Energy and Metabolism

• Cells obtain energy in many forms, and have mechanisms that convert energy from one form to another.
• Radiant energy is the ultimate source of energy for life.
• Photosynthetic organisms capture about 0.02% of the sun’s energy that reaches Earth, and convert it to chemical energy in bonds of organic molecules.
• Flavin adenine dinucleotide (FAD) is a nucleotide that accepts hydrogen atoms and their electrons.
• The iron component of cytochromes accepts electrons from hydrogen atoms, then transfers the electrons to some other compound.
• The reduced form of Flavin adenine dinucleotide (FAD) is FADH2.
• Cytochromes are proteins that contain iron.
• For cellular respiration, photosynthesis, and many other chemical processes, redox reactions are crucial.
• The ATP concentration in a cell is about 10 times the concentration of ADP due to the high ratio of ATP to ADP.
• Energy is transferred through the transfer of electrons from one substance to another in redox reactions.
• In redox reactions, oxidation is when a substance loses electrons, and reduction is when a substance gains electrons.
• NAD+ and NADH are electron carriers.
• Energy is then transferred through a series of reactions that result in formation of ATP.
• Exergonic reactions release energy, which drives endergonic reactions.
• In cellular respiration, NADH transfers electrons to another molecule.
• In cells, redox reactions usually involve the transfer of a hydrogen atom.
• Electrons of NADPH are used to provide energy for photosynthesis.
• An electron, along with its energy, is transferred to an acceptor molecule such as nicotinamide adenine dinucleotide (NAD+), which is reduced to NADH.
• NADP+ is not involved in ATP synthesis.
• Redox reactions often occur in a series of electron transfers.
• An electron progressively loses free energy as it is transferred from one acceptor to another.
• Cells use energy that is temporarily stored in ATP.
• Hydrolysis of ATP yields ADP and inorganic phosphate.
• ATP donates energy to endergonic reactions in cells, such as the formation of sucrose.
• In a living cell, the exergonic reaction often involves the breakdown of ATP.
• A thermodynamically favorable exergonic reaction provides the energy required to drive a thermodynamically unfavorable endergonic reaction.
• Endergonic reactions are coupled to exergonic reactions.
• Hydrolysis of ATP can be coupled to endergonic reactions in cells, such as the formation of sucrose.
• The intermediate reaction in the formation of sucrose is a phosphorylation reaction: a phosphate group is transferred to glucose to form glucose-P.
• Cells Drive Endergonic Reactions by Coupling Them
• Endergonic reactions require an input of energy from the environment.
• Free energy decreases as entropy increases.
• A certain amount of activation energy is required to initiate every reaction, even a spontaneous one.
• Δ G is a negative number for exergonic reactions.
• Δ G has a positive value for endergonic reactions, meaning the free energy of the products is greater than the free energy of the reactants.
• A cell must expend energy to produce a concentration gradient.
• A concentration gradient is an orderly state with a region of higher concentration and another region of lower concentration.
• Free energy decreases during an exergonic reaction, which releases energy and is a "downhill" reaction, from higher to lower free energy.
• Diffusion is an exergonic process, as randomly moving particles diffuse down their own concentration gradient.
• Free energy increases during an endergonic reaction, which is a reaction in which there is a gain of free energy.