Cells+Interaction+with+Environment

= __Content Summary:__ =

**// Unit Overview: //**
Cells have many ways of getting the oxygen and nutrients that they need and maintaining homeostasis in the process. The plasma membrane is a very unique aspect of the cell because it is a semi-permeable, phospholipid bilayer. It allows certain molecules through, but not other ones. Because of the phospholipid bilayer, some molecules need different ways of getting into the cell (protein channels, etc.) because they are too big or are polar and can't get through the membrane by themselves. Learning how the cell interacts with its environment is of vital importance to physiology.

//__1. Cell Transport:__//
There are three different types of cell transport: bulk transport, passive transport, and active transport. There are two types of **bulk transport** which is when a large quantity of molecules is either taken into the cell or transported out of the cell. //Endocytosis// is when the membrane reaches out and brings material into the cell. One type of endocytosis is called __pinocytosis__ or "cell drinking". This is when the cell brings in extracellular fluid. The membrane is drawn inward with the extracellular fluid inside of the vesicle (small compartment) that it formed and the membrane that didn't form the vesicle fuses over the opening that pinocytosis left. Another type of endocytosis is called __phagocytosis__ or "cell eating". In this type of transport, the cell extends pseudopodia which surround the molecule(s) that it wants to "eat". These cellular extensions then fuse together and form a vacuole (large compartment) and the material is broken down. White blood cells perform phagocytosis on a daily basis to rid the body of infectious material. Another type of endocytosis is __receptor-mediated endocytosis__ which is a specific reaction. When certain molecules bind to receptor proteins on the cell membrane, it causes the membrane to close in on itself and pull the molecules into the cell. The molecules are then packaged in a vesicle and brought elsewhere in the cell.



The second type of bulk transport is exocytosis which is when a vesicle from inside the cell fuses with the plasma membrane and the cell releases the materials that were in the vesicle out into the extracellular fluid. This is how the cell gets rid of waste products secretes hormones, enzymes, mucus, etc. or adds more surface area to the cell membrane.



//__2. Passive Transport:__//
**Passive transport** is bringing substances into or out of the cell that does NOT require the use of energy. There are three different types of passive transport: diffusion, osmosis, and facilitated diffusion. __//Diffusion//__ is the movement of particles in a solution because of constant, random thermal movement. Molecules in a solution always diffuse from an area of higher concentration to an area of lower concentration. In other words, these molecules move "down" the concentration gradient. A concentration gradient is the difference in the concentration of two different locations. For example, if the concentration of glucose is higher outside of the cell, the glucose will want to move into the cell in order to balance out the concentration gradient. In order for the molecules to be able to get into the cell, though, the cell membrane must be permeable to that substance. Molecules can either diffuse across the plasma membrane, through channel proteins, or move through specific carrier proteins (facilitated diffusion - covered later). If there is a greater concentration gradient, greater membrane surface area, increased permeability of membrane, or increased temperature, the molecules will diffuse across the membrane more quickly.

__//Osmosis//__ is the diffusion of water across the plasma membrane (through aquaporins - channel proteins for water) that does NOT require energy. Osmosis occurs when the plasma membrane is not permeable to the solute dissolved in the water. This process is driven by the concentration of solutes on either side of a membrane, but it's the water that moves to balance out the concentration gradient - not the solutes. Water diffuses from the more dilute solution (less solutes, more water) to the more concentrated solution (more solutes, less water).

//Osmotic pressure// is the amount of pressure that would have to be applied to stop the influx of water across a plasma membrane. If there is a higher amount of molecules in a solution, the osmotic pressure increases. //Osmolarity// is the amount of solute particles per liter of solution. Osmosis will occur from an area of higher water concentration to one of lower concentration. //Tonicity// is the comparison of how a solution affects osmotic movement of water. //Isotonic// solutions are solutions in which the solute concentration on the inside and outside of the cell are equal and there is no osmotic movement. //Hypertonic// solutions have a higher solute concentration (and a lower water concentration) outside of the cell and thus, the water moves out of the cell and shrinks in size. //Hypotonic// solutions, on the other hand, have a lower solute concentration (and a higher water concentration) outside of the cell, so water moves into the cell which causes the cell to swell.

The third type of passive transport is __facilitated diffusion__ which is a type of carrier-mediated transport. In this type of passive transport, the molecules still diffuse along the concentration gradient, but carrier proteins are required. The higher concentration gradient on either side of the plasma membrane promotes the binding of the molecule to the carrier protein and the lower concentration gradient on the other side promotes the release of the specific molecule.



//__3. Active Transport:__//
Cell membranes also utilize active transport in order to get some of the needed nutrients into the cell. In active transport, carrier proteins are used to move molecules "uphill" or against the concentration gradient, but this type of transport DOES require energy (ATP) to be performed. The calcium (Ca2+) pump is a good example of this type of transportation. If the calcium concentration inside the cell gets too high, calcium and ATP bind with a carrier protein. The ATP molecule gives the carrier protein a phosphate group (phosphorylation) which promotes the protein to change shape and expel the calcium into the extracellular fluid (against its concentration gradient).

