SAS' Cell Biology Page

Cell Structure

In this section, I will talk exclusively about eukaryotes. Eukaryotes, unlike the prokaryotes, contains nuclei, and several other membrane-bound organelles. Cells have three main parts, the cell membrane, the nucleus, and cytoplasm. A cell's cytoplasm lies between the cell membrane and the nuclear membrane.

Cytoplasm contains molecules, ions, water, and every cell organelle, except the nucleus. The liquid of the cytoplasm acts like a buffer, maintain a pH that promotes life, helps chemical reactions to work optimally.

Cell wall are peculiar to plants. Cell walls are made from a polysaccharide of glucose called cellulose. Look at bark, and you are looking mainly at cellulose. The fibers of grass are cellulose. Cellulose is a tough macromolecule that has been used in rope, and paper. The cell wall lies outside of the cell membrane of plants. Cell walls function to provide structure, support, and protection for plant cells.

The nucleus acts like a safe. Within the confines of the nucleus lies the cell's DNA, protected within a nuclear membrane, also called the nuclear envelope, which is as tough as the membrane surrounding the entire cell. The nuclear envelope contains thousands of nuclear pores, through which RNA leaves the nucleus, in its job to make proteins for the cell. Also found within the nucleus are chromatin, which is granular. Chromatin consists of DNA bound to protein. Usually it is spread throughout the nucleus, yet during mitosis, the chromatin condenses to form chromosomes. Nucleolus, also found in the nucleus, are small, dense regions. It is within these regions where ribosomes are made.

Ribosomes may have one of the most important jobs within cells. Ribosomes carries out the job of assembling proteins, based upon the blueprint found in the DNA molecule. Ribosomes uses both mRNA, and tRNA to make proteins. Ribosomes are found on sections of another organelle called the endoplasmic reticulum. When the ribosomes make proteins, the proteins directly enter the endoplasmic reticulum.

Ribosomes are made from 50 proteins, several segments of rRNA (ribosomal RNA), broken into two subunits. The two subunits come together, combining with a chain of RNA called mRNA when the cell is making a protein.

The endoplasmic reticulum is a long internal system of membranes, running throughout a cell. The section of the endoplasmic reticulum that has ribosomes is called the rough endoplasmic reticulum. It is called rough because of the rough, bumpy look that the ribosomes gives the endoplasmic reticulum. Proteins made in the rough endoplasmic reticulum are destine for use outside of the cell. So the endoplasmic reticulum serves as a passage leading to the outside of a cell, like a road leading from a factory within a city to another city where the product will be used. Where you do not have ribosomes, the endoplasmic reticulum is called the smooth endoplasmic reticulum. Within this section of the endoplasmic reticulum, specialized enzymes can be found, making things like lipids.

Proteins are made in ribosomes found on the rough endoplasmic reticulum. These proteins move into a stack of membranes called the Golgi apparatus. The golgi apparatus finishes the proteins, adding carbohydrates and lipids to the protein. From the Golgi apparatus, proteins are finally sent on their way through the endoplasmic reticulum, to the outside of the cell.

An advantage of eukaryotes over prokaryotes is the membrane bound organelles. This is particularly the case when the cell makes too many proteins, like digestive enzymes. Vacuoles are the storage organelles of the cell. Vacuoles hold water, salts, proteins, carbohydrates, and sugars. Specialized vacuoles may contain very strong digestive enzymes. In this case, we call the organelle lysosomes. Lysosomes contain enzymes the can rapidly breakdown proteins, nucleic acids, lipids, and carbohydrates, so the lysosome acts as the stomach of the cell. Those organelles that hold enzymes that can convert fats into carbohydrates, or can convert hydrogen peroxide into water are called peroxisomes.

Two important organelles involved converting energy into a usable form are chloroplast and mitochondrion. Chloroplast contains chemicals that converts sunlight into sugars. Mitochondrion contains enzymes that can convert sugars and other high-energy molecules to ATP, the cells fuel, or gasoline. Once the energy stored in sugar has been released to ATP, the cell has the extra energy that it needs to run complex processes.

When one look at one single-cell organism, the ameba, you see a blob that can alter its shape to suite its needs of movement, eating, and disposing of waste. But many cells have specific shapes. Cytoskelteton are networks of proteins the helps the cell maintain its shape without the need of extra energy to contract the cell membrane. The cytoskeleton is composed of two groups, microtubules and microfilaments. Microtubules are hollow, protein tubes that act as "tracks" along which organelles can move through a cell. Microfilaments are long, thin protein fibers that helps cells move, and/or provide some shape and structure to the cell. Microfilaments are much thinner than microtubules, 7nm and 25nm respectively.