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OCR A A-Level Biology > Unit 2 - Basic Components of Living Things > Flashcards

Unit 2 - Basic Components of Living Things Flashcards

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1
Q

When were light microscopes developed?

A

In the 16th and 17th centuries.

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2
Q

What does cell theory state?

A
  • Both plant and animal tissue is composed of cells
  • Cells are the basic unit of all life
  • Cells only develop from existing cells
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3
Q

What are the two lenses on a light microscope called and what do they do?

A

Objective lens - produces a magnified image of the specimen
Eyepiece lens - magnifies the image produced by the objective lens

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4
Q

Give two advantages of light microscopes.

A

Any two of:
- Easily available
- Relatively cheap
- Easy to use
- Can be used in the field
- Can be used to observe living organisms as well as dead organisms

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5
Q

Give two disadvantages of light microscopes.

A

Any two of:
- Low magnification
- Low resolution
- Specimen has to be thin and transparent
- Can’t give detailed information about internal cell structure

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6
Q

How is a slide prepared using the dry mount method?

A

Solid specimens are viewed whole or cut into thin slices. The specimen is placed on the centre of a slide and covered with a cover slip.

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7
Q

Which specimens are viewed with the dry mount method?

A

Hair, pollen, dust, insect parts and muscle/plant tissue.

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8
Q

How do you prepare a slide using the wet mount method?

A

The specimen is suspended in liquid with the cover slip placed on at an angle.

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9
Q

Which specimens are viewed with the wet mount method?

A

Aquatic samples and other living organisms.

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10
Q

How do you prepare squash slides?

A

A wet mount is prepared, then a lens tissue is used to press down the cover slip.

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11
Q

Which specimens are viewed as squash slides?

A

Soft samples such as root tip squashes.

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12
Q

How do you prepare smear slides?

A

The edge of a slide is used to smear the sample, creating a thin, even coating on another slide. A cover slip is then placed over the sample.

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13
Q

Which specimens are viewed as smear slides?

A

Blood

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14
Q

Why do specimens have to be thin in slide preparation for light microscopes?

A

So enough light can pass through the specimen to produce a clear image.

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15
Q

Why, when preparing a wet mount, should the refractive index of the medium be roughly the same as that of glass?

A

So the light passes through without bending too much and the image isn’t distorted.

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16
Q

Why must the cover slip on a wet mount be placed at an angle?

A

To avoid air bubbles forming between the cover slip and the specimen.

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17
Q

Why are stains used in slide preparation?

A

To increase the contrast of structures in the cell. The different components stain to different degrees so they are more visible and it is easier to identify them.

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18
Q

How are slides prepared before being stained?

A

The specimen is placed on a slide and allowed to air dry, then passed through a flame. This fixes the specimen to the slide, allowing it to take up stains.

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19
Q

What are two positively charged stains used?

A

Crystal violet and methylene blue. Because these stains are positively charged, they are attracted to the negatively charged material in the cytoplasm, leading to the staining of cell components.

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20
Q

What are two negatively charged stains used?

A

Nigrosin and Congo red. These stains are repelled by the negatively charged material in the cytoplasm, so leave cells unstained. This means the cells stand out against an unstained background, and is called a negative stain technique.

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21
Q

What is differential staining?

A

Staining used to distinguish between two types of organisms that would otherwise be difficult to identify.

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22
Q

How are Gram positive bacteria and Gram negative bacteria different in structure?

A

Both have a layer of peptoglycan (mesh-like layer of sugars and amino acids) on the cell wall but Gram positive bacteria has a thicker layer than Gram negative bacteria. Gram negative bacteria also has an extra layer of lipopolysaccharide.

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23
Q

Give two examples of Gram positive bacteria.

A

Staphylococcus spp. and streptococcus spp.

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24
Q

Give two examples of Gram negative bacteria.

A

E. coli and salmonella.

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25
Q

What are the four steps of the Gram stain technique?

