2Boardworks AS Biology Cell Structure Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
3Boardworks AS Biology Cell Structure What is a cell?Boardworks AS BiologyCell StructureCells are the basic unit of life. They are small membrane-bound structures containing several smaller structures called organelles.There are two main categories of cell, each of which have important different structural properties:eukaryotic cell, including the cells of animals and plantsprokaryotic cell, including bacterial cells.
4Boardworks AS Biology Cell Structure A brief historyBoardworks AS BiologyCell StructurePhoto (Start and 1665): Page from Robert Hooke’s Micrographia, showing his drawing of cork cells. Available atPhoto credit (1676): John Innes Rare Books Collection,Drawing of Leeuwenhoek's microscopePhoto credit (1838): Wellcome Library, LondonPortrait of M J SchleidenPhoto (1861): Portait of Louis PasteurPhoto credit (1931): NASA Langley Research Center (NASA-LaRC)Scanning electron microscope at NASA’s Langley Research Centre
5The cell membrane and cytoplasm Boardworks AS BiologyCell StructureAll 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 cytoplasm comprises a liquid called cytosol, and all the organelles suspended in it (except the nucleus in eukaryotes).Teacher notesSee the ‘Cell Membranes’ presentation for more information about the structure and roles of membranes.The cell membrane and cytoplasm are universal features of the living cell.
6The universal energy carrier Boardworks AS BiologyCell StructureCells 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).ATPADPATP diffuses throughout the cell, and breaks down into adenosine diphosphate (ADP), releasing chemical energy wherever it is needed.
7Boardworks AS Biology Cell Structure Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
8Boardworks AS Biology Cell Structure What is a eukaryote?Boardworks AS BiologyCell StructureA eukaryote is any organism consisting of one or more cells that contain DNA in a membrane-bound nucleus, separate from the cytoplasm.Eukaryotes include:animalsplantsfungiPhoto credit: Jupiterimages CorporationTeacher notesSee the ‘Classification’ presentation for more information about the relationship between the eukaryote kingdoms.a diverse group known as the protists (or protoctists).All eukaryotic cells contain a large number of specialized, membrane-bound organelles.
9The organelles of protein synthesis Boardworks AS BiologyCell StructureTeacher notesIt may be worth pointing out to students that the membranes surrounding the organelles in this animation are all based on the same structure – the phospholipid bilayer – but are coloured differently for clarity. In addition, it could be noted that the membrane of endoplasmic reticulum is an extension of the nuclear membrane.See the ‘Nucleic Acids and the Genetic Code’ presentation for information about DNA, transcription and translation.
10Boardworks AS Biology Cell Structure MitochondriaBoardworks AS BiologyCell StructureThe mitochondrion is an energy-generating organelle.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 membranecristaematrixTeacher notesThe folds in the inner membrane (the cristae) provide a large surface area for ATP synthesis. The inner membrane is studded with stalked particles on which the enzyme ATPase (which catalyzes the creation of ATP) is located.inner membraneThe inner membrane is coated in enzymes, which catalyze the reactions of aerobic respiration to produce ATP.
11Microtubules and the cytoskeleton Boardworks AS BiologyCell StructureTeacher notesStudents should be reminded that the spindle fibres that align and separate chromosomes during mitosis and meiosis are microtubules.Cilia are not present on all cells, and are rare in plants. In mammals, cilia are present on epithelial cells lining the trachea (windpipe), where they sweep dirt and mucus out of the lungs. In females, they also line the Fallopian tubes, and beat to move the egg from the ovary into the uterus.
12Boardworks AS Biology Cell Structure Which organelle?Boardworks AS BiologyCell Structure
13Boardworks AS Biology Cell Structure Plant cellsBoardworks AS BiologyCell StructurePlant 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.chloroplastvacuoleEvery plant cell is surrounded by a cell wall, and contains one or more permanent vacuoles.cell wall
14Boardworks AS Biology Cell Structure ChloroplastsBoardworks AS BiologyCell StructureChloroplasts 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.granastromaTeacher notesChloroplasts use the light energy absorbed by chlorophyll to build sugars. These are converted into complex carbohydrates to store energy for all the plant’s needs.thylakoid membraneThe thylakoid membranes are the site of photosynthesis.
