Chapter
6 - Energy Flow in the Life of a Cell
Where to start with this chapter. This is a hard chapter to teach due to the fact that all of you seem to come in at a slightly different level of comfort with the core material. So let us look at the nuts and bolts first.
All metabolism is about the cycling of resources (materials, energy, wastes, etc.). No life-form on this planet exist in isolation, they are all connected in some way to an network of other organisms that either supply energy and/or materials for it or derive energy and/or resources from it. In this chapter we are going to be looking at the various ways in which an organism can either produces energy or utilizes energy.
The first part of the chapter deals with the idea of a chemical reaction. A chemical reaction is when molecules interact and form new molecules. This reaction can either release energy (exothermic or exergonic) or require energy (endothermic or endergonic). The key point to this idea is that the release or absorption of energy is a net measurement. All chemical reactions require energy to start (activation energy) and whether they produce less or more energy than this activation energy is the determining factor as to whether they are exothermic or endothermic. Say you need to borrow money from a friend to buy lunch. If the friend gives you five dollars but lunch only cost three, you will absorb the additional two dollars (endothermic), but if lunch cost seven you will have to give an additional two to pay (exothermic). I know it isn't the best example, but I'll try to come up with a new one.
The second part of the chapter deals with the concept of catalysts and enzymes. Catalyst lower the activation energy of a reaction (i.e. make it easier to start). Most importantly, they do not change the starting or final energy of a reaction, just the activation energy. In other words, an exothermic reaction is always exothermic and an endothermic reaction is always endothermic. Enzymes are biological catalysts, typically proteins. While almost all enzymes are proteins, not all proteins are enzymes.
more to come...
I. What is Energy?
A. Energy – the capacity to do work.
1. Kinetic Energy – the energy of movement
2. Potential Energy – stored energy
3. Forms of energy can be interconverted in the right situations.
B. The Laws of Thermodynamics describe the basic properties of energy.
1. The First Law of Thermodynamic – assuming no added energy, the amount of energy in a system remains constant.
a. Also known as “The Law of Conservation of Energy”
b. All systems on Earth receive an influx of energy from the Sun.
2. The Second Law of Thermodynamics – When energy is converted from one form to another, the total amount of useful energy diminishes.
a. Energy that is random and of no use is called Entropy.
b. Concentrated energy tends to be highly ordered.
3. Life on Earth is possible due to the a loss of energy from the Sun, the total entropy of the solar system is increasing, even as highly ordered structures are made on Earth.
II. How does Energy flow in chemical reactions?
A. Chemical Reaction – a process that forms or breaks a chemical bond.
1. Chemical reactions convert one set of chemical substances (reactants), into another set (products).
A + B à C + D
Reactants Products
2. Exergonic – describes a reaction that releases energy
A + B à C + D + Energy
3. Endergonic – describes a reaction that requires an influx of energy
A + B + Energy à C + D
B. Exergonic Reactions
1. The Glucose oxidation reaction
Glucose + 6 O2 à 6 CO2 + 6 H2O + Energy
2. Two important concepts
a. Since this is an exergonic reaction the reaction will proceed to completion without a net input of energy.
b. Even exergonic reactions require an initial push to start; this initial input of energy is called the activation energy.
C. Endergonic reactions require an input of energy.
1. Remember that all reactions are theoretically reversible.
2. Glucose reaction in reverse is the equation for photosynthesis.
6 CO2 + 6 H2O + Energy à 6 O2 + Glucose
3. In an endergonic reaction the activation energy is greater than the energy released upon completion of the reaction.
D. Coupled Reactions – An exergonic reaction provides the energy for an endergonic reaction.
III. How is cellular energy carried between coupled reactions?
A. The energy released by the oxidation of glucose can not be used directly, it has to go through an intermediate.
B. The energy from a reaction is transported to an intermediate material, an energy-carrier molecule.
C. ATP is the principal energy carrier in cells.
1. ATP – Adenosine Triphosphate – is the most common energy carrier.
ATP à ADP + Energy
ADP + Energy à ATP
D. During the reactions that a biological organism has to accomplish, some energy is lost to heat.
E. ATP is well suited to be an energy carrier.
1. The final bond requires a large amount of energy to form, resulting in a large amount of potential energy.
2. The final bond is unstable, which makes it easy to recover energy from. (low activation energy to initiate the breakage)
F. Energy in the cell can also be transferred in the form of electrons.
1. Two most common types
a. Nicotinamide adenine dinucleotide (NAD)
b. Flavin adenine dinucleotide (FAD)
IV. How do cells control their metabolic reactions?
A. Metabolism – the sum of all chemical reactions in the cell.
B. Sometimes sequences of reactions are linked together for a final end-product, these are called metabolic pathways.
C. At body temperature even spontaneous reactions occur too slowly to sustain life.
D. Catalysts reduce the activation energy of a reaction.
1. Catalysts are compounds that reduce the activation energy of a reaction, speeding them up, without being used up or permanently altered.
a. Catalysts speed up reactions
b. Catalyst will only speed up a reaction that can occur spontaneously (i.e. are exergonic)
c. Catalyst are not used up in a reaction.
2. Enzymes are biological catalysts, usually made of proteins.
a. Enzymes are highly specific, catalyzing only a few types of chemical reactions
b. Enzymes are often regulated by the molecules they act upon.
E. The structure of enzymes allows them to catalyze specific reactions.
1. Enzymes have a complex three-dimensional shape.
2. Enzymes also have a “pocket” where the reactions happen, this is called the active site.
3. Reactants, called substrates, enter the active site for reactions.
4. How does an enzyme catalyze a reaction.
a. The shape and charge of the active site forces substrates to enter in a specific orientation.
b. When the substrates enter the active site both the active site and the substrate change shape.
c. The novel shapes of the substrate-enzyme complex force the reaction forward.
5. Typically reactions happen in multiple steps, each with a unique enzyme, thereby lowering high activation energy into more manageable steps.
F. Cells regulate the amount and activity of their enzymes
1. Cells regulate the production of enzymes according to their needs.
2. Cells produce enzymes in inactive form, only activating them when needed.
3. When adequate products for a specific enzymes reaction are already present in the cell, those products can inhibit the enzyme, a process called feedback inhibition.
4. A small molecule can serve as a regulator of an enzyme, even though it is not part of the enzymes reaction. This is called allosteric regulation.
5. Two similar compounds can compete for the same site in an enzyme, leading to competition inhibition (this is how a number of poisons act, ie methanol and ethanol).
G. The activity of enzymes is regulated by the environment
1. Enzymes depend on their three-dimensional structure which in turn is dependent on their environmental conditions.
2. pH – The pH of a solution can alter the structure of an enzyme and change its charge properties.
3. Salt – Too much or too little salt can change a enzymes function
4. Temeprature
5. Coenzymes – organic molecules that bind to some enzymes and alter the active site to its proper configuration.