8.1-8.5 Cellular respiration

Cellular respiration

Is a cellular process that breakdown nutrient molecules produced by photosynthesis with the concomitant production of ATP

• Cellular respiration is an aerobic process

It usually involves the complete breakdown of glucose to CO2 and H2O

Energy is extracted from the glucose molecule

• Released step-wide

• Allows ATP to produce efficiently

Oxidation - reduction enzymes include NAD+ and FAD as coenzymes

The breakdown of glucose

Electrons are removed from substrates and received by oxygen, which combines with H+ to become water.

Glucose is oxidized and O2 is reduced

NAD+ (nicotinamide adenine dinucleotide)

As a coenzyme of oxidation-reduction, it is,

• Oxidized when it gives up electrons

• Reduced when it accepts electrons

Each ___ molecule is used over and over again

FAD (flavin adenine dinucleotide)

Also a coenzyme of oxidation-reduction

• Sometimes used instead of NAD+

• Accepts two electrons and two hydrogen ions (H+) to become FADH2

Phases of Cellular respiration

Glucolysis

Preparatory (prep) reaction

Citric acid cycle (krebs cycle)

Electron transport chain

Glycolysis - simple

Is the breakdown of glucose into two molecules of pyruvate

• Occurs in the cytoplasm

• ATP is formed

• It does not utilize oxygen (anaerobic)

Preparatory (prep) reaction - simple

Both molecules of pyruvate are oxidized and enter the matrix of mitochondria

• Electron energy is stored in NADH

• Two carbons are released as CO2 (one for each pyruvate)

Citric acid cycle (krebs cycle) - simple

Occurs in the matrix of the mitochondria and produces NADPH and FADH.

• A series of reactions, releases 4 carbons as CO2

• Turns twice per glucose molecule (once for each pyruvate)

• Produces two immediate ATP molecules per glucose molecule

Electron transport chain (ETC) - simple

A series of carries on the cristae of the mitochondria

• Extracts energy from NADH and FADH2

• Passes electrons from higher to lower energy states

• Produces 30 or 34 molecules of ATP by chemiomosis

Glycolisis

Occurs in the cytoplasm outside mitochondria

Energy investment step

• Two ATP are used to activate glucose

• Glucose splits into two G3P molecules

Energy harvesting steps

Oxidation of G3P occurs by removal of electrons and hydrogen ions.

Two electrons and one hydrogen ion accepted by NAD+ resulting in two NADH.

Four ATP are produced by substrate-level ATP synthesis

Input of glycolysis

6C glucose

2 NADP+

2 ATP

4 ADP + 4 phosphate

Outputs of glycolysis

2(3C) Pyruvate

2 NADH

2 ADP

4 ATP total

Fermentation

Pyruvate is a pivotal metabolic in cellular respiration. If O2 is not available to the cell, ___ occurs in the cytoplasm

If O2 is available to the cell, pyruvate enters the mitochondria for aerobic respiration

Fermentation process

Is an anaerobic process that reduces pyruvate to either lactate or alcohol and CO2. NADH transfers its electrons to pyruvate. Alcoholic fermentation, carried out by yeasts, produces carbon dioxide and ethyll alcohol.

• Used in the production of alcoholic spirits and breads

  Lactic acid fermentation, carried out by certain bacteria and fungi, produces lactic acid (lactate)

• Used commercially in the production of cheese, yogurt and sauerkraut

  Other bacteria produce chemicals anaerobically, including isopropanol, batyric acid, propionic acid, and acetic acid

Advantages of fermentation

Provides a burst of ATP energy for muscular activity.

• When muscles are working vigorously for short periods of time, lactid acid fermentation provides ATP

Disadvantages of fermentation

Lactate and alcohol are toxic to cells. Lactate changes pH and causes muscles to fatigue.

• Oxygen debt

  Yeast die from the alcohol they produce by fermentation

Efficiency of fermentation

Two ATP produced per glucose of molecule during fermentation is equivalent to 14.6 kilocalories. Complete oxidation of glucose can yield 686 kilocalories.

Efficiency is 21% of total possible for glucose breakdown. Only 2 ATP per glucose are produced, compared to 36 or 38 ATP molecules per glucose produced by cellular respiration.

