Master the basics - and everything after gets easier

What it is

Oxygen rides on haemoglobin in red blood cells from the lungs to every tissue.

Answer

Oxygen diffuses from the air sacs (alveoli) of the lungs into the blood, where it binds to the red pigment haemoglobin in red blood cells to form oxyhaemoglobin. The blood carries it to the tissues, where oxygen is released for respiration. Haemoglobin makes this transport efficient because it has a high affinity for oxygen.

What it is

Arteries carry blood away from the heart under high pressure; veins bring it back and have valves.

Answer

Arteries carry blood away from the heart, usually oxygenated (except the pulmonary artery), have thick elastic walls and no valves because blood flows under high pressure. Veins carry blood towards the heart, usually deoxygenated (except the pulmonary vein), have thinner walls and contain valves to prevent the backflow of low-pressure blood.

What it is

A reflex arc gives an instant, automatic response through the spinal cord - no thinking needed.

Answer

A reflex arc is the pathway followed by a nerve impulse during a reflex action, allowing a quick automatic response without conscious thought. When you touch a hot object, receptors in the skin send an impulse along a sensory neuron to the spinal cord; there a relay neuron passes it to a motor neuron, which carries the impulse to the muscles of the hand, causing you to pull your hand away instantly.

What it is

Sexual reproduction mixes two parents' genes to create variety; asexual makes identical copies.

Answer

In sexual reproduction two parents contribute genes, and the mixing of DNA during the formation of gametes and their random combination at fertilisation creates new gene combinations in the offspring. In asexual reproduction a single parent produces genetically identical copies, so there is little variation. Greater variation gives species a better chance of surviving changes in the environment.

What it is

Crossing two hybrids (Tt x Tt) gives a 3:1 tall-to-short ratio because T is dominant over t.

Answer

The F1 generation is all Tt (tall), because T is dominant over t. When F1 plants (Tt x Tt) are crossed, the F2 offspring are TT, Tt, Tt, tt. TT, Tt and Tt appear tall while tt is short, giving a phenotypic ratio of 3 tall : 1 short. The genotypic ratio is 1 TT : 2 Tt : 1 tt.

What it is

Photosynthesis is how green plants make their own food (glucose) from CO2 and water using sunlight.

Answer

Photosynthesis is the process by which green plants make their own food (glucose) from carbon dioxide and water using sunlight, releasing oxygen. The balanced equation is: 6CO2 + 6H2O -> C6H12O6 + 6O2. It requires chlorophyll (the green pigment), sunlight, carbon dioxide and water.

What it is

A food chain shows who eats whom, and only about 10 percent of energy passes to the next level.

Answer

A food chain is a series of organisms through which energy and nutrients pass as one organism eats another, for example grass -> deer -> lion. The ten percent law states that only about 10 percent of the energy at one trophic level is passed on to the next level; the rest is lost mainly as heat. This is why food chains usually have only three or four links.

What it is

A reflex action is a fast, automatic response to a stimulus, controlled by the spinal cord.

Answer

A reflex action is a sudden, automatic response to a stimulus that does not need conscious thinking by the brain. The path taken by the nerve impulse is called the reflex arc: a receptor detects the stimulus, a sensory neuron carries the impulse to the spinal cord, a relay neuron passes it to a motor neuron, and the motor neuron makes the effector (a muscle) act. Because the impulse is processed in the spinal cord and not the brain, the response is very quick.

What it is

Crossing two Tt plants gives a 3:1 ratio of tall to short - the classic monohybrid result.

Answer

When two Tt plants are crossed (where T for tall is dominant over t for short), the offspring appear in the genotype ratio 1 TT : 2 Tt : 1 tt. Because T is dominant, both TT and Tt plants are tall and only tt is short, giving a phenotype ratio of 3 tall : 1 short (3:1). This shows that a recessive trait, which stays hidden in the F1 generation, can reappear in the F2 generation.

What it is

Arteries carry blood away from the heart; veins carry it back to the heart.

Answer

1) Arteries carry blood away from the heart, while veins carry blood back towards the heart. 2) Arteries carry oxygenated blood (except the pulmonary artery), while veins carry deoxygenated blood (except the pulmonary vein). 3) Arteries have thick, elastic, muscular walls to handle high pressure and have no valves, whereas veins have thinner walls, carry blood at low pressure, and contain valves that stop the blood flowing backwards.

