Cellular Respiration and Energy

• Life requires energy, which in most ecosystems comes from the sun.
• Photosynthesis uses sunlight to rearrange carbon dioxide (CO2) and water (H2O) into organic molecules, releasing oxygen (O2).
• Cellular respiration breaks down organic molecules, consuming O2 and releasing CO2 and H2O, capturing energy as ATP.
• Photosynthesis occurs in prokaryotes, chloroplasts of plants, and algae.
• Cellular respiration occurs in prokaryotes and the mitochondria of eukaryotes (plants, animals, fungi, and protists).
• Energy makes a one-way trip through an ecosystem, while matter is recycled (CO2 and H2O from cellular respiration are used in photosynthesis to make sugars and O2, which are then used in respiration).

Respiration

• Respiration refers to the exchange of gases where an organism obtains O2 and releases CO2.
• Breathing and cellular respiration are closely related; lungs take up O2, which the bloodstream delivers to muscle cells for ATP production via cellular respiration, and the bloodstream carries CO2 back to the lungs to be exhaled.
• Breathing and eating provide reactants for cellular respiration, which generates ATP.
• Glucose (C6H12O6) is often the primary fuel source for cellular respiration but fats, proteins, and complex carbohydrates can also be used.
• During cellular respiration, the atoms of glucose and O2 rearrange to form CO2 and H2O with the chemical energy released stored in ATP (around 34%) and the rest released as heat.
• Heat released during cellular respiration helps maintain a constant body temperature of about 37°C or 98.6°F, which is critical for survival.
• Without ATP regeneration, you would use up your body weight in ATP daily.
• The brain requires a lot of energy, burning about 120 grams (a quarter of a pound) of glucose a day, accounting for about 20% of total energy consumption.
• Maintaining brain cells and other life-sustaining activities uses as much as 75% of the energy a person takes in as food during a typical day.
• Basal Metabolic Rate (BMR) energy needs for basic life-sustaining activities range from 1,300 to 1,800 kcal a day.
• The U.S. National Academy of Sciences estimates that the average adult needs about 2,200 kcal of energy per day, but the number varies based on age, sex, and activity level.
• During cellular respiration, electrons are transferred from glucose or other organic fuels to oxygen, releasing energy.
• The transfer of electrons from one molecule to another is an oxidation-reduction reaction, Redox reaction.
• Oxidation is the loss of electrons, while reduction is the gaining of electrons (OIL RIG: Oxidation Is Loss, Reduction Is Gain).
• In cellular respiration, glucose (C6H12O6) loses hydrogen atoms and is oxidized to CO2, while O2 gains hydrogen atoms and is reduced to H2O.
• Nicotinamide adenine dinucleotide (NAD+) is a coenzyme that cells use to shuttle electrons in redox reactions and it accepts electrons to become NADH.

Cellular Respiration Stages

• Cellular respiration consists of Glycolysis, Pyruvate oxidation and the citric acid cycle, and Oxidative phosphorylation.
• In prokaryotic cells, these steps occur in the cytosol, and the electron transport chain is in the plasma membrane.
• Glycolysis occurs in the cytosol and breaks glucose into two molecules of pyruvate.
• Pyruvate oxidation and the citric acid cycle take place within the mitochondria and completes the breakdown of glucose to carbon dioxide.
• Oxidative phosphorylation involves electron transport and chemiosmosis; NADH and FADH2 shuttle electrons to electron transport chains, and most ATP is generated by oxidative phosphorylation.

