Cell Metabolism

Food undergoes a transformative journey within our bodies, necessitating both mechanical and chemical processes to translate it absorbable for energy utilization. This intricate process centers on specialized enzymes orchestrating chemical digestion.  In this laboratory exploration, you will delve into the enzyme-driven hydrolysis of proteins, lipids, and carbohydrates, unraveling how our bodies extract energy from these essential biomolecules.

To comprehend this biological marvel, let’s first establish some foundational vocabulary:

• Digestion: The intricate process of breaking down food into absorbable nutrients.
• Digestive enzymes: Specialized proteins synthesized within specific cells, deployed into the digestive tract to expedite the extracellular breakdown of biological nutrients, including proteins, carbohydrates, and lipids.
• Mechanical digestion: The physical manipulation of food, encompassing actions like chewing, chyme mixing, and peristalsis.

Metabolism, the aggregate of all chemical reactions in the human body, can be classified into two categories: catabolism and anabolism. Catabolism entails breaking down large molecules into smaller ones, whereas anabolism involves synthesizing larger molecules from smaller components.

The absorption of nutrients from ingested food mandates prior digestion, in which food is broken down into easily absorbable units.  This process encompasses both mechanical actions, such as chewing and peristalsis, and chemical processes catalyzed by digestive enzymes.

Once synthesized within specialized cells, digestive enzymes are dispatched into the digestive tract to accelerate the extracellular breakdown of biological nutrients present in consumed food and fluids.  The resultant products of chemical digestion are subsequently absorbed into the bloodstream or lymphatic vessels of the small intestine, guided to bodily cells, and utilized in intracellular metabolism. Familiar metabolic processes include aerobic respiration, glycolysis, lipolysis, and protein synthesis.

The Small Intestine: The absorptive surface of the small intestine. (Figure by OpenStax is used under a Creative Commons Attribution license.)

Extracellular digestion relies on enzyme-catalyzed hydrolysis, wherein hydrolytic enzymes, or hydrolases, catalyze the addition of water to facilitate the breakdown of larger food molecules into their constituent monomers.

Proteins        +       H20     +      enzyme         ⟶           amino acids       +          enzyme

Fats   +   H20     +    enzyme ⟶    fatty acids   +      glycerol (or monoglyceride)      +       enzyme

Carbohydrates      +    H20     +   enzyme       ⟶         monosaccharides        +            enzyme

Each of these reactions within the body necessitates the presence and activity of specific hydrolytic enzymes. Notably, enzymes remain unchanged throughout the reaction, present on both the substrate and product sides of the equation. Factors influencing enzyme activity include pH, temperature, enzyme and substrate concentrations, and cofactors.

Temperature and pH fluctuations are meticulously regulated within narrow ranges in the body to maintain optimal conditions for enzymatic activity.  These organic molecules play a pivotal role in the body’s metabolic state.  During the absorptive or fed state, glucose is the primary energy source, with minimal utilization of absorbed fats and amino acids.  In contrast, the postabsorptive or fasting state prompts the body to rely on its energy reserves to fulfill its energy demands.

Proteins

Proteins, also known as polypeptides, consist of amino acids – monomers covalently bonded together by peptide bonds.  These amino acids share a common chemical backbone but differ in their R groups, or side chains, which determine their classification as polar, charged, or nonpolar.  While all proteins exhibit hydrophilic properties (affinity for water), the degree of hydrophilicity varies.  Some proteins feature a higher proportion of nonpolar side chains, rendering them less hydrophilic compared to others.

Amino Acid Structure. (Figure by OpenStax is used under a Creative Commons Attribution license.)

Proteins possess intricate structures, transitioning from a primary to secondary, tertiary, and often quaternary structure to fulfill specific functions.  Subsequent enzymes maintain their optimal temperature is crucial for preserving their final structure.

Protein Structure: Primary, Secondary, Tertiary, and Quaternary Structure. (Figure by OpenStax is used under a Creative Commons Attribution license.)

The process of protein digestion commences in the stomach with the enzyme pepsin.  Typically, inactive when the stomach is empty, pepsin activates upon food entry.  Produced by the chief cells of the stomach, pepsin catalyzes the hydrolysis of proteins into smaller peptides, consisting of short strands of amino acids.  Pepsin operates most effectively in the acidic environment of the stomach, with an optimal pH of 2. However, it denatures as chyme, partially digested food, transitions into the small intestine, where the pH rises to around 9.

