Life Processes

All living things must keep working just to stay alive. The basic jobs a body does to get energy, build itself and stay healthy are called life processes. They go on every moment — even when you are asleep or sitting still.

The main life processes you study in this chapter are:

• Nutrition — taking in food to get energy and materials for growth and repair.
• Respiration — breaking down food inside the cells to release energy.
• Transportation — carrying food, oxygen, water and wastes to and from the cells.
• Excretion — removing the harmful waste products made in the body.

Control and coordination is also a life process, but it is studied in a separate chapter.

Why does even a resting body need energy? Living things are made of well-ordered molecules, and this order tends to break down over time. Energy is needed all the time to repair and maintain the body and keep its molecular order in place. This energy, and the raw materials needed to build and mend the body, come from food.

Different organisms obtain food in different ways. This gives two main modes of nutrition:

• Autotrophic nutrition — the organism makes its own food from simple substances. Green plants (and some bacteria) do this by photosynthesis.
• Heterotrophic nutrition — the organism takes in food made by others. Animals and fungi feed in this way.

Autotrophic nutrition is the mode of nutrition in which an organism makes its own food from simple inorganic substances. Green plants are autotrophs — they carry out photosynthesis, the process by which they prepare food (glucose) from carbon dioxide and water in the presence of sunlight and chlorophyll.

The raw materials are carbon dioxide, taken from the air through tiny pores called stomata, and water, absorbed from the soil by the roots and carried up to the leaves. The conditions needed are sunlight (the energy source) and chlorophyll (the green pigment that traps this light). The products are glucose, much of which is stored in the plant as starch, and oxygen, which is released into the air.

Photosynthesis takes place mainly in the green parts of the plant, inside cell structures called chloroplasts, which contain chlorophyll. It happens through three events:

1. Absorption of light energy by chlorophyll.
2. Conversion of light energy to chemical energy, and splitting of water molecules into hydrogen and oxygen.
3. Reduction of carbon dioxide to carbohydrate (glucose) using the hydrogen released.

Remember that these three events need not always take place one immediately after the other. For example, some desert plants take in carbon dioxide at night and complete the rest during the day.

Stomata are tiny pores found mainly on the surface of leaves. Most exchange of gases — carbon dioxide in, oxygen out — happens through them. Each stoma is bordered by two guard cells that swell to open the pore and shrink to close it. A large amount of water is also lost as vapour through open stomata, a process called transpiration; so a plant tends to keep its stomata closed when it does not need carbon dioxide, to reduce this water loss.

Heterotrophic nutrition is the mode of nutrition in which an organism cannot make its own food from simple inorganic substances and instead depends on other organisms for its food. Such organisms are called heterotrophs. All animals and fungi are heterotrophs.

Depending on how the food is obtained, heterotrophic nutrition is of three types:

• Saprophytic nutrition: the organism feeds on dead and decaying matter. It secretes digestive juices (enzymes) onto the food, digests it outside the body, and then absorbs the simple, soluble food. Examples: fungi such as bread mould, yeast and mushrooms.
• Parasitic nutrition: the organism (parasite) lives on or inside the body of another organism (the host) and takes ready-made food from it, usually harming the host. Examples: Cuscuta (amarbel), ticks, lice, tapeworm and leeches.
• Holozoic nutrition: the organism takes in whole (solid or liquid) food and then digests it inside its body. Examples: Amoeba and humans.

Nutrition in Amoeba: Amoeba is a single-celled organism that shows holozoic nutrition. It pushes out temporary finger-like projections called pseudopodia around the food particle and engulfs it. This traps the food inside a food vacuole. Inside the food vacuole the food is digested by enzymes; the digested food is then absorbed into the cytoplasm and used for growth. Finally, the undigested waste is thrown out of the cell as the Amoeba moves.

Human digestion happens inside a long tube called the alimentary canal, which runs from the mouth to the anus. As food moves along, glands add juices with enzymes that break big food molecules into small, absorbable ones. Let us follow the food from start to finish.

In the mouth, the teeth chew and grind the food into smaller pieces, and the tongue helps mix it well. The salivary glands release saliva, which contains the enzyme salivary amylase (also called ptyalin). This enzyme begins the digestion of starch, breaking it into sugar. From the mouth, the food is pushed into the oesophagus (food pipe), which carries it down to the stomach by rhythmic muscular movements called peristalsis.

In the stomach, the gastric glands release three things. Hydrochloric acid makes the medium acidic, kills harmful germs in the food, and creates the right conditions for the enzyme to work. The enzyme pepsin then digests proteins. A layer of mucus coats and protects the inner lining of the stomach from the strong acid.

The small intestine is the main site of complete digestion and absorption. It receives two important secretions:

• Bile from the liver — it makes the medium alkaline and emulsifies fats, breaking large fat globules into small droplets so enzymes can act on them easily.
• Pancreatic juice from the pancreas — it contains trypsin (digests proteins), lipase (digests emulsified fats), and amylase (digests carbohydrates).

The inner wall of the small intestine has many tiny finger-like projections called villi. These greatly increase the surface area, so the digested food is absorbed efficiently into the blood. Finally, the undigested food enters the large intestine, which mainly absorbs water. Whatever remains is passed out as faeces through the anus.

Respiration is how cells break down food, mainly glucose, to release the energy locked inside it. This energy is captured in a molecule called ATP, which the cell then spends on all its work.

The first step is the same in every cell. In the cytoplasm, one 6-carbon glucose molecule is broken into two molecules of pyruvate (a 3-carbon compound). What happens to the pyruvate next depends on whether oxygen is available.

