ATP and Cellular Respiration

• Individuals require food as a source of energy for all cells.
• Cells require energy for all activities except diffusion and osmosis.
• They produce energy in the form of adenosine triphosphate (ATP).

ATP Composition

• Building blocks for making ATP: adenosine monophosphate (AMP) and phosphate groups
• These building blocks come from food sources.
• AMP is a nucleotide derived from eating RNA.
• Phosphates are plentiful in a variety of foods.
• Cells can store adenosine diphosphate (ADP) for some time because it is relatively stable.
• Energy is needed to attach the third phosphate to ADP to make ATP.
• The third phosphate is attached to ADP with a high-energy bond.

Cellular Respiration

• Cellular respiration provides the energy to attach the third phosphate onto ADP.
• During cellular respiration, glucose (most often) is broken down into carbon dioxide and water in the presence of oxygen.
• Some of the energy is lost as heat energy, which warms the body but cannot be used by cells for work.
• Enough energy is released from glucose breakdown to make 36 ATP molecules used wherever work needs to be done in the cell.
• The enzyme ATPase breaks the high-energy bond holding the third phosphate of ATP, releasing energy to do work.
• ATP molecules produced during cellular respiration power all cells to stay alive, maintain homeostasis, repair, synthesize proteins, perform active transport, and undergo mitosis.
• They also enable each differentiated cell to perform its unique tasks.

Stages of Cellular Respiration

• Glycolysis: Glucose is broken down into two 3-carbon molecules: pyruvic acid or pyruvate.
• Glycolysis is a multistep process in the cytoplasm, does not require oxygen (anaerobic), and results in a net gain of only 2 ATPs.
• 4 of the 12 hydrogens from glucose are removed.
• Krebs Cycle (KC): 2 pyruvic acids and 4 hydrogens undergo further processing in the mitochondria, in the central matrix.
• The first 3-carbon pyruvic acid is converted to a 2-carbon molecule called acetyl coenzyme A, forming one CO2 molecule and removing one hydrogen.
• Acetyl-coA breaks down in a multistep process making 2 extra CO2 molecules and removing 3 more hydrogens (total of 3 CO2 molecules and 4 hydrogens removed from the first pyruvic acid).
• The second pyruvic acid is processed the same way, producing 3 more CO2 molecules and 4 more hydrogens.
• 8 hydrogens removed during the Krebs cycle, plus 4 from glycolysis, results in all 12 hydrogens from the original glucose being transported to the cristae of the mitochondria where the ETC takes place.
• Most hydrogens are brought to the cristae by hydrogen acceptors/carriers NADH and FADH2, yielding a net gain of only 2 ATPs during the Krebs cycle.
• Electron Transport Chain (ETC): Source of the remaining 32 ATPs produced during CR, along the inner membranes of the mitochondria called cristae.
• Each of the 12 hydrogens (with their electrons) are brought to the ETC by NADH and FADH2.
• As they pass along acceptors/carriers (cytochromes) on the cristae, energy is released with each transfer from one acceptor to the next and used to make 32 ATPs via chemiosmosis.
• An acceptor (carrier) that takes a hydrogen (and its electron) becomes reduced, and when it passes it to the next acceptor, it becomes oxidized.
• The entire process of passing 12 hydrogens (and their electrons) down the ETC is called an oxidation-reduction or "redox" chain.
• The final hydrogen acceptor along the ETC is oxygen.
• Each oxygen takes 2 hydrogens at a time to produce a water molecule (H2O), so 6 oxygens are needed to take 12 hydrogens, forming 6 H2O molecules as products of the ETC.

Absorptive State

• Cellular respiration must occur every second in a cell's life to provide ATP for survival, whether in the absorptive state (food being absorbed during mealtime with a surplus) or the post-absorptive state (resources are short, glucose levels dropping).
• It lasts for 4 hours during/after mealtime when glucose levels are high.
• Insulin primarily regulates the body's responses.

Carbohydrates in Absorptive State

• Glucose molecules are absorbed into the superior mesenteric capillaries and delivered to all cells for CR and carbohydrate synthesis.
• The liver can convert any monosaccharide to glucose.
• Excess glucose molecules are delivered to the liver and skeletal muscles for conversion to glycogen (glycogenesis).
• Still excess glucose molecules are delivered to adipose tissues for conversion to fat (lipogenesis).

Lipids in Absorptive State

• Glycerol and fatty acids are absorbed into the cells of the intestinal mucosa.
• There, they are recombined into triglycerides, coated with protein to become water-soluble chylomicrons.
• Chylomicrons are absorbed into the lacteals and travel through the lymphatic system to the blood.
• Some chylomicrons remain in the blood, while others are delivered to adipose tissue and converted to fat (lipogenesis).

Proteins in Absorptive State

• Amino acids are absorbed into the superior mesenteric capillaries and delivered to all cells for protein synthesis.
• Excess amino acids are delivered to the liver, where they are deaminated (amino group removed).
• The remaining amino acid molecule is converted to fat (lipogenesis).
• After deamination, ammonia (NH3) is formed, which is toxic.
• The liver converts most ammonia to urea before releasing both into the blood for removal by the kidney.

