UPDATED Biology AS Level, OCR EXAM STUDYPACK WITH VERIFIED SOLUTIONS 2023/2024

What is cell theory? - ANSWER It states that the cell is the fundamental structure of all living matter, and that cells can only develop from other existing cells. Why was cell theory not fully developed before the mid-19th century? - ANSWER Before then, microscope magnification was not as powerful, and so scientists were unable to see and identify single cells or cell components. Describe the process of calibrating a microscope. - ANSWER 1. Place the stage micrometer on the stage and align it with the eyepiece graticule. 2. Calculate the value of one eyepiece unit. 3. Replace the stage micrometer with a specimen to measure. Each division on the stage micrometer represents how many micrometres? - ANSWER 10 How do you calculate the size of one eyepiece unit? - ANSWER Divide the number of eyepiece divisions by the number of micrometres. Define magnification. - ANSWER How many times larger the image is than the actual size of the object. How does a compound light microscope work? - ANSWER It has two lenses - the objective lens near the specimen, and the eyepiece lens, through which the specimen is viewed. What is a dry mount? - ANSWER A mount used to observe solid specimens, where the specimen is sectioned and a cover slip is placed over the top. What is a wet mount? - ANSWER A mount used to observe specimens suspended in water or immersion oil. What is a squash slide? - ANSWER A mount used to observe living specimens where the sample is squashed gently between the slide and the cover slip. What is a smear slide? - ANSWER A mount used to observe liquid samples by smearing the sample across the slide with a cover slip, creating a thin, even coating of the substance. What is the gram stain technique used for? - ANSWER To separate bacteria into gram-positive and gram-negative bacteria. Describe the process for the gram stain technique. - ANSWER 1. Add crystal violet dye to a bacterial specimen. 2. Fix the dye using iodine. 3. Wash the slide with alcohol. How does the gram stain technique differentiate gram positive bacteria from gram negative bacteria? - ANSWER Gram-positive bacteria will retain the dye while gram- negative will not, as they have thinner cell walls. What is the acid fast technique used for in staining? - ANSWER To differentiate Mycobacterium from other bacteria. Describe the process for the acid-fast technique. - ANSWER A lipid solvent carries dye into the cells, which are then washed with a dilute acid-alcohol solution. Mycobacterium retain the stain. Why do we need to stain samples? - ANSWER By providing contrast, it allows us to differentiate between cell components and cell types. Define the term 'resolution'. - ANSWER The shortest distance between two objects that can still be seen as separate entities. How does TEM (Transmission Electron Microscopy) work? - ANSWER Electrons are transmitted THROUGH a specimen and focused to produce an image. How does SEM (Scanning Electron Microscopy) work? - ANSWER Electrons are sent across the SURFACE of a specimen, and the reflecting electrons are collected, giving 3D images. Is it easy to prepare a sample for light microscopy? - ANSWER Yes - the simple sample preparation means that distortion rarely occurs. Is it easy to prepare a sample for electron microscopy? - ANSWER No - the complex sample preparation often leads to distortion. What colour are images produced from electron microscopes? - ANSWER Black and white, but can be coloured digitally. Magnification of a light microscope? - ANSWER Up to 2000x Magnification of an electron microscope? - ANSWER Over 500,000x Resolving power of light microscope? - ANSWER 200nm Resolving power of TEM? - ANSWER 0.5nm Resolving power of SEM? - ANSWER 3-10nm How can we calculate magnification? - ANSWER image size/ object size What is an artefact? - ANSWER A visible structural detail caused by processing the specimen, and is not actually a feature of the specimen itself. Define the term 'metabolism'. - ANSWER This refers to the chemical reactions of both the synthesis and the breaking down of molecules. What is the function of the nucleus? - ANSWER To contain genetic information in the form of DNA molecules. What is the function of the nuclear envelope? - ANSWER To provide a double membrane to protect DNA from damage in the cytoplasm. What is the function of the nucleolus? - ANSWER Made from proteins and RNA, it's main role is to produce ribosomes to form rRNA. What is the function of the nuclear pores? - ANSWER To allow molecules, such as mRNA, to move in/out of the nucleus. What is the function of the SER? - ANSWER To synthesize carbohydrates and lipids, and act as storage. What is the function of the RER? - ANSWER To synthesize and transport proteins. What is the function of the ribosomes? - ANSWER They are the site of protein synthesis. Suggest how long strands of DNA are packed into a cell only 50 micrometres in diameter. - ANSWER DNA associates with proteins called histones, which form complexes known as chromatin. Chromatin then coils and condenses to form chromosomes which only become visible when cells are preparing to divide. What are mitochondria? - ANSWER They are the site of the final stages of cellular respiration. Describe the structure of a mitochondrion. - ANSWER They have a double membrane, and the inner membrane is folded to form cristae, which encases the fluid interior known as the matrix. What is the function of the vesicles? - ANSWER They are membranous sacs that have important roles in storage and transportation of substances. What are lysosomes? - ANSWER These are specialised vesicles that contain hydrolytic enzymes, used to break down waste material in cells. What is the cytoskeleton? - ANSWER It is a network of fibres necessary for the shape and stability of a cell. Describe the structure and function of the microfilaments in the cytoskeleton. - ANSWER They are contractile fibres made from actin which bring about cell contraction during cytokinesis. Describe the structure and function of the microtubules in the cytoskeleton. - ANSWER They are formed from tubulin and form scaffold-like structures used in the movement of organelles. Describe the function of the intermediate fibres in the cytoskeleton. - ANSWER They give mechanical strength to the cell. What are centrioles? - ANSWER They are components in the cytoskeleton of most eukaryotic cells, and are made from microtubules. What is a centrosome? - ANSWER It is formed from two centrioles and is involved in the organisation of the spindle fibres during cell division. What are the functions of flagella? - ANSWER They are used primarily for movement, but in some cells can also be used as sensory organelles to detect changes in the environment. What is the function of cilia? - ANSWER These can move to create a current, causing fluids or objects around the cell to move.