Other Organelles - GRE Subject Test: Biology

Peroxisomes are primarily responsible for the breakdown of very long chain fatty acids and D-amino acids, as well as the synthesis of special types of phospholipids known as plasmalogens. If the cell cannot catabolize very long chain fatty acids, the issue is most likely within the peroxisomes.

The Golgi apparatus is very important for protein packaging. Mitochondria are crucial to generating energy for the cell. The smooth endoplasmic reticulum has various functions, including lipid and carbohydrate metabolism.

The function of breaking down cellular contents is done by the lysosome. Lysosomes have an acidic interior, which is useful for breaking down macromolecules that are no longer being used in the cell.

The smooth endoplasmic reticulum, in contrast, helps to break down foreign material, such as toxins. Mitochondria primarily serve to produce ATP for cellular energy. The Golgi apparatus works in tandem with the rough endoplasmic reticulum and helps to group and package proteins for vesicular transport.

Proton pumps use ATP to continuously pump hydrogen ions into the interior of the lysosome, thus maintaining an acidic environment in which the enzymes are most optimally efficient at breaking down the debris/macromolecules. The membrane of the lysosome is selectively permeable due to these types of transporters, and protects the rest of the cell from the very acidic environment. Recent journal articles suggest that the lysosome's membrane will be deliberately disrupted, releasing the acid and the hydrolases into the cytosol during apoptosis.

Chloroplasts are found primarily in the mesophyll cells. Mesophyll cells are the green colored tissue that is found within the leaf. It is the abundance of chloroplasts containing chlorophyll that give the mesophyll cells their green coloring. Parenchyma, sclerenchyma, and collenchyma cells are all found within the shoot and root of the plant, not in the leaves. These cell types have little to no chloroplasts within their membranes.

ATP synthase and cytochromes stud the inner membrane, and more surface area means that more of them can be present in each mitochondrion. This increases the capacity to generate ATP.

Eukaryotic ribosomes are 80S with one large 60S subunit and one small 40S subunit. Prokaryotic ribosomes are 70S, with one large 50S subunit and one small 30S subunit. "S" refers to the sedimentation coefficient (Svedberg), which is a particles sedimentation velocity for a given applied acceleration.

Ribosomes can be classified as ribonucleoproteins because the proteins are associated with ribonucleic acids (RNA). Thus, RNA-protein complex is the best descriptor.

Interestingly, ribosomes are one of the only examples of an RNA structure that has enzymatic activity. The primary enzymatic function of the ribosome is one of a peptidyl transferase; that is, the catalysis of peptide bond formation between amino acids as those acids are brought to the nascent strand.

On its own, G-actin (globular actin) is not likely to nucleate and begin to form chains of F-actin (fibrous actin); therefore, it is useful to have proteins to help the nucleation process. One of these protein complexes is Arp2/3. Arp2/3 is especially known for its function of nucleating actin chains that branch off of previously established actin chains. Myosins are motor proteins that interact with actin chains to perform various functions, such as muscle contraction and transporting vesicles. Cofilin is a protein that binds G-actin monomers and helps them dissociate from F-actin.

Each choice describes a distinct function of the cytoskeleton. The cytoskeleton is involved in supporting various organelles, helping to anchor them in various locations around the cell and maintaining their shape and integrity. It also has the important function of helping with vesicle trafficking by associating with various motor proteins that carry vesicles from one part of the cell to another. The cytoskeleton is also a part of several different types of cell junctions (e.g. adherens junctions). Finally, the cytoskeleton is also an important part of various motile structures, such as cilia and flagella.

The three major components of the cytoskeleton in cells are microtubules, intermediate filaments, and microfilaments. Microtubules are the larger filaments and make up the mitotic spindle, as well as flagella and cilia. Intermediate filaments are used in structural maintenance.

Microfilaments are the smaller filaments and make up the polymerized actin filament in muscle fibers.

Microfilaments and microtubules are both polarized, and can be used in vesicular transport. Intermediate filaments lack polarity and serve only structural functions.

Intermediate filaments form a veil right next to the nuclear membrane, are of intermediate thickness with respect to the other two cytoskeletal filaments, and they almost exclusively play structural roles. Actin filaments brace cells against surfaces and allow contractions in striated muscle. Also, actin filaments provide structural support and have a role in determining cell shape. Microtubules form the mitotic spindle and comprise cilia and flagella. They are also the "freeways" on which motor proteins move and transport vesicles throughout the cell.

GDP-GTP cycling can regulate a number of these cytoskeletal components, although GTP-binding is especially crucial to microtubule polymerization. In addition, microtubule polymers are comprised of alpha and beta tubulin, while this is not the case for the other components listed. Microtubules are the only choice that fit all of the criteria posed in the question.

Membrane-bound ribosomes are found on the surface of the rough endoplasmic reticulum. Proteins synthesized by membrane-bound ribosomes are translocated into the endoplasmic reticulum while they are being translated.

These proteins generally undergo modification in the rough endoplasmic reticulum and are packaged into vesicles by the Golgi apparatus. Vesicles from the Golgi apparatus then interface with the plasma membrane of the cell or the membranes of other organelles. Proteins from the interior of the vesicle can be released into the extracellular space or interior of other organelles. Proteins in the membranes of the vesicle become embedded in the plasma membrane of the cell or the organelle.

Proteins synthesized on free ribosomes commonly remain in the cytosol and are modified by different processes than protein synthesized on the rough endoplasmic reticulum.

The smooth endoplasmic reticulum serves several different purposes in the cell, one of which is lipid synthesis. This structure is responsible for producing phospholipids and cholesterol, as well as some steroid hormones and other lipid molecules. It also helps to eliminate foreign toxins from the cell.

The rough endoplasmic reticulum houses ribosomes that synthesize proteins that are destined for the membrane or extracellular environment. The Golgi apparatus is responsible for packaging proteins in vesicles, allowing for transport out of the cell. Ribosomes are involved in protein synthesis (translation).

Nuclear import and export is tightly regulated by importin and exportin proteins. These proteins interact with target proteins to carry them into or out of the nucleus, respectively. Proteins cannot passively diffuse into the nucleus and they are not translocated into the nucleus during translation.

All of these choices are correct regarding the nucleus. Nuclear pore complexes are incredibly important for regulating nuclear import and export. Ribosomal subunits are assembled in the nucleolus, which is a portion of the nucleus. The nucleus is covered by two lipid bilayers, which constitute the nuclear envelope.

The correct answer is lamina. This is a fibrous network of filaments within the nucleus that provides architectual support similar to the cytoskeleton. Chromatin also is held in place by association with the lamina.