Cells are the fundamental units of life. At its most basic, a cell is a mass of cytoplasm enclosed by a membrane. Cellular composition refers to the collection of molecules and structures that constitute a cell. These components work together to carry out the processes that define life, and understanding them provides insight into how organisms function, grow, and reproduce.
The Four Major Macromolecules of Life
At the chemical level, cells are primarily composed of water and organic molecules. Water is the most abundant molecule, making up 70% or more of a cell’s total mass. Its polar nature, with a slight positive charge on its hydrogen atoms and a slight negative charge on its oxygen atom, makes it an excellent solvent for the many chemical reactions that occur within the cell. Dissolved in this water are various inorganic ions, such as sodium, potassium, and calcium, which are important for numerous cellular processes.
The bulk of a cell’s dry weight, about 80-90%, is comprised of four major classes of organic macromolecules. Proteins are the most diverse of these, carrying out a vast array of tasks. Their functions include:
- Acting as structural components
- Transporting and storing small molecules
- Transmitting information between cells
- Defending against infection
Each protein’s specific function is determined by its unique three-dimensional shape.
Carbohydrates, composed of sugar monomers, are a primary source of chemical energy for cells. Glucose is broken down to release energy that powers cellular activities, and some carbohydrates are also used for structural purposes. Lipids, or fats, form the membranes that enclose cells and their internal compartments. They are also a more concentrated form of energy storage than carbohydrates.
Finally, nucleic acids, DNA and RNA, are the informational molecules of the cell. Deoxyribonucleic acid (DNA) contains the genetic blueprint for the entire organism, dictating which proteins a cell will make. Ribonucleic acid (RNA) has several roles, including carrying the genetic instructions from the DNA to the protein-making machinery of the cell.
Cellular Structures and Organelles
The macromolecules within a cell are organized into structures and compartments called organelles. Each organelle has a specific function, contributing to the overall operation of the cell. The entire cell is enclosed by the cell membrane, a double layer of lipids and proteins that controls what enters and leaves. This selective barrier maintains the cell’s internal environment.
The cytoplasm is the gel-like substance that fills the cell and surrounds the organelles. It is the site of many metabolic reactions and provides a medium for the movement of materials within the cell. Within the cytoplasm of eukaryotic cells is the nucleus, a large organelle that houses the cell’s DNA. The nucleus acts as the control center, directing the cell’s activities by regulating gene expression.
Energy for the cell’s many functions is generated primarily by the mitochondria. These organelles convert the chemical energy stored in food molecules into a usable form. Protein synthesis is carried out by ribosomes. These small structures can be found freely floating in the cytoplasm or attached to another organelle, the endoplasmic reticulum.
Variations in Cellular Makeup
While all cells share some basic components, their composition can vary significantly. The most fundamental difference is between prokaryotic and eukaryotic cells. Prokaryotic cells, such as bacteria, are simpler in structure and lack a nucleus and other membrane-bound organelles. Their genetic material is located in a region of the cytoplasm called the nucleoid.
Eukaryotic cells, which make up plants, animals, fungi, and protists, are generally larger and more complex. They have a true nucleus that contains their DNA, as well as a variety of other organelles that prokaryotes lack. Within the eukaryotes, there are also notable differences in composition, particularly between plant and animal cells.
Plant cells have several structures not found in animal cells. A rigid cell wall, located outside the cell membrane, provides structural support and protection. They also contain chloroplasts, the organelles where photosynthesis occurs, converting light energy into chemical energy. Additionally, many plant cells have a large central vacuole that stores water, nutrients, and waste products, and helps maintain turgor pressure against the cell wall.
Linking Composition to Cellular Function
The specific composition of a cell is directly related to its function. For example, muscle cells are packed with mitochondria to meet the high energy demands of contraction. The large number of these energy-producing organelles allows for the sustained conversion of food into usable energy, powering movement.
Another example is the red blood cell, which is specialized for oxygen transport. To maximize its capacity for carrying oxygen, a mature red blood cell ejects its nucleus and most other organelles. This adaptation provides more space for hemoglobin, the protein that binds to oxygen.
Nerve cells, or neurons, have a unique structure that is suited for transmitting signals over long distances. Their cell membrane contains specialized proteins that act as channels and pumps, allowing for the rapid movement of ions that creates an electrical impulse. The long, thin extension of a neuron, the axon, is a testament to how cellular composition can be adapted for a highly specific function.