What can cells be compared to

ANIMAL CELL BIOLOGY

Cells are the structural and functional unit of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, or have many cells—an estimated 100,000,000,000,000 cells! Each cell can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.

Cell Organization

It is important to know what organism the cell comes from. There are two general categories of cells: prokaryotes and eukaryotes. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Eukaryotes are about 10 times the size of a prokaryote and can be as much as 1000 times greater in volume. The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bounded compartments in which specific metabolic activities take place, and have small specialized structures called organelles that are dedicated to performing certain specific functions. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the eukaryotic cell’s DNA.

Cell Structures: The Basics

The Plasma Membrane—A Cell's Protective Coat

The outer lining of a eukaryotic cell is called the plasma membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of proteins and lipids, fat-like molecules. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell. A form of plasma membrane is also found in prokaryotes, but in this organism it is usually referred to as the cell membrane.

The Cytoskeleton—A Cell's Scaffold

The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis (the uptake of external materials by a cell); and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell’s structure by directing, bundling, and aligning filaments.

The Cytoplasm—A Cell's Inner Space

Inside the cell there is a large fluid-filled space called the cytoplasm, sometimes called the cytosol. In prokaryotes, this space is relatively free of compartments. In eukaryotes, the cytosol is the "soup" within which all of the cell's organelles reside. It is also the home of the cytoskeleton. The cytosol contains dissolved nutrients, helps break down waste products, and moves material around the cell. The nucleus often flows with the cytoplasm changing its shape as it moves. The cytoplasm also contains many salts and is an excellent conductor of electricity, creating the perfect environment for the mechanics of the cell. The function of the cytoplasm, and the organelles which reside in it, are critical for a cell's survival.

Genetic Material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms are made of DNA, but a few viruses have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence.

Prokaryotic genetic material is organized in a simple circular structure that rests in the cytoplasm. Eukaryotic genetic material is more complex and is in units called genes. The nuclear genome is divided into 24 DNA molecules, each contained in a different chromosome.

Organelles

The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs", called organelles, which are adapted and/or specialized for carrying out one or more vital functions. Organelles are found only in eukaryotes and are always surrounded by a protective membrane. It is important to know some basic facts about the following organelles.

The Nucleus—A Cell's Center

The nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occurs. The nucleus is spheroid in shape and separated from the cytoplasm by a membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing.

The Ribosome—The Protein Production Machine

Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNA and proteins, and is responsible for processing the genetic instructions carried by mRNA. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell.

Ribosomes float freely in the cytoplasm or sometimes bind to another organelle called the endoplasmic reticulum.

Mitochondria--The Power Generator

Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called cristae. The number and shape of cristae in mitochondria differ depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane. Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process involves a number of complex pathways. They are the powerhouses of the cell.

The Endoplasmic Reticulum and the Golgi Apparatus—Macromolecule Managers

The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER and the smooth ER. The rough ER is labeled as such because it has ribosomes adhering to its outer surface, whereas the smooth ER does not. The smooth ER serves as the recipient for those proteins synthesized in the rough ER. Proteins to be exported are passed to the Golgi apparatus, sometimes called a Golgi body or Golgi complex, for further processing, packaging, and transport to a variety of other cellular locations.

Lysosomes and Peroxisomes—The Cellular Digestive System

Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.

Note: The animal eukaryotic cell concept map is based upon this article, and these two are meant either to be given out as homework or to be done in class as a team exercise to fill in the concept map blanks. I think it is best utilized as a normal homework assignment to clarify in the students’ minds exactly how the parts of the cell are connected.

Comparing the Cell to a Factory (modified from A Busy Factory, Imagine a busy factory making the latest must-have toy. Whether they make bicycles, cell phones, or hot air balloons, most factories are set up the same way. All factories have outside walls that protect and support them and inside walls that create different work areas. They usually have a production line where a product is put together and an executive department that decides what product is made. A finishing department processes and prepares the product for shipping, and a packaging department wraps the product. In addition, a factory has a receiving department that brings in the parts it needs to make its product, a communications department that allows it to contact suppliers, and a power plant that provides the energy it needs to run. Finally, a custodial staff keeps everything clean and in good working order. Cells are similar to factories. To stay alive and function properly, cells have a division of labor similar to that found in factories. All eukaryotic cells are composed of a plasma membrane, a nucleus, and cytoplasm. These structures can be compared with a factory's departments.

