Cell studies, scientifically known as cytology, define the smallest functional unit of the body rather than its smallest organ. Robert Hooke first discovered cells by examining dead samples under a microscope, while Antonie van Leeuwenhoek identified the first living cells. The cell theory, proposed by Schleiden and Schwann, states that all living organisms are composed of cells, making them the fundamental building blocks of life. Different organisms and body parts host cells of various sizes, with mycoplasma being the smallest globally and ostrich eggs being the largest.
In humans, sperm cells are the smallest, while female eggs are the largest, showcasing the diversity within a single species. Neurons in the human brain are the longest cells, but they lack the ability to divide, making brain injuries particularly dangerous. Conversely, liver cells possess the highest capacity for rapid cell division, allowing the organ to regenerate after significant damage. This process of division is essential for replacing dead or damaged cells throughout the rest of the body.
The cell membrane, or plasma membrane, acts as a protective layer for all internal components and is found in both plant and animal cells. It is primarily composed of protein and houses various organelles like mitochondria and the nucleus. Plants have an additional outer layer called the cell wall, made of cellulose, which providing extra structural support and shape. This wall is exclusive to plant cells and is the reason why wood and paper are rich in cellulose.
Mitochondria are the powerhouses of the cell, where oxygen is used to create ATP through the Krebs cycle. A total of 38 ATP molecules combine to form one glucose molecule, providing the energy necessary for bodily functions. The Golgi body performs the vital task of transporting this energy from where it is generated to where it is most needed. This system ensures that parts of the body used most frequently, like a runner's legs or a teacher's tongue, receive consistent energy.
Vacuoles serve as storage units within cells, primarily holding water which make up about 65% of the human body. Plant cells feature much larger vacuoles than animal cells to store consistent water supplies. The endoplasmic reticulum acts as an internal skeleton, providing structural support to prevent cell collapse. It consists of smooth parts that produce fats and rough parts, populated by ribosomes, that facilitate protein production.
Ribosomes are known as the protein factories of the cell, synthesizing the essential proteins required for cellular health. There are two main types of ribosomes, 70S and 80S, differentiated by their sedimentation coefficients and presence in prokaryotic or eukaryotic cells. The nucleus serves as the brain or controller of the cell, governing all other organelles and ensuring they function correctly. It contains chromosomes housing DNA and RNA, which determine the cell's genetic characteristics and functions.
Lysosomes contain powerful digestive enzymes that break down waste within the cell, acting as its waste management system. If a cell malfunctions or dies, the lysosome can rupture to digest the entire cell, a process known as autophagy. The entire interior of the cell is filled with protoplasm, a colorless fluid that is crucial for maintaining life and preventing organelle collisions. Protoplasm is divided into cytoplasm, which fills the cell body, and nucleoplasm, which is contained within the nucleus.