5 Membranes and Transport (DIY)

• Fluid Mosaic Model

  • Both phospholipid molecules and some proteins are free to move laterally (and transversely)
  • random arrangement of such proteins embedded into the membrane resembles a mosaic pattern
  • Cell surface membrane is a complex structure composing of many components
  • Relative amounts of such components vary from membrane to membrane, depending on its function (Asymmetrical, because the membrane is never identical on both sides)
• Phospholipid (STRUCTURE)

  Amphitatic Molecule (Negatively-charged, hydrophilic phosphate head + 2 non-polar, hydrophobic hydrocarbon chains of fatty acid tails)
• Phospholipids (FUNCTIONS - 1)

  • Form stable bilayers spontaneously as they line up with phosphate heads facing water, and their fatty acid tails facing away from water in the formation of a hydrophobic core
  • Phosholipids in the membrane are held together by weak hydrophobic interactions
  • membrane fluidity
• Phospholipids (FUNCTIONS - 2)

  • Saturated fatty acid tails: Membrane less fluid (Straight chains allowing maximum interaction of the fatty acid tails → increased hydrophobic interactions)
  • Unsaturated fatty acid tails: Membranes more fluid (Presence of C=C that causes kinks in the fatty acid tails → prevents close packing of phospholipid molecules → decreased hydrophobic interactions)
• Proteins (STRUCTURE)

  • Peripheral/ Extrinsic Proteins
  • Loosely attached at the surface of the phospholipid bilayer
  • Rich in amino acids with hydrophilic side chains --> permitting interaction with surrounding water and the charged hydrophilic phosphate heads of the bilayer
  • Integral/ Intrinsic Proteins
  • Either partially embedded or span across the phopholipid bilayer entirely (Transmembrane proteins)
  • Contain both hydrophilic and hydrophobic regions which interact with the charged phosphate heads and the non-polar fatty acid tails of the phopholipids respectively
• Proteins (FUNCTIONS)

  • Transport proteins: function as carrier proteins/ channel proteins
• Proteins (EMBEDDING)

  1. Membrane proteins are synthesised by bound ribosomes at RER and modification may occur in RER (Glycosylation)
  2. A transport vesicle with the glycoproteins embedded on its membrane buds off the RER and brings it to the cis face of the GA where further chemical modifications may occur
  3. A secretory vesicle with the glycoproteins embedded on its membrane buds off from the trans face of the GA and transports the glycoproteins to the cell surface membrane
  4. The secretory vesicle fuses with the cell surface membrane, and the glycoproteins are embedded in the cell surface membrane
• Glycoproteins (STRUCTURES)

  • Carbohydrates in the membrane covalently associated with either lipids or proteins
• Glycoproteins (FUNCTIONS)

  • Cell-cell recognition
  • Cell-cell adhesion
• Cholesterol (STRUCTURE)

  • Composed by mainly a non-polar carbon skeleton, consisting of 4 interconnected ring (hydrophobic)
  • Polar hydroxyl (-OH) groups of cholesterol (hydrophilic) is able to interact with the phosphate head of phospholipids, forming hydrophobic interactions with the fatty acids
• Cholesterol (FUNCTIONS - 1)

  • Presence helps to regulate fluidity
  • LOW temperatures: phospholipid molecules have less kinetic energy → cholesterol prevents them from being too closely packed and solidifying → helps to maintaining membrane fluidity when temperature drops
  • HIGH temperature: phopholipid molecules have more kinetic energy → cholesterol prevents them from moving too far apart as they hold the phopholipids together → restricts membrane fluidity when temperature rises, stabilising the membrane structure
• Chloesterol (FUNCTIONS - 2)

  • WITHOUT cholesterol
  • LOW temperatures: Phospholipids are rigid and not as fluid/ flexible → may break
  • HIGH temperatures: Phospholipids are too fluid/ flexible and would not hold shape → Membrane structure becomes wide-spread
• Function of Membrane (1)