What it is

Population density is the number of individuals living per unit area.

Answer

Population density = number of individuals / area = 500 / 25 = 20 deer per km^2. If the same number of animals had to live in a smaller area, the density - and the competition for food and space - would rise.

What it is

A population changes as births add individuals and deaths remove them.

Answer

New population = initial + births - deaths = 500 + 60 - 40 = 520. When births exceed deaths the population grows; when deaths exceed births it falls (here we ignore migration).

What it is

The number of heartbeats is the heart rate multiplied by the time.

Answer

Heartbeats = heart rate x time = 72 x 10 = 720 beats. A resting heart beating about 72 times a minute beats roughly 100,000 times in a whole day, pumping blood non-stop.

What it is

Only about 10% of the energy at one trophic level is passed on to the next level.

Answer

Energy passed on = 10% of the energy = 1000 x 10 / 100 = 100 units. The other 90% is used up in life processes and lost as heat, so less and less energy is available at each higher level.

What it is

The number of breaths taken is the breathing rate multiplied by the time.

Answer

Breaths = breathing rate x time = 16 x 10 = 160 breaths. A resting adult breathes about 12 to 16 times a minute; the rate rises during exercise to bring in more oxygen.

What it is

The population growth rate is the net change (births minus deaths) expressed as a percentage of the population.

Answer

Growth rate = (births - deaths) / population x 100 = (60 - 40) / 500 x 100 = 4% per year. If deaths exceed births the rate is negative, meaning the population is shrinking.

What it is

A Punnett square predicts the genotypes and phenotypes of offspring from the alleles of the two parents.

Answer

Each Tt parent gives half T and half t gametes. Filling the Punnett square gives TT, Tt, Tt, tt - a genotype ratio of 1 : 2 : 1. Since T (tall) is dominant, three of the four are tall and one is short, a phenotype ratio of 3 : 1.

What it is

A reflex arc is the short nerve pathway that produces an automatic, rapid response to a stimulus.

Answer

The stimulus (heat) is detected by receptors in the skin. A sensory neuron carries the message to the spinal cord, which acts as the relay and decides the response without waiting for the brain. A motor neuron then carries the command to the effector - a muscle - which contracts and pulls your hand away. This whole pathway is the reflex arc, and it makes the response fast and automatic.

What it is

The human heart has four chambers that keep oxygenated and deoxygenated blood completely separate.

Answer

The right atrium receives deoxygenated blood from the body and passes it to the right ventricle, which pumps it to the lungs. The left atrium receives oxygenated blood from the lungs and passes it to the left ventricle, which pumps it to the whole body. The atria receive blood while the ventricles pump it out; the left ventricle has the thickest wall because it pushes blood the farthest. Keeping the two sides separate prevents oxygenated and deoxygenated blood from mixing.

What it is

Autotrophs like green plants make their own food from carbon dioxide and water using sunlight in photosynthesis.

Answer

Photosynthesis is the process by which green plants (autotrophs) synthesise their own food in the form of glucose using carbon dioxide and water, in the presence of sunlight and the green pigment chlorophyll. The balanced equation is: 6CO2 + 6H2O, in the presence of sunlight and chlorophyll, gives C6H12O6 + 6O2. It occurs mainly in the chloroplasts of the mesophyll cells of the leaf. Oxygen is released as a by-product.

What it is

Human digestion breaks down food in the alimentary canal with the help of enzymes so that nutrients can be absorbed.

Answer

Digestion in humans begins in the mouth, where saliva contains the enzyme salivary amylase that breaks starch into sugar. In the stomach, gastric juice with hydrochloric acid and the enzyme pepsin digests proteins and kills germs. In the small intestine, bile from the liver emulsifies fats, and pancreatic and intestinal juices complete the digestion of carbohydrates, proteins and fats. The digested food is absorbed into the blood through finger-like projections called villi, and the undigested waste passes to the large intestine and is egested.

What it is

Aerobic respiration uses oxygen and releases more energy; anaerobic respiration occurs without oxygen and releases less energy.

