Cells are the building blocks of life, performing essential functions like metabolism, energy production, and protein synthesis. They divide, communicate, and specialize to form complex organisms. Understanding cells is crucial for grasping how living things work.

Cell structure directly relates to function. The cell membrane controls what enters and exits, while organelles like mitochondria and the endoplasmic reticulum carry out specific tasks. This organization allows cells to efficiently perform their roles in organisms.

Fundamental Principles of Cell Biology

Cells as life's basic unit

• Smallest functional units of living organisms
  • All living things composed of one or more cells (bacteria, plants, animals)
  • Cells carry out essential life processes (metabolism, reproduction, growth)
• Cell theory states that:
  • All living organisms composed of cells
  • Cells are the basic unit of structure and function in living organisms
  • All cells arise from pre-existing cells through cell division (mitosis, meiosis)
• Cells contain genetic material (DNA) directs cellular activities
  • DNA passed from parent cells to daughter cells during cell division ensures continuity of genetic information

Essential functions of cells

• Metabolism: sum of all chemical reactions in a cell
  • Catabolism breaks down complex molecules to release energy (glucose, fatty acids)
  • Anabolism synthesizes complex molecules using energy (proteins, lipids, carbohydrates)
• Energy production: cells convert energy from nutrients into usable forms ($ATP$)
  • Cellular respiration breaks down glucose to produce $ATP$ (glycolysis, Krebs cycle, electron transport chain)
  • Photosynthesis in plants and some microorganisms converts light energy into chemical energy (chloroplasts, light-dependent and light-independent reactions)
• Protein synthesis: cells synthesize proteins based on genetic instructions
  1. Transcription: DNA transcribed into $mRNA$ (nucleus)
  2. Translation: $mRNA$ translated into proteins by ribosomes (cytoplasm)
• Cell division: cells reproduce to replace damaged or dead cells and for growth
  • Mitosis divides somatic cells, resulting in two genetically identical daughter cells (interphase, prophase, metaphase, anaphase, telophase)
  • Meiosis divides germ cells, resulting in four genetically diverse haploid cells (meiosis I, meiosis II)
• Cell signaling: cells communicate with each other and respond to external stimuli
  • Receptors on cell surface or inside the cell detect signals (hormones, neurotransmitters)
  • Signal transduction pathways relay information to target molecules inside the cell (protein kinases, second messengers)

Cell specialization in organisms

• Cell specialization: cells develop specific structures and functions to perform particular tasks
  • Enables efficient division of labor within an organism
  • Examples: nerve cells (signal transmission), muscle cells (contraction), epithelial cells (barrier function)
• Cell differentiation: process by which a less specialized cell becomes a more specialized cell type
  • Occurs during embryonic development and throughout an organism's life
  • Driven by differential gene expression (transcription factors, epigenetic modifications)
• Advantages of cell specialization and differentiation:
  • Allows for the development of complex tissues and organs (brain, heart, lungs)
  • Enables organisms to perform a wide range of functions efficiently (locomotion, digestion, reproduction)
  • Facilitates adaptation to different environments and niches (aquatic, terrestrial, aerial)

Cell structure vs function

• Cell membrane: selectively permeable barrier controls movement of substances in and out of the cell
  • Composed of a phospholipid bilayer with embedded proteins (integral, peripheral)
  • Regulates cell-cell communication and cell signaling (receptors, adhesion molecules)
• Nucleus: contains the cell's genetic material (DNA) and controls cellular activities
  • Nuclear envelope: double membrane separates the nucleus from the cytoplasm (nuclear pores)
  • Nucleolus: site of ribosomal $RNA$ synthesis and ribosome assembly
• Cytoskeleton: network of protein filaments provides structure, support, and movement
  • Microfilaments (actin filaments) involved in cell movement and division (muscle contraction, cytokinesis)
  • Microtubules maintain cell shape and assist in intracellular transport (centrioles, spindle fibers)
  • Intermediate filaments provide mechanical strength and support (keratin, lamin)
• Organelles: specialized structures perform specific functions within the cell
  • Mitochondria: sites of cellular respiration and $ATP$ production (cristae, matrix)
  • Endoplasmic reticulum ($ER$): synthesis and transport of proteins and lipids (rough $ER$, smooth $ER$)
  • Golgi apparatus: modification, packaging, and sorting of proteins and lipids (cis, medial, trans)
  • Lysosomes contain digestive enzymes for intracellular digestion and waste removal (hydrolases, pH)