OCR A Level Biology Revision Notes 2.1 Cell Structure PDF

OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ CONT...

OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ CONTENTS 2.1.1 Studying Cells 2.1.2 Using a Microscope 2.1.3 Drawing Cells 2.1.4 Magnification & Resolution 2.1.5 Eukaryotic Cells 2.1.6 Eukaryotic Cells Under the Microscope 2.1.7 Organelles & the Production of Proteins 2.1.8 The Cytoskeleton 2.1.9 Prokaryotic & Eukaryotic Cells 2.1.1 STUDYING CELLS Use of Microscopy Microscopes can be used to analyse cell components and observe organelles Magnification and resolution are two scientific terms that are very important to understand and distinguish between when answering questions about microscopy (the use of microscopes): Magnification tells you how many times bigger the image produced by the microscope is than the real-life object you are viewing Resolution is the ability to distinguish between objects that are close together (i.e. the ability to see two structures that are very close together as two separate structures) There are different types of microscopes: Optical microscopes (sometimes known as light microscopes) Electron microscopes Laser scanning confocal microscopes © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 1 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Optical (light) microscopes Optical microscopes use light to form an image This limits the resolution of optical microscopes Using light, it is impossible to resolve (distinguish between) two objects that are closer than half the wavelength of light The wavelength of visible light is between 500-650 nanometres (nm), so an optical microscope cannot be used to distinguish between objects closer than half of this value Optical microscopes have a maximum resolution of around 0.2 micrometres (µm) or 200 nm Therefore optical microscopes can be used to observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts Optical microscopes cannot be used to observe smaller organelles such as ribosomes, the endoplasmic reticulum or lysosomes The maximum useful magnification of optical microscopes is about ×1500 Electron microscopes Electron microscopes use electrons to form an image This greatly increases the resolution of electron microscopes compared to optical microscopes, giving a more detailed image A beam of electrons has a much smaller wavelength than light, so an electron microscope can resolve (distinguish between) two objects that are extremely close together Electron microscopes have a maximum resolution of around 0.0002 µm or 0.2 nm (i.e. around 1000 times greater than that of optical microscopes) This means electron microscopes can be used to observe small organelles such as ribosomes, the endoplasmic reticulum or lysosomes The maximum useful magnification of electron microscopes is about ×1,500,000 There are two types of electron microscopes: Transmission electron microscopes (TEMs) Scanning electron microscopes (SEMs) © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 2 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Transmission electron microscopes (TEMs) TEMs use electromagnets to focus a beam of electrons This beam of electrons is transmitted through the specimen Denser parts of the specimen absorb more electrons This makes these denser parts appear darker on the final image produced (produces contrast between different parts of the object being observed) Advantages of TEMs: They give high-resolution images (more detail) This allows the internal structures within cells (or even within organelles) to be seen Disadvantages of TEMs: They can only be used with very thin specimens or thin sections of the object being observed They cannot be used to observe live specimens (as there is a vacuum inside a TEM, all the water must be removed from the specimen and so living cells cannot be observed, meaning that specimens must be dead, unlike optical microscopes that can be used to observe live specimens) The lengthy treatment required to prepare specimens means that artefacts can be introduced (artefacts look like real structures but are actually the results of preserving and staining) They do not produce a colour image (unlike optical microscopes that produce a colour image) Scanning electron microscopes (SEMs) SEMs scan a beam of electrons across the specimen This beam bounces off the surface of the specimen and the electrons are detected, forming an image This means SEMs can produce three-dimensional images that show the surface of specimens Advantages of SEMs: They can be used on thick or 3-D specimens They allow the external, 3-D structure of specimens to be observed Disadvantages of SEMs: They give lower resolution images (less detail) than TEMs They cannot be used to observe live specimens (unlike optical microscopes that can be used to observe live specimens) They do not produce a colour image (unlike optical microscopes that produce a colour image) © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 3 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Laser scanning confocal microscopes These microscopes are relatively new technology The cells being viewed must be stained with fluorescent dyes A thick section of tissue or small living organisms are scanned with a laser beam The laser beam is reflected by the fluorescent dyes Multiple depths of the tissue section/organisms are scanned to produce an image Think of it like the laser beam is building up the image layer by layer Advantages: They can be used on thick or 3-D specimens They allow the external, 3-D structure of specimens to be observed Very clear images are produced. The high resolution is due to the fact that the laser beam can be focused at a very specific depth You can even see the structure of the cytoskeleton in cells Disadvantages: It is a slow process and takes