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3 of 37© Boardworks Ltd 2008 What is a cell? Cells are the basic unit of life. They are small membrane- bound structures containing several smaller structures called organelles. eukaryotic cell, including the cells of animals and plants prokaryotic cell, including bacterial cells. There are two main categories of cell, each of which have important different structural properties:
4 of 37© Boardworks Ltd 2008 A brief history
5 of 37© Boardworks Ltd 2008 The cell membrane and cytoplasm All cells, and all true organelles, are contained within a membrane, based on a phospholipid bilayer. The cell membrane holds a cell together and controls what enters and leaves the cytoplasm, as it is a selectively permeable barrier. The cell membrane and cytoplasm are universal features of the living cell. The cytoplasm comprises a liquid called cytosol, and all the organelles suspended in it (except the nucleus in eukaryotes).
6 of 37© Boardworks Ltd 2008 The universal energy carrier Cells need a constant supply of energy to carry out vital processes such as protein synthesis, DNA replication and cell division. This energy originally comes from fuel molecules, such as glucose, consumed by the organism. These are broken down during aerobic or anaerobic respiration, and the energy released is used to make adenosine triphosphate (ATP). ATP diffuses throughout the cell, and breaks down into adenosine diphosphate (ADP), releasing chemical energy wherever it is needed. ATP ADP
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8 of 37© Boardworks Ltd 2008 What is a eukaryote? A eukaryote is any organism consisting of one or more cells that contain DNA in a membrane-bound nucleus, separate from the cytoplasm. plants a diverse group known as the protists (or protoctists). All eukaryotic cells contain a large number of specialized, membrane-bound organelles. Eukaryotes include: fungi animals
9 of 37© Boardworks Ltd 2008 The organelles of protein synthesis
10 of 37© Boardworks Ltd 2008 Mitochondria The mitochondrion is an energy-generating organelle. The inner membrane is coated in enzymes, which catalyze the reactions of aerobic respiration to produce ATP. It is surrounded by two membranes. The inner layer folds inwards to form the cristae. The cristae project into a liquid called the matrix. outer membrane matrix inner membrane cristae
11 of 37© Boardworks Ltd 2008 Microtubules and the cytoskeleton
12 of 37© Boardworks Ltd 2008 Which organelle?
13 of 37© Boardworks Ltd 2008 Plant cells Plant cells share all the common features of animal cells, but also contain some additional organelles. Plants gain all their energy from sunlight; cells in their leaves contain many chloroplasts to convert this into a useful form. Every plant cell is surrounded by a cell wall, and contains one or more permanent vacuoles. chloroplast vacuole cell wall
14 of 37© Boardworks Ltd 2008 Chloroplasts Chloroplasts use carbon dioxide, water and light energy to build sugars. They are present in all green plants. The chloroplast is surrounded by a double membrane. It is filled with a liquid called the stroma, and contains stacks of thylakoid membranes called grana. stroma thylakoid membrane The thylakoid membranes are the site of photosynthesis. grana
15 of 37© Boardworks Ltd 2008 Permanent vacuoles only exist in plant cells. Animal cells can contain temporary vacuoles but they are not common features. A vacuole consists of a membrane called the tonoplast, filled with cell sap – a watery solution of different substances, including sugars, enzymes and pigments. The vacuole is important in keeping the cell firm. When the vacuole is full of sap the cell is said to be turgid. Vacuoles
16 of 37© Boardworks Ltd 2008 The cell wall does not seal off a cell completely from its neighbours. There are pores within the walls called plasmodesmata. These connect two cells together by their cytoplasm, enabling the exchange and transport of substances. The cell wall The cell wall of a plant cell gives it support and structure. It is made of the polysaccharide cellulose, and can function as a carbohydrate store by varying the amount of cellulose it holds.
17 of 37© Boardworks Ltd 2008 Eukaryotic organelles
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19 of 37© Boardworks Ltd 2008 What is a prokaryote? A prokaryote is any organism – usually single-celled – whose DNA is suspended freely in the cytoplasm. The word means ‘before the nucleus’. bacteria archaea. Prokaryotes have simpler structure than eukaryotes, lacking organelles such as the nucleus, ER and Golgi. Prokaryotes can be divided into two groups:
20 of 37© Boardworks Ltd 2008 Features of the bacteria and archaea
21 of 37© Boardworks Ltd 2008 Bacterial capsules Many pathogenic bacteria are surrounded by a mucous-like protective layer called a capsule. The capsule is usually composed of polysaccharides, and also contains water to protect against desiccation (drying out). capsulecell wall The capsule protects bacteria from viruses, or attack from a host organism’s immune system, by hiding antigens on the cell surface.
22 of 37© Boardworks Ltd 2008 Some prokaryotic cells feature one or more flagella. These are long helical tubes extending out of the cell wall, which rotate to provide locomotion. Flagella and pili Many bacteria also feature pili. These are hollow protein structures used during bacterial conjugation – the transfer of genetic material from one bacterium to another. Flagella are powered by protein motors and can propel bacteria at a rate of more than 50 lengths per second.
23 of 37© Boardworks Ltd 2008 Plasmids Bacterial cells often contain several plasmids – small continuous loops of DNA. Plasmids are replicated independently of a bacterium’s genophore (e.g. during bacterial conjugation), and may confer an advantage, such as antibiotic resistance. Plasmids are commonly used in genetic engineering to make copies of genes or large quantities of proteins or hormones. pilus draws bacteria together replication of plasmid
24 of 37© Boardworks Ltd 2008 Structure of a bacterium
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26 of 37© Boardworks Ltd 2008 How small is a cell?
27 of 37© Boardworks Ltd 2008 Light microscopes Light (or optical) microscopes use lenses to project a magnified image of an object onto the eye. Light microscopes are limited to a magnification of 1500× by their resolving power (resolution). This is a measure of their ability to distinguish between two separate points. A light microscope cannot resolve two points that are closer than half a wavelength of visible light (250 nm). Magnification is a measure of how many times bigger the image is than the object: size of image actual size of the object magnification =
28 of 37© Boardworks Ltd 2008 Virtual microscope
29 of 37© Boardworks Ltd 2008 Electron microscopes
30 of 37© Boardworks Ltd 2008 Preparing a specimen for microscopy Electron microscopes contain a vacuum as air particles would interfere with the beam of electrons. Optical microscopes can be used to view living specimens. Coloured dyes (stains) can be used to make specific structures more visible under a light microscope. Water boils at room temperature in a vacuum, so the specimen must be dried out completely (dead).
31 of 37© Boardworks Ltd 2008 Extracting organelles from cells
32 of 37© Boardworks Ltd 2008 Microscope characteristics
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34 of 37© Boardworks Ltd 2008 Glossary
35 of 37© Boardworks Ltd 2008 What’s the keyword?
36 of 37© Boardworks Ltd 2008 Eukaryotes vs. prokaryotes
37 of 37© Boardworks Ltd 2008 Multiple-choice quiz
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