Glucose → Pyruvate → Lactate → Continues production of lactate accumulate lactic acids in muscles

In presence of oxygen

In absence of oxygen

Inputs of fermentation

Glucose

2 ADP + 2 Phosphate

Outputs of fermentation

2 lactate or

2 alcohol and 2 CO2

2 ATP net gain

The preparatory (prep) reactions

Connects glycolysis to the citric acid cycle (krebs cycle). End product of glycolysis, pyruvate, enters the mitochondrial matrix. It is then converted to a 2-Carbon acetyl group

• Attached to coenzyme A to form aceryl-CoA

• Electrons picked up (as hydrogen atom) by NAD+, producing NADH

• CO2 is released and transported out of the mitochondria into cytoplasm

• Occurs twice per glucose molecule

Citric Acid Cycle process

Also called the Krebs cycle, occurs in the matrix of mitochondria

• Begins with the addition of a C2 acetyl group (from acetyl - COA) to a C4 molecule (oxaloacetate), forming a C6 molecule (citric acid)

• NADH and FADH2 capture energy-rich electrons

• ATP formed by substrate level phosphorylation

• Turns twice for one glucose molecule (once for each pyruvate)

Produces 4 CO2, 2 ATP, 6 NADH, and 2 FADH2 per glucose molecule

Citric acid cycle

1) The C2 acetyl group combines with a C4 molecule to produce citrate, a C6 molecule

2) Oxidation reaction produce two NADH + H+

3) The loss of two CO2 results in a new C4 molecule

4) One ATP is produced by substrate-level ATP synthesis

5) Additional oxidation reactions produce an FADH2 and another NADH + H+ and regenerate original C4 molecule

Inputs of citric acid cycle

2(2C) Acetyl groups

6 NAD+

2 FAD

2 ADP + 2 phosphate

Outputs of citric acid cycle

4 CO2

6 NADH

2 FADH 2

2 ATP

Electron transport chain

Location: Cristae mitochondria.

• Aerobic prokaryotes-plasm membrane

Series of carrier molecules

Pass energy rich electrons successively from one to another

• complex arrays of proteins and cytochrome

• Proteins with heme groups with central iron atoms

The electron transport chain

• Receives electrons from NADH and FADH

• Produces ATP by oxidative phosphorylation

Oxygen final electron acceptors

Combine with hydrogen ion to form water

Cycling of carriers 1

The fate of the hydrogens:

• Hydrogens from NADH deliver enough energy to make 3 ATPs

• Those from FADH2 have only enough for 2 ATPs

• “Spent” hydrogen combine with oxygen

Recycling of coenzymes increases efficiency

• Once NADH delivers hydrogens, it returns (as NAD+) to pick up more hydrogens

• However, hydrogens must be combined with oxygen to make water

• If O2 is not present, NADH cannot release H+

• It is no longer recycled back to NAD+

Cycling of carriers 2

The electron transport chain complexes pump H+ from the matrix into the intermembrane space of the mitochondrion. H+ therefore becomes more concentrated in the intermembrane space, creating an electrochemical gradient.

ATP synthase allows H+ to flow down its gradient. The flow of H+ drives the synthesis of ATP from ADP nd inorganic phosphate by ATP synthase .

Process is called chemiosmosis

• ATP production is linked to the establishment of H+ gradient

  ATP moves out of mitochondria and is used for cellular work

• It can be broken down to ADP and inorganic phosphate

• These molecules are returned to the mitochondria for more ATP production

Energy yield from glucose metabolism

Net yield per glucose

• Form glycolysis - 2 ATP

• Form citric acid cycle - 2 ATP

Form electrol transport chain - 32 or 34 ATP

Energy content

• Reactant (glucose) 686 kilocalories

• Energy yield (36 ATP) 263 kilocalories

• Efficiency is 39%

• Rest of energy from glucose is lost as heat

Foods

• Sources of energy-rich molecules

• Carbohydrates, fats, and proteins

Degradative reactions (catabolism)

Break down molecules

• Tend to be exergonic releasing energy

Synthetic reactions (anabolism)

Build molecules

• Tend to be endergonic (consume energy)

Catabolism

Glucose is broken down in cellular respiration. Fats breaks down into glycerol and three fatty acids. Amino acids breaks into carbon chains and amino groups.

Deamination (NH2 removed) Occurs in the liver

• Results in poisonous ammonia (NH3)

• Quickly converted to urea

  Different R groups from amino acids are processed differently. Fragments enter respiratory pathways at many different points

Anabolism

All metabolic compounds are part of the metabolic pool. Intermediates from respiratory pathways can be used for ____

Synthetic reactions of metabolism: Carbohydrates

• Start with acetyl - 6A

• Basically reverses glycolysis(but different pathway)

Fats

• G3P converted to glycerol

• Acetyl groups are connected in pairs to form fatty acids

Anabolism protiens

They are made of combinations of 20 different amino acids. Some amino acids (11) can be synthesized by adult humans. However, other amino acids (9) cannot be synthesized by humans

• Essential amino acids

• Must be present in the diet

The energy of organelles revisited

Similarities between photosynthesis and cellular respiration: use of membrane

• Chloroplasts inner membrane forms thylakoids

• Mitochondria’s inner membrane form cristae

Enzymes

• In chloroplast, stroma has calvin cycle enzymes

• In mitochondria, matrix contains enzymes of citric acid