COLLEGE PHYSICS - BIOLOGY 2E - Chapter 4: Cell Structure and Function

Cell Structure and Function

Cell Size and Visualization

  • Cell Size Variation: Most cells are too small to be seen by the naked eye, necessitating the use of microscopes.

  • Microscopes: Devices that make small cells visible.

    • Light Microscope: Utilizes light photons to illuminate objects, viewed through lenses. Provides a visible image for general observation.

    • Electron Microscope: Employs electrons to illuminate objects, offering higher magnification and resolution.

      • Scanning Electron Micrograph (SEM): Generates a 3D image, which can be colorized. Useful for observing surface structures (e.g., largest and smallest human cells).

      • Transmission Electron Micrograph (TEM): Produces an image of a very thin cross-section (slice) through the cell, revealing internal structures (e.g., plant cell).

Minimum Components of a Cell

  • For a cell to function as a living organism, the absolute minimum components required are:

    • Cell Membrane: Encloses the cell and regulates passage of substances.

    • Cytoplasm: The internal contents of the cell.

    • DNA: Genetic material carrying hereditary information.

    • RNA: Involved in various aspects of gene expression and regulation.

    • Ribosomes: Cellular machinery for protein synthesis.

    • Proteins: Perform a vast array of functions within the cell.

  • Minimal cells, such as those discussed in research, may operate with as few as 493 genes.

Classification of Cells

According to Morphology (Shape and Features)

  • Prokaryotes: Organisms that lack a membrane-bound nucleus and other membrane-bound organelles.

    • Their chromosomes are located in a dense area within the cytoplasm called the nucleoid.

  • Eukaryotes: Organisms that possess a membrane-bound nucleus, which contains all their chromosomes, and other membrane-bound organelles.

According to Phylogeny (Evolutionary History)

  • Based on fossil evidence, biochemistry, and genetic relatedness, life is divided into three domains:

    • Bacteria: Unicellular prokaryotic cells.

    • Archaea: Unicellular prokaryotic cells, often found in extreme environments.

    • Eukarya: Composed of eukaryotic cells; can be unicellular or multicellular organisms.

Living at the Extremes: Extremophiles

  • Habitability: Organisms can live almost anywhere on Earth.

  • Extremophiles: Organisms that thrive in environments at the planetary extremes of conditions, including:

    • Temperature (very hot or very cold)

    • pH (highly acidic or highly alkaline)

    • Mineral concentrations

    • Salt concentrations

    • Pressure

    • Oxygen levels

    • Light or dark conditions

Astrobiology: Candidates for Life in Our Solar System

  • Essential Criteria for Microbial Habitability: These generally include the presence of liquid water, a source of energy, and essential chemical elements.

  • Mars:

    • Description: Fourth planet, arid, rocky, and cold.

    • Water: Most water is ice; some transient liquid water at high latitudes (poles). Icy poles are composed of CO2 and H2O ice.

    • Atmosphere: Thin, mostly CO_2 atmosphere, with winds and seasons; large dust storms occur.

    • Radiation: Receives intense solar and cosmic radiation due to the absence of a magnetic field.

    • Minerals: Abundant water-rich minerals found in the regolith (loose surface rocks).

    • Evidence of Water: Strong evidence of actively flowing water in the past and possible subglacial lakes currently.

  • Europa (Moon of Jupiter):

    • Description: Ice shell with a global subsurface ocean.

    • Core: Rocky core with active geological processes, including hydrothermal vents.

    • Challenges for Life: Life on Europa would need to adapt to low temperatures, high pressure, high surface radiation, variable ocean salinity, low pH, and a minimal O_2 atmosphere.

    • Potential: Oxygen, CO2, H2O_2, and other organics could be dissolved in the oceans.

    • Habitats: Areas on the ocean floor, such as benthic habitats (hard substrates, soft sediments, and hydrothermal vents), might support microbial life.

  • Enceladus (Moon of Saturn):

    • Description: Ice shell with a global subsurface ocean.

    • Core: Rocky core with active geological processes, including hydrothermal vents.

    • Ocean Chemistry: Water is rich in organic molecules. Ocean water pH is between 9 and 11, and it is less salty than Earth's oceans.

