Energy Transfer

1st Law of Thermodynamics The energy of the universe is constant. Energy can be transformed & transferred, but it cannot be created or destroyed.

2nd Law of Thermodynamics Every energy transfer or transformation makes the universe more disordered. In other words,every energy transfer or transformation increases the entropy of the universe

Kinetic energy is the energy of motion. Moving objects perform work by imparting motion to other matter. Heat is kinetic energy that results from the random movement of molecules. If an object is not moving, it still has the capacity to do work. This is called potential energy. Chemical energy is a form of potential energy. It is stored in molecules as a result of the arrangement of atoms in those molecules. It is released as the molecules are broken down. In most energy transformations, ordered forms of energy are at least partly converted to heat. Free energy is the portion of a systems energy that can perform work when temperature is uniform throughout the system; it is represented by G.

Based on free energy changes, reactions can be classified as exergonic (energy outward) or endergonic (energy inward). An exergonic reaction proceeds with a net release of free energy. They occur spontaneously. An endergonic reaction is one that absorbs free energy from its surrounds; the energy is stored in the product; they are nonspontaneous.

Energy coupling is the use of an exergonic process to drive an endergonic process. The hydrolysis of ATP is exothermic; it is coupled with an endothermic reaction to provide:

  • Transport work (as in the Na/K pump)
  • Mechanical work (as in muscle contraction)
  • Chemical work (as in energy requirements to synthesize macromolecules such as proteins)

ATP is used like an “on-off” switch to control chemical reactions and to send messages. ATP + H2O  ADP + Pi

When ATP is hydrolyzed in a test tube, the release of free energy heats up the surrounding water. In the cell, this would be inefficient and dangerous; with the help of enzymes the cell is able to couple the energy of ATP hydrolysis directly to endergonic processes by transferring a phosphate group from ATP to some other molecule. The receipient of the phosphate group is said to be phosphorylated. Nearly all cellular work depends on ATPs energizing of other molecules by transferring phosphate groups. For instance, when ATP powers the movement of muscles, it transfers phosphate to contractile proteins.ATP must constantly be regenerated in a cell. A working muscle cell regenerates its entire pool of ATP about once every minute.

Enzymes

Even a spontaneous chemical reaction may occur so slowly that it is imperceptible. A catalyst is a chemical agent that changes the rate of a reaction without being consumed in the reaction. Enzymes are biological catalysts, most enzymes are proteins.

Every chemical reaction involves bond breaking and bonds forming. The initial investment of energy for starting a reaction (the energy required for breaking bonds in the reactant molecules) is called the activation energy. You can think of the activation energy as the amount of energy needed to push the reactants over an energy barrier or hill, so that the “downhill” part of the reaction can begin.

Characteristics of Enzymes

  • Most are proteins
  • Substrate specific
  • Have an optimum T and pH
  • Have an active site
  • Can exist in active & inactive form
  • Can be denatured
  • May require a cofactor
  • Subject to mutation

Cellular Respiration Cellular respiration and photosynthesis are both examples of oxidation reduction reactions. Oxidation is a loss of electrons; reduction is a gain of electrons. Draw arrows to show which is which-

C6H12O6 + 6O2  6CO2 + 6H2O + ATP

What is oxidized? What is reduced?

An overview of the relationship between cellular respiration and photosynthesis-

Cellular Respiration is a catabolic process. Molecules are broken down and energy is released. Cellular respiration is the most prevalent and efficient catabolic pathway. It requires oxygen and is sometimes called aerobic respiration. Fermentation is the partial breakdown of sugars that occurs without the use of oxygen.

Cellular respiration has 3 stages-

  1. Glycolysis (breaks down glucose into two molecules of pyruvate)
  2. Citric Acid Cycle aka Krebbs Cycle- (completes the breakdown of glucose)
  3. Oxidative Phosphorylation (accounts for most of the ATP synthesis)

NOTE- you will see these two terms used to describe ATP synthesis

Substrate level phosphorylation- glycolysis and the Krebs cycle decompose glucose and other organic fuels. The energy derived is used to phosphorylate a small amount of ADP.

