Organelles:
What's inside the Cell

 

What is inside a cell?

 

The cells pictured below are very high qualty micrographs taken with a very powerful microscope.  This wasn't the case for early scientists - in fact, many of the organelles that are present aren't normally visible because everything is the same color in the cell.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Both of these cells are Prokaryotes. The cell on the left is a Heterotroph, and the cell to the right is an autotroph.  Different structures can be seen in each.

 

The first thing you notice when you look at these cells is a dark spot.  This is probably the largest structure inside the cell, and was noted by van Leeuwenhoek (of microscope fame).

 

The nucleus is evident in both animal and plant cells, although there are some Eukaryote cells that do not contain a nucleus, this is the exception.  Prokaryotes, being simpler forms, do not have a nucleus.

It was  during the mid-1800's that scientists noted that the nucleus of a sperm cell enters the the egg and fuses (combines) with its nucleus. When this data was confirmed with many types of animals, and even plants, it became accepted that the nucleus played a role in heredity (the passing of traits and characteristics from parent to offspring).  The function of the nucleus as carrier of genetic information became clear only later, after mitosis was discovered and the Mendelian rules were rediscovered at the beginning of the 20th century; the chromosome theory of heredity was then developed.

 

So what exactly is an organelle?

Scientists needed terminology to help describe what they were seeing.  Organelles were defined as intracellular (inside the cell) membrane-bound structures in eukaryotic cells (not prokaryotic), usually specialized for a particular function.  The nucleus, described above, is an example of an organelle. Organelles are analogous to the organ systems in a multicellular organism.  Just as an animal needs to eat, digest, assimilate nutrition and then excrete waste, and so must a single cell. The organelles ar elike organs in a multi-cellular organism.

 

The next organelles described, surprisingly, included the cell membrane - the seperator of the cell from its surroundings.  It may seem obvious that there must be a demarkation between cell and non-cell, this feature was only described fully in 1925.

 

We must remember that all of the organelles and parts of the cell are esentially the same color, so the quality of microscope, and technique used to stain different parts of a cell were paramount in identifying organelles.  We now have super-powerful electron scanning miscroscopes that have extraordinary ability to enlarge, butthis has only been the case for the past 50 years or so.  Prior to this time, light microscopes, with all of their limitations, were the only tool available to identify what lay inside the cell.

 

Because (or in spite of) of these limitations, mitochondria and golgi bodies were seen.  These are so small they are at the limit of a light microscopes ability to discern them.  Scientists began to take cells apart, and were able to identify organelles by function even before they were seen. 

 

Further identification was made by examining the chemical components located inside the cell. 

 

Determining the funcgtion of these structures was difficult.  Identification was facilitated when scientists became able to break the cell membrane and seperate out the specific organelles by filtration.  Experiments and data collection became possible as a quantity of organelles had been collected.

 

Is there a list of organelles we need to know about?

There are a lot of different structures inside a cell, and more are discovered every year.  We continue to discover new structures, and better understand the make-up of other structures because of technological advances.  In the past, we were limited to what we could see with our microscopes; now we can determine the chemical structure and shape of the individual molecules inside a cell and determine what each does.  This type of discovery will continue as our tchnology continues to advance.

 

The cell needs to perfrom certain functions, and there is an organelle for each of these functions.  Let's do an activity to understand the inner workings of a cell.

 

 

Homeostasis

Homeostasis is the maintenance of a constant environment of a cell or in the body. 

  • Homoios (Greek) – similar

  • Stasis (Greek) – standing still

 

Cells work best if they have a correct and constant environment, for example:

  • Temperature

  • Oxygen level

  • Glucose concentration

  • Acidity

  • Fluid volume

 

An organism that can’t maintain these environmental factors steady will die.  For example, organisms need to stay in a narrow temperature range to stay alive.  If the temperature goes much above 40  C, or much lower than 0 C, the organism dies.  Likewise, if a plant cell looses more than 60% of its water, the plant will wilt and die.  Animals kept in oxygen starved environments will die in a similar fashion.

Most organelles are enclosed in a membrane, much as the cell and nucleus are contained in a membrane.  Prokaryotic cells contain few types of organelles, and do not contain any organelles with a membrane.  The only membrane in a prokaryotic cell is the cell membrane.

 

Membranes can have openings in them called Pores that control what can pass through the membrane.  These pores act as a gatekeeper, and can open or close depending on the molecule approaching.

