The Human Cell / The Human Cell

submitted by Dr. Gary Farr Last Updated May, 31, 2002

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Attached to the nuclear membrane is an elongated membranous sac called the endoplasmic reticulum.

This organelle tunnels through the cytoplasm, folding back and forth on itself to form a series of membranous stacks. Endoplasmic reticulum takes two forms: rough and smooth. Rough endoplasmic reticulum (RER) is so called because it appears bumpy under a microscope. The bumps are actually thousands of ribosomes attached to the membrane's surface. The ribosomes in eukaryotic cells have the same function as those in prokaryotic cells—protein synthesis—but they differ slightly in structure. Eukaryote ribosomes bound to the endoplasmic reticulum help assemble proteins that typically are exported from the cell. The ribosomes work with other molecules to link amino acids to partially completed proteins. These incomplete proteins then travel to the inner chamber of the endoplasmic reticulum, where chemical modifications, such as the addition of a sugar, are carried out. Chemical modifications of lipids are also carried out in the endoplasmic reticulum.
 

The endoplasmic reticulum and its bound ribosomes are particularly dense in cells that produce many proteins for export, such as the white blood cells of the immune system, which produce and secrete antibodies. Some ribosomes that manufacture proteins are not attached to the endoplasmic reticulum.

Ribosomes are the protein builders of the cell. When they build proteins, scientists say that they SYNTHESIZE the proteins. Ribosomes are found either floating around in the CYTOPLASM or attached to the Endoplasmic Reticulum (ER). The floating ribosomes synthesize proteins that will be used inside the cell. These so-called free ribosomes are dispersed in the cytoplasm and typically make proteins—many of them enzymes—that remain in  the cell.The ribosomes attached to the ER make proteins that will be used inside the cell AND sent outside the cell.

Making the Proteins

Ribosomes are a totally important factor in the creation of proteins. When the cell needs to create proteins, something called mRNA is created in the nucleus. mRNA stands for MESSENGER RNA. The mRNA is sent from the nucleus to the ribosome. Normally the ribosome is in two parts called the small and large SUBUNITS. When it is time to make a protein, the two pieces come together.

One Amino Acid at a Time

When the two subunits come together it's like two pieces of a bun and then you slide the hot dog inside. It starts with mRNA combining with the small subunit. Then, with tRNA (TRANSFER RNA), the large subunit connects to the small subunit. Once all of the pieces have combined, the two subunits create proteins one amino acid at a time.

The second form of endoplasmic reticulum, the smooth endoplasmic reticulum (SER), lacks ribosomes and has an even surface. Within the winding channels of the smooth endoplasmic reticulum are the enzymes needed for the construction of molecules such as carbohydrates and lipids. The smooth endoplasmic reticulum is prominent in liver cells, where it also serves to detoxify substances such as alcohol, drugs, and other poisons.

Pronounce it like this - Golgi is like GOAL and GEE. It's like the ROUGH Endoplasmic Reticulum, the Golgi Apparatus is made up of a stack of flattened out sacs. Imagine a loose stack of pancakes. Or, imagine a stack of French berets with gelatin inside. That is what the Golgi Complex looks like. Plant cells can have a bunch of these stacks while animal cells only have a few.

Proteins are transported from free and bound ribosomes to the Golgi apparatus. It is packed with enzymes that complete the processing of proteins. These enzymes add sulfur or phosphorous atoms to certain regions of the protein, for example, or chop off tiny pieces from the ends of the proteins. The completed protein then leaves the Golgi apparatus for its final destination inside or outside the cell. During its assembly on the ribosome, each protein has acquired a group of from 4 to 100 amino acids called a signal. The signal works as a molecular shipping label to direct the protein to its proper location.

WHAT'S IT DO?

The Golgi Complex takes simple molecules and combines them and pieces them together to make larger molecules. Then it takes those big molecules and puts them into packs called GOLGI VESCILES. Time to imagine again. Think about building a model of a ship (that's the molecule). Then take that model and put it in a bottle (that's the vesicle).

When a protein is made in the ER, something called a TRANSITION VESICLE is made. This vesicle or sac floats through the cytoplasm to the Golgi Apparatus and is absorbed. After the Golgi does its work on the molecules inside the sac, a Golgi vesicle is created and let loose into the cytoplasm. From there the vesicle moves to the cell membrane and the molecules are released out of the cell.

We've already learned about the Golgi Apparatus and the Endoplasmic Reticulum. These organelles also create something called ENZYMES. Enzymes are molecules that speed up chemical reactions. We talk about enzymes in lysosomes. Enzymes are the molecules used to break down large molecules. When the enzymes are packaged into vesicles, they are called lysosomes.

