1.1 The Microscope in Cell Studies.pdf

Head to savemyexams.com for more awesome resources YOUR NOTES AS Biology CIE...

Head to savemyexams.com for more awesome resources YOUR NOTES AS Biology CIE  1.1 The Microscope in Cell Studies CONTENTS 1.1.1 The Microscope in Cell Studies 1.1.2 Magnification Calculations 1.1.3 Eyepiece Graticules & Stage Micrometers 1.1.4 Resolution & Magnification 1.1.5 Calculating Actual Size Page 1 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources 1.1.1 The Microscope in Cell Studies YOUR NOTES  Microscope Slide Preparation In order to observe cellular material in more detail, specimens can be prepared for viewing under a light microscope Samples need to be thin enough to allow light to pass through The type of preparation that is appropriate is dependent on the cellular material that needs to be viewed Slide preparation methods table Samples sometimes need to be stained, as the cytosol and other cell structures may be transparent or difficult to distinguish To stain a 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 As with the type of preparation required, the type of stain used is dependent on what type of specimen is being used Common microscope stains & uses table Page 2 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Drawing Cells YOUR NOTES To record the observations seen under the microscope (or from photomicrographs taken)  a labelled biological drawing is often made Biological drawings are line pictures which show specific features that have been observed when the specimen was viewed There are a number of rules/conventions that are followed when making a biological drawing The conventions are: The drawing must have a title The magnification under which the observations shown by the drawing are made must be recorded A sharp HB pencil should be used (and a good eraser!) Drawings should be on plain white paper Lines should be clear, single lines (no thick shading) No shading The drawing should take up as much of the space on the page as possible Well-defined structures should be drawn 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 Label lines should be kept to one side of the drawing (in parallel to the top of the page) and drawn with a ruler Drawings of cells are typically made when visualizing cells at a higher magnification power, whereas plan drawings are typically made of tissues viewed under lower magnifications (individual cells are never drawn in a plan diagram)  Exam Tip When producing a biological drawing, it is vital that you only ever draw what you see and not what you think you see.To accurately reflect the size and proportions of structures you see under the microscope, you should get used to using the eyepiece graticule. Page 3 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources 1.1.2 Magnification Calculations YOUR NOTES  Magnification Calculations Magnification is how many times bigger the image of a specimen observed is in comparison to the actual (real-life) size of the specimen The magnification (M) of an object can be calculated if both the size of the image (I), and the actual size of the specimen (A), is known An equation triangle for calculating magnification  Worked Example An image of an animal cell is 30 mm in size and it has been magnified by a factor of X 3000. What is the actual size of the cell? To find the actual size of the cell: Page 4 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources The size of cells is typically measured using the micrometre (μm) scale, with cellular YOUR NOTES structures measured in either micrometers (μm) or nanometers (nm)  When doing calculations all measurements must be in the same units. It is best to use the smallest unit of measurement shown in the question To convert units, multiply or divide depending if the units are increasing or decreasing Magnification does not have units Converting units of measurement There are 1000 nanometers (nm) in a micrometre (µm) There are 1000 micrometres (µm) in a millimetre (mm) There are 1000 millimetres (mm) in a metre (m) Page 5 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources YOUR NOTES  Worked Example  Step 1: Check that units in magnification questions are the same Remember that 1mm = 1000µm 2000 / 1000 = 2, so the actual thickness of the leaf is 2 mm and the drawing thickness is 50 mm Step 2: Calculate Magnification Magnification = image size / actual size = 50 / 2 = 25 So the magnification is x 25 Page 6 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources 1.1.3 Eyepiece Graticules & Stage Micrometers YOUR NOTES  Eyepiece Graticules & Stage Micrometers An eyepiece graticule and stage micrometer are used to measure the size of the object when viewed under a microscope The type of microscope and magnification used can vary signficantly so the eyepiece graticule needs to be calibrated each time when measuring objects The calibration is done using a stage micrometer, this is a slide with a very accurate known scale in micrometres (µm) The eyepiece graticule is a disc placed in the eyepiece with 100 divisions, this has no scale To know what the graticule divisions equal at each magnification the eyepiece graticule is calibrated to the stage micrometer at each magnification Using stage micrometer & eyepiece graticule A stage micrometer alongside an eyepiece graticule. In the diagram, the stage micrometer has three lines each 100 µm (0.1 mm) apart Each 100 µm division has 40 eyepiece graticule divisions 40 graticule divisions = 100 µm 1 graticule division = number of micrometres ÷ number of graticule division 1 graticule division = 100 ÷ 40 = 2.5 µm this is the magnification factor The calibrated eyepiece graticule can be used to measure the length of the object The number of graticule divisions