FLS Metabolism Lectures.pdf

# Structure and function of biological membranes Biological membranes are dynamic, fluid structures essential for cellular integrity and function, composed of a phospholipid bilayer interspersed with proteins, following the fluid mosaic model [1](#page=1). ### 1.1 The fluid mosaic model The fluid mosaic model describes the cell membrane as a flexible and dynamic entity rather than a rigid barrier, emphasizing the presence of membrane proteins that facilitate cell signaling, communication, and selective permeability [1](#page=1). ### 1.2 Phospholipids Phospholipids are the primary lipids in eukaryotic cell membranes, characterized by a hydrophilic head and hydrophobic tails, giving them an amphipathic nature. The hydrophilic heads face the extracellular and intracellular spaces, while the hydrophobic tails are oriented inward. Each phospholipid consists of two fatty acid chains, a glycerol molecule, and a phosphate group. The fatty acid chains are crucial in determining membrane fluidity [1](#page=1). #### 1.2.1 Types of phospholipids * **Phosphatidylethanolamine:** Possesses a small head, enabling tight packing in membrane regions requiring curvature [1](#page=1). * **Phosphatidylserine:** Carries a negative charge and is typically confined to the inner membrane surface; its externalization signals apoptosis [1](#page=1). * **Phosphatidylcholine:** Abundant in mammalian membranes and usually found on the outer surface, its cylindrical shape contributes to stable bilayer structures, and a cis double bond in its tail enhances fluidity [1](#page=1). * **Sphingomyelin:** Built on a sphingosine backbone with long, straight fatty acid tails that pack tightly with cholesterol, forming rigid lipid rafts vital for cell signaling [1](#page=1). ### 1.3 Lipid structures Lipids can form two primary structures: micelles and bilayers [2](#page=2). #### 1.3.1 Lipid micelles Lipids with a single fatty acid tail are cone-shaped and spontaneously assemble into spherical micelles to shield their hydrophobic tails from water [2](#page=2). #### 1.3.2 Lipid bilayers Membrane phospholipids, having two fatty acid tails, are cylindrical and arrange side-by-side to form a flat, two-layered sheet. These bilayers spontaneously close to create sealed compartments, resolving the energetic instability of exposed edges by curving to form a spherical structure where all hydrophobic tails are shielded [2](#page=2). ### 1.4 Fluidity of cell membranes Biological membranes exhibit fluidity, a property critical for cellular processes such as endocytosis, cell motility, equal distribution of membranes during cell division, and rapid diffusion of signaling molecules and proteins for cell communication. Membrane fluidity is influenced by its composition; in colder conditions, organisms synthesize shorter fatty acid chains with increased unsaturation to maintain fluidity by reducing inter-chain interactions [2](#page=2). #### 1.4.1 Measuring membrane fluidity Fluorescence Recovery After Photobleaching (FRAP) is a technique used to quantify membrane fluidity [2](#page=2). 1. **Labeling:** Lipids or proteins are labeled with a fluorescent dye, causing the entire membrane to glow evenly [2](#page=2). 2. **Photobleaching:** A strong laser beam bleaches the fluorescent dye in a small, specific area, rendering it non-fluorescent [2](#page=2). 3. **Recovery:** Over time, the fluid nature of the membrane allows unbleached molecules from surrounding areas to diffuse into the bleached spot, while bleached molecules diffuse out, leading to a recovery of fluorescence [3](#page=3). 4. **Results:** More fluid membranes exhibit faster fluorescence recovery due to quicker molecular movement compared to less fluid membranes [3](#page=3). ### 1.5 Role of cholesterol Cholesterol intercalates between phospholipids in the membrane, aligning its polar head with phospholipid heads and its non-polar structure with hydrophobic tails. Cholesterol modulates membrane fluidity through distinct mechanisms [3](#page=3): * **At body temperature:** Its rigid steroid rings immobilize adjacent fatty acid tails, increasing lipid packing, reducing membrane flexibility, and decreasing permeability [3](#page=3). * **At low temperatures:** Cholesterol acts as a spacer, preventing fatty acid tails from packing too tightly and solidifying [3](#page=3). ### 1.6 Lipid bilayer assembly The synthesis and assembly of lipid bilayers occur in the smooth endoplasmic reticulum (SER) [3](#page=3). 1. Fatty acids are synthesized in the cytoplasm and delivered to the SER [3](#page=3). 2. Fatty acids embed into the outer leaflet of the SER membrane [3](#page=3). 3. Enzymes attach glycerol, phosphate, and a head group to form phospholipids [3](#page=3). 4. Initially, only the outer half of the membrane grows, leading to instability [3](#page=3). 5. The enzyme Scramblase catalyzes the movement of new lipids from the outer to the inner leaflet, ensuring an equal distribution of lipids across both sides without requiring ATP [3](#page=3). 6. Once balanced, membrane fragments bud off as vesicles and are transported to the cell surface, where asymmetry is established [3](#page=3). 