Cell Membrane

Proteins can be associated with either the intracellular (cytosolic) side or extracellular side of membranes.
These proteins may be attached to other proteins or lipids.
Main categories of proteins regarding membrane association:
- Transmembrane Proteins:
- Span the entire membrane.
- Can be involved in a variety of functions, notably transport.
- Peripheral Proteins:
- Located on one side of the membrane (extracellular or cytosolic).
- Often serve functions like anchoring to other structures.

Membrane spanning sections of proteins typically consist of:
- Alpha helices:
- Make up transmembrane segments through hydrophobic interactions.
- Beta pleated sheets:
- Form larger structures known as beta barrels; these tend to have larger openings, resulting in a nonspecific transport function.

Focus on two primary types of transport proteins:
- Transporters:
- Move specific types of ions or molecules across the membrane (e.g., glucose, amino acids).
- Channels:
- Provide passages for ions and other small particles, often characterized by specificity.
Example of a specialized additional transport protein:
- Sodium-Potassium Pump: Active transporter critical for maintaining ionic gradients across the membrane.

Proteins can move fluidly within the plane of the membrane unless restricted by structures such as tight junctions.
Tight Junctions:
- Prevent movement of proteins between the apical surface (top) and the basal surface (bottom) of epithelial cells, thereby maintaining polarity and function.

Carbohydrates are always located extracellularly and can be represented as hexagonal structures (monosaccharides).
Functions of carbohydrates in the membrane include:
- Cell identity recognition.
- Providing mechanical protection through glycan layers.

Mechanism of Selective Permeability:
- Phospholipids create a barrier to ions and large polar molecules while allowing nonpolar and small molecules to diffuse freely.
Examples of molecules permeable through lipid membrane include:
- Nonpolar gases (O₂, CO₂) and steroid hormones (estrogen, testosterone).
Restrictions for larger or charged molecules:
- Larger polar molecules, ions cannot permeate without transport mechanisms.

Simple Diffusion:
- Movement of small nonpolar molecules from high to low concentration without energy use.
Facilitated Diffusion:
- Utilizes channels or transporters to move larger or polar molecules across membranes, also from high to low concentration without energy expenditure.

Electrochemical gradients are essential for ion movement and involve both concentration and electrical gradients.
Concentration Gradient:
- Refers to the differences in concentration of ions/molecules across a membrane.
Electrical Gradient:
- Results from differences in charge distribution across the membrane, affecting the movement of charged ions (e.g., sodium, potassium).
Example with Sodium Ions:
- Sodium concentration is higher outside the cell, encouraging sodium to move inside (positively charged, attracted to the negatively charged interior).

Driving Force (DF):
- Refers to the force determining the direction in which an ion moves (calculated based on membrane potential).
- Membrane Potential (Vm):
- Generally negative inside a cell (~ -70 mV) under resting conditions.
- New membrane potential at equilibrium after ion movement can be calculated and will change based on ion concentrations and charges.

Structure of Ion Channels:
- Comprised of protein subunits with selective filters that allow specific ions through while excluding others.
Na⁺ ions have a specific impediment due to their size compared to K⁺ ions within selectivity filters, affecting transport efficiency and specificity.

Ions such as Na⁺ and K⁺ rely on channels to cross membranes, facts rooted in their respective concentrations and electrical charges.
Understanding these principles establishes a foundation for exploring mechanisms of neurotransmission, muscle contraction, and homeostasis regarding ion balance in biological systems.