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The Cytoplasmic Membrane
Inside a prokaryotic cell is a cellular barrier that decides what enters and what leaves the cell at any given time. This barrier is often referred to as a cytoplasmic membrane.
It is important in allowing nutrients to come into the cell, as well as allowing cellular wastes to exit. It is composed of two layers called a phospholipid bilayer. Taking it a step further, the phospholipid bilayer is made up of two sets of molecules known as hydrophobic and hydrophillic lipid tails.
Now a cytoplasmic membrane is selectively permeable, which means that it can control what enters and exits the cell. You should note that the cytoplasmic membrane also forms a concentration gradient which basically means that the concentration of certain chemicals can be high or low inside or outside of the cell.
This fact will be important when we discuss active transport and passive transport. Let's first start with passive transport.
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One of the most simple types of prokaryotic cell transport is passive transport. Passive transport comes in three varieties: simple diffusion, facilitated diffusion, and osmosis. Unlike active transport, passive transport does not require energy to operate.
Simple diffusion is described by Weber as "the movement of chemicals down their concentration gradient, from an area of higher concentration to an area of lower concentration." Almost like a balancing act, simple diffusion allows the equal balance in areas inside and outside of the prokaryotic cell.
Facilitated diffusion is different from simple diffusion in that it allows molecules to diffuse through the cytoplasmic membrane through various proteins. Proteins along the cytoplasmic membrane can open or close to allow certain molecules to diffuse through the membrane. Like simple diffusion, facilitated diffusion does not need energy.
The next type of passive transport is osmosis which deals with the diffusion of water. Like simple diffusion, osmosis allows water to shift from an area of higher concentration to an area of lower concentration across the cytoplasmic membrane. Water molecules are sometimes helped by proteins called "aquaporins" which are similar to the proteins in facilitated diffusion.
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Types of Solutions for Osmosis
Osmosis depends on certain solutions a prokaryotic cell is located in. For example, it is important to know that in a solution, a "solute" is the various chemicals that are being dissolved in a medium called a "solvent."
When the water concentration outside of the cell is greater than the inside, and the solute concentration inside of the cell is greater than the outside; the water travels into of the cell to allow the shift of water from an area of higher concentration to an area of lower concentration. This is called a "hypotonic solution" and the cell often enlarges (because it has gained water).
When the water concentration inside the cell is greater than the outside, and the solute concentration outside of the cell is greater than the inside, the water travels out of the cell to compensate its concentration. This is called a "hypertonic solution" and often the cell shrinks (because it has lost water).
When both the water and solute concentrations are equal, nothing happens and this is called an "isotonic solution."
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Rather than passive transport, active transport needs energy in order for it to go against the concentration gradient. Why would a prokaryote cell need active transport? Gary E. Kaiser writes that "active transport enables [prokaryotes] to successfully compete with other organisms for limited nutrients." In other words, active transport is used for survival.
When prokaryotes use active transport, they allow "transport proteins" to move certain chemicals across the cytoplasmic membrane by using ATP (energy). This expenditure of energy allows prokaryotes to move chemicals even if the concentration gradient is unequal. This is important to allow prokaryotes to acquire food even if the concentration gradient is unequal.
So in review, we have two main processes that help power the cell transport of prokaryotes. Whether it is from passive to active transport, both functions are important for a prokaryotic cell to thrive in our harsh environment.
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Kaiser, Gary E. "The Prokaryotic Cell: Bacteria." http://student.ccbcmd.edu/courses/bio141/lecguide/unit1/prostruct/cm.html
Weber. "Functional Anatomy of Prokaryotic Cells." http://www2.raritanval.edu/departments/Science/full-time/Weber/Microbiology%20Majors/SoftChalkeCoursesubmission/chapter4sub/chapter4sub_print.html
LadyofHats. "Prokaryote Cell." http://commons.wikimedia.org/wiki/File:Average_prokaryote_cell-_en.svg
Sascheme. "Hypertonic." http://commons.wikimedia.org/wiki/File:Hypertonic.jpg