15. Life Processes in Living Organisms

‘d. Growth-irrelevant movement’ does not match any item; examples are the folding of touch-me-not leaves and the closing of the Venus fly trap, which are not related to growth.

Letting go of the hot vessel at once, without thinking, is a reflex action controlled by the spinal cord. The impulse travels from the dendrites to the cell body and then along the axon; at the axon terminal, chemicals cross the synapse and start an impulse in the next neuron, until it finally reaches the muscles of the hand, which contract.

The cells on the surface of roots are in contact with water and minerals in the soil. Due to the difference in concentration, water and minerals enter these cells, which become turgid. These turgid cells exert pressure on the adjacent cells; this pressure is called root pressure. Under its effect, water and minerals reach the xylem of the root and are continuously pushed forward, forming a column of water that moves upward. Root pressure is sufficient to lift water in shrubs, small plants and small trees, and it plays the important role of pushing water up during the night, when there is little transpiration.

The loss of water in the form of vapour through the stomata of leaves is called transpiration. Each stoma is surrounded by two guard cells, which control its opening and closing. As water evaporates from the leaves, the water level in the leaf cells decreases, and water is pulled up through the xylem from the roots to make up the loss. This pull, called transpiration pull, helps in the absorption of water and minerals by the roots and their distribution to all parts of the plant, even in very tall trees. An oak tree gives out about 1,51,000 litres of water in a year by transpiration.

Nerve cells or neurons are the structural and functional units of the nervous system. They are the longest cells in the human body, up to about a metre long, and can generate and conduct electro-chemical impulses. A neuron has a cell body with a nucleus, many short branched dendrites that collect information, and a long axon covered with a myelin sheath, which ends in axon terminals. Impulses travel from the dendrites to the cell body, then along the axon to its terminals. There, chemicals are released that cross a minute gap, the synapse, to the next neuron. According to function, neurons are sensory, motor or association neurons. Neuroglia are cells that support neurons. (See the labelled diagram in Q5.)

The brain is the main controlling part of the nervous system. It lies safely in the cranial cavity, protected by the meninges and cerebro-spinal fluid. An adult brain weighs about 1300–1400 g and has about 100 billion neurons. It has three main parts:

• Cerebrum: the largest part (two-thirds of the brain), with two hemispheres whose surface has deep convolutions. It controls voluntary movements, concentration, planning, decision-making, memory, intelligence and intellectual activities.
• Cerebellum: below the cerebrum at the back, with shallow grooves. It co-ordinates voluntary movements and maintains the body’s balance.
• Medulla oblongata: the hind-most part, which continues as the spinal cord. It controls involuntary activities such as the heartbeat, blood circulation, breathing, sneezing, coughing and salivation.

The left side of the brain controls the right side of the body, and the right side controls the left.

An immediate and involuntary response given to a stimulus from the environment is called a reflex action. It happens without our thinking and without the intervention of the brain; the spinal cord acts as the centre of co-ordination. For example, when our hand touches a hot object or a sharp pin, we pull it back at once.

Reflex actions protect the body from harm, because the response is very fast.

• Growth hormone — stimulates the growth of bones.
• Adrenocorticotropic hormone (ACTH) — stimulates the adrenal gland.
• Thyroid stimulating hormone (TSH) — stimulates the thyroid gland.
• Prolactin — stimulates milk production.
• Follicle stimulating hormone (FSH) — controls the growth of the gonads.
• Luteinizing hormone (LH) — controls the menstrual cycle and ovulation.
• Oxytocin — contracts the uterus during childbirth.
• Antidiuretic hormone (ADH) — regulates the water level in the body.

• Thyroxine — controls the growth of the body and metabolic activities.
• Calcitonin — controls calcium metabolism and the calcium level in blood.

• Adrenaline and noradrenaline — control behaviour in crisis and emotional situations; stimulate the heart and metabolic processes.
• Corticosteroids — maintain the balance of Na⁺ and K⁺ and stimulate metabolism.

• Thymosin — controls the cells which give rise to immunity.

• Testosterone — stimulates the development of male secondary sexual characteristics such as beard, moustache and a deep voice.

• Oestrogen — stimulates the growth of the endometrium and the development of female secondary sexual characteristics.
• Progesterone — prepares the endometrium for conception and maintains pregnancy.

Besides the nervous system, control and co-ordination in the human body is brought about by chemical substances called hormones, secreted by endocrine (ductless) glands. These glands have no ducts to store or carry their secretions, so hormones are released directly into the blood and reach all parts of the body. Nerve impulses are fast but short-lived, while the action of hormones is slow but long-lasting; the two systems work together.

Hormones are secreted only in the required quantity, controlled by a special mechanism. For example, when the blood-glucose level rises, certain cells of the pancreas are stimulated and release more insulin, which brings the level back to normal.

• Insulin (pancreas) — converts excess blood glucose into glycogen.
• Glucagon (pancreas) — converts glycogen into glucose.
• Thyroxine (thyroid) — controls growth and metabolism.
• Adrenaline (adrenal) — prepares the body for emergencies.
• Growth hormone (pituitary) — stimulates the growth of bones.
• Parathormone (parathyroid) — controls calcium and phosphorus metabolism.

