The chemical digestion of food is facilitated by the secretion of the hormone gastrin, which stimulates the secretion of hydrochloric acid. Insulin is secreted by the pancreas in response to high levels of blood glucose. Glucagon is secreted in response to low levels of blood glucose. Aldosterone is secreted by the adrenal cortex and increases reabsorption of salt in the kidney, subsequently increasing water retention and blood volume. Thyroxine is secreted by the thyroid and increases the metabolic rate of most body cells.

Mitochondria are membrane-bound organelles found in both plant and animal cells, and provide energy for the cell by converting different forms of reactants into ATP. The cell membrane is present in both types of cells and separates the environment from the inside of the cell, and provides cell structure and protection. The rough endoplasmic reticulum is present in both types of cells and is responsible for the storage and packaging of proteins and vesicles. The nucleus is present in both types of cells and controls the general activities of the cell, including gene expression and cell division.

Of these answer choices, only chloroplasts are present in plants cells, but not animal cells. Plant cells are photosynthetic and create their own form of energy from sunlight. The photosynthesis occurs in the chloroplasts of plant cells, which contain the pigment chlorophyll.

Darwin's finches were distinct from one another due to the size and shape of their beaks. Darwin noted that these distinct beaks allowed the different birds to eat specific food groups. The varying food groups allowed the animals to coexist in the same habitat, without resorting to competing for natural food resources. Each species had evolved to occupy a unique ecological niche in order to survive with minimal competition for resources.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.

As amino acids are added to a polypeptide during translation, tRNA molecules will enter the A site of the ribosome. The tRNA is then transferred to the P site, where a peptide bond is formed between the amino acid residue and the amino acid chain. Finally, the tRNA moves to the E site to release its tRNA and exit the ribosome.

Eukaryotes and prokaryotes are distinguished based on a structure called the nucleus. Eukaryotes have a nucleus, which houses their genetic material. Prokaryotes do not have a nucleus; instead their DNA is localized in an area (i.e. it is not membrane bound). So if the researcher were to stain a sample containing both types of cells, the eukaryotic cells would have darker, more defined stains within their cells. This would indicate the presence of a more compacted and localized DNA than that found in prokaryotes.

The larynx, or voice box, contains the vocal cords, which are the source of sound production. The pharynx, or throat, has a dual role in respiration and digestion. The pharynx receives air from the nasal cavity and food and drink from the oral cavity. The trachea, or windpipe, is a rigid tube that transports air from the larynx to the bronchi. The bronchi in turn transport air into the lungs. The alveoli are thin-walled sacs where the exchange of gasses between the lungs and the blood occurs.

Histones are proteins found in the nucleus of eukaryotic cells. DNA wraps itself around histones to further condense. Also, depending on how tightly the DNA is wrapped around the histones, it may or may not be availible for activity (e.g. replication or transcription). Cells modify the interaction between DNA and histones around certain genes under certain conditions to make those genes available or unavailable as needed.

A sugar phosphate backbone is characteristic of a DNA sequence. While fully functional ribosomes are composed of a mix of functional RNA and protein the sugar-phosphate components within the RNA do not comprise the backbone of the entire ribosome. This question also requires the knowledge that an amino acid chain has an N and C terminus. Last, it is important to note that the Peptidyl site is where the bond between two amino acids is synthesized.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.