🔬General Biology I Unit 4 – Cell Structure

Key Concepts

• Cells are the fundamental unit of life and the building blocks of all living organisms
• Cell theory states that all living things are composed of cells, cells are the basic units of structure and function in living things, and all cells come from pre-existing cells
• Cells can be classified as prokaryotic or eukaryotic based on their internal structure and organization
  • Prokaryotic cells lack a nucleus and other membrane-bound organelles (bacteria and archaea)
  • Eukaryotic cells have a nucleus and other membrane-bound organelles (animals, plants, fungi, and protists)
• The cell membrane is a selectively permeable barrier that controls the movement of substances in and out of the cell
  • Composed of a phospholipid bilayer with embedded proteins
  • Allows for the transport of essential molecules while maintaining cellular homeostasis
• Organelles are specialized structures within a cell that perform specific functions
  • Examples include the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus
• The cytoskeleton provides structural support and enables cell movement
  • Consists of microfilaments, intermediate filaments, and microtubules
• Cells communicate with each other through various mechanisms, such as cell signaling and cell junctions, to coordinate cellular activities and maintain tissue and organ function

Cell Theory and History

• Cell theory was developed in the 19th century by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow
  • Schleiden and Schwann proposed that all living organisms are composed of cells
  • Virchow added that all cells come from pre-existing cells through cell division
• The invention of the microscope in the 17th century by Antonie van Leeuwenhoek and Robert Hooke was crucial to the development of cell theory
  • Hooke coined the term "cell" after observing the structure of cork under a microscope
• The cell theory has three main postulates:
  1. All living organisms are composed of one or more cells
  2. The cell is the basic unit of structure and function in living organisms
  3. All cells come from pre-existing cells through cell division
• The cell theory is a unifying concept in biology that explains the fundamental similarities among all living organisms despite their diverse forms and functions
• The discovery of cells and the development of cell theory marked a significant milestone in the history of biology and laid the foundation for modern cellular and molecular biology

Types of Cells

• Cells can be broadly classified into two main types: prokaryotic and eukaryotic cells
• Prokaryotic cells are simpler and smaller than eukaryotic cells
  • Lack a nucleus and other membrane-bound organelles
  • Have a single circular DNA molecule located in the cytoplasm
  • Examples include bacteria and archaea
• Eukaryotic cells are more complex and larger than prokaryotic cells
  • Have a nucleus and other membrane-bound organelles
  • DNA is organized into linear chromosomes within the nucleus
  • Examples include cells of animals, plants, fungi, and protists
• Plant cells and animal cells are both eukaryotic but have some distinct differences
  • Plant cells have a cell wall, chloroplasts, and a large central vacuole
  • Animal cells lack a cell wall, chloroplasts, and a large central vacuole but have centrioles
• Specialized cells are differentiated to perform specific functions within multicellular organisms
  • Examples include neurons, muscle cells, and epithelial cells in animals, and guard cells and root hair cells in plants

Cell Membrane Structure

• The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that separates the cell's interior from the external environment
• Composed of a phospholipid bilayer with embedded proteins and other molecules
  • Phospholipids have a hydrophilic head and two hydrophobic tails
  • Arranged in a bilayer with the hydrophilic heads facing the aqueous environment and the hydrophobic tails facing each other
• Membrane proteins are embedded in the phospholipid bilayer and perform various functions
  • Integral proteins span the entire membrane and may function as channels, carriers, or receptors
  • Peripheral proteins are attached to the surface of the membrane and may serve as enzymes or structural components
• The fluid mosaic model describes the dynamic nature of the cell membrane
  • Phospholipids and proteins can move laterally within the membrane
  • Membrane fluidity is influenced by temperature, lipid composition, and the presence of cholesterol
• The cell membrane is involved in various cellular processes, such as selective permeability, cell signaling, and cell adhesion

