Cell Theory and Cell Types Questions and Answers

Answer: A

It is examined under a fluorescence microscope.

A fluorescence microscope illuminates the specimen with light of the excitation wavelength (often ultraviolet or blue) and uses filters to pass only the longer-wavelength light that the fluorescent dye emits. The labelled structures shine against a dark background, which is how fluorescent stains and antibody labels are viewed.

Stereozoom and dissecting microscopes are low-power light microscopes for viewing whole specimens in three dimensions with ordinary light, and an electron microscope uses an electron beam rather than light, so none of them detects fluorescence emission.

Answer: B

The usual electron source is tungsten.

The standard electron gun uses a hairpin tungsten filament heated to about 2,700 K, from which electrons boil off by thermionic emission and are accelerated by the high voltage. Tungsten is chosen because it has the highest melting point of any metal (about 3,400 °C) and so can be heated enough to emit electrons without melting. Brighter sources such as lanthanum hexaboride crystals and field-emission tips are also used.

Iron, silver and copper melt at far lower temperatures and would fail long before giving useful emission. (Copper is used for the specimen grids, not the filament.)

Answer: A

The order is nucleus, mitochondria, microsomes, cytosol.

In differential centrifugation a homogenate is spun at increasing speeds, and the largest, densest particles sediment first. Nuclei come down at low speed (about 600-1,000 g), mitochondria (with lysosomes and peroxisomes) at medium speed (about 10,000-20,000 g), and microsomes, the fragments of ER, at high speed (about 100,000 g). The supernatant left at the end is the cytosol.

Every other option puts the cytosol or the microsomes too early; the cytosol never forms a pellet, so it is always last.

Answer: C

The fixative for plant material in electron microscopy is glutaraldehyde.

Glutaraldehyde is a five-carbon dialdehyde whose two aldehyde groups cross-link proteins rapidly and firmly, preserving fine structure at the level the electron microscope resolves. It is normally followed by post-fixation in osmium tetroxide, which fixes lipids and adds contrast.

Alcohol and acetic alcohol are coagulating fixatives used for light microscopy and chromosome work; they distort ultrastructure. Formaldehyde penetrates fast but cross-links weakly and reversibly, so on its own it is not the standard EM fixative.

Answer: C

Colocalization is usually visualized by confocal microscopy.

Confocal laser scanning microscopy excites the sample with lasers and uses a pinhole to reject out-of-focus light, giving sharp optical sections. Two proteins tagged with different fluorophores (for example GFP and RFP) are imaged in separate channels and the images overlaid; overlap of the two signals in the same pixels indicates colocalization.

Phase contrast microscopy shows unstained structures by refractive index and does not detect fluorescence. Scanning electron and atomic force microscopy image surfaces and cannot distinguish two fluorescent labels.

Answer: D

The correct combination is 4 and (iv): prokaryotic cells are less than 5 micrometres across and have no membrane around any organelle.

Typical bacteria are 0.5 to 5 micrometres long, and they lack a nucleus and membrane-bound organelles such as mitochondria, chloroplasts, the endoplasmic reticulum and the Golgi apparatus; their ribosomes are not membrane-bound.

The other pairs are wrong. Prokaryotic cells are not more than 10 micrometres (pair 1 and ii). Eukaryotic cells, usually 10 to 100 micrometres, do not lack membranes around their organelles (pair 2 and i). Pair 3 and iii is wrong on both counts: eukaryotic cells are usually larger than 10 micrometres, and they have membranes around nearly all their organelles, not just some.

Answer: D

The statement that is not part of cell theory is that cells in a multicellular organism are derived from noncells.

Cell theory, from Schleiden and Schwann and completed by Virchow, holds that all organisms are made of one or more cells, that the cell is the basic structural and functional unit of life, and that all cells arise from pre-existing cells (omnis cellula e cellula).

The first three options restate these tenets. Derivation from non-cellular material is spontaneous generation of cells, which Virchow's principle specifically rejects.

Answer: C

Prokaryotes are characterised by the absence of membrane surrounding organelles.

A prokaryotic cell has no nucleus bounded by a membrane and no membrane-bound organelles such as mitochondria, chloroplasts, endoplasmic reticulum or Golgi. Its DNA lies in a region of the cytoplasm called the nucleoid.

'Presence of nucleus' is the eukaryotic condition, so that option fails even though prokaryotes do have a nucleoid. Prokaryotes certainly have DNA, and no cell encloses its RNA in a membrane.

Answer: C

The cytoplasm is all the fluid part and structures within the plasma membrane except the nucleus.

Cytoplasm means the cytosol together with the organelles (mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes and so on) enclosed by the plasma membrane, excluding the nucleus. The nuclear contents are called the nucleoplasm.

The first option covers only the fluid, which is the cytosol. The second and fourth include everything within the membrane, and the fourth explicitly includes the nucleus; that whole living content is the protoplasm, not the cytoplasm.

Answer: C

The feature that does not describe the TEM is a three-dimensional image.

In a transmission electron microscope the beam passes through ultrathin sections stained with heavy metals, giving a flat, two-dimensional projection image at high resolution in which intracellular organelles and membranes are clearly seen.

So ultrathin stained sections, visible organelles and two-dimensional images are all features of TEM. Images with a three-dimensional appearance come from the scanning electron microscope, which scans the surface of the specimen.