Nucleic Acids in Absorptive State

• Nucleotides from RNA and DNA are absorbed into the superior mesenteric capillaries and delivered to all cells for RNA/DNA synthesis and repair (and ATP).

Post-Absorptive State

• Occurs after 4 hours after mealtime when glucose shortages begin.
• The goal is to raise glucose levels so that CR continues and to provide substitutes for glucose in CR.
• Since CR continuation is critical for survival, hormones from numerous glands help the body respond: glucagon (pancreas), glucocorticoids like cortisol (adrenal cortex), growth hormone (anterior pituitary gland), thyroxine or T4 and triiodothyronine or T3 (thyroid gland), and epinephrine (adrenal medulla).

Carbohydrates in Post-Absorptive State

• Glycogen in the liver and skeletal muscles is broken down to glucose and released into the blood for all cells to use.
• This process is referred to as glycogenolysis.

Mobilization of Fats

• Adipose cells break down fats into glycerol and fatty acids, releasing them into the blood.
• Chylomicrons are broken down by plasma enzymes into glycerol and fatty acids.
• Some glycerol is converted to glucose by the liver (gluconeogenesis) and released for all cells to use in CR.
• Some glycerol can be absorbed directly by cells and converted to pyruvic acid for CR.
• Fatty acids are converted to ketones (keto-acids) by the liver and released.
• Most cells absorb ketones and convert them to acetyl-coA for CR.
• The process of converting fatty acids to acetyl-coA is called beta-oxidation.

Proteins in Post-Absorptive State

• Amino acids are deaminated by the liver to form NH3 and urea.
• The remainder of each amino acid molecule is converted to glucose (gluconeogenesis) for all cells to use in CR.

Vitamins and Minerals

• Vitamins and minerals provided in food play important roles in metabolic reactions.

Fat-Soluble Vitamins

• A: Maintains epithelia and is needed for the synthesis of visual pigments.
• D: Needed for normal bone growth via calcium and phosphorus absorption.
• E: Antioxidant.
• K: Used for the synthesis of clotting proteins.

Water-Soluble Vitamins

• B1 (thiamine): Coenzyme leading to the Krebs cycle.
• B2 (riboflavin): Part of FAD.
• Niacin: Part of NAD.
• B6 (pyridoxine): Coenzyme in amino acid, lipid metabolism, protein synthesis, antibody and nucleic acid synthesis.
• Folacin (folic acid): Coenzyme in amino acid, nucleic acid synthesis, metabolism, and blood cell production.
• B12 (cobalamin): Coenzyme in nucleic acid metabolism, carbohydrate and fat metabolism, and RBC production.
• Biotin: Coenzyme leading to the Krebs cycle, amino acid and fatty acid metabolism, and nucleic acid synthesis.
• Pantothenic acid: Part of acetyl-coA.
• C (ascorbic acid): Coenzyme for amino acid metabolism, collagen production, promotes Fe absorption, synthesis of steroid hormones from cholesterol, and is an antioxidant.

Minerals

• Na (sodium) and K (potassium): Nerve and muscle function.
• Cl (chlorine): pH and water balance.
• Ca (calcium): Nerve, muscle, hemostasis, and skeletal function.
• P (phosphorus): ATP, creatine phosphate, nucleotides, and skeletal functions.
• Mg (magnesium): Membrane functions, cofactor for enzymes, and bone formation.
• S (sulfur): Component of hormones, vitamins, and many proteins.
• Fe (iron): Hemoglobin, myoglobin, and cytochromes.
• Zn (zinc): Cofactor for more than 70 enzymes, like carbonic anhydrase, and contributes to growth and taste.
• Cu (copper): Hemoglobin synthesis, cytochromes, and melanin.
• Mn (manganese): Cofactor for enzymes, hemoglobin synthesis, urea formation, and activation of ATPase.
• F (fluorine): Bones and teeth.
• Co (cobalt): Part of vitamin B12, erythropoiesis.

Basal Metabolic Rate (BMR)

• BMR is the energy required to maintain the body's functions per unit of time.
• Measure under basal conditions: awake, at rest, room temperature, no exercise for 2-3 hours, and no food for 12 hours.
• Oxygen consumption/min. measures how much CR occurs per minute under basal conditions.
• Unit of energy used in BMR: Calorie (vs. calorie).
• When 1 liter of O2 is breathed to oxidize an average mixture of food (carbohydrates, lipids, and proteins), 4.825 Calories are consumed.
• Under basal conditions, ~14L of O2 are breathed/hr. and ~70 Calories/hr. are consumed (14L x ~5 Calories).
• To maintain the body for a day (24 hrs.), ~1680 Calories (24 hrs. x 70 Calories) are needed to provide enough energy to carry out basic functions (basal conditions).
• Temperature Regulation: Body heat from cellular respiration.
• To raise body temperature when it's too low: Shiver and constrict superficial blood vessels.
• To lower body temperature when it's too high: Sweat and dilate superficial blood vessels.