15Boardworks AS Biology Cell Structure VacuolesBoardworks AS BiologyCell StructurePermanent 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.Photo credit: Dr Jeremy Burgess / Science Photo LibraryColoured Transmission Electron Micrograph (TEM) of mesophyll cells in a young leaf of Zinnia elegans. These cells contain large numbers of chloroplasts (green), and are therefore active centres of photosynthesis. The chloroplasts are situated on the edge of each cell; starch granules (pink) inside them are a form of stored sugar. At the centre of each cell is a large vacuole (blue) filled with cell sap. The nucleus is coloured red. Mesophyll cells are loosely arranged cells in the leaf, with large air spaces (white) between them, allowing for gas exchange during photosynthesis. Magnification: x2,000 at 6x4.5cm size.Teacher notesThe vacuole contains enzymes, like a lysosome. Enzymes are produced by the endoplasmic reticulum, then packaged into lysosomes by the Golgi apparatus. These lysosomes may then fuse with a vacuole, depositing their contents.See the ‘Transport Across Membranes’ presentation for more information about turgidity.The vacuole is important in keeping the cell firm. When the vacuole is full of sap the cell is said to be turgid.
16Boardworks AS Biology Cell Structure The cell wallBoardworks AS BiologyCell StructureThe 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.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.Photo credit: Eye Of Science / Science Photo LibraryColoured scanning electron micrograph (SEM) of the walls of plant cells (oblong) from a bamboo (family Gramineae). Unlike animal cells, plant cells are enclosed in a protective rigid cell wall. Channels (round) in the walls, known as plasmodesmata, allow transport and communication between the cells. Magnification: x1250 when printed at 10 centimetres wide.Teacher notesSee the ‘Biological Molecules 1: Water & Carbohydrates’ presentation for more information about cellulose.
17Eukaryotic organelles Boardworks AS BiologyCell Structure
18Boardworks AS Biology Cell Structure Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
19Boardworks AS Biology Cell Structure What is a prokaryote?Boardworks AS BiologyCell StructureA prokaryote is any organism – usually single-celled – whose DNA is suspended freely in the cytoplasm. The word means ‘before the nucleus’.Prokaryotes can be divided into two groups:bacteriaarchaea.Photo credit: Jupiterimages CorporationMany prokaryotes, particularly archaea, are extremophiles: they are capable of surviving in extreme conditions due to their simple cell structure. This image shows a hot spring in Yellowstone National Park, populated by Thermus aquaticus – a heat-loving bacterium, or ‘thermophile’.Teacher notesThe archaea are more closely related to eukaryotes than bacteria, but the two groups share such similar cell structure that they can be considered together.See the ‘Classification’ presentation for more information about the relationship between, and classification of, bacteria, archaea and eukaryotes.Prokaryotes have simpler structure than eukaryotes, lacking organelles such as the nucleus, ER and Golgi.
20Features of the bacteria and archaea Boardworks AS BiologyCell StructureTeacher notesNote that this activity only lists structures that are common to all prokaryotes/bacteria and, as such, does not include structures such as plasmids and flagella that only appear in certain species.The DNA of bacteria is sometimes referred to as a ‘chromosome’ but this is misleading as it does not form chromatin.Bacteria can be divided into two groups based on the composition of their cell wall:Gram-positive bacteria have a thick layer of peptidoglycan and certain acids. These bacteria appear blue/violet under a light microscope after staining with the Gram stainGram-negative bacteria have a much thinner layer of peptidoglycan, which is surrounded by a second plasma membrane containing lipopolysaccharides or lipoproteins. These bacteria do not take up the Gram stain and appear red/pink under a light microscope.The structure of its cell wall can determine a bacteria’s susceptibility to different antibiotics. Vancomyxin, for example, is ineffective against gram-negative bacteria such as Haemophilus influenzae.
21Boardworks AS Biology Cell Structure Bacterial capsulesBoardworks AS BiologyCell StructureMany pathogenic bacteria are surrounded by a mucous-like protective layer called a capsule.capsulecell wallThe capsule protects bacteria from viruses, or attack from a host organism’s immune system, by hiding antigens on the cell surface.Teacher notesBacteria surrounded by a capsule include Haemophilus influenzae B, Streptococcus pneumoniae and Salmonella typhi.The polysaccharides in bacterial capsules are often used in conjugated vaccines.The capsule is usually composed of polysaccharides, and also contains water to protect against desiccation (drying out).
22Boardworks AS Biology Cell Structure Flagella and piliBoardworks AS BiologyCell StructureSome prokaryotic cells feature one or more flagella. These are long helical tubes extending out of the cell wall, which rotate to provide locomotion.Flagella are powered by protein motors and can propel bacteria at a rate of more than 50 lengths per second.Photo credit: David Gregory & Debbie Marshall, Wellcome ImagesElectron micrograph of Escherichia coli, computer-coloured red. Close-up showing pili (shorter, thinner) and flagella (longer, thicker).Teacher notesThe flagella of bacteria and archaea are superficially similar, but are thought to have evolved separately for similar functions.Many bacteria also feature pili. These are hollow protein structures used during bacterial conjugation – the transfer of genetic material from one bacterium to another.