can then be multiplied by the magnification factor: graticule divisions x magnification factor = measurement (µm) Page 7 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Exam Tip YOUR NOTES  The calculations involving stage micrometers and eyepiece graticules are often  seen in exam questions, so make sure that you are comfortable with how to calibrate the graticule and calculate the length of an object on the slide. Page 8 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources 1.1.4 Resolution & Magnification YOUR NOTES  Resolution & Magnification Magnification Magnification is how many times bigger the image of a specimen observed is in compared to the actual (real-life) size of the specimen A light microscope has two types of lens: An eyepiece lens, which often has a magnification of x10 A series of (usually 3) objective lenses, each with a different magnification To calculate the total magnification the magnification of the eyepiece lens and the objective lens are multiplied together: eyepiece lens magnification x objective lens magnification = total magnification Resolution Resolution is the ability to distinguish between two separate points If two separate points cannot be resolved, they will be observed as one point The resolution of a light microscope is limited by the wavelength of light As light passes through the specimen, it will be diffracted The longer the wavelength of light, the more it is diffracted and the more that this diffraction will overlap as the points get closer together Electron microscopes have a much higher resolution and magnification than a light microscope as electrons have a much smaller wavelength than visible light This means that they can be much closer before the diffracted beams overlap The concept of resolution is why the phospholipid bilayer structure of the cell membrane cannot be observed under a light microscope The width of the phospholipid bilayer is about 10nm The maximum resolution of a light microscope is 200nm (half the smallest wavelength of visible light, 400nm) Any points that are separated by a distance less than 200nm (such as the 10nm phospholipid bilayer) cannot be resolved by a light microscope and therefore will not be distinguishable as “separate” Page 9 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources YOUR NOTES  The resolving power of an electron microscope is much greater than that of the light microscope, as structures much smaller than the wavelength of light will interfere with a beam of electrons Comparison of the electron microscope & light microscope Light microscopes are used for specimens above 200 nm Light microscopes shine 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 The specimens can be living (and therefore can be moving), or dead Light microscopes are useful for looking at whole cells, small plant and animal organisms, tissues within organs such as in leaves or skin Electron microscopes, both scanning and transmission, are used for specimens above 0.5 nm Electron microscopes fire a beam of electrons at the specimen either a broad static beam (transmission) or a small beam that moves across the specimen (scanning) The electrons are picked up by an electromagnetic lens which then shows the image Due to the higher frequency of electron waves (a much shorter wavelength) compared to visible light, the magnification and resolution of an electron microscope is much better than a light microscope Electron microscopes are useful for looking at organelles, viruses and DNA as well as looking at whole cells in more detail Electron microscopy requires the specimen to be dead however this can provide a snapshot in time of what is occurring in a cell eg. DNA can be seen replicating and chromosome position within the stages of mitosis are visible Page 10 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Light v Electron Microscope Table YOUR NOTES  Page 11 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources 1.1.5 Calculating Actual Size YOUR NOTES  Calculating Actual Size When investigating the size of organisms and biological structures you will use a microscope of a specific magnification to produce an image Photomicrographs are images obtained from a light microscope, these are used for specimens above 200 nm (a bacteria cell is about 1000 nm) Electron micrographs are images obtained from electron microscopes, both scanning and transmission, these are used for specimens above 0.5 nm Electron microscopes are useful for looking at organelles and biological molecules, eg. DNA can be seen replicating To better understand the images we produce using microscopes we need to know the actual size of the specimen Worked example: Calculating the actual size of a specimen A scientist looks at a sample of red blood cells under a light microscope. The eyepiece lens of the microscope has a magnification of x10 and an objective lens of x40 was used to view the blood cells. The scientist takes a photomicrograph of the blood cells, in which the average size of each cell is 3 mm. What is the average size of the red blood cells in the sample? Give your answer in micrometres. Known values: Eyepiece lens magnification: x10 Objective lens magnification: x40 Image size: 3 mm Step 1: Calculate the total magnification of the specimen eyepiece lens magnification x objective lens magnification = total magnification x10 x x40 = x400 Step 2: Calculate the image size in the units asked for (micrometres) 1 mm = 1000 μm 3 mm = 3000 μm Step 3: Calculate the actual size of the red blood cell Page 12 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Therefore, the average size of a red blood cell in this sample is 7.5 micrometres YOUR NOTES  Page 13 of 13 © 2015-2023 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers