Membrane Structure and Function

• Syllabus covers membrane structure and function, transport across membranes, passive transport, active transport, endocytosis, and exocytosis.
• Students should be able to explain the fluid mosaic model, outline membrane functions, explain substance movement across membranes, and apply this knowledge to problem-solving.

Membrane Structure and Function

• Membranes are made of a phospholipid bilayer, with embedded/attached proteins, glycoproteins, glycolipids, and cholesterol. The membranes include the:
  • Plasma membrane
  • Membranes surrounding the organelles (e.g. nuclear envelope).
• Membranes are 5-10 nm, while cell surface membranes are about 7 nm thick.

Phospholipid Bilayer

• Amphipathic phospholipids have hydrophilic phosphate heads facing outwards and hydrophobic fatty acid chains facing inwards.
• The hydrophilic heads contact the aqueous exterior and interior of the cell; the hydrophobic tails are sandwiched between the heads, shielded from the aqueous environment.
• Some phospholipid fatty acid chains are saturated, some are unsaturated.
• Saturated fatty acids allow closer packing of phospholipids, whereas unsaturated fatty acids (with C=C double bonds) cause kinks, preventing close packing.
• More unsaturated fatty acids increase membrane fluidity and prevent freezing at low temperatures.

Fluid Mosaic Model

• Describes the arrangement of phospholipids and proteins in cell membranes.
• Phospholipids and proteins are in constant motion (dynamic) unless anchored.
• Phospholipids and proteins move laterally within the membrane plane.
• Hydrophobic interactions primarily hold phospholipids together, and membrane proteins are scattered within the phospholipid sea.

Membrane Asymmetry

• Membrane composition and function differ between the outer and inner layers.
• The amounts of proteins, phospholipids, and cholesterol vary between layers.
• Glycoproteins and glycolipids of the cell surface membrane face the cell exterior, involved in cell recognition, communication, and adhesion.

Cholesterol

• It is amphipathic
• The hydrophobic hydrocarbon skeleton interacts with phospholipid fatty acid chains, and the hydrophilic –OH group interacts with phosphate heads.
• It regulates membrane fluidity.
  • Prevents fluidity from fluctuating too much at extreme temperatures.
  • At low temperatures, it increases fluidity by preventing close packing.
  • At high temperatures, it decreases fluidity by interacting with phospholipid and glycolipid fatty acid chains

Membrane Proteins

• Scattered in the phospholipid "sea," forming a mosaic.

Types of Membrane Proteins

• Intrinsic/Integral Proteins:
  • Embedded in the membrane.
  • Not easily removed without detergents.
  • The Non-polar amino acid residues interact with hydrophobic fatty acid chains. The Hydrophilic amino acid residues interact with hydrophilic phosphate heads. The Transmembrane proteins span the entire phospholipid bilayer.
• Extrinsic/Peripheral Proteins:
  • Largely hydrophilic.
  • Loosely attached to the membrane surface and easily removed.

Functions of Membrane Proteins include

• Transport (channel/carrier proteins):
  • Transmembrane proteins form a hydrophilic channel to shield polar molecules and ions, helping them cross the hydrophobic bilayer.
• Enzymes:
  • Active sites exposed to substrates in the cytosol for enzymatic reactions.
• Receptors (cell signalling):
  • Binding site exposed to the exterior for ligand binding.
  • Allows cells to detect and respond to external stimuli, triggering a cellular response.
  • Usually have a carbohydrate side-chain.
• Cell Recognition/Communication:
  • Glycoproteins bind to proteins/glycoproteins/glycolipids of other cells.
  • Acts as receptors involved in cell-cell recognition.
• Cell Adhesion:
  • Glycoproteins/proteins bind to adjacent cells in correct orientation.
  • Important for regulating cell growth/division and tissue formation.
• Attachment to Cytoskeleton:
  • Components of the cytoskeleton bind to membrane proteins.
  • Helps maintain cell shape and stabilize the location of certain membrane proteins.
• Glycoproteins and glycolipids are involved in cell-cell recognition, communication, and adhesion.
• All membranes form a hydrophobic barrier between the external environment and cytoplasm or between the cytoplasm and organelles, regulating movement of substances.
• It also provides compartmentalization for constant internal environment and optimal conditions.

