DRAFT. Classification. Sorting things out. Chloroplasts and nervous systems

Transcription

1 Classification Sorting things out Imagine that you have downloaded lots of MP3 tracks but are finding it difficult to locate the right piece of music for the right occasion. Sorting the tracks into playlists of similar types of music would make finding the right music much easier. This is an example of classification. FIGURE 1: A playlist on an MP3 player. Chloroplasts and nervous systems A plant is made up of millions and millions of cells. > Many of the cells contain tiny structures called chloroplasts. > Within each chloroplast is a compound called chlorophyll. > It is chlorophyll that makes plants green and enables plants to make their own food. > The food-making process of plants is called photosynthesis. FIGURE 2: Chloroplasts in plant cells. Magnification Define the word classification. 2 List two important differences between plants and animals. Animals can move about. In many animals, movement occurs when an animal s muscles act on its skeleton. > Our skeleton is made of bones. > Contraction (shortening) of the muscles acts on the skeleton by pulling on the bones. > The result is that we move! Muscles contract because they are stimulated by nerve impulses sent to them by nerve cells. Bundles of nerve cells form nerves, which in turn are organised into the nervous system. nervous system spinal cord cranial nerves spinal nerves You will find out: > about classification > the main characteristics of vertebrates and invertebrates > about the five groups of vertebrate animals > the issues surrounding classification brain FIGURE 3: At full stretch a cheetah s nervous system enables it to run at 110 km/h. Having chloroplasts, that allow photosynthesis, is a characteristic that most plants have in common. Having a nervous system, that allows movement, is a characteristic that most animals have in common. These different characteristics are used to sort plants and animals into groups. Sorting living things which have characteristics in common into groups is called classification. For example: > plants have chloroplasts but no nervous system > animals have nervous systems but cannot make their own food. 2 Animal classification Comparing vertebrates and invertebrates

2 Unit B1, Topic 1: 1.1, 1.2, 1.3, 1.4 Grouping living things Living things are grouped together using the characteristics they have in common. Some characteristics are unique to the group, for example, feathers are a unique characteristic of birds. The backbone is another characteristic of birds. However, it is also a characteristic of fish, amphibians, reptiles and mammals. Having a backbone unites these groups (including birds) into a larger group: > animals with backbones are called vertebrates > animals without backbones (like worms and snails) are called invertebrates. fish > Most fish covered with wet scales. > Oxygen and carbon dioxide are exchanged between the fish and water across its gills Problems of classification (Higher tier only) Classifying some organisms is difficult. For example, viruses are not even cells. They do not seem to need food and to reproduce (replicate) must first enter a living cell. Once inside a cell, viruses take over the cell s biological machinery to reproduce new viruses. Viruses cause diseases in plants and animals. vertebrates (backboned animal with a skeleton made of bone) amphibians > Most amphibians have smooth, moist permeable skin (allows substances to pass across it in solution) FIGURE 5: Viruses seen in an electron microscope. Magnification Did you know? If dried and crystallised like minerals, viruses can be stored unchanged in sealed jars for years. Adding water to crystallised viruses renews their activity. reptiles > Most reptiles have dry, scaly skin 3 Give reasons why a bird is not an invertebrate animal. 4 Explain why most amphibia can live out of water for only short periods of time. birds > Most birds are covered with feathers and have a beak FIGURE 4: There are five main groups of vertebrate animals classified by their characteristics. Bacteria are difficult to classify because the characteristics used to organise them into groups can mammals > Most mammals have hair and females produce milk quickly change from one generation to the next. Their shape helps to identify different types of bacteria. Fungi (for example, mushrooms) used to be classified as plants, even if rather strange ones without chloroplasts or leaves. Nowadays fungi are classified in their own group because we know that many of their characteristics are very different from plants. Algae come in various colours and sizes. Some are red, some brown, while others are green. Their cells contain chloroplasts with varying structure in different algae groups. Body shape ranges from strap-like fronds of seaweeds common along the seashore, to thread-like filaments of pond weed, to minute cells that smother tree trunks with a powdery covering. The range of size and differences in the structure of the chloroplasts make it very difficult to classify algae with any certainty. FIGURE 6: Rod-shaped Escherichia coli bacteria seen in a scanning electron microscope. Magnification Explain why it is difficult to decide whether viruses are alive or not. 6 How would you identify different types of bacteria? 7 Why are fungi no longer classified as plants? Classification of living things 3