7. Flippase, an ATP-dependent and specific enzyme, actively flips certain phospholipids (e.g., Phosphatidylserine) from the extracellular to the inner side to create the correct membrane identity [4](#page=4). ### 1.7 Asymmetry of the lipid bilayer The inner and outer layers of the lipid bilayer possess distinct compositions and functional importance [4](#page=4). #### 1.7.1 The inner layer * Is concentrated with Phosphatidylserine, which carries a negative charge crucial for binding and activating Protein Kinase C (PKC), thereby translating extracellular signals into intracellular ones [4](#page=4). * Phosphatidylserine's presence on the outer membrane during apoptosis signals neighboring cells to initiate phagocytosis of the dying cell [4](#page=4). #### 1.7.2 The outer layer * Is concentrated with glycolipids, composed of sphingosine lipids and sugars, which are exclusively found on the outer surface due to glycosylation in the Golgi [4](#page=4). * These glycolipids form a protective coat and serve as markers for cell-to-cell recognition [4](#page=4). ### 1.8 Vesicular transport Transport vesicles facilitate material transfer between organelles, beginning with synthesis in the ER and transport to the Golgi for modification. Vesicles then bud from the Golgi to deliver cargo to various destinations, including the plasma membrane, where they fuse and release contents. During fusion, the vesicle's inner layer becomes the plasma membrane's outer layer, and newly synthesized phospholipids incorporated into the vesicle become part of the plasma membrane. This process explains the exclusive localization of glycolipids on the outer membrane [4](#page=4). ### 1.9 Membrane fusion Membrane fusion preserves topology, ensuring the outer membrane consistently faces the cytosol and the inner membrane faces the cell's interior [5](#page=5). 1. A vesicle buds off from a donor membrane [5](#page=5). 2. The vesicle encapsulates cargo and forms a sealed sphere [5](#page=5). 3. Upon reaching the plasma membrane, the vesicle and plasma membranes merge [5](#page=5). 4. The vesicle's luminal membrane becomes the cell's outer membrane [5](#page=5). 5. The vesicle's cytosolic membrane becomes the cell's inner membrane [5](#page=5). 6. The cargo is secreted into the extracellular space [5](#page=5). ### 1.10 Integral and peripheral membrane proteins Membrane proteins are broadly categorized into integral and peripheral types. #### 1.10.1 Types of integral membrane proteins * **Transmembrane Proteins:** These span the entire lipid bilayer and are composed of hydrophobic amino acids that interact with the lipid monolayer. Beta barrels are a specific type that form pores or channels [5](#page=5). * **Lipid-Linked Proteins:** Covalently bonded to a lipid anchor inserted into the membrane, these proteins are held at the surface without direct membrane interaction. Integral proteins are permanently attached and their removal damages the membrane [5](#page=5). #### 1.10.2 Types of peripheral membrane proteins * **Protein-Attached Proteins:** Loosely bound, these proteins do not interact directly with the membrane but bind non-covalently to integral membrane proteins. Their weaker bonds allow for gentle removal without membrane destruction [5](#page=5). ### 1.11 Functions of membrane proteins Membrane proteins perform diverse functions: * Transport of molecules [6](#page=6). * Enzymatic activity [6](#page=6). * Signal transduction [6](#page=6). * Cell-to-cell recognition [6](#page=6). * Intercellular joining [6](#page=6). * Attachment to the cytoskeleton and extracellular matrix (ECM) [6](#page=6). ### 1.12 Protein translocation and integration * **Translocation of Secreted Proteins:** Water-soluble proteins destined for export are synthesized on ribosomes on the rough ER (RER) and directed by an N-terminal hydrophobic signal peptide to a translocator channel in the ER membrane. The growing polypeptide chain is threaded through this channel and across the membrane; the signal peptide is then cleaved off by signal peptidase, releasing the protein into the ER lumen [6](#page=6). * **Integration of Single-Pass Transmembrane Proteins:** An N-terminal start-transfer peptide guides the polypeptide to a translocator. A subsequent hydrophobic stop-transfer sequence halts translocation and causes the channel to release the protein sideways into the lipid bilayer. The signal peptide is cleaved, anchoring the protein in the membrane with its N-terminus in the ER lumen and C-terminus in the cytosol [6](#page=6). ### 1.13 Protein glycosylation Protein glycosylation, the addition of sugar units to proteins, occurs in the ER and Golgi lumens. As these sugar residues are added internally, they face the extracellular side of the plasma membrane upon transport to the cell surface. Glycoproteins, glycolipids, and proteoglycans form the glycocalyx, a carbohydrate-rich layer that protects the cell, facilitates cell-to-cell recognition, and mediates inflammatory responses [6](#page=6). --- # Cellular transport mechanisms Cellular transport mechanisms are essential for regulating intracellular conditions, nutrient uptake, and waste removal by facilitating the movement of substances across cell membranes [7](#page=7). ### 2.1 Types of membrane transport There are two primary categories of membrane transport: passive and active transport. The key distinctions lie in their energy requirements and the direction of solute movement relative to their concentration gradients [7](#page=7). #### 2.1.1 Passive transport Passive transport moves solutes along their concentration gradients and does not require cellular energy in the form of ATP [7](#page=7). ##### 2.1.1.1 Simple diffusion Simple diffusion is the movement of a solute across a membrane without the assistance of membrane proteins. The ease of passage depends on the solute's concentration gradient, charge, and size [7](#page=7). * **Most permeable:** Small, hydrophobic, uncharged molecules [7](#page=7). * **Moderately permeable:** Uncharged, polar molecules pass at a slower rate [7](#page=7). * **Impermeable:** Large, polar, uncharged molecules and ions [7](#page=7). Solutes moving via simple diffusion always travel from an area of high concentration to an area of low concentration to achieve equilibrium [7](#page=7). ##### 2.1.1.2 Facilitated diffusion Facilitated diffusion utilizes specialized membrane proteins to transport inorganic ions and small molecules that cannot readily cross the lipid bilayer via simple diffusion. This process relies on two protein classes: ion channels and carrier proteins (uniporters) [7](#page=7). ###### 2.1.1.2.1 Ion channels Ion channels are integral membrane proteins that facilitate the passage of ions across the membrane. They are selective, allowing specific ions to pass based on size and charge, and are driven by electrochemical gradients. These channels can be regulated, opening and closing in response to specific stimuli [7](#page=7). * **Electrochemical gradient:** This combines the concentration gradient and the membrane potential, acting as the driving force for charged solutes [7](#page=7). * **Potassium channels:** These are prevalent in all cells and are highly specific for potassium ions, conducting them about 10,000 times more effectively than sodium ions [8](#page=8). * **K+ leak channels:** A subset of potassium channels that are continuously open, allowing for background ion movement [8](#page=8). * **Transport process for K+:** 1. Negatively charged amino acids at the pore entrance attract hydrated K+ ions into a vestibule [8](#page=8). 