Answer

Aerobic respiration takes place in the presence of oxygen and completely breaks down glucose into carbon dioxide and water, releasing a large amount of energy; it occurs in the mitochondria. Anaerobic respiration takes place in the absence of oxygen and breaks glucose down incompletely, releasing much less energy; in our muscle cells it produces lactic acid (causing cramps during vigorous exercise), while in yeast it produces ethanol and carbon dioxide (used in baking and brewing).

What it is

Air passes through the nose, trachea and bronchi to the alveoli of the lungs, where gases are exchanged.

Answer

During breathing, air enters through the nostrils, passes into the nasal cavity where it is filtered, warmed and moistened, then travels down the pharynx and larynx into the windpipe (trachea). The trachea divides into two bronchi, one entering each lung, which further branch into finer tubes called bronchioles. Each bronchiole ends in tiny balloon-like sacs called alveoli. It is in the alveoli, which have thin walls and a rich blood supply, that oxygen from the air diffuses into the blood and carbon dioxide diffuses out to be exhaled.

What it is

The human heart has four chambers, and blood passes through it twice in one complete cycle - double circulation.

Answer

The human heart has four chambers: two upper atria and two lower ventricles. Double circulation means that blood passes through the heart twice in each complete cycle. In pulmonary circulation, deoxygenated blood from the right side of the heart goes to the lungs and returns oxygenated. In systemic circulation, this oxygenated blood from the left side is pumped to the whole body and returns deoxygenated. Double circulation is necessary because it keeps oxygenated and deoxygenated blood completely separate, ensuring an efficient supply of oxygen to the body, which is needed by warm-blooded animals to maintain their body temperature.

What it is

Xylem transports water and minerals upward; phloem transports food in both directions by translocation.

Answer

In plants, water and dissolved minerals absorbed by the roots are transported upward through the xylem. This upward movement is mainly driven by transpiration pull, the suction created when water evaporates from the leaves. Food (mainly sucrose) prepared in the leaves during photosynthesis is transported to all parts of the plant, including growing and storage regions, through the phloem; this movement is called translocation and requires energy from ATP.

What it is

The kidneys filter blood to form urine; the nephron is the functional unit that carries out filtration and reabsorption.

Answer

The functional unit of the kidney is the nephron. Urine is formed in three main steps. First, in glomerular filtration, blood is filtered under pressure in the glomerulus, and water, glucose, salts and urea pass into the nephron. Second, in selective reabsorption, useful substances such as glucose, most water and essential salts are reabsorbed back into the blood as the filtrate flows along the tubule. Third, the remaining liquid, containing urea, excess salts and water, forms urine, which passes to the urinary bladder through the ureters and is removed from the body.

What it is

A reflex action is a quick, automatic response to a stimulus, carried out through a pathway called the reflex arc.

Answer

A reflex action is a sudden, rapid and automatic (involuntary) response of the body to a stimulus, which protects us from harm, such as quickly withdrawing the hand on touching a hot object. It occurs through a pathway called the reflex arc. The path is: a receptor in the skin detects the stimulus and sends an impulse along a sensory neuron to the spinal cord; there, a connector (relay) neuron passes the impulse to a motor neuron, which carries it to the effector (a muscle) that brings about the response. The spinal cord processes reflexes quickly without waiting for the brain.

What it is

A neuron is the structural and functional unit of the nervous system that carries information as electrical impulses.

Answer

A neuron (nerve cell) has three main parts: dendrites, which receive information; the cell body (cyton), which contains the nucleus; and a long axon, which carries the impulse away. A nerve impulse is picked up by the dendrites, travels as an electrical signal through the cell body and along the axon to its endings. At the end of the axon, the electrical impulse causes the release of chemicals called neurotransmitters into the tiny gap called the synapse, which then start a new electrical impulse in the dendrite of the next neuron. Thus impulses travel in one direction.

What it is

The brain has three major regions - the forebrain, midbrain and hindbrain - each with specific functions.

Answer

The human brain has three main parts. The forebrain, whose main part is the cerebrum, is the centre for thinking, memory, reasoning and voluntary actions, and it receives sensory information. The midbrain controls reflex movements of the head, neck and eyes. The hindbrain includes the cerebellum, which maintains posture and balance and coordinates precise movements, and the medulla oblongata, which controls involuntary actions such as heartbeat, breathing and blood pressure.

