ASVAB General Science: Cells
Cells are the fundamental units of life, serving as the building blocks for all living organisms. While cells can vary greatly in their complexity and specialization, they all share certain basic structures that allow them to carry out the essential functions of life.
At the core of every cell is the cell membrane, also known as the plasma membrane. This selectively permeable barrier separates the cell's internal environment from the external world. Composed of a phospholipid bilayer with embedded proteins, the cell membrane plays a crucial role in controlling the movement of substances in and out of the cell, maintaining cell shape, facilitating cell-to-cell communication, and providing anchoring sites for the cytoskeleton.
Inside the cell membrane, we find the cytoplasm, a gel-like substance that fills the cell and surrounds the organelles. The cytoplasm consists of the cytosol (the liquid component), various organelles (specialized structures that perform specific functions), and the cytoskeleton (a network of protein filaments that provides structure and facilitates movement within the cell).
One of the most prominent structures within eukaryotic cells is the endoplasmic reticulum (ER), a network of membranous tubules and sacs that extends throughout the cytoplasm. The ER comes in two varieties: rough ER, which is studded with ribosomes and involved in protein synthesis and modification, and smooth ER, which lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage.
Working in tandem with the ER is the Golgi apparatus, also called the Golgi complex. This stack of flattened membrane sacs is responsible for modifying, sorting, and packaging proteins and lipids, as well as preparing molecules for secretion or transport to other cellular locations.
Cellular maintenance and waste management are handled by lysosomes, membrane-bound organelles containing digestive enzymes. These structures are responsible for breaking down cellular waste, debris, and foreign materials, as well as recycling old or damaged cell components.
Energy production in cells is primarily the domain of mitochondria, often referred to as the "powerhouses" of the cell. These organelles generate most of the cell's supply of adenosine triphosphate (ATP) through cellular respiration. Additionally, mitochondria play important roles in cell signaling, cell differentiation, and cell death.
The control center of the cell is the nucleus, which contains most of the cell's genetic material in the form of DNA. The nucleus is surrounded by a nuclear envelope, a double membrane that protects the genetic material. Within the nucleus, we find the nucleolus, a structure where ribosomal RNA is produced, and chromatin, the complex of DNA and proteins that makes up chromosomes.
Protein synthesis occurs on ribosomes, small granular structures composed of RNA and protein. These can be found free in the cytoplasm or attached to the rough ER. Ribosomes are responsible for translating genetic information into proteins, a process essential for cell function and survival.
In plant cells and some protists, we find chloroplasts, organelles responsible for conducting photosynthesis and converting light energy into chemical energy. Like mitochondria, chloroplasts contain their own DNA and can reproduce independently within the cell.
Another structure found in plant cells, as well as in fungi and some protists, is the cell wall. This rigid structure provides structural support and protection, maintains cell shape, and prevents over-expansion when water enters the cell. It's worth noting that animal cells do not have a cell wall.
The diversity of life on Earth can be broadly categorized into two types of cells: prokaryotic and eukaryotic. Prokaryotic cells, found in bacteria and archaea, have a simpler, smaller structure compared to eukaryotic cells. They lack a membrane-bound nucleus and organelles, and their DNA is circular and located in the nucleoid region. Eukaryotic cells, on the other hand, are found in plants, animals, fungi, and protists. They have a more complex, larger structure with a membrane-bound nucleus and organelles, and their DNA is linear and organized into chromosomes within the nucleus.
The differences between prokaryotic and eukaryotic cells are significant. Prokaryotes lack a true nucleus, while eukaryotes have a membrane-enclosed nucleus. Prokaryotes generally lack membrane-bound organelles, whereas eukaryotes have various specialized organelles like mitochondria, ER, and Golgi apparatus. In terms of size, prokaryotic cells are typically smaller (0.1-5 μm) than eukaryotic cells (10-100 μm). The arrangement of DNA also differs, with prokaryotes having a single circular chromosome, while eukaryotes have multiple linear chromosomes. Even the ribosomes in these cell types differ in size, with prokaryotes having smaller ribosomes (70S) compared to eukaryotes (80S).
The way these cells divide is also distinct. Prokaryotes undergo binary fission, while eukaryotes divide through mitosis or meiosis. Overall, eukaryotic cells demonstrate a higher degree of internal organization and compartmentalization compared to their prokaryotic counterparts.