Biological and Medical Physics I - Lecture Notes

Biological Membranes

1.1. Main functions of biological membranes

• Organizing and functioning of cells: Biological membranes are crucial for organizing and ensuring correct cellular function

• Cellular Isolation: Membranes isolate cellular components to maintain a stable environment

• Selectively Permeable Barriers: Act as selectively permeable barriers to regulate what enters and exits the cell

• Various functions: Cell membrane carries out many critical cellular functions, including signaling, antigen presentation and communication.

Functions in detail:

• Barrier function: Membranes physically separate cellular compartments and contents, controlling biochemical reactions and maintaining high concentrations of necessary compounds while removing harmful ones.

• Transport function: Membranes transport molecules across their structures using proteins like channels, carriers, and pumps, preserving cellular/electrical gradients.

  • These systems maintain concentrative/electrical gradients and form electrical potential across membranes.

  • Ion gradients are important in transduction of intracellular/extracellular signals.

  • Gradients are used in cell energy generation.

• Signaling function: Proteins on the membrane's outer surface regulate tissue and organ formation via cell activities (proliferation, differentiation, grouping).

  • Membranes transmit external signals into the cell via specific receptors.

  • Ligand-receptor interactions affect internal cell activities (growth, death, ATP, protein synthesis).

  • Density-dependent inhibition (DDI) involves signal termination upon monolayer formation in normal cells, which is often lacking in tumor cells.

• Antigen function: Glycoproteins and glycolipids on the membrane's outer surface form intercellular contacts and define immune properties to specific surface antigens.

  • Antigens are crucial for cell recognition by the immune system.

  • Cell surface antigen compatibility is essential during blood transfusion/tissue transplantation.

  • These antigens' determinants are polypeptide chains of transmembrane proteins coded by the main histocompatibility complex genes.

• Cell-to-cell communication: Membranes facilitate direct cell-to-cell communication via chemical & physical mechanisms including adhesion.
  -Adhesion process: It involves chemical groups on cell surfaces, forming mechanical or chemical bonds.
  -Intercellular liquid contains Ca^{2+} ions containing complexes. Reduction in calcium ions in intercellular liquid induced a reduction in cell adhesion and promoted the disintegration of tissue into single cells
  -Membrane structural features also support communication stability (e.g., invagination in cardiomyocytes or desmosomes in epithelium).
  -Adhesive proteins (cadherins, integrins) are involved in cell-cell contact and recognition.
  -Tissue stability depends on cells' surface charge as well; carbohydrate groups bearing a negative charge cause cell repulsion.
  -Malignant cells exhibit low adhesion due to degradation of binding structures, calcium content, or increased surface negative charge; this promotes metastasis.

1.2. Structure of biological membrane

• Key components: Lipids and proteins make up 95%, while carbohydrates make up only about 5%. Lipids are organized in a bilayer (60-100 Å thick) with proteins scattered throughout.

• Fluid-mosaic model: Proposed by Singer and Nicolson, illustrating proteins are inserted into the bilayer.

The fluid-mosaic model encompasses components:

1. Membranes make a form of a lipid bilayer.

2. Membranes are two-dimensional fluids with lipids and proteins diffusing freely.

3. Proteins are embedded discontinuously lipid bilayer throughout the lipid bilayer

4. Both lipids and proteins are distributed asymmetrically across the bilayer.

• Asymmetry: Lipids and proteins are distributed asymmetrically.

The ratio of lipids: proteins: carbohydrates varies for different cell types.

1.2.1. Membrane lipids

• Amphipathic lipids: Lipids that contain with both hydrophilic and hydrophobic parts

Main structural unit of the membrane: Phospholipids are the most common type, derived from:

• sphingosine (sphingolipids)

• glycerol (phospho-glycerides).

• Fatty acid tails: Phospholipids feature fatty acid "tails" with an even number of carbon atoms, averaging 14-24. Saturated tails have no double bonds, while unsaturated tails contain double bonds between carbon atoms.

• Glycolipids They are Significantly less abundant compared to phospholipids and exclusively on the extracellular side of the plasma membrane. They produce glycocalyx (glycoprotein and glycolipids)

  • Functions: toxins Recognizing (antigen function).

  • Structure containing a hydrophilic polar head and “tails”.

• Cholesterol: It is component of animal cells which regulates viability & proliferative activity (50-66 Mol%). Cholesterol is positioned in the middle slice of the phospholipid bilayer; its hydroxyl group is directed toward the “heads” of amphipathic phospholipids

1.2.2. Driving forces for bilayer formation

• Amphiphilicity: The amphipathic properties of phospholipids drive lipid bilayer formation in water.

• Hydrophobic effect: Insoluble compounds aggregate; water is repelled.
  Explanation of hydrophobic effect includes:

• Covalent bonds: formed when atoms share electron pairs (high energy, 300-4000 kJ/Moll)

• Non-covalent bonds: weaker interactions (10-100 times weaker) between atoms, molecules, fragments, and ions
  * Electrostatic bonds: strong non-covalent interactions between ions/charged molecules
  * Hydrogen bonds: weak interactions between polar molecules with hydrogen bound to larger atoms (O, N, S).
  * Van der Waals forces: interactions between neutral molecules, resulting from polarization.

• Water properties: Water molecules are polar and interact via hydrogen bonds, which is stronger than lipid molecule interactions. This drives the hydrophobic effect.

