Cambridge IGCSE Biology

Characteristics of living organisms

Every living organism carries out seven processes, and something that does not do all of them is not alive in the ordinary sense.

• Movement is an action by an organism, or by part of it, that changes its position or place. A plant does not usually change place, but its shoots and leaves move as they grow towards light.
• Respiration is the set of reactions inside cells that split nutrient molecules apart and release the energy that metabolism needs.
• Sensitivity is the ability to detect changes in the internal or external environment and respond to them.
• Growth is a permanent increase in size and dry mass. Dry mass is used because fresh mass changes with the water content, which can go up and down without any real growth.
• Reproduction is the set of processes that make more organisms of the same kind.
• Excretion is the removal of the waste products of metabolism and of substances present in excess of what the body needs.
• Nutrition is taking in materials that provide energy, growth and development.

Species and naming

Organisms are sorted into groups according to the features they share. The basic group is the species, a group of organisms that can reproduce with each other to produce fertile offspring. A horse and a donkey can breed, but their offspring (a mule) is sterile, so they are separate species.

Scientists everywhere use the binomial system, which gives each species a two-part Latin name agreed internationally. The first part is the genus, written with a capital letter, and the second is the species, written in lower case. The whole name is italicised or underlined, so humans are Homo sapiens. Two species in the same genus share the first word, which tells you they are closely related. Common names differ between languages and even between regions, and the binomial name removes that confusion.

Dichotomous keys

A dichotomous key identifies an organism by a series of paired statements, each based on an observable feature, where every choice either gives a name or sends you to another numbered pair. The features used must be ones that can be seen without dissection or measurement that depends on age, such as the number of legs or whether there is a shell.

A key for four small invertebrates might read:

To build a key, choose a feature that splits the group into two parts, then split each part again with a different feature until every organism is on its own.

Classification and evolutionary relationships

Organisms that share features are grouped together, and the five largest groups are the kingdoms. Supplement only: a classification system aims to reflect evolutionary relationships, so that organisms placed together share a recent common ancestor. Because the sequence of bases in DNA is passed on and changes slowly through mutation, comparing sequences gives a way of classifying. Organisms with a more recent common ancestor have DNA base sequences that are more alike than those of organisms whose shared ancestor lived long ago, and this lets biologists test groupings that were first made from appearance alone.

The animal and plant kingdoms

Animals are multicellular. Their cells have no cell walls or chloroplasts, and they feed on other organisms, taking in organic food. Plants are multicellular too, with cells that have cellulose walls and chloroplasts, and they make their own food by photosynthesis.

Vertebrates

Vertebrates have a backbone. They are divided into five groups.

Arthropods

Arthropods are animals without a backbone that have jointed legs, a hard external skeleton and a segmented body. There are four groups you need to tell apart.

Ferns and flowering plants

Supplement only: plants are divided into ferns and flowering plants. Ferns have leaves called fronds, and they have roots and stems, but no flowers, and they reproduce by releasing spores. Flowering plants have flowers and produce seeds inside fruits. They split into two groups by the number of seed leaves (cotyledons) in the seed.

The five kingdoms

Supplement only: all organisms are placed in one of five kingdoms.

Viruses

Supplement only: a virus is not a cell. It has genetic material surrounded by a protein coat and nothing else, it has no cytoplasm or ribosomes, and it can reproduce only by taking over a host cell. Viruses are not placed in any of the five kingdoms.