Glycolysis

• Glycolysis (glyco = sweet, lysis = split) splits sugar, beginning with one glucose molecule and ending with two pyruvate molecules, each representing a three-carbon atom (glucose has six).
• Glucose is oxidized, reducing two molecules of NAD+ to two NADH, resulting in a net gain of two ATP molecules.
• ATP is formed in glycolysis by substrate-level phosphorylation.
• The breakdown of glucose to pyruvate releases energy stored in ATP and NADH.
• Pyruvate molecules still possess about 90% of the energy from the original glucose.
• The subsequent sequential steps of glycolysis happen in a metabolic pathway.
• Compounds formed between the initial reactant and final product are intermediates.
• The steps of glycolysis are grouped into two phases: an energy investment phase and an energy payoff phase.
• The energy investment phase consumes energy, using two molecules of ATP to add a phosphate group to the glucose molecule which then splits into small sugars.
• In the energy payoff phase the cell yields energy where two G3P molecules occur after glucose has been split, generating NADH in a redox reaction (step 5) and producing ATP.
• The net gain is two ATP molecules for each glucose that enters glycolysis because the first phase uses two molecules of ATP.
• The two ATP molecules from glycolysis equals about 6% of the energy that a cell can harvest from a glucose molecule.
• Some organisms (yeasts, bacteria) meet their energy with the ATP produced by glycolysis alone, and muscle cells may use anaerobic ATP for short periods without enough O2.
• After glucose is oxidized to pyruvate in glycolysis, the pyruvate is transported from the cytosol into a mitochondrion. Each pyruvate enters a series of redox reactions that produce acetyl CoA and NADH.

Citric Acid

• This cycle consists of a circular series of enzyme-catalyzed reactions where one acetyl CoA makes two CO2 molecules, one ATP, three NADH, and one FADH2 molecule.
• Because two acetyl CoA molecules come from two pyruvate molecules, the cycle runs twice, and its outputs are doubled for each glucose processed.
• The ATP from the citric acid cycle can be used immediately, most of the energy is stored in NADH and FADH2 to be shuttled to an electron transport chain. One turn of the citric acid cycle begins as an enzyme strips the CoA portion from acetyl CoA and combines the remaining two-carbon group with the four-carbon molecule oxaloacetate.
• In steps 2 and 3, NADH, ATP, and CO2 are generated during redox reactions.
• In steps 4-6, further redox reactions generate FADH2 and more NADH.
• Succinate is oxidized as the electron carrier FAD is reduced to FADH2. Fumarate is converted to malate, which is then oxidized as one last NAD+ is reduced to NADH.
• One turn of the citric acid cycle is completed with the regeneration of oxaloacetate, which is ready to start the next cycle by accepting an acetyl group from acetyl CoA.
• Almost 90% of the ATP generated in cellular respiration occurs in Stage 3, by oxidative phosphorylation, (electron transport and chemiosmosis).
• Energy releases as electrons move down the "energy staircase" is used to pump H+ into the intermembrane space and the H+ gradient is used for the process chemiosmosis.
• In chemiosmosis concentration gradient results and drives H+ through ATP synthase producing a high energy payoff to begin with a molecule of glucose.

Glucose

• Glycolysis oxidizes glucose to two molecules of pyruvate, produces 2 NADH, and a net gain of two ATP by substrate-level phosphorylation.
• The oxidation of 2 pyruvate yields 2 NADH and 2 acetyl CoA.
• The 2 acetyl CoA feeds into the citric acid cycle and produces 6 NADH and 2 FADH2, as well as 2 ATP by substrate-level phosphorylation.
• Glucose is now completely oxidized to CO2 and the NADH and FADH2 deliver electrons to the electron transport chain, they are passed to O2, forming H2O.
• The energy released as electrons move down the “energy staircase” is used to pump H+ into the intermembrane space, where the H+ gradient is tapped by ATP synthase to produce about 28 molecules of ATP by oxidative phosphorylation.
• Capturing energy from the oxidation of molecules via an electron transport chain to phosphorylate ADP to ATP gives rise to the name oxidative phosphorylation.
• The total yield of ATP molecules per glucose is about 32 but, ATP molecules cannot be stated exactly.