    • Activity: Geysers at the south polar terrain (tiger stripes) spew plumes of water containing methane, ethane, and oxygen.

    • Cycles: Active hydrological cycles, hydrothermal vents, and cryovolcanoes help transfer organic material to the surface.

  • Astrobiology Mission Candidates (Hypothetical Choices):

    • First-choice terrestrial world candidate: Mars (Planet 4).

    • First-choice ocean world candidate: Enceladus (moon of Saturn) or Europa (moon of Jupiter).

    • Where to Look for Microbial Life: Besides Earth, likely candidates are Mars, Europa, and Enceladus.

Prokaryotic Cell Structure

  • General Characteristics: All prokaryotic cells (bacteria and archaea) lack a membrane-bound nucleus and membrane-bound organelles.

  • Structural Similarities (Most Prokaryotes):

    • Plasma Membrane: Encloses the cytoplasm.

    • DNA: A single circular DNA chromosome located in the nucleoid region.

    • Plasmids: Many also possess small, circular DNA plasmids, which carry accessory genes.

    • Ribosomes: Essential for protein synthesis.

    • Cell Wall: A stiff outer wall surrounding the plasma membrane.

  • Internal Structures (within cytoplasm):

    • All contain ribosomes but lack an Endoplasmic Reticulum (ER).

    • Many have internal photosynthetic membranes (e.g., in cyanobacteria).

    • Some possess membrane- or protein-enclosed organelles or microcompartments (e.g., magnetosomes in magnetotactic bacteria, which help them orient towards or away from oxygen).

    • Many are supported by a cytoskeleton composed of long, thin protein filaments.

  • External Structures:

    • Flagella: Some prokaryotes have tail-like flagella on the cell surface that spin like a propeller to move the cell.

    • Cell Wall: A tough, fibrous cell wall surrounds the plasma membrane in most prokaryotes, providing structural support and protection.

    • Glycocalyx: Many species have an additional layer outside the cell wall, composed of glycolipids, which can form a capsule or slime layer.

Introduction to Eukaryotes

  • Diversity: Eukaryotes range from microscopic algae to large organisms like 100-meter-tall redwood trees.

  • Cellularity: Many eukaryotes are multicellular (e.g., plants and animals), while others are unicellular (e.g., protists and some algae).

  • Size Comparison: Most eukaryotic cells are significantly larger than most prokaryotic cells.

    • On average, prokaryotes are about 10 times smaller than eukaryotic cells in diameter and approximately 1000 times smaller in volume.

Eukaryotic Cell Structure and Compartmentalization

  • Surface-to-Volume Ratio: Eukaryotic cells generally have a small surface-to-volume ratio (large volume relative to surface area) compared to prokaryotic cells.

  • Rate of Diffusion: Due to their larger size, the rate of diffusion within the cell would be too slow for the speed of life if they lacked internal organization.

  • Compartmentalization: Eukaryotic cells overcome this limitation by breaking up their large cell volume into several smaller, membrane-bound organelles.

  • Advantages of Compartmentalization:

    1. Separation of Incompatible Chemical Reactions: Different reactions can occur simultaneously without interference.

    2. Increased Efficiency of Chemical Reactions: Specific enzymes and substrates can be concentrated in particular organelles.

    3. Storage of Pre-made Molecules: Allows cells to store molecules needed for specific processes, ready for use.

Eukaryotic Cell Structure: Plant vs. Animal Cells

Generalized Plant Cell Organelles

  • Shared Organelles: Nuclear envelope, nucleolus, chromosomes, nucleus, Rough Endoplasmic Reticulum (RER), ribosomes, Smooth Endoplasmic Reticulum (SER), Golgi apparatus, peroxisome, mitochondrion, plasma membrane, cytoskeletal elements.

  • Plant-Specific Structures:

    • Cell Wall: Stiff outer layer providing structural support and protection.

    • Chloroplast: Site of photosynthesis.

    • Large Central Vacuole: Stores water, nutrients, and waste, helps maintain turgor pressure.

    • Plastids: A group of organelles including chloroplasts, chromoplasts, and amyloplasts.

