Movement of Substances Across the Plasma Membrane Study Notes

CHAPTER 3: MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE

3.1 STRUCTURE OF PLASMA MEMBRANE

  • Permeability of a Plasma Membrane

    • S.J. Singer and G. Nicholson proposed a membrane model called the Fluid Mosaic Model to describe the structure of plasma membrane.

    • Components of the Plasma Membrane:

    • Comprised of a phospholipid bilayer and various types of protein molecules embedded within it.

    • Both phospholipids and proteins are not static but are constantly moving, forming a dynamic and fluid structure.

    • This movement allows phospholipids and proteins to glide sideways within the membrane, exhibiting a fluid characteristic.

    • Mosaic Pattern:

    • The various proteins integrated into the plasma membrane create a mosaic pattern across the surface.

  • Types of Molecules in the Membrane:

    • Glycoproteins and Glycolipids:

    • Act as receptors for hormones.

    • Stabilize the membrane by forming hydrogen bonds with water.

    • Function as antigens for cell identification.

    • Cholesterol:

    • Strengthens and modulates the flexibility of the phospholipid bilayer.

    • Reduces permeability to water-soluble (hydrophilic) substances, such as ions.

  • Membrane Permeability Types:

    • Permeable

    • Impermeable

    • Selectively Permeable

3.2 CONCEPT OF MOVEMENT OF SUBSTANCES ACROSS A PLASMA MEMBRANE

  • Passive Transport

    • Characteristics of substances able to move across a plasma membrane:

    • Phospholipid Bilayer:

      • Small non-polar molecules (e.g., oxygen, carbon dioxide).

      • Lipid-soluble molecules (e.g., fatty acids, glycerol, vitamins A, D, E, K, and steroid compounds).

      • Small polar molecules (e.g., water).

    • Channel Protein:

      • Small charged mineral ions (e.g., calcium ions, magnesium ions, sodium ions).

      • Small polar molecules (e.g., water).

    • Carrier Protein:

      • Slightly larger uncharged polar molecules (e.g., glucose, amino acids).

    • Movement occurs down the concentration gradient:

    • From an area of higher concentration to an area of lower concentration.

    • This passive transport continues until dynamic equilibrium is achieved.

    • Energy Requirement: Does not require energy.

  • Factors affecting movement through the plasma membrane:

    • Size of molecules

    • Ionic charge

    • Polarity of molecules

    • Hydrophilic molecules: Polar/charged water-soluble molecules.

    • Hydrophobic molecules: Non-polar, uncharged lipid-soluble molecules.

  • Specific Transport Processes:

    1. Simple Diffusion:

    • Occurs with lipid-soluble molecules and small non-polar molecules within the phospholipid bilayer.

    • It is a passive movement of molecules down the concentration gradient until dynamic equilibrium is reached.

    • Can occur with or without the presence of a semi-permeable membrane; requires no energy or transport proteins.

    • Incorrect vs. Correct Statement:

      • Incorrect - water enters the cells.

      • Correct - water diffuses into cells by osmosis.

    1. Osmosis:

    • Net movement of water molecules from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration) through a semi-permeable membrane until dynamic equilibrium is achieved.

    • Moves down the water potential gradient without energy.

    1. Facilitated Diffusion:

    • Passive movement of molecules across the plasma membrane through specific carrier proteins down the concentration gradient without using energy.

    • Substances able to pass through:

      • Channel Protein: Small charged mineral ions.

      • Carrier Protein: Slightly larger uncharged polar molecules (e.g., glucose, amino acids).

    • Mechanism:

      • A glucose molecule from a region of high concentration outside the cell enters the carrier protein and binds to the specific binding site.

      • Carrier proteins then change shape to allow glucose molecules to pass through and enter the cell.

      • Carrier proteins revert to their original shape and are ready to transport other molecules.

  • Active Transport

    • Definition:

    • Movement of molecule or ionic substances across a plasma membrane occurs against their concentration gradient: from a region of low concentration to a region of high concentration.

    • Energy Requirement: Requires energy in the form of ATP.

    • Carrier Proteins: Specific carrier proteins are required, with specific sites to bind with certain molecules or ions.

    • Results: Accumulation or elimination of molecules or ions in or from the cell.

    • Proton Pump:

    • Energy from ATP enables hydrogen ions to move across the carrier protein out of the cell.

    • Results in the accumulation of hydrogen ions that increase the acidity of stomach contents, providing an acidic medium for protein digestion.

    • Sodium-Potassium Pump:

    • Sodium ion concentration (Na+) is high outside the cell and low inside the cell; potassium ion concentration (K+) is low outside the cell and high inside the cell.

    • This pump simultaneously transports sodium ions into the extracellular fluid and potassium ions, through the carrier protein, into the cell, using ATP.

    • Vital for muscle contractions and heart function.

    • Mechanism:

      • Three sodium ions in the cytoplasm bind to the carrier protein (sodium-potassium pump).

      • Binding of sodium ions stimulates breakdown of ATP into ADP and phosphate (P).

      • The phosphate group binds to the carrier protein, providing energy to change the shape of the carrier protein, allowing sodium ions to exit the cell.

      • Two potassium ions bind from the outside, and upon release of the phosphate group, the carrier protein returns to its original shape, facilitating the entry of potassium ions into the cell.

ENDOCYTOSIS & EXOCYTOSIS

  • Both processes do not involve direct passage through the plasma membrane; both require energy in the form of ATP for fusing and breaking of the plasma membrane.

  • Features:

    Feature

    Endocytosis

    Exocytosis


    Direction

    Into the cell

    Out of the cell


    Vesicle Formation

    Plasma membrane folds inward to form a vesicle

    Vesicle fuses with the plasma membrane


    Purpose

    Brings in large molecules (e.g., nutrients, pathogens)

    Expels waste or secretes substances (e.g., enzymes, hormones)


    Example

    Phagocytosis (e.g., Amoeba sp., white blood cell)

    Secretion of digestive enzymes

    • Mechanism:

    • In endocytosis, the plasma membrane surrounds a part of the exterior environment and buds off as an internal vesicle.

    • In exocytosis, a vesicle fuses with the plasma membrane, releasing its contents, and its membrane integrates with the plasma membrane.

STRUCTURED QUESTIONS

  • Question Types Overview:

    • Various structured questions assessing understanding of plasma membrane structure, types of movements, and specific transport mechanisms.

  • Characteristics and Functions of Specific Structures Identified:

    • P, Q (structures within diagrams) and their respective roles.

    • Differentiating between types of movements (e.g., substance P and Q) regarding concentration levels and binding interactions with carrier proteins.

HOMEWORK QUESTIONS

  • Exercises aimed at reinforcing the concepts of membrane structure, movement mechanisms, and practical applications of those concepts across various cellular environments.