CHAPTER 9 CELLULAR RESPIRATION

9-1 Chemical Pathways

Why We Need Food - provides us with chemical building blocks needed to grow and reproduce - a source of raw materials from which cells can make new parts - source of energy

A. Chemical Energy and Food - food contains large amounts of energy - 1 gm of glucose when burned in the present of oxygen, releases 3,811 calories of heat energy - a calorie is the amount of energy needed to raise the temperature of 1 gm of water 1 degree Celsius - Calorie (capital C) is used on food labels and equals 1000 calories or 1 kilocalorie - cells release the energy from glucose and the process begins with glycolysis - glycolysis releases very little energy, but if oxygen is present, can lead to pathways that release large amounts of energy - if oxygen is not present, glycolysis leads to another pathway

B. Overview of Cellular Respiration

Cellular respiration - the process that releases energy by breaking down food molecules in the presence of oxygen - it begins with glycolysis in the cytoplasm and is followed by the Krebs Cycle and the electron transport chain in the mitochondria - requires oxygen and glucose and gives off CO2, H2O, and energy

The equation for cellular respiration is:

Oxygen + glucose carbon dioxide + water + energy

6O2 + C6H12O6 6CO2 + 6 H2O + energy

- the energy is made a little bit at a time and the cell traps the energy as it is made by making ATP

C. glycolysis (Stage 1) - the process in which one molecule of glucose is broken in half to produce two molecules of pyruvic acid (each a 3-carbon compound)

1. ATP production - glycolysis used 2 ATPs to get started and produces 4 ATPs when complete with a net gain of 2 ATP molecules 2. NADH production - the glycolysis reaction removes 4 high-energy electrons and passes them to an electron carrier called NAD+ (nicatinamide adenine dinucleotide) - each NAD+ molecule accepts a pair of electrons and becomes NADH holding those electrons until they can be transferred to other molecules - NAD+ helps pass the energy from glucose to other pathways in the cell

Advantages of glycolysis - energy yield from glycolysis is small, but produced very quickly (thousands of ATP molecules in just milliseconds) - glycolysis does not require oxygen, therefore, energy can still be supplied to cells even when oxygen is absent

Disadvantage - NAD+ can get filled up with electrons very quickly and cannot keep glycolysis going, therefore, ATP production stops

D. Fermentation - the pathway which releases energy from food molecules when oxygen is not present - cells convert NADH to NAD+ by passing high-energy electrons back to pyruvic acid - the conversion of NADH to NAD+ allows glycolysis to continue producing a steady supply of ATP - fermentation is anaerobic (occurring without oxygen)

Two main types of fermentation 1. alcoholic fermentation - used by yeasts and other microorganisms to produce energy and the byproducts of ethyl alcohol and CO2

Equation

Pyruvic acid + NADH alcohol + CO2 + NAD+

- alcoholic fermentation causes bread dough to rise with the production of CO2 gas and the small amount of alcohol produced evaporates during baking

2. lactic acid fermentation - this types of fermentation produces lactic acid and also regenerates NAD+ (a high energy electron carrier) so glycolysis can continue

Equation

Pyruvic acid + NADH lactic acid + NAD+

- lactic acid is produced during vigorous muscle exercise - lactic acid build-up in muscles causes pain and burning (sore muscles)

9-2 The Krebs Cycle and Electron Transport

- the chemical energy produced during glycolysis is still locked in the high-energy electrons of pyruvic acid and must be released- oxygen acts as the electron acceptor in the final steps of cellular respiration- this pathway requires oxygen and is, therefore, said to be aerobic A. The Krebs Cycle - named after Hans Krebs who discovered it in 1937 - pyruvic acid is broken down into CO2 in a series of steps that release energy - it is also known as the citric acid cycle because citric acid is one of the first compounds formed

Steps:

1. Citric acid production - pyruvic acid (formed in glycolysis) enters mitochondrion - one C is removed forming CO2 and electrons are removed changing NAD+ to NADH - co-enzyme A attaches to the remaining 2-carbon molecule forming acetyl co-enzyme A - acetyl coenzyme A attaches the 2-carbon molecule to a 4-carbon compound forming citric acid

2. Energy extraction - citric acid is broken down to a 5-carbon compound and then to a 4-carbon compound - along the way, 2 CO2 molecules are released and electrons join NAD+ and FAD (flavine adenine dinucleotide) forming NADH and FADH2 (electron carriers) with one molecule of ATP generated - the energy extracted from one pyruvic acid molecule equals 4 NADH, 1 FADH2, and 1 molecule of ATP

What happens to the Krebs Cycle products?

1. All CO2 produced is released as we breathe out or exhale. 2. ATP produced is used for cellular activities 3. In the presence of O2, high-energy electron carriers like NADH and FADH2 can release their electrons to generate huge amounts of ATP

B. Electron Transport - the electron carriers pass their electrons obtained in the Krebs Cycle to the electron transport chain to convert ADP to ATP Steps:

1. High-energy electrons from NADH and FADH2 are passed into the electron transport chain- the electron transport chain consists of a series of carrier proteins located in the inner membrane of the mitochondrion of eukaryotes. It is located in the cell membrane of prokaryotes.- the high-energy electrons are carried from one protein carrier to the next- at the end of the chain, an enzyme combines the electrons with hydrogen ions and O2 to form water- O2 is the final electron acceptor of the electron transport chain and is necessary for getting rid of the waste products of cellular respiration (low-energy electrons and hydrogen ions)

2. As one pair of high-energy electrons move through the electron transport chain, their energy is used to transport hydrogen ions across the membrane- Hydrogen ions build up in the intermembrane space making it positively charged, while the outside is now negatively charged.

3. ATP synthase (enzyme) is contained in the inner membrane. It captures hydrogen ions as they pass through a channel. Once the ions are captured in the enzyme, the enzyme spins and as it rotates, it grabs an ADP and attaches a phosphate forming the high-energy compound ATP

****one pair of electrons produces enough energy to convert 3 ADP molecules to 3 ATP molecules C. The TotalsThe complete breakdown of 1 glucose molecule through glycolysis ( 2 ATP) and cellular respiration ( 34 ATP) equals 36 ATP molecules

D. Energy and Exercise

1. Quick energy - produced by lactic acid fermentation - stored ATP is used up quickly within a few seconds which results in ATP formation through lactic acid fermentation (seen in sprinters) - O2 is all used up creating an oxygen debt which results in heavy breathing. Oxygen is required to recycle (get rid of) lactic acid buildup in the muscles.

Sprinters

2. Long term energy - needed to soccer players or marathon runners - cellular respiration is the only way to generate a continuing supply of ATP - cellular respiration releases energy more slowly than fermentation (athletes must pace themselves) - stored carbohydrate (glycogen) lasts 15-20 minutes after which other stored molecules are broken down (fats, etc.) for energy

Long-term energy required

Cellular Respiration: An Overview

E.Comparing photosynthesis and cellular respiration

Photosyn. Cell. respiration

Function energy storage energy release

Location chloroplasts mitochondria

Reactants CO2 and water O2 and glucose

Products O2 and glucose CO2 and water

Equation CO2 + H2O sugar + O2 O2 + sugarCO2+H2O+ATP

- almost nearly opposite processes - cellular respiration takes place in all eukaryotes and some prokaryotes - photosynthesis occurs only in plants, algae, and some bacteria

Comparing PS and CR