There are two general types of cells:

• PROKARYOTIC - no membrane-bounded nucleus or other organelles
• EUKARYOTIC - has membrane-bounded nucleus and other organelles

Everything else (Plants, Animals, Fungi and "Protists") are EUKARYOTIC.

Let's take a look at some illustrations of the parts of a eukaryotic cell, and discuss their functions. (If you missed class, well...what can I say?)

Organisms may be:

• unicellular (composed of only one cell)
• colonial (composed of many cells, but without a division of labor among them)
• multicellular (composed of many cells, each with a specific job)
  Depending on the species, multicellular organisms may...
  • lack true, distinct tissues, or
  • have true tissues, each composed of specialized cells with special jobs.

• All the similar organisms which can breed in nature to produce viable (i.e., able to live), fertile offspring comprise a SPECIES.
• All the members of a single species living in a defined geographic area comprise that area's POPULATION (of that species)
• All the different populations of species living in a defined geographic area comprise that area's COMMUNITY
• The community plus the non-living components of the environment (earth, climate, water, etc.) form the ECOSYSTEM.
• All ecosystems on earth collectively comprise the BIOSPHERE.

WHAT MAKES SOMETHING "ALIVE?"

• ANATOMY - structural organization
• REPRODUCTION
• GROWTH AND DEVELOPMENT
• UTILIZATION OF ENERGY
• ABILITY TO RESPOND TO ENVIRONMENTAL STIMULI - adaptation
• HOMEOSTASIS - ability to maintain a constant internal environment
• EVOLUTION - genetic change in populations over time

The smallest item on the "hierarchy" list we've been studying that meets all these criteria is the CELL.

Major Components of the Prokaryotic Cell

• plasma membrane (the "skin" of the cell)
• cell wall (a protective covering of a special protein/carbohydrate compound called peptidoglycan that covers the plasma membrane)
• protoplasm (the gel matrix inside the cell)
• ribosomes (tiny structures involved in the manufacture of protein)
• chromosome (one large, circular chromosome consisting of DNA, and organized in a central region called the "nucleoid region")

SOME Major Components of the Eukaryotic Cell

• plasma membrane (like the prokaryote plasma membrane, this serves as a barrier between the inner and outer environment of the cell. Protein channels allow specific substances to enter and exit the cell.)

• cell wall - PLANTS ONLY have a cell wall outside the plasma membrane, but it is composed of cellulose ("fiber")--not peptidoglycan.

• cytoplasm - The gel matrix (full of tubules and fibers that give the cell structural integrity and internal transport) just inside the plasma membrane, but outside the nucleus.

• ribosomes - similar to those found in bacteria, but different in several important ways, these tiny structures are composed of RNA and protein, and are the machinery the cell uses to build proteins. They are found lining the rough endoplasmic reticulum, a "canal system" in the cytoplasm that facilitates transport of proteins to other areas in the cell.

• Golgi apparatus - a cluster of "sacs" in which proteins manufactured at the ribosomes are further processed and constructed into finished products. These are packaged in vesicles (small membrane sacs) "pinched off" the Golgi and sent to other areas in the cell, including the plasma membrane for "export."

• vacuole - membrane-bounded storage structures which may contain anything from air to liquids such as pigment solutions, lipids (for energy storage) or other substances.

• nucleus - Bounded by a double layer of plasma membrane, this houses the DNA in the form of chromosomes. Unlike bacterial chromosome, which is circular, eukaryotic chromosomes are linear, and the number of individual chromosomes depends on the species. The gel matrix inside the nucleus is called NUCLEOPLASM, and transport between the nucleoplasm and cytoplasm is accomplished via NUCLEAR PORES in the nuclear membrane. These are selectively permeable: only certain substances are allowed into and out of the nucleus.

• nucleolus - Not an organelle, but a dark-staining region inside the nucleus, it's essentially a cloud of ribosome parts in the process of being constructed. There may be several nucleoli, all arranged around a genes on the DNA that code for the special RNA molecules that form parts of the ribosomes. Once the ribosome parts are constructed, they are exported to the cytoplasm and incorporated into the rough endoplasmic reticulum.

