Chapter Five: The Working Cell

Questions:

1. What is an exergonic reaction?  What is an endergonic reaction? What are their differences?
2. Why is selective permeability important to the plasma membrane?
3. What are the differences between an isotonic solution, a hypertonic solution, and a hypotonic solution?

Answers:

1. An exergonic reaction is a chemical reaction that releases energy.  This means that it begins with reactants whose covalent bonds possess more energy than those in the products.  In this kind of reaction, an amount of energy equal to the difference in potential energy between the reactants and the products is released to the surrounding atmosphere.  An example of this reaction is the burning of wood.  As it burns, the wood releases heat and light.  An endergonic reaction requires an input of energy in order to take place.  This means that their products are high in potential energy.  However, the reactants have little and must absorb energy from their surroundings.  This is potential energy that is stored in the covalent bonds of the product molecules.  
2. Selective permeability is important because it allows some substances to cross the plasma membrane more easily than others.  This helps to keep the cells functioning properly because it gives the cell control of what enters the cell.  
3. In an isotonic solution there is no net movement of water across the plasma membrane because water moves at the same rate in both directions.  In a hypertonic solution the cell will lose water to its surroundings.  This means that their are more solutes in the water around the cell so the water moves to the higher concentration of solutes.  And so, since the cell loses water to the environment, it will shrivel and may die.  In a hypotonic solution water enters the cell faster than it leaves because there are fewer solutes in the water around the cell.  So the water outside the cell moves into the cell where there is a higher concentration of solutes.  Because of this the cell will swell and may rupture.  

Important Facts: 

1. Membranes are made up of proteins and phospholipids and are often described as fluid mosaics.  The surface appears mosaic because of the proteins embedded in the phospholipids.  The proteins roam around on the phospholipids making the membrane fluid.  Most phospholipids are made from unsaturated fatty acids that have kinks in their tails which makes it so that they cannot be packed together tightly.  This tendency keeps them fluid.   
2. In active transport, substances are moved against their concentration gradient using energy in the form of ATP.  An example of active transport is the sodium-potassium pump,  a transmembrane protein that pumps sodium out of the cell while pumping potassium in.  In passive transport, substances are diffused across the cell membrane without requiring any energy whatsoever.  
3. Diffusion is the process in which particles move from an area of more concentrated particles to an area where they are less concentrated.  The particles diffuse down their concentration gradient until they reach equilibrium.  
4. Environmental conditions, including temperature and pH, influence enzyme activity.  For example, human enzymes function best at 37 degrees Celsius (body temperature).  If temperature is too high, enzymes will denature, rendering them useless because the function of an enzyme is directly connected to its structure.  If the structure changes, the enzyme can not perform its function.   
5. The path of proteins begins in the nucleus.  mRNA is transcribed from DNA and then travels out of the nucleus into the cytoplasm until they reach the ribosomes.  Some of these ribosomes are connected with the rough endoplasmic reticulum where the mRNA are translated into proteins.  The proteins then carry out their metabolic function in the cell.  Some act as enzymes, others as structural components etc. 

Diagram: This shows an enzyme catalyzing a cellular reaction.  You can see the substrate (the specific reactant that an enzyme acts on) bind to the enzyme's active site.  Then the products are released and the enzyme is free to bind once again.

Key Terms:
Tonicity- describes the ability of a solution to cause a cell to gain or lose water.
Isotonic- indicates that the concentration of a solute is the same on both sides.
Facilitated Diffusion- A type of passive transport that does not require energy.  Aquaporins are required.
Exocytosis- used to export bulky molecules (proteins, polysaccharides)
Endocytosis- used to import substances useful to the livelihood of the cell.
Phagocytosis- engulfment of a particle by wrapping cell membrane around it, forming a vacuole.
Chemical Energy- potential energy because of its energy available for release in a chemical reaction.
Thermodynamics- the study of energy.  First law- energy is constant.  Second law- energy conversions increase the disorder of the universe.  (Entropy- measure of disorder)
Exergonic Reaction- A chemical reaction that releases energy in covalent bonds of the reactions.
Endergonic Reaction- A chemical reaction that requires energy and yields products rich in potential energy.
Metabolism- the combination of an organism's exergonic and endergonic reactions.
Metabolic Pathway- A series of chemical reactions that either break down or build a complex molecule.
ATP- Adenosine triphosphate, renewable source of energy for the cell.

Summary: This chapter is mainly about the structure and function of the plasma membrane.  It describes many of the important processes such as diffusion, passive transport and active transport.  It also talks about hypotonic, hypertonic, and isotonic solutions as well as exocytosis and endocytosis.  The chapter then goes on to explain the role of energy in the cell as well as the main types of chemical reactions.  Before the chapter ends, it also describes how enzymes function, both as catalysts and as inhibitors.

Video on Diffusion and Osmosis:
http://www.youtube.com/watch?v=aubZU0iWtgI&feature=fvw