a long time to obtain an image The laser has the potential to cause photodamage to the cells Exam Tip This is a lot of information to learn! First, make sure you know the basics of how each type of microscope works. Then learn the advantages and disadvantages of each type of microscope. In particular, make sure you can compare and contrast the different microscopes in terms of their relative advantages and disadvantages. In an exam question, you could be given a situation and then asked which type of electron microscope would be most suitable to use and why. A good revision idea is to make a table of the advantages and disadvantages of each type of microscope…then learn them! © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 4 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 2.1.2 USING A MICROSCOPE Preparation of Microscope Slides Many biological structures are too small to be seen by the naked eye Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied For example, the movement of chromosomes during mitosis can be observed using a microscope How optical microscopes work Light is directed through the 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 Apparatus 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 used: Forceps Scissors Scalpel Coverslip Slides Pipette Staining solution © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 5 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Image showing all 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 to cut a small sample of the tissue Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps) © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 6 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ The tissue needs to be thin so that the light from the microscope can pass through Apply a stain Gently place a coverslip on top and press down to remove any air bubbles Or Some tissue samples need to be treated with chemicals to kill/make the tissue rigid This involves fixing the specimen using formaldehyde (preservative), dehydrating it using a series of ethanol solutions, impregnating it in paraffin/resin for support then cutting thin slices from the specimen using a microtome The paraffin is removed from the slices/specimen, a stain is applied and the specimen is mounted using a resin and a coverslip is applied Or Freeze the specimen in carbon dioxide or liquid nitrogen Cut the specimen into thin slices using a cryostat Place the specimen on the slide and add a stain Gently place a coverslip on top and press down to remove any air bubbles 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 Using a graticule to take measurements of cells: 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 As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer) By using the two scales together the number of micrometers each graticule unit is © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 7 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ worth can be worked out After this is known the graticule can be used as a ruler in the field of view The stage micrometer scale is used to find out how many micrometers each graticule unit represents 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 can not be seen The treatment of specimens when preparing slides could alter the structure of cells Exam Tip Remember the importance of calibration when using a graticule. If it is not calibrated then the measurements taken will be completely arbitrary! © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 8 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Staining in Light Microscopy Many tissues that are used in microscopy are naturally transparent, they let both light and electrons pass through them This makes it very difficult to see any detail in the tissue when using a microscope Stains are often used to make the tissue coloured/visible Staining for light microscopy Coloured dyes are used when staining specimens The dyes used absorb specific colours of light while reflecting others; this makes the structures within the specimen that have absorbed the dye visible Certain tissues absorb certain dyes, which dye they absorb depends on their chemical nature Specimens or sections are sometimes stained with multiple dyes to ensure the different tissues within the specimen show up – this is known as differential staining It is important to remember that most of the colours seen in photomicrographs (image taken using a light microscope) are not natural Chloroplasts don’t need stains as they show up green, which is their natural colour Toluidine blue and phloroglucinol are common stains used Toluidine blue turns cells blue Phloroglucinol turns cells red/pink © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 9 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Toluidine blue and phloroglucinol have been used to stain this tissue specimen taken from a leaf Staining for electron microscopy When using Transmission electron microscopes (TEMs) the specimen must be stained in order to absorb the electrons Unlike light, electrons have no colour The dyes used for staining cause the tissues to show up black or different shades of grey Heavy-metal compounds are commonly used as dyes because they absorb electrons well Osmium tetroxide and ruthenium tetroxide are examples Any of the colour present in electron micrographs is not natural and it is also not a result of the staining Colours are added to the image using an image-processing software © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 10 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ The internal structure of the mitochondrion can be seen using a TEM and staining A spiracle found on the exoskeleton of an insect. No colours have been added to this image using image-processing software. © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 11 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 2.1.3 DRAWING 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 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 Guidelines for microscope drawings 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) © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 12 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ An example of a tissue plan drawn from a low-power image of a transverse section of a root. There is no cell detail present. An example of a cellular drawing taken from a high-power image of phloem tissue. 