Cell Membrane

All cells have common needs:

Communication with other cells, allowing for coordination and interaction within tissues and organ systems.
Taking in food/water, which provides the necessary nutrients and hydration for cellular functions.
Eliminating wastes to prevent toxic buildup that can harm the cell.
All of these needs are met by the cell membrane, which acts as a selective barrier.

Regulates (controls) what goes in/out of the cell: The membrane ensures that essential nutrients enter the cell while waste products are expelled, maintaining homeostasis.
Protects the cells: It serves as a barrier that shields the internal environment from external threats, such as pathogens and harmful substances.
Supports the cells: Provides structural integrity and shape to the cell, allowing it to maintain its function in different environments.

The cell membrane is composed of three primary types of molecules:

Phospholipids: These molecules form the foundation of the membrane structure.
- Heads: Hydrophilic (“water-loving”) and face outward toward the aqueous environment.
- Tails: Hydrophobic (“water-fearing”) and face inward, away from water.
- Saturated fats: Have straight tails, leading to tighter packing.
- Unsaturated fats: Contain bends caused by double bonds, introducing fluidity to the bilayer.
- The cell membrane forms a bilayer (two layers) of phospholipids, which is crucial for its semi-permeable nature.
Proteins: Integral to the membrane's functionality, proteins can be classified into:
- Peripheral proteins: Located on the membrane surface, involved in signaling and structural support.
- Integral proteins: Span the bilayer, facilitating the transport of materials that cannot pass directly through the hydrophobic lipid layer.
- Function: Allows materials in/out of the cell that cannot pass through the bilayer, crucial for nutrient uptake and waste removal.
Carbohydrates: Primarily found on the exterior of the membrane:
- Typically attached to peripheral proteins (glycoproteins) or lipids (glycolipids).
- Function: Cell-to-cell recognition, enabling cells to identify and communicate with each other, important in immune responses and tissue formation.

Cholesterol is a sterol found only in animal cells.
Purpose: It plays a vital role in maintaining membrane structure and fluidity, preventing the fatty acid chains of phospholipids from packing too closely together, which can hinder movement and flexibility.
The OH group (hydroxyl) is polar, allowing interaction with phospholipid heads, while the rest of the cholesterol molecule is nonpolar, integrating smoothly into the membrane's hydrophobic core.

The cell membrane is semi-permeable, selectively allowing substances to cross based on size and charge.
- Ions (such as Na+, Cl-) and large, uncharged polar molecules CANNOT pass through the cell membrane due to their size and charge.
- Small uncharged polar molecules (H2O) and hydrophobic molecules (O2, CO2) CAN pass through the cell membrane freely, facilitating essential processes like respiration and nutrient transport.