• mitochondrion This is the site of energy metabolism in all cells. Here, sugar is broken down via CELLULAR RESPIRATION to yield energy, which can be used by the cell to do work. The mitochondria also contain a circular chromosome of DNA remarkably similar to that found in bacteria!

• chloroplast This is the site of energy metabolism in PLANT cells. Here, sugar is manufactured from water and carbon dioxide (in the presence of light) via PHOTOSYNTHESIS. This sugar is stored for later use in cellular respiration, to yield energy for cellular work. Like the mitochondria, the chloroplasts contain a circular chromosome of DNA remarkably similar to that found in bacteria!

All living things on earth have one thing in common: the utilization of DEOXYRIBONUCLEIC ACID (DNA) as the permanent genetic "blueprint" for how to "build and run" themselves. This genetic code is replicated and passed on from generation to generation, and not always without change.

Over the course of the next few weeks, we will be studying GENETICS, the study of genes and inheritance. Genetics is one area of BIOLOGY, and like other areas of biology, genetics is a NATURAL SCIENCE.

The Natural Sciences (Physics, Chemistry, Biology, Geology, etc.) are all governed by the necessity of their adherents to utilize

...to add to the knowledge of their field.

The Scientific Method is a precise set of rules followed by researchers/investigators in the natural sciences. One of its earliest formulators was Sir Karl Popper (an Austrian philosopher), who made powerful and convincing arguments that in order to be valid, a scientific hypothesis must be testable and FALSIFIABLE.

The Scientific Method consists of the following steps...

• OBSERVATION - The investigator notes a phenomenon that poses a problem/question.
• HYPOTHESIS FORMULATION - The investigator poses the question in such a way that it can be tested by rigorously designed experiments or field observations.
  • Null hypothesis - Stated in terms of "no difference between observed results and expected results" of an experiment.
  • Alternative hypothesis - The opposite of the null, and actually the statement of interest. (We'll give an example in class!)
• PREDICTION - The investigator makes a statement about what s/he believes is true about the hypothesis.
• EXPERIMENTAL DESIGN - The investigator designs an experiment which will yield data to either support or refute the hypothesis.
• DATA COLLECTION - The experiments are run, and data are collected.
• DATA ANALYSIS - The data are subjected to rigorous analysis via quantification and statistical tests to determine whether any deviation from the expected result is truly meaningful, or merely due to chance.
• CONCLUSION - Investigator accepts or rejects the null hypothesis.

"The process known as the Scientific Method outlines a series of steps for answering questions, but few scientists adhere rigidly to this prescription. Science is a less structured process than most people realize. Like other intellectual activities, the best science is a process of minds that are creative, intuitive, imaginitive, and social. Perhaps science is distinguished by its conviction that natural phenomena ,m including the processes of life, have natural causes--and by its obsession with evidence. Scientists are generally skeptics." (from Biology by Neil A. Campbell

So don't confuse The Scientific Method with Science, in general. And also note that if something is outside the realm of scientific testability, the wise scientists will not presume that it is not true, or that it does not exist. It is simply outside the realm of Science, and may not be answerable with the Scientific Method.

• Induction - reasoning from a specific case to the general.
• Deduction - reasoning from a general observation to a specific conclusion. For example:
  • If all organisms are composed of cells
  • and humans are organisms
  • ...then humans are composed of cells.

A common theme in scientific endeavors is the use of HYPOTHETICO-DEDUCTIVE reasoning: The formulation of hypotheses (a tentative answer to a question) and the execution of experiments from which one may deduce a general answer to the hypothesis.

Important aspects of hypotheses...

• A hypothesis is nothing more than a POSSIBLE EXPLANATION of a particular phenomenon.
• A hypothesis is based on past experience about the phenomenon. It's an "educated guess."
• Multiple hypotheses make good science. (If you have only one possible answer, you may bias your experiment and your analysis.)
• Hypotheses should be testable via experimental procedures or field studies based on the hypothetico-deductive approach.
• Hypotheses can be refuted (proven wrong, or falsified), but they CANNOT BE PROVEN CORRECT. (It is impossible to perform enough experiments to be certain that the answer will always be the same, and that the same explanation will hold true.)

Let's try it ourselves and see! Think of a question in genetics, and we'll run it through the process to see how it goes.