Cell Structure PDF

# CELL STRUCTURE ## 1. The microscope in cell studies ### Microscope slide preparation In order to observe cellular material in detail, specimens must be prepared for viewing under a light microscope (Samples need to be thin enough to allow light to pass through). | Type | Specimens | Preparatio...

# CELL STRUCTURE ## 1. The microscope in cell studies ### Microscope slide preparation In order to observe cellular material in detail, specimens must be prepared for viewing under a light microscope (Samples need to be thin enough to allow light to pass through). | Type | Specimens | Preparation | Uses | |---|---|---|---| | Dry Mount | Solid specimens | Thin slices called sectioning. Cover slip placed on top | Hair, pollen, dust, muscle tissue, and plant tissue | | Wet Mount | Wet specimens | Suspended in water or immersion oil. Cover sip placed at an angle | Aquatic samples and other living organisms | | Squash Slides | Soft specimens | Wet mount squashed between slide and cover sip | Root cells to look at cell division | | Smear Slides | Body fluid specimens | Edge of the slide used to smear the sample, creating a thin, even coating | Blood smears to view erythrocytes | * Samples sometimes need to be stained as the cytosol and other cell structures may be transparent or difficult to distinguish. * To stain the slide - the sample needs to be first air-dried and then heated by passing it through a Bunsen burner flame - this will allow the sample to be fixed to the slide and to take up the stain. | Stain | Uses | |---|---| | Crystal Violet | Stains cell walls purple, used in Gram staining | | Methylene Blue | Stains the nuclei in animal cells to give contrast | | Congo Red | Negative stain not taken up by the cell, provides contrast between the cell and background | ### Biological drawings of cells * Biological drawings are line pictures which show specific features that have been observed when the specimen was viewed through a microscope or photomicrographs * The conventions/rules for biological drawing * The drawing must have a title. * The magnification under which the observations shown by the drawing are made must be recorded. * Use sharp HB pencil, clear lines, no shading, well defined structures. * Drawings should be on plain white paper. * The drawing should take up as much of the space on the page as possible. * The drawing should be made with proper proportions. * Label lines should not cross or have arrowheads and should connect directly to the part of the drawing being labelled. ## Magnification Calculation * Magnification is how many times bigger the image of a specimen observed is in compared to the actual (real-life) size of the specimen. * $Magnification = \frac{Image Size}{Actual Image}$. * $Image Size = Actual \times Magnification$. * A light microscope has two types of lenses: * An eyepiece lens, which often has a magnification of x10. * A series of (usually 3) objective lenses, each with a different magnification. * Total magnification = eyepiece lens magnification x objective lens magnification. ## Resolution * Resolution is the ability to distinguish between two separate points (i.e., the minimum distance apart that two objects can be in order for them to appear separate items). * Shorter the wavelength (higher frequency), higher the resolution. Higher resolution, clearer (more detail) the specimens are. * Electron microscopes have a much higher resolution and magnification than a light microscope as electrons have shorter wavelength than visible light. * Any points that are separated by a distance less than 200 nm (such as the 10 nm phospholipid bilayer) cannot be resolved by a light microscope and therefore will not be distinguishable as “separate.” So, use an electron microscope. ## Light and electron microscopes ### Light microscope * Used for specimens above 200 nm. * Maximum magnification x1500. * It shines light through the specimen –> this light is then passed through an objective lens (which can be changed) and an eyepiece lens (x10) which magnify the specimen to give an image that can be seen by the naked eye. * Specimens can be living (moving), or dead. * Useful for looking at whole cells, small plant and animal organisms, tissues within organs such as in leaves or skin. * Total magnification = eyepiece lens magnification x objective lens magnification ### Electron microscope * Both scanning, and transmission, are used for specimens above 0.5 nm. * Maximum magnification x500 000. * It fires a beam of electrons at the specimen either a broad static beam (transmission - can see internal structure but not 3D, can’t see surface contours) or a small beam that moves across the specimen (scanning) –> the electrons are picked up by an electromagnetic lens which then shows the image. * Specimen should be dead. * Useful for looking at organelles, viruses, and DNA, as well as looking at whole cells in more detail. * Shorter wavelength (higher frequency) of electron waves compared to visible light, the magnification and resolution is better than light microscopes. ## Eyepiece graticules & stage micrometres * An eyepiece graticule and stage micrometre are used to measure the size of an object when viewed under a microscope. * Stage micrometre, is a slide with a very accurate scale in micrometres (calibrated - 10 units, 100 mini stage units). The eyepiece graticule is a disc with 100 divisions placed in the eyepiece, this has no scale (not calibrated). * Coincide the stage micrometre with the eye piece graticule, and calculate the actual length in the below ways. * In the diagram, the stage micrometre has three lines each 10 µm apart. So, each 10 µm division has 40 eyepiece graticule divisions (scale: 40 graticule divisions = 10 µm). 