Concentration Gradients

• The selective permeability of membranes allows for the formation of concentration gradients of solutes across the membrane
  • Concentration Gradients: “Difference of concentration between two substances”
• Diffusion: movement of molecules so they spread out evenly
  • Steeper concentration gradient or higher temp = faster diffusion

Passive Transport

• The net movement from high concentration to low concentration without direct input of metabolic energy (Does NOT need ATP)
  • Plays a primary role in the import of material and the export of waste
  • Includes facilitated diffusion & simple diffusion: the molecule is hydrophobic

• Net movement of molecules is random/constant and independent from other molecules
  • Net = is eventual movement = not all molecules go down gradient

Facilitated Diffusion

• Passive transport sped up by proteins → still down concentration gradient
• Carrier & channel proteins can facilitate movement of ions and larger molecules like amino acids or glucose
• Ion channels (channel): transport ions, many are gated channels which open/close in response to stimuli
• Aquaporins (channel): Integral membrane proteins that allow for the passage of water into the cell
  • Very few water molecules can go through the membrane because are polar so need aquaporins that speed up process

Active Transport

• Proteins move molecules from a region of low concentration to region of high concentration
  • It moves AGAINST the gradient
  • It DOES need ATP
• Unlike passive, does NOT result from random movement of molecules → moves specific solutes across a membrane
• ATP hydrolysis causes integral protein to change shape & shuttle molecules across membrane & against the gradient

Bulk Flow

• Movement of substances (solvent & solutes) in the same direction bcuz of pressure

Vesicular Transport (Bulk Transport)

• Uses vesicles to move substances across the plasma membrane; in/out of the cell
• Endocytosis: cell takes in substance outside of cell when plasma membrane merges to engulf it → substance enters cytoplasm enclosed in a vesicle; 3 types
  • Vesicle then merges with lysosome to break down food

1. Phagocytosis: occurs when undissolved (solid) material enters the cell

• Plasma membrane wraps around solid material and engulfs it → forms phagocytic vesicle → phagocytic cells attack and engulf bacteria this way

2. Pinocytosis: occurs when dissolved substances enter cell
3. Receptor-mediated: form of pinocytosis when ligand bind to specific receptors in plasma membrane pits

• Membrane pits, receptors and ligands fold inwards and vesicle forms

Osmosis

• “Water diffuses out or in of a cell”; when it does so osmotic pressure may build up → cell expands as it volume increases
• Turgor Pressure: osmotic pressure that develops when water enters cells
  • Presses cytoplasm against cell → makes plants rigid and controls rate of osmosis
• Higher water potential = less solutes, lose water; Lower water potential = more solutes, gain water
• Plasmolysis: movement of water out of a cell that results in the collapse of a cell
• Cell Lysis: water enters the cell, causing it to swell and burst
• More common in animal cells and others that lack a cell wall
• Hypertonic solution: more solutes
  • Cell in hypertonic solution has higher water potential (more water) → water leaves cell → cell will shrivel and die
• Isotonic: no net movement, same amount of water goes in and out
• Hypotonic: less solutes
  • Cell in hypotonic in solution has lower water potential (more solutes) → water enters cell → cell will swell and burst; plants become turgid
• Animal cells prefer isotonic solution; plant cells prefer hypotonic cuz water stored in vacuoles

Ion Pumps:

• Ions diffuse across membranes thru ion channels down their electrochemical gradient                                           
• Ion channels/pumps work together to establish ion gradient across membrane, resulting in negative charge inside membrane
• Electrochemical gradient: difference in charge across plasma membrane → determines direction of ionic diffusion. Composed of chemical force & electrical (voltage)
  • Chemical: concentration gradient of ions
  • Electrical: effect of membrane potential
• Membrane potential: resting voltage across membrane that affects the movement of ions
• Active Potential: rapid rise and fall in voltage/membrane potential across a cellular membrane
• Electrogenic pump: primary active transporters that hydrolyze ATP and use released energy to transport ions
  • Generates voltage (potential energy of ions) & results in diff of charge
  • Ex: Sodium-Potassium Pump
  • Plants have proton pump that pumps H+ rather than sodium and potassium and increases potential energy

Cotransport:                                                                                                 

• 2 solutes, membrane protein uses downwards diffusion of ion to power upward transport of another against gradient (secondary active transport)
  • Glucose & Sodium transport:

Sodium-Potassium Pump Review

• Keeps cell polarized
  • Has (-) & (+) side
• Cell has higher concentration of K+ and lower Na+ than extracellular solution                                   
• Protein goes back and forth between two forms:An inward-facing form with high affinity for sodium (and low affinity for potassium) and an outward-facing form with high affinity for potassium (and low affinity for sodium)

• The protein can be toggled back and forth between these forms by the addition or removal of a phosphate group (ATP hydrolysis)
• Overall charge of cell is negative because cations (K+) leave cell at faster rate than Sodium (Na+) enters l
  • K+ more permeable

Primary vs Secondary Active Transport

• Primary directly uses energy source (ie ATP) to move molecules against gradient
• Secondary transport (ex: cotransport) uses electrochemical gradient (formed thru active) to move molecules against gradient (no ATP)