Chapter 6 How Cells Harvest Chemical Energy Overview: Cellular Respiration Reactions involved in cellular respiration Glycolysis Krebs Cycle Electron Transport Fermentation Food used to produce ATP ATP used to produce food

INTRODUCTION TO CELLULAR RESPIRATION Photosynthesis and cellular respiration provide energy for life Nearly all the cells in our body break down sugars for ATP production Cellular respiration occurs in mitochondria Cellular respiration is a chemical process that harvests energy from organic molecules Cellular respiration yields CO2, H2O, and a large amount of ATP

The ingredients for photosynthesis are carbon dioxide and water CO2 is obtained from the air by a plant’s leaves H2O is obtained from the damp soil by a plant’s roots Chloroplasts rearrange the atoms of these ingredients to produce sugars (glucose) and other organic molecules Oxygen gas is a by-product of photosynthesis

The Relationship Between Cellular Respiration and Breathing Cellular respiration and breathing are closely related Cellular respiration requires a cell to exchange gases with its surroundings Breathing exchanges these gases between the blood and outside air

Cellular respiration banks (stores) energy in ATP molecules Cellular respiration breaks down glucose molecules and banks their energy in ATP The process uses O2 and releases CO2 and H2O Glucose Oxygen gas Carbon dioxide Water Energy

Connection: The human body uses energy from ATP for all its activities ATP powers almost all cell and body activities A calorie is the amount of energy that raises the temperature of 1 gram of water by 1 degree Celsius

Cells tap energy from electrons transferred from organic fuels to oxygen Glucose gives up energy as it is oxidized Loss of hydrogen atoms Energy Glucose Gain of hydrogen atoms

Redox Reactions Chemical reactions that transfer electrons from one substance to another are called oxidation-reduction reactions Redox reactions for short The loss of electrons during a redox reaction is called oxidation The acceptance of electrons during a redox reaction is called reduction

[Glucose loses electrons (and hydrogens)] Oxidation [Glucose loses electrons (and hydrogens)] Glucose Oxygen Carbon dioxide Water Reduction [Oxygen gains electrons (and hydrogens)]

Why does electron transfer to oxygen release energy? When electrons move from glucose to oxygen, it is as though they were falling This “fall” of electrons releases energy during cellular respiration

NADH and Electron Transport Chains The path that electrons take on their way down from glucose to oxygen involves many stops An enzyme called dehydrogenase and a coenzyme called NAD+ (nicotinamide adenine dinucleotide) play important role in oxidizing glucose.

The first stop is an electron acceptor called NAD+ The transfer of electrons from organic fuel to NAD+ reduces it to NADH The rest of the path consists of an electron transport chain This chain involves a series of redox reactions These lead ultimately to the production of large amounts of ATP

All of the reactions involved in cellular respiration can be grouped into three main stages Glycolysis The Krebs cycle Electron transport

Stage 1: Glycolysis A molecule of glucose is split into two molecules of pyruvic acid Glycolysis breaks a six-carbon glucose into two three-carbon molecules These molecules then donate high energy electrons to NAD+, forming NADH Glycolysis occurs in the cytoplasm

Glycolysis makes some ATP directly when enzymes transfer phosphate groups from fuel molecules to ADP (This process is called substrate-level phosphorylation) Enzyme

Stage 2: The Krebs Cycle The Krebs cycle completes the breakdown of sugar In the Krebs cycle, pyruvic acid from glycolysis is first “prepped” into a usable form, Acetyl-CoA

The Krebs cycle extracts the energy of sugar by breaking the acetic acid molecules all the way down to CO2 The cycle uses some of this energy to make ATP The cycle also forms NADH and FADH2 The Krebs cycle and the electron transport chain occur in the mitochondria

Stage 3: Electron Transport Electron transport releases the energy your cells need to make the most of their ATP The molecules of electron transport chains are built into the inner membranes of mitochondria The chain functions as a chemical machine that uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membrane These ions store potential energy

When the hydrogen ions flow back through the membrane, they release energy The ions flow through ATP synthase ATP synthase takes the energy from this flow and synthesizes ATP

The electrons from NADH and FADH2 travel down the electron transport chain to oxygen Energy released by the electrons is used to pump H+ into the space between the mitochondrial membranes In chemiosmosis, the H+ ions diffuse back through the inner membrane through ATP synthase complexes, which capture the energy to make ATP

Cyanide, carbon monoxide Connection: Certain poisons interrupt critical events in cellular respiration Rotenone Cyanide, carbon monoxide Oligomycin

Fermentation is an anaerobic alternative to aerobic respiration Some of your cells can actually work for short periods without oxygen For example, muscle cells can produce ATP under anaerobic conditions Fermentation The anaerobic harvest of food energy Under anaerobic conditions, many kinds of cells can use glycolysis alone to produce small amounts of ATP

Aerobic metabolism Anaerobic metabolism Anaerobic metabolism When enough oxygen reaches cells to support energy needs Anaerobic metabolism When the demand for oxygen outstrips the body’s ability to deliver it Anaerobic metabolism Without enough oxygen, muscle cells break down glucose to produce lactic acid Lactic acid is associated with the “burn” associated with heavy exercise If too much lactic acid builds up, your muscles give out

Fermentation in Human Muscle Cells Human muscle cells can make ATP with and without oxygen They have enough ATP to support activities such as quick sprinting for about 5 seconds A secondary supply of energy (creatine phosphate) can keep muscle cells going for another 10 seconds To keep running, your muscles must generate ATP by the anaerobic process of fermentation

Fermentation in Microorganisms Various types of microorganisms perform fermentation Yeast cells carry out a slightly different type of fermentation pathway This pathway produces CO2 and ethyl alcohol The food industry uses yeast to produce various food products

In alcoholic fermentation, pyruvic acid is converted to CO2 and ethanol This recycles NAD+ to keep glycolysis working

In lactic acid fermentation, pyruvic acid is converted to lactic acid As in alcoholic fermentation, NAD+ is recycled Lactic acid fermentation is used to make cheese and yogurt

INTERCONNECTIONS BETWEEN MOLECULAR BREAKDOWN AND SYNTHESIS Cells use many kinds of organic molecules as fuel for cellular respiration Polysaccharides can be hydrolyzed to monosaccharides and then converted to glucose for glycolysis Proteins can be digested to amino acids, which are chemically altered and then used in the Krebs cycle Fats are broken up and fed into glycolysis and the Krebs cycle

Food molecules provide raw materials for biosynthesis In addition to energy, cells need raw materials for growth and repair Some are obtained directly from food Others are made from intermediates in glycolysis and the Krebs cycle Biosynthesis consumes ATP

Biosynthesis of macromolecules from intermediates in cellular respiration

The fuel for respiration ultimately comes from photosynthesis All organisms have the ability to harvest energy from organic molecules Plants, but not animals, can also make these molecules from inorganic sources by the process of photosynthesis