Clase 3 Fisiología - Transporte a través de la membrana (VERSIÓN ANTIGUA)(NUEVA VERSIÓN DESCRIPCIÓN)

Introduction to Membrane Transport

Overview of Topics

• Eduardo Paiva introduces the third class on physiology, focusing on substance transport across membranes.

• Key topics include general concepts, diffusion, osmosis, and active transport.

Intracellular vs. Extracellular Fluid

• The intracellular fluid (ICF) has high potassium levels while the extracellular fluid (ECF) has low potassium levels; sodium is high in ECF and low in ICF.

• Chloride ions are abundant in ECF, whereas ICF contains various phosphates and proteins contributing to cellular electronegativity.

Concentration Differences

Ion Concentration Details

• Sodium concentration: 142 mEq/L in ECF vs. 10 mEq/L in ICF; Potassium: 4 mEq/L in ECF vs. 140 mEq/L in ICF.

• Oxygen pressure (Po2): 35 mmHg; Carbon dioxide pressure (Pco2): 46 mmHg; pH level averages at 7.4.

Diffusion Explained

Definition and Importance

• Diffusion is defined as the movement of solute from a region of higher concentration to one of lower concentration through a selectively permeable membrane.

• Understanding diffusion is crucial for studying cardiac physiology and electrolyte imbalances.

Mechanism of Diffusion

• A visual representation shows how solutes move from areas of higher concentration to lower concentration across a lipid bilayer membrane.

Types of Diffusion

Simple vs. Facilitated Diffusion

• Simple diffusion involves kinetic movement through membrane openings without protein interaction; influenced by solute concentration and temperature.

• Higher concentrations lead to faster diffusion rates; at absolute zero, diffusion ceases entirely.

Facilitated Diffusion Characteristics

Role of Proteins

• Facilitated diffusion requires specific transporter proteins but does not consume ATP energy.

Factors Affecting Speed

• Lipid solubility affects how quickly substances pass through membranes; lipophilic substances diffuse rapidly compared to hydrophilic ones which require channels like aquaporins for water transport.

Conclusion on Transport Mechanisms

Summary Insights

Understanding Molecular Polarities and Protein Channels

The Influence of Polarities on Molecular Movement

• The diffusion rate of small molecules is affected by their polarity; a negatively charged cell will repel negative molecules, making it harder for them to enter.

• In contrast, positively charged ions are attracted to the negatively charged cell, facilitating easier entry due to opposite charges attracting.

Characteristics of Protein Channels

• Protein channels exhibit selective permeability, allowing specific substances to pass through and can open or close via gates.

• Sodium channels measure 0.3 by 0.5 nanometers and have a strong negative charge that attracts dehydrated sodium ions into the cell.

• Potassium channels are smaller (0.3 by 0.3 nanometers), do not carry a negative charge, and allow potassium ions to pass without dehydration.

Mechanisms of Channel Activation

Voltage-Gated Channels

• Voltage-gated channels can be activated by changes in membrane potential; they remain closed until the voltage reaches -65 mV, at which point they open.

• Once opened, these channels allow sodium ions to flow from extracellular fluid into intracellular fluid.

Ligand-Gated Channels

• Ligand-gated channels require a chemical signal (ligand), such as acetylcholine, which binds to receptors and opens the channel for ion passage.

• This binding alters the state of the protein channel, enabling sodium influx from outside the cell into the cytoplasm.

Understanding Osmosis and Osmotic Pressure

• Osmosis is defined as the movement of solvent across a selectively permeable membrane from an area of lower solute concentration to one of higher concentration.

• The osmotic pressure is necessary to halt osmosis; applying pressure in one compartment can prevent solvent movement between compartments A and B.

Distinguishing Between Osmolality and Osmolarity

• Osmolality refers to the molecular weight in grams of an osmotically active solute per kilogram of solvent.

Understanding Osmolarity and Active Transport

Introduction to Osmolarity

• Osmolarity is a crucial measurement expressed in osmol per liter of solution, focusing on the concentration rather than molecular weight.

• It emphasizes measuring osmolarity in liters instead of grams, which aligns with molarity concepts.

Active Transport Mechanisms

Definition and Energy Requirement

• Active transport involves moving molecules across a cell membrane from low to high concentration, opposing the concentration gradient.

• This process requires energy, specifically ATP, due to its nature of working against the gradient.

Types of Active Transport

Primary Active Transport

• Primary active transport derives energy directly from ATP hydrolysis. A key example is the sodium-potassium pump (Na+/K+ pump).

• The Na+/K+ pump transports three sodium ions out and two potassium ions into the cell, maintaining cellular equilibrium through ATP hydrolysis.

Secondary Active Transport

• Secondary active transport utilizes energy indirectly derived from primary active transport processes.

• It can be categorized into:

• Cotransport: Molecules are transported alongside another molecule that is actively transported.

• Contratransport: Molecules move in opposite directions relative to an actively transported molecule.

Examples of Cotransport and Contratransport

Cotransport Mechanism

• In cotransport, glucose and amino acids are moved along with sodium ions via secondary active transport.

• This mechanism illustrates how sodium's primary active transport facilitates the secondary movement of glucose into cells.

Contratransport Mechanism