Study guide: Membrane Structure and Transport

📋 Course Outline

1. Fluid Mosaic Membrane Structure
2. Membrane Fluidity and Adaptation
3. Membrane Proteins and Carbohydrates
4. Selective Permeability and Transport Proteins
5. Passive Transport and Osmosis
6. Active Transport and Membrane Potential
7. Bulk Transport by Vesicles

📖 1. Fluid Mosaic Membrane Structure

🔑 Key Concepts & Definitions

• Fluid mosaic model: A fluid phospholipid bilayer with a mosaic of proteins embedded within it or attached to its surface.

• Phospholipid: An amphipathic molecule with both hydrophobic and hydrophilic regions.

★ Must-know

• Membranes consist mainly of lipids and proteins, with carbohydrates also present, and phospholipids are the most abundant plasma-membrane lipid.

Further detail

• In 1935, Hugh Davson and James Danielli proposed a sandwich model in which a phospholipid bilayer lies between two layers of globular proteins.

• In 1972, J. Singer and G. Nicolson proposed a mosaic of proteins dispersed within the phospholipid bilayer, with only hydrophilic protein regions exposed to water; freeze-fracture studies supported this model.

💡 Memory Hook

Picture a fluid phospholipid sea with proteins embedded like scattered islands.

📖 2. Membrane Fluidity and Adaptation

★ Must-know

📌 Membranes rich in unsaturated fatty acids are more fluid than membranes rich in saturated fatty acids.

• At warm temperatures such as 37°C, cholesterol restrains phospholipid movement, while at cool temperatures it prevents tight packing and helps maintain membrane fluidity.

Further detail

• Phospholipids move laterally within the bilayer, adjacent molecules switch places about 10^7 times per second, and transverse flip-flop is rare.

• Membrane proteins move more slowly than lipids because they are larger, and proteins anchored to cytoskeletal or extracellular-matrix elements do not move.

• Membrane fluidity must be balanced because solidification changes permeability and inactivates some enzymes, while excessive fluidity prevents membranes from performing their role properly.

• Cold-water fish have more unsaturated fatty acids in their plasma membranes, and winter wheat increases unsaturated lipids in its membranes during fall to help prevent winter solidification.

💡 Memory Hook

Unsaturated lipids keep membranes fluid in cold; cholesterol restrains warmth but prevents cold packing.

📖 3. Membrane Proteins and Carbohydrates

★ Must-know

📌 Integral proteins penetrate the hydrophobic core of the membrane, and most are transmembrane proteins that span it; peripheral proteins bind to the membrane surface or to other membrane proteins.

• Membrane proteins perform these functions:

  • Transport
  • Enzymatic activity
  • Signal transduction
  • Cell-cell recognition
  • Intercellular joining
  • Attachment to the cytoskeleton and extracellular matrix

• Short carbohydrate chains of fewer than 15 monomers occur on the external plasma-membrane surface, covalently attached to lipids as glycolipids or, more commonly, to proteins as glycoproteins.

Further detail

• The hydrophobic regions of integral proteins contain one or more stretches of nonpolar amino acids, often arranged as alpha helices.

• Transmembrane proteins expose hydrophobic nonpolar regions within the membrane and hydrophilic polar regions to the extracellular fluid and cytosol.

• Surface carbohydrates vary among species, individuals, and cell types and act as markers for cell recognition; human blood types A, B, AB, and O differ in carbohydrate portions of red-cell glycoproteins.

• The endoplasmic reticulum and Golgi apparatus establish the asymmetric distribution of proteins, lipids, and associated carbohydrates as membranes are built.

📖 4. Selective Permeability and Transport Proteins

🔑 Key Concepts & Definitions

• Selective permeability: The property by which a plasma membrane regulates molecular traffic, allowing some substances to cross more easily than others.

• Channel protein: A hydrophilic passage through the membrane for particular molecules or ions.
• Carrier protein: Binds a molecule and changes shape to shuttle it across the membrane.

★ Must-know

📌 Hydrophobic nonpolar molecules such as hydrocarbons, carbon dioxide, and oxygen dissolve in the lipid bilayer and cross rapidly, whereas polar molecules such as sugars, ions, and water do not cross it easily.

Further detail

• Aquaporins are channel proteins that facilitate water passage, with each aquaporin allowing up to 3 billion (3×10^9) water molecules per second.

📌 Transport through membrane proteins can be passive or active without requiring the morphological changes involved in bulk transport by endocytosis or exocytosis.

💡 Memory Hook

Small nonpolar molecules cross the bilayer; hydrophilic solutes need transport proteins.