3 Naming species What s in a name? When Lewis Carroll wrote Through the Looking-Glass, he imagined Alice meeting Humpty Dumpty and talking about names. Humpty s reply to Alice suggests that his name describes his appearance. Since nobody else looks like Humpty, his name (and the appearance it describes) identifies him. FIGURE 1: Alice and Humpty Dumpty talk about names. Alice: Must a name mean something? Humpty Dumpty: Of course it must, my name means the shape I am. Confusing names A species is a particular type of living thing. The species of plant shown in Figure 2 is called different everyday names depending on where you are in the world. On the other hand different species may be called the same everyday name. For example, the North American robin is a different species from what is called a robin in the UK. You will find out: > about defining the term species > about the binomial system of naming > the importance of classification FIGURE 3: The the UK robin and the North American robin. How are these birds different from and similar to one another? The confusion over everyday names was cleared up by scientists agreeing to give each type of living thing a name in two parts: > a genus name which begins with a capital letter > a species name which begins with a small letter. The two-part name is printed in italics. For example, the two-part name of humans is Homo sapiens: > Homo is our genus name > sapiens is our species name. FIGURE 2: This plant is called cuckoo pint, lords and ladies, parson-in-thepulpit, depending on where you are in the UK. 1 Explain why everyday names are not a good way to describe living things. 2 How is the name of a genus and species printed? 4 Naming organisms Species identification

4 Unit B1, Topic 1: 1.5, 1.6, 1.7 Classifying organisms The Swedish scientist Carl Linnaeus tackled the problem of confusing everyday names. His book Systema Naturae (published in 1735) laid the foundations for the system we use today to name organisms, with each name in two parts. Since each name has two parts, the Linnaean method of naming is called the binomial system. So, the robins in Figure 3 become Erithacus rubecula (the UK robin) and Turdus migratorius (the North American robin). Groups within groups Linneaus system is more than just a list of names. Living things are grouped together using the characteristics they have in common. Some characteristics are unique to a species and identify it. However, other characteristics are shared with other species. This means that different species which have characteristics in common can be put together into a group. This group may be part of a larger group, and so on. Different groups are named following Linnaeus system. This system therefore not only gives genus and species names, but also creates groups within groups. The largest group of all is the kingdom. The animal kingdom is an example of this type of group: > each kingdom contains a number of phyla > a phylum (singular of phyla) contains a number of classes > a class contains a number of orders and so on, finishing with > a genus, which contains one or more species. Classifying living things identifies them. Accurate classification is important because it enables us to communicate information about the many species with which we share planet Earth. So far we know of about five million species, but this figure is only a fraction of the species awaiting discovery. The word biodiversity refers to the number of different species living in a particular environment. This is another reason why accurate classification is important. If we cannot identify species properly, describe them and know what they do, we cannot conserve them. Species become extinct and biodiversity suffers. Humans are then affected because our food supplies, medicine, new drugs and how we live ultimately depend on maintaining global biodiversity. What is a species? Why is classification important? (Higher tier only) Most biologists agree that if organisms are able to sexually reproduce offspring which themselves are able to reproduce sexually (the offspring are fertile) then they belong to the same species. However, sometimes individuals of closely related species can mate and reproduce. The offspring are hybrids. A mule is the hybrid offspring of a mating between a horse and a donkey. It is sterile which means that it cannot reproduce. Different species of plant can also produce hybrids between themselves. Unlike most animal hybrids, hybrid plants are often fertile and able to reproduce offspring. This questions what is a species?. FIGURE 4: The mule is a hybrid. 3 Explain the meaning of the word hybrid and give an example. 4 Rearrange the following groups into their correct order according to size: species, class, order, genus, phylum. Then provide an example of a species, a genus and a phylum. Did you know? Over 600 new species of beetle have been discovered living in just one type of tree that grows in the rainforests of Costa Rica. 5 Explain why the definition of a species is not entirely satisfactory. 6 Discuss the classification of living things and why classification is important. Plant hybrids Biodiversity hotspots 5

5 Identifying species and variation Identification During the Second World War ( ) gun operators were trained to spot aircraft. Charts showing the outlines (silhouettes) of different aircraft helped them with their work. The charts were keys which enabled the gunners to describe what they saw and arrive at an identification. What happened next depended on whether the aircraft were identified as friend or foe. Using keys How many living things can you see in the photograph? Describing the animals collected from the dead leaves littering the woodland floor, and matching your description of them with the descriptions in a key will help you to identify them. FIGURE 2: A woodland floor. 1 Explain how you would use a key to identify an organism you don t recognise. 2 Look at the different fruits shown below. Make a key of opposite statements using descriptions of the fruits appearance. FIGURE 1: Silhouettes of Second World War fighter aircraft. You will find out: > how to use keys > about variation The descriptions on the key shown in Figure 3 are clues to identifying the animals found in leaf litter. > Notice that the descriptions come as pairs of opposite statements. > Choosing one statement of a pair leads you to the next paired statement, and so on. > By working through the key in this way you arrive at a statement that identifies the animal in question. Therefore, in biology, a key is a guide to a name. Different keys are used to identify different living things. No legs Has the animal got legs? Legs No shell Shell 6 legs 8 legs 14 legs Over 14 legs No Segments segments Insects Woodlice Slugs Snails Body in one part Body in Centipedes two parts Worms Pupae and larvae Spiders Millipedes FIGURE 3: Using a key. Crane fly (Daddy longlegs) 6 Using biological keys Human variation

6 Unit B1, Topic 1: 1.8, 1.9, 1.10 Variation Humans all belong to one species Homo sapiens. However, people are different from one another. When you are next in a crowd look at the people near to you. Notice their differently coloured skin, eyes and hair, and differently shaped faces. The term variation refers to the differences in the particular characteristics of a species. Continuous and discontinuous variation The variation in some characteristics is spread over a range of values. We say that the characteristic shows continuous variation. Height of people is an example. A few people are either very short or very tall and there is a full range of in-betweens. Try to roll your tongue. Are you a tongue roller or can you not manage the trick, even though you try and try? Tongue rolling is a characteristic that shows discontinuous variation. There are no in-betweens (intermediates). People can be put into distinctly different groups: people who can roll their tongue and people who cannot there are no half-rollers! The A, B, AB and O blood grouping system in humans is another example of discontinuous variation. Different types of variation can be shown by using different types of graphs. Height as an example of continuous variation is best displayed by a line graph (see Figure 4). Whereas, tongue rolling as an example of discontinuous variation is more clearly shown by a bar graph (see Figure 5). Causes of variation Variations in characteristics resulting from genetic causes may be inherited by offspring from their parents. Recall that the characteristics may vary continuously or discontinuously. Characteristics which vary continuously are usually the result of the activity of numerous sets of genes. Characteristics which vary discontinuously are usually the result of the activity of one set of genes. Variations in characteristics resulting from environmental causes are not inherited. We say that these variations are acquired. For example, training (environmental cause) helps a weightlifter to develop bulging muscles. However, this does not mean that his or her children will have bulging muscles unless they train and take up weightlifting as well. number of people (%) height (cm) rollers tongue rolling non-rollers 3 Define the word variation and give examples of how people vary. 4 Suggest what examples of continuous variation and discontinuous variation you can see in yourself? number of people (%) Did you know? FIGURE 4: Height of people: an example of continuous variation. What is the height range of most people? FIGURE 5: Tongue rolling: an example of discontinuous variation. Plants usually grow faster, the brighter the light and the warmer it is (within limits of course!). These are examples of environmental variation. 5 Explain the difference between inherited characteristics and acquired characteristics. 6 Give a genetic explanation of why characteristics vary either continuously or discontinuously. Inherited characteristics Acquired characteristics 7