Another example of this type of transport is the sodium/potassium pump (Na+/K+ pump) which can pump sodium and potassium ions to opposite sides of the membrane at the same time. It carries three sodium ions and two potassium ions for each cycle of the pump. This pump is important for electrical impulses in nerves and for driving other cotransport systems. Cotransport is when the carrier protein is able to carry two molecules at the same time and in the same direction.



= __Application:__ = = = Cell transport is extremely important in the medical field. For example, patients with kidney disease or kidney failure may have to be put on **dialysis** in order to keep their bodies functioning correctly and to keep them alive (maybe even for a possible kidney transplant). When an individual's kidneys only have 10-15% of the function remaining or when there is an excessive accumulation of waste products in the blood, a patient is required to have their blood filtered three times a week for about four hours per visit to sustain their life.

The patient has a particular access point surgically inserted into his arm in order to be hooked up to the dialyzer. The dialyzer is a very interesting machine. It has two different parts to it: one for the patient's blood and one for the washing fluid that filters the waste out of the blood. Thin dialysis tubing separates the two parts and when the blood passes through the dialyzer, the waste products diffuse through the semi-permeable membrane and are washed away with the washing fluid. The components of the blood, though, are too large to diffuse through the tubing, so they get put back into the body and begin circulating like normal.

For patients with kidney failure, dialysis is literally a life saver. It's amazing at how far ahead technology has brought us compared to just a little while ago. Research is extremely important in the world that we live in right now. Without advancements in our knowledge and technology, our life expectancy wouldn't be nearly as good as it is right now. //(From: http://www.kidney.org/atoz/content/hemodialysis.cfm)//



= __Essential Questions:__ =

===<span style="font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 110%;">**//1. Compare and contrast passive and active transport. Include characteristics and examples of each type of transport cell used to maintain homeostasis. Why do cells need to bring molecules in and out of the cell membrane?//** ===

<span style="font-family: Georgia,serif; font-size: 110%;">Passive and active transport are both a means of getting substances into and out of the cell, but they also have many differences. Passive transport does NOT require energy while active transport does. Active transport uses different types of pumps (Na+/K+ pump, Ca2+ pump, Cotransport, etc.) to transport substances against their concentration gradient while passive transport uses diffusion across the cell membrane and channel proteins (aquaporins, gated channels, etc.). Passive transport moves substances from greater concentration to lower concentration through random thermal molecular movement while active transport moves substances against their gradient (low concentration to high concentration).

<span style="font-family: Georgia,serif; font-size: 110%;">Cells have to perform passive and active transport in order to get molecules into and out of the cell. The cell needs nutrients in order to function properly and it also needs to get rid of its waste products. If the cell doesn't get nutrients, it dies. If the cell doesn't get rid of waste, it dies. Transportation of molecules is very important for cell life and for the well-being of our bodies.

===<span style="font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 110%;">**//2. How do cells communicate? Be specific! What happens when communication doesn't work like it should? Research a disorder that is caused by faulty cell communication and post what you learned.//** ===

<span style="font-family: Georgia,serif; font-size: 110%;">Cells communicate by sending out and receiving several types of signals that tell other cells what to do or direct that particular cell about what to do. Cells have receptor proteins on their cell membrane, and if the signal from another cell fits and binds to those receptors, that cell will get the message and respond. Paracrine regulators are chemical signals given off by cells of the same organ as the target cells (cells receiving the signal). Neurotransmitters are chemicals released from one cell that diffuses across a gap (synapse) and is interpreted by another cell. And hormones are chemicals released into the bloodstream and "dropped off" at the target cells.

<span style="font-family: Georgia,serif; font-size: 110%;">If communication between cells doesn't work and signals get lost in the process, illness and disease may occur in the cells of the body. Parkinson's disease is a common disorder that is related to a decrease in cell communication. The cells that are supposed to secrete dopamine in the brain decrease their secretion which results in decreased communication between two important parts of the brain (the substantia nigra and the corpus striatum) that are associated with smooth voluntary movements. When communication between these two parts of the brain is decreased, movement becomes becomes more uncontrolled. The less dopamine secreted, the more uncontrollable the movements become.This disease is associated with trembling of the extremities, stiffness, slowness, and unsteady gait, but the severity depend upon the individual and how their body reacts to the disease.

<span style="font-family: Georgia,serif; font-size: 110%;">Other parts of the brain also atrophy, but lead to different symptoms not related to movement of the individual. Some of these secondary symptoms could be anxiety, confusion, memory loss, depression, dysphagia, decreased senses, slower, more quiet speech, etc. (From: http://www.medicinenet.com/parkinsons_disease/article.htm)

<span style="font-family: Georgia,serif; font-size: 110%;">Parkinson's has no cure and is not preventable. It's a very serious disease and all from miscommunication from the smallest living part of the body.