0610 · 2.1Cell structure

Animal cell structures and functions

0:00In 1665, a scientist called Robert Hooke looked at a thin slice of cork through an early microscope. He saw that it was made of thousands of tiny empty boxes, and he called them cells. Today we know that every living thing is made of cells, from a single bacterium to a blue whale. But a living cell is not an empty box. It is packed with smaller structures, and each one has a job to do.

0:22By the end of this video you will be able to describe the structure of a plant cell, an animal cell and a bacterial cell, say what each part does, and pick out each part in a diagram or in a photo taken through a microscope. You will also meet six specialised cells, and see how cells are built up into tissues, organs, organ systems and whole organisms. You only need one idea before we start.

0:44In the last chapter you saw that every living organism is made of one or more cells, and that the chemical reactions inside cells are called metabolism. A cell is the smallest unit that can carry out the processes of life. Everything an organism does, from growing to respiring, is really happening inside its cells. Let us start with an animal cell, like one of the cells lining the inside of your cheek.

1:06It is a rounded blob with no fixed shape. Around the outside is a thin layer called the cell membrane. Inside is a jelly-like liquid called the cytoplasm, and in the cytoplasm sits a large, round structure called the nucleus. Look closer and you would also find many small, sausage-shaped mitochondria, and huge numbers of tiny dots called ribosomes. Now let us see what each of these parts does. Start with the cell membrane.

1:31The membrane holds the cell together, and it controls which substances enter and leave the cell. It lets useful substances like oxygen and glucose in, lets waste like carbon dioxide out, and keeps the contents of the cell inside. Think of it as the security gate around a building. The cytoplasm is where most of the chemical reactions of the cell take place. It is mostly water with many substances dissolved in it, and the other structures of the cell sit in it.

1:56The nucleus contains the genetic material, in the form of chromosomes made of DNA. This genetic material carries the instructions that control the activities of the cell. If the cell were a factory, the nucleus would be the office that holds all the instruction manuals. Mitochondria are where aerobic respiration happens. Aerobic respiration uses oxygen to release energy from glucose, so the mitochondria release most of the energy that the cell needs.

2:22Cells that use a lot of energy, like muscle cells, have a lot of mitochondria. The word is mitochondrion for one, and mitochondria for more than one. Ribosomes are where proteins are made. On the ribosomes, the cell joins small molecules called amino acids together to build proteins. Ribosomes are so small that you cannot see them with an ordinary light microscope. We only know what they look like from far more powerful electron microscopes.

Plant cells and animal cell comparison

2:48Now a plant cell, like one from inside a leaf. A plant cell has everything an animal cell has: a cell membrane, cytoplasm, a nucleus, mitochondria and ribosomes. But it also has three extra structures. These are a cell wall, chloroplasts and a large permanent vacuole. The cell wall is a tough layer outside the cell membrane. In plants it is made of cellulose. The cell wall gives the cell its shape and supports it, and it stops the cell from bursting when it takes in a lot of water.

3:17Most substances pass straight through the cell wall. It is the membrane just inside it that controls what enters and leaves. Chloroplasts are green oval structures in the cytoplasm. They contain a green pigment called chlorophyll, which absorbs light energy. The plant uses this energy for photosynthesis, the process in which it makes its own food. So chloroplasts are where photosynthesis happens. The vacuole is a large space in the middle of the cell, filled with a watery liquid called cell sap.

3:46Cell sap contains dissolved sugars and mineral ions. When the vacuole is full, it pushes the cytoplasm outwards against the cell wall. This keeps the cell firm, a bit like air inside a tyre, and firm cells help to hold the plant upright. That is why a plant that has lost too much water starts to droop. Animal cells can have vacuoles too, but they are small and temporary, and they come and go.

4:08Only plant cells have a large, permanent vacuole full of cell sap. In an exam, always write large permanent vacuole, not just vacuole, when you compare the two kinds of cell. So here is the comparison the syllabus asks for. Plant and animal cells both have a cell membrane, cytoplasm, a nucleus, mitochondria and ribosomes. Only plant cells have a cell wall, chloroplasts and a large permanent vacuole. Of the structures on this list, there is none that an animal cell has and a plant cell lacks.

4:38Here is a trap. Not every plant cell has chloroplasts. A root cell grows underground in the dark, where there is no light for photosynthesis, so it has no chloroplasts at all. It still has a cell wall and a large permanent vacuole, so it is still clearly a plant cell. If you are asked how you can tell that a cell comes from a plant, the cell wall and the large permanent vacuole are the safest answers.

