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biology !!! > 2.1 Cell Structure > Flashcards

2.1 Cell Structure Flashcards

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1
Q

What is magnification regarding microscopes?

A
  • Magnification tells you how many times bigger the image produced by the microscope is than the real-life object you are viewing.
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2
Q

What is resolution regarding microscopes?

A
  • Resolution is the ability to distinguish between objects that are close together (i.e., the ability to see two structures that are very close together as two separate structures).
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3
Q

What are the 4 different types of microscopes?

A
  1. Optical (light) microscopes
  2. Transmission electron microscopes
  3. Scanning electron microscope
  4. Laser scanning confocal microscopes
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4
Q

Describe optical microscopes

A
  • Optical microscopes use light to form an image.
  • This limits the resolution of optical microscopes.
    —> Using light, it is impossible to resolve (distinguish between) two objects that are closer than 0.5 the wavelength of light.
    —> The wavelength of visible light is between 500-650 nanometres so an optical microscope cannot be used to distinguish between objects closer than half of this value.
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5
Q

What is the maximum resolution of optical microscopes? What organelles can it be used to identify?

A
  • 0.2 micrometers or 200 nanometres
  • Therefore it can be used to observe eukaryotic cells, their nuclei, and possibly mitochondria and chloroplasts.
  • Cannot be used to observe smaller organelles such as ribosomes, the endoplasmic reticulum, or lysosomes.
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6
Q

What is the maximum useful magnification of optical microscopes?

A
  • x1500
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7
Q

Describe electron microscopes

A
  • Electron microscopes use electrons to form an image which greatly increases the resolution of the microscope compared to optical microscopes, giving a more detailed image.
  • A beam of electrons has a much smaller wavelength than light, so an electron microscope can resolve two objects that are extremely close together.
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8
Q

What is the maximum resolution of electron microscopes? What can they be used to observe?

A
  • 0.2 nanometres, i.e., around 1000x greater than that of optical microscopes.
  • Used to observe small organelles such as ribosomes, the endoplasmic reticulum, or lysosomes.
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9
Q

What is the maximum useful magnification of electron microscopes?

A
  • x1.5 million
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10
Q

What are the 2 types of electron microscopes?

A
  1. Transmission electron microscope (TEMs)
  2. Scanning electron microscope (SEMs)
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11
Q

How do transmission electron microscopes work?

A
  • TEMs use electromagnets to focus a beam of electrons.
  • This beam of electrons is transmitted through the specimen.
  • Denser parts of the specimen absorb more electrons which makes them appear darker on the final image produce, showing a contrast between different parts of the object being observed.
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12
Q

What are advantaged of transmission electron microscopes?

A
  • They give high-resolution images (more detail).
  • This allows the internal structures within the cells, or even within the organelles, to be seen.
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13
Q

What are the disadvantages of transmission electron microscopes?

A
  • They can only be used with very thin specimens or thin sections of the object being observed.
  • They cannot be used to observe live specimens as there is a vacuum inside a TEM.
  • The lengthy treatment required to prepare specimens means that artefacts can be introduced (artefacts look like real structures but are actually rhetorical results of preserving and staining).
  • They don’t produce a colour image unlike optical microscopes.
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14
Q

Describe scanning electron microscopes.

A
  • Scanning electron microscopes scan a beam of electrons across the specimen.
  • This beam bounces off the surface of the specimen and the electrons are detected, forming an image.
  • This means SEMs can produce 3-dimensional images that show the surface of specimens.
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15
Q

What are the advantaged of scanning electron microscopes?

A
  • They can be used on thick, or 3d specimens.
  • They allow the external, 3d structure for specimens to be observed.
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16
Q

What are disadvantages of scanning electron microscopes?

A
  • They give lower resolution images (less detail) than transmission electron microscopes.
  • They cannot be used to observe live specimen (unlike optical microscopes that can be used to observe live specimens).
  • They do not produce a colour image (unlike optical microscopes that produce a colour image).
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17
Q

Describe laser scanning confocal microscopes.

