Edexcel International AS Biology Cell Structure & Organisation PDF

Head to www.savemyexams.com for more awesome resources Edexcel International AS Biology Your notes Cell Structure & Organisation Contents 3.1 Cell Theory 3.2 Levels of Organisation of Cells 3.3 Eukaryotic Cells 3.4 The R...

Head to www.savemyexams.com for more awesome resources Edexcel International AS Biology Your notes Cell Structure & Organisation Contents 3.1 Cell Theory 3.2 Levels of Organisation of Cells 3.3 Eukaryotic Cells 3.4 The Rough Endoplasmic Reticulum & Golgi 3.5 Prokaryotic Cells 3.6 Electron Microscopy of Animal Cells 3.7 Microscopy: Magnification & Resolution 3.8 Core Practical 5 - Light Microscopy Page 1 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 3.1 Cell Theory Your notes The Cell as a Unit of Life Until microscopes became powerful enough to view individual cells, no-one knew for certain what living organisms were made from A scientist called Robert Hooke is thought to be the first person to view cells, using the term 'cell' to describe these newly discovered structures Matthias Schleiden and Theodor Schwann were two other scientists who studied animal and plant cells In 1837 they came up with the idea that all living organisms are made of cells This idea is known as ‘cell theory’ The cell theory is a unifying concept in biology, meaning that it is universally accepted Cell theory includes three main ideas All living organisms are made up of one or more cells Cells are the basic functional unit in living organisms New cells are produced from pre-existing cells The cells of all living organisms share some common features Cell surface membrane Cytoplasm DNA Ribosomes Beyond these common features different cell types contain different structural elements and combinations of organelles, e.g. Prokaryotic cells have no internal membranes and smaller ribosomes Eukaryotic cells have several internal membrane-bound organelles and larger ribosomes When examined under a microscope it is possible to see the ultrastructure of different cell types Cell ultrastructure refers to the internal structures of the cell Page 2 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 3.2 Levels of Organisation of Cells Your notes Multicellular Cell Organisation Cell theory states that cells are the basic functional unit of all living organisms Cells can become specialised for specific functions, e.g. Epithelial cells in the small intestine are specialised to absorb food efficiently Red blood cells are specialised to transport oxygen Xylem cells in plants are specialised to allow the transport of water around a plant In multicellular organisms specialised cells of the same type group together to form tissues A tissue is a group of cells that work together to perform a particular function, e.g. Epithelial cells group together to form epithelial tissue the function of which, in the small intestine, is to absorb food Muscle cells group together to form muscle tissue, the function of which is to contract in order to move parts of the body Different tissues can group together to form organs An organ is a group of tissues working together to perform a particular function, e.g. Many different tissues, including cardiac muscle tissue, blood vessel tissues and connective tissue, group together to form the heart, enabling it to function to pump blood around the body Tissues including palisade mesophyll, spongy mesophyll, and vascular tissue, group together in plants to form leaves, enabling them to perform photosynthesis effectively Different organs work together to form organ systems An organ system is a group of organs working together to perform a particular function, e.g. The heart and blood vessels work together to form the circulatory system, the job of which is to allow blood to circulate around the body The stomach, pancreas, small intestine, and large intestine work together to form the digestive system, the job of which is to digest food and absorb nutrients Levels of Organisation Table Page 3 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Examples of Specialised Cells and their Associated Tissues, Organs and Organ Systems Table Page 4 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 3.3 Eukaryotic Cells Your notes Organelle Structures Cells can be divided into two broad types; eukaryotic and prokaryotic cells Eukaryotic cells have a more complex ultrastructure than prokaryotic cells The term ultrastructure refers to the internal structure of cells Eukaryotic cells are larger than prokaryotic cells Eukaryotic cells range in diameter from around 10-100 μm Prokaryotic cells range in diameter from around 0.1-5 μm The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles Animal and plant cells are both types of eukaryotic cells that share key structures such as Membrane-bound organelles, including a nucleus Larger ribosomes known as 80S ribosomes Key differences between animal and plant cells include Animal cells contain centrioles and some have microvilli while plant cells do not Microvilli are folded regions of the cell surface membrane that increase cell surface area for absorption, e.g. in the small intestine Plant cells have a cellulose cell wall, large permanent vacuoles, and chloroplasts while animal cells do not Page 5 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Animal cells are a type of eukaryotic cell Page 6 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Plant cells are eukaryotic cells that have a cellulose cell wall, permanent vacuole, and chroroplasts Page 7 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Organelle Functions Cell surface membrane Your notes All cells are surrounded by a cell surface membrane which controls the exchange of materials between the internal cell environment and the external environment The membrane is described as being partially permeable, meaning that some substances can pass through the membrane while others