2. To pass the selectivity filter, K+ must shed its water shell (dehydrate) to interact with carbonyl oxygens lining the filter [8](#page=8). 3. Upon exiting the filter, the ion becomes re-hydrated [8](#page=8). ###### 2.1.1.2.2 Uniporter carrier proteins Uniporter carrier proteins bind to specific solutes and undergo conformational changes (switching between State A and State B) to transport them across the membrane. Their specificity arises from the solute's physical binding to a site within the carrier. This binding and flipping mechanism makes them relatively slow compared to ion channels [8](#page=8). * **Glucose transporters (GLUTs):** These are integral membrane proteins that function as uniporters, exclusively transporting glucose. They are typically 12-pass transmembrane proteins that alternate conformations to move glucose [8](#page=8). * **GLUT1 deficiency syndrome:** A condition characterized by a lack of functional GLUT transporters, leading to seizures, microcephaly, and developmental delays [8](#page=8). * **Transport of glucose via GLUT1:** Glucose moves from an area of high concentration (blood) to low concentration (erythrocyte). To prevent back-diffusion, erythrocytes immediately convert incoming glucose to glucose-6-phosphate, a form not recognized by GLUT1. This maintains the concentration gradient and unidirectional transport [8](#page=8). > **Example:** The Glucose transporter (GLUT2) in the gut epithelia is an example of a uniporter carrier protein [8](#page=8). ### 2.2 Active transport Active transport is crucial for cells to maintain their electrochemical gradients, osmotic balance, and electrical forces across the membrane. It moves solutes against their electrochemical gradients and requires energy, typically in the form of ATP [9](#page=9). * **Tip:** Without active transport, ions would flow down their electrochemical gradients, disrupting critical cellular balances [9](#page=9). Cells employ three main mechanisms for active transport: #### 2.2.1 ATP-driven pumps These pumps directly use the energy released from ATP hydrolysis to move solutes against their gradients [9](#page=9). * **Sodium and potassium ion transport (Na+/K+-ATPase):** This is a classic example of an ATP-driven pump, essential for maintaining ion gradients [9](#page=9). 1. The pump opens to the intracellular side, where 3 sodium ions bind [9](#page=9). 2. ATP hydrolysis occurs, phosphorylating the pump and causing a conformational change that opens it to the extracellular side [9](#page=9). 3. The 3 sodium ions are released extracellularly [9](#page=9). 4. 2 extracellular potassium ions bind, triggering dephosphorylation and a return to the original conformation [9](#page=9). 5. The pump opens intracellularly, releasing the 2 potassium ions into the cell [9](#page=9). #### 2.2.2 Coupled transporters These transporters utilize the energy derived from the movement of one solute down its gradient to power the movement of another solute against its gradient [9](#page=9). * **Symporters:** Transport two different solutes in the same direction [9](#page=9). * **Antiporters:** Transport two different solutes in opposite directions [9](#page=9). > **Example:** The glucose symporter in the gut epithelia is a form of coupled transport, relying on the sodium gradient rather than direct ATP hydrolysis [9](#page=9). * **Glucose symporter (SGLT1):** Located on the apical membrane facing the gut lumen, it binds sodium ions when their concentration is high [10](#page=10). 1. Sodium binding increases the protein's affinity for glucose (cooperative binding) [10](#page=10). 2. Despite low glucose concentrations in the gut, it binds to the carrier [10](#page=10). 3. Once both are bound, SGLT1 flips to face the cell interior [10](#page=10). 4. Sodium is released into the cell where its concentration is low, causing SGLT1 to lose affinity for glucose and release it [10](#page=10). 5. The released glucose is then transported out of the cell into the blood via facilitated diffusion by GLUT2, which is located on the basolateral membrane [10](#page=10). 6. This transcellular transport is enforced by tight junctions blocking paracellular pathways [10](#page=10). > **Example:** The calcium antiporter uses the inward movement of sodium ions to drive the outward movement of calcium ions, helping to relax cardiac muscle. Drugs like Ouabain and Digoxin inhibit this antiporter to strengthen cardiac contractions, but overdoses can cause arrhythmias [10](#page=10). #### 2.2.3 Light-driven pumps These pumps move solutes against their gradients by using energy derived from light [9](#page=9). --- # Body water compartments and osmosis The distribution and movement of water within the body are governed by distinct fluid compartments and the fundamental principle of osmosis, crucial for maintaining cellular and overall physiological balance. ### 3.1 Body fluid compartments Bodily fluids are categorized into different compartments, each with specific locations and functions [11](#page=11). #### 3.1.1 Intracellular water Intracellular water is the