23Boardworks AS Biology Cell Structure PlasmidsBoardworks AS BiologyCell StructureBacterial 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.pilus draws bacteria togetherreplication of plasmidPlasmids are commonly used in genetic engineering to make copies of genes or large quantities of proteins or hormones.
24Structure of a bacterium Boardworks AS BiologyCell Structure
25Boardworks AS Biology Cell Structure Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
26Boardworks AS Biology Cell Structure How small is a cell?Boardworks AS BiologyCell StructureTeacher notesNote that the slider uses a logarithmic scale.
27Boardworks AS Biology Cell Structure Light microscopesBoardworks AS BiologyCell StructureLight (or optical) microscopes use lenses to project a magnified image of an object onto the eye.Magnification is a measure of how many times bigger the image is than the object:size of imageactual size of the objectmagnification =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).
28Boardworks AS Biology Cell Structure Virtual microscopeBoardworks AS BiologyCell StructurePhoto credit (first slide): Med. Mic. Sciences Cardiff Uni, Wellcome ImagesColour-enhanced scanning electon micrograph of rod-shaped Clostridium difficile clinging to the microvilli of the human gut. This bacterium is often responsible for holiday diarrhoea and is of increasing concern as a hospital acquired infection, particularly affecting the elderly.Photo credit (second slide): University of Edinburgh, Wellcome Images Colour-enhanced electron micrograph of cells showing the nuclei in green, mitochondria in red, two lysosomes in brown and large amounts of rough endoplasmic reticulum in white.Photo credit (third slide): M I Walker, Wellcome Images A light micrograph of the leaf of a foliaceous liverwort showing cells packed with chloroplasts, Nomarski optics and difference interference microscopy.Photo credit (fourth slide): University of Edinburgh, Wellcome Images Cell from the proximal convoluted tubule in the kidney cortex. The nucleus (purple) has a prominant nucleolus, and many elongated mitochondria (red) fill the cell. The basal lamina and intercellular space is shown in bluePhoto credit (fifth slide): M I Walker, Wellcome Images Epidermal cells in the skin of an onion (Allium).Photo credit (sixth slide): M I Walker, Wellcome Images Light micrograph (dark ground illumination) of ciliates and bacteria from water in a flower vase.
29Boardworks AS Biology Cell Structure Electron microscopesBoardworks AS BiologyCell StructurePhoto credit (start): WM Keck Electron Microscopy Laboratory, University of MassachusettsPhoto credit (TEM): CDC / Cynthia Goldsmith / Jackie KatzThis transmission electron micrograph (TEM), taken at a magnification of 150,000x, reveals the ultrastructural details of an avian influenza A (H5N1) virion, a type of bird flu virus which is a subtype of avian influenza A. At this magnification, one may note the stippled appearance of the roughened surface of the proteinaceous coat encasing the virion.Photo credit (SEM): Kelly Bateman, Cefas (www.cefas.co.uk)A ciliate parasite. Hair-like projections, cilia, can be seen protruding from the organism.Teacher notesAlthough an electron microscope is based on the same principle as an optical microscope, it is not possible to observe the image directly. Electrons cannot be seen, so they are projected onto a fluorescent screen to create a visible image.
30Preparing a specimen for microscopy Boardworks AS BiologyCell StructureElectron microscopes contain a vacuum as air particles would interfere with the beam of electrons.Water boils at room temperature in a vacuum, so the specimen must be dried out completely (dead).Photo credit: Steve Gschmeissner / Science Photo LibraryColoured scanning electron micrograph (SEM) of starch grains (green) in the parenchyma of a Clematis sp. plant.Teacher notesA sample for electron microscopy is often coated with a heavy metal, such as gold, after desiccation to provide a better barrier to the passage of electrons (for a clearer image).Electron micrographs are always initially monochromatic, but are often given false colour to make specific structures stand out.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.
31Extracting organelles from cells Boardworks AS BiologyCell StructureTeacher notesThe centrifuge in this animation is shown holding three extra test tubes, though only one is studied. This illustrates that a centrifuge must always be balanced to prevent damage from the high forces experienced at very high rates of revolution.Note that the term ‘supernatant’ is, in fact, an adjective and should only be used in the form ‘a supernatant liquid’: the correct term to use when referring to the actual liquid above a sediment or precipitate is the noun ‘supernate’, i.e. ‘the supernate’.
32Microscope characteristics Boardworks AS BiologyCell Structure
33Boardworks AS Biology Cell Structure Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
34Boardworks AS Biology Cell Structure GlossaryBoardworks AS BiologyCell Structure
35Boardworks AS Biology Cell Structure What’s the keyword?Boardworks AS BiologyCell Structure
36Eukaryotes vs. prokaryotes Boardworks AS BiologyCell Structure
37Boardworks AS Biology Cell Structure Multiple-choice quizBoardworks AS BiologyCell Structure