Osmosis, Protein Channels and Active Transport

Class Details

  • Greeting: Good afternoon, class.

  • Service Learning Groups: Preston and Aya should speak with the instructor after class regarding the draft submissions.

  • Exam Information: Exam scheduled for Friday; confirmed to be posted on Canvas.

  • Study Reminder: Reminder to bring pencils, not pens, for exam completion. One student previously bubbled answers in pen and was given a special exception for pencil use, which is unlikely to be repeated.

Important Concepts in Osmosis

  • Definition of Osmosis: Focuses solely on water movement across membranes.

  • Osmotic Terms:

    • Tonicity: The measure of the osmotic pressure gradient, characterized by three terms:

    • Hyperosmotic: Higher solute concentration outside the cell compared to inside.

    • Hypoosmotic: Lower solute concentration outside the cell compared to inside.

  • Key Points:

    • Forward Movement of Water: Water moves from outside to inside the cell in hyperosmotic conditions.

    • Solute Movement: Solutes will move from areas of high concentration to areas of low concentration.

Transport Proteins

General Information

  • Two primary types of transport proteins involve channels and carriers

  • Rate of Transport:

    • Channel Proteins: Allow high rates of transport, between 10,000 to 100,000 molecules per second.

    • Carrier Proteins: Generally slower due to the binding process; they need to be free to transport additional solutes.

Specificity and Regulation

  • Channels and carriers are highly specific for their respective solute(s) and are regulated (i.e., they open/close based on cellular needs).

  • Energy Requirements: Movement occurs from high concentration to low concentration without energy expenditure.

Types of Channels

  1. Leaky Channels:

    • Always partially open; help maintain resting membrane potential (often exemplified with potassium channels).

    • Allow potassium ions to flow across the membrane, contributing to housekeeping functions such as circulation and respiration during rest.

  2. Regulated Channels:

    • Ligand-Gated Channels: Open in response to the binding of signaling molecules (ligands).

    • Intracellular Regulatory Protein Channels: Signals binding to these channels come from within the cell.

    • Phosphorylated Channels: Phosphate binds covalently causing a conformation change, allowing channel activity.

  3. Voltage-Gated Channels:

    • Channel opening and closing depend on the membrane potential.

  4. Mechanosensitive Channels:

    • Sensitive to changes in mechanical forces, such as pressure changes in the inner ear, can be affected by sound and noise levels.

Carriers and Transport Types

  • Carriers facilitate diffusion, emphasizing the concept of facilitated diffusion where no energy is required.

  • Differences in Transport Rates:

    • Carriers have a saturation point due to binding availability.

  • Types of Carriers:

    • Uniporter: Transports one solute across the membrane.

    • Cotransporters: Transport more than one solute; divided into:

    • Symporter: Both solutes move in the same direction.

    • Antiporter: Solutes move in opposite directions, maintaining their concentration gradients.

Active Transport

General Characteristics

  • Active transport moves solutes against their concentration gradient and requires energy (ATP).

  • Types of Active Transport:

    1. Primary Active Transport: Directly uses energy from ATP hydrolysis to transport solutes.

    2. Secondary Active Transport: Uses the energy derived from the gradient created by primary transport.

Sodium-Potassium Pump

  • *Mechanism Overview:

    • Involves altering conformation states (E1 to E2) to facilitate the transport of sodium and potassium.

Transport Details

  1. Conformational Change (E1):

    • Affinity to bind with 3 sodium ions; requires ATP hydrolysis for energy to change to E2.

    • Sodium ions released to the extracellular environment against their concentration gradient.

  2. Conformational Change (E2):

    • Following sodium release, potassium ions bind, leading to a new conformational change to return to E1 and push potassium inside against its gradient.

    • Results in a net balance across the membrane, creating an electrochemical gradient important for cell function.

    • After the complete cycle, the pump is noted as an electrogenic pump due to a difference of charge maintained across the membrane (3 sodium out for every 2 potassium in).

Endocytosis and Exocytosis

  • Processes used for cell uptake or export of materials, especially when size goes beyond transport protein capabilities.

Endocytosis:

  • Two forms:

    • Phagocytosis (Cell Eating): Involves the uptake of solid particles (e.g., pathogens).

    • Pinocytosis (Cell Drinking): Involves the uptake of liquid substances; crucial for nutrient absorption.

    • Example scenario: Absorption within intestinal lining cells to collect nutrients.

Exocytosis

  • Involves the packaging of proteins and larger molecules into vesicles for secretion out of the cell (e.g., hormones, neurotransmitters).

    • Cargo examples: Collagen and insulin.

    • Vesicles fuse with the plasma membrane, expelling contents outside the cell.