Cell Anatomy


     The cell is the smallest self-sustaining unit of any living organism. The human body is made up of trillions of cells.
Composition of Cells Chemical_makeup_of_the_human_body
       The major elements that make up cells are C, H, O and N. About 60 percent of the cell is H2O, and the cells are surrounded by an aqueous (watery) environment similar to seawater. Most of cell structure is due to protein.
       Other elements present in smaller quantities but equally important for life include:
Ca (calcium) – found in bones and teeth, required for muscle contraction, nerve cell communication and blood clotting.
P (phosphorus) – found in bones and teeth, found in nucleic acids and part of ATP
K (potassium) – major cation inside the cell, necessary for nerve impulses and muscle contraction.
S (sulfur) – component of many protein molecules.
Na (sodium) – major cation outside the cell, necessary for nerve impulses and muscle contraction.
Cl (chlorine) – major anion in the cell.
Fe (iron) – component of hemoglobin (the molecule that carries oxygen throughout the body).
I (iodine) – part of thyroid hormone, the hormone that controls metabolism.


Anatomy of the Cell
     There is no such thing as the “typical” cell. Cells are very diverse in structure and function. We will use an imaginary cell to explore the structures that can be found in cells in general.
  Major regions of the cell
     The major regions of the cell are the nucleus, plasma membrane and the cytoplasm. Each of these will now be examined in greater detail.
     The nucleus is the command center of the cell. It contains the genetic material encoded in the form of DNA, that provides the basic instructions for all cellular activity. The nucleus has three major regions:
  Nuclear envelope
     The nuclear envelope is a double membrane with a fluid-filled space in between. The double membrane fuses in places to form openings called nuclear pores that permit the passage of large molecules.
     This is a dark-staining center within the nucleus where ribosomes are assembled.
     When the cell is not dividing the genetic material, the DNA and its associated protein, is loose and spread out. In this form the genetic material is called chromatin.
  Plasma Membrane (cell membrane)
     The plasma membrane forms the boundary between the cell and its environment. It is essentially only two molecules thick! Phospholipids, molecules that possess charged (the head) and uncharged (the tail) parts, arrange themselves in water so that the part that is attracted to water, the head, face outward, and the part that is not attracted to water, the tail, face each other. This arrangement of molecules forms a membrane (sheet) that blocks penetration by water and any substance dissolved in water. Cholesterol molecules also are a part of the membrane and help to stabilize it.
     Although lipids are essential to the structure of the membrane they only make up about 50% of the membrane by weight. The other essential components of membranes are protein molecules.
     Proteins are scattered within the bilayer of the plasma membrane. Proteins perform a variety of functions including:
  Structural such as anchoring the cell to extracellular structures or other cells.
  Receptor cells receive signals from other cells by means of proteins.
  Transport proteins may facilitate the passage of materials across the membrane by acting as channels or carriers.
  Enzymatic proteins acting to speed up reactions may be attached to the membrane.
     Glycolipids and Glycoproteins
       Although most of the molecules in the cell membrane are lipids or proteins, carbohydrates become attached the outward-facing lipids and the outward-facing parts of protein molecules to form glycolipids and glycoproteins, respectively. Glycolipids and glycoproteins play an important role in cellular interactions. For example, blood type is determined by glycoproteins in the cell membrane of red blood cells.
     The cytoplasm is all the material between the plasma membrane and the nucleus.

Cytoplasm includes:

  Cytosol – is the fluid in which other components of the cytoplasm are suspended.
  Inclusions – refer to the non-functioning aggregates of stored nutrients or cell products that are suspended in the cytosol.

Organelles – are functional structures within the cytoplasm. Organelles may be bounded by their own membranes, similar to the plasma membrane, which creates a compartment within the cell where a specialized function can be performed.

      The organelles include:

  Mitochondria (Sing. Mitochondrion)
        The mitochondria have two membranes, an outer smooth membrane and an inner membrane with a larger surface area that is thrown into shelf-like folds called cristae.  
          The mitochondria have enzymes that break down glucose and other high energy molecules and convert the energy that is released into ATP molecules. Ninety-five percent of the ATP produced by the cell is produced in mitochondria and mitochondria have been referred to as the powerhouse of the cell.  
          Ribosomes are tiny structures composed of protein and ribosomal RNA. Ribosomes are the protein-assembly sites of the cell.
  Endoplasmic Reticulum (ER)
       The endoplasmic reticulum has about half of the membrane present in the cell. This membrane encloses a continuous, fluid-filled compartment within tubules and shelf-like spaces formed by the membrane.
       The endoplasmic reticulum provide a site for protein and lipid synthesis and modification of these molecules, creates a transportation network for moving molecules around the cell, and serves as a compartment for storage of calcium ions.
       There are two kinds of endoplasmic reticulum:
  Rough ER
  Rough ER is “rough” because ribosomes attach to the outer surface of the membrane. Because ribosomes are protein-assembly sites, rough ER is associated with protein synthesis.
  Smooth ER
  Smooth ER is continuous with rough ER and lacks ribosomes. It is not involved with protein synthesis but functions in lipid metabolism (both synthesis and breakdown of cholesterol and fats) and detoxification of toxins. Steroid hormones (testosterone, estrogen, cortisol etc.) are synthesized in smooth ER.
  Golgi Apparatus
       The Golgi apparatus consists of a stack of flattened membranous sacs. The Golgi apparatus is the site where proteins delivered to it by transport vesicles from the ER are modified, sorted and packaged.
       The product produced by the Golgi apparatus leaves it in the form of a vesicle. These vesicles form secretory vesicles that dump their contents to the outside of the cell and fuse with the cell membrane to add to its surface area. Vesicles may form that also contains enzymes that are destined for use by lysosomes.
       The lysosome is a membrane enclosed compartment that contains digestive enzymes that are capable of breaking down unusable cell structures and engulfed foreign material. The enzymes lysosomes contain are produced in the endoplasmic reticulum and are packaged by the Golgi apparatus.
       Peroxisomes are membrane enclosed compartments that contain oxidative enzymes which use O2 to detoxify harmful or poisonous substances including alcohol and formaldehyde. Peroxisomes help the cell get rid of harmful free radicals which are a byproduct of cell metabolism. In the process they produce H2O2 which is broken down into H2O and O2 by catalase.
       The cytoskeleton consists of a network of proteins within the cell that serves to maintain its shape, move organelles within the cytoplasm and stabilize their positions, and enable cellular movements. In a sense it is the “bones and muscles” of the cell.
       The proteins that make up the cytoskeleton include:
  Intermediate filaments
     Intermediate filaments are intermediate in size and include a variety of proteins that are similar in structure. These filaments enable the cell to maintain its physical structure by resisting pulling and stretching forces.
     Microfilaments consists of actin molecules. Microfilaments provide structural support for the cell but are also involved in cell motility and changing the shape of the cell.
     Microtubules are tube-like and determine the overall shape of the cell and the distribution of organelles.
       Centrioles made up of microtubules that form two cylindrical structures that are oriented at right angles to one another. Centrioles are the organizing center for microtubules and form the spindle apparatus. Centrioles also migrate to the plasma membrane where they form cilia and flagella.
       Cilia and flagella are both involved in movement. Cilia move materials across the surface of the cell and flagella propel the cell itself (in humans, the sperm cell).