BIOL 115 Chapter 6

Chapter 6: Lipids, Membranes, and Cells

Overview of Molecules of Life

  • All living organisms are composed of four primary classes of large biological molecules:

    1. Proteins

    2. Nucleic acids

    3. Carbohydrates

    4. Lipids

Lipids

  • Lipids do not include true polymers.

  • Unifying feature is their poor mixing with water.

  • Types of lipids include:

    1. Fats

    2. Steroids

    3. Phospholipids

Phospholipids

  • Characterized as amphipathic:

    • Hydrophobic (nonpolar) tails and hydrophilic (polar) heads.

    • Structure includes:

      • Polar head made of phosphate and glycerol.

      • Two fatty acid tails.

Importance of Membranes in Biology

  • Membranes serve crucial functions:

    1. Separate the interior of cells from their environment.

    2. Regulate the passage of materials into and out of the cell.

    3. Separate interior compartments/processes.

Defining Characteristics of Life

  • Organisms are classified based on five fundamental characteristics:

    1. Energy: All organisms acquire and utilize energy.

    2. Cells: Comprised of membrane-bound cells.

    3. Information: Process hereditary and environmental information.

    4. Replication: Capability of reproduction.

    5. Evolution: Continuous evolution of populations.

Studying Membranes

  • Artificial membranes used include:

    • Lipid micelles

    • Lipid bilayers

  • Key questions include:

    • Is it permeable?

    • What variations exist in permeability?

    • What conditions influence permeability?

Selective Permeability

  • Membranes display selective permeability:

    • They allow certain substances to pass while restricting others.

  • Examples of permeabilities based on the permeability scale (cm/sec):

    • High: O2, CO2, N2

    • Moderate: H2O, glycerol

    • Low: Glucose, sucrose, ions (Na+, K+)

Factors Affecting Membrane Permeability

  • Saturated vs. Unsaturated Phospholipids

    • Saturated fatty acids lead to lower permeability.

    • Unsaturated fatty acids increase fluidity and permeability.

  • Cholesterol

    • Cholesterol can reduce membrane permeability.

    • In experimental setups, it fills gaps between phospholipids affecting fluidity.

  • Temperature

    • Lower temperatures reduce fluidity; higher temperatures generally increase it.

Fluidity of Membranes

  • Dynamic Nature: Membranes are fluid, allowing for lateral movement of phospholipids.

  • Affected by:

    • Type of fatty acids (saturated/unsaturated)

    • Temperature fluctuations

    • Cholesterol presence keeping balance between rigidity and fluidity.

Protein Composition of Membranes

  • Membrane composition is lipid-rich but also includes many proteins:

    • Peripheral proteins: Bound to surface.

    • Integral proteins: Span the membrane, assisting in transport and communication.

Membrane Transport Mechanisms

  • Channels: Allow specific molecules to pass through based on concentration gradients without energy.

  • Carriers/Transporters: Change shape to transport molecules, may or may not require energy.

  • Pumps: Move substances against their concentration gradients, requiring energy (active transport).

Classification of Molecules Diffusing Across Membranes

  • Factors that influence diffusion:

    1. Charge of the molecule.

    2. Size of the molecule.

    3. Presence of transport proteins.

Membrane Transport Analogy**

  • Comparing permeation to crossing a river:

    • Notes the significance of characteristics of both the crossing entity and the medium itself in permeation.

Problem Set Summary

  1. Discuss structural influences of lipids on membrane function.

  2. Analyze factors determining membrane permeability including:

    • Transport proteins

    • Energy requirements for transport

  3. Rank diffusion likelihood based on characteristic molecule types.