Membrane Structure and Transport

Overview

• Unit 5: Cell Transport covers chapters 5.7–5.10 of the textbook.
• Central question: Does the structure of a plasma membrane affect its permeability?
• Major topics: plasma membrane structure, diffusion, osmosis, passive and active transport, membrane proteins, and transport mechanisms (endocytosis/exocytosis, pumps).
• Assessments and deadlines noted throughout (vocabulary worksheets, labs, QCER, unit test).

Plasma Membrane Structure

• Phospholipid bilayer: polar (hydrophilic) heads face water; nonpolar (hydrophobic) tails face inward.
• Fluid mosaic model: membrane made of many moving parts (phospholipids, proteins).
• Membrane proteins are embedded and perform transport and regulatory roles.
• Functions:
  • Protect the cell.
  • Control incoming and outgoing substances.
  • Maintain ion concentrations.
  • Maintain homeostasis via selective permeability.

Diffusion (Passive Transport)

• Definition: net movement of particles from high to low concentration (down the concentration gradient).
• No ATP required.
• Factors affecting diffusion rate:
  • Steepness of gradient: steeper = faster.
  • Molecular size: smaller = faster.
  • Temperature: higher temperature = faster.
  • Electrical or pressure gradients.
• Simple diffusion: small nonpolar (hydrophobic) molecules cross lipid bilayer directly.
• Facilitated diffusion: polar molecules and ions move through membrane protein channels (integral proteins); still passive.

Osmosis and Tonicity

• Osmosis: diffusion of water across a semipermeable membrane.
• Tonicity is relative; compares solute concentration between two solutions.
  • Hypotonic: solution has lower solute (higher water) relative to another.
  • Hypertonic: solution has higher solute (lower water) relative to another.
  • Isotonic: equal solute concentrations; no net water change.
• Water moves from hypotonic → hypertonic.
• Effects on cells:
  • Animal cells in hypotonic solution: water into cell → may lyse (burst).
  • Animal cells in hypertonic solution: water out → shrivel (crenate).
  • Plant cells: turgid in hypotonic (normal), flaccid in isotonic, plasmolysed in hypertonic.

Active Transport

• Requires ATP energy to move substances against concentration gradients (low → high).
• Mechanisms:
  • Pumps (e.g., Na+/K+ pump, Ca2+, Mg2+ pumps) move specific ions using ATP.
  • Endocytosis (pinocytosis: cell drinking; phagocytosis: cell eating) brings materials into the cell.
  • Exocytosis expels materials via vesicle fusion with plasma membrane.
• Ion pumps maintain concentration differences and electrochemical gradients.

Comparison: Passive vs Active Transport

• Passive:
  • No energy required.
  • Moves down concentration gradients.
  • Includes simple diffusion, facilitated diffusion, osmosis.
• Active:
  • Requires ATP.
  • Moves against gradients.
  • Includes pumps, endocytosis, exocytosis.

Labs, Observations, and Practical Notes

• Diffusion lab: set up bags and test tubes; record before/after colors, starch tests, and data analysis.
• Osmosis lab with de-shelled eggs: predict and observe changes in egg mass/appearance in solutions (water, corn syrup, salt water, 7% NaCl).
• Record for each solution: egg state (shriveled, swollen, same), water movement (into, out, none), tonicity (hypo/hyper/isotonic).
• Clean-up steps: dispose of bags with fluid, rinse glassware, remove labels, return materials, wash hands.
• Discussion prompts: effects of salt on slugs (osmotic water loss), purity of water and safety.

Key Terms and Definitions

• Phospholipid bilayer: two-layered membrane of phospholipids forming cell boundaries.
• Hydrophilic / Hydrophobic: water-attracting head / water-repelling tail of phospholipids.
• Selectively permeable: membrane that allows certain substances to pass.
• Concentration gradient: difference in concentration between regions.
• Facilitated diffusion: passive transport via membrane proteins.
• Osmosis: diffusion of water through a semipermeable membrane.
• Hypertonic / Hypotonic / Isotonic: relative solute concentrations and resulting water movements.
• ATP (adenosine triphosphate): cellular energy currency used in active transport.
• Endocytosis / Exocytosis: vesicle-mediated import and export.

Learning Targets and Review Focus

• Define hypotonic, hypertonic, isotonic and predict water movement by osmosis.
• Explain diffusion’s role in cell transport and differences between diffusion and osmosis.
• Describe plasma membrane chemical structure and the fluid mosaic model.
• Distinguish passive vs active transport and list active transport mechanisms.
• Interpret lab evidence (egg and bag experiments) to support claims about tonicity and permeability.
• Prepare QCERs and review vocabulary for unit test.

Action Items / Next Steps

• Complete vocabulary worksheet and assessment by due dates.
• Finish diffusion and osmosis lab write-ups and record observations clearly.
• Study comparisons: passive vs active, osmosis vs diffusion, endocytosis vs exocytosis.
• Review sodium-potassium pump and other ion pumps for active transport examples.
• Prepare for Unit 5 test (scheduled end of unit).