What it is

Hormones are chemical messengers secreted by endocrine glands that coordinate slow, long-term body activities.

Answer

Four important hormones are: insulin, secreted by the pancreas, which controls the level of sugar (glucose) in the blood; thyroxine, secreted by the thyroid gland, which controls the rate of metabolism and needs iodine; adrenaline, secreted by the adrenal glands, which prepares the body for emergencies (the fight-or-flight response); and growth hormone, secreted by the pituitary gland, which controls the growth of the body. A deficiency of insulin causes diabetes and a deficiency of iodine (for thyroxine) causes goitre.

What it is

Plant hormones control growth, and tropisms are directional growth responses to stimuli like light and gravity.

Answer

Tropisms are directional growth movements of a plant in response to an external stimulus. In phototropism, shoots grow towards light (positive) and roots away from it (negative). In geotropism, roots grow downward towards gravity (positive) and shoots grow upward (negative). Other examples are hydrotropism (response to water) and chemotropism (response to chemicals). The plant hormone auxin is responsible for phototropism: it accumulates on the shaded side of the shoot, making those cells grow longer, so the shoot bends towards the light. Other plant hormones include gibberellins, cytokinins and abscisic acid.

What it is

Asexual reproduction produces offspring from a single parent by methods such as fission, budding, fragmentation, regeneration and spore formation.

Answer

Three methods of asexual reproduction are: binary fission, in which a single parent cell splits into two, as in Amoeba; budding, in which a small outgrowth (bud) develops on the parent, grows and detaches to form a new individual, as in Hydra and yeast; and spore formation, in which the parent produces many spores in a structure that burst and grow into new organisms under favourable conditions, as in the bread mould Rhizopus. Other methods include fragmentation (Spirogyra) and regeneration (Planaria).

What it is

In flowering plants, pollination transfers pollen to the stigma, and fertilisation forms a seed inside the fruit.

Answer

Pollination is the transfer of pollen grains from the anther (male part) to the stigma (female part) of a flower; it may be self-pollination (within the same flower) or cross-pollination (between different flowers), carried out by agents such as wind, water and insects. Fertilisation is the fusion of the male gamete (from the pollen) with the female gamete (egg) inside the ovule, forming a zygote. After fertilisation, the ovule develops into a seed and the ovary develops into a fruit.

What it is

The male system produces and delivers sperm; the female system produces eggs, and supports fertilisation and development of the baby.

Answer

In the male reproductive system, the testes produce sperms and the male hormone testosterone; the sperms are carried by the vas deferens and, along with secretions from glands, form semen that is released through the urethra. In the female reproductive system, the ovaries produce eggs (ova) and the hormones oestrogen and progesterone; the egg travels through the oviduct (fallopian tube), where fertilisation occurs; the fertilised egg then implants and develops into a baby in the uterus (womb), nourished through the placenta.

What it is

The menstrual cycle is the roughly monthly preparation of the female body for a possible pregnancy.

Answer

The menstrual cycle is a series of changes, of about 28 days, that occurs in the reproductive system of a woman to prepare for a possible pregnancy. In each cycle, an egg matures and is released from the ovary (ovulation), and the wall of the uterus thickens with blood vessels to receive a fertilised egg. If the egg is not fertilised, the thickened lining of the uterus breaks down and is shed along with blood through the vagina; this is called menstruation and lasts a few days. Methods used to prevent pregnancy are called contraception.

What it is

Mendel's experiments on pea plants gave the laws of dominance, segregation and independent assortment.

Answer

Mendel studied inheritance in pea plants and gave laws of heredity. The law of dominance states that in a pair of contrasting characters (alleles), one is dominant and expresses itself, while the other is recessive and remains hidden in the first generation; for example, in a cross between tall (TT) and short (tt) plants, all offspring are tall (Tt). The law of segregation states that the two factors (alleles) of a character separate during the formation of gametes, so each gamete carries only one factor of a pair, and they are randomly combined at fertilisation.

What it is

A monohybrid cross between two hybrids (Tt x Tt) gives a phenotypic ratio of 3:1 in the second generation.

