• 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
  1. Pinocytosis: occurs when dissolved substances enter cell
  2. 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


  • “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


  • 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)