📖 5. Passive Transport and Osmosis

🔑 Key Concepts & Definitions

• Passive transport: The diffusion of a substance across a membrane down its concentration gradient without energy investment.

• Diffusion: The tendency of molecules to spread evenly through available space; absent other forces, a substance moves from higher to lower concentration without energy input.

• Osmosis: The diffusion of water across a selectively permeable membrane from lower solute concentration, where water concentration is higher, toward higher solute concentration, where water concentration is lower.

★ Must-know

📌 Tonicity depends on nonpenetrating solutes: an isotonic solution has the same solute concentration as the cell and causes no net water movement, a hypertonic solution has greater concentration and makes the cell lose water, and a hypotonic solution has lower concentration and makes the cell gain water.

Further detail

• A plant cell in a hypotonic solution becomes turgid as it swells until the wall opposes water uptake, while an isotonic environment leaves it flaccid and a hypertonic environment can cause plasmolysis as the membrane pulls away from the wall.

• Paramecium is hypertonic to pond water and limits water uptake through a membrane less permeable to water and a contractile vacuole that acts as a pump.

💡 Memory Hook

Passive transport moves down a gradient without energy; osmosis is specifically water diffusion.

📖 6. Active Transport and Membrane Potential

🔑 Key Concepts & Definitions

• Active transport: Uses energy, directly or indirectly, to move solutes against their concentration gradients through carrier proteins.
• Membrane potential: The voltage difference across a membrane, typically ranging from −50 to −200 mV, with the cell interior negative relative to the outside.

★ Must-know

📌 Primary active transport uses energy directly from ATP, whereas secondary active transport uses energy indirectly through a gradient established by active transport.

• In primary active transport, ATP-powered pumps are phosphorylated, changing conformation and moving a solute across the membrane.

• The sodium-potassium pump is a major electrogenic pump in animal cells and moves 3 Na+ out and 2 K+ in per cycle, producing a net movement of one positive charge outward; proton pumps are the main electrogenic pumps in plants, fungi, and bacteria.

📌 A symporter moves two solutes in the same direction, whereas an antiporter moves them in opposite directions.

Further detail

• An electrochemical gradient combines the ion's concentration gradient with the electrical effect of membrane potential to drive ion diffusion.

💡 Memory Hook

ATP-powered pumps build gradients; those gradients can then drive coupled transport.

📖 7. Bulk Transport by Vesicles

🔑 Key Concepts & Definitions

• Bulk transport: Moves large molecules such as polysaccharides and proteins across the membrane in vesicles and requires energy.
• Endocytosis: The uptake of macromolecules by forming vesicles from the plasma membrane.

★ Must-know

• 🔄 Exocytosis proceeds as follows:

  1. Secretory vesicles travel from the Golgi apparatus to the plasma membrane
  2. The vesicles fuse with the plasma membrane
  3. The vesicle contents are released outside the cell
  4. The vesicle membrane becomes part of the plasma membrane

• The three types of endocytosis are:

  • Phagocytosis
  • Pinocytosis
  • Receptor-mediated endocytosis

📌 Pinocytosis nonspecifically takes up extracellular fluid and dissolved molecules in small vesicles formed by plasma-membrane infoldings, whereas receptor-mediated endocytosis selectively concentrates specific substances bound to membrane receptors in coated pits.

Further detail

• Exocytosis exports:

  • Insulin from pancreatic cells
  • Neurotransmitters from neurons
  • Plant cell-wall components

• 🔄 Phagocytosis proceeds as follows:

  1. Pseudopodia extend around a particle
  2. The particle is enclosed in a food vacuole
  3. The vacuole fuses with a lysosome containing hydrolytic enzymes
  4. The particle is digested

• In receptor-mediated endocytosis, receptors bind specific solutes, cluster in coated pits that form vesicles, and are recycled to the plasma membrane after unloading the solutes.

• Familial hypercholesterolemia can result from absent or defective LDL receptors, preventing cells from taking up LDL cholesterol and causing blood cholesterol accumulation and increased risk of atherosclerosis and cardiovascular disease.

💡 Memory Hook

Exocytosis sends vesicle contents out; endocytosis brings material in.

📊 Synthesis Tables

Membrane Transport Types

Transport type	Direction or cargo	Energy and mechanism
Passive transport	Down concentration gradient	No energy investment
Active transport	Against concentration gradient	Energy used directly or indirectly through carrier proteins
Bulk transport	Large molecules in vesicles	Requires energy and membrane-shape change