cannot Cell membrane is formed from a phospholipid bilayer spanning a diameter of around 10 nm Many organelles inside cells are surrounded by cell membrane, so when referring to the outer membrane of a cell it is always a good idea to refer to it as the cell surface membrane The cell surface membrane can also be referred to as the plasma membrane The cell surface membrane surrounds the cell, separating it from its external environment Nucleus Present in all eukaryotic cells, the nucleus is relatively large and separated from the cytoplasm by a double membrane called the nuclear envelope, which has many pores Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes, e.g. DNA polymerases, and signalling molecules to travel in The nucleus contains chromatin, the material from which chromosomes are made Chromosomes are made of sections of linear DNA tightly wound around proteins called histones Usually, at least one or more darkly stained regions of the nucleus can be observed under a microscope; these regions are individually termed nucleolus (plural nucleoli) and are the sites of ribosome production Page 8 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The nucleus of a eukaryotic cell is surrounded by the nuclear envelope and contains chromatin as well as a region called the nucleolus. Note that the nucleus is shown here surrounded by another organelle; the endoplasmic reticulum Mitochondria The site of aerobic respiration within eukaryotic cells, mitochondria (singular mitochondrion) are just visible with a light microscope Mitochondria are surrounded by a double-membrane with the inner membrane folded to form structures called cristae The matrix of mitochondria contains enzymes needed for aerobic respiration, producing ATP Small circular pieces of DNA, known as mitochondrial DNA, and ribosomes are also found in the matrix These are needed for replication of mitochondria before cell division Page 9 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Mitochondria are the site of aerobic respiration in eukaryotic cells Ribosomes Ribosomes can be found as free organelles in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells They are not surrounded by a membrane Each ribosome is a complex of ribosomal RNA (rRNA) and proteins 80s ribosomes are found in eukaryotic cells 70s ribosomes are found in prokaryotes, mitochondria, and chloroplasts Ribosomes are the site of translation Page 10 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein Endoplasmic reticulum There are two types of endoplasmic reticulum; rough and smooth Rough Endoplasmic Reticulum (RER) RER is formed from folds of membrane continuous with the nuclear envelope The surface of RER is covered in ribosomes The role of the RER is to process proteins made on the ribosomes Smooth Endoplasmic Reticulum (SER) SER is also formed from folds of membrane but its function is distinct from the RER, being involved in the production, processing and storage of lipids, carbohydrates and steroids SER does not have ribosomes on its surface Page 11 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The RER and SER are visible under the electron microscope; the presence or absence of ribosomes helps to distinguish between them Golgi apparatus The Golgi apparatus consists of flattened sacs of membrane similar in appearance to the smooth endoplasmic reticulum The Golgi apparatus is sometimes known as the Golgi body The Golgi can be distinguished from the SER by its regular, stacked appearance; it can be described as looking like a wifi symbol! The role of the Golgi apparatus is to modify proteins and lipids before packaging them into Golgi vesicles The vesicles then transport the proteins and lipids to their required destination Proteins that go through the Golgi apparatus can be Exported from the cell, e.g. hormones such as insulin Put into lysosomes, e.g. hydrolytic enzymes Delivered to other membrane-bound organelles Page 12 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The Golgi apparatus; the cis face lies near the rough endoplasmic reticulum, while the trans face lies near the cell membrane Lysosomes Lysosomes are specialist forms of vesicle which contain hydrolytic enzymes The role of lysosomes is to break down waste materials such as worn-out organelles, Lysosomes are used extensively by cells of the immune system and in programmed cell death, known as apoptosis Page 13 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Lysosomes contain digestive enzymes Centrioles Centrioles are made of hollow fibres knows as microtubules Microtubules are filaments of protein that can be used to move substances around inside a cell, as well as to support the shape of a cell from the inside Two centrioles at right angles to each other form a centrosome which organises the spindle fibres during cell division Centrioles are not found in plants and fungi Page 14 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Centrioles are structures formed from microtubules; they are involved with the process of nuclear division in animal cells Page 15 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 3.4 The Rough Endoplasmic Reticulum & Golgi Your notes Formation of Extracellular Enzymes In cells, many organelles are involved in the production and secretion of proteins Organelles are specialised parts of a cell that carry out a particular function Some organelles are membrane-bound, meaning that they are surrounded by membrane The organelles involved in protein synthesis include Nucleus Transcription of the DNA code occurs here Ribosomes Free ribosomes and those on the RER produce proteins in the process of translation Rough endoplasmic reticulum (RER) Golgi apparatus Cell surface membrane Proteins formed within the cell are secreted here Rough endoplasmic reticulum Ribosomes on the RER produce proteins that can be secreted out of the cell or become attached to the cell surface membrane Proteins that have been