Cells and Organelles

        There are two main types or categories of cells: prokaryotic cells and eukaryotic cells. Both of these types of cells have several things in common. All cells are surrounded by a plasma membrane, which is made of a double layer (a bilayer) of phospholipids. Within this membrane, is the cytoplasm which is composed of the fluid and organelles of the cell.Bacteria (Kingdom Monera) are prokaryotes. They do have DNA, but it is not organized into a true nucleus with a nuclear envelope around it. Also, they lack many other internal organelles such as mitochondria and chloroplasts. 

Organisms in the other four kingdoms are eukaryotes. Their DNA is organized into a true nucleus surrounded by a nuclear envelope which consists of two bilayer membranes. The nucleus of eukaryotic cells contains the genetic material which chemically directs all of the cell’s activities. Usually this is in the form of long strands of chromatin made of DNA and affiliated proteins. When a cell is getting ready to divide, the chromatin coils and condenses into individual, distinguishable chromosomes. Because the nuclear envelope consists of two bilayer membranes, there is a space between these two membranes called a lumen.


Branching off from and continuous with the outer membrane of the nuclear envelope is a double walled space which zigzags through the cytoplasm. This is the endoplasmic reticulum (ER for short) and its central space or lumen is a continuation of the lumen between the membranes of the nuclear envelope. There are two kinds of ER: smooth ER and rough ER. Typically ER closer to the nucleus is rough and that farther away is smooth. Smooth ER is a transition area where chemicals like proteins the cell has manufactured are stored in the lumen for transportation elsewhere in the cell. Pieces of the smooth ER called vesicles pinch off from the smooth ER and travel other places in the cell to transfer their contents. Rough ER gets its name because it has other organelles called ribosomes attached, which give it a rough appearance when viewed by an electron microscope. Rough ER and its associated ribosomes are involved in protein synthesis, with the new polypeptide being threaded into the lumen of the ER as it is formed.
Ribosomes are special organelles that are directly involved in protein synthesis. They are made of RNA (ribonucleic acid) and protein and are manufactured in the nucleus (from a DNA template), then go out into the cytoplasm to function. Ribosomes of prokaryotes and eukaryotes are chemically different enough that some of our antibiotics such as tetracycline, streptomycin, and the new Zithromax® (azithromycin), can interfere with bacterial ribosomes’ ability to do protein synthesis without also interfering with our ribosomes.
Vacuoles and vesicles are similar in that both are storage organelles. Generally, vacuoles are larger than vesicles. Plant cells generally have one large central vacuole that takes up most of the space within the cell and is used for storage of all sorts of molecules. Paramecium have a special type called a contractile vacuole that serves to excrete water from the cell, sort of like our kidneys excrete water from our bodies. Vesicles are small enough and mobile enough that they are often used to move chemicals to other locations in the cell where they might be needed.
One of the places to which vesicles travel is the Golgi apparatus or Golgi bodies. These look like stacks of water-balloon-pancakes. They are sort of like the shipping and receiving department of the cell. Materials are received as vesicles unite with the Golgi apparatus, and sent elsewhere as other vesicles pinch off. Materials are temporarily stored in the Golgi bodies, and some further chemical reactions do take place there.
Mitochondria are found in nearly all eukaryotic cells, usually several or many per cell. They burn sugar for fuel in the process of cellular respiration: they’re the “engine” of the cell. Mitochondria consist of a smooth outer membrane and a convoluted inner membrane separated by an intermembrane space. The convolutions of the inner membrane are called cristae and the space inside the inner membrane is the mitochondrial matrix. As sugar is burned for fuel, a mitochondrion shunts various chemicals back and forth across the inner membrane (matrix to/from intermembrane space).

Plant cells normally contain another type of organelle that is not found in animals: chloroplasts. Chloroplasts convert light energy (from the sun) to chemical energy via the process of photosynthesis. The main pigment (green color) located in chloroplasts and involved in photosynthesis is chlorophyll. Chloroplasts are surrounded by an outer membrane and inner membrane separated by an intermembrane space. The fluid within the center of the chloroplast is called stroma. Within this fluid is an interconnected system of stacks of disks, kind of like more water-balloon-pancakes. Each sack is called a thylakoid. and has chlorophyll and other useful pigments built into its membranes. A stack of thylakoids is called a granum.
It has been suggested that mitochondria and chloroplasts may have originally arisen from prokaryotic invaders. Evidence for this includes the fact that both of these organelles contain their own DNA (separate from that in the nucleus) and program some, but not all, of their own protein synthesis. They control their own replication within the cell, and often can move around within the cell and change shape. They are both surrounded by two bilayer membranes suggesting one membrane originated from the plasma membrane of the cell and one from the plasma membrane of the hypothetical invader. Interestingly, because of the way human eggs and sperm are formed and unite, while half of the DNA in the nucleus of the newly formed embryo comes from the mother and half comes from the father, since sperm do not pass any of their mitochondria to the offspring, the mitochondrial DNA comes only from the mother. This has enabled some rather interesting studies to be done tracing relationships among various ethnic groups of people around the world based on mitochondrial DNA.
The cytoskeleton is made of various types of special proteins. Microtubules are hollow tubes made of globular proteins. Most notably, they are found in cilia, flagella, and centrioles. The arrangement of microtubules in cilia and flagella consists of nine doublets around the edge and two single microtubules in the center, all running the length of the structure. This is referred to as the “nine-plus-two formula.”
In centrioles, microtubules are arranged in 9 sets of 3 each. Animal cells typically have a pair of centrioles located just outside the nucleus and oriented at right angles to each other. These function in cell division.
Microfilaments are also part of the cytoskeleton and are made of solid rods of globular proteins.