Bacterial cell structure

5:00Now for the third type, the bacterial cell. You met bacteria in the last chapter as single-celled organisms with no nucleus. Bacteria are much smaller than plant and animal cells. A typical bacterium is only one or two thousandths of a millimetre long, so hundreds of them could fit side by side across a full stop. A bacterial cell has a cell wall, a cell membrane, cytoplasm and ribosomes, and these do the same jobs as in the other cells.

5:27The cell wall gives the cell its shape, supports it and stops it from bursting, although it is not made of cellulose. The membrane controls what enters and leaves. Most of the reactions happen in the cytoplasm, and the ribosomes make proteins. The big difference is the genetic material. A bacterial cell has no nucleus. Instead, its DNA is one long loop, called circular DNA, lying loose in the cytoplasm.

5:51This circular DNA carries most of the genes, and it controls the activities of the cell, just as the nucleus does in plant and animal cells. Bacteria also have plasmids. Plasmids are small extra rings of DNA, separate from the main circular DNA. They carry a few extra genes, for example genes that help the bacterium survive an antibiotic. Later in the course you will see that scientists use plasmids in genetic modification.

6:16Notice what bacteria do not have. There is no nucleus, there are no mitochondria and there are no chloroplasts. A bacterium still respires and releases energy, but it does this without any mitochondria.

Identifying cells in diagrams

6:28The syllabus also expects you to identify all of these structures in diagrams, and in photos taken through a microscope, which are called micrographs. So here is how to recognise each one. In a diagram, the cell wall is a thick outer line around a plant cell or a bacterium, with the membrane drawn as a thin line just inside it. The nucleus is the large, round structure, often shaded darker.

6:51The vacuole is the big, empty-looking space in the middle of a plant cell. Chloroplasts are small ovals, coloured green or shaded, in the thin layer of cytoplasm around the edge. Mitochondria are small sausage shapes, often with a folded line inside. Ribosomes are drawn as tiny dots. And in a bacterium, look for the tangled loop of circular DNA, and one or two small separate rings, which are the plasmids.

7:14In a photo taken with a light microscope, things look different. You can usually see the cell wall, the membrane, the nucleus, the cytoplasm, the vacuole and the chloroplasts. Mitochondria are only just visible as tiny specks, and ribosomes cannot be seen at all, because they are too small. To see ribosomes and the inside of mitochondria clearly, you need a photo taken with an electron microscope. The shape is a useful clue too.

7:39Plant cells usually have a regular, boxy shape because of their stiff cell wall. Animal cells have irregular, rounded shapes. Bacteria are far smaller, and they have no nucleus. Let us try an exam-style question. A diagram shows a cell with a thick outer wall, a large central vacuole and a nucleus, but no chloroplasts. The question asks whether this is a plant cell or an animal cell, and wants two reasons from the diagram.

8:05Check each feature in turn. A cell wall is present, and animal cells never have a cell wall. A large permanent vacuole is present, and animal cells never have one of those either. There are no chloroplasts, but that does not rule out a plant cell, because root cells have none. And it has a nucleus, so it cannot be a bacterium. So the answer is a plant cell, because it has a cell wall and a large permanent vacuole.

8:28Notice that each reason names a structure you can see in the diagram, and one that only plant cells have. Here is a second one. State two structures found in a bacterial cell that are not found in an animal cell. Think about what is special to bacteria. A cell wall is one, because animal cells have none. Circular DNA is another, and plasmids are a third. Any two of these would score.

8:50Ribosomes or a cell membrane would not, because animal cells have those as well.

Cell division and specialised cells

8:54So where do all these cells come from? The syllabus wants you to state one simple fact. New cells are produced by division of existing cells. A cell grows, copies its genetic material, and then splits into two. Those two can each divide again, making four, then eight, and so on. Cells never appear from nothing. Every cell in your body came from the division of a cell before it, all the way back to the single fertilised egg cell that you started as.

9:20Cell division is how a many-celled organism grows, how it replaces worn out cells and repairs damage such as a cut in your skin, and how single-celled organisms like bacteria reproduce. As that fertilised egg divides into millions of cells, the cells become different from each other. A cell that has become suited to one particular job is called a specialised cell. The syllabus lists six examples, and for each one you need to state its function.

9:46It also helps to know the feature that suits it to that job, because the feature is what makes the function make sense. First, ciliated cells. These line the trachea and bronchi, the tubes that carry air down to your lungs. Their top surface is covered in tiny hairs called cilia. Other cells nearby make sticky mucus, which traps dust and bacteria from the air you breathe in. The cilia beat back and forth together and sweep the mucus up, away from the lungs, to the throat, where it is swallowed.