1 graticule division = $\frac{number\ of\ micrometres}{number\ of\ graticule\ division}$. 1 graticule division = $\frac{10}{40} = 0.25 \mu m$. * The number of graticule divisions can then be multiplied by the magnification factor to find the actual length. $\frac{graticule \ divisions}{magnification \ factor} = actual \ length (\mu m)$ For example, if a leaf cell’s eyepiece graticule division is 4, then the actual length is 4×0.25=1 µm. ## 2. Cell as the basic units of living organisms ### 2.1 Plant and animal cells #### Plant cells * Cellulose cell wall present (in addition to the cell surface membrane). * Plasmodesmata present. * Chloroplasts present in large numbers to carry out photosynthesis. * Mature cells normally have a large single, central permanent vacuole filled with cell sap which is surrounded by a vacuolar membrane (tonoplast). * Centrioles absent in higher plants. * Cilia absent in higher plants. * Microvilli absent. * Starch grains (or smaller starch granules) used for carbohydrate storage. * Cytoplasm normally confined to a thin layer at the edge of the cell because of vacuole. * Nucleus at the edge of the cell. * Golgi bodies smaller and greater in number. #### Animal cells * Cell wall absent – only a cell surface membrane surrounds the cell. * No cell wall and therefore no plasmodesmata. * Chloroplasts absent. * Temporary vacuoles, if present, are small and scattered throughout the cell. * Centrioles present. * Cilia may be present. * Microvilli may be present. * Glycogen granules used for carbohydrate storage. * Cytoplasm present throughout the cell. * Nucleus anywhere in the cell, but often central. * One or a few Golgi bodies, generally larger. ## 2.2 Cell organelles ### Cell surface membrane * All cells are surrounded by a cell surface membrane which controls the exchange of materials between the internal cell environment and the external environment. * It is partially permeable. * The cell membrane is formed from a phospholipid bilayer (hydrophilic heads and hydrophobic tails) containing proteins – allows substances based on size and solubility. ### Nucleus, nuclear envelope, and nucleolus * Controls cell division (A cell without a nucleus cannot reproduce), controls cell development and controls cell activities. * Present in all eukaryotic cells. * Nuclear envelope (double membrane) separates the nucleus from the cytoplasm which has many pores. * Nuclear pores allow the passage of large molecules like mRNA and ribosomes to travel out of the nucleus and allowing enzymes (eg. DNA polymerases) and signalling molecules to travel in. * The nucleus contains chromatin (made up of protein and DNA – condenses to form chromosome). * Nucleolus (centre of nucleus) produces and assembles ribosomes. ### Mitochondria * Surrounded by double-membrane to control entry and exit of substances. Cristae is the infolding of the inner membrane. * Matrix is a fluid that contains enzymes for aerobic respiration and ATP production. * Small circular DNA (mitochondrial DNA – replica of DNA in nucleus) and 70s ribosomes are also found in the matrix, for replication (mitosis). * The site of aerobic respiration – supplies energy to cells as they synthesis (produce) ATP molecules from carbohydrate and other respiratory substances. * Cells use ATP from respiration for energy-requiring processes. ### Ribosomes * Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells. (can be viewed only through electron microscope). * Site of protein synthesis. * Each ribosome contains two subunits. Both contains ribosomal RNA (rRNA) and proteins. * 80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells. * 70S ribosomes (composed of 50S and 30S subunits) ribosomes in prokaryotes, mitochondria, and chloroplasts. ### Rough endoplasmic reticulum * Surface covered in 80s ribosomes, giving a rough appearance. * Flattened sacs and have cisternae. Continuous folds with external nuclear membranes, organised. * Synthesis, transport and modification of proteins. * Provides large surface area for protein synthesis. ### Smooth endoplasmic reticulum * Does not have ribosomes on the surface. Tubular structure. Not continuous with nuclear membrane. Disorganised. * Involved in the production, synthesis of lipids (eg cholesterol). ### Golgi body * Flattened sacs of membrane called cisternae. Not continuous with nuclear membrane. * Golgi body collects, processes, and modifies proteins/lipids. Packages them into vesicles called Golgi vesicles. * Golgi vesicles are also used to make lysosomes that contain hydrolytic enzymes. ### Lysosomes * Contain digestive hydrolytic enzymes responsible for the breakdown (digestion) of unwanted structures such as old organelles, dead cells, and bacteria. Breaks the bonds of these substances by hydrolysis. ### Microtubules * Tiny tubules made from protein (alpha tubulin and beta tubulin). * Form the cytoskeleton is used to provide support and movement of the cell. * The spindle (in mitosis) on which chromosomes separate during nuclear division is made of microtubules. ### Centrioles * Made of 27 microtubules, the centre microtubule is called centrosome. * Form cytoskeleton – support. * Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division. ### Cilia * Hair-like projections made from microtubules. * Whip like movement to allows substances to move over the cell surface (eg: mucus). ### Flagella * Similar in structure to cilia, made of longer microtubules. * Contract to provide cell movement for example in sperm cells. ### Microvilli * Finger-like extensions of the cell surface membrane that increase the surface area for absorption. ### Chloroplasts * Contains membrane-bound sacs called thylakoids containing chlorophyll. Thylakoids stack to form grana. * Chloroplast envelope is double membrane that controls the entry and exit of substances. * Also contain small circular DNA and 70s ribosomes used to synthesise proteins needed in photosynthesis and chloroplast replication (mitosis). * Chloroplasts are the site of photosynthesis: * The light-dependent stage takes place in the thylakoids (light energy absorbed by chlorophyll in chloroplast). * The light-independent stage takes place in the stroma (contains enzymes for photosynthesis – converts CO2 to glucose). ### Cell wall * Cell walls are formed outside of the cell membrane and offer structural support to the cell. * Structural support is provided by the cellulose (polysaccharide) in plants, and peptidoglycan in most bacterial cell. Fully permeable. ### Plasmodesmata * Plasmodesmata (pore like structure), connect the cytoplasm of adjacent/ neighbouring cells for substances to pass through (without having to pass through the cell membrane or cell wall). * The channel/path through which they pass through are lined with cell surface membrane. ### Large permanent vacuole and tonoplast * Sac in plant cells surrounded by the tonoplast, selectively permeable membrane (single membrane). * Creates a pressure that is exerted on the cell wall to provide support. * Store substances in the form of cell sap – solution of ions, sugars, and other pigment. ## 2.3 Eukaryotic and prokaryotic cells | | Prokaryotic cells | Eukaryotic cells | |---|---|---| | _Nucleus_ | No true nucleus or nuclear envelope (so DNA is free in the cytoplasm) | Distinct nucleus, with a nuclear envelope (so DNA is enclosed) | | _Nucleolus_ | No nucleolus (0.1 to 0.5 micrometre) | Nucleolus is present | | _DNA_ | DNA is not associated with histone proteins (naked DNA) | DNA is associated with histone proteins to form chromosomes | | _DNA shape_ | Circular (closed loop) DNA | Linear DNA | | _Cell wall_ | Cell wall made of peptidoglycan (murein) | Where present, cell wall is made mostly of cellulose (plants) or chitin (fungi) | | _Organelles_ | No double membrane bounded organelles | Double membrane bounded organelles such as mitochondria and chloroplasts (e.g. in plant cells) are present | | _Ribosomes_ | Ribosomes are 70S, smaller | Ribosomes are 80S, larger | | _Plasmids_ | Plasmids may be present | No plasmids present | | _Flagella_ | Flagella (if present) lack internal 9 + 2 microtubule arrangement | Flagella, where present, have a 9 + 2 internal microtubule arrangement | | _Endoplasmic reticulum_ | No endoplasmic reticulum or associated Golgi body and lysosomes | Endoplasmic reticulum present along with Golgi body and lysosomes | ## 2.4 Viruses * Viruses are non-cellular infectious particles. * They are relatively simple in structure; much smaller than prokaryotic cells (with diameters between 20 and 300 nm). * Structures: * A nucleic acid core (their genomes are either DNA or RNA, and can be single or double-stranded). * A protein coat called a capsid. * Some viruses have an outer layer called an envelope made of phospholipid bilayer with glycoproteins. * All viruses are parasitic in that they can only reproduce by infecting living cells and using their protein-building machinery (ribosomes) to produce new viral particles. ## Summary of cell organelles * Cell organelles visible under light microscope – Nucleus, nucleolus, chloroplast, vacuole, cell membrane, cell wall. * Cell organelles visible under electron microscope – Mitochondria, Ribosomes, Rough Endoplasmic Reticulum (RER), Smooth Endoplasmic Reticulum (SER), Golgi body, Vesicles.