Cambridge IGCSE Biology: every topic, by paper

What are the seven characteristics of living organisms?

Movement, respiration, sensitivity, growth, reproduction, excretion and nutrition (often remembered as MRS GREN).

Define excretion.

The removal of waste products of metabolism, toxic materials and substances in excess of requirements from the body.

Define growth as a characteristic of living things.

A permanent increase in size and dry mass.

Name four structures found in both animal and plant cells.

Nucleus, cytoplasm, cell membrane and mitochondria (also ribosomes).

Name three structures found in plant cells but not animal cells.

A cellulose cell wall, chloroplasts and a large permanent vacuole.

What is the function of the nucleus?

It contains the genetic material (DNA in chromosomes), which controls the activities of the cell.

Define diffusion.

The net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient), as a result of their random movement.

Where does the energy for diffusion come from?

The kinetic energy of the random movement of molecules and ions. No energy from respiration is needed.

Give four factors that increase the rate of diffusion.

A larger surface area, a higher temperature, a steeper concentration gradient and a shorter diffusion distance.

Which chemical elements make up carbohydrates?

Carbon, hydrogen and oxygen.

Which chemical elements make up fats and oils?

Carbon, hydrogen and oxygen.

Which chemical elements make up proteins?

Carbon, hydrogen, oxygen and nitrogen (some also contain sulfur).

What is an enzyme?

A protein that functions as a biological catalyst, speeding up metabolic reactions.

Define a catalyst.

A substance that increases the rate of a chemical reaction and is not changed by the reaction.

What is the active site of an enzyme?

The region of the enzyme where the substrate binds and the reaction takes place.

Define photosynthesis.

The process by which plants make carbohydrates from raw materials using energy from light.

Give the word equation for photosynthesis.

Carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).

Give the balanced symbol equation for photosynthesis.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy, chlorophyll).

What is a balanced diet?

A diet containing all the nutrients in the right amounts: carbohydrates, fats, proteins, vitamins, minerals, fibre and water.

What is the main use of carbohydrates in the diet?

A source of energy.

What is the main use of protein in the diet?

Growth and repair of body tissues.

What does xylem transport?

Water and mineral ions, from the roots to the rest of the plant.

What does phloem transport?

Sucrose and amino acids (dissolved food), both up and down the plant.

Describe the structure of xylem vessels.

Long, hollow tubes of dead cells with no end walls, strengthened with lignin; they also support the plant.

What is a double circulation?

A circulatory system in which blood passes through the heart twice for each complete circuit of the body: once to the lungs and once to the rest of the body.

What is the advantage of a double circulation?

Blood returning from the lungs is pumped again, so it reaches the body's organs at high pressure and flows faster, delivering oxygen quickly.

How does the circulation of a fish differ from that of a mammal?

A fish has a single circulation: blood passes through the heart once per circuit, going from the heart to the gills and then straight on to the body.

What is a pathogen?

A disease-causing organism, such as a bacterium, virus, fungus or protist.

What is a transmissible disease?

A disease in which the pathogen can be passed from one host to another.

Give ways pathogens can be spread.

Direct contact (e.g. blood and other body fluids) or indirect contact (contaminated surfaces, food, water, animals or the air).

List the parts of the respiratory system in the order air passes through them.

Nose or mouth → larynx → trachea → bronchi → bronchioles → alveoli.

Why does the trachea have rings of cartilage?

They keep the trachea open so it does not collapse when air pressure inside falls during inhalation.

List four features of the alveoli that make gas exchange efficient.

A very large total surface area, walls one cell thick (short diffusion distance), a good blood supply from capillaries, and good ventilation.

Define respiration.

The chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

Give five uses of the energy released by respiration.

Muscle contraction, protein synthesis, cell division, active transport and growth; also nerve impulses and maintaining a constant body temperature.

Where in the cell does aerobic respiration take place?

Mainly in the mitochondria.

Define excretion.

The removal of waste products of metabolism, toxic materials and substances in excess of requirements from the body.

Name three excretory products in humans and the organ that removes each.

Carbon dioxide (lungs), urea (kidneys) and excess water and salts (kidneys, and some in sweat through the skin).

Where is urea made, and from what?

In the liver, from excess amino acids (deamination).

What makes up the central nervous system (CNS)?

The brain and spinal cord.

What is the peripheral nervous system?

The nerves outside the brain and spinal cord that connect the CNS to the rest of the body.

Name the three types of neurone.

Sensory neurones, relay neurones and motor neurones.

Define a hormone.

A chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Compare nervous and hormonal control.

Nervous: electrical impulses along neurones, very fast, short-lived and precise. Hormonal: chemicals carried in the blood, slower, longer-lasting and often more widespread.

Where is adrenaline made, and what are its effects?

In the adrenal glands; it prepares the body for "fight or flight" by increasing heart rate and breathing rate, widening the pupils and raising blood glucose.

What is a drug?

Any substance taken into the body that modifies or affects chemical reactions in the body.

What is an antibiotic?

A drug that kills bacteria or stops their growth.

Why do antibiotics not work against viruses?

Viruses do not have the structures or processes that antibiotics target, and they reproduce inside the host's own cells.

What is asexual reproduction?

Reproduction involving one parent, producing offspring that are genetically identical to the parent (and to each other).

What is sexual reproduction?

Reproduction involving the fusion of the nuclei of two gametes (sex cells) to form a zygote, producing genetically different offspring.

Define fertilisation.

The fusion of the nuclei of a male gamete and a female gamete to form a zygote.

What is a chromosome?

A thread-like structure of DNA, carrying genetic information in the form of genes.

What is a gene?

A length of DNA that codes for a protein.

What is an allele?

An alternative form of a gene.

What is variation?

Differences between individuals of the same species.

Compare continuous and discontinuous variation.

Continuous variation gives a range of phenotypes between two extremes (e.g. height, mass); discontinuous variation gives a limited number of distinct phenotypes with no intermediates (e.g. blood group).

What usually causes discontinuous variation?

Genes alone.

Define a population.

A group of organisms of one species, living in the same area at the same time.

Define a community.

All of the populations of different species in an ecosystem.

Define an ecosystem.

A unit containing the community of organisms and their environment, interacting together.

What is biodiversity?

The variety of different species (and genetic variety) in an area.

Why is high biodiversity important?

It makes ecosystems more stable, reduces dependence on one species for food or materials, and preserves species that may be useful, e.g. for medicines.

Give four reasons for deforestation.

To clear land for farming (e.g. cattle, rice, palm oil), to build houses and roads, for timber, and for fuel.

Why are bacteria useful in biotechnology and genetic modification?

They reproduce rapidly and can make complex molecules; using them raises few ethical concerns, and their plasmids are easy to modify.

Why is yeast used in bread-making?

Yeast respires anaerobically, producing carbon dioxide, which makes the dough rise.

Why is yeast used to make biofuels such as ethanol?

Yeast ferments sugars from plant material anaerobically, producing ethanol.