passed into the lumen of the rough endoplasmic reticulum are folded and processed here The term lumen refers to the inside space of the RER Note that free ribosomes found within the cytoplasm make proteins that stay within the cytoplasm rather than being moved to another organelle or being exported from the cell Golgi apparatus Processed proteins from the RER are transported to the Golgi apparatus in vesicles which fuse with the Golgi apparatus, releasing the proteins into the Golgi The Golgi apparatus modifies the proteins, preparing them for secretion Proteins that go through the Golgi apparatus are usually Exported, e.g. extracellular enzymes The term extracellular refers to 'outside the cell' Put into lysosomes, e.g. hydrolytic enzymes Delivered to other membrane-bound organelles The modified proteins then leave the Golgi apparatus in vesicles Page 16 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The RER and Golgi apparatus are involved with producing, packaging and transporting proteins in a cell. This process can be used to produce and export extracellular enzymes. Page 17 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 3.5 Prokaryotic Cells Your notes Cell Components: Structures Animal and plant cells are types of eukaryotic cells, whereas bacteria are a type of prokaryote Prokaryotic cells are much smaller than eukaryotic cells They also differ from eukaryotic cells in having A cytoplasm that lacks membrane-bound organelles Ribosomes that are smaller (70 S) than those found in eukaryotic cells (80 S) No nucleus, instead having a single circular bacterial chromosome that is free in the cytoplasm and is not associated with proteins A cell wall that contains the glycoprotein murein Murein is sometimes known as peptidoglycan In addition, many prokaryotic cells also have the following structures Loops of DNA known as plasmids Capsules Flagella (singular flagellum) Pili (singular pilus) A cell membrane that contains folds known as mesosomes Page 18 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Bacteria are prokaryotic cells Prokaryotes vs eukaryotes There are a number of important structural and physiological differences between prokaryotic and eukaryotic cells These differences affect their metabolic processes and how they reproduce Comparison of Prokaryotic and Eukaryotic Cells Table Page 19 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Examiner Tip You will need to know all the differences between prokaryotic and eukaryotic cells; remember that not all of the structures mentioned here are present in all prokaryotic cells Remember that size is not a structural feature so if you are asked for a structural difference between a prokaryotic and eukaryotic cell don't include size in your answer. Page 20 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cell Components: Functions The ultrastructure of prokaryotic cells includes several features that are not found in eukaryotic cells, Your notes e.g. Plasmids Capsule Flagella Pili Mesosomes Circular DNA Ribosomes Some of these features are found in all prokaryotes, while some are only present in some Additional Prokaryotic Structures Table Page 21 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Examiner Tip Note that the specification does not include the role of mesosomes in prokaryotic cells. Page 22 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 3.6 Electron Microscopy of Animal Cells Your notes Electron Microscopy of Animal Cells Organelles under the electron microscope There are two types of electron microscope Transmission electron microscopes (TEMs) Scanning electron microscopes (SEMs) Transmission Electron Microscopes TEMs use electromagnets to focus a beam of electrons This beam of electrons is transmitted through a thin specimen Denser parts of the specimen absorb more electrons; these denser parts appear darker on the final image, producing contrast between different parts of the object being observed The internal structures within cells, or even within organelles can be seen as a 2D image The resolution of these images is very high Scanning Electron Microscopes SEMs scan a beam of electrons across a specimen This beam bounces off the surface of the specimen and the electrons are detected, forming an image This means SEMs can produce 3D images that show the surface of specimens Since they scan the outside surface it means that the specimen viewed does not have to be thin The images they form are of a lower resolution than TEMs Page 23 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A stained TEM microscope of the nucleus. It is clear this is a TEM micrograph as the image is 2D and in high resolution; the inside of the nucleus would not be clear to view at low resolution. Page 24 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A stained TEM micrograph of a mitochondrion. It is clear this is a TEM micrograph as the image is 2D and in high resolution; the inside of a mitochondrion would not be clear to view at low resolution. Page 25 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources A SEM of a spiracle (part of an insect). You can tell this is a SEM micrograph as the image is 3D. Your notes Cell organelles can be identified in a micrograph produced using a TEM Examiner Tip You need to be able to recognise organelles from electron microscope images; cells in real life are not always as easy to observe as cells in diagrams, so be sure to get practice at looking at electron micrographs of cells Generally, if you can see internal structures the image would have been taken with a TEM and if the image appears 3D then an SEM would have been used. Page 26 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 3.7 Microscopy: Magnification & Resolution Your notes Magnification & Resolution Magnification Magnification is how many times bigger the image of a specimen observed is in comparison 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 total