Understand functions of organelles in the cell - AP Biology

The endoplasmic reticulum is an organelle that manufactures proteins, steroid molecules, cholesterol, and other lipids. The rough endoplasmic reticulum is embedded with ribosomes to allow polypeptides to be translated into the interior of the organelle. The smooth endoplasmic reticulum is responsible for lipid synthesis and degradation of toxins.

The simple answer here is that all of these organelles have a membrane, except for ribosomes. Nuclei, mitochondria, chloroplasts, and cells as a whole have transmembrane proteins spanning the lipid bilayer, which can be used for transport or other purposes. Ribosomes are mostly comprised of rRNA and do not have membranes; thus, they will not bear transmembrane proteins.

The nucleolus is a structure found inside the nucleus. It is responsible for the production of rRNA, and also constructs the subunits of ribosomes.

Mitochondria are responsible for ATP production, and various chaperone proteins are responsible for directing substance transport within a cell. Several classes of proteins in the nucleus are involved in mRNA transcription and post-transcription modification, but these processes do not involve the nucleolus.

Desmosomes are the primary junction that helps keep tissues that are under a lot of stress, like skin, intact. Although tight junctions typically accompany desmosomes, their primary function is to prevent substances from passing between cells. Gap junctions are used for communication between cells, and intercalated discs are only found in cardiac muscle tissue.

The lysosome is an organelle that is used to digest broken cellular machinery or foreign particles. It maintains an acidic environment inside by pumping hydrogen ions in, not out. This environment helps denature the things it needs to digest, and is the most effective pH for the digestive enzymes that are inside.

The other structures do not maintain acidic environments, and would need the capability of removing protons.

The rough endoplasmic reticulum is given its name due to the ribosomes present within its membrane. These ribosomes are able to translate proteins, which are then modified within the endoplasmic reticulum and sent to the Golgi apparatus.

Ribosomes are the only cellular machinery capable of translating RNA into proteins.

The Golgi apparatus is a series of flattened membrane sacs found in the cell. It receives vesicles filled with proteins from the rough endoplasmic reticulum. The Golgi apparatus is responsible for recognizing proteins based on their signal sequences and sending concentrations of similar proteins to various parts of the cell. It can also deliver proteins out of the cell using secretory vesicles. The membrane sacs of the Golgi apparatus are constantly used and regenerated to create vesicles of packaged proteins.

Muscle contraction is accomplished using the function of microfilaments, namely actin and myosin. Microtubules are not a key player in muscle contraction, but are used to create the mitotic spindles, flagella, and cilia. Microtubules are a key element of the cytoskeleton, and are generally involved in structural aspects of the cell.

While free ribosomes are present independently in the cytosol, bound ribosomes are attached to the rough endoplasmic reticulum. Their presence gives this organelle its "rough" appearance and distinguishes it from the smooth endoplasmic reticulum, which does not contain ribosomes. The nuclear envelope, which surrounds and protects the eukaryotic nucleus, is also the site of some bound ribosomes and is connect to the rough endoplasmic reticulum. The Golgi apparatus, lysosomes, and mitochondria have no ribosomes bound to their membranes.

The dense, rigid exterior is most likely describing a cell wall. Cell walls are used to protect cells and help them maintain their shape. Plant cells, fungal cells, and most prokaryotic cells have a cell wall structure, though the composition of the cell walls varies between cell types. Plants, such as ferns, use cellulose in their cell walls. Fungi, such as mushrooms, generally use chitin. Prokaryotic cell walls vary in composition, but bacteria generally use peptidoglycan, which can help in identifying bacteria via Gram staining.

Animal cells, such as those found in the liver, do not have cell walls.

The Golgi apparatus is derived from the endoplasmic reticulum and consists of a series of flattened sacs. The function of the Golgi apparatus is to package and transport proteins to the cell surface within vesicles, which pinch off from the Golgi. These vesicles then interface with the cell membrane, releasing their contents into the extracellular space or integrating membrane proteins into the cell membrane.

The Golgi apparatus is not involved in differentiation between the smooth and rough endoplasmic reticulum. This difference comes from the presence of ribosomes, which are embedded in the surface of the rough endoplasmic reticulum, but absent from the smooth endoplasmic reticulum. The rough endoplasmic reticulum is responsible for synthesizing and modifying proteins that are destined for the Golgi apparatus for packaging. The smooth endoplasmic reticulum helps eliminate toxins from the cell.

