Topic one

The binomial system

There are millions of species on our planet. Although species can be very different from each other, many have similar features that allow us to put them into groups. This is called classification.Kingdoms

The first big division of living things in the classification system is to put them into one of five kingdoms. These are based on what an organism's cells are like.The five kingdoms are:. animals (all multicellular animals)

. plants (all green plants)

. fungi (moulds, mushrooms, yeast)

. prokaryotes (bacteria, blue-green algae)

. protoctists (Amoeba, Chlorella and Plasmodium)

Further divisions

Living things can then be ranked according to:phylumclass

order

family

genusspecies

The binomial system of naming species uses Latin words. Each name has two parts, the genus and the species. For example, human beings belong to the genus Homo, and our species is sapiens - so the scientific name is Homo sapiens.

Erithacus rubecula, the European RobinThe binomial system is important because it allows scientists to accurately identify individual species. For example, the European robin is Erithacus rubecula. It is much smaller than the American robin, Turdus migratorius, which belongs to a different genus.

Other classification systems

Cladistics is another way to classify organisms. It can use data from DNA or RNA sequences, rather than just physical characteristics. It emphasises the evolutionary relationships between different species.

Vertebrates

Vertebrates are animals with backbones. They can be classified according to their features, and include bony fish, amphibians, reptiles, birds and mammals.

Bony fish

Bony fish absorb oxygen through their gills and they have wet scales. Their body temperature varies according to their surroundings (organisms that do this are called poikilotherms). Bony fish reproduce by external fertilisation and lay eggs (they are oviparous).

Amphibians

Amphibians usually have lungs but they can also absorb oxygen through their moist skin. Their body temperature also varies according to their surroundings. Amphibians also reproduce by external fertilisation and lay eggs.

Reptiles

Reptiles have lungs and dry scales. As with bony fish and amphibians, their body temperature varies according to their surroundings. However, reptiles reproduce by internal fertilisation and lay eggs.

Birds

Birds have lungs, feathers and beaks. They can maintain their body temperature (organisms that do this are called homeotherms). Like reptiles, birds reproduce by internal fertilisation and lay eggs.

Mammals

Mammals have lungs and fur. Like birds, they can maintain their body temperature. Mammals reproduce by internal fertilisation and they give birth to live young (they are viviparous).

Invertebrates – arthropods

Invertebrates are animals without backbones. They include annelids, nematodes, molluscs and arthropods.Arthropods are an important phylum of invertebrates. They have:. a hard exoskeleton (their skeleton is on the outside

rather than on the inside)

. a segmented body (their body has different sections)

jointed legs

Other invertebrates

Other invertebrate groups include:annelids

nematodes

molluscs

Unlike arthropods, these invertebrates do not have jointed legs.

Annelids

Annelids are segmented worms with soft bodies. Most of the worm’s body consists of identical segments, giving it a ‘ringed’ appearance.

Nematodes

Nematodes are tiny, thin worms without segments. Many species are free-living but others are parasites that cause disease. For example, hookworms can infect humans.

Molluscs

Molluscs usually have a broad muscular foot and may also have a shell. Examples of molluscs include snails, slugs, squids and octopuses.Flowering plants

Flowering plants can be classified according to the nature of their seeds, seedlings and leaves. The two largest groups of flowering plants are:Monocotyledons ,eudicotyledons (also called dicotyledons)

Monocotyledons

Wheat plants are monocotyledonsThe seeds of monocotyledons each contain one embryonic leaf (the ‘cotyledon’). Many monocotyledons have leaves with parallel veins and the parts of their flowers come in threes. Examples of monocotyledons include:palms

orchids

grasses

Eudicotyledons

Sunflower plants are eudicotyledonsThe seeds of eudicotyledons each contain two embryonic leaves. Eudicotyledons have leaves with branching veins, and the parts of their flowers come in fours or fives. Examples of eudicotyledons include: oak trees, dandelions.

Keys and identification

Keys are used to identify different species. A key will usually ask questions based on easily identifiable features of an organism. Dichotomous keys use questions to which there are only two answers. They can be presented as a table of questions, or as a branching tree of questions.

Characteristics of living organisms

Living organisms have the following characteristics in common:. Movement - they can move and change their position.

. Reproduction – they can make more of the same kind of organism as themselves.

. Sensitivity – they can detect or sense stimuli and respond to them.

. Growth - they can permanently increase their size or dry mass by increasing the number or size of their cells.

. Respiration – they can create chemical reactions that break down

nutrient molecules in living cells to release energy.

. Excretion – they can excrete toxic materials, waste products of metabolism, and excess substances (note that excretion is not the same as egestion).

. Nutrition - they can take in and absorb nutrients such as organic substances and mineral ions. These nutrients contain the raw materials or energy needed for growth and tissue repair.