  1. Compartmentalisation

  • Membranes allow separate compartments to be formed within the cell
  • Maintains a constant internal environment within each compartment
  • The different internal environments provide optimal conditions for enzymes' activity in catalysing different metabolic processes and biochemical pathways within these organelles
• Function of Membrane (2)

  1. Selectively Permeable Barrier

  • Membranes act as selective barriers for regulating the passage of substances in and out of the cell
  • High composition of lipids which allow only non-polar, hydrophobic molecules to pass through, preventing large/ polar molecules and charged particles/ ions from getting across
  • Presence of specific integral transport proteins allows hydrophilic substances to avoid the hydrophobic core of the phospholipid bilayer by passing through the hydrophilic channel of the transmembrane proteins
• Functions of Membrane (3)

  1. Communication with Surroundings

  • Membrane serves as a point of communication between one cell with another cell and even with its surrounding environment
  • Proteins embedded into the membrane act as receptor sites for recognising external stimuli
• Functions of Membrane (4)

  1. Site of Chemical Reactions

  • Due to the presence of enzymes and proteins organised and localised in a sequential manner in a membrane
• Type of Movement: Passive

  • Substances move down a concentration gradient from an area of higher to lower concentration
  • Cells do not expend energy
• PASSIVE: Simple diffusion

  • The movement of molecules or ions from a region of high concentration to a region of low concentration, down a concentration gradient
  • DOES NOT REQUIRE ATP
  • Diffusion eventually results in an equal distribution when dynamic equilibrium is achieved --> no net movement of substances
• PASSIVE: simple diffusion (FACTORS affecting: 1-4)

  1. Molecular size: smaller particles = faster diffusion
  2. Surface area of membrane: larger surface area = faster rate of diffusion
  3. Concentration gradient: Steeper concentration gradient = faster rate of diffusion
  4. Lipid solubility: Polar molecules and charged molecules are not lipid soluble (repelled by hydrophobic core of bilayer) + Charged ions and molecules attract water molecules (form hydration shells which increases their size) = unable to diffuse
• PASSIVE: simple diffusion (FACTORS affecting: 5-7)

  1. Presence of pores: Existence of pores = enhanced rate of diffusion
  2. Distance: Shorter distance = faster rate of diffusion
  3. Temperature: Higher kinetic energy = higher rate of diffusion + Increased fluidity (increased formation of transient pores) = increased rate of diffusion
• PASSIVE: Osmosis

  • The movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
  • DOES NOT REQUIRE ATP
  • Water molecules move across the membrane through transient pores or aquaporins
  • Osmosis continues until dynamic equilibrium is achieved and there is no more difference in water potential --> no net movement of water molecules in or out of the cell
• PASSIVE: Facilitated diffusion

  • The movement of a substance across a membrane via transport proteins from a region of high concentration to a region of low concentration, down a concentration gradient, with no expenditure of ATP
  • DOES NOT REQUIRE ATP
  • Requires transport proteins (as ions or polar molecules are repelled by the hydrophobic core of the phospholipid bilayer of membranes)
  • Highly specific in nature
• PASSIVE: Facilitated diffusion (TYPES)

  1. Channel proteins

  • Transmembrane proteins which provides a hydrophilic pore through which a specific ion, charged or polar molecule can diffuse through to move across the membrane
  • Some are gated → can close to prevent access to the hydrophilic pore and open only when an electrical or chemical signal is received

  2. Carrier proteins

  • Transmembrane proteins which have a specific binding site to bind the solute to be transported
  • Binding of the solute results in conformational changes of the carrier proteins and solute is moved across the membrane
• PASSIVE: Facilitated diffusion (FACTORS affecting: 1-2)

  1. Temperature: increased temperature = increased kinetic energy = higher chance of effective collisions between transport proteins and the solutes = higher rate of facilitated diffusion
  2. No. of transport proteins: greater number of carrier/ channel proteins embedded on membrane = faster rate of facilitated diffusion
• PASSIVE: Facilitated diffusion (FACTORS affecting: 3-4)