  • Connections Between Cells: Plasmodesmata are channels that directly connect the cytoplasm of adjacent plant cells, allowing for communication and transport.

Generalized Animal Cell Organelles

  • Shared Organelles: Similar to plant cells, excluding plant-specific ones.

  • Animal-Specific Structures:

    • Centrioles: Components of the centrosome, involved in cell division.

    • Lysosome: Contains digestive enzymes for waste processing and recycling.

    • Centrosomes: Main microtubule-organizing center in animal cells.

  • Connections Between Adjacent Animal Cells:

    • Gap Junctions: Provide direct channels for communication, allowing passage of ions and small molecules.

    • Tight Junctions: Watertight seals between cells, preventing leakage of extracellular fluid.

    • Desmosomes:

Cell Structure and Function

#### Key Terms and Cell Components

Minimum Components for a Functional Cell:

  • Cell Membrane: Encloses the cell and regulates the passage of substances.

  • Cytoplasm: The internal contents of the cell.

  • DNA: Genetic material carrying hereditary information.

  • RNA: Involved in gene expression and regulation.

  • Ribosomes: Cellular machinery for protein synthesis.

- Proteins: Perform a vast array of functions within the cell.

Prokaryotic Cell Structures:

  • Plasma Membrane: Encloses the cytoplasm.

  • DNA: A single circular DNA chromosome located in the nucleoid region.

  • Plasmids: Small, circular DNA molecules carrying accessory genes.

  • Ribosomes: Essential for protein synthesis.

  • Cell Wall: A stiff outer wall surrounding the plasma membrane, providing structural support and protection.

  • Internal Photosynthetic Membranes: Found in some prokaryotes (e.g., cyanobacteria).

  • Membrane- or Protein-Enclosed Organelles/Microcompartments: Such as magnetosomes in magnetotactic bacteria, aiding orientation.

  • Cytoskeleton: Composed of long, thin protein filaments for support.

  • Flagella: Tail-like structures that spin like a propeller for cell movement.

- Glycocalyx: An additional layer outside the cell wall (capsule or slime layer) composed of glycolipids.

Eukaryotic Cell Structures (Generalized Plant Cell Organelles):

  • Nuclear envelope: Double membrane surrounding the nucleus.

  • Nucleolus: Site of ribosome synthesis within the nucleus.

  • Chromosomes: Structures containing genetic material (DNA).

  • Nucleus: Contains all chromosomes and regulates cell activities.

  • Rough Endoplasmic Reticulum (RER): Involved in protein synthesis and modification, studded with ribosomes.

  • Ribosomes: (Also found in prokaryotes) Sites of protein synthesis.

  • Smooth Endoplasmic Reticulum (SER): Involved in lipid synthesis, detoxification, and calcium storage.

  • Golgi apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

  • Peroxisome: Involved in metabolic processes, breaking down fatty acids and detoxifying harmful substances.

  • Mitochondrion: Site of cellular respiration, producing ATP.

  • Plasma membrane: Encloses the cell and regulates passage of substances.

  • Cytoskeletal elements: Provide structural support, aid in cell movement, and transport materials.

  • Cell Wall: (Plant-specific) Stiff outer layer providing structural support and protection.

  • Chloroplast: (Plant-specific) Site of photosynthesis.

  • Large Central Vacuole: (Plant-specific) Stores water, nutrients, and waste, helps maintain turgor pressure.

  • Plastids: (Plant-specific) Group of organelles including chloroplasts, chromoplasts, and amyloplasts.

- Plasmodesmata: (Plant-specific) Channels connecting the cytoplasm of adjacent plant cells for communication and transport.

Eukaryotic Cell Structures (Generalized Animal Cell Organelles):

  • Centrioles: (Animal-specific) Components of the centrosome, involved in cell division.

  • Lysosome: (Animal-specific) Contains digestive enzymes for waste processing and recycling.

  • Centrosomes: (Animal-specific) Main microtubule-organizing center in animal cells.

  • Gap Junctions: (Animal-specific cell connections) Provide direct channels for communication, allowing passage of ions and small molecules.

  • Tight Junctions: (Animal-specific cell connections) Watertight seals between cells, preventing leakage of extracellular fluid