fluid found within cells and constitutes the largest portion of bodily fluids [11](#page=11). #### 3.1.2 Extracellular water Extracellular water comprises fluids outside the cells, including plasma, interstitial fluid, and transcellular fluid [11](#page=11). ##### 3.1.2.1 Plasma Plasma is the fluid component of blood, accounting for 55% of its volume, with the remaining 45% being blood cells (hematocrit) [11](#page=11). ##### 3.1.2.2 Interstitial water Interstitial water fills the spaces between cells and plays a vital role in transporting nutrients and waste between the blood and cells [11](#page=11). ##### 3.1.2.3 Transcellular fluid Transcellular fluids are found in specialized, enclosed spaces within the body, distinct from interstitial fluids [11](#page=11). ###### 3.1.2.3.1 Examples of transcellular compartments Examples of transcellular fluid spaces include the peritoneal space (containing peritoneal fluid), cerebrospinal fluid (CSF) surrounding the brain and spinal cord, the pleural cavity (containing pleural fluid), and synovial fluid within joints. These fluids provide lubrication, cushioning, and nutrient supply [12](#page=12). ### 3.2 Body water content The total body water content varies among individuals. A 70-kilogram person typically has about 42 liters of total body fluid. This fluid is distributed as follows: approximately 5% plasma water, 20% fat, 16% interstitial fluid, 35% intracellular fluid, and 2% transcellular fluid. Men generally have a higher body water content than women, and water content decreases with age [11](#page=11). ### 3.3 Osmosis and cell volume regulation Osmosis is the movement of water across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. Water crosses cell membranes via aquaporins, which are specialized channels [12](#page=12). > **Tip:** While aquaporins facilitate rapid water transport, they do not alter the equilibrium of water balance. Significant changes in cell volume require alterations in solute concentration [12](#page=12). Cells employ several mechanisms to regulate their volume: * **Metabolic Regulation:** Cells can alter intracellular solute concentration by breaking down molecules like proteins or glycogen [12](#page=12). * **Volume-Regulated Anion Channels (VRACs):** When a cell swells, VRACs open to pump osmolytes out of the cell [12](#page=12). * **Stretch-Activated TRP Channels:** These channels open in response to cell swelling, allowing sodium and calcium ions to enter. The influx of calcium acts as a signal to rearrange the cytoskeleton, aiding the cell in physically adjusting to volume changes [12](#page=12). ### 3.4 Measuring body fluid volumes Various methods utilize dilution techniques to measure the volume of different body water compartments. #### 3.4.1 Measuring total body water 1. A known amount of water labeled with deuterium or tritium is administered [13](#page=13). 2. The labeled water equilibrates evenly throughout the body's water [13](#page=13). 3. A biological sample (urine or blood) is collected [13](#page=13). 4. The dilution of the labeled water is measured, and total body water is calculated using the formula: $$ \text{Total Body Water} = \frac{\text{Amount of labelled water}}{\text{Concentration of labelled water after mixing}} $$ [13](#page=13). #### 3.4.2 Measuring plasma volume 1. Labeled proteins or Evan's blue dye are injected and bind to plasma proteins [13](#page=13). 2. The marker distributes evenly within the bloodstream [13](#page=13). 3. A blood sample is taken to measure the marker's dilution [13](#page=13). 4. The same dilution formula used for total body water is applied to calculate plasma volume [13](#page=13). #### 3.4.3 Measuring extracellular fluid volume 1. A marker that can leave blood vessels but not enter cells (e.g., inulin) is injected [13](#page=13). 2. The marker is allowed to distribute throughout the extracellular spaces [13](#page=13). 3. A blood or urine sample is collected to measure the marker's dilution [13](#page=13). 4. The dilution formula is used to calculate the extracellular fluid volume [13](#page=13). ### 3.5 Osmotic pressure Osmotic pressure is the force required to oppose the net movement of water across a membrane due to differences in solute concentration. It is equal to the hydrostatic pressure needed to prevent osmosis [14](#page=14). * **Osmoles:** A measure of the number of molecules a compound dissociates into when dissolved in water [14](#page=14). * **Osmolality:** The number of osmoles per unit mass of solvent [14](#page=14). * **Osmolarity:** The number of osmoles per unit volume of solution [14](#page=14). #### 3.5.1 Oncotic pressure Oncotic pressure, also known as colloid osmotic pressure, is the specific portion of the total osmotic pressure of plasma generated by proteins, primarily albumin. While total plasma osmotic pressure is high, oncotic pressure is relatively low [14](#page=14). > **Example:** A decrease in plasma albumin (due to liver failure, kidney disease, or malnutrition) leads to reduced oncotic pressure. This causes fluid to leak from blood vessels into surrounding tissues, resulting in edema [14](#page=14). #### 3.5.2 Diffusion of solutions * **Isosmotic Solutions:** Solutions with the same osmolality [14](#page=14). * **Isotonic Solutions:** Solutions that exert the same osmotic pressure as body cells, resulting in no net movement of water across the cell membrane [14](#page=14). * **Urea Solutions:** Urea readily crosses cell membranes. Therefore, an isosmotic urea solution may not be isotonic, as it can increase intracellular osmotic pressure and cause cells to swell. This highlights that not all isosmotic solutions are isotonic [14](#page=14). The relationship between osmotic pressure, osmolality, and temperature can be approximated by the Morse equation: $$ \text{Osmotic Pressure} = nCRT $$ where $nC$ represents osmolality, $R$ is the ideal gas constant, and $T$ is the temperature. Osmotic pressure is