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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

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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.

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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.
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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.

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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.

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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

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33
Q

Define magnification.

A

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

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34
Q

Define resolution.

A

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

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35
Q

What is the equation for magnification?

A

Magnification = Image size / Actual size

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36
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.

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37
Q

When were electron microscopes developed?

A

In the 20th century.

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38
Q

How do electron microscopes work?

A

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

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39
Q

Give two advantages of electron microscopes.

A
  • High magnification (could be up to x500,000)
  • High resolution
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40
Q

Give two disadvantages of electron microscopes.

A
  • Expensive
  • It’s easier to damage the specimen
  • They require more preparation
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41
Q

What is the average resolution of a light microscope?

A

200nm.

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42
Q

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

A

Transmission: 0.5nm
Scanning: 3-10nm
TEM have a higher resolution.

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43
Q

What is the average magnification of a light microscope?

A

x1000.

44
Q

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

A

Transmission: x250,000
Scanning: x100,000
TEM have a higher magnification.

45
Q

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

A

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

46
Q

How do transmission electron microscopes work?

A

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

47
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.

48
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.

49
Q

Give an example of artefacts in light microscopy.

A

Air bubbles trapped under the cover slip.

50
Q

Define fluorescence.

A

The absorption and re-radiation of light.

51
Q

What produces green fluorescent protein?

A

The jellyfish Aequorea victoria.

52
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.

53
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.

54
Q

Define eukaryote.

A

A multicellular organism containing DNA in a membrane-bound nucleus, separate from the cytoplasm. All eukaryotic cells contain a large number of specialised, membrane-bound organelles.

55
Q

What is the structure of the nucleus?
Include:
Nuclear envelope
Chromatin
Nucleolus

A

The nucleus is surrounded by a nuclear envelope - a dense spherical structure made of an inner and outer membrane separated by fluid containing nuclear pores and a viscous liquid called nucleoplasm. The nucleus contains chromatin (made of DNA and proteins called histones) and a nucleolus made of proteins and RNA.

56
Q

What is the function of the nucleus?

A

The nucleus houses nearly all of the cell’s genetic material. It controls the cell’s activities by controlling the transcription of DNA, which directs protein synthesis.

57
Q

What is the function of the chromatin within the nucleus?

A

Some of the proteins in the chromatin regulate the cell’s activity. It coils to form chromosomes in cell division.

58
Q

What is the function of the nucleolus?

A

The nucleolus makes rRNA (ribosomal RNA) from RNA, which is made into ribosomes.

59
Q

What is the function of the nuclear envelope?

A

The nuclear envelope protects the nucleus from damage. The nuclear pores allow substances like RNA to move between the nucleus and the cytoplasm. Nucleoplasm provides structural support for the chromatin.

60
Q

What is the structure of the mitochondria?

A

Mitochondria are surrounded by two membranes. The inner membrane folds inwards to make cristae. Inside the cristae is the matrix, containing enzymes which catalyse aerobic respiration. Mitochondria contain a small amount of DNA.

61
Q

What is the function of the mitochondria?

A

Mitochondria are the site of respiration, where ATP is produced. Mitochondria can also produce their own enzymes and reproduce themselves.

62
Q

What is the function of the cell-surface membrane?

A

The cell-surface membrane controls the exchange of substances between the cell and its surroundings. It also has receptor molecules on it, which allow it to respond to chemicals like hormones.

63
Q

What is the structure of ribosomes?

A

Ribosomes, which can be found either in the cytoplasm or attached to the RER (they are also in mitochondria and chloroplasts), consist of a small sub-unit and a large sub-unit made of proteins and RNA.

64
Q

What is the function of ribosomes?

A

Ribosomes are the site of protein synthesis in a cell.

65
Q

What is the structure of the Golgi apparatus?

A

The Golgi apparatus is a specialised ER made of a stack of cisternae. The membrane of the cisternae has two layers surrounding an area of fluid.

66
Q

What is the function of the Golgi apparatus?

A

The Golgi apparatus modifies proteins received from the RER (sometimes adding sugar molecules) and packages them into vesicles. It also makes lysosomes.

67
Q

What is the structure of lysosomes?

A

Lysosomes are specialised vesicles - spherical sacs surrounded by a single membrane. They contain digestive enzymes.

68
Q

What is the function of lysosomes?

A

Digestive enzymes in lysosomes are important for the immune system as they break down pathogens engulfed by phagocytic cells. They also digest invading cells and break down worn out cell components.

69
Q

Why is the membrane surrounding a lysosome important?

A

It keeps the digestive enzymes separate from the cytoplasm so they don’t digest the cell.

70
Q

What is the structure of the rough endoplasmic reticulum?

A

The RER consists of a stack of cisternae studded with ribosomes.

71
Q

What is the function of the rough endoplasmic reticulum?

A

The RER transports proteins made on the ribosomes.

72
Q

What is the structure of the smooth endoplasmic reticulum?

A

The SER consists of a stack of cisternae. It has no ribosomes, unlike the RER.

73
Q

What is the function of the smooth endoplasmic reticulum?