Oxidative phosphorylation- occurs at the inner membrane of the mitochondria. Electrons that were picked up by NAD+ (and FADH2) are passed to oxygen. The energy derived is used to generate the majority of the ATP produced in cellular respiration.

GLYCOLYSIS

Means “splitting of sugar”

Glucose is split into two molecules of pyruvate (ionized form of pyruvic acid)

Occurs in the cytosol

Occurs with or without oxygen

Consists of 10 steps, each catalyzed by a specific enzyme; the first 5 steps require energy, the last 5 steps are the energy “pay-off”

Energy yield from glycolysis is 2 ATP and 2 NADH

Glycolysis releases less than 25% of the chemical energy stored in glucose; most of the energy remains in pyruvate

IF O2 IS PRESENT IN THE CELL, PYRUVATE ENTERS THE MITOCHONDRIA.

Pyruvate is converted into a compound called acetyl coenzyme A (acetyl CoA)

This step is the junction between glycolysis and the Krebs cycle

One molecule of NADH results from this step and a molecule of CO2 is given off

KREBS CYCLE (CITRIC ACID CYCLE)

Occurs in the matrix of the mitochondria

Has eight steps, each catalyzed by a specific enzyme

Most of the energy harvested in the Krebs cycle is conserved in NADH (3 molecules)

One step transfers energy to FAD, resulting in FADH2 (1 molecule)

Only one molecule of ATP is produced for each “turn” of the Krebs cycle

Two molecules of CO2 are produced with each “turn”

So far, only 4 molecules of ATP have been generated: 2 from glycolysis and 1 from each molecule of acetyl CoA that entered the Krebs cycle. The majority of the ATP that results from cellular respiration occurs during oxidative phosphorylation which takes place on the inner mitochondrial membrane . At this point, most of the energy “harvested” during substrate level phosphorylation is in NADH and FADH2.

The electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion. The folding of the inner membrane to form cristae increases the surface area, providing space for thousands of copies of the electron transport chain in each mitochondrion. Most components of the electron transport chain are proteins. Other molecules associated with the proteins are oxidized and reduced as they accept and donate electrons. The electrons that travel along the electron transport chain come from NADH and FADH2.

This figure shows the sequence of electron carriers in the electron transport chain. Notice the molecules labeled “cyt”. These are proteins called cytochromes. They have a heme group and are very similar to hemoglobin.

The electrons released by NADH and FADH2 are passed along, releasing energy as they go. Ultimately they are “accepted” by oxygen.

The electron transport chain produces no ATP directly. The energy derived from the “fall” of electrons is linked to a mechanism called chemiosmosis.

All along the inner membrane of the mitochondria are protein complexes called ATP synthase. This is the enzyme that actually catalyzed the formation of ATP from ADP and inorganic phosphate. Hydrogen ions “fuel” ATP synthase. For ATP synthase to function, there has to be a concentration gradient of H+. H+ will only flow down their concentration gradient if the concentration is greater in the intermembrane space than it is in the matrix.

How is the H+ concentration gradient maintained? It is maintained by the electron transport chain. The chain uses the energy released as electrons move along, to pump H+ into the intermembrane space. This “coupling” of H+ flow and ATP synthesis is called chemiosmosis.

The synthesis of ATP that results from the electron transport chain and chemiosmosis is referred to as oxidative phosphorylation. Oxidative phosphorylation yields a much greater amount of ATP- either 26 or 28 molecules of ATP for each molecule of glucose.

Oxygen serves as the final electron acceptor in oxidative phosphorylation (oxygen is extremely electronegative). Food can be oxidized without oxygen. NAD+ serves as the oxidizing agent in glycolysis; if oxygen is not available, pyruvate cannot enter the mitochondrion, and undergoes fermentation. This is a way for NADH to “unload” its H and be recycled.

**Note- the number of ATP molecules produced by oxidative phosphorylation may actually be less., possibly between 26-28 molecules. Regardless of the actual number, most of the ATP is produced in oxidative phosphorylation.

In plants, certain fungi and bacteria, ethanol and carbon dioxide are by-products of fermentation. Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce. The lactate that accumulates as a waste product may cause muscle fatigue and pain, but lactate is gradually carried away by the blood to the liver. Lactate is converted back to pyruvate by liver cells.

