CELL MEMBRANE & MEMBRANE TRANSPORT

PLASMA MEMBRANE

  • Separation of Fluids

    • Divides extracellular fluid from intracellular fluid.

  • Structure

    • Thin structure (7-10 nm)

    • Composed of a lipid bilayer consisting of:

      • Membrane Lipids

      • Membrane Proteins

MEMBRANE LIPIDS

  • Components of Lipid Bilayer

    • Phospholipids

    • Cholesterol

    • Glycolipids

  • Phospholipid Structure

    • Charged polar hydrophilic head

    • Two fatty acid chains (uncharged, non-polar, hydrophobic tail)

    • Orientation:

      • Heads face intracellular and extracellular compartments

  • Glycolipids

    • Located on the outer plasma membrane surface

  • Cholesterol

    • Stabilizes and increases mobility & fluidity of the membrane

GLYCOCALYX

  • Structure

    • Fuzzy, sticky carbohydrate-rich area at the cell surface

  • Components

    • Enriched by glycolipids & glycoproteins

  • Function

    • Acts as a biological marker for cell recognition (e.g., sperm-ovum, immune recognition)

MEMBRANE PROTEINS

  • Contribution to Mass

    • Comprise half of the plasma membrane mass

    • Essential for specialized membrane functions

  • Types of Membrane Proteins

    • Integral proteins

    • Peripheral proteins

Integral Proteins

  • Firmly embedded in the lipid bilayer

  • Transmembrane Proteins

    • Span the entire width of the membrane and protrude on both sides

    • Contain hydrophobic and hydrophilic regions

    • Some function as enzymes; most involved in transport

Peripheral Proteins

  • Loosely attached to integral proteins

  • Support the membrane from the cytoplasmic side with a network of filaments

  • Function as enzymes and motor proteins

  • Link cells together and may float freely or anchor to the cytoskeleton

FUNCTIONS OF MEMBRANE PROTEINS

  1. Transport

  2. Receptors for Signal Transduction

  3. Attachment to Cytoskeleton & Extracellular Matrix

  4. Enzymatic Activity

  5. Intercellular Joining

  6. Cell-Cell Recognition

Transport

  • Proteins facilitate movement across the membrane

  • Types of Transport Proteins

    • Carrier Proteins

      • Bind solutes and undergo shape change to deliver to the other side

    • Channel Proteins

      • Form hydrophilic pores allowing specific substances to pass through

Receptors for Signal Transduction

  • Membrane proteins with binding sites for hormones

  • Initiate Cascading chemical reactions upon activation

Attachment to Cytoskeleton & Extracellular Matrix

  • Anchoring of cytoskeleton and extracellular matrix elements to membrane proteins

  • Facilitates coordination of extracellular and intracellular changes, as well as maintains cell shape

Enzymatic Activity

  • Membrane proteins can serve as enzymes, catalyzing metabolic reactions

Intercellular Joining

  • Membrane proteins of adjacent cells interact through various types of junctions

Cell-Cell Recognition

  • Glycoproteins function as identification tags recognized by other cells

PLASMA MEMBRANE TRANSPORT

  • Permeability

    • Cells are permeable to lipid-soluble substances (e.g., CO2, O2, fatty acids, steroid hormones)

    • Low permeability to water-soluble substances (e.g., ions, glucose, amino acids)

  • Transport Methods

    • Passive Movement

    • Active Movement

PASSIVE MOVEMENT

  • Diffusion

    • Movement from high concentration to low concentration (downhill gradient)

  • Types of Diffusion

    • Simple Diffusion

      • No helper protein required, for lipid-soluble substances

    • Facilitated Diffusion

      • Requires a helper protein (carrier/channel) for substances like glucose and certain ions

Osmosis

  • Definition: The diffusion of water through a selectively permeable membrane

  • Water travels through specific channels known as aquaporins

ACTIVE MOVEMENT

  • Characteristics

    • Movement against concentration gradient (uphill)

    • Requires energy (ATP) and carrier proteins

  • Types of Active Transport

    • Primary Active Transport

      • Directly utilizes ATP for energy

    • Secondary Active Transport

      • Utilizes the gradient established by primary active transport indirectly

VESICULAR TRANSPORT

  • Overview

    • Moves fluid containing large particles and macromolecules across membranes using vesicles

  • Types of Vesicular Transport

    • Endocytosis

    • Exocytosis

Endocytosis

  • Processes for ingestion:

    • Moves substances from cell exterior to interior via protein-coated vesicles

    • Types of Endocytosis

      • Phagocytosis

      • Receptor-mediated Endocytosis

      • Pinocytosis

Exocytosis

  • Ejection of substances from cell interior into extracellular fluid

  • Stimulated by signals like hormone binding or changes in membrane voltage

  • Involves v-SNAREs (transmembrane proteins on vesicles)