Cell Biology: Structure and Organization

Cell Biology Overview

• Cell biology examines cells, including their structure, function, and behavior

Cell Structure and Organization

• Cells serve as the fundamental unit of life
• Living organisms consist of single or multiple cells
• Various organelles within cells perform specific functions
• The plasma membrane encloses the cell, separating its contents from the external environment
• A cell's genetic material (DNA) resides in the nucleus
• Protein synthesis occurs via ribosomes
• The endoplasmic reticulum (ER) participates in protein and lipid synthesis
• The Golgi apparatus modifies, sorts, and packages proteins and lipids
• Mitochondria facilitate cellular respiration and ATP production
• Lysosomes contain enzymes that break down cellular waste and debris
• The cytoskeleton offers structural support and aids cell movement

Cell Membrane

• A lipid bilayer with embedded proteins forms the cell membrane
• Phospholipids, with a hydrophilic head and a hydrophobic tail, primarily constitute the lipid bilayer
• Phospholipid hydrophobic tails face each other, creating a barrier for water-soluble molecules
• Membrane proteins are either integral (embedded) or peripheral (associated with the surface)
• Selective permeability characterizes the cell membrane, allowing some molecules to pass while blocking others
• Transport across the cell membrane involves passive or active mechanisms
• Passive transport doesn't require energy, including diffusion, osmosis, and facilitated diffusion
• Active transport needs energy, including primary and secondary active transport
• Cells import and export large molecules or particles through endocytosis and exocytosis

Cell Communication

• Cells communicate using signaling molecules like hormones, neurotransmitters, and growth factors
• Cell signaling includes:
  • Signaling molecule synthesis and release
  • Transport to the target cell
  • Detection by a receptor protein
  • Activation of intracellular signaling pathways
  • Changes in target cell behavior
• Receptors reside on the cell surface or inside the cell
• Hydrophilic signaling molecules bind to cell surface receptors
• Intracellular receptors bind to hydrophobic signaling molecules that diffuse across the membrane
• Signal transduction pathways employ protein interactions that amplify and relay signals
• Common signaling pathways include:
  • G protein-coupled receptor (GPCR) pathways
  • Receptor tyrosine kinase (RTK) pathways
  • Ion channel receptor pathways

Cell Cycle and Cell Division

• The cell cycle includes events leading to cell growth and division
• Four main phases of the cell cycle:
  • G1 phase (growth and preparation for DNA replication)
  • S phase (DNA replication)
  • G2 phase (growth and preparation for cell division)
  • M phase (mitosis or meiosis)
• Cell cycle checkpoints ensure proper DNA replication and chromosome segregation
• Mitosis produces two identical daughter cells
• Meiosis produces four genetically different daughter cells (gametes)
• Mitosis functions in growth, repair, and asexual reproduction
• Meiosis functions in sexual reproduction
• Uncontrolled cell division may lead to cancer

Cellular Respiration

• Cellular respiration breaks down glucose for ATP energy
• Four main stages of cellular respiration:
  • Glycolysis (glucose breakdown into pyruvate)
  • Pyruvate oxidation (pyruvate to acetyl CoA)
  • Citric acid cycle (acetyl CoA oxidation to produce ATP, NADH, and FADH2)
  • Oxidative phosphorylation (NADH and FADH2 use to generate a proton gradient for ATP production)
• Glycolysis takes place in the cytoplasm
• Pyruvate oxidation and the citric acid cycle occur in the mitochondria
• Oxidative phosphorylation happens in the inner mitochondrial membrane
• Cellular respiration can occur with or without oxygen
• Fermentation is used to produce ATP in the absence of oxygen
• Fermentation is less efficient than cellular respiration and produces less ATP

Cell Differentiation

• Cell differentiation specializes cells for specific functions
• Cells receive signals during development that establish their fate
• Signals activate or repress specific genes, leading to differences in cell structure and function
• Cell differentiation supports the formation of tissues and organs
• Stem cells are undifferentiated cells that divide and differentiate into specialized cell types
• Two main types of stem cells:
  • Embryonic stem cells (pluripotent, can differentiate into any cell type)
  • Adult stem cells (multipotent, can differentiate into a limited number of cell types)
• Potential for regenerative medicine and tissue engineering comes from stem cells

Explore the basic unit of life with this lesson on cell biology. Learn about the cell's structure, organelles, and their functions. Understand the roles of the plasma membrane, nucleus, ribosomes, endoplasmic reticulum, and more.