Cell Structure and Functions Quiz

Fundamental Unit of Life: The Cell

Cell Structure

A cell is the basic structural and functional unit of a living organism. According to cell theory postulates, a cell is the basic building block of life, which makes anything alive and is self-sufficient to carry out all the fundamental functions of an organism. Cells come in various shapes and sizes, from the smallest known cells such as mycoplasma bacteria, which are spheres as small as 0.2 μm in diameter, to the largest human cells, which are about 20 μm across.

The cell is surrounded by a plasma membrane, which forms a selective barrier that allows nutrients to enter and waste products to leave. Inside the cell, there are specialized compartments called organelles, each surrounded by a separate membrane. One major organelle, the nucleus, contains the genetic information necessary for cell growth and reproduction. Other organelles include mitochondria, responsible for energy transactions necessary for cell survival; lysosomes, which digest unwanted materials within the cell; and the endoplasmic reticulum and the Golgi apparatus, which play important roles in the internal organization of the cell by synthesizing selected molecules and then processing, sorting, and directing them to their proper locations. Plant cells also contain chloroplasts, which are responsible for photosynthesis, whereby the energy of sunlight is used to convert molecules of carbon dioxide and water into carbohydrates.

Cell Division

Cell division is the process by which a cell splits into two identical daughter cells. There are two main types of cell division: mitosis and meiosis. Mitosis results in two genetically identical daughter cells, while meiosis leads to four genetically unique daughter cells. Cell division is essential for growth, development, and the maintenance of life.

Cell Theory

Cell theory postulates that all living organisms are composed of cells, that cells are the basic units of structure and function, and that all cells come from preexisting cells. This theory has been the foundation of modern biology since the mid-19th century.

Cell Organelles and Their Functions

Cytoskeleton

The cytoskeleton is a network of protein fibers within the cytoplasm that provides both cell movement and stability. It is responsible for maintaining cell shape and facilitating movement of organelles and vesicles within the cell. The major components of the cytoskeleton are microtubules, intermediate filaments, and microfilaments.

Microtubules

Microtubules are small tubes made from the protein tubulin and found in cilia and flagella. They help provide pathways for secretory vesicles to move through the cell and are involved in cell division as they are a part of the mitotic spindle, which pulls homologous chromosomes apart.

Intermediate Filaments

Intermediate filaments are made of a variety of proteins, such as keratin and neurofilament, and are very stable. They help provide structure to the nuclear envelope and anchor organelles.

Microfilaments

Microfilaments are the thinnest part of the cytoskeleton and are made of actin, a highly-conserved protein. Actin is both flexible and strong, making it a useful protein in cell movement. In the heart, contraction is mediated through an actin-myosin system.

Plastids

Plastids are organelles found in plant cells that resemble mitochondria and are responsible for turning sunlight into useful energy for the cell through photosynthesis.

Eukaryotes vs Bacteria vs Archaea

Organelles are found only in eukaryotic cells, one of the three major cell divisions, while prokaryotes and archaea do not have organelles. Organelles are membrane-bound structures within a cell that have some sort of specialized function.

Membrane-Bound Organelles

Membrane-bound organelles, such as the nucleus, mitochondria, lysosomes, endoplasmic reticulum, and Golgi apparatus, are present in eukaryotic cells. These organelles are bounded by a double layer of phospholipids to insulate their little compartments within the larger cell.

Non-Membrane-Bound Organelles

Non-membrane-bound organelles, such as the cytoskeleton, ribosomes, and centrosome, are present in both prokaryotic and eukaryotic cells. They are distinct in their structures and functions and coordinate and function efficiently for the normal functioning of the cell.