Plasma Membrane 2.1

Plasma Membrane: A biological membrane that separates and protects the cell from its external environment.
Functions:
- Protection: Shields the internal components of the cell.
- Communication: Facilitates cellular communication.*
- Enzymatic Activities: Engages in various enzymatic processes essential for cell function.
- Structural Integrity: Ensures the maintenance of the cell’s shape and structural components.
- Homeostasis: Maintains the chemical composition of the cytoplasm and extracellular fluid.
- Regulation: Controls the entry and exit of substances to and from the cell.

Boundary: Acts as a boundary separating intracellular and extracellular environments.
Components:
- Phospholipid Bilayer:
- Composed of hydrophilic "heads" and hydrophobic fatty acid "tails."
- Forms the fundamental structure of the membrane that provides its fluidity.
Illustration: Displays the structural organization of the phospholipid bilayer.

Hydrophilic Heads: Attract water molecules, citing interactions with aqueous environments.
Hydrophobic Tails: Repel water, oriented inward away from the aqueous environment.
Composition Details:
- Typically consists of saturated and unsaturated fatty acids, which affect the membrane’s fluidity.
- Glycerol as part of the phospholipid structure.

Types of Membrane Proteins:
- Integral Proteins:
- Inserted into the membrane; cannot be removed without damaging the membrane.
- Often span the entire membrane: known as transmembrane proteins.
- Functions include acting as transport channels for various substances.
- Peripheral Proteins:
- Attached to the inner or outer surface.
- Can be removed without damaging the cell and may perform regulatory or enzymatic functions.

Transport: Facilitate selective transport of molecules across the plasma membrane.
Enzymatic Activity: Metabolic reactions catalyzed by enzymatic proteins embedded in the membrane.
Signal Transduction: Function as receptors to recognize and bind to chemical messengers, triggering cell responses.
Cell-Cell Recognition: Glycoproteins act as recognition sites for interaction among cells.
Cell-Cell Adhesion: Facilitate adhesion between adjacent cells.
Anchoring: Bind to the cytoskeleton or extracellular matrix for maintaining cell shape and coordinating changes.

Definition: The plasma membrane's ability to control what passes in and out of the cell, allowing only certain molecules to cross.
Factors Influencing Permeability:
- Substance Characteristics:
- Size
- Molecular shape
- Lipid solubility
- Electrical charge
- Membrane Characteristics:
- Types of lipids and proteins present
- Arrangement of these lipids and proteins

Hydrophobic Molecules:
- Easily pass through the bilayer.
- Examples: Oxygen (O₂), nitrogen (N₂), and steroids.
Small Uncharged Polar Molecules:
- Can pass through without transport proteins.
- Examples: Water (H₂O), glycerol, urea, and ethanol.
Large Uncharged Polar Molecules:
- Low permeability across the membrane.
- Examples: Glucose, sucrose.
Ions:
- Not permeable through the membrane.
- Examples: Sodium ions (Na⁺), potassium ions (K⁺), hydrogen ions (H⁺), calcium ions (Ca²⁺), and chloride ions (Cl⁻).

Transport Mechanisms:
- Non-Polar Uncharged Molecules: Move by simple diffusion.
- Small and Medium Sized Charged Molecules: Utilize facilitated diffusion.
- Small Charged Molecules (Water): Move via osmosis.

Passive Transport: Does not require ATP. Includes:
- Simple Diffusion: Substances move along the concentration gradient.
- Facilitated (Carrier-mediated) Diffusion: Large or charged molecules assisted across the membrane.
Active Transport: Requires ATP. Includes:
- Carrier-mediated Transport: Moves substances against the concentration gradient.
- Vesicular Transport: Encompasses endocytosis (uptake of materials) and exocytosis (release of materials).

Definition: The movement of substances across a membrane without energy input.
- Simple Diffusion: Passed based on concentration without influence by the membrane.
  - Example Substances: Lipids and soluble gases.
- Facilitated Diffusion: Assists larger or charged molecules through channels.
  - Membrane Channel Examples: Water and small water-soluble molecules, ions.
  - Carrier Molecule Examples: Hydrophilic or large molecules.

Definition: The diffusion of water across a semipermeable membrane to equalize solute concentrations between environments.
Aquaporins: Specialized channels that significantly increase the speed at which water molecules can cross membranes (up to $10^8$ molecules/s), passing through in single file due to channel structure.

Purpose: Maintain concentration gradients differing from the surroundings, utilizing energy (ATP).
Facilitated by: Specific plasma membrane proteins known as transporters or pumps.

Definition: Requires the movement of large macromolecules via vesicles across the plasma membrane.
Types:
- Endocytosis: Movement of substances into the cell by invaginating the membrane to form a vesicle around the target substance.
  - Example: Destruction of pathogens by immune cells (neutrophils).
- Exocytosis: Movement of substances out of the cell where transport vesicles fuse with the plasma membrane to release contents.
  - Example: Release of digestive enzymes from the pancreas.

Passive Transport: No ATP required.
Active Transport: ATP required.
Types of transport molecules: Include proteins with various roles such as transport, enzymatic functions, and cell recognition.
Selective permeability: Depends on the membrane’s structural composition and the characteristics of molecules trying to pass through.