Fermentation

• Fermentation is a way of harvesting energy that does not require oxygen, the pathway that generates ATP is glycolysis.
• Glycolysis generates a net gain of 2 ATP while oxidizing glucose to two molecules of pyruvate and reducing NAD+ to NADH and requires NAD+ as an electron acceptor.
• Fermentation provides an anaerobic path for recycling NADH back to NAD+..
• With lactic acid fermentation, NADH is oxidized back to NAD+ as pyruvate is reduced to lactate which muscle cells use when the need for ATP outpaces O2 delivery.
• In alcohol fermentation, yeasts and certain bacteria recycle NADH back to NAD+ while converting pyruvate to CO2 and ethanol.

Types of Cells

• Obligate anaerobes are prokaryotes that live in stagnant ponds and deep in the soil require anaerobic conditions and are poisoned by oxygen
• Facultative anaerobes like yeast and muscle cells, can make ATP either by fermentation or by oxidative phosphorylation, when O2 is available.
• Pyruvate is a fork in the metabolic road for facultative anaerobes where, if oxygen is available, the more productive aerobic respiration is used.
• Glycolysis is the universal energy-harvesting process of life as well as a metabolic heirloom that functions in fermentation and as the first stage in the breakdown of organic molecules by cellular respiration.
• Ancient prokaryotes used glycolysis to make ATP long before oxygen existed in Earth's atmosphere and it does not require any membrane-enclosed organelles of the eukaryotic cell.
• Cells use many kinds of organic molecules as fuel for cellular respiration for the production of proteins and fats.
• A wide range of carbohydrates can be funneled into glycolysis, digested from starch to glucose, which is then broken down by cellular respiration.
• Fats are hydrolyzed to glycerol (enters glycolysis as G3P) and fatty acids (broken into two-carbon fragments that enter the citric acid cycle as acetyl CoA), producing more than twice as much ATP as a gram of carbohydrates.
• Typically amino acids will be used to generate energy for biosynthetic production where excess amino acids will be converted to intermediates of glycolysis or the citric acid cycle, During the conversion, the amino groups are stripped off and later disposed of in urine.

Biosynthesis

• Biosynthesis is the production of organic molecules using energy-requiring metabolic pathways to make raw materials from amino acids/molecules from glycolysis.
• There are pathways by which cells can make three classes of organic molecules using some of the intermediate molecules of glycolysis and the citric acid cycle.
• The interconnections among these pathways produce a balanced metabolism that is regulated by a certain amino acids or feedback inhibition (end product inhibits an enzyme that catalyzes an early step in the pathway).
• There is Harvesting energy from the breakdown of organic molecules (cellular respiration, atoms of starting materials end up in carbon dioxide and water), and ability to make organic molecules from inorganic ones (photosynthesis). Overall, Photosynthesis and cellular respiration provide energy for life, Cellular respiration banks energy in ATP molecules; The human body uses energy from ATP for all its activities.
• Stage 1: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate with ATP usage; The citric acid cycle completes the energy-yielding oxidation of organic molecules where oxidation of pyruvate yields acetyl CoA, CO2, and NADH.
• Stage 3: Most ATP production occurs by oxidative phosphorylation. In mitochondria, electrons from NADH and FADH2 are passed down the electron transport chain to O2, which picks up H+ to form water.
• Multiple reactions in glycolysis splits glucose into two molecules. Steps 1–4 consume energy, while steps 5-9 yield energy
• The multiple reactions of the citric acid cycle finish off dismantling of glucose; Glycolysis enables cells to produce ATP without oxygen where cells recycle NAD+ from NADH as pyruvate is reduced to lactate (lactic acid fermentation) or alcohol and CO2 (alcohol fermentation);
• Cells use intermediates from cellular respiration and ATP for biosynthesis of other organic molecules and metabolic pathways are often are regulated by feedback inhibition.

Explore energy transformations in cells. Understand how cellular respiration converts glucose's stored energy and regenerates ATP to power cellular activities. Learn about the significance of body temperature and the role of basal metabolic rate in total energy consumption.