10:13So the function of ciliated cells is the movement of mucus in the trachea and bronchi. Second, root hair cells. These are found on the outside of young roots. Each one has a long, thin extension that reaches out between the soil particles, which gives the root a much bigger surface in contact with the soil. Their function is absorption, taking in water and mineral ions from the soil. And, being underground, they have no chloroplasts.

10:39Third, palisade mesophyll cells. These are tall, column-shaped cells, packed in a layer near the upper surface of a leaf, where they get the most light. Each one is full of chloroplasts. Their function is photosynthesis, and most of the photosynthesis in a leaf happens in these cells. Fourth, neurones, which are also called nerve cells. A neurone has a long, thin fibre that can stretch a long way through the body, in some cases about a metre.

11:05Their function is the conduction of electrical impulses. They carry messages as electrical impulses, for example from your eyes to your brain, or from your brain to your muscles. Fifth, red blood cells. These are packed with a red protein called haemoglobin, which carries oxygen. They have no nucleus, which leaves more room for haemoglobin, and each one is shaped like a disc that is pushed in on both sides.

11:26Their function is the transport of oxygen from the lungs to all the other cells of the body. Sixth, sperm and egg cells, which together are called gametes. A sperm cell has a tail that lets it swim towards an egg, and an egg cell is much larger and contains a store of food. When a sperm and an egg join, they form the first cell of a new organism.

11:45So the function of gametes is reproduction. A common mistake here is to mix up a feature with a function. If a question asks for the function of a root hair cell, the answer is absorption, of water and mineral ions. A long extension with a large surface is a feature that helps it to absorb, but it is not the function. Read the question carefully, and when it asks for the job, give the job.

Levels of organisation and recap

12:08Finally, how do cells build up into a whole organism? The syllabus asks you to describe five terms: cell, tissue, organ, organ system and organism. Each level is made from the one before it, a bit like letters building words, words building sentences, sentences building paragraphs, and paragraphs building a whole book. A cell is the basic unit of all living organisms. A tissue is a group of cells with similar structures, working together to perform a shared function.

12:35Muscle tissue is an example. It is made of muscle cells that all contract together. An organ is a structure made of a group of tissues, working together to perform specific functions. The heart is an organ. It contains muscle tissue, nerve tissue and other tissues, all working together to pump blood. An organ system is a group of organs with related functions, working together to perform body functions. The heart, together with the blood vessels, forms the circulatory system, which transports substances around the body.

13:06And an organism is a whole living thing. In a large animal like you, it is made of several organ systems all working together. Plants are organised in the same way. Palisade mesophyll cells are packed together to form palisade mesophyll tissue. A leaf is an organ, made of several tissues, including this one, working together to carry out photosynthesis. And together with the stem, the roots and the flowers, the leaves make up the whole plant, which is the organism.

13:32Watch out for two mistakes here. First, do not call the heart a tissue. It is made of several different tissues, so it is an organ. Second, when you describe a tissue, an organ or an organ system, always include the words working together, and mention the shared function. A group of cells is not a tissue just because the cells are next to each other. They must be similar cells, doing the same job together.

13:55Before the recap, let us collect the most common mistakes on the whole topic. The first is mixing up the cell wall and the cell membrane. The wall supports the cell and lets most substances through. It is the membrane that controls what enters and leaves. The second is saying that mitochondria make energy. Mitochondria release energy, by aerobic respiration. Energy is never made. And it is the ribosomes, not the nucleus, that make proteins.

14:20The third is saying that every plant cell has chloroplasts. Root cells have none. The fourth is giving bacteria a nucleus. Bacteria have circular DNA and plasmids, loose in the cytoplasm, and no nucleus. And the fifth is writing vacuole when you mean large permanent vacuole. Let us recap. Plant and animal cells both have a cell membrane, which controls what enters and leaves, and cytoplasm, where most reactions happen.

14:48They both have a nucleus, which contains the genetic material that controls the cell, mitochondria, where aerobic respiration releases energy, and ribosomes, where proteins are made. Plant cells also have a cellulose cell wall for support, chloroplasts for photosynthesis, and a large permanent vacuole full of cell sap that keeps the cell firm. Bacterial cells have a cell wall, a membrane, cytoplasm and ribosomes, but no nucleus. Their genetic material is circular DNA, with small extra rings of DNA called plasmids.

15:19And new cells are always made by the division of existing cells. Specialised cells have specific functions. Ciliated cells move mucus in the trachea and bronchi, root hair cells absorb water and mineral ions, palisade mesophyll cells carry out photosynthesis, neurones conduct electrical impulses, red blood cells transport oxygen, and sperm and egg cells are for reproduction. Finally, cells with a shared function form tissues, tissues form organs, organs form organ systems, and organ systems make up an organism.

15:52In the next video, we will find out exactly how small cells are, using the magnification formula.