The nucleus of a eukaryotic cell contains the genetic material of the cell and is surrounded by cytoplasm which contains many organelles. Ribosomes produce proteins, endoplasmic reticulum helps synthesize proteins and fats, and Golgi apparatus prepares proteins for use.

If no oxygen is present, cellular respiration is anaerobic and will result in fermentation where either lactic acid or alcohol is used instead of oxygen.

Unlike curved mirrors that operate on the principle of reflection, lenses utilize refraction. A convex lens is thicker in the middle than on the edges and converges light while a concave lens is thicker on the edges than in the middle and diffuses light. A common use for curved lenses is in eye glasses where a convex lens is used to correct farsightedness and a concave lens is used to correct nearsightedness.

The troposphere is the closest layer to Earth, followed by the stratosphere, mesosphere, and finally the thermosphere which borders space.

The respiratory system manages respiration which is the process by which blood cells absorb oxygen and eliminate carbon dioxide.