How Cells Harvest Chemical Energy

• Cellular respiration is the process by which energy is harvested by cells from sugar– Requires Oxygen (O2)

– Releases Carbon Dioxide (CO2) , water (H2O) and a large amount of ATP

• All of our cells harvest chemical energy (from food)

Cellular Respiration stores energy in ATP molecules

• A cell uses energy to build and maintain its structure, transport materials, manufacture products, move, grow and reproduce

• Cellular respiration involves mainly sugars, but other organic compounds can be broken down (nearly all the equations you’ll see use glucose as the representative food molecule)

• Our bodies require a continuous supply of energy just to stay alive

Organisms use energy from ATP for all its activities

• Breathing• Heart

pumping• Maintaining

body temperature

• Thinking, dreaming

www.sxc.hu/photo/437515

Organisms use energy from ATP for all its activities

• Above and beyond the energy we need for body maintenance, cellular respiration provides energy for voluntary activities

• The amount of energy it takes to perform these activities are expressed as kilocalories (a “calorie” on a nutritional label actually equals 1 kilocalorie)

• A kilocalorie is the quantity of heat required to raise the temperature of 1 kg water by 1 C ̊�

Energy consumed by various activities (kilocalorie consumed per hour)

• Running (from BIO 101 class) 979• Dancing (after passing BIO 101) 510• Bicycling (to test the potential/kinetic energy

reference) 490• Swimming (in a sea of knowledge) 408• Walking (to lab) 245• Sitting (and absorbing it all) 28

Maybe we should try teaching this in a spinning class???

How cells ‘tap’ energy

• Energy is contained in the arrangement of electrons in the chemical bonds that hold an organic molecule (such as glucose) together

• During cellular respiration, electrons are transferred to oxygen (why we need O2) as the Carbon-Hydrogen bonds of the molecule (in this case, glucose) are broken down.

• When these electrons are transferred, they lose potential energy, which is released as energy

Cellular Respiration

• Cellular respiration occurs in 3 stages– Glycolysis– Citric Acid Cycle– The Electron Transport Chain (Oxidative

Phosphorylation)

• All of these steps occur in and around the mitochondria in eukaryotic cells, and in the plasma membrane and cytoplasm in prokaryotic cells

Mitochondrion

CO2 CO2

NADH

ATP

High-energy electronscarried by NADH

NADH

CITRIC ACIDCYCLE

GLYCOLYSIS

PyruvateGlucose

andFADH2

Substrate-levelphosphorylation

Substrate-levelphosphorylation

OXIDATIVEPHOSPHORYLATION

(Electron Transportand Chemiosmosis)

Oxidativephosphorylation

ATPATP

CytoplasmInnermitochondrialmembrane

Occurs in the cytoplasm Occurs in the

mitochondria

The Electron Transport Chain

Cellular Respiration – Glycolysis

• What two words do you see immediately in the word “glycolysis”?

• Glycolysis literally means the “splitting of sugar”

• Glycolysis begins with a molecule of glucose (6-Carbon sugar) and ends with 2 molecules of pyruvate (a 3-Carbon compound)

• Requires the initial input of 2 ATP molecules

Glycolysis

• Glucose + 2ATP 2 Pyruvate (ionized form of pyruvic acid)

• Glycolysis is a multi-step pathway, which utilizes at least 6 different enzymes in a metabolic pathway that is anaerobic

• In the later stages of glycolysis, 4 ATP molecules are synthesized using the energy given off during the chemical reactions; net gain of 2 ATP

Glycolysis

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_glycolysis_works.html

Glycolysis

• The chemical energy stored in the bonds of the glucose molecule is used to form the high energy compounds, ATP

• Glycolysis is universal among organisms; simpler organisms (yeasts and bacteria) can satisfy their metabolic needs with the ATP produced by glycolysis alone. Most organisms have far higher energy demands and must undergo additional stages of cellular respiration to release more energy

The Citric Acid Cycle (aka “The Krebs Cycle”)

• As pyruvate is formed at the end of glycolysis, it is transported from the cytoplasm into a mitochondrion

• The Citric Acid Cycle breaks down pyruvate in the mitochondrial matrix

• Compared to glycolysis, the Citric Acid Cycle pays big energy dividends to the cell, producing 2 ATP molecules plus 10 NADH and 2 FADH2, two very high energy molecules

The Citric Acid Cycle

• The energy stored in NADH and FADH2 is released when these molecules ‘shuttle’ their high-energy electrons to the Electron Transport Chain (stage 3)

• The Electron Transport Chain is oxygen-driven and produces the largest amount of ATPs in the entire cellular respiration process

• For every glucose molecule that enters glycolysis, there is a total net production of 36-38 ATPs by the end of the Electron Transport Chain

The Electron Transport Chain

• The Citric Acid Cycle and the Electron Transport Chain occur in the mitochondria

• The many folds of the mitochondrial inner membrane enlarge its surface area, providing space for thousands of copies of the Electron Transport Chain occurring at once, producing many ATP molecules

What do cigarette smokers and musk ox have in common?

• Brown fat is 1 of 2 types of fat found in mammals

• Unlike white fat, it is brown in color and especially abundant in newborns and arctic animals

• Its primary function is to generate heat in animals (or in newborns who can not yet shiver)

• What does this have to do with cellular respiration?

Brown phat

• Brown fat has extremely high numbers of mitochondria

• One of the steps in the Electron Transport Chain involves the transfer of hydrogen ions (H+) across the mitochondrial inner membrane which stores energy as a proton (H+) gradient

• This energy is used to make ATP

That’s (brown) phat!