magnification = eyepiece lens magnification x objective lens magnification Resolution Resolution, or resolving power, is the ability to distinguish between two separate points If two separate points cannot be resolved, they will be observed as one point and the image will be unclear The resolution of a microscope limits the magnification that it is capable of; there is no point in magnifying an image at low resolution as this will just result in a big blur rather than a small blur! The resolution of a light microscope is limited by the wavelength of light; the wavelength of light is too long to allow for high resolution E.g. the phospholipid bilayer structure of the cell membrane cannot be observed under a light microscope The width of the phospholipid bilayer is about 10 nm The maximum resolution of a light microscope is 200 nm Any points that are separated by a distance less than 200 nm cannot be resolved by a light microscope and therefore will not be distinguishable as separate points on an image Electron microscopes have a much higher resolution, and therefore magnification, than a light microscope as electrons have a much smaller wavelength than visible light Page 27 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The resolving power of an electron microscope is much greater than that of the light microscope due to the smaller wavelength of electrons Comparing electron & light microscopes Light microscopes are used for specimens larger than 200 nm Light microscopes shine light through the specimen 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, and tissues within organs such as in leaves or skin Electron microscopes, both scanning and transmission, are used for specimens larger than 0.5 nm Electron microscopes fire a beam of electrons at the specimen The electrons are picked up by an electromagnetic lens which then shows the image Electron microscopy requires the specimen to be dead; this can provide a snapshot in time of what is occurring in a cell, e.g. DNA can be seen replicating and chromosome position within the stages of mitosis are visible Electron microscopes are useful for looking at organelles, viruses, and DNA, as well as looking at whole cells in more detail Page 28 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Light v Electron Microscope Table Your notes Page 29 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Staining Specimens Specimens to be viewed under a microscope sometimes need to be stained, as the cytoplasm and Your notes other cell structures may be transparent or difficult to distinguish Note that most of the colours seen in images taken using a light microscope are the result of added stains Chloroplasts are the exception to this; they show up green, which is their natural colour The type of stain used is dependent on what type of specimen is being prepared and what the researcher wants to observe within the specimen Different molecules absorb different dyes depending on their chemical nature Specimens or sections are sometimes stained with multiple dyes to ensure that several different tissues within the specimen show up; this is known as differential staining Some common stains include Haemotoxylin Stains plant and animal cell nuclei purple, brown or blue Methylene blue Stains animal cell nuclei blue Acetocarmine Stains chromosomes in dividing nuclei of plant and animal cells Iodine Stains starch-containing material in plant cells blue-black Toluidine blue Stains tissues that contain DNA and RNA blue Phloroglucinol Stains a chemical called lignin found in some plant cells red/pink Page 30 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Toluidine blue and phloroglucinol have been used to stain this tissue specimen taken from a leaf Page 31 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 3.8 Core Practical 5 - Light Microscopy Your notes Microscope Images Many biological structures are too small to be seen by the naked eye Optical, or light, microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied Light is directed through a thin layer of biological material that is supported on a glass slide This light is focused through several lenses so that an image is visible through the eyepiece The magnifying power of the microscope can be increased by rotating the higher power objective lens into place Preparation of microscope slides The key components of an optical microscope are The eyepiece lens The objective lenses The stage The light source The coarse and fine focus Other tools that may be used Forceps Scissors Scalpel Coverslip Slides Pipette Staining solution Page 32 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The components of an optical microscope Method Preparing a slide using a liquid specimen Add a few drops of the sample to the slide using a pipette Cover the liquid / smear with a coverslip and gently press down to remove air bubbles Wear gloves to ensure there is no cross-contamination of foreign cells Methods of preparing a microscope slide using a solid specimen Take care when using sharp objects and wear gloves to prevent the stain from dying your skin Use scissors or a scalpel to cut a small sample of the tissue Use forceps to peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide The tissue needs to be thin so that the light from the microscope can pass through Apply a stain to make cells more visible Gently place a coverslip on top and press down to remove any air bubbles Some tissue samples need to be treated with chemicals to kill cells or make the tissue rigid This involves fixing the specimen using the preservative formaldehyde, dehydrating it using a series of ethanol solutions, impregnating it with paraffin or resin for support and then cutting thin slices from the specimen The paraffin is removed from the slices and a stain is applied before the specimen is mounted and a coverslip is applied Page 33 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Slide