0610 · 1.3Features of organisms

Animal and plant kingdoms

0:00A bat flies. It has wings, it flaps them, and it hunts insects in the air, just like many birds do. So is a bat a bird? It is not. A bat is a mammal, in the same group as dogs, cows and you. Biologists do not decide where an organism belongs by what it does. They look at particular features of its body, and once you know which features to look for, sorting an organism into its group becomes straightforward.

0:23By the end of this video you will know the main features used to place organisms into the animal and plant kingdoms, the features of the five groups of vertebrates and the four groups of arthropods, and how to use those features to classify an organism you have never seen before. For the extended syllabus, we will also cover all five kingdoms, the groups within the plant kingdom, and the features of viruses.

0:44In the last video we saw that organisms are classified into groups by the features they share, and that the biggest groups are called kingdoms. Let us start with the two kingdoms you know best, animals and plants. The main features of animals are these. Animals are multicellular, which means they are made of many cells. Their cells have a nucleus, but no cell wall and no chloroplasts. And animals feed on organic substances made by other organisms.

1:09In other words, they cannot make their own food, so they have to eat plants or other animals. The main features of plants are these. Plants are also multicellular, and their cells have a nucleus. But plant cells contain chloroplasts, which they use to make their own food by photosynthesis. And every plant cell is surrounded by a cell wall made of cellulose. So the quickest test between the two kingdoms is this: chloroplasts and a cellulose cell wall mean a plant, and no chloroplasts and no cell wall mean an animal.

Classifying vertebrates

1:36Now let us look inside the animal kingdom. Animals with a backbone are called vertebrates. The syllabus asks you to know the main features of five groups of vertebrates: mammals, birds, reptiles, amphibians and fish. For each one, think about its skin covering, how it breathes and how it reproduces. Mammals have fur or hair on their skin. They have external ears, which are the flaps of skin on the outside of the head called pinnae.

2:01And female mammals have mammary glands, which produce milk to feed their young. Almost all mammals give birth to live young rather than laying eggs. That is why the bat is a mammal. It has fur, it has external ears, and it feeds its babies on milk. Its wings are made of skin, not feathers. Birds have feathers covering their body. They have a beak and no teeth, and their front limbs are wings.

2:23They lay eggs with a hard shell. Feathers are the feature to remember, because no other group has them. So a penguin cannot fly, but it has feathers and a beak, so it is a bird. Reptiles, such as lizards, snakes, crocodiles and turtles, have dry skin covered in scales. They lay eggs with a soft, rubbery shell, usually on land. Amphibians, such as frogs, toads and newts, have moist, smooth skin with no scales.

2:46They lay soft eggs covered in jelly, in water. Their young, like tadpoles, live in water and breathe using gills. The adults usually live on land and breathe using lungs, though they must keep their skin moist. That double life in water and on land is where the name amphibian comes from. Fish have scales covering their body, and fins for swimming. They breathe using gills, and most fish lay soft eggs in water.

3:10The trap here is to classify an animal by where it lives. A whale lives in the sea and has a fish-like shape, but it breathes air with lungs, has a little hair and feeds its young on milk, so it is a mammal. A snake has no legs, but it has dry scaly skin, so it is a reptile. Always use the body features, not the habitat or the way the animal moves.

Arthropod groups and classification

3:31Animals without a backbone are called invertebrates, and the biggest group of them is the arthropods. All arthropods have three features in common. They have an exoskeleton, which is a hard outer skeleton on the outside of the body. They have jointed legs, which bend at joints like ours. And their body is divided into segments. The syllabus asks you to know four groups of arthropods: myriapods, insects, arachnids and crustaceans.

3:54The features to count are body parts, legs and antennae. Myriapods, such as centipedes and millipedes, have a long body made of many segments, with legs on every segment. They have one pair of antennae. The name means many feet. Insects have a body in three parts: the head, the thorax and the abdomen. They have three pairs of legs, which is six legs in total, and all six are attached to the thorax, the middle part.

4:18They have one pair of antennae, and most insects have one or two pairs of wings. Arachnids, such as spiders, scorpions and ticks, have a body in two parts. They have four pairs of legs, which is eight legs, and they have no antennae at all. Students often call spiders insects, but count the legs and the body parts. A spider has eight legs and two body parts, so it is an arachnid, not an insect.

4:41Crustaceans, such as crabs, lobsters, prawns and woodlice, have more than four pairs of legs, and they have two pairs of antennae. Their exoskeleton is often very hard, and most of them are found in water. Woodlice are the odd ones out, because they are found on land, under logs and stones. But they have two pairs of antennae and seven pairs of legs, so they are still crustaceans. So to sort an arthropod, ask three questions.

5:03How many body parts does it have? How many pairs of legs? And how many pairs of antennae? Insects have three body parts, three pairs of legs and one pair of antennae. Arachnids have two body parts, four pairs of legs and no antennae. Crustaceans have more than four pairs of legs and two pairs of antennae. And myriapods have many segments with legs on each, and one pair of antennae.

5:26Now let us use these features to classify an organism, which is the next thing the syllabus asks. Here is an animal you may not know. It has an exoskeleton, jointed legs and a segmented body, so it is an arthropod. It has two body parts, four pairs of legs and no antennae. It also has large claws and a curled tail with a stinger. Which group is it in?

5:47Ignore the claws and the stinger for a moment, because they are not the features the groups are defined by. Two body parts, eight legs and no antennae make it an arachnid. This animal is a scorpion. In the exam, you will often be shown a drawing of an unfamiliar animal, and the skill is to pick out the features that matter, count them carefully, and ignore the rest. Try one more.

6:08An animal has a backbone and smooth, moist skin with no scales, and it lays its eggs in jelly in a pond. It has a backbone, so it is a vertebrate. Smooth, moist skin without scales and jelly-covered eggs laid in water are the features of amphibians, so it is an amphibian.

The other three kingdoms

6:23Now for the extended syllabus. Animals and plants are only two of the kingdoms. Biologists place all organisms into one of five kingdoms: animal, plant, fungus, prokaryote and protoctist. We have already seen the features of animals and plants, so let us look at the other three. Fungi include mushrooms, moulds and yeast. Most fungi are made of thin threads called hyphae, which spread through their food and together form a network called a mycelium.

6:49Yeast is an exception, because it is single-celled. Fungal cells have a nucleus and a cell wall, but the wall is made of a substance called chitin, not cellulose. Fungi have no chloroplasts, so they cannot photosynthesise. Instead, they feed by releasing enzymes onto dead or decaying material, such as a rotting log, and absorbing the digested food. This way of feeding is called saprotrophic nutrition. This is why fungi are not plants, even though mushrooms grow out of the ground and stay in one place.

7:16They have no chloroplasts, their cell walls are not made of cellulose, and they do not make their own food. Prokaryotes are the bacteria. Every prokaryote is a single cell, and these cells are much smaller than plant or animal cells. The most important feature is that a prokaryote has no nucleus. Its DNA is a single circular loop, lying free in the cytoplasm. Prokaryote cells have a cell wall, but it is not made of cellulose.

7:39Many also contain plasmids, which are small extra loops of DNA. Protoctists are organisms that are mostly single-celled, but unlike prokaryotes, their cells do have a nucleus. Some protoctists, such as Amoeba, are like tiny animal cells. They have no cell wall and they take in food. Others, such as Chlorella, have chloroplasts and photosynthesise, more like tiny plant cells. The protoctist kingdom is really a mixed group, made up of organisms that do not fit into any of the other kingdoms.

8:06The key comparison is between prokaryotes and protoctists, because both are usually single-celled. A prokaryote has no nucleus. A protoctist has a nucleus. If an exam question shows you a single cell and asks which kingdom it belongs to, look for the nucleus first.

Plant groups and viruses

8:22The extended syllabus also asks you to sort the plant kingdom into groups, limited to ferns and flowering plants. Ferns have leaves called fronds, which are often divided into many small parts like a feather. Ferns do not make flowers or seeds. Instead, they reproduce using spores, which are made in small brown clusters on the underside of the fronds. Flowering plants, as their name tells you, make flowers, and they reproduce using seeds, which are made inside the flower.

8:48The flowering plants are then split into two groups, the monocotyledons and the dicotyledons, often shortened to monocots and dicots. The names come from the seed. A cotyledon is a seed leaf, a simple leaf stored inside the seed that feeds the young plant when it starts to grow. A monocot seed has one cotyledon, and a dicot seed has two. But you usually cannot see inside a seed, so it helps to know two features you can see on the grown plant.

9:13The first is the leaf veins. Monocots, such as grasses, maize and lilies, have long narrow leaves with parallel veins, all running side by side along the leaf. Dicots, such as beans, roses and oak trees, usually have broader leaves with a network of branching veins. The second is the flower. The parts of a monocot flower, such as its petals, come in threes or multiples of three. The parts of a dicot flower usually come in fours or fives, or multiples of four or five.

9:40So a lily with six petals is a monocot, and a flower with five petals is a dicot. Let us classify one. A plant has flowers with five petals and broad leaves with a network of veins. It makes flowers, so it is a flowering plant, not a fern. Its petals come in fives and its veins form a branching network, so it is a dicot. Finally, the extended syllabus asks you to state the features of viruses, and this is limited to two things.

10:06A virus has a protein coat, and inside the coat it has genetic material. That is all. There is no cell membrane, no cytoplasm and no nucleus. A virus is not made of cells at all, which is why viruses are not placed in any of the five kingdoms. Think back to the seven characteristics of living things. On its own, a virus does not respire, grow, excrete or take in nutrition.

10:27It can only reproduce by getting inside a living cell and using that cell to make copies of itself. That is why most biologists do not think of viruses as living organisms. In the exam, if you are asked for the features of a virus, the two marks are for the protein coat and the genetic material.

Classification recap

10:44Watch out for these mistakes. Bacteria are prokaryotes, not protoctists, because they have no nucleus. A fungus is not a plant, because it has no chloroplasts and its cell wall is made of chitin. And a virus is not a bacterium. It is much smaller, it is not a cell, and it has only a protein coat and genetic material. Let us recap. Animals are multicellular, with no cell walls and no chloroplasts, and they feed on organic substances made by other organisms.

11:11Plants are multicellular, with chloroplasts for photosynthesis and cellulose cell walls. Mammals have fur or hair, external ears and mammary glands. Birds have feathers, a beak and wings, and lay hard-shelled eggs. Reptiles have dry scaly skin and lay rubbery-shelled eggs. Amphibians have moist skin with no scales and lay eggs in water. Fish have scales, fins and gills. All arthropods have an exoskeleton, jointed legs and a segmented body.

11:37Insects have three body parts, three pairs of legs and one pair of antennae. Arachnids have two body parts, four pairs of legs and no antennae. Crustaceans have more than four pairs of legs and two pairs of antennae. Myriapods have many segments, legs on every segment and one pair of antennae. For the extended syllabus, there are five kingdoms. Fungi have chitin cell walls and feed by saprotrophic nutrition.

12:02Prokaryotes are single cells with no nucleus. Protoctists are mostly single cells, but they have a nucleus. Ferns have fronds and reproduce by spores, while flowering plants make flowers and seeds. Monocots have one cotyledon, parallel veins and flower parts in threes, and dicots have two cotyledons, branching veins and flower parts in fours or fives. And a virus has just a protein coat and genetic material. To classify any organism, pick out the features that define each group, count them carefully, and let the features decide, not the habitat or the way it moves.