All living organisms carry out certain basic functions that are essential for their survival. These functions, such as nutrition, respiration, transportation and excretion, are called life processes. Nutrition provides the raw materials and energy needed by the body, respiration releases energy from food, transportation carries materials from one part of the body to another, and excretion removes metabolic wastes. Organisms also show growth, reproduction and response to stimuli, but these are not essential for the survival of the individual; the four basic life processes above are vital for every living being.
Life processes require energy, and this energy ultimately comes from the food that organisms take in. Autotrophs prepare their own food through photosynthesis, while heterotrophs depend on other organisms for food. The energy released from food through respiration is used to carry out all other life activities. Understanding life processes helps us appreciate how the human body works and how plants and animals carry out the same fundamental activities in different ways.
In this chapter, we will study nutrition in plants and animals, the human digestive system, respiration and the human respiratory system, transportation in humans and plants, the human circulatory system, and excretion in humans and plants. We will also learn important concepts such as photosynthesis, cellular respiration, blood composition, and the double circulation of blood.
Autotrophs synthesise their own food from simple inorganic substances. Green plants make food by photosynthesis, using sunlight, carbon dioxide and water. The raw materials are carbon dioxide from the air and water from the soil. Chlorophyll, present in the leaves, captures sunlight energy.
$$\text{6CO}2 + \text{6H}_2\text{O} \xrightarrow{\text{Sunlight, Chlorophyll}} \text{C}_6\text{H}_2$$}\text{O}_6 + \text{6O
The process takes place in the chloroplasts, which contain the green pigment chlorophyll. Photosynthesis occurs in two main phases: the light reaction, which occurs in the grana of the chloroplast and produces ATP, and the dark reaction, which occurs in the stroma and fixes carbon dioxide to form glucose. The factors affecting photosynthesis are light intensity, carbon dioxide concentration and temperature.
Organisms that cannot make their own food and depend on other organisms are called heterotrophs. Heterotrophic nutrition is of three types:
Amoeba captures food using pseudopodia (finger-like projections) and forms a food vacuole. Digestive juices are secreted into the vacuole, the digested food is absorbed, and the waste is expelled. This is intracellular digestion.
The human digestive system consists of the alimentary canal (mouth, oesophagus, stomach, small intestine, large intestine, anus) and associated glands (salivary glands, liver and pancreas).
Respiration is the process of releasing energy from food (glucose) in the presence or absence of oxygen. It occurs in all living cells.
In the presence of oxygen, glucose is completely oxidised to release a large amount of energy:
$$\text{C}6\text{H}$$}\text{O}_6 + \text{6O}_2 \rightarrow \text{6CO}_2 + \text{6H}_2\text{O} + \text{38 ATP (Energy)
In the absence of oxygen, glucose is incompletely broken down. In human muscle cells during vigorous exercise, it produces lactic acid:
$$\text{C}6\text{H}$$}\text{O}_6 \xrightarrow{\text{No oxygen}} 2\text{C}_3\text{H}_6\text{O}_3 + \text{Energy
In yeast, glucose is converted to ethanol and carbon dioxide:
$$\text{C}6\text{H}$$}\text{O}_6 \xrightarrow{\text{Yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + \text{Energy
Breathing is the physical process of taking in oxygen-rich air and giving out carbon dioxide-rich air. The respiratory system consists of the nasal cavity, pharynx, larynx, trachea, bronchi and lungs. The exchange of gases occurs in the alveoli, which are tiny air sacs in the lungs with a rich network of blood capillaries, providing a large surface area for gas exchange. During inhalation the diaphragm contracts and flattens and the ribcage moves up and out, increasing the chest volume, and air rushes in. During exhalation the reverse happens.
Plants exchange gases through stomata (in leaves), lenticels (in stems) and roots. At night, plants take in oxygen and give out carbon dioxide (respiration), while during the day, photosynthesis and respiration both occur, with photosynthesis dominating. Plants get rid of excess water through transpiration.
The circulatory system consists of blood, blood vessels and the heart. Blood carries oxygen, digested food, hormones, carbon dioxide and nitrogenous wastes.
The human heart has four chambers: two atria and two ventricles. The right side receives deoxygenated blood from the body and pumps it to the lungs for oxygenation, while the left side receives oxygenated blood from the lungs and pumps it to the rest of the body. This is called double circulation. The separation of the right and left sides prevents mixing of oxygenated and deoxygenated blood.
The heart beats, producing a pulse which can be felt at the wrist. The cardiac cycle has three phases: atrial systole, ventricular systole and joint diastole. High blood pressure, or hypertension, can result from narrowing of arteries.
Plants transport water and minerals through xylem and prepared food through phloem. Water and dissolved minerals move from roots to leaves through xylem vessels, driven mainly by the transpiration pull created by the loss of water vapour from the leaves. Food prepared in the leaves is transported to all parts of the plant through phloem in a process called translocation.
Excretion is the removal of nitrogenous waste products from the body. The excretory system in humans consists of a pair of kidneys, ureters, a urinary bladder and a urethra. Each kidney has millions of filtering units called nephrons.
A nephron has a Bowman's capsule, a glomerulus (a tuft of capillaries), and a long tube. Blood enters the glomerulus under high pressure, and water, glucose, amino acids, salts and urea are filtered out. Useful substances such as glucose and amino acids are reabsorbed in the tubules, while nitrogenous waste like urea is removed in the urine. Urine is about 95% water, 2.5% urea and 2.5% other waste products.
Plants get rid of waste products in various ways: some wastes are stored in the vacuoles of cells, others are removed as gums, resins, and the yellowing and falling of leaves in autumn. Carbon dioxide is released through stomata, and excess water through transpiration. Many waste products of plants are commercially useful, such as gums, rubber, resins and latex.
If a person's kidneys fail, waste products build up in the blood, a condition called uremia. This is treated by haemodialysis, in which an artificial kidney (dialyser) removes nitrogenous wastes from the blood. Dialysis is needed about two to three times a week and is performed for 3-4 hours each time.
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen required | Yes | No |
| End products | CO2 and H2O | Lactic acid / ethanol and CO2 |
| Energy released | Large (38 ATP) | Small (2 ATP) |
| Site of occurrence | Cytoplasm then mitochondria | Cytoplasm only |
| Example | Human muscle cells at rest | Yeast, muscle cells during exercise |
| Life Process | Definition | Main Organ/Structure |
|---|---|---|
| Nutrition | Intake of food and its use by the body | Digestive system |
| Respiration | Release of energy from food | Lungs, mitochondria |
| Transportation | Movement of substances in the body | Heart, blood, xylem, phloem |
| Excretion | Removal of metabolic wastes | Kidneys, nephrons, stomata |
Life processes are the very essence of being alive, and each process is beautifully designed to sustain the organism. Nutrition provides the fuel, respiration releases the energy, transportation delivers materials, and excretion removes the wastes. The human digestive, respiratory, circulatory and excretory systems work together in perfect coordination, and understanding each one reveals the elegance of the human body. Plants, though different in structure, carry out the same processes using leaves, roots, xylem, phloem and stomata. This chapter builds the biological foundation needed for higher studies in physiology and medicine, and it also creates awareness about healthy habits, such as balanced diet, exercise and kidney care. Master the diagrams and equations, and the concepts will become easy to recall in the examination.