  1. Chance Collision: increased chance collision between transport proteins and substrates = increased rate of facilitated diffusion
  2. No. of substrate binding sites on the carrier proteins: Increased no. of substrate binding sites = increased rate of facilitated diffusion
• TYPE of movement : ACTIVE

  • Active: substance move against a concentration gradient from an area of lower to higher concentration through transport proteins in the membrane
  • Require energy expenditure in the form of ATP
• ACTIVE: active transport

  • The transport of polar molecules, ions or charged molecules across a membrane against a concentration gradient by carrier proteins called pumps embedded in the cell membrane
  • ATP IS CONSUMED
  • One-directional only
• ACTIVE: bulk transport - endocytosis

  • The movement of macromolecules or large quantities of a substance in/ out of the cell
  • The uptake of substances into the cell
• ACTIVE: bulk transport - endocytosis (PROCESS - 1. Phagocytosis)


  Phagocytosis: takes up solid substances

  1. ATP is used for rearrangement of filaments in the cytoskeleton to form pseudopodia (outward extensions of the membrane which wrap around and engulf the particle)
  2. Pseudopodia wraps around the solid particle and its ends fuse, forming a phagocytic vesicle containing the solid particle
  3. In the cytoplasm, the phagocytic vesicle fuses with a lysosome which contains hydrolytic enzymes to digest the solid particle
• ACTIVE: bulk transport - endocytosis (PROCESS - 2. Pinocytosis)

  1. Pinocytosis: takes up liquid/ dissolved substances

  •  A small area of the cell surface membrane invaginated and pinches off, forming small vesicles containing extracellular fluid within the cells
• ACTIVE: bulk transport - endocytosis (PROCESS - 3. Receptor-mediated endocytosis)

  1. Receptor-mediated endocytosis

  • Helps cell to acquire bulk quantities of specific substances even though they may not be in very high concentration in the extracellular fluid
  • Involved specific receptor proteins embedded in the cell surface membrane → exposed to extracellular fluid and have binding sites for extracellular substances called ligands
• ACTIVE: bulk transport - exocytosis

  • The secretion of macromolecules
  • Commonly used in the export of secretory products via the endomembrane system
• ACTIVE: bulk-transport - exocytosis (PROCESS)

  1. Secretory vesicles that have budded off from the GA move along the cytoskeleton fibres (microtubules) towards the cell surface membrane
  2.  Secretory vesicle fuses with the cell surface membrane and contents of the secretory vesicle are then released to the outside of the cell
• Explain why it is called a fluid mosaic

  1. Fluid - Both phospholipid molecules and some proteins are free to move laterally and transversely
  2. Mosaic - Random arrangement of proteins embedded in the membrane resemble a mosaic pattern + the membrane is never identical on both sides / asymmetrical.
•  Suggest why there are no channels for insulin release across the membrane

  1. Insulin is a peptide hormone/protein that is too large to cross the membrane via a channel; (must have this point)
  2. A channel protein with a hydrophilic channel large enough for insulin will also allow many other molecules to pass through; thus making the cell surface membrane fully permeable;
  3. Insulin is packaged in secretory vesicles and released through process of exocytosis instead.
• Describe the role of proteins, glycolipids and cholesterol in the structure of cell surface membranes. (GLYCOLIPIDS/GLYCOPROTEINS)

  Acts as markers for cell-cell recognition to distinguish cells as ‘self’/’non-self’ as the

  basis of the immune system;

  7. Involve in adhesion of cells to neighbouring cells (cell-cell adhesion), allowing cells

  to be attached to one another to form tissues and organs;
• Describe the role of proteins, glycolipids and cholesterol in the structure of cell surface membranes (CHOLESTEROL)

  Cholesterol regulates membrane fluidity by preventing the membrane from being overly

  fluid at warmer temperatures and preserves membrane’s rigidity by restricting

  phospholipids’ lateral movement (restricting membrane fluidity);

  9. The membrane is also prevented from becoming overly solid / rigid at lower

  temperatures as cholesterol prevents the close packing of phospholipids and hence

  prevents its solidification/ crystallization (maintaining membrane fluidity);