Nutrition, Photosynthesis and Human Digestion

Introduction: What Keeps Us Alive?

Even when you are asleep, fast asleep with no muscles moving, your body is still busy. Your heart is beating, your lungs are moving air in and out, your kidneys are filtering blood, and your cells are burning food to produce energy. All these background jobs — the ones that keep a body running whether active or resting — are grouped by biologists under a single name: life processes.

How do we decide if something is actually alive? Movement alone is not enough — rivers move and clouds move, but neither is alive. Biologists look for something deeper: the constant building and breaking of molecules inside a body, carried out to keep the body organised and working. To run this maintenance, an organism needs food (nutrition), must release energy from that food (respiration), must shift materials around inside itself (transportation), and must get rid of wastes (excretion).

5.1 What Are Life Processes?

The four life processes studied in this chapter are:

• Nutrition — taking in food and converting it to usable forms
• Respiration — releasing energy from food, usually using oxygen
• Transportation — carrying materials (food, gases, wastes) from one part of the body to another
• Excretion — removing harmful metabolic wastes produced by the body

5.2 Nutrition

Every living cell needs a source of energy and raw materials to build and repair itself. The food that supplies this is the starting point of all life activity. Based on how organisms obtain food, nutrition is of two broad kinds.

5.2.1 Autotrophic Nutrition — Photosynthesis

Green plants trap sunlight and use it to build glucose from carbon dioxide and water. This remarkable food-making process, called photosynthesis, can be summarised as:

Three things happen simultaneously during photosynthesis: (i) light energy is absorbed by chlorophyll, (ii) water molecules are split into hydrogen and oxygen, and (iii) carbon dioxide is reduced to form carbohydrates. The site of this activity is the chloroplast, a green organelle packed with chlorophyll-containing discs called grana.

Where do the raw materials come from? Carbon dioxide diffuses in from the air through tiny pores on the leaf called stomata. Each stoma is bounded by two bean-shaped guard cells that swell with water to open the pore and become flaccid to close it. Water is taken up from the soil by roots and carried up through the xylem. Minerals such as nitrogen, phosphorus, iron and magnesium — also drawn from the soil — help build proteins and chlorophyll.

5.2.2 Heterotrophic Nutrition — Three Styles

Nutrition in Amoeba — Holozoic Nutrition at Cell Level

Amoeba, a single-celled organism, shows the full sequence of holozoic nutrition within one cell. When a food particle comes near, the cell pushes out finger-like extensions called pseudopodia. These flow around the particle and fuse at the far side, trapping the food inside a food vacuole. Digestive enzymes are secreted into the vacuole; the complex food is broken down into simpler molecules which diffuse into the cytoplasm. Whatever cannot be digested is later expelled when the vacuole meets the cell surface.

5.2.3 Nutrition in Human Beings — The Digestive System

Humans are holozoic. Food travels along a long tube — the alimentary canal — which begins at the mouth and ends at the anus. Different regions of the canal do different jobs, helped by secretions from glands such as the salivary glands, liver and pancreas.

The complete path is: Mouth → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus.

Region-by-region digestion

Mouth: Teeth cut and grind food into a pulp (mechanical digestion). Three pairs of salivary glands pour out saliva, which contains the enzyme salivary amylase. Amylase begins breaking down cooked starch into maltose, a simpler sugar.

Oesophagus: The chewed food (bolus) is pushed down to the stomach by wave-like muscular contractions called peristalsis. These waves occur throughout the canal.

Stomach: A J-shaped muscular bag. Its wall secretes gastric juice, which contains three important things:

• HCl (hydrochloric acid) — kills bacteria in food and creates the acidic medium needed to activate pepsin.
• Pepsin — an enzyme that begins protein digestion.
• Mucus — a slimy layer that protects the stomach wall from being digested by its own acid and enzymes.

Small intestine — the longest part (~6 m long in humans), where digestion is completed and absorption takes place. Three juices act here:

Absorption — the role of villi: The inner wall of the small intestine is covered with millions of finger-like projections called villi. They enormously increase the surface area for absorption and carry a rich network of blood capillaries and lymph vessels. Digested glucose, amino acids and minerals move into the blood; digested fats (fatty acids + glycerol) enter the lymph.

Large intestine: The unabsorbed food passes here. Its main job is to absorb water from the residue and form semi-solid faeces, which are stored in the rectum and expelled through the anus under voluntary control.

Worked example — following a chapati