Organelles are only found in eukaryotic cells which set them apart from prokaryotic cells.

Bacterial cells are prokaryotic and lack all membrane-bound organelles. We can thus eliminate mitochondria and nucleus as answers. These structures are not found in bacterial cells or in mature erythrocytes.

All cells, regardless of origin, are surrounded by a cell membrane. This structure is present in both prokaryotes and erythrocytes.

DNA is used to code for the genome of the organism and is found in both prokaryotes and eukaryotes. Bacteria contain a cytsolic genome known as the nucleoid. Red blood cells, however, lack DNA. This is partly due to their lack of a nucleus, and is considered evolutionarily advantageous because no viruses can target red blood cells.

Anaerobic environments lack oxygen, while aerobic environments contain oxygen. Generally, aerobic respiration is more energetically efficient than anaerobic respiration mechanisms. Aerotolerant organisms, however, will continue to use anaerobic means of energy production, even in the presence of oxygen. These are a special class of organism that can survive in either an aerobic or anaerobic environment.

Obligate anaerobes require an anaerobic environment and find oxygen to be toxic. These organisms will not be able to produce ATP in the presence of oxygen. Obligate aerobes require the presence of oxygen in order to survive and cannot produce ATP without oxygen utilization. Facultative anaerobes can switch between metabolism mechanisms, using cellular respiration in the presence of oxygen and fermentation or anaerobic respiration in its absence. Microaerophiles are aerobic organisms, but require a particularly low level of oxygen in order to survive.

Prokaryotes do not have membrane-bound organelles, such as the nucleus or mitochondria. In fact, endosymbiotic theory suggests that mitochondria evolved from symbiotic prokaryotic relationships. Prokaryotes are often associated with all of the other characteristics listed.

Bright pink cells under Gram stain are characteristic of Gram negative status. This is associated with an outer lipid membrane and reduced peptidoglycan. The reduction in peptidoglycan is what leads to a failure to retain the Gram stain, which would have colored the cells purple.

Facultative anaerobes are adaptive prokaryotes that can survive in either aerobic or anaerobic environments. These organisms are capable of both cellular respiration, through use of the electron transport chain along the cell membrane, and fermentation. They will adopt the more efficient process (cellular respiration) when oxygen is present, but can easily switch between one metabolism and the other.

Obligate anaerobes cannot produce energy in the presence of oxygen, and obligate aerobes cannot produce energy in the absence of oxygen.

Bacilli is a genus of bacteria that are characterized by their rod-shaped cells. Bacilli can be either gram positive, such as Bacillus anthracis, or gram negative, such as Salmonella enterica. Bacilli bacteria can be either aerobic (require oxygen) or facultative anaerobes (can thrive in both oxygen rich and oxygen poor environment). Bacilli are not exclusively aerobic, nor are they exclusively gram positive.

The main difference between a gram positive and a gram negative bacterial species is the thickness of their cell walls: gram positive bacteria have thicker cell walls than gram negative bacteria. The interior of each bacterium, however, contains the same types of structures. Recall that bacteria, in general, have no membrane-bound organelles; therefore, both gram positive and gram negative bacteria will have zero membrane-bound organelles. The internal structures of each bacterial class are essentially indistinguishable.

No animal cells will have cell walls. Trichocytes are specialized to be hard and resilient, but do not have cell walls.

Fungi, plants, and bacteria do use cell walls. The distinction between gram positive and gram negative bacteria refers to the composition of the cell wall only; both positive and negative bacteria have a cell wall present.

The chief difference between prokaryotic and eukaryotic cells is the presence of membrane-bound organelles. Mitochondria and Golgi bodies are examples of such organelles, and are only found in eukaryotes. Nucleoids are only found in prokaryotes, while eukaryotes use membrane-bound nuclei.

Cell walls can be found in either prokaryotes or eukaryotes. Many prokaryotic bacteria use cell walls as protection from the external environment, while eukaryotic plants cells use cell walls to help maintain rigidity and prevent water loss.