Metabolism & Energy

METABOLISM? The term metabolism refers to the sum of all the chemical reactions that occur within the cell. Many times, due to energy constraints, the reaction required takes place in a sequential series of chemical reactions called a metabolic pathway. (A  B  C  D  E) Metabolic reactions can be categorized as: –Catabolic – those which break larger substances down into smaller ones to release energy. –Anabolic – those which consume energy to build large molecules from smaller ones.

ENERGY Energy is simply the capacity to do work. We can categorize energy into two main types: –Kinetic Energy – The energy of motion. –Potential Energy – Stored energy. The cell often will transform potential energy into kinetic energy. (Remember the electrostatic gradient?...) Other types of energy fall into one of these two categories. For example… –Heat – Kinetic energy of moving particles. –Chemical Energy – Potential energy contained within the bonds of a molecule.

BOND ENERGY Whenever a bond is formed between two atoms – energy is released. And it is the same amount of energy required to break the bond. Bond energy is the energy required to break (or form) a chemical bond. Here is the weird part… –Because unbonded, free atoms can form bonds and give off energy – they are considered to have more chemical energy that the compounds they form… –Think about it – if energy is given off when the bonds of a compound form – where did it come from?…The free atoms that built the compound!!! BUT – most atoms will want to form compounds for the sake of stability so we often are comparing the bond energy values of compounds as they turn into other compounds.

THE WEIRDNESS CONTINUES! CC H H H H O O O O H E Energy released M when bonds form I C Energy released A when bonds form L ECH 4 + 2O 2 N E Net energy released R in the combustion G reaction YCO 2 + 2H 2 O

LAWS of THERMODYNAMICS We are energy beings – without it, we die real quick! Most energy transfer in the cell is done so as heat. Too much heat denatures proteins so the question is how do we run all of the reactions needed without too much heat build up or loss? Thermodynamics – the study of the transfer and transformation of thermal energy (heat). These laws discuss the energy transfer between a “system” and its “surroundings” – the cell is the system and extracellular fluid is the surroundings. It is important to note that biological systems are considered to be “open systems” because they can exchange matter and energy.

FIRST LAW OF THERMODYNAMICS “Energy cannot be created or destroyed, but it can be transformed from one type of energy into another and transferred from one object to another.” This simply means that when a chemical reaction occurs – the amount of energy that enters the reaction must exit the reaction. Jogging… –Chemical energy from food converts to kinetic energy of muscle movement. –Heat built up in body is lost to the environment when you cool off.

SECOND LAW OF THERMODYNAMICS “During any process, the universe tends toward disorder.” Entropy (S) – measure of disorder in a system. This means that energy transformations tend convert matter from order to disorder. WAIT A FREAKIN’ SECOND!!! We know that life is highly organized – this breaks the law…does anarchy reign supreme? –No – the second law only applies to closed systems – biological systems are open systems. –We use inputs of matter & energy to reduce entropy (randomness). –All energy we need to survive comes from the Sun!

TURNING UP THE HEAT Heat makes it easier to break chemical bonds – it increases the kinetic energy of the atoms so they are easier to pull apart. Heat is measured using temperature (T). Chemical bonds have energy and forming these bonds reduces entropy. The value of the energy in the bonds of a molecule is called enthalpy (H). So…When a chemical reaction occurs… –There is a change in entropy (ΔS) – order or disorder. –There is a change in enthalpy (ΔH) – bonds are broken and made into new bonds with different energies. –There could be a change in temperature – BUT – many living things maintain a stable internal body temperature. (37°C for ourselves)

FREE ENERGY Free energy (G) is the energy from a chemical reaction that is available to do work. We know that reactions can give off or absorb energy. Free energy of a chemical reaction is calculated using the formula: ΔG = ΔH - TΔS The free energy value (ΔG) tells us a lot about a reaction.

ENDERGONIC & EXERGONIC If ΔG has a positive value, it means that the bond energy of the products is higher than that of the reactants. –This means that energy was absorbed by the reaction and you have gone from disorder to order. These reactions are termed endergonic. If ΔG has a negative value, it means that the bond energy of the products is lower than that of the reactants. –This means that energy was given off by the reaction and you have gone from order to disorder. These reactions are termed exergonic.

THERMODYNAMICS & METABOLISM Reactions in our cells can have a very large gap between the energy of the reactants and products. This means a large absorption or release of energy within the cells. This energy is often transferred as heat so this could significantly alter the stable temperature of the cell and disrupt protein function leading to cell death. The cell has several clever ways of getting around this problem while still ensuring the reactions it needs are still being carried out.

KEEPING IT STABLE One of the best ways a cell can cope with energy transfer is to have a series of smaller reactions take place in stead of one big reaction. Cellular Respiration C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O If this were to occur in one step – like burning the glucose with a match – the sudden, massive energy release would literally combust the cell…So instead we do this… C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O The energy is now released in a series of reactions in smaller bursts that are more manageable for the cell. The overall energy yield is the same because we are using bond energies to calculate this and the reactants and products are still the same.

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