A
  • Relatively new technology.
  • The cells being viewed must be stained with fluorescent dyes.
  • A thick section of tissue or small living organisms are scanned with a laser beam, and the laser beam is reflected by the fluorescent dyes.
  • Multiple depths of the tissue section/organisms are scanned to produce an image (as if the laser beam is building up the image layer by layer).
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18
Q

What are advantages of laser scanning confocal microscopes?

A
  • They can be used on thick or 3d specimens.
  • They allow the external, 3d structures of specimens to be observed.
  • Very clear images are produced. The high resolution is due to the fact that the laser beam can be focused at a very specific depth.
    —> you can even see the structure of the cytoskeleton in cells.
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19
Q

What are the disadvantages of laser scanning confocal microscopes?

A
  • It is a slow process and takes a long time to obtain an image.
  • The laser has the potential to cause photodamage to the cells.
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20
Q

How do optical microscopes work?

A
  • Light is directed through the thin layer of biological material that is supported on a glass slide.
  • The light is focused through several lenses so that an image is visible through the eyepiece.
  • The magnifying power of the microscope can be increased by rotating the higher power objective lens into place.
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21
Q

What are they key components of an optical microscope (5)?

A
  1. Eyepiece lens
  2. Objective lenses
  3. Stage
  4. Light source
  5. Coarse and fine focus
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22
Q

For an optical microscope, how do you prepare a slide using a liquid specimen?

A
  1. Add a few drops of the sample to the slide using a pipette.
  2. Cover the liquid/smear with a coverslip and gently press down to remove air bubbles.
  3. Wear gloves to ensure there is no cross-contamination of foreign cells.
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23
Q

What is the basic method of preparing an optical microscope using a solid specimen?

A
  1. Take care when using sharp objects and wear gloves to prevent the stain from dying your skin.
  2. Use scissors to cut a small sample of the tissue.
  3. Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps).
    —> tissues need to be thin so that light from the microscope can pass through.
  4. Apply a stain.
  5. Gently place a coverslip on top and press down to remove any air bubbles.
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24
Q

What does formaldehyde do?

A
  • Some tissue samples need to be treated with chemicals to kill/make the tissue rigid before being observed with a microscope.
  • This involves fixing the microscope using formaldehyde (preservative), dehydrating it using a series of ethanol solutions, impregnating it in paraffin for support then cutting thin slices from the specimen using a microtome.
  • The paraffin is removed from the slices, a stain is applied and the specimen is mounted using a resin and a coverslip is applied.
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25
Q

Why should you always start with the low power objective lens when using an optical microscope?

A
  • It is easier to find what you are looking for in the field of view.
  • This helps prevent damage to the lens or coverslip in case the stage has been raised too high.
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26
Q

How do you prevent the dehydration of tissue when using a microscope?

A
  • Adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being damaged by dehydration.
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27
Q

When using an optical microscope, how do you deal with unclear or blurry images?

A
  • Switch to the lower power objective lens are try using the coarse focus to get a clearer image.
  • Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly.
  • There could be cross-contamination with foreign cells or bodies.
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28
Q

How do you use a graticule to take measurements of a cell?

A
  1. A graticule is a small disc that has an engraved ruler. It can be placed into the eyepiece of a microscope to act as a ruler in the field of view.
  2. As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a stage micrometer (scale engraved on a microscope slide).
  3. By using these two scales together, the number of micrometers each graticule unit is worth can be worked out.
  4. After this is known the graticule can be used as a ruler in the field of view.
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29
Q

What are limitations of optical microscopes?

A
  • Size of cells or structures of tissues may appear inconsistent in different specimen slides as cell structures are 3d and the different tissue samples will have been cut at different planes resulting in inconsistencies when viewed on a 3d slide.
  • Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that cannot be seen.
  • The treatment of specimens when preparing slides could alter the structure of cells.
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30
Q

What do dyes do in staining?

A
  • Dyes used absorb specific colours of light while reflecting others; this makes the structures within the specimen that have absorbed the dye visible.
  • Certain tissues absorb certain dyes, which dye they absorb depends on their chemical nature.
31
Q

What is differential staining?

A
  • Specimens or sections are sometimes stained with multiple dyes to ensure the different tissues within the specimen show up.
32
Q

What are 2 common stains that are used and what colour do they turn cells?