A
  1. Staining with crystal violet which will leave both variants blue/purple.
  2. Specimen is treated with a mordant (solution of iodine and potassium iodide) which fixes the dye.
  3. Decolourisation with a mixture of alcohol and acetone.
    Gram positive bacteria keeps the crystal violet stain while Gram negative bacteria is decolourised because it has thinner cell walls.
  4. Counterstaining with either safranin or fuchsin dye.
    Gram positive bacteria is blue/purple from the crystal violet while Gram negative bacteria will be red/pink
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26
Q

What is the acid-fast technique used for?

A

To differentiate species of acid-fast bacteria from other bacteria.

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27
Q

Give an example of acid-fast bacteria.

A

Mycobacterium

28
Q

What is the structure of acid-fast organisms?

A

Acid-fast organisms have high levels of mycolic acid, fatty acids, waxes and complex lipids in their cell walls, meaning the cell walls are wax-like and virtually impermeable.

29
Q

What are the four stages of the acid-fast technique?

A
  1. A lipid solvent is used to carry carbolfuchsin dye into the cells of the specimen. Both variants will be coloured red.
  2. The red stain is fixed by heating the slide.
  3. Cells are washed with a dilute acid and alcohol solution.
    The acid-fast bacteria retains the stain, but other bacteria will lose the stain
  4. Counterstaining with methylene blue.
    The acid-fast bacteria will be red and other bacteria will be blue.
30
Q

How is carbolfuchsin dye able to permeate the cell walls of acid-fast bacteria?

A

Carbolfuchsin contains phenol, which helps dissolve the waxy cell wall.

31
Q

Why can acid-fast bacteria retain a carbolfuchsin stain after applying a decolouriser?

A

The dilute acid and alcohol solution cannot permeate the acid-fast bacteria’s cell walls, due to the high level of mycolic acid.

32
Q

What are the stages of producing pre-prepared slides?

A

Fixing - preserving specimens in as close to a natural state as possible
Sectioning - specimens are dehydrated with alcohol and placed in a mould with wax/resin to form a hard block, then sliced thinly with a microtome
Staining - often multiple stains are used
Mounting - specimens are secured to a microscope slide and a cover slip is placed on top

33
Q

What are the stages of producing pre-prepared slides?

A

Fixing - preserving specimens in as close to a natural state as possible
Sectioning - specimens are dehydrated with alcohol and placed in a mould with wax/resin to form a hard block, then sliced thinly with a microtome
Staining - often multiple stains are used
Mounting - specimens are secured to a microscope slide and a cover slip is placed on top

34
Q

Define magnification.

A

Magnification describes how many times larger the image is than the actual object.

35
Q

Define resolution.

A

Resolution is the ability to distinguish between two separate objects which are close together.

36
Q

What is the equation for magnification?

A

Magnification = Image size / Actual size

37
Q

How are microscopes calibrated using an eyepiece graticule and stage micrometer?

A

The eyepiece graticule (inside the eyepiece lens) is a glass disk with a 1-100 scale (no units). The size of the divisions between the scale numbers differs between the different objective lenses (which all give a different magnification). The stage micrometer, a slide with an accurate measuring scale, is used to determine the size of the divisions at each magnification.

38
Q

When were electron microscopes developed?

A

In the 20th century.

39
Q

How do electron microscopes work?

A

A beam of electrons with a wavelength under 1nm is used to illuminate the specimen.

40
Q

Give two advantages of electron microscopes.

A
  • High magnification (could be up to x500,000)
  • High resolution
41
Q

Give two disadvantages of electron microscopes.

A
  • Expensive
  • It’s easier to damage the specimen
  • They require more preparation
42
Q

What is the average resolution of a light microscope?

A

200nm.

43
Q

What is the average resolution of an electron microscope (transmission and scanning)?

A

Transmission: 0.5nm
Scanning: 3-10nm

44
Q

What is the average magnification of a light microscope?

A

x1000.

