Bio Ch5 Pt1

Selective Uptake and Export of Ions

  • Function of the plasma membrane: Encloses cells, acting as a gateway for substances entering or exiting.

Structure and Composition of Plasma Membrane

  • Composed of:
    • Phospholipids
    • Proteins
    • Cholesterol
  • Plasma membrane is common to all cells: Prokaryotic and eukaryotic.
  • Fluid mosaic model: Concept describing the need for membranes to be fluid for transport processes.
    • Enables synthesis and trafficking of membrane components.

Selective Permeability of Plasma Membrane

  • Definition: Not all substances can cross the membrane; it controls entry and exit based on:
    • Size
    • Charge
    • Lipid solubility
    • Presence of channels and transporters
  • Contribution to cell structure:
    • Separates extracellular fluid (interstitial fluid) from cytosol (fluid inside the cell).

Key Functions of Biological Membranes

  • Selective uptake and export of ions and molecules.
  • Compartmentalization: Regions within cells compartmentalize biochemical reactions.
  • Protein sorting discussed in Chapter 4: Orientation towards lysosomes, plasma membranes, or secretion.
  • Anchoring of cytoskeleton: Membranes interact with microtubules, microfilaments, and intermediate filaments for shape and support.
  • Energy production: Involvement in ATP and NADPH production.
  • Cell signaling: Membrane role in processes related to division (cytokinesis).
  • Adhesion between cells through specialized proteins.

Historical Models of Plasma Membrane Structure

  • Sandwich Model (1935) by Davson and Danielli:
    • Membrane as a phospholipid bilayer sandwiched between two protein layers.
    • Limited by inability to account for membrane differences and asymmetry.
  • Fluid Mosaic Model (1972) proposed by Singer and Nicholson:
    • Recognized different compositions and arrangements of proteins in membranes.
  • Development of high-resolution electron microscope in the 1940s enabled freeze fracture studies.

Freeze Fracture Studies

  • Methodology:
    • Rapid freezing of biological samples.
    • Fracturing along the lipid bilayer resulting in two leaflets.
    • Metal layer applied to create a replica viewed under a transmission electron microscope.
  • Results:
    • Showed asymmetrical distribution of proteins: More integral proteins in the cytoplasmic layer than in the extracellular layer.

Fluid Mosaic Model Overview

  • Membranes characterized by:
    • Fluidity: Lipids and proteins moving relative to each other.
    • Basic structure: Phospholipid bilayer; additional cholesterol present.
    • Integral membrane proteins span the membrane; peripheral proteins are noncovalently attached.
    • Glycolipids predominantly in the extracellular leaflet with carbohydrate portions protruding.

Membrane Fluidity

  • Individual mobility in a biological membrane:
    • Lipids can rotate and move laterally; flip-flop movement is unfavorable due to energetic costs.
    • Enzyme flippase aids lipid transport between leaflets, requiring ATP.
  • Factors influencing fluidity:
    • Presence of double bonds in lipid tails: Create kinks, enhancing fluidity (unsaturated lipids are more fluid).
    • Cholesterol's dual effect:
    • At high temperatures: Decreases fluidity.
    • At low temperatures: Increases fluidity, preventing freezing.

Adaptation to Temperature Changes

  • Organisms not maintaining constant body temperature adjust membrane lipid composition based on seasonal temperatures.
    • Increase in unsaturated fatty acids in colder months to prevent solidification.

Transmembrane Proteins and Their Movement

  • Transmembrane proteins can also move laterally but slower than lipids due to size.
  • Experiment by Fry & Edidin (1970):
    • Created mouse-human cell hybrids to study protein movement.
    • Fluorescent labeling demonstrated that proteins can move if cells are incubated at higher temperatures.

Membrane Protein Characteristics

  • Amphipathic Nature:
    • Integral proteins have hydrophobic and hydrophilic regions.
    • Hydrophobic regions integrate into the membrane, while hydrophilic regions protrude.
  • Mobility restrictions:
    • Some proteins bound to cytoskeletal filaments or extracellular matrix.

Functions of Membrane Proteins

  • Types of proteins and functions:
    • Transport proteins: Channel and transporter proteins facilitate movement across membranes (active and passive transport).
    • Enzymatic functions: Catalyze reactions at the membrane surface.
    • Signal Reception: Receive external signals (e.g., hormones, neurotransmitters).
    • Cell Recognition and Adhesion: Glycoproteins interact with glycolipids for cell recognition, mediating adhesion.
    • Structural Support: Maintain cell shape through connections with cell cytoskeleton.

Protein Sorting in Eukaryotic Cells

  • Sorting Signals: Direct transmembrane proteins to their specific destinations.
    • Include ER signal sequences that guide proteins to the endoplasmic reticulum.
    • Transmembrane segments formed by nonpolar amino acids.
    • Vesicle-mediated transport to other cell regions (Golgi, lysosomes, etc.).

Glycosylation Process

  • Glycosylation: Attachment of carbohydrates to proteins or lipids.
    • Glycolipid formation: Carbohydrate plus lipid.
    • Glycoprotein formation: Carbohydrate plus protein.
  • Functional consequences of glycosylation:
    • Critical for cell surface recognition and interaction.

Conclusion

  • Review of chapter conclusions and transition to next topics on membrane transport and selective permeability.