Aerobic respiration takes place in the presence of oxygen, inside the mitochondria (often called the powerhouse of the cell). Here pyruvate is broken down completely into carbon dioxide and water, releasing a large amount of energy.

Anaerobic respiration takes place without oxygen and releases much less energy. There are two examples you must know:

• In yeast, pyruvate is converted into ethanol and carbon dioxide. This is called fermentation.
• In our muscles during heavy exercise, when oxygen runs short, pyruvate is converted into lactic acid. The build-up of lactic acid causes muscle cramps.

Breathing brings in the oxygen that aerobic respiration needs and throws out the carbon dioxide. Air travels through the nostrils, down the trachea (windpipe), into the bronchi, and reaches the lungs, ending in the alveoli. The alveoli are tiny balloon-like air sacs with very thin walls and a rich blood supply. Here oxygen passes from the air into the blood and carbon dioxide passes out to be breathed away. The oxygen is then carried all over the body by haemoglobin in the red blood cells.

Our body is large, so simple diffusion is not enough to move materials around. We need a proper transport system. In humans this is the circulatory system, made of blood, the heart and blood vessels. It carries oxygen, digested food, hormones and wastes to and from every cell.

Blood is a fluid connective tissue. It has these parts:

• Plasma — the liquid part that carries digested food, carbon dioxide (CO₂), nitrogenous wastes and other dissolved substances.
• Red blood cells (RBCs) — contain the red pigment haemoglobin, which binds and carries oxygen to the tissues.
• White blood cells (WBCs) — fight infection and protect the body against germs.
• Platelets — help the blood to clot at a wound and stop bleeding.

The heart is a muscular organ that pumps blood. It has four chambers — two upper chambers called atria and two lower chambers called ventricles. The atria receive blood and the thicker-walled ventricles pump it out. This four-chambered design keeps oxygen-rich and oxygen-poor blood completely separate.

In humans there is double circulation: blood passes through the heart twice in one complete round of the body. In pulmonary circulation, the right side sends deoxygenated blood to the lungs to pick up oxygen. In systemic circulation, the left side sends oxygenated blood to the rest of the body. Because the two sides do not mix, warm-blooded animals get an efficient, steady supply of oxygen to keep their bodies warm and active.

Blood vessels are of three types:

• Arteries — carry blood away from the heart. They have thick, elastic walls to take the high pressure of blood pumped by the heart.
• Veins — carry blood back to the heart. They have thinner walls and valves that prevent the backward flow of blood.
• Capillaries — very thin vessels with walls only one cell thick. Here the actual exchange of oxygen, food and wastes between blood and body cells takes place.

Lymph is another fluid that helps in transport. It is a colourless fluid formed when some plasma leaks out of the capillaries into the tissues. Lymph carries digested and absorbed fats from the intestine and drains the extra fluid from the tissues back into the blood.

A plant has no heart, yet it still needs to move things around its body. It uses two separate conducting tissues for two different jobs. Remember them clearly, because xylem and phloem are one of the most commonly mixed-up pairs in the exam.

Xylem transports water and dissolved minerals in one direction only — upward, from the roots to the leaves. What pulls the water up so high? During the day, the main force is transpiration — the evaporation of water from the leaves through tiny pores called stomata. As water evaporates, it creates a suction that draws more water up through the xylem. This suction is called the transpiration pull, and it is the chief driving force by day. At night, when the stomata are mostly closed and transpiration is low, root pressure plays the bigger part in pushing water upward.

Phloem carries out translocation — the transport of food (mainly sucrose) that is made in the leaves during photosynthesis. This food is delivered to all other parts of the plant that cannot make their own — the roots, growing tips, flowers, fruits and storage organs. Unlike xylem, translocation uses energy from ATP, and it can move food in different directions as the plant's needs change (for example, towards a growing shoot in one season and towards a storage root in another).

So the simple contrast to lock into memory:

• Xylem — moves water and minerals, travels one way (upward only), driven by transpiration pull (day) and root pressure (night), and does not use much energy.
• Phloem — moves food (sucrose), can travel both ways, and uses energy (ATP).

Excretion is the biological process of removing harmful nitrogenous wastes (mainly urea) and other metabolic wastes from the body. If these wastes build up, they poison the cells, so the body must throw them out regularly.

In humans, this work is done by the excretory system, which is made up of:

• two kidneys (they filter the blood and make urine),
• two ureters (tubes that carry urine down from the kidneys),
• a urinary bladder (stores the urine), and
• the urethra (passes urine out of the body).

The functional (basic filtering) unit of the kidney is the nephron. Each nephron works in two main steps. First, blood entering the kidney is filtered in a fine cluster of capillaries called the glomerulus, which sits inside a cup-shaped Bowman's capsule. As this filtrate then passes along the long tubule, useful substances are selectively reabsorbed back into the blood:

• all the glucose and amino acids,
• most of the water, and
• most of the useful salts.

What is left behind is urine — mainly water, urea and some salts. Urine collects in the kidney and flows through the ureters into the urinary bladder, where it is stored until it is passed out through the urethra.

If a person's kidneys stop working (kidney failure), wastes like urea start collecting in the blood. Such patients can be kept alive using an artificial kidney, a process called dialysis, in which a machine filters the blood and removes the nitrogenous wastes.

In plants, waste removal is simpler because plants have no special excretory organs. Gaseous wastes leave through the stomata: carbon dioxide is released as a waste of respiration, while oxygen is given out as a by-product of photosynthesis during the day. Excess water is removed as water vapour by transpiration. Many other plant wastes are simply stored — as resins and gums, in old xylem, or gathered in the leaves that are later shed.