Cell Definition
A cell is the basic unit of life that makes up all living organisms and tissues of the body. It is the smallest unit that can live on its own and has three main parts: the cell membrane, cytoplasm, and nucleus. Cells are responsible for all of life’s processes and are composed of fundamental molecules.
Function of Cells
Cells are the basic unit of life and are responsible for all of life’s processes. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body’s hereditary material and can make copies of themselves.
The cytoplasm is a jelly-like fluid that makes up most of the cell’s volume. It contains several biomolecules like proteins, nucleic acids, and lipids.
The cytoskeleton is a network of long fibers that make up the cell’s structural framework. It determines cell shape, participates in cell division, allows cells to move, and provides a track-like system that directs the movement of organelles within cells.
The nucleus serves as the cell’s command center, sending directions to the cell to grow, mature, divide, or die. It houses DNA (deoxyribonucleic acid), which is the cell’s hereditary material. The Golgi apparatus packages molecules processed by the endoplasmic reticulum to be transported out of the cell.
Lysosomes and peroxisomes are organelles that digest foreign bacteria that invade the cell, rid the cell of toxic substances and recycle worn-out cell components.
Cells have several functions including the movement of substances across their membrane, protein synthesis, and division to make new cells for growth repair or replacement in the body.
Proteins synthesized in cells function as structural materials, enzymes that regulate chemical reactions hormones, and other vital substances.
Cellular research has already led to cancer treatments and uncovers new ways to treat diseases by learning about how cells work -and what happens when they don’t work properly- teaching us about biological processes that keep us healthy.
How Cells Work
Cells are the smallest form of life and provide structure and function for all living things, from microorganisms to humans.
They house the biological machinery that makes the proteins, chemicals, and signals responsible for everything that happens inside our bodies. Cells take in nutrients from food, convert those nutrients into energy, and carry out specialized functions.
Cells have several critical components called organelles that perform different functions to keep the cell alive and healthy. The most important organelle is the nucleus which controls all actions that the cell undertakes because it contains DNA – the genetic blueprint for the cell that contains all the necessary information for cells to live, grow, and reproduce.
Other organelles include cytoplasm which is a jelly-like fluid made up of a liquid environment packed full of cellular machinery and structural elements.
- Cytoskeleton is a network of long fibers that make up the cell’s structural framework.
- Golgi apparatus packages molecules processed by the endoplasmic reticulum to be transported out of the cell.
- Lysosomes and peroxisomes are recycling centers of the cell.
- Mitochondria convert organic molecules into other forms that might help a cell meet its energy needs or build support structures.
- Ribosomes make proteins.
- The endoplasmic reticulum synthesizes lipids detoxifies harmful substances, and stores calcium ions.
Cells communicate with each other through signals from neighboring cells and their environment. Within this membrane, a cell’s interior environment is water-based called cytoplasm.
This liquid environment is packed full of cellular machinery and structural elements. In fact, concentrations of proteins inside a cell far outnumber those on the outside.
Carbohydrates are another important type of organic molecule found in cells. Simple carbohydrates are used for immediate energy demands while complex carbohydrates serve as intracellular energy stores.
Studying cells helps us understand how they work together with similar cells to form structures called tissues. Tissues make up different organs in our bodies.
Learning about how cells work -and what happens when they don’t work properly- teaches us about biological processes that keep us healthy. It also uncovers new ways to treat disease.
Cell Types
There are two main types of cells: prokaryotic cells and eukaryotic cells
Prokaryotes Cell
Prokaryotes are single-celled organisms that lack a nucleus and membrane-bound organelles. They belong to the domains of Bacteria and Archaea. Prokaryotic cells are much smaller than eukaryotic cells, ranging from 0.1 to 5.0 micrometers in diameter. All prokaryotic cells are encased by a cell wall, and many also have a capsule or slime layer made of polysaccharides.
Prokaryotic cells have various shapes, including sphere, rod, or spiral shapes. The majority of prokaryotic DNA is found in a central region of the cell called the nucleoid, which typically consists of a single large loop called a circular chromosome.
Prokaryotes generally have a single circular chromosome that occupies a region of the cytoplasm called a nucleoid. They also may contain small rings of double-stranded extra-chromosomal DNA called plasmids.
Prokaryotes are simpler than eukaryotes because they lack membrane-bound organelles such as mitochondria or most other membrane-bound organelles that characterize eukaryotic cells.
Instead, many reactions happen within the cytoplasm of the cell. Prokaryotic cells have ribosomes that produce proteins and vacuoles like eukaryotic cells but do not have distinct organelles bound by membranes like eukaryotic cells do.
Eukaryotes Cell
Eukaryotes are cells or organisms that possess a clearly defined nucleus. The eukaryotic cell has a nuclear membrane that surrounds the nucleus, in which the well-defined chromosomes containing the hereditary material are located.
Eukaryotic cells also contain organelles, including mitochondria (cellular energy exchangers), a Golgi apparatus (secretory device), an endoplasmic reticulum (a canal-like system of membranes within the cell), and lysosomes (digestive apparatus within many cell types). Eukaryotic cells are typically much larger than those of prokaryotes, having a volume of around 10,000 times greater than the prokaryotic cell.
