Cellular Respiration

Heterotrophs need to obtain energy from consuming autotrophs.  Remember autotrophs turn light energy into chemical energy, which is stored as the sugar glucose.  The process of releasing energy by breaking down glucose and other food in the presence of oxygen is called cellular respiration.

Cellular respiration is not exactly, but can be roughly viewed as, the reverse process as photosynthesis.  Oxygen and glucose combine to breakdown and reassemble as carbon dioxide, water, and energy.

6O2 + C6H12O6 -> 6CO2 + 6H2O + ATP

oxygen + glucose -> carbon dioxide + water + energy

This process is simplified of course.  There’s a whole confusing mess of glycolysis, ATP, and NADH which the average high school bio teacher would want you to know, but honestly it’s overkill (…it’s not even in our state standards…).

Now there are cases when cells don’t get enough oxygen.  In this case, the cells produce nasty waste products that they remove from their body.  Some microorganisms, such as yeast, produce alcohol in the absence of oxygen.  This is called alcoholic fermentation.  Other organisms, such as yourself, produce lactic acid in the absence of oxygen.  Fittingly, this is called lactic acid fermentation.  Fermentation is vital in our food system.  Production of alcohol is quite the large business throughout the world; as is production of foods such as yogurt and pickles which utilize lactic acid.

One last point of overkill: the Krebs cycle and electron transport chain.  Unless you go to college for Biology, you have no need for this… and honestly, I went to college for Biology and I don’t have a use for it, short of torturing students with it if I was an evil person.

The Cell II

After covering the animal cell, I’d like to talk just briefly about a few other types of cells.  We’ll go over plant cells and prokaryote cells.

Plant cells

Plant cells are very similar to animal cells.  Major differences are that plants have a cell wall outside of their cell membrane.  The cell wall provides structure, support, and protection for the cell.  Plant cells also have chloroplasts which are an organelle used to capture sunlight energy and make glucose (food).  The chloroplasts are what give plants their green color; chloroplasts are filled with a green pigment called chlorophyll.  The last major difference is that plant cells tend to have one large, central vacuole (when present in animal cells, they are smaller) which serves to house waste and add structural support.

Prokayote cells

Prokaryotes, organisms without a nucleus (like bacteria), are even more different from plant and animal cells.  They also lack other membrane-bound organelles such as mitochondria and chloroplasts.  Prokaryotes tend to be unicellular, unlike plants and animals.  While they do have DNA, it is in the form of chromosomes you are used to seeing; it tends to be in simple circle or tangled-looking loop shape.

Spontaneous Generation: A Brief History Of Disproving It.

In discussing experiments, it must be mentioned that frequently experiments are improved upon after they’re finished.  Ideally, you would be able to foresee all problems with an experiment before you begin, but that’s not always the case.  Sometimes you think you’ve done something wonder and had the results published, only to find out later that someone else sees a flaw in your work.  Maybe someone else continues your work and publishes their findings.  Let’s look at a case that is rooted in Biology’s history: spontaneous generation.  For centuries, people have realized the correlation between sex and reproduction in humans.  But some living things were thought to come to life on their own–to spontaneously generate.  In other, living organisms came to life from nonliving matter.

In 1668, an Italian physician named Francesco Redi came up with a hypothesis to disprove the idea of spontaneous generation–specifically, the thought that maggots could come to life from meat.  Redi observed that after meat sat out, flies would be attracted to it, and a few days after that maggots would appear.  Redi thought that maggots were from fly eggs too small to be seen.  Redi set up an experiment–with the control and variable groups–to prove his hypothesis that flies produce maggots.  In the experiment, the control group was a piece of meat in an uncovered jar.  The variable group was a piece of meat in a jar covered with gauze; the gauze allowed air through, but not the flies.  After a few days, Redi observed that the control group had maggots on the meat and the variable group didn’t. He then concluded that maggots only form when flies come in contact with meat and that spontaneous generation is not at play.

In the 1700s, an English scientist proposed that spontaneous generation was possible and performed an entirely different experiment that he suggested proved it.  Later, another Italian scientist, improved on that experiment and concluded that Redi was indeed correct the first time.  So for almost 200 years after Redi, there was still much debate as to whether or not spontaneous generation could happen.

Until there was Pasteur.  Louis Pasteur, in 1864, settled the argument once and for all.  Taking the basic idea of the two scientists from the 1700s and answering critics that said air was necessary for life, Pasteur developed a special flask.  It had a curved neck that allowed air in, but would trap any microorganisms and not let them contaminate his findings.

Pasteur's Experiment
Click for full image

Pasteur showed that his flask was free from microorganisms, even though it was open to the air.  For a year, there was no microbial growth. Until Pasteur broke the neck of the flask.  And when microorganisms appeared, he proved to the world that life could only come from other life.  Because of his findings in this and many other experiments throughout his life, Louis Pasteur is considered one of the greatest Biologists in history.


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