Characteristics and classification of living organisms

Every living thing shares the same seven characteristics 特征 — seven life processes 生命过程 that all organisms 生物体 carry out. You can remember them with the letters MRS GREN. The examiner gives marks for the exact wording, so learn each definition.

The seven life processes spell MRS GREN

• Movement 运动 — an action by an organism, or part of an organism, that changes its position or place.
• Respiration 呼吸作用 — the chemical reactions in cells 细胞 that break down nutrient molecules 营养物质 and release energy 能量 for metabolism 新陈代谢.
• Sensitivity 应激性 — the ability to detect and respond to changes in the internal or external environment 环境.
• Growth 生长 — a permanent increase in size and dry mass 干重.
• Reproduction 生殖 — the processes that make more of the same kind of organism.
• Excretion 排泄 — the removal of the waste products 废物 of metabolism, and of substances the body has in excess (more than it needs).
• Nutrition 营养 — the taking in of materials for energy, growth and development.

Metabolism means all the chemical reactions that happen inside an organism's cells.

There are millions of kinds of living thing. To study them, scientists classify 分类 them — they sort them into groups by the features they share. Organisms in the same group share more features with each other than with organisms in other groups.

Species and the binomial system

The smallest common group is the species 物种. A species is a group of organisms that can reproduce to make fertile offspring 可育后代 ("fertile" means the offspring can themselves go on to have young).

A scientific name: a capital-letter genus and a small-letter species

Each species has a two-part scientific name 学名. This way of naming is the binomial system 双名法 — a system agreed all over the world, so every scientist uses the same name.

• The first part is the genus 属 (a larger group that the species belongs to). It starts with a capital letter.
• The second part is the species. It starts with a small letter.

The whole name is written in italics, for example Homo sapiens (humans). Two rats named Rattus norvegicus and Rattus rattus belong to the same genus but are different species.

Dichotomous keys

A dichotomous key 二歧检索表 is a tool that helps you identify an unknown organism. "Dichotomous" means "splitting into two". At each step the key gives you two choices about a feature. You pick the one that fits your organism, and that choice sends you on to the next pair of choices. You repeat this until you reach the organism's name.

A dichotomous key: each step gives two choices until you reach the organism

When you build a key, choose clear features you can see — for example "has wings / has no wings" — not features that change, such as size.

Worked example. A key reads: step 1, has wings, go to 2; no wings, go to 3. Step 2, one pair of wings, it is a housefly; two pairs of wings, it is a butterfly. An unknown insect has two pairs of wings. Step 1 only asks "wings or no wings?", so having wings sends you to step 2. At step 2, "two pairs" gives butterfly. Follow the route the key gives you, one step at a time - never jump straight to the name that looks right.

Classification and DNA (Supplement)

Modern classification tries to show evolutionary relationships 进化关系 — how closely different organisms are related. Organisms that are more closely related share a more recent ancestor 祖先.

To measure this, scientists compare the base sequence 碱基序列 of DNA (the order of the chemical "letters" that make up an organism's genes 基因). The rule is simple:

• Organisms that share a more recent ancestor (more closely related) have more similar DNA base sequences.
• Organisms that share only a distant ancestor have less similar base sequences.

So a DNA base sequence can be used both to classify organisms and to work out how closely two species are related.

The biggest groups that organisms are sorted into are called kingdoms 界. At Core level you place organisms into the first two kingdoms below; for Supplement you need all five kingdoms. You decide where an organism belongs by looking at its main features.

Living organisms are sorted into five kingdoms by their main features

The fungus, prokaryote and protoctist kingdoms are Supplement only.

A fungus: the fly agaric, a mushroom

A prokaryote (bacterium): E. coli seen under an electron microscope

Grouping the animal kingdom

The animal kingdom is split first into two: animals with a backbone and animals without one.

The animal kingdom splits into vertebrates (with a backbone) and arthropods (no backbone), each with its main groups

Vertebrates 脊椎动物 are animals with a backbone 脊柱. There are five main groups.

Arthropods 节肢动物 are animals with no backbone. They have a hard outer skeleton (an exoskeleton 外骨骼) and legs that bend at joints 关节. There are four main groups.

Grouping the plant kingdom (Supplement)

Plants are placed into groups too. You need two of them.

• Ferns 蕨类植物 — have roots, stems and leaves, but make no flowers or seeds. They reproduce using tiny spores 孢子.
• Flowering plants 开花植物 — make flowers, and form seeds 种子 inside the flower. They split into two groups by their seed-leaves (the first leaves inside a seed, called cotyledons 子叶):

Monocot leaves have parallel veins; dicot leaves have a net of branching veins

Viruses (Supplement)

Viruses 病毒 are not placed in any kingdom. They are not made of cells, so many scientists do not count them as living. A virus is very simple, with only two parts:

• an outer protein coat 蛋白质外壳, and
• genetic material 遗传物质 (its genes) inside.

A virus is just a protein coat around its genetic material

A virus cannot carry out the life processes on its own. It can only reproduce inside the living cells of a host 宿主 — the organism it infects.

• Learn the seven characteristics by heart, with their exact definitions. Write them as clear, full sentences.
• Digestion 消化 is not one of the seven characteristics — it is just one part of nutrition. "Breathing" is not one of the seven either.
• In a scientific name, the genus starts with a capital letter and the species with a small letter, and both are in italics.
• To use a dichotomous key, start at the first pair of choices and follow the route step by step. Do not jump ahead.
• More similar DNA base sequence → the organisms are more closely related (they share a more recent ancestor).