The first letter of each of these characteristics makes up the acronym ‘MRS GREN’. This is a good way of remembering them.

Living organisms can reproduceLiving organisms can also control their internal conditions, such as their temperature or water content.

Animals

Animals are multicellular organisms – they consist of many cells that work together. Examples of animals include mammals (such as humans) and insects (such as houseflies and mosquitoes).

Animal cell structure

The main parts of an animal cell are the nucleus, cell membrane and cytoplasm.

Animal cells:

. do not have cell walls

. do not contain chloroplasts, so animals cannot carry out photosynthesis

. may store carbohydrate as glycogenAnimals usually have nerves or nervous systems for coordination, and they are able to move from place to place.

Plants

Plants are also multicellular organisms – they consist of many cells that work together. Examples include cereals (such as maize), and peas and beans.

Plant cell structure

Plant cells contain the same parts as animal cells. They also have some additional ones:. chloroplasts

. cell wall made of cellulose

. permanent vacuole

Other features of plants

Plant cells contain chloroplasts so plants can carry out photosynthesis. They store carbohydrates as starch or sucrose.

Cell structure

Animal cells and plant cells have certain structures in common.Structure Function

Cytoplasm Where most of the chemical reactions happen

Nucleus Contains genetic material which controls the cell’s activities

Cell membrane

Controls the movement of substances in and out of the cell

Mitochondria

Where most energy is released in respiration

Ribosomes

Where protein synthesis happens

Plant cells have additional structures.Structure Function

Chloroplast Absorb light energy for photosynthesis

Cell wall Strengthens the cell and supports the plant

Permanent vacuole

Filled with cell sap to help keep the cell turgid

Animal cells may also have vacuoles, but these are small and temporary.

Fungi

Mushrooms, toadstools and moulds (such as Mucor) are multicellular fungi. Yeast is an example of a single-celled fungus.Fungal cell structure

Fungal cells have a cell wall made of chitin (remember that plant cell walls are made of cellulose).

A yeast cell

Athlete’s foot, caused by a fungusSome fungi are pathogens, for example the fungal infection which causes athlete’s foot.

Fungal structure

A multicellular fungus showing rounded spore cases and spores and thread-like hyphaeMulticellular fungi, such as Mucor, are organised into a mycelium - which is made from thread-like structures called hyphae.The hyphae contain many nuclei.

Fungal nutrition

Fungi cannot carry out photosynthesis. Instead they use saprotrophic nutrition. They secrete enzymes onto their food so that digestion happens outside the fungal cells. They then absorb the digested organic products.Fungal cells may store carbohydrate as glycogen (remember that plant cells store carbohydrate as starch).

Bacteria

Rod-shaped Escherichia coli (E.coli) bacteria,commonly found in the

intestines of animals

Bacteria are microscopic single-celled organisms. Examples of bacteria include:. Lactobacillus bulgaricus (a rod-shaped bacterium used to

make yoghurt from milk)

. Pneumococcus (a spherical bacterium that acts as the pathogen that causes pneumonia)

Bacterial cell structure

Bacterial cells have a cell wall made of polysaccharides and proteins. They do not have a nucleus, but instead they have a circular chromosome of DNA. They may also have small extra circles of DNA called plasmids.

Other features of bacteria

Some bacteria can carry out photosynthesis, but most bacteria feed from other organisms (living or dead).Some bacteria are pathogens, for example Pneumococcus (which causes pneumonia).

Protoctists

Protoctists are microscopic single-celled organisms.Some protoctists, such as Amoeba, have features like an animal cell. Others, such as Chlorella, have chloroplasts and are more like plants.Some protoctists are pathogens. For example, Plasmodiumis, the pathogen that causes malaria.

Viruses

Viruses are very small particles capable of infecting every type of living organism. They are parasitic and can only reproduce inside living cells. For example:. the tobacco mosaic virus – this stops chloroplasts

forming in tobacco plants and causes the tobacco leaves to become discoloured

. the influenza virus – this causes flu

. HIV (human immunodeficiency virus) – this causes AIDS

Virus structure

Virus particles have a variety of shapes. They do not have a cellular structure. Instead, they have a core of genetic material surrounded by a protein coat. Their genetic material can be DNA or RNA, but not both.

The structure of a hepatitis virus

Cells, tissues and organs

In order of increasing complexity, multicellular organisms consist of:organelles → cells → tissues → organs → organ systems

Structure Description

Organelle Cell structure that is specialised to carry out a particular function or Job

Cell Basic structural and functional unit of a living organism

Tissue Group of cells with similar structures, working together to perform a shared function

Organ Structure made up of a group of tissues, working together to perform specific functions

Organ system Group of organs with related functions, working together to perform body functions