DAT Cellular Respiration

Cellular Respiration

Glucose → ATP

1. Glycolysis

2. Pyruvate decarboxylation

3. TCA (Krebs cycle)

4. Electron transport chain

Glycolysis

• Break down of glucose

• Anaerobic (no O2)

• 1 glucose mol. (6 Carbons) → 2 pyruvate mols. (3 Carbons)

  • 2 ATP input → 4 ATP output & 2 NADH = 2 net ATP mols. & 2 NADH

• Occurs in cytosol (not in organelle)

• Steps:

  1. Hexokinase: phosphorylates glucose → Glucose-6-phosphate → irreversible rxn

  2. Phosphofructokinase (PFK): phosphorylates Glucose-6-phosphate → Fructose-1,6-bisphosphate → rate limiting step

Most oxidized form of Carbon

• CO2

• Waste product of cellular respiration (occurs via oxidation)

Oxidation rxns

• ADP + Pi → ATP (oxidized)

• NAD+ + FAD+ → FADH2 + NADH (oxidized)

Mitochondria

• Double layered

• Outer membrane

• Intermembrane space: H+ build up

• Matrix:

  • Krebs cycle → produces ATP

  • β-oxidation to break down fatty acids

• Inner membrane: many folds to ↑ surface area → ↑ electron transport chain output

Pyruvate decarboxylation

• Occurs in mitochondrial matrix

• Aerobic process

• 2 pyruvate molecules from glycolysis transported into matrix via secondary active transport using protons (doesn’t directly use ATP)

• 1 Pyruvate + Coenzyme A → Acetyl CoA + 1 NADH + 1 CO2

  • Pyruvate decarboxylate complex (PDC) catalyzes rxn

How many CO2 and NADH yield from the breakdown of 1 glucose?

2 CO2 + 2 NADH

TCA (Krebs cycle) / Citric Acid Cycle

• Occurs in mitochondrial matrix

• Aerobic process

• 1 Acetal CoA + oxaloacetate → citrate

  • Citrate further oxidizes until oxaloacetate is formed and the cycle repeats

• Full cycle yields: 3 NADH + 1 FADH2 + 1 GTP (ATP) + 2 CO2

Electron transport chain (ETC)

• Occurs in inner membrane / cristae of mitochondria

• Aerobic process

• Removes e- from glucose, pyruvate and Acetyl CoA

• Oxidative phosphorylation occurs here

• Carrier proteins (I, II, III, IV) in inner membrane (electron acceptors): receive electrons from electron transporters (NADH, FADH2) → pump protons against [ ] gradient into intermembrane space to supply energy to ATP synthase

  • Highly acidic environment in intermembrane space

  • CoQ (Ubiquinon): can be fully oxidized and reduced during passing of electrons b/w protein complexes

    • Soluble carrier

  • Cyt C (Cytochrome C): bound to Iron atom which transfers electrons b/w Complex III and Complex IV for redox rxns

    • Protein carrier

    • Used for genetic relations

• Final electron acceptor (after electrons have passed though all proteins): Oxygen → combines w/ H+ to form H2O

• ATP Synthase: drives protons down the gradient towards matrix (high [ ] → low [ ]) to catalyze ADP + Pi → ATP

  • pH + Electrical Gradient: Proton Motive Force

  • If pH of intermembrane space is higher than normal → less H+ → less cellular respiration occurring

• NADH creates more ATP (3x) than FADH2 (2x)

  • NADH pumps more protons to carrier proteins than FADH2 because NADH enters the protein complex earlier than FADH2 and it enters Complex I (FADH2 enters Complex II)

• Total glucose produced = 36 ATP in eukaryotes and 38 in prokaryotes (no mitochondria so don’t need to pump NADH into matrix → saving 2 ATP during glycolysis)

Oxidative phosphorylation

Process of ADP → ATP from NADH and FADH2 via passing of e- through various carrier proteins in the electron transport chain

Electron carriers

NADH and FADH2

What are the products of 1 Glucose molecule that has __**only**__ undergone the Krebs cycle?

1 glucose → 2 pyruvate → 2 acetyl CoA → 6 NADH + 2 FADH2 + 2 GTP + 4 CO2

Fermentation

• Occurs when [O2] is too low to carry out aerobic processes in mitochondria

• NAD+ formation prioritized to form NADH

• Alcohol fermentation

• Lactic Acid fermentation

Alcohol fermentation

• Fungi (yeast), bacteria, plants

• Reduces pyruvate (from glycolysis) → acetaldehyde + CO2 → ethanol (by product) in a process that oxidizes NADH → NAD+

• Acetaldehyde is the final e- acceptor from NADH

Lactic Acid Fermentation

• Occurs in muscle cells

• Use glycolysis to produce 2 pyruvate mols.