A
  1. Toluidine blue: turns cells blue
  2. Phloroglucinol: turns cells pink/red
33
Q

How does staining work for electron microscopes?

A
  • When using transmission electron microscopes, the specimen must be stained in order to absorb the electrons.
  • Unlike light, electrons have no colour so the dyes used for staining cause the tissues to show up black or different shades of grey.
  • Any of the colour present in electron micrographs is not natural and is also not a result of the staining, so sometimes colours are added to the image after using an image-processing software.
34
Q

What dyes are used for electron microscopes?

A
  • Heavy-metal compounds because they absorb electrons well, e.g., osmium tetroxide and ruthenium tetroxide.
35
Q

What is the formula for magnification?

A

Magnification = image size/actual size of image

36
Q

How many millimetres are there in a metre?

A

1000

37
Q

How many micrometers are there in a millimetre?

A

1000

38
Q

How many nanometres are there in a micrometer?

A

1000

39
Q

What two types of lenses does a light microscope have?

A
  1. Eyepiece lens which often has a magnification of x10
  2. A series of (usually 3) objective lenses, each with a different magnification.
40
Q

How do you calculate the total magnification?

A

Total magnification = eyepiece lens magnification x objective lens magnification

41
Q

How do you calculate the total magnification?

A

Total magnification = eyepiece lens magnification x objective lens magnification

42
Q

What is the resolution of a light microscope limited by?

A

The wavelength of light

43
Q

What happens to light as it passes through a specimen?

A

It will be diffracted.

44
Q

What happens to light as it passes through a specimen?

A

It will be diffracted.

45
Q

The _ the wavelength of light, the more it is diffracted, and the more that this diffraction will _ as the points get closer together.

A
  • longer
  • overlap
46
Q

The _ the wavelength of light, the more it is diffracted, and the more that this diffraction will _ as the points get closer together.

A
  • longer
  • overlap
47
Q

Why can the phospholipid bilayer not be observed under a light microscope?

A
  • The width of the phospholipid bilayer is about 10nm.
  • The maximum resolution of a light microscope is 200nm.
  • Any points separated by a distance less than 200nm (such as the 10nm phospholipid bilayer) cannot be resolved by a light microscope and therefore will not be distinguishable as separate.
48
Q

Light vs electron microscope (6)

A
  1. Electrons microscopes are large and cannot be moved easily whereas light microscopes are small and easy to carry.
  2. Electron microscopes require a vacuum whereas light microscopes don’t.
  3. Electron microscopes involve a complicated sample preparation whereas light microscopes involve an easy sample preparation.
  4. Electron microscopes have over x500 000 magnification whereas light microscopes have only up to x2000 magnification.
  5. Electron microscope’s resolution is 0.5nm while light microscope’s resolution is 200nm.
  6. Electron microscopes use only dead specimen whereas specimens can be either living or dead.
49
Q

What is a cell membrane formed from?

A
  • Phospholipid bilayer of phospholipids spanning a diameter of around 10nm.
50
Q

Describe the cell wall.

A
  • Found in plant cells but not in animal cells.
  • Cell walls are formed outside the cell membrane.
  • Structural support is provided by the polysaccharide cellulose in plant, and Peptidoglycan in most bacterial cells.
  • Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells.
51
Q

Describe the nucleus.

A
  • The nucleus of a cell contains chromatin which is the genetic material of the cell.
  • Present in all eukaryotic cells (except red blood cells), the nucleus is relatively large and separated from the cytoplasm by a double membrane (the nuclear envelope) which has many pores.
  • Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes (e.g., DNA polymerases) and signalling molecules to travel in.
  • Usually, at least one or more darkly stained regions can be observed—these regions are individually termed ‘nucleolus’ and are the sites of ribosome production.
52
Q

What is chromatin?

A
  • A complex of DNA and histone proteins which is the genetic material of the cell.
  • The material from which chromosomes are made.
    —> chromosomes are made from sections of linear DNA tightly wound around proteins called histones.
53
Q

Describe mitochondria.

A
  • The site of aerobic respiration within all eukaryotic cells.
  • Visible simply under a light microscope.
  • Surrounded by a double-membrane with the inner membrane folded to form cristae.
  • The matrix formed by cristae contains enzymes needed for aerobic respiration, producing ATP.
  • Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication).
54
Q

Describe chloroplasts.

