Describe the basic structure of the cell membrane. What is the cell membrane’s main function?

The cell membrane's core structure is the **lipid bilayer**. This bilayer consists of two layers of phospholipids. Each phospholipid molecule has a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails.
The hydrophilic heads face outward, towards the watery environments inside and outside the cell.
This arrangement creates a hydrophobic interior that is impermeable to most water-soluble molecules. This property helps the cell maintain a distinct internal environment, crucial for cellular functions.
The lipid bilayer is semi-permeable, allows certain substances to pass while blocking others. This selective permeability is vital for maintaining the cell's integrity and environment.

**Membrane proteins** are essential components of the cell membrane. These proteins are embedded within the lipid bilayer and perform various critical functions.
There are two main types of membrane proteins: integral and peripheral. Integral proteins span the entire lipid bilayer, while peripheral proteins are attached to the bilayer's surface.
Functions of membrane proteins include:

• Transporting molecules across the membrane
• Acting as enzymes to speed up biochemical reactions
• Serving as receptors for signaling molecules

By interacting with the cell's exterior environment, membrane proteins play a significant role in communication and the cell's response to external signals.

**Cell signaling** is the process by which cells communicate with each other and respond to their environment. This communication relies heavily on the cell membrane.
Receptors in the cell membrane proteins bind to signaling molecules, such as hormones or neurotransmitters. This binding triggers a series of intracellular events, leading to a specific cellular response.
There are different types of cell signaling mechanisms:

• Autocrine signaling: Cells respond to signals they release themselves.
• Paracrine signaling: Cells respond to nearby signaling molecules.
• Endocrine signaling: Hormones released into the bloodstream affect distant cells.
• Direct signaling: Cells communicate through direct contact.

Effective cell signaling is crucial for everything from growth and development to immune responses.

**Cellular transport** refers to the movement of substances across the cell membrane. This process is vital for the cell to obtain nutrients, expel waste, and maintain homeostasis.
There are two primary types of cellular transport: passive and active.
Passive transport involves the movement of molecules from high to low concentration without energy use (e.g., diffusion and osmosis).
Active transport requires energy (ATP) to move molecules against their concentration gradient.
Examples of active transport mechanisms include:

• Sodium-potassium pump: Moves sodium ions out and potassium ions into the cell.
• Endocytosis: Engulfs external substances into the cell.
• Exocytosis: Expels substances from the cell.

Through these transport mechanisms, cells can regulate their internal environment effectively.