• Lipid bilayers: Hydrophobic tails face inward, shielded from water, while hydrophilic heads face out. The membrane remains stable because water is attracted to itself and lipid heads than the lipid tails. This mechanism aids formation of vesicular and micelles phospholipid structures

1.2.3. Physical properties of membrane lipids

• Role: Phospholipids are building blocks and modulators of cellular processes, causing by their physicochemical properties.
  The major characteristics: are solubility, formation of closed spaces, permeability, mobility, asymmetry, and electric resistance.

• Temperature: At low temperatures, the lipid bilayer “freezes,” restricting lipid movement. Increasing temperature increases fluidity. Transformation happens between high-temperature membrane liquefaction and Low-temperature sealing membrane.

• Membrane fluidity factors: four main factors contribute to membrane fluidity: temperature, membrane lipid tail length, the degree of unsaturation of membrane lipid tails and the cholesterol contents.

• Optimal temperature: (36-37°C) gives amorphous structure (fluid-crystal).
  Living organisms regulate membrane through the adaptation of lipid composition to temperature.

Four main factors contribute to membrane fluidity:

• Temperature

• Membrane lipid tail length

• The degree of unsaturation of membrane lipid tails

• The cholesterol contents

• Membrane lipid tail length: As melting temperature is higher for lipids with longer tails, membranes become less fluid as tail length increases.

• The degree of unsaturation: Double bonds cause kinks, reducing density packing and increasing fluidity.

• The cholesterol contents:

To maintain membrane fluidity at low temperatures, cold-adapted species incorporate  more unsaturated lipids into their membrane.
Saturated triglycerides containing animal  fat (butter) are solids at room temperature, while unsaturated triglycerides containing oils (from  plants) are liquid at room temperature.

Melting points of saturated fatty acids increase with increasing the length of lipids tails and decreasing  of double bonds number.
Consequently, various types of cellular membranes are characterized by different fluidity and viscosity.
Table 2. Dependence of membrane fluidity on the unsaturated phospholipids

• Charge: Membrane lipids are high electroneutral (phosphatidylcholine, sphingomyelin, phosphatidyl-ethanolamine) . Cell membranes may not be different by the quantitative content of phospholipids, but they vary by the composition of phospholipids.

How eukaryotic membranes with different lipids compositions are established  and maintained?

• Lipid movement: Membranes components (lipids, proteins) are in constant motion. Using Electron  paramagnetic resonance spectroscopy, it was evidenced that phospholipids move in the membrane plane at a  high speed during 10-7  seconds.

• Lateral diffusion: Is the movement of phospholipids and proteins in the membrane mono-layer  plane.
  Rapid =10-8 cm².5-1
  Lateral diffusion D = 1 μm².s-1
  Tranvers diffusion (flip-flop)
  Very slow
  (10's)
  Movement  Flip-flops
  (frequent)(rare)
  Gensitic
  Consequently, lipid arrangement in the membrane is genetically determined, and any alternations can be achieved only with the help of inductors (catalysts).
  Thus, lipid motion in the bilayer is carried out by the following mechanisms:

• Nonpolar tails undergo wagging due to rotation around the C-C single bonds.

• The whole lipid molecule is free to rotate around its vertical axis.

• Transverse diffusion of lipid molecules within a leaflet (lateral diffusion).

• Flipping motion (flip-flop) occurs when a lipid molecule shushes from one leaflet to another.
  Low frequency of movement between lipid molecules in monolayers ensures lipid compositionconstancy of membrane monolayers and membrane asymmetry (phospholipids asymmetricdistribution among monolayers), which is very important for the maintenance of cell structure\and function.

Lipid bilayers have different thicknesses depending on their composition as: saturated fatty  acids increase thickness and sterols decrease thickness.

Asymmetry: Membrane inner and outer surfaces vary in lipid composition.
Membrane lipids asymmetric distribution between inner and outer monolayers of lipid matrix is
observed in almost every cellular and subcellular membrane. The asymmetry, it is established that phospholipids that contain choline
(phosphatidylcholine, sphingomyelin) are mostly positioned in the outer monolayer of the
membrane, and phospholipids that contain amine groups (phosphatidyl-ethanolamine,
phosphatidylserine) are positioned in the inner layer of the membrane (towards the cytoplasm).

1.2.4. Cholesterol effects on the membrane futures and functions

• Roles: it plays a role of a key regulator of membrane fluidity.

• Interaction with lipids: Interacts via the hydroxyl group with water and other lipid heads, embedding its hydrophobic tail in the membrane's interior. It molecules are inserted in the membrane with the same orientation as the molecules of phospholipids.

Cholesterol's dual influence on the lipid bilayer organization and properties:

• At physiological temperature: additional van der Waals force between acyl chains of additional van der Waals force between acyl chains of phospholipids and restricts the free movement  and vibration of the lipids, decreases membrane fluidity.

• additional van der Waals force between acyl chains of  additional van der Waals force between acyl chains of phospholipids and restricts the free movement  and vibration of the lipids, decreases membrane fluidity additional van der Waals force between acyl chains of phospholipids and restricts the free movement  and vibration of the lipids, decreases membrane fluidity decreases the density of carbohydrate chains At low-temperature cholesterol, increases the volume  of free  space surrounding phospholipid tails and as a result increases the fluidity of the membrane inner layer.

Membrane stability greatly depends on the concentration of cholesterol in the membrane. As higher the cholesterol concentration in the membrane, the greater is  membrane stiffness, which decreases membrane fluidity, deformability, and permeability.
Various types membranes differ by cholesterol content.
Bacterial cells do not have cholesterol, whereas in animal cells it is represented in a high percentage