Each of these answers correctly describes characteristics of mitochondria. Mitochondria are semi-autonomous: they are capable of synthesizing some of their own proteins with DNA and ribosomes. The proteins are then embedded in the inner mitochondrial membrane, and function in the electron transport chain. Mitochondria have two membranes, an inner membrane and an outer membrane. Mitochondria are the sites of cellular respiration and ATP production, often called the "powerhouse" of the cell. After glycolysis, pyruvate is transported to the mitochondrial matrix for the Krebs cycle and electron transport chain. Mitochondria are not involved in the endomembrane system; they are separate organelles.

The smooth endoplasmic reticulum contains enzymes responsible for the metabolism of small chemical molecules such as alcohol, pharmaceutical drugs, and environmental contaminants. Expansion of the smooth endoplasmic reticulum can occur under conditions of new and continued exposure to a given chemical or stimulus as an adaptation to metabolize and eliminate the drug or chemical.

Smooth endoplasmic reticulum is particularly abundant in hepatocytes, or liver cells. Blood absorbs nutrients and ingested toxins from the small intestine and immediately travels to the liver through the portal vein. Hepatocytes then filter and metabolize the absorbed toxins before they can circulate through the body.

Mitochondria and chloroplasts are organelles within eukaryotic and plant cells, respectively. All eukaryotic organisms contain mitochondria, but only photoautotrophs will contain chloroplasts. The function of these organelles is to produce energy for the cell to survive and function. Mitochondria produce energy through respiration and chloroplasts produce energy through photosynthesis. These organelles have their own DNA to express their own set of genes and have ribosomes to make the proteins necessary to make energy for the cell. The proteins for the electron transport chain, for example, are transcribed and translated within the mitochondrial matrix before being implanted in the inner mitochondrial membrane.

The mitochondria are membrane-bound organelles involved in the process of cellular respiration. Specifically, the Krebs cycle takes place in the matrix of the mitochondria and the electron transport chain takes place along the inner mitochondrial membrane. During aerobic respiration, oxygen is used as the final electron receptor of the electron transport chain and generates water as a byproduct. Without oxygen, the mitochondria cannot perform oxidative phosphorylation, and the cell must rely on glycolysis for energy.

Lysosomes contain hydrolytic enzymes that can digest macromolecules from phagocytosis, endocytosis, and autophagy. When damage occurs to the membrane of the lysosome, these enzymes can be released and cause damage to the cell, leading to apoptosis.

Peroxisomes contain peroxidases, which help to eliminate hydrogen peroxide from the cell and prevent the creation of free radicals. The nucleus houses DNA and would be severely damaged by the introduction of hydrolytic enzymes. The plasma membrane is used to contain the cytoplasm and organelles and the endoplasmic reticulum is used to modify and package proteins.

Homeostasis is the mechanism that allows for organisms to maintain equilibrium of their internal environments. Certain variables of the internal environments are regulated such that the conditions of the organism remain stable and relatively constant. This is often accomplished via negative feedback loops and hormone regulation.

Reproduction refers to an organism's ability to generate offspring. Respiration refers to the method of gas exchange in the lungs. Synthesis refers to the generation of certain molecules. Mitosis refers to cellular division. While any of these processes could be used to affect the equilibrium state of an organism, they are all mediated by homeostatic mechanisms in order to maintain biological stability.

Centrioles are cell structures that are mainly composed of tubulin and are involved in the organization of the mitotic spindle and cytokinesis during cell division.

Cellular respiration primarily occurs on the inner mitochondrial membrane with the aid of membrane proteins. Glycolysis and gluconeogenesis occur in the cytoplasm and are facilitated by proteins. Lipids are mostly stored in bones and adipose tissue. These processes do not require intervention from the cytoskeleton or microtubules that compose the centrioles.

Nuclear lamins are fibrous proteins (intermediate filaments) that provide the structural function and the transcriptional regulation in the nucleus of a cell. Note that lamins differ from laminins, which are a protein component of basement membranes.

Euchromatin is “light” chromatin that represents DNA that is currently active for transcription. The fact that it is “light” implies that it is relatively decondensed and accessible by polymerases. Euchromatin is generally present during interphase, when most transcription and replication takes place, and is converted to heterochromatin during mitosis.

Translation occurs outside of the nucleus and uses mRNA as a template, not DNA.