Organelle List:

 

Nucleolus:  The Nucleolus is found inside the Nucleus of the cell, and does not have its own membrane.  The nucleolus is a concentration of genetic material that specifically codes for Ribosome (a type of organelle) production and assembly.  The Nucleolus makes the RNA portion of a Ribosome, passes this through the nuclear membrane to combine with a protein portion of a Ribosome, and then returns to the Nucleolus for assembly into the working Ribosome.

 

Nucleus:  The Nucleus is enclosed with a membrane.  It is the portion of the cell that contains all of the genetic material (DNA) that contains the instructional code for all cell functions.  The Nucleus protects the DNA, and segregates it from the general cell contents.

 

Ribosomes:  Ribosomes are built and assembled in the Nucleolus, which is inside the Nucleus.  Ribosomes are the place that genetic instructional code is translated and used to build proteins.  Ribosomes are found floating freely in the cytoplasm, as well as attached to the outside of the Rough Endoplasmic Reticulum (Rough ER).

 

Vesicles/Vacuoles:  Vesicles are membrane bound containers that are used to package newly created substances for transport throughout the cell. 

 

Rough Endoplasmic Reticulum (Rough ER):  The Rough ER is a series of folded membranes attached to the Nucleus.  The Rough ER is covered with Ribosome son its outer surface.  The Rough ER provides a scaffold for proteins as they are assembled by the Ribosomes.  The Rough ER will package these assembled proteins into vesicles for transport throughout the cell.

 

Golgi Body/Golgi Assembly:  The Golgi Body is made of membranes that repackage materials in Vesicles.  The Golgi Body can combine vesicle contents to make specialized vesicles (like Lysosomes) so materials can be properly processed by the cell.

 

Cytoskeleton:  The Cytoskeleton is not enclosed in a membrane – it is the internal structure of the cell. The Cytoskeleton provides support and structure for the cell.  All of the Organelles are attached to parts of the Cytoskeleton and are not free-floating inside the cell.  The Cytoskeleton is made up of two different types of components – the main Compressional supports are Microtubules, while the thinner torsional supports are made of Microfibers.  Transport proteins can “walk” along these structures to move vesicles and organelles from one part of a cell to another.

 

Smooth Endoplasmic Reticulum (Smooth ER):  The Smooth ER differs from the Rough ER in that it is not covered with Ribosomes.  The Smooth ER is a series of folded membranes that provides a scaffold for lipid production, and detoxification inside the cell.   The Smooth ER makes membranes for use by the cell and its organelles.

 

Mitochondria:  The Mitochondria are made of folded membranes inside a membrane envelope.  The Mitochondria provide energy to the cell through cellular respiration.  The Mitochondria produce energy for the cell by breaking down Sugar (C6H12O6) in the presence of Oxygen (O2) releasing energy and producing Carbon Dioxide (CO2) and water (H2O).  This process is the exact opposite of what happens inside a Chloroplast.

 

Cytoplasm: While not strictly an Organelle, Cytosol is the semi-liquid portion contained by the cell membrane.  It contains all of the chemical components needed by the cell to function.

 

Central Vacuole:  A large, Central Vacuole is only present in plant cells (Autotrophic Eukaryotes), although small vacuoles (vesicles) can be present in animal cells (Heterotrophic Eukaryotes).  Vacuoles are membrane bound containers.  Plant cells contain a large central vacuole that stores water for the cell.  Animal cell vesicles can contain proteins packaged by the Rough ER and Golgi Bodies, and can be the result of Endocytosis and Exocytosis.

 

Lysosome:  The Lysosome is a specialized type of vesicle (Membrane bound container) that is used to digest and recycle portions of the cell that are no longer functioning or needed. Golgi Bodies creates Lysosomes by combining vesicles containing digestive enzymes vesicles containing materials to be recycled.

 

Centriole:  The centriole is found only in animal cells (Heterotrophic Eukaryotes).  The Centriole determines where each of the organelles and Nucleus are located inside a cell.  It plays an important role in cell division.

 

Other Parts of the cell:

 

DNA: The genetic code that is enclosed and protected by the Nucleus.  The DNA contains all of the instructions the cell need to function.  DNA contains instructions that code for proteins – the molecules that control all of the cell functions, and also control all of the organisms’ characteristics. DNA is a double-stranded molecue that is very long.

 

Chromosomes:  DNA molecules that are tightly coiled.  The DNA coils to prevent tangle and damage to this very important molecule.

 

Cholesterol: A type of lipid (fat).  Special lipids make up all of the membranes in the cell.  Sometimes we think that cholesterol is bad for our bodies, but it is actually an important component of our cells.

 

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