Answer

When two heterozygous tall plants (Tt x Tt) are crossed, the possible combinations of gametes give the genotypes TT, Tt, Tt and tt, that is a genotypic ratio of 1 : 2 : 1. Since T (tall) is dominant over t (short), the plants TT, Tt and Tt are all tall, while tt is short. Therefore the phenotypic ratio is 3 tall : 1 short, that is 3:1. This shows that the recessive short trait reappears in the F2 generation.

What it is

The sex of a child in humans is determined by the sex chromosome (X or Y) inherited from the father.

Answer

In humans, there are 23 pairs of chromosomes, of which one pair is the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). All eggs produced by the mother carry an X chromosome, while sperms produced by the father are of two kinds: half carry X and half carry Y. If a sperm carrying an X chromosome fertilises the egg, the child is a girl (XX); if a sperm carrying a Y chromosome fertilises the egg, the child is a boy (XY). Therefore, the sex of the child is determined by the father.

What it is

Homologous organs have a similar structure but different functions; analogous organs have different structures but similar functions.

Answer

Homologous organs are organs that have a similar basic structure and origin but perform different functions in different animals; for example, the forelimbs of a human (arm), a bird (wing) and a whale (flipper) have the same basic bone structure but do different jobs. They indicate a common ancestor. Analogous organs are organs that have different basic structures but perform a similar function; for example, the wings of a bird and the wings of an insect are used for flight but are built differently. Homologous organs show evolutionary relationships.

What it is

A food chain shows the flow of energy from producers to consumers through trophic levels.

Answer

A food chain is a series of organisms through which energy flows as one organism eats another. It begins with producers (green plants) that make their own food, followed by consumers that feed on them. The different steps or feeding levels are called trophic levels: the first trophic level is the producers (grass); the second is herbivores or primary consumers (deer); the third is small carnivores or secondary consumers (snake); and the next is larger carnivores or tertiary consumers (eagle). Energy flows in one direction from one trophic level to the next.

What it is

Only about 10% of the energy at one trophic level is passed on to the next; the rest is lost mainly as heat.

Answer

The ten percent law, given by Lindeman, states that only about 10 percent of the energy available at one trophic level is transferred to the next higher trophic level; the remaining 90 percent is used by the organisms for their life processes and is lost, mostly as heat, to the environment. Because so much energy is lost at each step, less and less energy is available at higher levels, so food chains usually have only three to five trophic levels and cannot support very many top carnivores.

What it is

Decomposers break down dead matter and recycle nutrients; the ozone layer shields life from harmful UV rays.

Answer

Decomposers are micro-organisms such as bacteria and fungi that break down the dead remains and waste products of plants and animals into simpler substances. This releases nutrients back into the soil, keeping the environment clean and recycling materials so that the cycle of nutrients continues. The ozone layer, present in the upper atmosphere, is important because it absorbs most of the harmful ultraviolet (UV) radiation from the Sun, protecting living organisms from its damaging effects such as skin cancer. Man-made chemicals called chlorofluorocarbons (CFCs) damage the ozone layer.

What it is

Natural resources are managed sustainably by the three R's - reduce, reuse and recycle.

Answer

The three R's for the management of natural resources are: reduce, meaning to use fewer resources and avoid wastage (for example, switching off unnecessary lights); reuse, meaning to use items again instead of throwing them away (for example, reusing bottles and envelopes); and recycle, meaning to collect and process used materials such as paper, plastic and metal to make new products. Sustainable development is the careful use of resources to meet the needs of the present generation without harming the ability of future generations to meet their own needs.

What it is

Forests and water are vital resources conserved through community participation, such as the Chipko movement and water harvesting.

Answer

Forests can be conserved by preventing indiscriminate cutting of trees, by afforestation (planting more trees), and by involving local communities in their protection. The Chipko movement (Hug the Trees movement) was a community effort in a village in Uttarakhand in which local people, especially women, hugged trees to stop contractors from felling them, showing how community participation protects forests. Water can be conserved by rainwater harvesting, in which rainwater is collected and stored or allowed to seep underground to recharge groundwater, by building check dams, and by preventing water pollution and wastage.