2.1 Cellular Structure and Function

Cells are the building blocks of life, forming the foundation of human physiology. Understanding their structure and function is crucial for grasping how our bodies work at the most basic level.

From the cell membrane to the nucleus, each part plays a vital role in keeping us alive. We'll explore how cells communicate, generate energy, and create proteins, connecting these processes to the broader picture of human anatomy and physiology.

Cell Structure

Cell Membrane and Cytoplasm

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• Cell membrane consists of a phospholipid bilayer with embedded proteins that separates the interior of the cell from the outside environment
• Selectively permeable membrane controls the passage of substances in and out of the cell (ions, nutrients, waste products)
• Cytoplasm is the gelatin-like fluid inside the cell where organelles are suspended
• Contains cytoskeleton, a network of filaments and tubules that provides structure, support, and movement to the cell (actin filaments, microtubules)

Nucleus and Endoplasmic Reticulum

• Nucleus is the control center of the cell, containing the cell's genetic material (DNA) and directing cellular activities
• Nuclear envelope is a double membrane that surrounds the nucleus, separating it from the cytoplasm
• Endoplasmic reticulum (ER) is a network of membranous tubes and sacs that extends from the nuclear envelope
• Rough ER is studded with ribosomes and is involved in protein synthesis and transport
• Smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage

Mitochondria, Golgi Apparatus, and Other Organelles

• Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration (converting glucose and oxygen into energy)
• Golgi apparatus is a stack of flattened membrane sacs that modifies, packages, and distributes proteins and lipids (secretory pathway)
• Lysosomes are membrane-bound organelles containing digestive enzymes that break down cellular waste, debris, and foreign invaders (intracellular digestion)
• Peroxisomes are organelles that detoxify harmful substances and break down fatty acids (liver cells)
• Centrosome is an organelle that organizes the cell's microtubules and is involved in cell division (mitotic spindle)

Cell Functions

Cell Signaling and Membrane Transport

• Cell signaling involves communication between cells through chemical messengers (hormones, neurotransmitters) that bind to receptors on the cell surface
• Membrane transport is the movement of substances across the cell membrane
• Passive transport occurs without energy input and includes simple diffusion (movement of small, nonpolar molecules) and facilitated diffusion (movement of larger, polar molecules through protein channels)
• Active transport requires energy (ATP) to move substances against their concentration gradient (sodium-potassium pump)

Cellular Respiration and Protein Synthesis

• Cellular respiration is the process of breaking down glucose to generate ATP, the cell's energy currency
• Occurs in the mitochondria and involves glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), and electron transport chain (inner mitochondrial membrane)
• Protein synthesis is the process of creating new proteins based on the genetic instructions in DNA
• Transcription occurs in the nucleus, where DNA is transcribed into mRNA
• Translation occurs in the cytoplasm, where ribosomes read the mRNA and assemble amino acids into polypeptide chains (proteins)