Cells make up all living organisms, from microscopic bacteria to plants to the largest animals on earth. As basic units of life, cells form the foundation of tissues, bark, leaves, algae and so much more. Organisms can be unicellular, meaning they're comprised of one cell, or multicellular, meaning that they're comprised of more than one cell. Bacteria are an example of a unicellular organism. Animals and plants are made of many cells.
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Cells make up all life on earth. Their functions vary depending on their location and their species type. The structures inside a cell determine its function.
Prokaryotes vs. Eukaryotes
Organisms are categorized as being prokaryotes or eukaryotes. Bacteria and archaea encompass prokaryotes. Prokaryotes display a relative simplicity. Their small cells are sheathed in a membrane or cell wall. Within the cell membrane, their genetic material, deoxyribonucleic acid (DNA), floats freely in a circular strand rather than in a defined nucleus.
Eukaryotes, such as plants, animals and fungi, in contrast, contain far more sophisticated cells with organelles. Organelles, small structures housed within eukaryotic cells, provide different capabilities. One such organelle, the nucleus, houses linear DNA. Organelles known as mitochondria provide power for the cells to use in their various functions.
Scientists think eukaryotes arose in the distant past, when mitochondria may have existed as small bacteria and were consumed by larger bacteria. The mitochondria formed a symbiotic relationship, beneficial to it and the overtaking host cell, leading to most of the higher life forms seen on earth today.
Cellular Structure and Function: Organelles
Cells provide both structure and function to entire organisms. But inside cells, structure and function also work together.
A protective plasma membrane provides a boundary around a cell. Made of fatty acids, this membrane forms a lipid bilayer, with hydrophilic heads on the outside and inside of the layers, and hydrophobic tails between the layers. Numerous channels dot the surface of this plasma membrane, allowing movement of materials into and out of the cell.
The cell’s cytoplasm is a gelatinous material throughout the cell, made mostly of water. This is where the cell’s organelles are located. The organelles drive the cell’s functions. While plants and animals share many of the same kinds of organelles, there are differences.
The cell’s nucleus, the largest organelle, contains DNA and a smaller organelle called the nucleolus. The DNA carries the genetic code of the organism. The nucleolus makes ribosomes. These ribosomes are made of two subunits, which work together with messenger ribonucleic acid (RNA) to assemble proteins for various functions.
Cells contain an organelle called endoplasmic reticulum (ER). The ER forms a network in the cell’s cytoplasm, and is called rough ER when ribosomes attach to it, and conversely smooth ER when no ribosomes are attached.
Another organelle, the Golgi complex, sorts proteins made by the endoplasmic reticulum. The Golgi complex creates lysosomes to break down large molecules and remove waste or recycle material.
Mitochondria are the power-producing organelles inside the eukaryotic cell. They convert food into molecules of adenosine triphosphate (ATP), the chief energy source of the body. Cells that require a great deal of energy, such as muscle cells, tend to have more mitochondria.
In plants, chloroplasts are organelles that convert sunlight’s energy into chemical energy. That in turn makes starches. Vacuoles, found in plant cells, store water, sugars, and other materials for the plant. Plant cells also have cell walls, which do not allow easy passage of material into the cell. Made mostly of cellulose, cell walls can be rigid or flexible. Plasmodesmata, small openings in the cell wall, allow material exchange in a plant cell.
Other organelles include vesicles, small transporter organelles that move materials within and outside the cell, and centrioles, which help animal cells divide.
The cell’s cytoskeleton, which is scaffolding found throughout the cell, is comprised of microtubules and filaments. These proteins aid in cell movement or motility. Cells move for immune system response, in cancer metastasis, or for morphogenesis. In morphogenesis, dividing cells move to form tissues and organs. Bacteria require movement to find food. Sperm cells rely on swimming to reach egg cells for fertilization. White blood cells and bacteria-eating macrophages relocate to damaged tissue to battle infection. Some cells actually crawl to their destination, which is the most common form of cell motility. Cells crawl by using cytoskeleton biopolymers (protein structures) called actin, microtubules, and intermediate filaments. These biopolymers work in tandem to adhere to a substrate, protrude the cell at the leading edge, and de-adhere the cell body at the rear of the cell.
The Importance of Cells
Cells group together with other cells of similar function to form tissue. Cells and tissue make up organs, such as livers in animals and leaves in plants.
A human body contains trillions of cells, which fall under roughly two hundred types. These include bone, blood, muscle and nerve cells called neurons, among many others. Each type of cell serves a different function. For example, red blood cells carry oxygen molecules. Nerve cells send signals to and from the central nervous system to direct movement and thought.
Cell division, or mitosis, occurs a few times an hour. This helps to build or repair tissue. Mitosis produces two new cells with the same genetic information as the parent cell. Bacteria can divide and form a large colony in a short period of time.
In reproduction, egg cells and sperm cells divide via meiosis. Meiosis produces four “daughter” cells differing genetically from the parent cell.
Cells provide the makeup for all living organisms. They form tissue, send messages, repair damage, fight disease and in some cases spread disease. Cells’ structure helps determine their function. Studying cells gives scientists vast knowledge in how organisms work and interact with the world around them.