Organelles and Their Functions

• Organelles are specialized structures within a cell that perform specific functions
• The nucleus is the control center of the cell and contains the cell's genetic material (DNA)
  • Surrounded by a double membrane called the nuclear envelope
  • Nuclear pores allow for the selective transport of molecules between the nucleus and cytoplasm
• Mitochondria are the powerhouses of the cell and generate ATP through cellular respiration
  • Have a double membrane structure with the inner membrane folded into cristae
  • Contain their own DNA and ribosomes for protein synthesis
• The endoplasmic reticulum (ER) is a network of membranous channels involved in protein and lipid synthesis and transport
  • Rough ER has ribosomes attached to its surface and is involved in protein synthesis and modification
  • Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification
• The Golgi apparatus is a stack of flattened membranous sacs that modifies, packages, and sorts proteins and lipids for transport to their final destinations
• Lysosomes are membrane-bound organelles that contain digestive enzymes and break down cellular waste, damaged organelles, and foreign particles
• Peroxisomes are organelles that break down fatty acids and detoxify harmful substances, such as alcohol and hydrogen peroxide
• Ribosomes are small organelles composed of RNA and protein that synthesize proteins
  • Can be found free in the cytoplasm or attached to the rough ER

Cytoskeleton and Cell Movement

• The cytoskeleton is a network of protein filaments that provides structural support and enables cell movement
• Consists of three main types of filaments: microfilaments, intermediate filaments, and microtubules
• Microfilaments are thin, flexible filaments composed of actin proteins
  • Involved in cell movement, cell division, and maintaining cell shape
  • Enable muscle contraction and the formation of cellular extensions, such as pseudopodia and microvilli
• Intermediate filaments are thicker and more stable than microfilaments
  • Provide mechanical strength and resistance to shear stress
  • Examples include keratin filaments in epithelial cells and neurofilaments in neurons
• Microtubules are hollow, cylindrical structures composed of tubulin proteins
  • Involved in cell division, organelle transport, and the formation of cilia and flagella
  • Organize the mitotic spindle during cell division and form the centrioles
• Motor proteins, such as myosin and kinesin, interact with the cytoskeleton to generate movement
  • Myosin interacts with actin filaments to enable muscle contraction and cell crawling
  • Kinesin and dynein move along microtubules to transport organelles and vesicles within the cell

Cell Communication

• Cell communication is the process by which cells exchange information and coordinate their activities
• Cells can communicate through direct contact (juxtacrine signaling) or by releasing signaling molecules (paracrine and endocrine signaling)
• Cell junctions are specialized structures that allow direct communication between adjacent cells
  • Tight junctions seal the space between cells and prevent the passage of molecules
  • Gap junctions allow the direct exchange of small molecules and ions between cells
  • Desmosomes and adherens junctions provide mechanical attachment between cells
• Signaling molecules, such as hormones, neurotransmitters, and growth factors, bind to specific receptors on the target cell's surface or interior
  • Receptor-ligand binding triggers a series of intracellular signaling cascades that alter the cell's behavior or gene expression
• Signal transduction pathways relay the information from the receptor to the cell's interior and amplify the signal
  • Examples include G protein-coupled receptor (GPCR) pathways and receptor tyrosine kinase (RTK) pathways
• Cell communication is essential for the coordination of cellular activities, tissue and organ development, and the maintenance of homeostasis in multicellular organisms

Practical Applications

• Understanding cell structure and function has numerous practical applications in fields such as medicine, biotechnology, and agriculture
• In medicine, knowledge of cell biology is crucial for the development of targeted therapies and personalized medicine
  • Cancer therapies often target specific cellular pathways or organelles to selectively kill cancer cells
  • Stem cell research and regenerative medicine rely on understanding cell differentiation and cell-cell communication
• In biotechnology, cell culture techniques are used to produce biopharmaceuticals, such as insulin and monoclonal antibodies
  • Genetic engineering of cells allows for the production of recombinant proteins and the development of genetically modified organisms (GMOs)
• In agriculture, understanding plant cell structure and function is essential for crop improvement and pest management
  • Genetic modification of crops can increase yield, improve nutritional value, and enhance resistance to pests and environmental stresses
  • Knowledge of plant cell communication and signaling pathways can help develop more efficient and sustainable agricultural practices
• Cell biology research has also contributed to the development of advanced imaging techniques, such as fluorescence microscopy and electron microscopy, which have revolutionized our understanding of cellular structure and function
• The study of cell biology has implications for environmental science, as understanding the effects of pollutants and toxins on cellular processes can help develop strategies for environmental protection and remediation