The Plasma Membrane

What if you needed to find a job in the factory? What could you do? If you do not have any manufacturing skills, and you are not management material, you would probably be placed in an entry-level position. Perhaps you'd be assigned to the warehouse. Here, you would be responsible for shipping and receiving. A factory requires a constant supply of raw materials, as well as a way to send out the finished product. This department is usually located along an outside wall of the factory. Working here, you would be one of the factory's contacts with the outside world. You might take a job as a receptionist and sit at a desk near the front door of the factory. A phone would allow you to contact anyone else in the building. Also, all incoming and outgoing calls would go through you. As a receptionist, you may speak for the factory and allow it to communicate with other people and businesses. Maybe you wouldn't want to lift heavy crates in the warehouse or answer phones. Another possibility might be to take a job with the security department. Security personnel are posted at every entrance to the building. You would be responsible for checking ID cards and admitting only those individuals who belong in the factory. Mail, reception, and security are separate departments in a real factory. But in a cell, these jobs are all performed by the plasma membrane.

The Nucleus

In a factory, the chief executive officer controls everything that happens. What would it be like to have this job in a cell factory? You would have your own office (which would be nice) but you would also have many responsibilities. You would need to keep track of all the blueprints kept in your office. And you would tell the workers which products to build and when to build them. The cell factory contains a large inventory of blueprints dating all the way to its founding. Some of these blueprints are out of date, and some are for parts and products that are no longer made. Part of your job would entail sorting through everything, finding the correct blueprints, copying them, and sending the copies out to the assembly line at the correct time. When the factory gets too large, it is difficult to run properly; thus, another factory must be built. To prepare for this, you have to provide the new factory with its own set of all the blueprints. Sounds a bit daunting? Now you may have a better appreciation for what the nucleus does in the cell.

The Cytoplasm

The real work of the cell occurs in the cytoplasm, the cell's "factory floor." The term "cytoplasm" refers to everything between the cell membrane and the nuclear membrane. It consists mostly of water, salts, some proteins, and many small structures called organelles (or little organs). These structures perform several different functions for the cell which generally fall under the categories of production, maintenance, and energy transformation. This tour of the cell includes several stops on the "factory floor." Let's start with the production team.

The Endoplasmic Reticulum

The Ribosomes

Ribosomes, the factory workers that build proteins, are manufactured by the nucleolus. They consist of two separate subunits: a large, lower subunit and a small, upper subunit. Ribosomes attach to the rough ER. Now let's take a look at how final processing occurs.

The Golgi Apparatus

What happens to all the products that are built on the assembly line of a factory? The final touches are put on them in the finishing and packing department. Workers in this part of the plant are responsible for making minor adjustments to the finished products. They inspect the products for flaws; clean them of any extra material added during their manufacture, wrap them, and target them for packing. The Golgi apparatus performs all the finishing and packing in the cell.

Lysosomes and The Cytoskeleton

Any factory needs a good maintenance crew to keep everything orderly, to get rid of the trash, and to take apart and throw away old machinery. The maintenance crew also functions as a second line of defense. If someone gets past the security guard at the front door, it is usually the maintenance crew who catches the trespassers and chases them out. In a cell, the role of building maintenance crew is filled by the lysosomes.

LYSOSOMES

The word "lysosome" is Latin for "kill body." This is a very colorful description for some of the most unusual organelles in the cell. LYSOSOMES are organelles produced by the Golgi apparatus that contain powerful protein digesting enzymes. Maintenance crews do not get the glory of chief executive officers, or even that of the production worker, but they serve a very important purpose. Without lysosomes, the cell would accumulate too much junk and would not be able to function for very long. Support Beams There is another major department in a cell factory, although it usually isn't given a department name in a regular factory. It's the walls, floors, and ceilings of a factory. Within the cytoplasm of the cell are many protein fibers that act as support structures. In a cell, the cytoskeleton supports the cell.

Mitochondria and Chloroplasts

Any factory needs power to operate. The power must be in a usable form. Most factories have power plants in which generators burn fuel to produce heat. This heat energy is used to make steam, which is then used to make electricity. Building proteins is the main function of cells. But for this to occur, a cell must have an energy source, and that energy must be in a form that the cell can use. The mitochondrion and the chloroplast are the two organelles responsible for energy transformation (neither organelle truly produces energy). Like our factory's power plant, mitochondria and chloroplasts transform one form of energy to another.