Cell Physiology: Structure and Membrane Transport Notes

Purpose and Requirements of Cell Physiology

• Understanding Basic Structure: To understand the basic structure of the cell membrane.
• Mastering Membrane Dynamics: To master transport through the cell membrane, bioelectrical activities, and neuromuscular transmission.
• Mastering Contraction Mechanisms: To master the excitation-contraction coupling and understand the mechanism and mechanics of skeletal muscle contraction.

The Cell: Basic Functional Unit of Life

• Definition: The cell is the building block and functional unit of life. It is the smallest form of life and is the basic unit making up every living organism.
• Replication: Almost all cells possess the ability to replicate on their own.
• Diversity in the Human Body: The human body consists of more than 200 distinct cell types. Examples include:     * Egg cells     * Sperm cells     * Muscle cells     * Rod cells in the eye     * Hair cells     * Nerve cells
• Basic Characteristics of Cells:     * Metabolic Energy: All cells utilize oxygen, which combines with carbohydrates, fat, or protein to release energy essential for the maintenance of life.     * Metabolic Output: Cells deliver their metabolic end-products into the surrounding fluids.     * Classification: Cells are categorized into two primary types: Prokaryotic and Eukaryotic.

Fundamental Structure of the Eukaryotic Cell

• Cell Membrane (Plasma Membrane): A selective barrier with a thickness of approximately $7.5\,nm$ to $10\,nm$.
• Organelles and Components:     * Nucleus: Contains the nucleolus, chromatin, and is surrounded by a nuclear envelope with nuclear pores.     * Cytoplasm: The internal fluid environment of the cell.     * Mitochondrion: The power engine of the cell.     * Endoplasmic Reticulum (ER): Divided into Smooth ER and Rough ER (the latter contains ribosomes).     * Golgi Complex: Involved in protein processing and packaging.     * Lysosome: Involved in digestion.     * Centriole: Plays a role in cell division.     * Vacuole: Storage space.     * Cytoskeleton: Comprised of microfilaments and microtubules.

Structure and Composition of the Cell Membrane

• Functions of the Plasma Membrane:     * Acts as a selective barrier.     * Supports and retains the cytoplasm.     * Contains transport systems.     * Facilitates communication.
• The Fluid Mosaic Model: Developed by Singer and Nicholson in 1972. This model describes the membrane as a fluid structure where proteins flow laterally within the lipid bilayer.
• Chemical Composition:     * Proteins: $55\%$     * Lipids: $42\%$. Primarily phospholipids ($Phosphatidylcholine$, $Phosphatidylserine$, $Phosphatidylethanolamine$, $Phosphatidylinositol$), cholesterol, and sphingolipids.     * Carbohydrates: $3\%$. Found as glycolipids and glycoproteins on the extracellular surface.
• The Lipid Bilayer:     * Structure: Composed of phospholipid molecules with a hydrophilic (water-loving) head and a hydrophobic (water-fearing) hydrocarbon tail.     * Characteristics: It is fluid rather than solid and exhibits selective permeability.     * Functions: Forms the framework of the membrane and acts as a major barrier to prevent water penetration.

Membrane Proteins and Their Roles

• Integral (Intrinsic) Proteins: These proteins penetrate through the lipid bilayer.
• Peripheral (Extrinsic) Proteins: These are attached to the surface (inside or outside) of the membrane.
• Functional Categories:     * Carrier Proteins: Involved in transporting substances across the membrane.     * Marker Proteins: Used for cell identification.     * Receptor Proteins: Involved in receiving chemical signals.

