Cell Membrane Structure and Chemical Bonds: Foundations for Anatomy & Physiology

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Chemical Bonds and Interactions in Physiology

Covalent Bonds

Covalent bonds are strong chemical bonds formed when atoms share one or more pairs of electrons. These bonds provide the backbone for biological molecules such as proteins, carbohydrates, nucleic acids, and lipids.

Definition: A covalent bond involves the sharing of electron pairs between atoms.
Example: Peptide bonds between amino acids in proteins.
Application: Covalent bonds give molecules their structure and function.

Ionic Bonds

Ionic bonds are attractions between oppositely charged ions. They are important for maintaining electrolyte balance and enabling nerve signaling.

Definition: Ionic bonds form when one atom donates an electron to another, resulting in positive and negative ions that attract each other.
Example: Sodium (Na+) and potassium (K+) gradients across cell membranes drive action potentials.

Hydrogen Bonds (Weak Bonds)

Hydrogen bonds occur when hydrogen in polar molecules is attracted to oxygen, nitrogen, or fluorine atoms. These bonds are crucial for molecular shape and specificity.

Definition: A hydrogen bond is a weak attraction between a hydrogen atom and an electronegative atom.
Example: Base pairing in DNA (A–T, C–G) relies on hydrogen bonding.

Van der Waals Forces

Van der Waals forces are weak, transient attractions between atoms that stabilize protein folding and interactions between biomolecules.

Definition: Van der Waals forces are weak interactions due to temporary dipoles in molecules.
Example: Antibody–antigen binding involves Van der Waals contacts.

Hydrophobic Interactions

Hydrophobic interactions occur when nonpolar molecules cluster together to avoid water, driving cell membrane formation and protein folding.

Definition: Hydrophobic interactions are the tendency of nonpolar molecules to aggregate in aqueous solutions to minimize contact with water.
Example: Phospholipid bilayers form because fatty acid tails avoid water, creating compartments for life.

Physiological Relevance

Buffers: Substances like bicarbonate in blood stabilize pH, allowing biological reactions to occur efficiently.
Protein–ligand binding: The binding of drugs, hormones, and substrates depends on noncovalent interactions.
Temperature and pH: Shifts in temperature and pH can disrupt hydrogen and ionic bonds, leading to protein denaturation (loss of function).

Cell Membrane Structure and Composition

Membranes Are Mostly Lipid and Protein

Biological membranes consist primarily of lipids and proteins, with only a small amount of carbohydrates. The ratio of protein to lipid varies among cell types and is related to metabolic activity.

Example: Mitochondria have a high protein content due to their role in ATP production.
Function: Membrane composition affects permeability and cell function.

Molecular Arrangement of the Cell Membrane

The cell membrane is organized as a phospholipid bilayer, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward. This arrangement creates a selective barrier.

Phospholipid bilayer: Two layers of lipids with hydrophilic heads and hydrophobic tails.
Thickness: Cell membranes are typically about 8 nm thick.
"Butter sandwich" model: A clear layer of lipids is sandwiched between two dark layers of proteins (as seen in electron micrographs).

Micelles, Liposomes, and Bilayers

Micelles: Spherical structures formed by phospholipids in water, with hydrophilic heads facing outward and hydrophobic tails inward. Important for digestion and absorption of fats.
Liposomes: Larger spheres with bilayer phospholipid walls, forming a hollow center that can carry water-soluble molecules. Used in drug delivery and cosmetics.
Phospholipid bilayer: The basic structure of cell membranes, providing compartmentalization.

Cholesterol and Sphingolipids

Cholesterol and sphingolipids are important components of cell membranes, affecting fluidity and permeability.

Cholesterol: Inserts between phospholipids, making membranes less permeable to small water-soluble molecules and more flexible over a range of temperatures.
Sphingolipids: Have fatty acid tails and may be either phospholipids or glycolipids. They are slightly longer than phospholipids.

Membrane Proteins

Types of Membrane Proteins

Integral proteins: Span the membrane and are tightly bound. They can only be removed by disrupting the membrane.
Peripheral proteins: Attach to the membrane surface or to integral proteins and can be removed without disrupting the membrane.
Lipid-anchored proteins: Covalently bound to lipid tails and associated with membrane phospholipids.

Transmembrane Protein Families

Transmembrane proteins are classified by the number of times they cross the membrane. Many have seven segments, but some may cross up to twelve times.

Function: Transmembrane proteins are crucial for transport, signaling, and cell adhesion.
Example: GPI-anchored proteins are held by a glycosyl-phosphatidylinositol anchor.

Membrane Protein Functions

Receptors: Bind signaling molecules and initiate cellular responses.
Ion channels: Allow ions to pass through the membrane, regulating electrical activity.
Enzymes: Catalyze reactions at the membrane surface.
Cell adhesion molecules: Bind cells to each other and to the extracellular matrix.

Membrane Carbohydrates

Attachment to Lipids and Proteins

Membrane carbohydrates are sugars attached to membrane proteins (glycoproteins) or lipids (glycolipids). They play roles in cell recognition and immune response.

Example: The ABO blood group antigens are determined by glycoproteins and glycolipids on red blood cells.
Function: Carbohydrates help cells identify each other and interact with the extracellular matrix.

Intracellular Compartments and Cell Specialization

Overview

Cells are divided into compartments: the cytoplasm and the nucleus. The cytoplasm contains cytosol (fluid), inclusions (insoluble particles), fibers (cytoskeleton), and organelles (membrane-bound structures).

Cytosol: Semi-gelatinous fluid containing nutrients, proteins, and waste products.
Inclusions: Insoluble particles such as glycogen granules and lipid droplets.
Fibers: Protein filaments that form the cytoskeleton.
Organelles: Specialized compartments with specific functions (e.g., mitochondria, endoplasmic reticulum).

Functions of the Cytoskeleton

Maintains cell shape and prevents deformation.
Organizes cell organelles internally.
Facilitates cell division and movement of chromosomes.
Provides mechanical strength.
Enables cytoplasmic streaming for movement of substances within the cell.

Inclusions Are in Direct Contact with the Cytosol

Inclusions are not surrounded by membranes and are in direct contact with the cytosol. They store nutrients and other materials for cell function.

Examples: Glycogen granules, lipid droplets, and protein-RNA complexes.

Cell Death: Apoptosis and Necrosis

Apoptosis

Apoptosis is a programmed form of cell death that is essential for development and tissue homeostasis.

Definition: Apoptosis is a controlled process where cells die without causing inflammation.
Function: Removes damaged or unnecessary cells.

Necrosis

Necrosis is an uncontrolled form of cell death resulting from injury or disease, often leading to inflammation.

Definition: Necrosis occurs when cells die due to external factors such as trauma or infection.
Function: Can damage surrounding tissues and provoke an immune response.

Summary Table: Types of Chemical Bonds in Physiology

Bond Type	Strength	Role in Physiology	Example
Covalent	Strong	Structural backbone of biomolecules	Peptide bonds in proteins
Ionic	Moderate	Electrolyte balance, nerve signaling	Na+/K+ gradients
Hydrogen	Weak	Molecular shape, specificity	DNA base pairing
Van der Waals	Very weak	Protein folding, molecular interactions	Antibody-antigen binding
Hydrophobic	Variable	Membrane formation, protein folding	Phospholipid bilayer

Key Equations

Buffer equation (Henderson-Hasselbalch):

Electrochemical gradient (Nernst equation):

Additional info:

Some context and definitions have been expanded for clarity and completeness.
Examples and applications have been added to support understanding.