directly proportional to osmolality [14](#page=14). --- # Metabolic pathways for nutrient utilization and energy production This section explores the interconnected metabolic pathways that allow the body to extract energy from carbohydrates, fats, and proteins, along with mechanisms for nutrient storage and regulation under different nutritional states [15](#page=15). ### 4.1 Nutrient storage The body stores excess nutrients primarily as triglycerides and glycogen, with proteins having no specific storage form but being readily available in muscle tissue [15](#page=15). #### 4.1.1 Triglycerides * Stored in adipose tissue [15](#page=15). * Broken down into fatty acids and glycerol via lipolysis for later energy use [15](#page=15). * Provide approximately 9 calories per gram [15](#page=15). #### 4.1.2 Carbohydrates * Stored as glycogen in the liver and muscles [15](#page=15). * Glycogen is a highly branched glucose polymer with alpha 1-4 and alpha 1-6 glycosidic bonds [15](#page=15). * Broken down into monosaccharides via glycogenolysis [15](#page=15). * Provide approximately 4 calories per gram [15](#page=15). * **Liver glycogen:** Can be released into the bloodstream for systemic use [15](#page=15). * **Muscle glycogen:** Primarily for local use within muscle cells due to the lack of glucose-6-phosphatase, an enzyme needed for glucose transport into the bloodstream [15](#page=15). #### 4.1.3 Protein * Does not have a dedicated storage site but is abundant in muscle tissue [15](#page=15). * Can be broken down into amino acids, which can be converted to glucose via gluconeogenesis [15](#page=15). ### 4.2 Nutritional states The body's metabolic activity shifts significantly based on nutrient availability, transitioning through fed, fasted, and starved states [16](#page=16). #### 4.2.1 Fed state (0-4 hours) * Glucose, lipids, and amino acids are absorbed from food [16](#page=16). * Triglycerides are delivered to adipose tissue [16](#page=16). * Glucose is distributed to the liver, muscles, adipose tissue, and the brain [16](#page=16). * Amino acids are utilized by muscles as a fuel source [16](#page=16). #### 4.2.2 Fasted state (4-12 hours) * Stored nutrients are utilized [16](#page=16). * Glucose is primarily derived from liver glycogen stores [16](#page=16). * Muscle glycogen is used locally for energy [16](#page=16). * Liver and muscles use free fatty acids as fuel [16](#page=16). #### 4.2.3 Starved state (12-20 hours onwards) * Characterized by low insulin and high glucagon levels [16](#page=16). * Glucagon promotes glycogenolysis and gluconeogenesis [16](#page=16). * Muscles continue to use free fatty acids as fuel [16](#page=16). * The liver produces glucose and ketones for the brain [16](#page=16). * Ketones can cross the blood-brain barrier to serve as an alternative fuel for the brain [16](#page=16). ### 4.3 Cellular respiration Cellular respiration is the process by which biochemical energy from food is converted into usable energy, primarily ATP [16](#page=16). * One glucose molecule typically yields 28-32 ATP molecules [16](#page=16). #### 4.3.1 Glucose uptake Glucose enters cells via facilitated diffusion through hexose transporters [17](#page=17). * **GLUT 1:** Blood, blood-brain barrier, heart [17](#page=17). * **GLUT 2:** Liver, pancreas, small intestine [17](#page=17). * **GLUT 3:** Brain, neurons, sperm [17](#page=17). * **GLUT 4:** Skeletal muscle, adipose tissue, heart; translocated to the plasma membrane by insulin [17](#page=17). #### 4.3.2 Glycolysis overview Glycolysis is the initial stage of cellular respiration occurring in the cytoplasm, breaking down glucose into pyruvate, producing a net of 2 ATP, 2 NADH, and 2 pyruvate molecules [17](#page=17). * **Energy Investment Phase:** 1. Glucose is phosphorylated to glucose-6-phosphate (G6P) by hexokinase, consuming 1 ATP [17](#page=17). 2. G6P is isomerized to fructose-6-phosphate by phosphoglucose isomerase [17](#page=17). 3. Fructose-6-phosphate is converted to fructose 1-6 bisphosphate by phosphofructokinase-1 (PFK-1), consuming another ATP [17](#page=17). 4. Fructose 1-6 bisphosphate is cleaved by aldolase into glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP) [17](#page=17). 5. DHAP is converted to G3P by triose phosphate isomerase, ensuring both 3-carbon molecules proceed [17](#page=17). * **Energy Generation/Pay-off Phase:** 6. G3P is oxidized and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase to form 1-3 bisphosphoglycerate (1-3 BPG), reducing 2 NAD+ to 2 NADH [18](#page=18). 7. 1-3 BPG is converted to 3-phosphoglycerate (3PG) by phosphoglycerate kinase, producing 2 ATP via substrate-level phosphorylation [18](#page=18). 8. 3PG is rearranged to 2-phosphoglycerate (2PG) by phosphoglycerate mutase [18](#page=18). 9. 2PG is dehydrated by enolase to form phosphoenolpyruvate (PEP), releasing water [18](#page=18). 10. PEP is converted to pyruvate by pyruvate kinase, producing another 2 ATP via substrate-level phosphorylation [18](#page=18). * **Net Products:** 2 ATP, 2 NADH, and 2 pyruvate molecules [18](#page=18). #### 4.3.3 Glycogen entry into glycolysis Glycogen can enter glycolysis as glucose-1-phosphate after being acted upon by glycogen phosphorylase, bypassing the initial ATP-consuming hexokinase step [18](#page=18). #### 4.3.4 Carbohydrate inputs Various sugars can enter glycolysis at different points: * Galactose is converted to glucose-1-phosphate [18](#page=18). * Lactose breaks down into glucose and galactose [18](#page=18). * Sucrose breaks down into glucose and fructose [18](#page=18). * Fructose can be phosphorylated to fructose-1-phosphate or fructose-6-phosphate [18](#page=18). * Mannose enters as mannose-6-phosphate, then converts to fructose-6-phosphate [18](#page=18). ### 4.4 Gluconeogenesis Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors, primarily occurring in the liver when blood glucose is low. [page=19 [page=20 This process bypasses the irreversible