Energy and Cells. Appendix 1. The two primary energy transformations in plants are photosynthesis and respiration.

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1 Energy and Cells Appendix 1 Energy transformations play a key role in all physical and chemical processes that occur in plants. Energy by itself is insufficient to drive plant growth and development. Enzymes are required to ensure that rates of biochemical reactions are high enough to support life. The two primary energy transformations in plants are photosynthesis and respiration. 1

2 Energy and Work Energy is a measure of the capacity to do work. In biology, work describes displacement against any of the forces living things encounter: mechanical, electrical, osmotic, chemical. KEY POINT: Plant cells must have the capacity to do work. This requires input of energy. No biological system can do work without a supply of energy! The First Law of Thermodynamics states that energy cannot be created or destroyed, but only converted to other forms. Energy is always conserved. 2

3 ΔU = ΔQ + ΔW The net amount of energy put into a system (ΔU) depends on the amount of heat (ΔQ) absorbed by the system and the amount of work k( (ΔW) done on the system. Internal energy is the difference between energy gained from the surroundings and energy lost to the surroundings. Energy stored by a leaf = energy absorbed energy dissipated 3

4 No matter how hard you try, you can t get more energy out of a plant than is put in. There are no exceptions! The Second Law of Thermodynamics states that total entropy of a system always increases. Entropy is the amount of energy that is not available for doing work. KEY POINTS: A low-entropy system has a high capacity to do work. Low Entropy 4

5 A high-entropy system has a low capacity to do work. Work must be done to convert a high-entropy system to a low entropy system. That requires input of energy. High Entropy What does a plant cell require: high entropy (equilibrium) or low entropy (non-equilibrium)? Free Energy 5

6 The potential energy of a compound is contained in its chemical bonds. When these bonds break, the energy that is released can be used to do work, such as form other bonds. The amount of energy available to do work is the free energy (G) of the molecule. Chemical reactions change the free energy. We denote this change as ΔG. KEY POINT: Spontaneous reactions (exergonic reactions) release heat, so that ΔG is negative. They form products with less free energy than their reactants. Spontaneous reactions move toward equilibrium and increase the entropy of the system. - ΔG 6

7 An example of an exergonic reaction is oxidation of glucose to form CO 2 and H 2 O. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + energy This reaction has a ΔG of -2.8 MJ mol -1, which means that t oxidation of 1 mole of glucose releases 2.8 MJ of energy. KEY POINT: Processes moving away from equilibrium require an input of energy, and the reactions are endergonic. Endergonic reactions have a +ΔG, and create products with more free energy than the reactants (substrates). + ΔG 7

8 An example of an endergonic reaction is the photosynthetic reaction, 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2. This reaction has a +ΔG, which means energy must be supplied to create products with more energy than the reactants. What provides the energy for this reaction? Adenosine Triphosphate (ATP) as an Energy Source When cells need energy, they hydrolyze ATP. 8

9 KEY POINT: ATP + H 2 O ADP + Pi + energy ΔG = kj mol -1 Only about half the energy released by hydrolysis of ATP is used to drive cellular reactions. What happens to the rest of the energy? Accomplishing all the work in a plant requires large amounts of ATP. The plant doesn t run out of ATP because it is recycled quickly. Conversion of ADP back to ATP via the following reaction requires input of energy. ADP + Pi + energy ΔG = kj mol -1 ATP 9

10 Cells couple the breakdown of ATP to other reactions that occur at the same time. Oxidation and Reduction Most energy transformations involve oxidation and reduction. Oxidation is loss of electrons, either alone or with hydrogen, from a donor to an acceptor. 10

11 Reduction is addition of electrons, either alone or with hydrogen, to a molecule. Reduction reactions require a net input of energy. Oxidation and reduction occur in concert. If something is reduced, something else is oxidized. Oxidation of Iron 2Fe 2+ + ½ O 2 + 2H + 2Fe +3 + H 2 O 2Fe 2+ 2Fe e - oxidation i ½ O 2 + 2H + + 2e - H 2 O reduction 11

12 Electrochemical Potential Membrane permeable to cations but not anions Anions in 2 not shown Diffusion of cations due to concentration differences generates a negative membrane potential in compartment 2. +ΔG or ΔG? Transport of cations against the concentration gradient and the electrochemical potential gradient requires work. +ΔG or ΔG? 12

13 The electrical potential across the plasma membrane is negative in most cells. Cations tend to diffuse in, but must be pumped out. Protons as an Energy Source Membranes of mitochondria and chloroplasts use energy stored in proton gradients to make ATP from ADP and P. 13

14 Energy is harvested from protons as they diffuse down a concentration gradient through ATP synthase complexes in the membranes. Energy must be used to maintain the proton gradient. Where does this energy come from in chloroplasts? In mitochondria? At other membranes in the cell, proton pumps (H + -ATPase) hydrolyze ATP to power transport of protons out of the cytosol, and establish membrane potentials. 14

15 nnaky&feature=related Enzymes Enzymes are proteins that act as biological Catalysts, greatly increasing rates of biochemical reactions in cells. Typical rate enhancements of enzyme-catalyzed reactions are of the order of 10 8 to Enzymes lower the energy barrier between substrates and products. 15

16 K m is the substrate concentration that halfsaturates the enzyme 16