The Microscope in Cell Studies (AQA A Level Biology): Flashcards

Early scientists prepared samples in different ways to see if a structure appeared consistently, helping to distinguish real cell structures from artefacts. Improved techniques have reduced artefacts and increased confidence in identifying organelles.

A starch grain is a structure inside plant cells where starch is stored as the main storage polysaccharide.

Starch grains are commonly found in the stroma of chloroplasts, storage organs such as potato tubers, and the seeds of cereals and legumes.

Sugars formed during photosynthesis are stored as starch inside starch grains.

Iodine in potassium iodide solution is used to stain starch grains, and the presence of starch is indicated by a blue-black colour.

Starch grains are large enough to be seen with an optical microscope, but they first require staining in order to be seen easily.

True.

Iodine in potassium iodide solution is used as a stain and turns blue-black in the presence of starch.

An optical microscope is a tool that uses visible light and lenses to magnify and view biological structures that are too small to be seen by the naked eye.

1. Place the slide on the stage.

1. Select the lowest power objective lens.

1. Use the coarse focus to lower the lens towards the slide.

1. Use the fine focus to raise the lens until the specimen is in focus.

1. Switch to higher power objectives and refocus if necessary.

A graticule is a small disc with an engraved scale that can be placed into the eyepiece of a microscope to act as a ruler.

To calibrate an eyepiece graticule, use a stage micrometer to compare its scale with the graticule scale and calculate how many micrometers each graticule unit represents for each objective lens.

False.

A graticule has no fixed units. It must be calibrated for each objective lens using a stage micrometer.

A biological drawing is a line picture that shows specific features observed when viewing a specimen through a microscope, following standard drawing conventions.

Three important conventions are: add a title and magnification, use clear single lines with no shading, and draw label lines straight with a ruler and all on one side.

When producing a biological drawing, only draw what you see, not what you think you see.

By using an eyepiece graticule to measure structures, you can ensure that your biological drawings are accurate in size and proportion.

Magnification is the number of times larger an image appears compared to the specimen's actual size.

Resolution is the ability to distinguish between two separate points.

The formula for total magnification is: eyepiece magnification × objective magnification.

Resolution in light microscopes is limited by the wavelength of light.

True.

Electron microscopes have a much higher resolution because electrons have a shorter wavelength than visible light.

Light microscopes cannot resolve structures smaller than about 200 nm.

A micrometre (μm) is a unit of length equal to one millionth of a metre (10⁻⁶ m).

Magnification = image size / actual size.

To convert from micrometres (μm) to nanometres (nm), multiply by 1000.

False.

Magnification is a ratio and does not have any units.

The actual size is 0.01 mm or 10 μm. (Actual size = 30 mm ÷ 3000)

1000 nanometres (nm) = 1 micrometre (μm).

The resolution of a microscope determines how close two points can be while still being seen as separate.

Cell fractionation is a process used to isolate specific organelles from cells for closer study, such as under an electron microscope or to investigate organelle function.

During homogenisation, cells are broken up using a homogeniser which releases organelles into a solution called the homogenate.

The solution must be cold to slow enzyme activity, isotonic to prevent osmotic damage to organelles, and buffered to maintain pH and avoid protein or enzyme denaturation.

The supernatant is the solution above the pellet after centrifugation, containing organelles not yet sedimented.

After filtration, the organelles remain in the filtrate, while large debris is removed.

During ultracentrifugation, the order of organelle separation from heaviest to lightest is: nuclei, chloroplasts, mitochondria, lysosomes, endoplasmic reticulum, ribosomes.

Ultracentrifugation is the process of separating organelles by spinning cell filtrate at increasing speeds to form pellets of decreasing weight organelles.

False.

The centrifuge separates organelles by spinning at increasing speeds to isolate organelles of different densities.