Metabolic Processes and Cell Communication

Chemical Reactions and Metabolism

• Metabolism is the chemical processes allowing cells to function.
• Key reactions are concerned with extracting energy from food.
• Two types of metabolism exist: anabolism (synthesis) and catabolism (breakdown).

Carbohydrate Metabolism and Formation of ATP (Chapter 69)

• After absorption, fructose and galactose are converted to glucose in the liver.
• Glucose is the primary pathway for carbohydrate uptake.

Transport of Glucose Through Cell Membrane

• Glucose cannot diffuse through cell membrane pores.
• Active transport or facilitated diffusion is needed.
• Insulin plays a critical role in this transport.

Phosphorylation

• Glucose combines with a phosphate radical to become glucose-6-phosphate.
• This form of glucose can be used immediately for cellular energy or stored.

Glycogenesis

• Glycogenesis is the process of forming glycogen, a storage form of glucose.

Glycogenolysis

• Glycogenolysis is the breakdown of glycogen to glucose in the liver.
• This makes glucose available for use.

Release of Energy from Glucose

• Glucose undergoes glycolysis, releasing energy.
• The energy is used to form ATP.

Glycolytic Pathway

• A single gram-mole of glucose releases 686,000 calories.
• Only 12,000 calories are required to form 1 gram-mole of ATP
• Enzymes help break down glucose stepwise to maximize energy extraction.
• The 38 ATP molecules are formed through various steps.

Glycolysis

• Glycolysis is the primary energy-releasing process of glucose.
• 10 steps are involved in converting glucose into pyruvic acid
• This produces ATP in the cytoplasm.

Glycolysis (2)

• Net gain from one glucose molecule is 2 pyruvic acid+2ATP molecules+ 4 H. The efficiency of ATP formation is 43%.
• Pyruvic acid combine with Coenzyme A to form Acetyl CoA.

Citric Acid Cycle (Krebs Cycle)

• The citric acid cycle occurs in the mitochondrial matrix.
• Acetyl CoA combines with oxaloacetic acid to form citric acid.
• The process continuously produces oxaloacetic acid.
• 24 hydrogen atoms are released per glucose molecule.

Citric Acid Cycle (2)

• 20 of the H atoms bond with NAD+ via dehydrogenase
• Water and carbon dioxide are released.
• 2 ATP molecules are formed

Net Reaction per Molecule of Glucose in Citric Acid Cycle

• 2 Acetyl CoA + 6 H2O + 2 ADP → 4 CO2 + 16 H+ + 2 CoA + 2 ATP

Oxidative Phosphorylation

• Oxygen is essential for this process in the mitochondria.
• NADH is split - energy captured creating a proton gradient
• This is used to form ATP.
• Electron Transport Chain + Chemiosmosis = Oxidative Phosphorylation

Summary of ATP Formation

• 38 ATP are formed per glucose molecule - 66% efficient
• Feedback controls regulate ATP production

Anaerobic Glycolysis

• Anaerobic glycolysis yields pyruvic acid + NADH + H+ → lactic acid
• Used to evaluate cell oxygen consumption
• Lactic acid can be converted back into glucose when oxygen is available.

Storage of Glucose

• Glycogen stores are utilized first, and excess is stored as fat.

Gluconeogenesis

• When glucose is insufficient, it's synthesized from fats (glycerol) and proteins (amino acids).
• This process is triggered by low blood glucose levels.
• Cortisol plays a crucial role in initiating gluconeogenesis

Lipid Metabolism (Chapter 69)

• Lipids are the most efficient cellular energy storage form.
• Lipids include triglycerides (neutral fat), phospholipids, and cholesterol.

Fatty Acids

• Fatty acids are long-chain organic hydrocarbon molecules with a carboxylic group (-COOH).
• Palmitic acid is an example of a fatty acid.

Triglycerides

• Triglycerides consist of three fatty acids bound to a glycerol molecule.
• Tristearin is an example of a triglyceride.

Absorption of Fats

• Dietary triglycerides break down into monoglycerides and fatty acids in the intestines.
• Chylomicrons are formed and absorbed into the lymphatic system.
• Plasma turbidity increases after a high-fat meal.

Uptake Into Cells

• Lipoprotein lipase breaks down triglycerides into fatty acids and glycerol in capillaries.
• Fatty acids then diffuse into cells for use or storage as triglycerides

Transport Through Body

• When lipids are needed, fat cells break down triglycerides into fatty acids and glycerol.
• These molecules combine with albumin to become free fatty acids for transport.

Fat Deposits

• Adipose tissue stores fat deposits.
• Fat cells contain high percentages of triglycerides, mostly for long term energy stores.