1. System: humans have a well-developed excretory system — kidneys, ureters, urinary bladder and urethra; plants have no special organ or system for excretion.
2. Process: excretion in humans is complex, and wastes are filtered from blood by nephrons in the kidneys; in plants it is simple.
3. Waste materials: humans excrete mainly urea, uric acid and excess salts and water as urine, and also through sweat and exhaled air. Plants give out gases by diffusion, and store other wastes as resin, gum, latex and crystals (raphides).
4. Removal: humans remove urine through the urethra; plants store wastes in the vacuoles of leaf cells, flowers, fruits and bark, which later fall off, and some wastes pass out through roots into the soil.
5. Usefulness: human wastes are of little direct use, but many plant wastes such as gum, resin and rubber latex are useful to humans.

Plants have no nervous or muscular system, but they respond to stimuli, mainly by movements brought about by hormones and electro-chemical impulses. Plant cells change their shape by increasing or decreasing their water content, and so bring about movements. There are two kinds of movements:

1. Growth-relevant movements (tropisms):

• Phototropic: the shoot grows towards light, as when a plant on a window sill bends towards the window.
• Gravitropic: the root grows downward, in the direction of gravity.
• Hydrotropic: roots grow towards water.
• Chemotropic: the pollen tube grows towards the ovule.
• Thigmotropic: tendrils of climbers coil around a support when they touch it.

2. Growth-irrelevant movements: the leaves of touch-me-not (Mimosa) fold up when touched; the Venus fly trap closes on an insect; lotus flowers open in the day and tuberose at night; balsam fruits burst open to disperse seeds.

Plant hormones: auxin helps in the enlargement of cells; gibberellins help stem elongation; cytokinins help cell division; and abscisic acid prevents and slows down growth and causes wilting of leaves.

In a multicellular organism, many organ systems work at the same time. Systematic regulation of these different processes is called control, and bringing them about in the proper sequence, so that they work together smoothly in response to internal changes and outside stimuli, is called co-ordination.

Example: when a batsman hits a ball, his eyes judge the ball, the brain decides the shot, and the muscles of the arms, legs and back move together in the right order. If there is any confusion at one step, as when we bite our own tongue or choke while eating in a hurry, the activity goes wrong. Proper co-ordination also keeps body temperature, water level and enzyme levels steady; this state of balance is called homeostasis.

• Metabolism produces harmful wastes such as urea, uric acid and ammonia. Urea is produced in the liver and comes into the blood.
• The human excretory system has a pair of kidneys, a pair of ureters, the urinary bladder and the urethra. Each kidney has about 10 lakh filtering units called nephrons.
• Each nephron has a cup-shaped Bowman’s capsule containing a network of capillaries, the glomerulus. When blood flows through the glomerulus, water, urea and other small molecules are filtered into the capsule.
• As this liquid passes through the tubule of the nephron, water and useful substances are reabsorbed into the blood. The remaining liquid, full of wastes, is urine.
• Urine is carried by the ureters to the urinary bladder, where it is stored, and is then passed out through the urethra. The bladder is under nervous control, so urination can be controlled.
• The skin (sweat) and lungs (carbon dioxide) also help in excretion. If the kidneys fail, blood is purified artificially by dialysis.

• Gum: from acacia (babhul) and other trees, used as an adhesive and in foods and medicines.
• Resin: from pine and other trees, used in varnishes, paints and incense.
• Latex: from the rubber tree, used to make natural rubber for tyres, tubes, footwear and gloves.
• Plant wastes such as tannins, essential oils and alkaloids are used in leather tanning, perfumes and medicines.
• Oxygen, given out as a by-product of photosynthesis, is essential for our breathing.
• Fallen leaves, bark, flowers and fruits, which store wastes, decompose and enrich the soil, or are used as fuel and compost.

Plants have two conducting tissues, which connect all their parts: the xylem conducts water and minerals, and the phloem conducts food.

• Transport of water and minerals: root hairs absorb water and minerals from the soil because of differences in concentration. The turgid root cells exert root pressure, which pushes water into the xylem and upward, especially at night. During the day, water evaporates from the leaves through the stomata (transpiration), creating a transpiration pull that draws water up through the xylem even in tall trees.
• Transport of food (translocation): food made in the leaves, such as sucrose, is carried by the phloem to all parts of the plant, both upward and downward, using energy from ATP. When sucrose enters a part, water follows by diffusion; the pressure increases and pushes the food on to cells where the pressure is lower. Excess food is stored in roots, fruits and seeds. During flowering, stored sugar is carried to the floral buds.

• (1) Evolution of the vertebrate brain: compare the brains of fish, amphibians, reptiles, birds and mammals. Note how the cerebrum becomes larger and more convoluted, and how the cerebellum develops more in birds and mammals, as the animals become more complex.
• (2) ‘Why I Am Important?’: each student plays one endocrine gland (pituitary, thyroid, adrenal, pancreas, and so on) and tells the class, in the first person, where it is located, which hormones it makes and what happens if it makes too much or too little.
• (3) ‘Human beings are intelligent’: collect points such as the large and highly convoluted cerebrum, the ability to speak and use language, reasoning, planning, memory, use of tools, art, science and technology, and passing on knowledge through writing.