grinding food. |
| Molars | 6 | Grinding food into a fine paste. |
Dental Formulas
A dental formula represents the number of each type of tooth in one half of the upper and lower jaw.
- Human Milk Teeth: $\frac{2120}{2120} \times 2 = 20$ (Premolars are absent).
- Human Permanent Teeth: $\frac{2123}{2123} \times 2 = 32$.
Structure of a Tooth
A tooth is divided into three main regions:
- Crown: The visible white part above the gums. Covered by Enamel (the hardest substance in the human body).
- Neck: The narrow part between the crown and the root.
- Root: The part fixed into the jawbone by a bone-like material called Cement.
Internal Layers:
- Dentine: The bulk of the tooth, located under the enamel.
- Pulp Cavity: The central space containing blood vessels and nerves.
Tooth Decay (Dental Caries):
Bacteria in the mouth act on sugar from leftover food to produce acids. These acids dissolve the enamel and dentine, forming cavities. A yellow sticky layer of bacteria and food is called Plaque.
Did you know? The enamel on your teeth is even harder than your bones because it is almost entirely made of minerals!
The tongue is a fleshy, muscular organ attached at the back to the floor of the buccal cavity. It is free at the front and can move in all directions.
Functions of the Tongue:
- Mixing: It mixes food with saliva during chewing.
- Swallowing: It helps in pushing the food into the food pipe.
- Taste: It has Taste Buds that detect different flavors: Sweet, Salty, Sour, and Bitter.
- Speech: It is essential for speaking clearly.
Salivary Glands and Chemical Digestion
Saliva: Our mouth contains three pairs of salivary glands that secrete saliva. Saliva contains an enzyme called Salivary Amylase (Ptyalin).
Role of Saliva:
- It moistens and lubricates the food, making it easier to swallow.
- Starch Digestion: Salivary Amylase breaks down about 30% of complex starch into simpler sugars like Maltose.
- It contains Lysozyme, which helps kill harmful bacteria.
The Oesophagus (Food Pipe)
Once swallowed, the food passes into the Pharynx and then the Oesophagus. The oesophagus is a 25 cm long muscular tube that connects the mouth to the stomach.
Peristalsis:
Food does not simply "fall" into the stomach. The walls of the alimentary canal undergo a series of wave-like muscle contractions and relaxations called Peristalsis. This movement pushes the food downward throughout the entire digestive tract.
[attachment_0](attachment)The Epiglottis (The Traffic Controller):
The pharynx is a common passage for both food and air. To prevent food from entering the Trachea (windpipe), a flap-like cartilage called the Epiglottis closes the windpipe during swallowing. If we talk or laugh while eating, food may enter the windpipe, causing choking.
Fun Fact: Humans produce about 1.5 to 1.7 liters of saliva every single day—that's enough to fill a large soda bottle!
The Stomach: The Churning Bag
The stomach is a thick-walled, flattened J-shaped bag. It is the widest part of the alimentary canal. Food stays here for about 4 to 5 hours and is mixed with gastric juices to form a semi-liquid paste called Chyme.
The Gastric Juice "Recipe":
- Hydrochloric Acid (HCl): Kills harmful bacteria and makes the medium acidic so enzymes can work.
- Mucous: Protects the inner lining of the stomach from being damaged by the acid.
- Pepsin: Breaks down proteins into simpler Polypeptides.
- Rennin: Helps in the digestion of milk by turning it into curd (important in infants).
The Small Intestine: The Digestion Center
Structure: A highly coiled tube, about 7.5 meters long. It is where the final digestion and absorption of food take place.
1. Secretions from Liver and Pancreas
The first part of the small intestine (Duodenum) receives juices from two major glands:
- Liver: The largest gland in the body. It secretes Bile Juice (stored in the Gall Bladder). Bile has no enzymes but breaks large fat droplets into tiny ones (Emulsification).
- Pancreas: Secretes Pancreatic Juice containing Trypsin (for proteins), Amylase (for starch), and Lipase (for fats).
2. Final Breakdown Summary
By the time food leaves the small intestine, it is broken into its simplest units:
| Complex Nutrient | End Product (Simpler Form) |
|---|---|
| Carbohydrates | Glucose |
| Fats | Fatty Acids and Glycerol |
| Proteins | Amino Acids |
Did you know? The liver is the only organ in the human body that can completely regenerate itself even if 75% of it is removed!
Absorption: The Work of Villi
Digested food is absorbed into the blood vessels in the walls of the small intestine. To make this process efficient, the inner walls have millions of tiny, finger-like outgrowths called Villi (singular: Villus).
Why are Villi important?
- Surface Area: They vastly increase the surface area for rapid absorption of food.
- Blood Supply: Each villus has a network of thin blood capillaries to transport the absorbed nutrients to the rest of the body.
Assimilation and Egestion
Assimilation: The process of using absorbed nutrients for energy, growth, and tissue repair in the body's cells.
Egestion: The removal of undigested and unabsorbed food material from the body through the anus.
The Large Intestine:
The undigested food passes into the large intestine, which is about 1.5 meters long. It is wider but shorter than the small intestine.
- Function: Its main job is to absorb water and some salts from the undigested waste.
- The Rectum: The remaining waste becomes semi-solid (faeces) and is stored in the rectum until it is expelled.
How Digestion was Discovered
In 1822, a soldier named Alexis St. Martin was accidentally shot in the stomach. The wound left a permanent hole (fistula) that didn't close properly. His doctor, William Beaumont, used this "window" to observe the stomach in action!
Beaumont's Discoveries:
- The stomach churns food mechanically.
- The stomach secretes fluids (gastric juices) that chemically digest food.
- Digestion is a process that occurs in stages.
Diffusion and Digestion:
For nutrients to pass from the alimentary canal into the blood, they must be in a soluble form. This movement of molecules from an area of high concentration to low concentration is called Diffusion. Insoluble starch cannot diffuse, which is why it must be broken down into soluble sugar (Glucose) first.
Interesting Fact: The small intestine is called "small" because of its diameter, not its length. If you stretched it out, it would be as long as a two-story building!