A
  • Found in plant cells.
  • Larger than mitochondria, and surrounded by a double-membrane.
  • Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana.
  • Grana are joined together by lamellae (thin and flat thylakoid membranes).
  • Chloroplasts are the site of photosynthesis.
  • Also contain small circular pieces of DNA and ribosomes used to synthesise proteins in chloroplast replication and photosynthesis.
55
Q

Where does the light-dependent stage of photosynthesis take place?

A

In the thylakoids within the chloroplasts.

56
Q

Where does the light-independent (Calvin cycle) stage of photosynthesis take place?

A

Stroma

57
Q

Describe ribosomes

A
  • Ribosomes are formed in the nucleolus and are composed of almost equal parts of RNA and protein.
  • Found in all cells.
  • Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells.
  • Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
  • Site of translation (protein synthesis).
58
Q

Describe ribosomes

A
  • Ribosomes are formed in the nucleolus and are composed of almost equal parts of RNA and protein.
  • Found in all cells.
  • Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells.
  • Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
  • Site of translation (protein synthesis).
59
Q

What ribosomes are found in eukaryotic cells?

A
  • 80S ribosomes composed of 60S and 40S units.
60
Q

What ribosomes are found in eukaryotic cells?

A
  • 80S ribosomes composed of 60S and 40S units.
61
Q

What ribosomes are found in prokaryotic cells?

A
  • 70S ribosomes composed of 50S and 30S subunits.
62
Q

Describe the rough endoplasmic reticulum (RER).

A
  • Found in plant and animal cells.
  • Surface covered in ribosomes.
  • Formed from continuous folds of membrane continuous with the nuclear envelope.
  • Processes proteins made by the ribosomes.
63
Q

Describe the smooth endoplasmic reticulum (RER).

A
  • Found in plant and animal cells.
  • No embedded ribosomes on the surface.
  • Involved in the production, processing, and storage of lipids, carbohydrates, and steroids.
64
Q

Describe the Golgi apparatus.

A
  • Found in plant and animal cells.
  • Flattened sacs of membrane similar to the smooth endoplasmic reticulum.
  • Modifies proteins and lipids before packaging them into Golgi vesicles.
    —> vesicles then transport the proteins and lipids to their required destination.
    —> proteins that go through the Golgi apparatus are usually exported (e.g., hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles.
65
Q

Describe large permanent vacuoles.

A
  • A sac in plant cells surrounded by the tonoplast, a selectively permeable membrane.
  • Vacuoles in animal cells are normally permanent and small.
66
Q

Describe large permanent vacuoles.

A
  • A sac in plant cells surrounded by the tonoplast, a selectively permeable membrane.
  • Vacuoles in animal cells are normally permanent and small.
67
Q

Describe vesicles.

A
  • Found in plant and animal cells.
  • A membrane-bound sac for transport and storage.
68
Q

Describe lysosomes

A
  • Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down).
  • Break down waste materials such as worn-out organelles.
  • Used extensively be cells of the immune system and in apoptosis (programmed cell death).
69
Q

Describe centrioles.

A
  • Hollow fibres made of microtubules.
  • 2 centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division.
  • Not found in flowering plants and fungi.
70
Q

Describe microtubules.

A
  • Found in all eukaryotic cells.
  • Makes up the cytoskeleton of the cell about 25 nm in diameter.
  • Made of up alpha and beta tubulin combined to form dimers, the dimers are then joined into protofilaments.
    —> 13 protofilaments in a cylinder make a microtubule.
  • The cytoskeleton is used to provide support and movement of a cell.
71
Q

Describe microvilli.

A
  • Found in specialised animal cells.
  • Cell membrane projections.
  • Used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances.
72
Q

Describe microvilli.

A
  • Found in specialised animal cells.
  • Cell membrane projections.
  • Used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances.
73
Q

Describe cilia.

A
  • Hair-like projections made from microtubules.
  • Allows the movement of substances over the cell surface.
74
Q

Describe flagella.

A
  • Found in specialised cells.
  • Similar in structure to cilia, but made up of longer microtubules.
  • Contract to provide cell movement in sperm cells.

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