<span>Carbon dioxide is a colorless and odorless gas that is vital to life on Earth. This naturally occurring chemical compound is made up of a carbon atom covalently double bonded to two oxygen atoms. Carbon dioxide exists in Earth's atmosphere as a trace gas at a concentration of about 0.04 percent by volume. Natural sources include volcanoes, hot springs and geysers, and it is freed from carbonate rocks by dissolution in water and acids. Because carbon dioxide is soluble in water, it occurs naturally in groundwater, rivers and lakes, in ice caps and glaciers and also in seawater. It is present in deposits of petroleum and natural gas.</span>
The exacted number sciences don't know.
Answer: Cell membrane is layer which is present in all living cells. It is present under the cell wall but in some cases such as plant cells.
Explanation: It is composed of lipid bilayer and proteins embedded in it. Phospholipids, glycolipids and sterols is the composition. Its function is to protect the cell from extracellular environment. It is semipermeable membrane that js allow only selected molecules to pass.
Physical structures..........
Macrophages as they are versatile cells which reside in the tissue