steps of glycolysis [16](#page=16) [19](#page=19) [20](#page=20). * **Bypass 1 (Pyruvate to PEP):** Requires two enzymes: pyruvate carboxylase (pyruvate to oxaloacetate) and phosphoenolpyruvate carboxykinase (oxaloacetate to PEP) [19](#page=19). * **Bypass 2 (Fructose 1-6 bisphosphate to Fructose-6-phosphate):** Catalyzed by fructose 1-6 bisphosphatase. [page=20 [19](#page=19) [20](#page=20). * **Bypass 3 (Glucose-6-phosphate to Glucose):** Catalyzed by glucose-6-phosphatase. [page=20 [19](#page=19) [20](#page=20). * **Energy Expenditure:** Synthesizing one glucose molecule from two pyruvate molecules via gluconeogenesis requires 4 ATP, 2 GTP, and 2 NADH [20](#page=20). ### 4.5 Pathways for pyruvate Pyruvate, the end product of glycolysis, can follow several metabolic fates depending on oxygen availability and cellular needs [19](#page=19). #### 4.5.1 Aerobic respiration * Pyruvate enters the mitochondria. [page=21 [19](#page=19) [21](#page=21). * The **link reaction** involves pyruvate dehydrogenase, which irreversibly decarboxylates pyruvate, producing Acetyl-CoA, CO2, and NADH [21](#page=21). * Acetyl-CoA then enters the citric acid cycle. [page=21 [19](#page=19) [21](#page=21). #### 4.5.2 Anaerobic respiration (Fermentation) * In humans, pyruvate is converted to lactate by lactate dehydrogenase (LDH) [19](#page=19). * This reaction regenerates NAD+ from NADH, allowing glycolysis to continue producing ATP in the absence of oxygen [19](#page=19). * Lactate can be recycled to the liver via the Cori Cycle and converted back to glucose via gluconeogenesis [19](#page=19). #### 4.5.3 Transamination * Pyruvate can accept an amino group to form alanine, an amino acid used in gluconeogenesis [19](#page=19). ### 4.6 Citric acid cycle (Krebs cycle) The citric acid cycle, located in the mitochondrial matrix, is a central hub for the oxidation of fuel molecules (carbohydrates, amino acids, fatty acids) under aerobic conditions. [page=21 It also provides intermediates for biosynthesis [20](#page=20) [21](#page=21). * **Link Reaction (Pyruvate to Acetyl-CoA):** 1. Pyruvate is transported into the mitochondrial matrix [21](#page=21). 2. A carboxyl group is removed from each pyruvate, releasing CO2 [21](#page=21). 3. The remaining two-carbon molecule is oxidized, reducing NAD+ to NADH [21](#page=21). 4. The acetyl group attaches to Coenzyme A, forming Acetyl-CoA [21](#page=21). * **Steps of the Krebs Cycle (per Acetyl-CoA):** 1. **Citrate formation:** Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), catalyzed by citrate synthase [22](#page=22). 2. **Isomerization:** Citrate is isomerized to D-isocitrate by aconitase [22](#page=22). 3. **Primary Oxidative Decarboxylation:** Isocitrate dehydrogenase oxidizes D-isocitrate, releasing CO2 and reducing NAD+ to NADH, forming alpha-ketoglutarate (5C) [22](#page=22). 4. **Secondary Oxidative Decarboxylation:** Alpha-ketoglutarate dehydrogenase oxidizes alpha-ketoglutarate, releasing CO2 and reducing NAD+ to NADH, forming succinyl-CoA (4C) [22](#page=22). 5. **Phosphorylation:** Succinyl-CoA synthetase cleaves succinyl-CoA, producing GTP (which can be converted to ATP) and succinate (4C) [22](#page=22). 6. **Oxidation:** Succinate is oxidized to fumarate by succinic dehydrogenase, reducing FAD to FADH2 [22](#page=22). 7. **Hydrolysis:** Fumarase adds water to fumarate, forming malate (4C) [22](#page=22). 8. **Regeneration of Oxaloacetate:** Malate dehydrogenase oxidizes malate to regenerate oxaloacetate, reducing NAD+ to NADH [22](#page=22). * **Net Products per glucose molecule (2 Acetyl-CoA):** 6 NADH, 2 FADH2, 2 ATP (or 2 GTP) [22](#page=22). #### 4.6.1 ATP yield of coenzymes * NADH generates 2-3 ATP molecules during the electron transport chain [22](#page=22). * FADH2 generates 1-2 ATP molecules during the electron transport chain [22](#page=22). ### 4.7 Beta oxidation of fatty acids Beta oxidation is the process of breaking down fatty acids into acetyl-CoA units, primarily occurring in the mitochondria during fasted or starved states [25](#page=25). * Fatty acids are transported into the mitochondria via the carnitine shuttle [25](#page=25). * Through a series of reactions involving CoAs, FAD, and NAD, fatty acids are broken down into acetyl-CoA [25](#page=25). * This process yields a significant amount of energy, typically around 131 ATP molecules per fatty acid molecule [25](#page=25). #### 4.7.1 Energy utilization during exercise * **Low to moderate intensity exercise:** Primarily relies on fatty acid oxidation, with some contribution from glucose [25](#page=25). * **Short-term or intense exercise:** Requires glucose as the primary energy source, leading to high rates of glycolysis [25](#page=25). ### 4.8 Protein metabolism In fasted or starved states, protein can serve as an energy source [26](#page=26). * Proteins are broken down into amino acids [26](#page=26). * Amino acids can be converted into metabolic intermediates, such as pyruvate or Acetyl-CoA, entering central pathways [26](#page=26). * Alanine, Glycine, and Serine can convert to pyruvate [26](#page=26). * Leucine and Lysine convert to Acetyl-CoA [26](#page=26). #### 4.8.1 Transamination Transamination is the transfer of an amine group from an amino acid to a keto acid, catalyzed by aminotransferases [26](#page=26). 1. An amino acid transfers its amine group to $\alpha$-ketoglutarate, forming Glutamate [26](#page=26). 2. Glutamate can then either store the amine group or release it via oxidative deamination by glutamate dehydrogenase [26](#page=26). 