• Many H+ use an alternative route to generate heat, rather than producing ATP with the help of a protein, thermogenin

• Brown fat is especially rich in this protein which causes the proton gradient established during cellular respiration to generate heat rather than ATP

• Some ‘skinny’ people may naturally have more brown fat than ‘heavier’ people

How now brown fat

• Brown fat production in humans usually disappears after infancy, but can be stimulated by exposure to severe cold

• So, what do smokers and musk ox have in common???

• Smoking (nicotine) and caffeine stimulate thermogenin production!– Less ATP, more heat– Why you gain weight when you quit smoking (and

why I won’t give up my caffeine addiction…)

And you thought biology was boring???!!!

www.flickr.com/photos/73234878@N00/2388306833/

Cellular Respiration: Review

• Beginning with 1 molecule of glucose, glycolysis and the citric acid cycle produce a net total of 4 ATP

• NADH and FADH2 are high-energy molecules produced by glycolysis, the ‘grooming’ of pyruvate, and the citric acid cycle

• NADH and FADH2 are electron carriers; they transfer electrons (& their energy) to other molecules releasing energy and producing ATP

Cytoplasm

Glucose

FADH2

Mitochondrion

Maximum per glucose:

OXIDATIVEPHOSPHORYLATION

(Electron Transportand Chemiosmosis)

CITRIC ACIDCYCLE

Electron shuttleacross membrane

2 NADH

2 NADH

2 NADH

6 NADH 2(or 2 FADH2)

2 AcetylCoA

GLYCOLYSIS2

Pyruvate

About38 ATP

about 34 ATPby substrate-levelphosphorylation

by oxidative phosphorylation2 ATP

by substrate-levelphosphorylation

2 ATP

Cellular Respiration: Review

• Oxygen is the final electron acceptor in the Electron Transport Chain

• Without oxygen, electrons cannot be removed from the system, and their energy cannot be released to produce ATP

• In animals, breathing is the essential process that brings oxygen into the body for delivery to the cells to participate in cellular respiration

Fermentation enables cells to produce ATP without oxygen

• In the absence of oxygen, ATP may still be produced via a process called fermentation

• Fermentation is the process of deriving energy from organic compounds such as carbohydrates in the absence of oxygen

• Cellular respiration always begins with glycolysis; the presence or absence of oxygen will then determine whether cellular respiration or fermentation will follow

Fermentation

• Remember, glycolysis results in the formation of 2 3-carbon pyruvate molecules from 1 6-carbon glucose molecule, and 2 ATP molecules are released

• When oxygen is lacking, certain organisms (such as yeasts) can convert pyruvate into ethyl alcohol and CO2 which removes electrons and allows continuous ATP production

• Yeasts are able to perform this process because they have the necessary enzyme to convert pyruvate into ethyl alcohol

Fermentation

• CO2 and ethyl alcohol… this sounds familiar…• For thousands of years, people have used

alcohol fermentation for brewing beer, winemaking and baking

• Yeasts (a type of fungi) are used in the process; the CO2 generated by fermentation causes bread to rise (and makes our libations bubbly), while the ethyl alcohol, well, you know…

Fermentation

• Normally, ethyl alcohol is released to the surroundings (or burnt off during baking), but in a wine vat, the yeast will die when the alcohol content reaches ~15% (toxic)

• Some organisms, such as yeast can alternate between cellular respiration or fermentation, depending upon whether oxygen is available or not; others may only use fermentation and are poisoned by the presence of oxygen

Fermentation continued…

• In muscle cells, another type of fermentation takes place

• When muscle cells contract too quickly (e.g., strenuous exercise), they rapidly use up their oxygen supply, which slows ATP production

• Muscle cells, however, have the ability to produce a small amount of ATP through glycolysis in the absence of oxygen = lactic acid fermentation

• The muscle cells convert glucose to pyruvate, and then an enzyme in the muscle cells converts pyruvate into lactic acid, releasing 2 ATP molecules

• Lactic acid is toxic and must be removed by the liver (& converted back to pyruvate); heavy breathing after exercise helps restore oxygen back to the muscle cells

How we use food for energy

• While we frequently focus on glucose as the starting molecule of cellular respiration or fermentation, many other organic compounds are utilized by our cells for energy

The Far Side; Gary Larson

How we use food for energy

• When you eat a bag of peanuts, for example, you are consuming lipids, proteins, and carbohydrates

• All of these compounds are used in the process of glycolysis, while fats (rich in carbon and hydrogen, and thus many energy-rich electrons) are broken into 2-carbon fragments which enter the citric acid cycle, and yield twice as many ATP as a carbohydrate

Food, such aspeanuts

ProteinsFatsCarbohydrates

Glucose

OXIDATIVEPHOSPHORYLATION(Electron Transportand Chemiosmosis)

CITRICACID

CYCLE

AcetylCoA

GLYCOLYSIS

Pyruvate

Amino acidsGlycerolSugars Fatty acids

Amino groups

G3P

ATP

Food also provides raw materials

• In addition to producing ATP for energy, many food molecules are used directly as raw materials which the cell uses to construct its structures and perform its functions

• For example, many proteins are broken down into amino acids by the cell to make its proteins

• In return, cells also use ATP to make biomolecules that are not present in food

How a cell harvests energy

• The cells of all living organisms have the ability to harvest energy from the breakdown of organic molecules; the atoms of the starting materials end up being released CO2 and H2O

• In contrast, the ability to make organic molecules using CO2 and H2O is not universal

• Animal cells lack this ability, but plants can actually produce organic molecules from inorganic ones using the energy of the sun (photosynthesis)