Membranes [moevement of substances]

Why must substances move across membranes?

• Obtain nutrients for energy and raw materials

• Excrete waste substances

• Secrete useful substances

• Generate ionic gradients essential for nervous, muscular activity and ATP

• Maintain suitable pH and ionic concentrations within the cell for enzyme activity

What are 5 ways in which substances move across a membrane?

Passive processes down a concentration gradient that do not require ATP

• Simple diffusion

• Facilitated diffusion

• Osmosis

Active processes against a concentration gradient that require ATP

• Active transport

• Bulk transport

What is simple diffusion?

Movement of non polar molecules and small polar water molecules directly across a membrane down a concentration gradient

• Diffusion of particles continues until particles are evenly distributed with net movement of particles in any direction

What factors affect the rate of simple diffusion? (6)

• Molecular size

  • Smaller particles → Increased rate of diffusion

• Solubility in lipid bilayer

  • Non polar membrane → Non polar substances move through the membrane faster

• Concentration gradient

  • Steeper concentration gradient → Faster rate of diffusion

• Kinetic energy and temperature of molecules

  • Higher temperature → Higher EK of molecules → Higher rate of diffusion

  • Higher temperature → Higher EK of phospholipids → Higher membrane fluidity → More formation of transient pores → Higher rate of diffusion

  • Temperatures too high → Loss of selectivity

• Surface area of membrane

  • Larger surface area → Increased rate of diffusion

• Diffusion distance

  • Longer distance → Decreased rate of diffusion

What is facilitated diffusion?

• Similar to simple diffusion BUT

• Transport proteins needed ← Ions or polar molecules cannot diffuse through the hydrophobic core

  • Channel proteins

  • Carrier proteins

  • Each is specific for a particular solute

What are channel proteins?

• Transmembrane protein

• Provides a hydrophilic pore → Only a particular ion or polar molecule can diffuse readily through

• Some are gated → Close to prevent access to the pore, opens with the arrival of chemical or electrical stimulus

What are carrier proteins?

• Transmembrane protein (usually)

• 2 alternate conformations → Conformation change occurs when the solute binds to its binding site → Solute is now exposed to other side of the membrane

• Hydrophilic interior contains a binding site for the solute

What affects the flow of substances across the carrier protein?

• Relative concentrations of solutes across the membrane → Direction of flow (bidirectional)

• Transport depends on the chance of collision between transport protein and solutes

• Number of carriers increases → Rate of facilitated diffusion increases

What is osmosis?

Movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane

• Simple diffusion → Direct diffusion of molecules through transient pores

• Facilitated diffusion → Aquaporin proteins

What is active transport?

The transport of polar molecules or ions across a membrane against a concentration gradient with the expenditure of ATP

• Involves specialised carrier proteins called pumps e.g. sodium potassium pump

• Substances only move in one direction

How does the sodium potassium pump work?

1. 3 cytoplasmic Na+ bind to pump → Stimulates phosphorylation by ATP → Causes conformational change in protein

2. Na+ is expelled to the outside → 2 extracellular K+ bind to protein → Release of phosphate group

3. Protein returns to the original conformation → Releases K+ inside the cell

4. Na+ sites become receptive again

What kinds of bulk transport are there?

• Endocytosis

  • Phagocytosis

  • Pinocytosis

  • Receptor-mediated endocytosis

• Exocytosis

What is endocytosis?

• Uptake of substances into the cell

• Infolding or extension of the membrane to form a vesicle → Cells can acquire macromolecules and particulate matter

What is phagocytosis?

• Solid material taken up during endocytosis

• Filaments in the cytoskeleton rearranged to help form pseudopodia (with the help of ATP) → Ends of pseudopodia fuse → Vesicle containing solid matter is pinched off → Moves into cytoplasm

• Pseudopodia = outward extensions of the membrane that wrap around and engulf the particle

What is pinocytosis?

• Liquid material taken up during endocytosis

• A small area of plasma membrane invaginates to form tiny vesicles within the cell

What is receptor-mediated endocytosis?

• Specific transport

• Protein receptors are embedded in membranes exposed to extracellular fluid

• Ligands = Extracellular substances that bind to receptors → Invagination of membrane occurs to form vesicles containing ligand-receptor complexes → Transported within cell/organelle

How do cells acquire large quantities of specific substances during receptor-mediated endocytosis?

• Coat proteins e.g. clathrin help deepen the pit to form vesicles

• Receptor proteins are usually clustered in regions of the membrane called coated pits

What is exocytosis?

Secretion of macromolecules by the fusion of vesicles with the plasma membrane

How does exocytosis occur?

Secretory vesicle budded from Golgi Apparatus moves to the cell surface membrane → Membrane of vesicle fuses with cell surface membrane to release contents into the extracellular environment