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. You should be able to describe and interpret photomicrographs, electron micrographs and drawings of typical animal cells. © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 13 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 14 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 2.1.4 MAGNIFICATION & RESOLUTION Magnification Formula 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 © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 15 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 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: The size of cells is typically measured using the micrometre (μm) scale, with cellular 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 © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 16 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 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) © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 17 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 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 © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 18 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Magnification & Resolution 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” © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 19 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 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 © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 20 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Light v Electron Microscope Table © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 21 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 2.1.5 EUKARYOTIC CELLS Eukaryotic Cell Structure Cell surface membrane The structure of the cell surface membrane – although the structure looks static the phospholipids and proteins forming the bilayer are constantly in motion 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’ The cell membrane is formed from a phospholipid bilayer of phospholipids spanning a diameter of around 10 nm © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 22 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Cell wall The cell wall is freely permeable to most substances (unlike the plasma membrane) Found in plant cells but not in animal cells Cell walls are formed outside of the cell membrane and offer structural support to cell Structural support is provided by the polysaccharide cellulose in plants, and peptidoglycan in most bacterial cells Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 23 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Nucleus The nucleus of a cell contains chromatin (a complex of DNA and histone proteins) which is the genetic material of the cell Present in all eukaryotic cells (except red blood cells), the nucleus is relatively large and separated from the cytoplasm by a double membrane (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 (eg. 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 can be observed – these regions are individually termed ‘nucleolus’ (plural: nucleoli) and are the sites of ribosome production © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 24 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Mitochondria A single mitochondrion is shown – the inner membrane has protein complexes vital for the later stages of aerobic respiration embedded within it The site of aerobic respiration within all eukaryotic cells, mitochondria are just visible with a light microscope Surrounded by double-membrane with the inner membrane folded to form cristae The matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication) © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 25 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Chloroplasts Chloroplasts are found in the green parts of a plant – the green colour a result of the photosynthetic pigment chlorophyll Found in plant cells Larger than mitochondria, also surrounded by a double-membrane Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana Grana are joined together by lamellae (thin and flat thylakoid membranes) Chloroplasts are the site of photosynthesis: The light-dependent stage takes place in the thylakoids The light-independent stage (Calvin Cycle) takes place in the stroma Also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 26 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Ribosomes Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein Found in all cells Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells Each ribosome is a complex of 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) in prokaryotes, mitochondria and chloroplasts Site of translation (protein synthesis) © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 27 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Endoplasmic reticulum The RER and ER are visible under the electron microscope – the presence or absence of ribosomes helps to distinguish between them Rough Endoplasmic Reticulum (RER) Found in plant and animal cells Surface covered in ribosomes Formed from continuous folds of membrane continuous with the nuclear envelope Processes proteins made by the ribosomes Smooth Endoplasmic Reticulum (ER) Found in plant and animal cells Does not have ribosomes on the surface, its function is distinct to the RER Involved in the production, processing and storage of lipids, carbohydrates and steroids © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 28 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Golgi apparatus (golgi complex) The structure of the Golgi apparatus Found in plant and animal cells Flattened sacs of membrane similar to the smooth endoplasmic reticulum Modifies 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 are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 29 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Large permanent vacuoles The structure of the vacuole A sac in plant cells surrounded by the tonoplast, selectively permeable membrane Vacuoles in animal cells are not permanent and small Vesicles The structure of the vesicle Found in plant and animal cells A membrane-bound sac for transport and storage © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 30 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Lysosomes The structure of the lysosome Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down) Break down waste materials such as worn-out organelles Used extensively by cells of the immune system and in apoptosis (programmed cell death) © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 31 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Centrioles The structure of the centriole Hollow fibres made of microtubules Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division Not found in flowering plants and fungi © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 32 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Microtubules The structure of the microtubule Found in all eukaryotic cells Makes up the cytoskeleton of the cell about 25 nm in diameter Made of α and β tubulin combined to form dimers, the dimers are then joined into protofilaments Thirteen protofilaments in a cylinder make a microtubule The cytoskeleton is used to provide support and movement of the cell © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 33 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Microvilli The structure of the microvilli Found in specialised animal cells Cell membrane projections Used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 34 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Cilia The structure of the cilia Hair-like projections made from microtubules Allows the movement of substances over the cell surface Flagella The structure of the flagella Found in specialised cells Similar in structure to cilia, made of longer microtubules Contract to provide cell movement for example in sperm cells © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 35 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 2.1.6 EUKARYOTIC CELLS UNDER THE MICROSCOPE Photomicrographs of Eukaryotic Cells There are some features or structures that can help to identify whether a cell shown in an image is a plant cell or animal cell Structures found only in animal cells: centrioles and microvilli Structures found only in plant cells: the cellulose cell wall, large permanent vacuoles and chloroplasts The ultrastructure of an animal cell shows a densely packed cell – the ER and RER and ribosomes form extensive networks throughout the cell in reality. © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 36 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Plant cells have a larger, more regular structure in comparison to animal cells. Describing and interpreting photomicrographs, electron micrographs and drawings of typical animal/plant cells is an important skill The organelles and structures within cells have a characteristic shape and size which can be helpful when having to identify and label them in an exam © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 37 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ TEM electron micrograph of an animal cell showing key features. Notice the lack of a cell wall. TEM electron micrograph of a plant cell showing key features. Notice the presence of a cell wall and vacuole. © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 38 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ More detailed structures can be seen and identified in electron micrographs compared to photomicrographs This is because electron microscopes have greater maximum magnification and resolution than light (optical) microscopes Mucus producing goblet cells (found in the lining of trachea, bronchi and larger bronchioles) are shown in a photomicrograph © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 39 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Details of the structures inside the goblet cell can be seen in an electron micrograph Exam Tip Make sure to learn the key identifying features of animal cells vs plant cells! It might also help to familiarise yourself with the shapes and sizes of important structures and organelles found in cells by finding some more photomicrographs and electron micrographs. © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 40 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 2.1.7 ORGANELLES & THE PRODUCTION OF PROTEINS Organelles & the Production of Proteins 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 The organelles involved in protein synthesis include: Nucleus Ribosomes Rough endoplasmic reticulum (RER) Golgi apparatus Cell surface membrane The nucleus stores the DNA (that codes for the production of proteins) and also contains the nucleolus, which manufactures ribosomes (required for protein synthesis) The DNA from the nucleus is copied into a molecule of mRNA via a process known as transcription The mRNA strand leaves the nucleus through a nuclear pore and attaches to a ribosome on the rough endoplasmic reticulum The ribosome ‘reads’ the genetic instructions contained within the mRNA and uses this code to synthesise a protein via a process known as translation This protein then passes into the lumen (the inside space) of the rough endoplasmic reticulum to be folded and processed Cells that produce a large amount of proteins (e.g. enzyme or hormone-producing cells) have an extensive rough endoplasmic reticulum The processed proteins are then transported to the Golgi apparatus (also known as the Golgi body or Golgi complex) in vesicles, which fuse with the Golgi apparatus, releasing the proteins The Golgi apparatus modifies the proteins, preparing them for secretion Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to other membrane-bound organelles The modified proteins then leave the Golgi apparatus in vesicles Finally, these vesicles (containing the final proteins) fuse with the cell surface membrane, releasing the proteins © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 41 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Many organelles are involved in the production and secretion of proteins © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 42 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 2.1.8 THE CYTOSKELETON The Cytoskeleton Within the cytoplasm of cells, there is an extensive network of protein fibres This is known as the cytoskeleton The cytoskeleton is made up of two main types of protein fibres: microfilaments and microtubules Microfilaments are solid strands that are mostly made of the protein actin. These fibres can cause some cell movement and the movement of some organelles within cells by moving against each other Microtubules are tubular (hollow) strands that are mostly made of the protein tubulin. Organelles and other cell contents are moved along these fibres using ATP to drive this movement Intermediate filaments (a third type of fibre) are also found within the cytoskeleton The importance of the cytoskeleton The cytoskeleton is important as it has several different functions, including: Strengthening and support: The cytoskeleton provides the cell with mechanical strength, forming a kind of ‘scaffolding‘ that helps to maintain the shape of the cell It also supports the organelles, keeping them in position Intracellular (within cell) movement: The cytoskeleton aids transport within cells by forming ‘tracks’ along which organelles can move Examples of this include the movement of vesicles and the movement of chromosomes to opposite ends of a cell during cell division Cellular movement: The cytoskeleton enables cell movement via cilia and flagella These structures are both hair-like extensions that protrude from the cell surface and contain microtubules that are responsible for moving them © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 43 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ The cytoskeleton provides mechanical strength to cells, aids transport within cells and enables cell movement Exam Tip For the exam, you only need to be aware of the two main types of protein fibres within the cytoskeleton: microfilaments and microtubules. The third type (intermediate filaments) are shown here to give extra detail on the composition of the cytoskeleton. © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 44 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ 2.1.9 PROKARYOTIC & EUKARYOTIC CELLS Comparison of Prokaryotic & Eukaryotic Cells Animal and plant cells are types of eukaryotic cells, whereas bacteria are a type of prokaryote Prokaryotic cells are much smaller than eukaryotic cells (between 100 – 1000 times smaller) Prokaryotic cells also differ from eukaryotic cells in having: A cytoplasm that lacks membrane-bound organelles Their ribosomes are structurally smaller (70 S) in comparison to those found in eukaryotic cells (80 S) No nucleus (instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins) A cell wall that contains murein (a glycoprotein) In addition, many prokaryotic cells have a few other structures that differentiate them from others and act as a selective advantage, examples of these are: Plasmids Capsules Flagellum Plasmids are small loops of DNA that are separate from the main circular DNA molecule Plasmids contain genes that can be passed between prokaryotes (e.g. genes for antibiotic resistance) Some prokaryotes (e.g. bacteria) are surrounded by a final outer layer known as a capsule. This is sometimes called the slime capsule It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism Flagellum (plural = flagella) are long, tail-like structure that rotate, enabling the prokaryote to move (a bit like a propeller) Some prokaryotes have more than one © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 45 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Structures unique to prokaryotic cells © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 46 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Prokaryotic cells are often described as being ‘simpler’ than eukaryotic cells, and they are believed to have emerged as the first living organisms on Earth. 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 © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 47 REVISION NOTES TOPIC QUESTIONS PAST PAPERS  OCR A Level Biology Revision Notes savemyexams.co.uk YOUR NOTES 2.1 Cell Structure ⬇ Prokaryotic & Eukaryotic Cells Comparison Table Exam Tip You will need to know all the differences between prokaryotic and eukaryotic cells. Remember – the features in the table above are not present in all prokaryotes so keep this in mind when answering exam questions. Also, 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. © 2015-2021 Save My Exams Ltd OCR A Level Biology Resources Page 48 REVISION NOTES TOPIC QUESTIONS PAST PAPERS