Glycolysis is common to fermentation and respiration. The end product of glycolysis, pyruvate, represents a fork in the catabolic pathways of glucose oxidation. In a cell capable of both respiration and fermentation, pyruvate is committed to one of those two pathways, usually depending on whether or not oxygen is present.

Glycolysis can accept a wide range of carbohydrates for catabolism. In the digestive tract, starch is hydrolyzed to glucose, which can be broken down in the cells by glycolysis and the Kreb’s cycle. Glycogen and sucrose are also broken down to glucose.

Proteins can be used for fuel, but they must be broken down into amino acids. Fats are broken down into fatty acids and glycerol.

Photosynthesis

6CO2 + 6H2O C6H12O6 + 6O2

Like cellular respiration, photosynthesis is an oxidation-reduction reaction. The energy flow is reversed in photosynthesis; water molecules are split and electrons are transferred to carbon dioxide, reducing it to sugar. (Carbon dioxide gains electrons (and H+); this is reduction.)

Describe how the structure of a leaf facilitates photosynthesis.

Explain the process of transpiration. Review the structure of the chloroplast and chlorophyll. Be able to relate each structure to the process that occurs there.

Photosynthesis consists of two complex stages, each with multiple steps. The first stage (the light reactions) occur on the thylakoid membranes and the second stage (the Calvin cycle ) occurs in the stroma. In a thylakoid membrane, chlorophyll is clustered with proteins and other smaller organic molecules into photosystems. Most photosystems contain chlorophyll a, chlorophyll b and carotenoids. This allows each photosystem to “harvest” energy from a wide array of light wavelengths. No matter which pigment absorbs the energy, it must eventually be passed to a particular chlorophyll a in the photosystem called the reaction center chlorophyll.

There are two types of photosystems in the thylakoid membrane- photosystem I and photosystem II (named in order of their discovery). Each photosystem has a particular type of chlorophyll as its reaction center chlorophyll. The reaction center chlorophyll of Photosystem I is called P700 (because it is best at absorbing light having a wavelength of 700 nm,) The reaction center chlorophyll at the center of Photosystem II is called P680.

Noncyclic Electron Flowaka linear electron flow

There are two different ways that electrons might flow in the light reactions. Noncyclic (linear) electron flow is the predominant route. Follow along with the picture-

  1. Light energy excites the electrons in Photosytem II. The reaction center chlorophyll is oxidized (now it needs an electron)
  2. An enzyme splits water; the electrons are supplied to the chlorophyll molecules; oxygen combines with another oxygen forming O2
  3. The original excited electron passes along an electron transport chain (goes from PSII to PSI).
  4. The energy from the transfer of electrons is used to pump protons (H+) creating a concentration gradient. This gradient will be used in chemiosmosis to phosphorylate ATP.
  5. Light energy has also excited the PSI chlorophyll, resulting in the donation of electrons. These electrons are replaced by the ones coming from PSII.
  6. The electrons donated by PSI move along another electron transport chain and eventually to NADP+. NADP+ accepts the electrons and is reduced to NADPH.

.Cyclic Electron Flow

Occasionally electrons take an alternative route which uses Photosystem I, but not Photosystem II. This is called cyclic electron flow. This route produces ATP, but no NADPH. The production of ATP by this system is called cyclic photophosphorylation.

CHEMIOSMOSIS IN THE LIGHT REACTIONS: As water is split in noncyclic photophosphorylation, protons (H+) are stored in the thylakoid compartment. As a proton gradient is established, H+ are pumped through ATP synthase embedded in the thylakoid membrane. ADP is phosphorylated.

THE CALVIN CYCLE: ATP and NADPH produced in the light reactions are used to convert CO2 to sugar. As CO2 enters the Calvin cycle it is attached to a five carbon sugar called ribulose biphosphate (RuBP). The enzyme that catalyzes this step is RuBP carboxylase, better known as rubisco. This six carbon product is rearranged in a series of steps that requires the phosphate from ATP and the H+ and electrons from NADPH. The product is a 3 carbon sugar called G3P and a 5 carbon sugar which is recycled to generate more RuBP. The molecule of G3P can combine with another molecule of G3P to form glucose. G3P molecules are also used for biosynthesis or the energy needs of the cell.

