OCR A-level biology chapter 2 (Answered)

What is the cell theory? - ANSWER The cell is the basic unit of structure and function of all living organisms. Describe how to create a dry mount. - ANSWER 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. - ANSWER 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. - ANSWER 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. - ANSWER 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? - ANSWER Stains increase contrast as different parts of cells absorb more/less stain making them more visible. How do you prepare a sample for staining? - ANSWER 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? - ANSWER 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? - ANSWER Distinguishes between types of organisms What is the gram staining technique used for? - ANSWER It is used to separate bacteria into two groups (gram positive and gram negative). Describe the gram staining technique. - ANSWER Crystal violet then iodine is applied to a sample, then it is washed with alcohol. Gram + retain the crystal violet so turn blue/purple. Gram - have thin walls so lose the stain, they are then stained with safranin dye (a counter stain) so they appear red. What is the acid fast technique used for? - ANSWER Differentiating species of mycrobacterium from other bacteria.