Breakin' It Down

Lysosomes combine with the food taken in by the cell. The enzymes in the lysosome bond to the food and start to digest it. Smaller molecules are released and they are absorbed by the mitochondria. Lysosomes also break down old organelles and cells. When an organelle no longer works, the lysosome attaches and breaks it down like food (kind of like a cannibal). Lysosomes can also destroy the cell if it breaks open accidentally. The enzymes inside the lysosome spread throughout the cell and digest it.

A Good Question...

Guess what? Scientists don't know everything. They can't figure out how the membrane of the lysosome can contain the enzymes it holds. The question is . . .

If the enzymes in a lysosome can break down anything in a cell, why don't they break down the lysosome too? Hmmm, something to think about.

The big thing you need to remember about MITOCHONDRIA is that they are the cell's little powerhouses. They are the thing that lets cells survive. Their whole purpose is to break down food molecules so that the cell has the energy to live. You eat and your intestines break down the food for you to use. A cell eats and the mitochondria break down the molecules for the cell to use.

Let's say there is a cow standing out in the middle of a field. He's eating the grass, chomping away. The food goes into one of his many stomachs (one of four). The grass is digested and broken down into small molecules. These molecules are absorbed by cells. The mitochondria in the cells break down the molecules and release the energy stored inside. It's just that simple. For now.

LOOKING INSIDE THE POWERHOUSE

Mitochondria are very tiny organelles. You should remember that scientists use the word organelles to describe the different parts of the cell. There can be several thousand mitochondria in one cell, depending on what the cell's job is. If a cell needs a lot of energy, it will have more mitochondria. The actual structure has two membranes (see that sketch). The OUTER MEMBRANE covers the mitochondria and the INNER MEMBRANE folds many times. There is a purpose in the folding. That folding of the membrane increases the SURFACE AREA. The surface area inside the mitochondria is like the table top where the reactions to break down food can take place. The more tabletop space you have, the more energy you can create. All around inside the mitochondria is a fluid called MATRIX.
 

WHAT'S GOING ON INSIDE?

That matrix is totally important in the function of mitochondria. The matrix is a fluid that has water and proteins all mixed together (like a solution). It is those proteins that take the food molecules and combine them with oxygen. The mitochondria are the only places in the cell where oxygen can be combined with food to release the energy inside.

So, the mitochondria are the powerhouses of the cell. Within these long, slender organelles, enzymes convert the sugar glucose and other nutrients into adenosine triphosphate (ATP). This molecule, in turn, serves as an energy battery for countless cellular processes, including the shuttling of substances across the plasma membrane, the building and transport of proteins and lipids, the recycling of molecules and organelles, and the dividing of cells. Muscle and liver cells are particularly active and require dozens and sometimes up to a hundred mitochondria per cell to meet their energy needs. Mitochondria are unusual in that they contain their own DNA in the form of a prokaryote-like circular chromosome; have their own ribosomes, which resemble prokaryotic ribosomes; and divide independently of the cell.
 

C. Eukaryotic Plant Cells

Unlike the tiny prokaryotic cell, the relatively large eukaryotic cell requires structural support. The cytoskeleton, a dynamic network of protein tubes, filaments, and fibers, crisscrosses the cytoplasm, anchoring the organelles in place and providing shape and structure to the cell. Many components of the cytoskeleton are assembled and disassembled by the cell as needed. During cell division, for example, a special structure called a spindle is built to move chromosomes around. After cell division, the spindle, no longer needed, is dismantled. Some components of the cytoskeleton serve as microscopic tracks along which proteins and other molecules travel like miniature trains. Recent research suggests that the cytoskeleton also may be a mechanical communication structure that converses with the nucleus to help organize events in the cell.

Plant cells have all the components of animal cells and boast several added features, including chloroplasts, a central vacuole, and a cell wall. Chloroplasts convert light energy—typically from the Sun—into the sugar glucose, a form of chemical energy, in a process known as photosynthesis. Chloroplasts, like mitochondria, possess a circular chromosome and prokaryote-like ribosomes, which manufacture the proteins that the chloroplasts typically need.

The central vacuole of a mature plant cell typically takes up most of the room in the cell. The vacuole, a membranous bag, crowds the cytoplasm and organelles to the edges of the cell. The central vacuole stores water, salts, sugars, proteins, and other nutrients. In addition, it stores the blue, red, and purple pigments that give certain flowers their colors. The central vacuole also contains plant wastes that taste bitter to certain insects, thus discouraging the insects from feasting on the plant.

In plant cells, a sturdy cell wall surrounds and protects the plasma membrane. Its pores enable materials to pass freely into and out of the cell. The strength of the wall also enables a cell to absorb water into the central vacuole and swell without bursting. The resulting pressure in the cells provides plants with rigidity and support for stems, leaves, and flowers. Without sufficient water pressure, the cells collapse and the plant wilts.