Cellular Structures and Functions - GRE Subject Test: Biochemistry, Cell, and Molecular Biology

Eukaryotic flagella are primarily composed of microtubules and the motor protein dynein. Hydrolysis of ATP by dynein produces a sliding movement of the microtubule filaments that produces movement.

Kinesin is a motor protein that associates with microtubules, but it is not present in flagella. Myosin is a motor protein that associates with actin microfilaments.

Flagella are long, hair-like extensions of the plasma membrane. The movement of flagella propel some cells through fluid.

Mitochondria are the site of the reactions of aerobic metabolism and ATP synthesis. Centrioles consist of microtubules at the base of cilia and flagella, and are involved in spindle cell formation during cell division. The cytoskeleton is a network of protein fibers in the cytoplasm that give shape to a cell and is typically involved in cell movement. A pilus is a hair-like projection located on the surface of certain bacteria and is typically used to attach a bacterium to another cell.

Cell walls can vary in composition based on what organism is being discussed. In the case of plant cells, cellulose is the primary component. Bacteria use peptidolgycan in their cell walls, and fungi use chitin.

The cell wall is composed of cellulose and encloses a plant cell in a rigid framework. Plant cells have both cell membranes and cell walls. Plant cell walls cannot be digested by humans due to the beta glycosidic bond in its structure. The cell membrane forms the outer boundary of a cell and isolates cell contents from the environment. The cell membrane regulates movement of materials into and out of the cell. The cell membrane is also referred to as the plasma membrane. The nuclear envelope regulates movement into and out of the nucleus. The cytoskeleton gives shape and support to cells and is typically involved in cell movement.

The most abundant molecule in bacterial cell walls is peptidoglycan. Recall that peptidoglycan contains repeating NAG-NAM units, which are made from glucose. In addition, peptidoglycan also has peptides attached to these oligosaccharides. These units form bonds with each other to create a strong and a robust cell wall. Peptidoglycan is synthesized in cytoplasm and on cell membrane before finally being transported to the cell wall.

Recall that phosphate head groups are found on phospholipids, which make up cell membranes (not cell walls).

Gram stain is a laboratory technique used to distinguish bacteria. Gram positive and Gram negative bacteria are distinguished based on their cell walls. Gram positive bacteria have a single thick cell wall (outside its cell membrane). Upon Gram staining, these bacteria will hold the Gram stain because of their thick cell wall. The Gram negative bacteria also have only one cell wall; however, it is a lot thinner and is sandwiched between two cell membranes. The thin nature of the cell wall makes it easier for the Gram stain to leak out of the bacterial cell.

Cell walls are robust structures found in most bacteria, fungi, and plants. Gram stain depends on the properties of the cell wall. Some bacteria don’t stain well with Gram stain (such as Mycobacterium that causes tuberculosis) because of unique cell walls (not lack thereof). Fungal and plant cell walls are also made up of polysaccharides. Fungal cell walls contain a special molecule called chitin whereas plant cell walls contain cellulose. Recall that plants obtain energy from sunlight via photosynthesis; therefore, photosynthetic organisms do contain cell walls.

One of the biggest function of a cell wall is to prevent cellular swelling. Recall that a cell placed in a hypoosmolar solution will swell due to the influx of water; however, this cellular swelling will not occur in a cell containing cell wall because the robust cell wall prevents contents of the cell from expanding, thereby preventing cellular lysis.

Cellulose is a polymer of glucose linked by beta (1-4) linkages. Humans lack the enzyme that catalyzes the cleavage of this bond; thus cellulose (dietary fiber) is indigestible and passes through the digestive system unchanged.

The chloroplast has a very simular structure to the mitochondrion, as it is a double-membraned organelle. The chloroplast is used to house the processes of photosynthesis. The light reactions take place in the thylakoid membrane, while the light independent reactions take place in the stroma.

The contractile ring formed during cytokinesis is composed of actin microfilaments (a component of the cytoskeleton) and the motor protein myosin. The extracellular matrix can be anchored to the cell via the actin cytoskeleton or the intermediate filament cytoskeleton, via adherens junctions or desmosomes.

Post-translational modifications of proteins commonly occur in the endoplasmic reticulum or the Golgi apparatus and do not involve the cytoskeleton. The same logic applies for post-transcriptional modifications of mRNA, except these occur in the nucleus.

One of the most likely explanations for why intermediate filaments have no known motor proteins is because they are nonpolar. What this means is that there are no distinct ends to the filament. Motor proteins have ways of detecting which direction they are moving along actin or microtubules because they have distinct ends (polarization). Size should not affect the ability of a motor protein to function, especially because actin is smaller than intermediate filaments and microtubules are larger (hence the name "intermediate").

Eukaryotic flagella are composed of nine pairs of microtubules. In contrast, prokaryotic flagella are composed of the protein flagellin.

G-actin subunits that polymerize form microfilaments, also known as actin filaments. The other cytoskeletal components listed are formed from different proteins, but only microfilaments are actin-based structures.

Tubulin is a key protein found in microtubules. Microtubules are large cytoskeletal elements that have several functions inside a cell. One of the biggest use of microtubules arises during mitosis. Recall that chromosomes are pulled apart to opposite ends during the anaphase of mitosis. The rope-like substances that pull these chromosomes to opposite ends of cell are microtubule segments. In addition, microtubules are also involved in axonal transport in neurons. Some nerves (such as sciatic nerve) can have extremely long axons. To maintain the integrity of these distal ends of the axons, nutrients and other important molecules from the cell body of a neuron are carried, along microtubule segments, to axon terminals; therefore, microtubules are also essential for axonal transport.

Recall that tubulin is an essential component of microtubules. Administering Vincristine to cancer patients will halt the synthesis of microtubules in cancer cells, thereby inhibiting several essential cellular processes (such as mitosis). This will prevent proliferation of the cancer cells. There are three cytoskeletal elements; microtubules, microfilaments, and intermediate filaments. Of the three, microtubules are the biggest, followed by intermediate filaments. Recall that microtubules are also involved in axonal transport in neurons; therefore, halting microtubule synthesis in neurons could result in nerve problems and neuropathies.

Actin-myosin interactions involved in muscle contraction do not depend on the microtubules; therefore, Vincristine will not affect muscle contraction. Note that actin filaments make up microfilaments; therefore, any damage to actin (and microfilaments) will affect muscle contraction.

Microfilaments are the smallest of the three cytoskeletal elements and are made up of actin filaments (myosin is not found in microfilaments). The 9+2 arrangement is found in microtubules, the biggest cytoskeletal elements. The third cytoskeletal element is called the intermediate filament and is made up of related proteins unique to each intermediate filament.

The smooth endoplasmic reticulum lacks ribosomes on its surface, meaning it does not participate in protein translation. It does play a key role in the production of various lipids, such as phospholipids and triglycerides.

The endoplasmic reticulum, while crucial to protein synthesis, is also an intracellular calcium storage organelle, and calcium can be released in response to signaling cascades as needed. None of the other organelles listed participates in storage of calcium.

The endoplasmic reticulum (ER) is the cellular organelle responsible for membrane synthesis. Products from the ER are moved to the Golgi, where they are tagged and shipped off to their final destination. Lysosomes are important for degradation and ribosomes play a role in translation. Mitochondria are for energy production in the form of ATP.