Chapter 6 How Cells Harvest Chemical Energy

Chapter 6How 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

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 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 • The first stop is an electron acceptor called NAD+

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 calledsubstrate-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

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

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

When enough oxygen reaches cells to support energy needs • Anaerobic metabolism • When the demand for oxygen outstrips the body’s ability to deliver it • Aerobic metabolism • 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 • Yeast cells carry out a slightly different type of fermentation pathway • This pathway produces CO2 and ethyl alcohol • Various types of microorganisms perform fermentation • The food industry uses yeast to produce various food products

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

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

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

Chapter 6 How Cells Harvest Chemical Energy