For the net synthesis of one G3P molecule this cycle must occur 3 times (carbon dioxide molecules enter one at a time),The Calvin cycle consumes 9 molecules of ATP and six molecules of NADPH for each molecule of G3P produced.

ALTERNATIVE METHODS OF CARBON FIXATION

Sometimes there must be a compromise between photosynthesis and water loss by a plant. What structural feature is responsible for this? The type of photosynthesis described so far occurs in plants called C3 plants (the first organic product of carbon fixation is a 3 carbon compound called 3-phosphoglycerate). Important commercial C3 plants include rice, wheat and soybeans. These plants produce less food when their stomata close on a dry hot day. Less CO2 enters the Calvin cycle, so rubisco accepts O2 instead and adds it to ribulose biphosphate. The product is eventually broken down in the mitochondria and perioxisome into CO2 . This is called photorespiration. It is a wasteful process which generates no ATP and no food.

In other plants, alternate modes of carbon fixation have evolved that minimize photorespiration even in hot, arid climates. The two most important photosynthetic adaptations are C4 photosynthesis and CAM photosynthesis.

C4 Plants have two kinds of photosynthetic cells- bundle sheath cells and mesophyll cells. The two stages of photosynthesis are separated structurally. Notice in the picture, that CO2 fixation occurs in the cytoplasm of the mesophyll cells. The enzyme involved is PEP carboxylase and the product is oxaloacetate (a four carbon sugar). Oxaloacetate is exported into the bundle sheath cells, which break it down, releasing carbon dioxide. The carbon dioxide in the bundle sheath cell is converted into carbohydrates through the normal Calvin cycle. This modification keeps the carbon dioxide levels high enough to supply rubisco; this minimizes photorespiration.

CAM Plants have adaptations that separate the stages of photosynthesis to day and night (temporal). This photosynthetic adaptation evolved in succulents, cacti, pineapples and several other plant families. These plants open their stomata at night and close them during the day (the reverse of other plants). At night these plants take up CO2 and incorporate it into a variety of organic acids. This mode of carbon fixation is called crassulacean acid metabolism or CAM (named after the plant family Crassulaceae). The mesophyll cells of CAM plants store the organic acids they make at night in their vacuoles until morning when the stomata close. During the day, when the light reactions can supply ATP and NADPH for the Calvin cycle, CO2 is released from the organic acids made the night before and incorporated into sugar.

The Fate of Photosynthetic Products

The products of photosynthesis supply the chloroplasts with chemical energy and carbon skeletons to synthesize all the major organic molecules of cells. About 50% of the organic material made by photosynthesis is consumed as fuel for cellular respiration in the mitochondria of plant cells.

Carbohydrates can be transported out of the leaf cells as sucrose. Sucrose provides raw material for cellular respiration and anabolic pathways that synthesize proteins, lipids and other products. A considerable amount of sugar is used to make cellulose, the most abundant molecule in the plant. Plants stockpile extra sugar by synthesizing starch and storing it in the cells of roots, tubers, seeds and fruits.

Here is a food pyramid that begins with producers and ends with tertiary consumers. If the producer level contains 25,000 kJ of energy and this pyramid follows the 10% rule, then how much energy gets transmitted to the tertiary consumers?

Carbon Flow in a Grassland Ecosystem

How much carbon (g/m2) is released into the atmosphere as a result of the metabolic activity of the herbivores?

ΔG = ΔH – TΔS

G = Free Energy

H = Enthalpy

S = Entropy

T = Temperature in Kelvin (K = C + 273)

Δ represents change in value over time

An experiment determined that when a protein unfolds to its denatured (D) state from the original folded (F) state, the change in Enthalpy is ΔH = H(D) – H(F) = 56,000 joules/mol. Also the change in Entropy is ΔS = S(D) – S(F) = 178 joules/mol. At a temperature of 20⁰C, calculate the change in Free Energy ΔG, in j/mol, when the protein unfolds from its folded state.