Eukaryotic cells are found in both unicellular and multicellular organisms. Animals, plants, fungi, and protists are examples of eukaryotes. Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion.
In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four haploid daughter cells that act as sex cells or gametes.
Eukaryotic cells have membrane-bound organelles such as a nucleus, mitochondria, and an endoplasmic reticulum. These organelles allow for more complex cellular processes such as protein synthesis and energy production. Prokaryotic cells lack membrane-bound organelles and their DNA simply floats around the cytoplasm.
Examples of Cells
Archaebacteria
Archaebacteria are single-celled microorganisms that lack a cell nucleus and membrane-bound organelles, making them prokaryotic cells. They have an outer cell membrane that serves as a protective barrier between the cell and its environment.
Within the membrane is the cytoplasm, where the living functions of the archeon take place. Archaeal cells have unique properties separating them from the other two domains, Bacteria and Eukaryota.
The cell wall of archaea differs from bacterial cell walls in their chemical composition and lack of peptidoglycans. The cell wall of archaea is composed of S-layers and lacks peptidoglycan molecules with the exception of methanobacteria which have pseudo peptidoglycan in their cell wall.
Bacteria
Bacteria are classified as prokaryotes, which means they lack a membrane-bound nucleus and other internal structures. The term “prokaryote” was first introduced as a result of electron microscope studies showing a shared simple cell structure among bacteria.
Bacterial cells are extremely small and range in size from large cells such as Bacillus anthracis to small cocci like Staphylococcus aureus. They have characteristic shapes such as cocci, rods, long filamentous branched cells, and comma-shaped cells.
Bacterial cells have a surface-to-volume ratio that is very high, about 100,000. They have a cell envelope made up of two to three layers: the interior cytoplasmic membrane, the cell wall, and — in some species of bacteria — an outer capsule. The cytoplasmic membrane encloses the interior of the bacterium and regulates the flow of materials in and out of the cell.
The bacterial cell wall is composed of peptidoglycan, which gives the cell its shape and surrounds the cytoplasmic membrane, protecting it from the environment. It also helps to anchor appendages like pili and flagella.
Bacteria reproduce by binary fission where one bacterium divides into two identical daughter cells. Binary fission begins when the DNA of the bacterium divides into two (replicates). The bacterial cell then elongates and splits into two daughter cells each with identical DNA to the parent cell. Each daughter cell is a clone of the parent cell.
Plant Cells
Plant cells are eukaryotic cells that have a membrane-bound nucleus and organelles. They have large central vacuoles, cell walls containing cellulose, and plastids such as chloroplasts and chromoplasts. There are different types of plant cells, including parenchymal, collenchymal, and sclerenchymal cells.
Parenchyma cells are living cells that have functions ranging from storage and support to photosynthesis (mesophyll cells) and phloem loading (transfer cells). Collenchyma cells provide stretchable support without elastic snap-back.
Sclerenchyma is a tissue composed of two types of cells, sclereids, and fibers that have thickened, lignified secondary walls laid down inside the primary cell wall. The secondary walls make the cell rigid and give it strength.
Plant cell walls surround the cell membrane. They are composed of cellulose, hemicelluloses, and pectin. Plant cell walls have multiple functions upon which plant life depends.
They provide mechanical support for the plant’s structure, protect against pathogens, regulate water uptake by the roots, store carbohydrates for later use in growth or reproduction, and help maintain turgor pressure in the cell.
Plastids are membrane-bound organelles with their own DNA. Chloroplasts contain chlorophyll and carry out photosynthesis while chromoplasts make and store other pigments responsible for flower petals’ bright colors. Leucoplasts are found in non-photosynthetic tissues of plants used for storing protein, lipids, or starch.
Animal Cells
Animal cells are eukaryotic cells that lack a cell wall and have a true, membrane-bound nucleus along with other cellular organelles. Animal cells are enclosed by a plasma membrane and contain a membrane-bound nucleus and organelles.
Unlike plant and fungi cells, animal cells do not have a cell wall. The lack of a rigid cell wall allowed animals to develop greater diversity in cell types, tissues, and organs.
There are several types of animal cells such as skin cells, muscle cells, blood cells, fat cells, nerve cells, Schwann cells, glial cells, etc. Skin cells form the outer layer of the skin. Muscle cells form muscles that allow movement. Blood cells carry oxygen throughout the body. Fat Cells store energy in the form of fat droplets.
Nerve Cells transmit signals throughout the body. Schwann Cells produce myelin sheath around axons in the peripheral nervous system while Glial Cells provide support to neurons in the central nervous system.
Animal cell structure includes several organelles such as the nucleus which contains chromosomes and nucleolus surrounded by a nuclear envelope perforated with nuclear pores. Centrioles are present only in animal cells and help organize cell division.
The cytoskeleton is an internal framework of the animal cell consisting of actin filaments, intermediate filaments, and microtubules that control cell shape and maintain intracellular organization for movement.