Use of Triglycerides for Energy

• Triglycerides contribute significantly to the energy needs of the body.
• Glycerol is converted to glycerol-3-phosphate to enter the glycolytic pathway for glucose breakdown.
• Fatty acids are oxidized in the mitochondria.

Beta-Oxidation

• Fatty acids are gradually converted into Acetyl-CoA through beta-oxidation.
• Acetyl-CoA enters the citric acid cycle for further energy extraction.
• Tremendous amounts (146) of ATP molecules are formed from the complete oxidation of a single molecule of stearic acid.

Ketosis

• When carbohydrates aren't available, fats are oxidized to supply energy.
• These metabolites (ketone bodies) include B-hydroxybutyric acid, acetoacetic acid, and acetone.
• High levels of ketone bodies may result in ketosis.
• Starvation, diabetes mellitus, and high-fat/low-carb diets can trigger ketosis

Regulation of Fat Utilization

• Epinephrine and norepinephrine enhance triglyceride lipase's activity, leading to the increased release of fat.
• Other factors like corticotropin and glucocorticoid release from pituitary and adrenal cortex also increase lipolysis.
• Insulin secretion decreases in tandem with rising fatty acid production

Phospholipids

• Phospholipids contain a fatty acid molecule, phosphoric acid radical, and nitrogenous base.

Uses of Phospholipids

• Essential components of cell membranes and lipoproteins.
• They are involved in blood clotting (thromboplastin).
• A key component of myelin sheaths around nerve cells

Uses of Cholesterol

• Important for bile acid production in the liver.
• Precursor for steroid hormones (estrogen, progesterone, and testosterone).
• A crucial component for building waterproof skin.

Atherosclerosis

• A disease where fatty lesions form on the walls of large arteries.
• Connective tissue builds around these lesions, making blood vessel walls stiff and narrow.
• Possible plaque rupture is a risk for blood clot formation.

Factors that Lead to Atherosclerosis

• Physical inactivity and obesity
• Diabetes mellitus
• Hypertension
• Hyperlipidemia
• Smoking

Prevention of Atherosclerosis

• A low-fat diet is critical.
• Avoiding smoking is key.
• Exercise is essential for overall health.
• Maintaining blood pressure and glucose levels is important.
• Oat bran can help bind bile acids in the gut.

Protein Metabolism (Chapter 70)

• Proteins constitute a significant portion of the body.
• Composed of 20 different amino acids linked by peptide bonds and hydrogen bonding.
• Proteins serve various functions (structure, enzymes, oxygen transport).

Regulatory Proteins

• Regulatory proteins transmit messages within cells.
• Often utilize G protein-coupled receptors and internal second messenger systems for signal transduction.

Transport of Amino Acids

• Protein digestion produces amino acids.
• These are absorbed by active transport or facilitated diffusion.
• Excess amino acids can be lost through urine.

Storage of Amino Acids

• Amino acids are immediately incorporated into new proteins.
• Released when blood levels are low.
• A balance between amino acids and proteins exist.

Major Plasma Proteins

• Albumin: crucial for colloid osmotic pressure in the blood.
• Globulins: encompass enzymes and proteins for the immune system.
• Fibrinogen: plays a vital role in blood coagulation.

Dietary Amino Acids

• 10 essential amino acids cannot be synthesized in the body.
• Dietary intake is essential.
• 10 nonessential amino acids can be synthesized, also important for protein synthesis.

Use of Proteins for Energy

• Protein breakdown starts in the liver via deamination.
• Amino group (-NH2) is removed—aminotransferases catalyze this.
• The resulting molecules are used to form urea.

Urea

• Urea is formed from ammonia to remove it from the blood.
• Ammonia is a neurotoxin.
• Urea is synthesized in the liver and excreted in the kidneys.

Oxidation of Deaminated Amino Acids

• Deaminated amino acid products enter the citric cycle.
• These undergo further oxidation to produce ATP.
• This process of turning amino acids into keto acids is called ketogenesis.

Obligatory Degradation of Proteins

• 20-30 g of protein is degraded and utilized, irrespective of dietary intake.
• Starvation can occur with insufficient protein intake.
• Carbohydrates can reduce the need to use protein as an energy source.

Hormonal Regulation of Protein Metabolism

• Hormones influence protein metabolism.
• Growth hormone encourages AA uptake and protein synthesis.
• Insulin promotes AA uptake.
• Glucocorticoids increase breakdown of proteins.
• Testosterone promotes muscle protein synthesis.
• Thyroxine regulates metabolic rates affecting either anabolism or catabolism.

Explore molecular changes in dehydration synthesis, ATP sources, and controlled energy release in cells. Learn about fructose and galactose conversion, glucose fate, and the role of insulin. Investigate glycogen formation, atherosclerosis prevention, G protein-coupled receptors, and glycogenolysis.