Cell Theory and Cell Types Questions and Answers

Answer: A

The order is nuclei, mitochondria, microsomes, and ribosomes.

In differential centrifugation, larger and denser particles sediment at lower forces. Nuclei, the largest organelles, pellet first at about 600 to 1000 g. Mitochondria (and lysosomes, peroxisomes) come down next at roughly 10,000 to 20,000 g. Microsomes, the vesicles formed from the endoplasmic reticulum, need around 100,000 g, and free ribosomes and large macromolecules need the highest forces, about 150,000 to 300,000 g.

The other orders put small particles such as ribosomes or microsomes ahead of the much larger nuclei or mitochondria, which reverses the dependence of sedimentation on size.

Answer: C

The organelle enclosed by a single membrane is the endoplasmic reticulum.

The ER is a network of tubules and sacs bounded by a single lipid bilayer, continuous with the outer membrane of the nuclear envelope. Lysosomes, the Golgi apparatus, vacuoles and peroxisomes are other single-membrane organelles.

Mitochondria have two membranes, an outer one and a folded inner one forming cristae. Ribosomes and the centrosome have no membrane at all: ribosomes are RNA-protein particles and the centrosome is built of centrioles and protein.

Answer: A

The main parts of a cell are the plasma membrane, cytoplasm and nucleus.

Every typical cell has a plasma membrane that encloses it and controls what enters and leaves, cytoplasm in which the organelles lie and most metabolism takes place, and a nucleus that holds the genetic material and directs the cell's activities.

A cell wall and chloroplasts are found in plant cells but not in animal cells, so they cannot be main parts of every cell. Mitochondria, ribosomes, the Golgi apparatus and lysosomes are organelles inside the cytoplasm, not the cell's main divisions.

Answer: B

Cytology is the study of cells.

The word comes from the Greek kytos, a vessel or cell, and logos, study. Cytology deals with the structure, function and chemistry of cells; when applied to diagnosis, it becomes cytopathology.

The study of disease is pathology. The study of tissues is histology, from the Greek histos, tissue. The study of bones is osteology, from osteon, bone.

Answer: B

Cells can supercool to only about \(-40\)°C.

Pure water does not freeze at 0°C unless ice can nucleate; small volumes free of nucleating particles stay liquid well below that. The limit is homogeneous nucleation, which happens spontaneously at about \(-40\)°C, so cell water, held in small compartments, can supercool down to roughly that temperature and no further. In cold-hardy trees this sets the lower limit of deep supercooling, which is why their northern range ends near the \(-40\)°C isotherm.

0°C is the normal freezing point, not a supercooling limit; \(-20\)°C stops short of the limit, and \(-80\)°C lies beyond it.

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.