Monday, May 08, 2006

Cell Structure

BIO 101 - Bora Zivkovic - Lecture 1 - Part 2

The Cell

All living organisms are composed of one or more cells - the cell is the unit of organization of Life.

Most cells are very small. Exceptions? Ostrich egg is the largest cell. Nerve cell in a leg of a giraffe may be as long as 3m, but is very thin.

Basic Structure of the Cell

A cell is a small packet or bag of liquid. The liquid is cytoplasm (or cytosol), which is essentially salty water with various organic molecules suspended in it.

The cytoplasm is contained within a cell membrane. Cell membrane is a phospholypid bilayer - this means that it is composed of two layers of tighly packed molecules of fat. Within the membrane, proteins are embedded into the bilipid layer and are more or less free to move around within the membrane. These proteins are important for the communication between the inside and outside of the cell.

You can see a good image here.

On the outside of the membrane, some cells may have additional structures. For instance, many bacterial and plant cells have thick cell walls that confer more rigidity to the cell as well as better defense against mechanical, chemical or biological insults.

Some cells also have hair-like cilia on the surface (e.g., a protist called Silver Slipper), or long whip-like flagella at one end (e.g., sperm cells). Both of these structures allow the cell to move utilizing its own energy.

Inside every cell, there is hereditary material - DNA. Exceptions? Red blood cells which have a membrane and cytoplasm, but no hereditary material.

Differences between prokaryotes and eukaryotes:

Prokaryotes (bacteria) have a cell membrane and cytoplasm and no other organelles.
Eukaryotes (plants, animals, fungi, protista) have a number of different cell organelles.

The nuclear material in Prokaryotes is a single, circular strand of DNA.
The nuclear material in Eukaryotes is organized in multiple chromosomes contained with a nucleus.

Cell Organelles

Eukaryotic cells have organelles. Organelles are subcellular structures that provide internal compartmentalization and other functions.

Nuclues is a large membrane-bound organelle. Its function is to sequester the DNA from the rest of the cell. The nuclear membrane (or nuclear envelope), which is also a phospholipid bilayer, selectively allows molecules to pass between the nucleus and cytoplasm. Inside the nucleus, DNA is organized in chromosomes. A chromosome is a tighly coiled and wound strand of DNA packaged with various proteins (e.g,. histones).

Smooth endoplasmic reticulum is a system of membranes and is involved in carbohydrate and lipid synthesis.

Rough endoplasmic reticulum is a system of membranes that possesses ribosomes. Proteins are synthesized in the rough ER.

Golgi apparatus stores and packages various molecules. When a molecule is needed elsewhere in the cell, a portion of the Golgi membrane closes off and forms a vesicle that can be transported around the cell.

Some eukaryotic organelles contain a little bit of their own DNA: the mitochondria and the chloroplasts. These two organelles used to be intercellular parasites, i.e., different species of bacteria that, over time, became an integral part of a cell.

Chloroplasts are found in plant cells. Photosynthesis is the process that occurs in them.

Mitochondria are found in all Eukaryotic cells. Breakdown of glucose begins in the cytoplasm and ends in the mitochondria, where the final products of the breakdown are ATP, water, CO2 and heat. This process requires oxygen - that is why we breath: to provide the oxygen for the mitochodria and to get rid of carbon dioxide produced in the mitochondria.

ATP (adenosine triphosphate) is the energy currency of the living world. Every cellular process that requires energy gets it from ATP. Thus, mitochondria are sometimes refered to as "factories of the cell".

The final portion of the process of glucose digestion (the Krebs cycle) is, like any process, not 100% efficient. Errors happen and not every atom of every glucose molecule ends up where it should: in ATP, water or CO2. The result of this inefficiency is production of heat and production of highly reactive small molecules called free radicals (e.g., hydrogen peroxyde, H2O2). Free radicals tend to quickly react with whatever molecule they first encounter upon leaving the mitochondria. Such reactions damage those molecules, be they proteins, lipids, sugars or nucleic acids. The intercellular damage caused by free radicals is one aspect of the process of aging.

Some animals - birds and mammals - have harnessed the heat production by the mitochondria to keep a stable internal temperature. The efficiency of the mitochondrial "machine" is held low under the control of hormones like thyroid hormones. As a result, there is a greater production of free radicals, so warm-blooded animals evolved particularly good mechanisms for neutralizing free radicals and for repairing the damage. If a person keeps a constant low temperature or constant low-grade fever, the first thing the physician will check is the function of the thyroid gland.

The cytoskeleton, composed of filaments and microtubules, anchors the organelles and gives a cell its shape. Microtubules move organelles, including vesicles, within a cell. They also move the membrane-embedded proteins around where they are needed.

References:

Peter H. Raven, George B. Johnson, Jonathan B. Losos, and Susan R. Singer, Biology (7th edition), McGraw-Hill Co. NY, Chapter 5

Previously in this series:
Biology and the Scientific Method

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