Preparation Table Your notes Using a microscope When using an optical microscope always start with the low power objective lens It is easier to find what you are looking for in the field of view This helps to prevent damage to the lens or coverslip in case the stage has been raised too high Preventing the dehydration of tissue The thin layers of material placed on slides can dry up rapidly Adding a drop of water to the specimen beneath the coverslip can prevent the cells from being damaged by dehydration Unclear or blurry images Switch to the lower power objective lens and try using the coarse focus to get a clearer image Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly There could be cross-contamination with foreign cells or bodies Limitations The size of cells or structures of tissues may appear inconsistent in different specimen slides Cell structures are 3D and the different tissue samples will have been cut at different planes resulting in this inconsistencies when viewed on a 2D slide Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that cannot be seen The treatment of specimens when preparing slides could alter the structure of cells Drawing cells To record the observations seen under the microscope, or from photomicrographs taken, a labelled biological drawing is often made Biological drawings are line drawings that show specific features that have been observed when the specimen was viewed There are a number of rules or conventions that are followed when making a biological drawing The drawing must have a title Page 34 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The magnification under which the observations shown by the drawing are made must be recorded A sharp pencil should be used Your notes Drawings should be on plain white paper Lines should be clear, single lines with no sketching 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 ideally be kept to one side of the drawing in parallel to the top of the page, and should be drawn with a ruler Only visible structures should be drawn; not structures that the viewer thinks they should be able to see! Drawings of cells are typically made when visualizing cells at a higher magnification power An example of a cellular drawing taken from a high-power image of phloem tissue Plan drawings that show the arrangement of cells within a tissue or organ are typically made using samples viewed under lower magnifications Individual cells are never drawn in a plan diagram Page 35 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes An example of a tissue plan diagram drawn from a low-power image of a transverse section of a root. Note that there is no cell detail present. Page 36 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Measurements of Microscopic Images Magnification is how many times bigger the image of a specimen observed is in comparison to the Your notes 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: total magnification = eyepiece lens magnification x objective lens magnification The magnification (M) of an object can also be calculated if both the size of the image (I), and the actual size of the specimen (A), is known magnification = image size ÷ actual size Remember to ensure that the image size (I) and the actual size (A) of the specimen have the same units before doing the calculation The equation for calculating magnification can be rearranged to calculate either actual size, image size, or magnification. Worked example An image of an animal cell is 30 mm in diameter and it has been magnified by a factor of x3000. What is the actual diameter of the cell? Page 37 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Using an eyepiece graticule & stage micrometer A graticule is a small disc that has an engraved ruler It can be placed into the eyepiece of a microscope to act as a ruler in the field of view, so is sometimes known as an eyepiece graticule As an eyepiece graticule has no fixed units it must be calibrated for the objective lens that is in use The graticule in the eyepiece remains the same size when the magnification of the microscope is altered, so recalibration is needed at each viewing magnification Calibration of the eyepiece graticule is done a microscope slide with an engraved scale known as a stage micrometer By using the eyepiece graticule and the stage micrometer together, the size of each graticule unit can be calculated After this is known the graticule can be used as a ruler to measure objects in the field of view Page 38 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The stage micrometer scale is used to find out how many micrometers each eyepiece graticule unit represents Page 39 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes Calculate the size of the units of the eyepiece graticule in the image below. Note that the large divisions in the top half of the image show the stage micrometer and that each stage micrometer division is 1 mm across. Step 1: Observe the number of eyepiece unit divisions per micrometer unit In the image, the stage micrometer has three lines Each micrometer division has 40 eyepiece graticule divisions within it Step 2: Calculate the size of each eyepiece graticule unit 40 graticule divisions = 1 mm (1000 µm) 1 graticule unit = 1000 ÷ 40 = 25 µm An object that spanned five eyepiece graticule units could therefore be measured as follows 5 x 25 µm = 125 µm Page 40 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes The biggest pitfall with these kinds of calculations is forgetting to convert the units so that they match before embarking on a calculation. E.g. if image size is measured in mm but the actual size of an object is given in µm then both need to be converted into µm before using the equation triangle above. To convert a measurement from mm into µm the measurement must be multiplied by 1000 (there are 1000 µm in 1 mm). Page 41 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers