OCR A-level biology chapter 2 | Questions and Answers with complete solution

What is the cell theory? - The cell is the basic unit of structure and function of all living organisms. Describe how to create a dry mount. - Solids are cut into thin slices (sectioning). The specimen should be placed on a slide and covered with a cover slip (small specimens can be viewed in this way without sectioning). Describe how to create a wet mount. - Specimens are put in a liquid (usually water or oil). A cover slip is placed at a 45° angle over the droplet and carefully dropped, allowing it to adhere to the slide. Describe how to create a squash slide. - First prepare a wet mount, then use a lens tissue to gently press down the cover slip. For weak materials use two microscope slides to press down the cover slip. Describe how to create a smear slide. - Use the edge of a slide to smear the sample coating thinly on the slide. Place a cover slip on top of the smeared substance. Why use staining? - Stains increase contrast as different parts of cells absorb more/less stain making them more visible. How do you prepare a sample for staining? - It must be dry, then heat fixed by passing through a flame. The specimen will stick to the microscope slide and take up stains. What do differently charged stains do to specimens? - Positively charged dyes are attracted to negatively charged chemicals leading to staining. Negatively charged dyes repel the negatively charged cytosol so they do not stain. What does differential staining do? - Distinguishes between types of organisms What is the gram staining technique used for? - It is used to separate bacteria into two groups (gram positive and gram negative).