A lysosome is a cell organelle that is part of the intracellular digestive system. Inside its limiting membrane there are hydrolytic enzymes capable of breaking down proteins and carbohydrates. Lysosomal enzymes contribute to the digestion of pathogens phagocytosed by a cell, and also to the tissue damage that accompanies inflammation. In short, they swallow up dead cells and debris, breaking down these compounds and recycling their components.

The mitochondria are frequently referred to as "powerhouses" because they are the site of the reactions of anabolic metabolism and most ATP synthesis. The nucleus is considered the "brain" of the cell because it controls the cell and contains the cell's genetic material. The ribosomes and endoplasmic reticulum are the sites of protein synthesis; smooth endoplasmic reticulum also synthesizes lipids and digests toxins.

The lysosome can be considered the "garbage man" of the cell. Old cellular parts that are no longer very useful to the cell are taken up by lysosomes and broken down into more fundamental molecules. Each of the functions listed above has been associated with the lysosome, except for the detoxification of ethanol, which is a specific function of the peroxisome.

The only statement that is true of those presented is that muscle contraction is controlled by the release of calcium ions. Calcium ions expose the myosin binding sites on actin filaments by causing the protein troponin to move out of the way. Static interaction between troponin, tropomyosin, and the actin binding sites prevents involuntary contraction of skeletal muscle, only allowing contraction during stimulation and subsequent calcium release.

Actin is known as the "thin" filament and myosin is known as the "thick" filament. The contraction process is highly dependent on ATP, and without it contraction will not occur. The basic unit of contraction is the sarcomere; the sarcoplasmic reticulum houses the calcium ions that will be released when stimulated by the appropriate neural pathways. 

Skeletal muscle has more t-tubules than cardiac muscle. Cardiac muscle does have larger T-tubules than skeletal, but not as many. All the other statements regarding cardiac muscle are true.

Voltage gated sodium channels allow the influx of sodium to quickly depolarize the neuron and generate the action potential. They are voltage gated, meaning they open in response to changes in membrane potential, thus providing the positive feedback required to get such a dramatic change in membrane potential once threshold has been reached. The other channels listed are not involved with the rising phase of the action potential. 

Of all the choices, axon and dendrite are the best answers to fill in the blanks because the axon is typically the pre-synaptic structure and the dendrite is the post-synaptic structure. In each of the other selections at least one of the choices does not make sense as a post/pre synaptic structure, or has nothing to do with synaptic transmission. 

Adherens junctions are formed by linking the actin cytoskeleton to transmembrane proteins known as cadherins. Cadherins are capable of interacting with other cadherins from neighboring cells on the exoplasmic face of the cell membrane. This interaction forms a physical link essentially connecting the actin cytoskeletons of the two adjacent cells, thus promoting force transduction.

Desmosomes are another type of cell junction that link the intermediate filaments of the cytoskeleton to cadherins.

Of the choices, only adherens junctions and hemidesmosomes are responsible for anchoring the extracellular matrix. This is accomplished by associating the actin cytoskeleton (for adherens junctions) or the intermediate filament cytoskeleton (for hemidesmosomes) with transmembrane proteins known as integrins. Integrins interact with the extracellular matrix.

Desmosomes are responsible for anchoring adjacent cells to one another by connecting their intermediate filament cytoskeletons with cadherins. Gap junctions connect the cytoplasm of adjacent cells and allow the free flow of small molecules between them. 

"Gap junctions prevent molecules and ions from traveling between cells in the extracellular space" is incorrect because this describes the function of tight junctions. Gap junctions electrically couple two cells by permitting the formation of small channels that connect two cells. 

Most of the other proteins listed are structural, but do not form any kind of pore or channel through which an electrical message can cross. Connexins are required for this function of gap junctions.