Cell Structure and Membrane Transport

All living organisms are composed of cells which serve as the fundamental structural and functional unit of life.
Life does not have a simple definition. Instead, living entities are recognized by their functions, characterized by a complex organization consisting of the following:
- Complex Cellular Organization: Living organisms exhibit a complex structure that contributes to their effectiveness in interacting with their surroundings.
- Energy Exchange and Homeostasis: Organisms can adjust their structure, function, and behaviors in response to changes in the environment. They require energy to maintain an ordered internal state and must manage waste release to maintain homeostasis.
- Controlled Movements: Living systems display precise movements that arise internally.

Cells are characterized by the following:
- Surrounded by a selectively permeable plasma membrane.
- Contain a semi-fluid cytosol with organelles and ribosomes (termed cytoplasm).
- Feature a central nucleus that houses chromosomes.

Definition: Metabolism encompasses all chemical reactions in an organism.
- Catabolism: Processes that release energy by breaking down complex molecules into simpler forms (e.g., sugars broken down in cellular respiration).
- Anabolism: Processes that consume energy to build complex molecules (e.g., synthesis of starch from glucose).
Energy Coupling: Energy from catabolic reactions often drives anabolic reactions, essential for maintenance.

Cell Division: Essential for growth, renewal, and repair; most somatic cells divide regularly, with notable exceptions (e.g., gametes, red blood cells).
Differentiation: Unspecialized cells (stem cells) can become specialized, leading to a diverse range of cell types due to specific gene expression.
- Ectoderm, Mesoderm, Endoderm: Germ layers differentiate into different cell types, influencing their specialized functions:
  - Example: Muscles contract to facilitate movement, nerve cells conduct electric signals, blood cells transport oxygen, and white blood cells defend against pathogens.

Phospholipid Bilayer: Composed of hydrophobic (nonpolar) fatty acid tails and hydrophilic (polar) heads, which form the core of the plasma membrane and dictate its selective permeability.
Membrane Proteins: Integral and peripheral proteins are embedded within the bilayer, influencing specific functions of the membrane.
Glycocalyx: Carbohydrate molecules on the external surface of the membrane that facilitate cell recognition processes.

Diffusion: Movement of molecules from high to low concentration without energy input (e.g., small hydrophobic molecules like O2 and CO2).
Facilitated Diffusion: Involves transport proteins to aid passive movement across the membrane; includes:
- Channel Proteins: Facilitate passage for ions or small polar molecules.
- Carrier Proteins: Assist larger polar molecules.
Osmosis: The diffusion of water across a selectively permeable membrane, maintaining homeostasis and regulating cell volume.
- Describes the behavior of red blood cells in hypertonic, hypotonic, and isotonic solutions:
  - Hypertonic: Cell shrivels.
  - Isotonic: Cell remains normal.
  - Hypotonic: Cell lyses (swells).

Definition: Movement of substances against their concentration gradient requiring energy (ATP).
- Primary Active Transport: Employs transport proteins (e.g., Na+/K+ pump) to regulate ion concentrations.
- Secondary Active Transport: Utilizes the concentration gradient established by primary active transport to move other molecules (e.g., Na+-glucose symport).

Endocytosis: Uptake of materials into the cell:
- Phagocytosis: Ingestion of large particles for digestion.
- Pinocytosis: Engulfing of tiny droplets of extracellular fluid.
- Receptor-mediated Endocytosis: Specific materials are ingested based on receptor clustering.
Exocytosis: Secretion of macromolecules from cells via vesicle fusion with the plasma membrane, crucial for secretory and nerve cells (e.g., release of digestive enzymes).