Unit 4: Membrane Transport and Enzymes Part 1

Membrane Structure and Transport Functions

Functions of the Plasma Membrane

• Separates cell from the external environment.
• Provides structural support, shape, and protection.
• Regulates entry and exit of substances (acts as a gatekeeper).

Fluid Mosaic Model

• Plasma membrane is described using this model.
• Mosaic of protein molecules in a fluid bilayer of phospholipids.
• Phospholipid bilayer constantly moves.
• Proteins also move throughout the membrane.

Phospholipid Bilayer

• Composed of hydrophilic heads (phosphate group, glycerol) and hydrophobic tails (fatty acid chains).
• Allows diffusion of water and neutral substances.

Cholesterol

• Lipid within phospholipids; four fused rings structure.
• Regulates membrane fluidity (stiffens and strengthens membrane).

Glycolipids

• Phospholipids with carbohydrate chains.
• Function as cell recognition markers.

Proteins in the Membrane

• Peripheral Proteins: Stabilize and shape the membrane; located intracellularly.
• Integral Proteins: Span the lipid bilayer.
  • Channel Proteins: Allow ions/molecules to cross.
  • Carrier Proteins: Selectively interact with specific molecules/ions.
  • Receptor Proteins: Bind specific molecules for cellular responses.
  • Enzymatic Proteins: Catalyze metabolic reactions.
  • Glycoproteins: Proteins with carbohydrate chains, aid in cell identification.

Cytoskeleton

• Stabilizes peripheral proteins; provides structural support.

Permeability of the Plasma Membrane

Selective Permeability

• Allows certain substances through.
• Permits water, non-charged molecules; restricts macromolecules and charged molecules.

Passive Transport

• Diffusion: Movement down a concentration gradient.
• Osmosis: Diffusion of water.
• Facilitated Transport: Movement via channel/carrier proteins.

Tonicity

• Hypertonic Solution: Solutes higher outside, cell shrinks.
• Hypotonic Solution: Solutes lower outside, cell swells.
• Isotonic Solution: Solutes equal inside and outside, no size change.

Active Transport

Characteristics

• Requires energy (ATP).
• Moves substances against a concentration gradient via carrier proteins.
• Example: Sodium-Potassium Pump.

Membrane-Assisted Transport

• Exocytosis: Vesicle fusion releases substances outside.
• Endocytosis: Plasma membrane engulfs substances inside.
  • Phagocytosis: Cell eating (engulfs large particles).
  • Pinocytosis: Cell drinking (engulfs liquids).

Factors Affecting Cell Transport

Rate of Diffusion Influenced By:

• Temperature, concentration gradient, pressure, particle size, diffusion distance, surface area.

Surface Area to Volume Ratio

• Larger ratio allows efficient transport.

Enzymes

Definitions

• Metabolism: Sum of all reactions in a cell.
• Catabolism: Breakdown reactions.
• Anabolism: Synthesis reactions.
• Metabolic Pathway: Series of linked reactions with enzymes.
• Negative Feedback: Product inhibits enzyme activity.

Enzyme Function

• Lower activation energy required for reactions.

Characteristics of Enzymes

• All enzymes are proteins, but not all proteins are enzymes.
• Enzymes are catalysts speeding up reactions.

Enzymatic Models

• Lock and Key: Substrate fits perfectly in enzyme.
• Induced Fit: Enzyme changes shape to fit substrate.

Factors Affecting Enzyme Activity

• pH: Optimal pH required.
• Temperature: Optimal temperature maximizes reactions.
• Substrate Concentration: Reaction rate increases with substrate concentration.
• Enzyme Concentration: Directly proportional to reaction rate.

Inhibitors

• Competitive Inhibitors: Compete with substrate for active site.
• Non-competitive Inhibitors: Bind elsewhere, changing enzyme shape.

End of Notes

For further information or clarification, refer to lecture notes available online or contact your instructor.