Passive Transport Mechanisms

• General Characteristics: No energy expenditure required; movement occurs from high concentration to low concentration (down the concentration gradient).
• Simple Diffusion:     * Mechanism: Substances soluble in lipids dissolve in the cell membrane and diffuse across it.     * Examples: Oxygen ($O_2$), Carbon Dioxide ($CO_2$), and Water ($H_2O$).     * Factors Affecting Diffusion Rate:         1. Membrane permeability.         2. Concentration difference across the membrane.         3. Membrane electrical potential difference.         4. Pressure difference across the membrane.
• Facilitated Diffusion:     * Mechanism: Diffusion of lipid-insoluble or water-soluble substances aided by membrane proteins.     * Types:         1. Carrier-mediated diffusion: Used for substances like glucose and amino acids. The carrier protein forms a compound with the substrate that is lipid-soluble to move through the membrane. It exhibits high selectivity, saturation phenomenon (limited number of carriers), and competitive inhibition.         2. Channel-mediated diffusion: Used for ions such as $Na^+$, $K^+$, and $Ca^{2+}$. High selectivity based on charge and size.

Detailed Ion Channel Physiology

• Ionic Channel Definition: Protein channels believed to be watery pathways through the interstices of protein molecules.
• Characteristics: High selective permeability, gating mechanisms, and time-dependent functional states.
• Functional States of Voltage-Gated Channels:     1. Resting State: Gate is closed but capable of opening.     2. Open State (Activated): Gate is open, allowing ion flow.     3. Closed State (Inactivated): Gate is closed and incapable of opening for a period of time.
• Types of Gating:     * Voltage Gating: Gate responds to the electrical potential across the membrane (e.g., voltage-dependent $Na^+$ channels with m-gates and h-gates).     * Chemical Gating: Binding of specific chemical molecules (ligands) causes a conformational change (e.g., $N_2-Ach$ nicotinic acetylcholine receptor channel).     * Mechanical Gating: Gate opens due to stretching of the cell membrane (e.g., hair cells in the inner ear).

Active Transport Mechanisms

• General Characteristics: Moves molecules or ions "uphill" against a chemical or electrochemical gradient by expending energy (ATP). It creates and maintains electrochemical gradients.
• Primary Active Transport:     * Definition: Energy is derived directly from the breakdown of adenosine triphosphate (ATP) by a transporter.     * Concentration Gradients (Standard Levels):         * $Na^+_{extracellular} = 140.0\,mmol/L$         * $Na^+_{intracellular} = 15.0\,mmol/L$         * $K^+_{extracellular} = 4.0\,mmol/L$         * $K^+_{intracellular} = 150.0\,mmol/L$     * Sodium-Potassium Pump ($Na^+-K^+$ ATPase):         * Directional transport: Pumps $3\,Na^+$ ions out of the cell and $2\,K^+$ ions into the cell.         * Requires ATP.         * Electrogenic process: Makes the cytoplasm slightly more negatively charged than the exterior.     * Physiological Roles of the Sodium-Potassium Pump:         1. Maintaining $Na^+$ and $K^+$ gradients.         2. Establishing a negative electrical potential inside the cell.         3. Controlling cell volume.         4. Providing energy for secondary active transport.
• Secondary Active Transport:     * Mechanism: Uses the ion gradient established by primary active transport (usually $Na^+$) to move other substances against their own gradients. ATP is not split directly by this carrier.     * Cotransport (Symport): Substances move in the same direction as the driving ion (e.g., $Na^+$-glucose, $Na^+$-amino acid cotransport).     * Counter Transport (Antiport): Particles move in opposite directions (e.g., $Na^+-Ca^{2+}$ exchange, $Na^+-H^+$ exchange).

Bulk Transport: Endocytosis and Exocytosis

• Endocytosis: Process where large amounts of materials are brought into the cell by the plasma membrane.     * Phagocytosis: "Cell eating."     * Pinocytosis: "Cell drinking."
• Exocytosis: Process where large amounts of materials (contained in vesicles) are moved out of the cell by fusing with the plasma membrane.

Questions & Discussion

• Concept Review: What is the difference between primary active transport and secondary active transport?
• Systematic Overview: Describe the various ways of membrane transport.
• Functional Importance: Describe the physiological role of the sodium pump.