Cell membranes

Topics for Exam 1

  • Membranes: Structure & function, ecological links

  • Key Concepts:

    • Polarity

    • Solubility

    • Lipids

    • Biological membranes

Key Functions of Membranes

  • Import substances for food and building blocks.

  • Export waste and keep out unwanted materials.

  • Serve as the basis for nerve and muscle function.

  • Involved in energy conversions in chloroplasts and mitochondria.

Membrane Structure and Function Summary

  1. Phospholipid Bilayer: Forms an impenetrable barrier around cells and sub-cellular spaces.

  2. Proteins: Provide selective, controlled passageways for the import of needed substances and export of unwanted substances, leading to selective permeability.

Fluid-Mosaic Model of Biological Membranes

  • Composition: A mosaic of phospholipids and membrane-spanning proteins.

  • Fluidity: Membranes need to maintain a fluid consistency similar to salad oil.

  • Temperature Dependence:

    • At high temperatures: Membranes may become too fluid (leaky).

    • At low temperatures: Membranes can lose fluidity and become too rigid.

Ecological Adaptations of Membrane Fluidity

  • Organisms either:

    • Invest energy to maintain a constant body temperature.

    • Adjust membrane fluidity by incorporating different types of fatty acids as needed, particularly in varying temperatures (cold versus hot environments).

Saturated vs. Unsaturated Fatty Acids

  • Saturated Fats: Tend to be solid at room temperature.

  • Unsaturated Fats:

    • Composed of one or more double bonds (C=C), leading to a liquid state due to kinks.

    • Examples include:

    • C18:0 Stearic acid (saturated, no kinks)

    • C18:1 Oleic acid (monounsaturated, one kink)

    • C18:2 Linoleic acid (polyunsaturated, two or more kinks)

    • C18:3 Alpha-Linolenic acid (polyunsaturated).

Predicting Membrane Fatty Acid Composition

  • Increasing Fluidity: Cells can increase membrane fluidity by:

    • Replacing phospholipids with fats.

    • Increasing the length of fatty acids in phospholipids.

    • Increasing the number of unsaturated fatty acids (which have kinks that increase fluidity).

  • In contrast, increasing saturated fatty acids will decrease fluidity.

Fatty Acid Composition in Different Environments

  • In hot environments: Cells should have more saturated fatty acids to prevent excessive fluidity.

  • In cold environments: Cells should have more unsaturated fatty acids to maintain fluidity of membranes.

Cholesterol in Membranes

  • Function:

    • Acts as a temperature buffer in membranes.

    • Prevents hydrophobic tails from packing too tightly, thereby increasing fluidity at low temperatures.

    • At high temperatures, it stabilizes membranes by limiting lateral phospholipid movement.

Membrane Transport Mechanisms

  • Simple Diffusion: Small hydrophobic molecules can pass through the phospholipid bilayer without assistance.

  • Facilitated Transport: Hydrophilic molecules require transport proteins for passage across the membrane:

    • Some parts of the transport proteins must interact with water, while others interact with fatty acid tails.

Molecule Types and Permeability

  • Hydrophobic Molecules: O2, N2, steroid hormones.

  • Small Uncharged Polar Molecules: H2O, urea, glycerol.

  • Large Uncharged Polar Molecules: Glucose, sucrose.

  • Ions: H+, Na+, K+, Ca²+.

  • Permeability Ranking: Permeability can be quantified using permeability coefficients.

Aquaporins

  • Functionality:

    • Aquaporins allow water passage through membranes.

    • In aquaporins, amino acids in contact with the hydrophobic membrane fatty acid tails need to be hydrophobic.

    • Amino acids lining the channel must be hydrophilic to facilitate H2O transport.

Roles of Aquaporins in Human Physiology

  • Mouth: Regulation of saliva output during eating.

  • Stomach & Small Intestines: Secretion of aqueous gastrointestinal juices.

  • Kidneys: Important for urine concentration and fluid balance (related issues include water retention and excessive thirst).

  • Lungs: Vital for hydration of lung tissues.

  • Eyes: Generation of tears to maintain moisture.

  • Skin: Assist in cooling by regulating sweating.

Key Terms and Definitions

  • Hydrophilic: Molecules that interact well with water.

  • Hydrophobic: Molecules that do not interact well with water.

  • Selectively Permeable: Property of membranes that allows some substances to pass through while blocking others.