Biology Cell Membrane Structure and Function

Definition: The cell membrane, also known as the plasma membrane, is a phospholipid bilayer that surrounds the cell.
Components:
- Phospholipids: Form the fundamental structure with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.
- Cholesterol: Interspersed within the bilayer to provide stability and fluidity.
- Proteins: Embedded within the bilayer, they can be integral (spanning the membrane) or peripheral (attached to the surface).
- Carbohydrates: Often attached to proteins (glycoproteins) or lipids (glycolipids), playing a role in cell recognition and signaling.
Functions:
- Barrier: Maintains cellular integrity and separates internal components from the external environment.
- Selective Permeability: Regulates the entry and exit of substances, allowing essential molecules in and waste products out.
- Communication: Contains receptors for signal transduction, enabling the cell to respond to external signals.

Types:
- Integral Membrane Proteins: Span the entire membrane; involved in transport and signal transduction.
- Peripheral Membrane Proteins: Located on the membrane's surface; involved in signaling and maintaining the cell's shape.
Functions:
- Transport Proteins: Facilitate the movement of ions and molecules across the membrane (e.g., channels, carriers).
- Receptor Proteins: Bind to specific molecules (ligands) to initiate cellular responses.
- Enzymatic Activity: Catalyze biochemical reactions at the membrane surface.
- Cell Recognition: Assist in identifying cells and pathogens, crucial for immune responses.

Passive Transport: Movement of molecules across the membrane without energy expenditure.
- Diffusion: Movement from an area of higher concentration to lower concentration.
- Facilitated Diffusion: Uses transport proteins to move substances down their concentration gradient (e.g., glucose transport).
- Osmosis: Diffusion of water through a selectively permeable membrane.
Active Transport: Requires energy (ATP) to move substances against their concentration gradient.
- Primary Active Transport: Directly uses ATP to transport molecules (e.g., sodium-potassium pump).
- Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient (e.g., glucose-sodium co-transport).
Vesicular Transport: Involves the movement of large particles or volumes of fluid.
- Endocytosis: Process of taking materials into the cell by engulfing them in a vesicle.
  - Phagocytosis: "Cell eating" - uptake of large particles.
  - Pinocytosis: "Cell drinking" - uptake of fluids.
- Exocytosis: Process of vesicles fusing with the membrane to release their contents outside the cell.

The cell membrane is a phospholipid bilayer that encases the cell.
Phospholipids have hydrophilic heads that attract water and hydrophobic tails that repel it, forming the bilayer structure.
Cholesterol molecules are embedded in the bilayer, providing structural stability and fluidity.
Membrane proteins can be integral, spanning the membrane, or peripheral, attached to its surface.
Carbohydrates attached to proteins (glycoproteins) or lipids (glycolipids) facilitate cell recognition and signaling.
The cell membrane acts as a barrier, maintaining the integrity of the cell by separating internal components from the outside environment.
It features selective permeability, controlling the movement of essential molecules in and waste products out of the cell.
Membrane receptors enable communication, allowing the cell to respond to external signals.

Integral membrane proteins span the membrane entirely and are crucial for transport and signal transduction.
Peripheral membrane proteins are found on the membrane's surface and play roles in signaling and maintaining cell shape.
Transport proteins facilitate the movement of ions and molecules across the membrane through channels or carriers.
Receptor proteins bind to ligands, triggering cellular responses.
Enzymatic activity occurs at the membrane surface, catalyzing biochemical reactions.
Cell recognition proteins help identify cells and pathogens, playing a vital role in the immune response.

Passive transport allows molecules to move across the membrane without the use of energy.
Diffusion is the process where molecules move from a region of high concentration to one of low concentration.
Facilitated diffusion utilizes transport proteins to help substances move down their concentration gradient.
Osmosis specifically refers to the diffusion of water through a selectively permeable membrane.
Active transport requires energy (ATP) to move substances against their concentration gradient.
Primary active transport uses ATP directly to transport molecules, exemplified by the sodium-potassium pump.
Secondary active transport leverages the energy generated by one substance's gradient to transport another substance.
Vesicular transport involves the movement of large particles or fluid volumes into or out of the cell.
Endocytosis is the process of engulfing materials into the cell via vesicles, with subtypes including phagocytosis ("cell eating") and pinocytosis ("cell drinking").
Exocytosis is the process where vesicles fuse with the membrane to release their contents outside the cell.