A

The SER synthesises and stores lipids and carbohydrates for the cell.

74
Q

What is the structure of centrioles?

A

The centrioles are made of nine bundles of microtubules (small protein cylinders).

75
Q

What is the function of centrioles?

A

Two centrioles form the centrosome, which is involved in organising spindle fibres in cell division.

76
Q

What is the structure of vesicles?

A

Vesicles are fluid-filled sacs in the cytoplasm, surrounded by membranes. Some are formed by the Golgi apparatus or ER, while others are formed at the cell surface.

77
Q

What is the function of vesicles?

A

Vesicles transport substances in and out of the cell via the cell-surface membrane and between organelles.

78
Q

What is the structure of cilia?

A

Cilia are small, hair-like structures found on the cell-surface membrane. They have an outer membrane and a ring of nine pairs of microtubules, with one pair in the middle.

79
Q

What is the function of cilia?

A

Stationary cilia (on the surface of cells) are important for sensory organs. Mobile cilia beat when the microtubules slide over one another and move fluid/objects adjacent to the cell.

80
Q

What is the structure of flagella?

A

Flagella have the same structure as cilia (an outer membrane and nine microtubule pairs in a ring surrounding a tenth pair) but they’re longer and stick out from the cell surface.

81
Q

What is the function of flagella?

A

Flagella are used to propel cells forward. In some cells they are used to detect changes in the cell’s environment.

82
Q

What are the three components of the cytoskeleton?

A

Microfilaments - contractile fibres formed from the protein actin
Microtubules - tubes made from polymers of globular proteins. They also compose spindle fibres.
Intermediate fibres.

83
Q

What is the function of the cytoskeleton?

A

The cytoskeleton holds organelles in place and controls cell movement and the movement of organelles within cells.

84
Q

What is the function of microfilaments within the cytoskeleton?

A

Microfilaments are responsible for cell movement and cell contraction during cytokinesis.

85
Q

What is the function of microtubules within the cytoskeleton?

A

Microtubules determine the shape of the cell and track the movement of organelles e.g. vesicles around the cell. Spindle fibres help separate chromosomes in cell division.

86
Q

What is the function of intermediate fibres within the cytoskeleton?

A

Intermediate fibres give cells mechanical strength and maintain their integrity.

87
Q

What is the function of chloroplasts?

A

To carry out photosynthesis - building sugars using carbon dioxide, water and light energy.

88
Q

Where are chloroplasts found?

A

In the stems and leaves of green plants.

89
Q

What is the structure of chloroplasts?

A

Chloroplasts consist of a double membrane filled with a liquid called the stroma. It has stacks of thylakoid membranes (membranes forming flattened sacs) called grana, which are joined by membranes called lamellae. Grana contain chlorophyll and provide a large surface area for photosynthesis. Chloroplasts also contain DNA and ribosomes, so they can make their own proteins.

90
Q

What is the function of the permanent vacuole?

A

The permanent vacuole maintains turgor - when the contents of the cell press against the cell wall, keeping the cell rigid.

91
Q

Where are permanent vacuoles found?

A

In the cytoplasm of plant cells - animal cells can only have temporary vacuoles.

92
Q

What is the structure of the permanent vacuole?

A

Permanent vacuoles are sacs filled with cell sap and bound by a selectively permeable membrane called the tonoplast.

93
Q

What is the function of the cell wall?

A

The cell wall gives the cell support and structure. It also acts as a defence mechanism against pathogens.

94
Q

What is the structure of the cell wall?

A

The cell wall is made of cellulose, a complex carbohydrate. It is freely permeable (substances can pass in and out of the cell) and contains pores called plasmodesmata, which allow substances to directly travel from the cytoplasm of one cell to that of another.

95
Q

How long are prokaryotic cells?

A

1-10 micrometres

96
Q

What is the main difference between eukaryotic and prokaryotic cells?

A

Prokaryotic cells are smaller and simpler than eukaryotic cells, containing no membrane-bound organelles.

97
Q

Do prokaryotic cells contain an endoplasmic reticulum?

A

No.

98
Q

Do prokaryotic cells contain a Golgi apparatus?

A

No.

99
Q

Do prokaryotic cells contain mitochondria?

A

No.

100
Q

Do prokaryotic cells contain a nucleus/DNA?

A

Prokaryotic cells have no nucleus. The DNA is found free in the cytoplasm as a single circular chromosome.

101
Q

Do prokaryotic cells contain a cell-surface membrane?

A

Yes.

102
Q

Do prokaryotic cells contain a cell wall?

A

Yes. It’s made of peptidoglycan/murein - a complex polymer of amino acids and sugars.

103
Q

Do prokaryotic cells contain ribosomes?

A

Yes. They contain 70s ribosomes, while eukaryotic cells contain 80s ribosomes.

104
Q

Do prokaryotic cells contain flagella?

A

Yes. They don’t contain the 9+2 arrangement found in eukaryotic cells.

105
Q

What are pili?

A

Pili are short extensions from the cell surface which are used to adhere to other cells/surfaces.

OCR A A-Level Biology (4 decks)

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