Continuing the hydrolysis of proteins into peptides, the duodenum of the small intestine hosts pancreatic enzymes such as trypsin, chymotrypsin, and carboxypeptidase.  These enzymes further break down peptides into amino acids.  Aminopeptidases and dipeptidases, enzymes produced in the brush border of the small intestine, play vital roles in this process by further degrading peptides into individual amino acids.

Following digestion, amino acids are absorbed into the capillaries of the small intestine and transported via the bloodstream to the liver through the hepatic portal vein.

Protein Metabolism

The breakdown process of proteins into amino acids, known as catabolism, is termed proteolysis, while the synthesis of proteins, termed anabolism, is referred to as proteogenesis.

Carbohydrates

Carbohydrates, also known as polysaccharides, consist of polymers of simple sugars, or monomers, linked together by glycosidic bonds.  Glucose, often spotlighted in physiology courses, stands out as the primary substrate for aerobic respiration and is also involved in anaerobic respiration.  While most carbohydrates contribute to energy metabolism, some serve in specialized cell coatings.

Monosaccharides: The five monosaccharides important in the human body. (Figure by OpenStax is used under a Creative Commons Attribution license.)

While their abundant hydroxyl, ketone, and aldehyde side groups, sugars render all carbohydrates hydrophilic.

The breakdown of carbohydrates, particularly starch, initiates with the enzyme amylase.  Salivary amylase originates from the salivary glands, while pancreatic amylase is produced in the pancreas.  Consequently, carbohydrate digestion occurs both in the mouth and in the duodenum. Once carbohydrates are broken down into simple sugars, these sugars become absorbable.  Thus, absorption of these simple sugars takes place in both the mouth and duodenum. It’s essential to note that carbohydrates are not inherently “bad”; rather, like anything else, excessive consumption can have adverse effects.  Nonetheless, this biomolecule plays a crucial role in maintaining our body’s homeostasis.

Polysaccharides: Three important polysaccharides. (Figure by OpenStax is used under a Creative Commons Attribution license.)

Amylose, a component of starch, serves as the substrate for the enzyme amylase, catalyzing its hydrolysis into maltose.  Maltose, a disaccharide, undergoes further breakdown into two glucose units, or monosaccharides, facilitated by the enzyme maltase, predominantly found in the brush border of the small intestine.  These monosaccharides then enter the bloodstream via the hepatic portal vein, eventually reaching the liver for metabolism.  Within the liver, glucose can be converted into ATP for cellular energy or stored as glycogen, a polysaccharide.

Carbohydrate Metabolism

The catabolic process of glucose to produce ATP is termed glycolysis, while the anabolic formation of glycogen from glucose is known as glycogenesis.  Gluconeogenesis involves the synthesis of new glucose molecules from alternative sources, such as proteins or lipids.  Conversely, glycogenolysis breaks down glycogen into glucose, while glycogenesis synthesizes glycogen.

Pancreatic hormones, including insulin and glucagon, play pivotal roles in regulating fuel metabolism.  Insulin facilitates glucose transport into cells, stimulates glycogenesis, and inhibits glycogenolysis and gluconeogenesis, thereby lowering blood glucose levels by promoting cellular glucose uptake.  Conversely, glucagon promotes glucose production, inhibits glycogenesis, and stimulates glycogenolysis and gluconeogenesis, elevating blood glucose concentrations.

Lipids

Lipids consist of fats and fatty acids, serving as their monomers, bonded together via simple covalent bonds, often in the form of ester bonds.  Notably, this group is predominantly hydrophobic, distinguishing it from other biomolecules.  Within human physiology, steroids and eicosanoids stand out, playing significant roles in cell signaling.  Moreover, they are frequently discussed in the context of cell membranes and lipid storage, particularly as triglycerides within adipocytes.

Triglycerides: These are composed of glycerol attached to three fatty acids. (Figure by OpenStax is used under a Creative Commons Attribution license.)

Because lipids are insoluble in water, they require emulsification before enzymes can efficiently break them down.  During emulsification, bile salts play a crucial role by breaking large lipid molecules into triglycerides (tiny oil droplets).  Produced by the liver, stored in the gallbladder, and released into the small intestine as needed, bile salts enable the formation of small oil droplets called micelles.  Micelles provide increased surface area for enzymes to act upon, facilitating the absorption of breakdown products of large lipids, such as small fats or fatty acids, as well as hydrophobic vitamins like A, D, E, and K.