• Pyruvate reduced to lactate (by-product) → oxidizes NADH → NAD+

  • Lactate (weak base) → Lactic acid (strong acid)

• Cori Cycle: lactate from muscle cells transported into blood stream → liver → converted to glucose → blood stream → used to generate ATP through glycolysis

Catabolic rxns

Releases energy by breaking down large molecules into smaller molecules

Anabolic rxns

Requires energy to build molecules from smaller molecules

Cellular metabolism

Anabolic & Catabolic rxns

What happens if a cell does not have glucose?

1. Uses other carbohydrates

2. Lipids

3. Proteins (last resort)

Carbohydrates as a source of energy

• Glycogen (polysacc.): storage for glucose

  • found mainly in muscle and liver cells

• Glycogenesis: formation of glycogen (glucose → glycogen)

• Glycogenolysis: break down of glycogen (glycogen→glucose)

• Glucose-6-phosphate main molecule for rxn

• Regulated by Insulin and glucagon

• First broken down in mouth → stomach → duodenum → small intestine

• Disaccharides hydrolyzed into monosaccharides → converted to glucose or glycolytic intermediates

• All cells can store glycogen but only skeletal muscle and liver cells can store large amounts

Gluconeogenesis

• Forming glucose from non-carbs

• Occurs in liver and kidney

Why is phosphate added to a glucose molecule?

To keep the molecule w/n the cell and prevent it from diffusing out of the cell

Insulin

• Endocrine hormone

• Released by pancreas when glucose ↑

• Triggers cells to:

  • make glycogen from glucose for storage

  • undergo glycolysis to form ATP → activates Phosphofructokinase (R.D.S)

Glucagon

• Released by pancreas when glucose ↓

• Similar to epinephrine (triggers formation of glucose)

• Triggers cells to:

  • Glycogenolysis to form glucose

  • Inhibit glycogenesis to inhibit glycogen production

Lipids as a source of energy

• Long hydrocarbon chains that are highly reduced → have more energy than carbs

• Triglycerides: 1 glycerol backbone bound to 3 fatty acid chains

• Lipases in adipose tissue are hormone sensitive (e.g., to glucagon)

• Lipolysis: break down of lipids into glycerol and the fatty acids by lipase enzymes

  • Glycerol → Glyceraldehyde 3 Phosphate (DAP/G3P/PGAL) → enters glycolysis

  • Fatty acid chains activated by 2 ATP → β-oxidation of saturated FA in mitochondrial matrix (breaks 2 carbons at the β position) → 1 NADH + 1 FADH2 and 1 Acetyl CoA → citrate in Krebs cycle → 120 ATP generated

  • β-oxidation of unsaturated FA → 1 less FADH2 for each double bond

• Lipids combine w/ soluble proteins → lipoproteins (contain Apoproteins)

  • Classified by density (fat : protein ratio)

  • B/w meals, most lipids in plasma are in the form of lipoproteins

• Lipoproteins large and less dense when ratio is also large

  • Chylomicrons: first fat transporters to leave enterocyte and enter lacteals (small lymphatic vessels)

  • LDL (low protein density): unhealthy due to high fat content

  • HDL (high protein density): healthy cuz transport fat away from tissues → liver → cholesterol for bile → expelled during digestion

What carries fatty acids in blood?

Albumin

Lipid digestion

• Stored as adipose tissue

• Only broken down in duodenum:

1. Bile released from gall bladder to emulsify fats & pancreatic lipase to break down lipids into FA chains and monoacylglycerides

2. Absorbed into enterocytes of small intestine

3. Reassembled into triglycerides, and then, along with cholesterol, proteins or phospholipids → packaged into chylomicrons

4. Chylomicrons move to lymph capillary → circulatory system

Proteins as a source of energy

• Excess amino acids used for energy

• Oxidative deamination: removal of amino group to form metabolic intermediates (Acetyl CoA, pyruvate, Oxaloacetate)

  • Directly removes ammonia from AA

  • Deamination in liver

  • By-product: NH3 (ammonia) → urea → excreted as urine in mammals

    • Uric acid in insects, birds, reptiles

• Digestion occurs in stomach: pepsin breaks down proteins into polypeptides

• In the small intestine: trypsin breaks down specific polypeptides into amino acids