45
Q

What is the average magnification of an electron microscope (transmission and scanning)?

A

Transmission: x250,000
Scanning: x100,000

46
Q

Which produces 2D images, transmission electron microscopes or scanning electron microscopes?

A

Transmission electron microscopes. Scanning electron microscopes produce 3D images.

47
Q

How do transmission electron microscopes work?

A

A beam of electrons is transmitted through the specimen and focused to produce an image.

48
Q

How do scanning electron microscopes work?

A

A beam of electrons is sent across the surface of the specimen and the reflected electrons are collected.

49
Q

What are artefacts in microscopy?

A

Artefacts are visible structural details caused by preparing the specimen. They are not a natural feature of the specimen and appear in both light and electron microscopy.

50
Q

Give an example of artefacts in light microscopy.

A

Air bubbles trapped under the cover slip.

51
Q

How does laser scanning confocal microscopy work?

A

A laser moves a single spot of focused light across a specimen. This causes fluorescence from the components labelled with a dye. The light emitted from these components is filtered through a pinhole aperture and light from other components is not.

52
Q

Define fluorescence.

A

The absorption and re-radiation of light.

53
Q

What is the focal plane in confocal microscopy?

A

The distance that gives the sharpest focus.

54
Q

Do confocal microscopes produce 2D or 3D images?

A

They produce 2D images, but 3D images can be formed by creating images at different focal planes.

55
Q

Give three advantages of laser scanning confocal microscopy.

A
  • it’s non-invasive
  • very high resolution images can be obtained
  • improved illumination
56
Q

Give two uses of laser scanning confocal microscopy.

A

Any two of:
- diagnosis of eye diseases
- endoscopic procedures
- development of new drugs
- virtual biopsies (in the future)

57
Q

What produces green fluorescent protein?

A

The jellyfish Aequorea victoria.

58
Q

What do GFP molecules do and how have they been engineered?

A

GFP molecules emit bright green light when illuminated with UV light. They have been engineered to fluoresce different colours so different components of a specimen can be studied at the same time.

59
Q

How are GFP molecules used to study the production and distribution of proteins in cells and organisms?

A

The gene coding for GFP has been isolated and can be attached by genetic engineering to genes coding for proteins under investigation. The fluorescence produced when a protein is made can be used to track where the protein goes within the cell or organism.

60
Q

How do atomic force microscopes gather information about specimens?

A

A mechanical probe on a cantilever is used to scan the surface of the specimen. When the probe is brought close to the surface, forces between the probe and the specimen cause deflections on the cantilever which are measured using a laser beam reflected from the top of the cantilever into a detector.

61
Q

Give two advantages of atomic force microscopy.

A

Any two of:
- Fixation and staining are not required
- Specimens can be viewed in normal conditions without cell damage
- Living specimens can be examined
- Very high resolution (about 0.1nm)

62
Q

Give two uses of atomic force microscopy.

A
  • in the pharmaceutical industry to identify potential drug targets on cellular proteins and DNA
  • to identify new drugs
63
Q

What is super resolved fluorescence microscopy?

A

A process allowing resolutions greater than 0.2 micrometres to be achieved in light microscopy.

64
Q

What is stimulated emission depletion?

A

The first principle of super resolved fluorescence microscopy. It uses two lasers which are slightly offset. One laser scans a specimen causing fluorescence, and the other laser then negates this fluorescence from all but a molecular sized area, building an image with a much higher resolution than normally produced in light microscopy.

65
Q

Who developed stimulated emission depletion?

A

Stefan Hell.

66
Q

What is the second principle of super resolved fluorescence microscopy?

A

Specimens are scanned multiple times with different molecules allowed to fluoresce each time. When the images are superimposed the resolution is at the molecular level, much higher than 0.2 micrometres.

67
Q

Who developed the second principle of super resolved fluorescence microscopy?

A

Eric Betzig and William E. Moerner. They worked independently.

OCR A A-Level Biology (2 decks)

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