3. After releasing the amine group, Glutamate reverts to a keto acid and releases ammonia for the urea cycle [26](#page=26). #### 4.8.2 The urea cycle This cycle occurs exclusively in the liver and detoxifies ammonia produced from amino acid metabolism by converting it into urea [26](#page=26). * It converts ammonia, carbon dioxide, and aspartate into urea and fumarate [26](#page=26). * Fumarate can enter the citric acid cycle [26](#page=26). * The process consumes 3 ATP molecules [26](#page=26). ### 4.9 Metabolic regulation in fed and fasted states Metabolic pathways are tightly regulated to manage energy storage and mobilization. [page=25 [24](#page=24) [25](#page=25). #### 4.9.1 Fed state regulation * High levels of metabolic intermediates like ATP, NADH, Acetyl-CoA, and citrate accumulate [24](#page=24). * High ATP and citrate allosterically inhibit PFK-1, slowing glycolysis and promoting energy storage [24](#page=24). * Accumulated glucose-6-phosphate (G-6P) can be diverted to the Pentose Phosphate Pathway (PPP) for NADPH and pentose production, or used for fatty acid synthesis [25](#page=25). #### 4.9.2 Fatty acid synthesis * Occurs in the cytoplasm via acetyl polymerization [25](#page=25). * The citrate shuttle system transports acetyl units from mitochondria to the cytosol [25](#page=25). * Acetyl-CoA carboxylase is activated by citrate and insulin, requiring biotin and NADPH [25](#page=25). * Acetyl-CoA and bicarbonate are converted to Malonyl-CoA, which then elongates the fatty acid chain [25](#page=25). #### 4.9.3 Fasted and starved state regulation * The liver produces glucose and ketones for the brain [25](#page=25). * Muscle tissue breaks down proteins into amino acids and lactate [25](#page=25). * Adipose tissue releases free fatty acids (FFA) and glycerol, which are converted to Acetyl-CoA in the liver [25](#page=25). * Low ATP and NADH levels stimulate fatty acid mobilization, catalyzed by hormone-sensitive lipase, which is activated by a cAMP-responsive kinase [25](#page=25). * Epinephrine stimulates this process, while insulin inhibits it [25](#page=25). --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Amphipathic | A molecule that has both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts, characteristic of phospholipids that form cell membranes. | | Fluid Mosaic Model | A model describing the cell membrane as a dynamic, flexible structure composed of a phospholipid bilayer with various proteins embedded or attached to it, allowing for movement and interaction of components. | | Phospholipid Bilayer | A two-layered arrangement of phospholipid molecules in cell membranes, with their hydrophobic tails facing inward and their hydrophilic heads facing outward, forming a stable barrier between intracellular and extracellular environments. | | Micelle | A spherical aggregation of lipids with a single fatty acid tail, where hydrophobic tails are shielded from water by hydrophilic heads, forming a structure that hides nonpolar regions in an aqueous environment. | | Fluorescence Recovery After Photobleaching (FRAP) | A technique used to measure the fluidity of cell membranes by labeling membrane components with a fluorescent dye, photobleaching a specific area with a laser, and observing how quickly fluorescence recovers as unbleached molecules diffuse into the bleached region. | | Cholesterol | A steroid lipid that is interspersed within the phospholipid bilayer of animal cell membranes, acting to regulate fluidity by immobilizing the fatty acid tails at higher temperatures and preventing tight packing at lower temperatures. | | Scramblase | An enzyme in the endoplasmic reticulum that catalyzes the movement of newly synthesized lipids from the outer leaflet to the inner leaflet of the membrane, helping to create a balanced lipid bilayer. | | Flippase | An ATP-dependent and highly specific enzyme that sorts and flips particular phospholipids across the lipid bilayer, crucial for establishing the asymmetrical distribution of lipids in cell membranes. | | Glycolipids | Lipids that have carbohydrate chains attached, typically found on the outer surface of the plasma membrane, where they play roles in cell recognition and form part of the glycocalyx. | | Vesicular Transport | A process where transport vesicles bud off from one membrane compartment and fuse with another, carrying proteins and lipids between organelles like the ER, Golgi apparatus, and plasma membrane. | | Integral Membrane Proteins | Proteins that are permanently embedded within the lipid bilayer, often spanning the entire membrane (transmembrane proteins) or covalently attached to lipids, and are difficult to remove without disrupting the membrane. | | Peripheral Membrane Proteins | Proteins that are loosely attached to the membrane surface, typically by non-covalent interactions with integral membrane proteins or the lipid head groups, and can be removed more easily. | | Translocation | The process by which a polypeptide chain is threaded across or through a membrane, often directed by a signal peptide, as seen in the synthesis of secreted and transmembrane proteins in the endoplasmic reticulum. | | Glycocalyx | A carbohydrate-rich layer surrounding the cell surface, formed by glycoproteins, glycolipids, and proteoglycans, which provides protection, aids in cell-to-cell recognition, and mediates inflammatory responses. | | Passive Transport | The movement of molecules across a cell membrane that does not require cellular energy (ATP), occurring down a concentration or electrochemical gradient. | | Active Transport | The movement of molecules across a cell membrane that requires cellular energy (ATP) to move substances against their concentration or electrochemical gradient. | | Simple Diffusion | The passive movement of small, hydrophobic, or uncharged molecules across a membrane directly down their concentration gradient, without the assistance of membrane proteins. | | Facilitated Diffusion | The passive movement of molecules across a membrane with the assistance of specific membrane transport proteins, such as channels or carrier proteins, down their electrochemical gradient. | | Ion Channels | Transmembrane proteins that form pores or channels through the lipid bilayer, allowing specific ions to pass through the membrane down their electrochemical gradient. | | Uniporter Carrier Proteins | Membrane proteins that bind to a specific solute and undergo a conformational change to transport it across the membrane down its concentration gradient; they are highly selective and relatively slow. | | Electrochemical Gradient | The combined influence of both the concentration gradient and the electrical potential difference across a membrane, which drives the movement of charged solutes like ions. | | ATP-Driven Pumps | Primary active transporters that use the energy released from ATP hydrolysis to move solutes against their electrochemical gradients. | | Coupled Transporters | Secondary active transporters that use the energy stored in an electrochemical gradient of one solute to drive the transport of another solute against its gradient. Symporters move both solutes in the same direction, while antiporters move them in opposite directions. | | Light-Driven Pumps | Active transporters that utilize energy from light to move solutes against their electrochemical gradients. | | Osmosis | The net movement of water molecules across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration, driven by differences in water potential. | | Aquaporins | Water channel proteins that facilitate the rapid passage of water across cell membranes, increasing the efficiency of osmosis without affecting the direction of net water movement at equilibrium. | | Osmotic Pressure | The pressure that needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane; it is a measure of the tendency of water to move into a solution by osmosis. | | Osmoles | The number of moles of solute particles that contribute to the osmotic pressure of a solution; it considers molecules that dissociate in water. | | Osmolality | The concentration of solute in a solution expressed as osmoles per kilogram of solvent. | | Osmolarity | The concentration of solute in a solution expressed as osmoles per liter of solution. | | Oncotic Pressure | The osmotic pressure exerted by plasma proteins, primarily albumin, which contributes to retaining fluid within blood vessels. | | Isotonic Solution | A solution that has the same osmotic pressure as a cell, resulting in no net movement of water across the cell membrane. | | Glycogen | A highly branched polymer of glucose stored in animal cells, primarily in the liver and muscles, as a readily accessible source of energy. | | Glycogenolysis | The metabolic process by which glycogen is broken down into glucose-1-phosphate, which is then converted to glucose-6-phosphate, to release glucose into the bloodstream or for cellular use. | | Gluconeogenesis | The metabolic pathway that synthesizes glucose from non-carbohydrate precursors, such as lactate, amino acids, and glycerol, primarily in the liver, especially during fasting or starvation. | | Glycolysis | The metabolic pathway that breaks down one molecule of glucose into two molecules of pyruvate, producing a net gain of ATP and NADH, occurring in the cytoplasm. | | Pyruvate | A three-carbon molecule that is the end product of glycolysis, which can be further metabolized under aerobic conditions (entering the citric acid cycle) or anaerobic conditions (fermentation). | | ATP (Adenosine Triphosphate) | The primary energy currency of the cell, a molecule that stores and releases energy through the hydrolysis of its phosphate bonds. | | NADH (Nicotinamide Adenine Dinucleotide) | An electron carrier that plays a crucial role in cellular respiration, accepting electrons during oxidation reactions and transferring them to the electron transport chain to generate ATP. | | Citric Acid Cycle (Krebs Cycle/TCA Cycle) | A series of metabolic reactions that occur in the mitochondrial matrix, where acetyl-CoA is oxidized to produce ATP, NADH, FADH2, and carbon dioxide, serving as a central hub for energy metabolism. | | Acetyl-CoA | A molecule that enters the citric acid cycle, formed by the breakdown of carbohydrates, fats, and proteins; it consists of a two-carbon acetyl group attached to coenzyme A. | | Mitochondria | The "powerhouse" of the cell, an organelle responsible for cellular respiration and the majority of ATP production through the citric acid cycle and oxidative phosphorylation. | | Beta Oxidation | The metabolic process by which fatty acids are broken down in the mitochondria to produce acetyl-CoA, NADH, and FADH2, which are then used to generate ATP. | | Oxidative Phosphorylation | The final stage of cellular respiration, where ATP is synthesized by the electron transport chain and chemiosmosis, utilizing the energy released from the oxidation of electron carriers (NADH and FADH2) and a proton gradient across the inner mitochondrial membrane. | | Electron Transport Chain (ETC) | A series of protein complexes embedded in the inner mitochondrial membrane that accept electrons from NADH and FADH2 and pass them along, releasing energy used to pump protons into the intermembrane space. | | Chemiosmosis | The process by which ATP is synthesized as a result of electron flow through the electron transport chain and the subsequent pumping of protons, creating an electrochemical gradient that drives ATP synthase. | | ATP Synthase | An enzyme complex located in the inner mitochondrial membrane that uses the energy of the proton gradient to catalyze the synthesis of ATP from ADP and inorganic phosphate. | | Uncoupling Protein 1 (UCP1) | A protein found in brown adipose tissue that allows protons to flow back into the mitochondrial matrix without passing through ATP synthase, releasing energy as heat instead of producing ATP. | | Transamination | A chemical reaction that transfers an amino group from an amino acid to a keto acid, forming a new amino acid and a new keto acid; it is a key step in amino acid metabolism and the urea cycle. | | Urea Cycle | A metabolic pathway occurring in the liver that detoxifies ammonia, a toxic byproduct of amino acid metabolism, by converting it into urea, which is then excreted by the kidneys. |