Membrane Transport

Unit 3.2: Membrane Transport

YPL Modified from @ Dr Sokhansanj

Lecture Objectives

  • Understand the general structure and function of the cell membrane.

  • Distinguish between active and passive transport.

  • Describe diffusion and its different forms.

  • Explain osmosis.

  • Discuss the importance of membrane transport in homeostasis and cellular function.

Cell Membrane Structure and Function

  • The cell membrane consists of a phospholipid bilayer that envelops the cell.

  • Functions as a dynamic barrier between the internal and external environment.

  • Maintains chemical and electrical gradients essential for cell function.

  • The plasma membrane maintains homeostasis by regulating what substances enter and leave the cell.

Overview of Membrane Composition

  • Extracellular fluid: The watery environment outside the cell.

  • The membrane includes:

    • Polar heads of phospholipid molecules

    • Carbohydrate portions of glycoproteins forming the glycocalyx

    • Bimolecular lipid layer containing proteins

    • Nonpolar tails of phospholipid molecules

    • Peripheral proteins located on one side of the membrane

    • Integral proteins that span the membrane

    • Filaments of cytoskeleton providing structural support

    • Cholesterol that stabilizes the membrane structure

    • Outward-facing and inward-facing layers of phospholipids

Transport Across the Plasma Membrane

  • Movement of substances across the plasma membrane is critical; some pass through easily while others do not.

  • The plasma membrane is selectively permeable, allowing only certain molecules to cross.

  • Two essential transport mechanisms are:

    • Passive transport: No energy (ATP) is required.

    • Active transport: Energy (ATP) is required.

Factors Affecting Diffusion

Pressure Gradient

  • Factors influencing diffusion include:

    • ΔConcentration: Difference in concentration between two areas.

    • Molecule size: Smaller molecules diffuse faster.

    • Temperature: Increased temperature raises kinetic energy, leading to faster diffusion.

    • Solvent viscosity: Affects the ease of movement through a medium.

Concentration Gradient

  • Solute: The substance being dissolved.

  • Diffusion: The movement of molecules from an area of high concentration to an area of low concentration.

  • Solvent: Often water, especially in the context of osmosis.

Speed of Diffusion

  • Influenced by three main factors:

    • Concentration: Greater concentration differences lead to faster diffusion.

    • Molecular size: Smaller molecules diffuse more rapidly.

    • Temperature: Higher temperatures increase diffusion speed due to greater kinetic energy.

  • Equilibrium is reached when there is no net movement of molecules in one direction.

Clinical Applications of Diffusion

  • Various substances diffuse differently based on their molecular weight as shown in the table:

    • Albúmina: 55,000 - 68,000

    • Mioglobina: 17,800

    • Beta 2 microglobulina: 11,800

    • Glucosa: 180

    • Urea: 60

    • Other substances range from 1,000 to 100

  • Implications for renal replacement therapies highlight the importance of diffusion in clinical settings.

Types of Membrane Transport

Passive Transport

  • Active transport requires energy (ATP).

  • Passive transport does not require energy and includes:

    • Simple Diffusion.

    • Facilitated Diffusion

    • Carrier-mediated diffusion

    • Channel-mediated diffusion

      • Leakage (nongated) channels

      • Gated-channels: chemically, voltage, mechanically-gated.

Passive Membrane Transport: Simple Diffusion

  • Molecules that can passively diffuse through the membrane include:

    • Nonpolar lipid-soluble (hydrophobic) substances (e.g., O2, CO2, steroid hormones, fatty acids).

    • Very small polar molecules like water.

Passive Membrane Transport: Facilitated Diffusion

  • Certain hydrophilic molecules (e.g., glucose, amino acids, and ions) are transported passively down their concentration gradient by:

    • Carrier-mediated facilitated diffusion: Substances bind to protein carriers.

    • Channel-mediated facilitated diffusion: Substances move through water-filled channels.

Carrier-mediated facilitated diffusion

  • Carriers are transmembrane integral proteins:

    • Transport specific polar molecules (e.g., sugars and amino acids) that are too large for membrane channels.

    • The binding of a molecule causes the carrier to change shape, moving the molecule across the membrane.

    • Saturation occurs when all carriers are bound to molecules.

Channel-mediated facilitated diffusion

  • Channels formed by transmembrane proteins transport molecules:

    • Specificity dependent on pore size and/or charge.

    • Water channels known as aquaporins (AQP).

    • Two types of channels:

    • Leakage channels: Always open.

    • Gated channels: Controlled by chemical or electrical signals.

Osmosis

  • Water diffuses across plasma membranes through:

    • Lipid bilayer despite being polar due to its small size.

    • Specific aquaporins (AQP).

  • Water flow occurs based on concentration differences across the membrane—this is crucial for maintaining cellular integrity.

Osmolarity and Tonicity

  • Osmolarity: Measures the concentration of total solute particles in a solvent.

  • Water concentration is inversely related to solute concentration:

    • Increase in solute concentration (hypertonic) leads to a decrease in water concentration.

    • Conversely, lower solute concentration (hypotonic) leads to higher water concentration.

  • Water moves by osmosis from areas of low solute concentration (high water; hypotonic) to areas of high solute concentration (low water; hypertonic).

Tonicity Types

Type

Description

Hypertonic

Has a higher solute concentration than another solution.

Isotonic

Has an equal solute concentration to another solution.

Hypotonic

Has a lower solute concentration than another solution.

Equilibrium in Diffusion & Osmosis

  • When different osmolarity solutions are separated by a permeable membrane:

    • Diffusion of solutes and osmosis occur until equilibrium is reached.

    • Equilibrium is characterized by the same concentration of solutes and water molecules on both sides of the membrane, with equal volume.

Body Water Distribution

  • Total body water varies by age and is distributed as follows:

    • Intracellular fluid (ICF): accounts for a significant portion of body water.

    • Extracellular fluid (ECF): which includes plasma and interstitial fluids.

  • Percentage of total body water by age typically shows a decline as individuals grow older.

Key Takeaways

  • The cell membrane, composed of a phospholipid bilayer, maintains homeostasis by selectively controlling substance movement in and out of the cell.

  • Passive transport processes (e.g., simple diffusion, facilitated diffusion, osmosis) allow substances to move down concentration gradients without energy use, critical for nutrient and gas exchange.

  • Active transport requires ATP to move substances against concentration gradients, ensuring vital ions and molecules are concentrated where needed.

  • Osmosis and tonicity principles govern water movement across membranes, essential for maintaining cellular, interstitial, and intravascular volumes.