The cell theory states that all living things are made of one or more cells, cells are the basic units of structure and function for living things, and cells can only arise from pre-existing cells. The cell, therefore, is integral to an understanding of life, and anatomically as well as physiologically important to comprehend.
From bacteria to human beings, cells all share some common features. The first is the cell membrane or plasma membrane. A flexible outer boundary of the cell, the plasma membrane acts to regulate the passage of materials into and out of the cell. Made of phospholipids, the membrane is a bilayer with hydrophobic (water-fearing) fatty acid tails on the inside and hydrophilic (water-loving) heads on the outside. The membrane has proteins embedded within it and on its surface that act as identification for the cell, and transport mechanisms for large molecules and charged molecules that cannot pass through the cell membrane on their own.
Another common cellular feature is cytoplasm. In the cell, the gooey substance filling the cell membrane is a mixture of water, salts, dissolved particles, and intracellular structures. These are collectively known as cytoplasm. The goo is specifically known as cytosol, or "cell solution". All cells have cytosol, and depending on the complexity of the cell some have very few structures suspended in it while other cells have many different types of structures. The cytoplasm of a cell functions to fill the space within the cell, suspend the organelles, and is also a place for chemical reactions to occur.
All cells also have small structures known as ribosomes. Ribosomes are the sites of protein synthesis, and are integral to successful cell functioning. Ribosomes are made of ribosomal RNA (rRNA) and have large and small subunits that clamp together and help to translate the genetic code of a protein that must be assembled.
Finally, all cells have genetic material, or DNA. In some cells, known as eukaryotic cells, the DNA is enclosed and protected in a central cell structure called the nucleus. In other cells, prokaryotic cells, the DNA in anchored to the cell membrane and floats in the cytoplasm. DNA contains all of the cell's instructions, and functions like blueprints, directing the building of proteins.
Simple cells, the aforementioned prokaryotes, are in plain English bacteria cells. These are the cells that are the smallest and most simple. Bacteria cells consist basically of the parts mentioned as being common to all cells. Larger, more complex cells are known as the eukaryotic cells, and include cells of plants and animals.
Eukaryotic cells are different from prokaryotic cells not only in their size and complexity, but in that eukaryotes have a nucleus and membrane-bound structures known as organelles. Prokaryotes do not.
Just as the body has organs, groups of tissues that work together to perform a specific function, cells have organelles, which means "little organs." Organelles in the eukaryotic cell can include mitochondria, chloroplasts, the Golgi Bodies, Lysosomes, Vacuoles, Endoplasmic Reticulum, and other structures specific to cell type.
Mitochondria are membrane-bound organelles in the shape of a kidney bean. They have a double outer membrane and a wavy inner membrane. The wavy inner membrane, site of much of Cellular Respiration's chemical reactions, is known as cristae. Approximately the size of bacteria, and perhaps descended from bacteria, they are the sites of cellular respiration, and so are responsible for breaking down food sugars to release energy. Mitochondria have their own DNA, and have been used to research human descent as the mitochondrial DNA (mDNA) is passed down from mothers to offspring.
The centrosome of the cell plays a role in cell reproduction. It contains the two organelles known as Centrioles, which during mitotic cell division, grow microtubules into a spindle apparatus responsible for sorting and arranging DNA. Centrosomes replicate so at some phases of the cellular life cycle there are two centrosomes.
The centrioles are the microtubule-manufacturing organelles that play a vital role in mitotic cell reproduction. Normally located in the centrosome, these structures migrate to the poles of the cell during mitosis, and make the spindle apparatus, which the parent cell needs to divide the copied DNA among the new daughter nuclei.
Chloroplasts are similar in both shape and function to mitochondria. Found in photosynthetic cells, these organelles are also membrane-bound with an outer protective membrane and inner membranes. In the chloroplasts, the inner membranous structures are more circular than wavy, and are called thylakoid membranes. These are where pigments are stored for capturing light energy and converting it to chemical energy in the process of photosynthesis. Thylakoid membranes are arranged in stacks called Grana, and the fluid that surrounds them is known as stroma.
The Golgi Bodies are membranous as well, and can pinch off membrane containers known as vesicles around cell molecules. Thus, the Golgi Bodies are able to package molecules for transport. They are similar to a post office in that they receive, package, and ship molecules.
Lysosomes and Vacuoles are both vesicles with specific functions. Lysosomes are membrane-bound enzymatic organelles. Within the membrane, the enzymes digest waste, worn-out cell parts, and other matter the cell must break down. Lysosomes are common in animal cells, but rare in plant cells.
Vacuoles, contrary to lysosomes, do not contain digestive enzymes. They are still membrane-bound sacs, but function as storage centers instead of locations of digestion and waste disposal. These organelles are for storage of molecules, including food and water for the cell. In a plant cell, the vacuole is very large and central to the cell, and stores the water that makes plant cells turgid.
The Endoplasmic Reticulum is a network of membranous tubes and channels throughout the cytoplasm of a cell. This network is for transportation of molecules. Some Endoplasmic pathways are studded with ribosomes, are these are primarily responsible for the movement of proteins. These pathways are known as Rough Endoplasmic Reticulum, or Rough ER, as the ribosomes make them appear bumpy. Other pathways are smooth, and the Smooth ER transports other materials including lipids (fats).
The cytoskeleton is a network of protein filaments known as Microfilaments and Microtubules. These structural proteins have the ability to break, bend, and reform, enabling the cell to change shape while still providing structural support.
Peroxisomes are structures similar to the lysosomes and vacuoles mentioned earlier. Small, membrane bound organelles filled with enzymes. In animal cells, the peroxisomes function to protect cells from hydrogen peroxide, which can be toxic to cells. In plant cells, the peroxisomes function to convert fatty acids to sugars, and help with photorespiration.
Cilia are short hair-like structures found on cell surfaces. Extensions of the cell membrane, cilia have various functions. Sometimes cilia can sweep back and forth, helping to move particles past the cell. These can help clean passages out, like cilia you have in the cells of your respiratory tract. Cilia can beat together to help move a cell through a liquid environment as well. Sometimes, cilia can help direct food into a cell.
The flagellum is a long whip-like tail found on some cells. This tail is primarily for propelling the cell. As it whips back and forth, it enables the cell to "swim." One example of this is the tail on sperm cells.
These structures comprise the basics of cellular anatomy and physiology. To say the least, cells are small but extremely complex! There are many other structures that can be found in cells, and each structure plays a vital role in the function of the living thing.