In this week’s experiment, you will observe the emulsification and digestion of lipids, represented by olive oil, using soap, which act similarly to bile salts.  Lipase, produced in the pancreas catalyze the hydrolysis reaction as follows:

Lipids Metabolism

The resulting glycerol and fatty acids are absorbed into the epithelial cells of the small intestine, where they are packaged into specialized lipoproteins called chylomicrons.  These chylomicrons are then absorbed into lacteals, or lymphatic vessels, of the small intestine and transported to the liver for intracellular metabolism. Chylomicrons, are often described as bubbles of lipids encased in a protein coat, serve as vehicles for transporting dietary lipids throughout the body following absorption in the intestine.  These lipid-rich particles are capable of being transported in the bloodstream, facilitating the distribution of essential fats to various tissues.  Upon formation, the majority of chylomicrons are directed towards adipose cells for storage, where they contribute to the body’s long-term energy reserves.  Additionally, a significant portion of chylomicrons are transported to muscles cells, where they are utilized as an immediate energy source during physical activity and metabolic processes.

In summary the body maintains blood glucose levels within a narrow range, typically between 70 to 100 mg/dL, to ensure adequate fuel supply for the brain, a critical homeostatic requirement.  During the absorptive state, also known as the fed state, which encompasses the period during an up to 3-5 hour after eating, organs utilize glucose as needed for immediate energy demands and store excess glucose for future use.  When blood glucose levels reach the upper limit of this range, the pancreas releases insulin, signaling a shift from catabolic to anabolic reactions.  This hormonal response promotes glycogenesis, the storage of glucose as glycogen in muscle and liver tissues, and lipogenesis, the conversion of excess carbohydrates, proteins, or lipids into triglycerides stored in adipose tissue.  While insulin facilitates these storage processes, protein synthesis can occur independently of insulin regulation, addressing cellular needs as required.

During the postabsorptive state, which sets in roughly 8-12 hours after meal, the gastrointestinal tract empties, and organs begin to rely on stored energy reserves.  As blood glucose levels decline, the pancreas releases glucagon, prompting a metabolic shift from anabolic to catabolic processes.  This transition leads to glycogenolysis in the liver, where stored glycogen is broken down and released into the bloodstream, ensuring a steady supply of glucose to meet the brain’s energy demands.  Glycogenolysis also occurs in skeletal muscle, but the muscle retains the glucose for its own use.  Additionally, lipolysis occurs in adipose tissue, releasing fatty acids for energy production.  When glycogen stores are depleted and blood glucose levels continue to drop, proteins can be broken down into amino acids.  These amino acids serve as substrates for gluconeogenesis, the synthesis of new glucose, but this process is initiated only when sugar and fat stores are low.

In this lab, students are tasked with correlating the biochemical phenomena that take place in different tissues during both fasting and fed states using posters provided in the lab.  They are expected to indicate whether each biochemical process increases or decreases during these states.  Group collaboration is encouraged to facilitate the completion of the posters.

Digestion

Food enters the oral cavity, or mouth, where the process of mastication, or chewing, begins.  Through the coordinated action of teeth and tongue movements, food is thoroughly mixed with saliva, forming a cohesive mass known as a bolus. This bolus, rich is saliva that aids in lubrication and contains enzymes, is then propelled by the tongue into the pharynx, where the pathways for food and air diverge.  From there, it travels down the esophagus via peristalsis, a rhythmic wave of contraction and relaxation of muscles, ultimately reaching the stomach.

Peristalsis: A process that moves food through the digestive tract by alternating contractions and relaxations of the muscles, creating wave movements. (Figure by OpenStax is used under a Creative Commons Attribution license.)

Peristalsis, crucial for the movement of a bolus along the esophagus, is governed by various factors.  These include the strength and duration of smooth muscle contractions in the gastrointestinal (GI) tract walls, the signaling mechanisms between the GI tract and smooth muscle, the composition of the chyme (partially digested food), and the level of hydration.  These factors collectively influence the rate at which peristalsis occurs, ensuing efficient digestion and transit of food through the digestive system.

The stomach performs the crucial task of pushing and thoroughly mixing food, transforming it into chyme – a highly acidic mixture.  This chyme then enters the small intestine, specifically the duodenum, where nutrient absorption occurs.  Subsequently, it travels through the large intestine, where water absorption takes place, before reaching the rectum and ultimately exiting through the anus, which is composed of both voluntary and involuntary muscles.  The voluntary muscles of the anus afford control over bowel movements, enabling individuals to regular when they need to use the restroom.

The Digestive Processes. (Figure by OpenStax is used under a Creative Commons Attribution license.)

Throughout this digestive journey, regulatory mechanisms within the gastrointestinal system work tirelessly to maximize the efficiency of digestion and absorption, ensuing that essential nutrients are properly absorbed while waste products are efficiently eliminated.