Transport Across Membranes

• Movement of substances into and out of cells across the cell surface membrane must occur continuously for cells to function properly.
• Transport processes are essential for:
  • Entry of useful substances like oxygen, glucose and nutrients.
  • Secretion of useful substances such as hormones and digestive enzymes.
  • Removal of waste products like carbon dioxide and urea.
  • Maintenance of optimal conditions such as pH and ionic concentrations.

Permeability of Phospholipid Bilayer

• Membranes are partially permeable.
• Hydrophobic fatty acid chains create a hydrophobic barrier, preventing polar molecules/ions from crossing directly.
• Only non-polar molecules can diffuse directly.

Passive and Active Transport

• Passive Transport:
  • Simple diffusion:
    • For small, nonpolar molecules down a concentration gradient.
  • Facilitated diffusion:
    • Polar molecules or charged ions diffuse down the concentration gradient, aided by channel or carrier proteins.
  • Osmosis:
    • It is movement of water molecules down a water potential gradient.
    • Water moves either directly across membrane or via aquaporins.
• Active Transport:
  • Moves molecules/ions against a concentration gradient, requiring energy (ATP) and specific transport proteins.
• Bulk Transport:
  • Endocytosis (Phagocytosis, pinocytosis, receptor-mediated endocytosis)
    • Uptake of large particles or large quantities of small molecules, forming vesicles from the cell surface membrane; requires energy.
  • Exocytosis releases large molecules by fusing vesicles with the cell membrane; requires energy.

Simple Diffusion

• Net movement of molecules from high to low concentration regions.
• Does not require energy.
• Factors increasing the rate of it include:
  • Higher temperature/kinetic energy.
  • Greater concentration gradient.
  • Larger surface area.
  • Increased hydrophobicity.
  • Shorter distance.
  • Smaller particle size.

Facilitated Diffusion

• Net movement of polar molecules or charged ions from high to low concentration, aided by specific transport proteins.
• No energy required.
• Transport proteins are transmembrane, with a hydrophilic interior.
• Transport proteins are specific to the molecules/ions being transported and can become saturated.
• Channel proteins are specific, intrinsic/transmembrane proteins with hydrophilic channels and can be gated (open/close in response to a stimulus).
• Carrier proteins exist in two conformations and change shape to transport molecules/ions.
• Inhibitors can inhibit carrier proteins.

Osmosis

• Net movement of water molecules from high to low water potential.
• No energy required.
• Water moves across a membrane, either directly or via aquaporins. Can cause cells to lyse, shrivel or plasmolysis.

Active Transport

• It's the movement of molecules/ions from low to high concentration
• It opposes concentration gradients through specific transport proteins and energy (ATP).
• Phosphate groups from ATP transfer to carrier proteins, causing a conformational change.
• Significance: Continually take up nutrients and remove wastes, maintaining optimal internal conditions.

Bulk Transport

• Requires Vesicles
• Endocytosis (into the Cell)
  • Uptake of large molecules. There are three types of it:
    • Phagocytosis:
      • Uptake of large particles by phagocytes, forming a phagocytic vesicle (phagosome) which fuses with lysosomes to digest the particles.
    • Pinocytosis:
      • Uptake of liquids or small, dissolved particles. Not selective.
    • Receptor-Mediated Endocytosis:
      • Specific particles bind to cell surface receptors, the membrane invaginates to form a coated pit and then a coated vesicle. It is selective.

Exocytosis

• Uses Secretory Vesicles
• Contents release OUT of the cell by travel secretions along cytoskeletal microtubules. The membrane fuses with the cell.