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Study Guide: Cells

Definition of a Cell

  • Basic Definition: The cell is the fundamental structural and functional unit of all living organisms.

  • Composition: Composed of cytoplasm encased within a membrane. Many contain organelles with specific functions.

  • Origin of the Term: Derived from the Latin word cellula, meaning 'small room'.

  • Visibility: Most cells require a microscope to be observed.

Discovery of Cells

  • Robert Hooke (1665): First to coin the term 'cell' while examining cork under a microscope, noting their resemblance to monk cells in a monastery.

  • Cell Theory (1839): Formulated by Matthias Jakob Schleiden and Theodor Schwann:

    1. All organisms are made of one or more cells.

    2. Cells are the basic unit of structure and function in living things.

    3. All cells arise from pre-existing cells.

Categories of Cells

  • Eukaryotic Cells: Cells with a nucleus.

    • Characteristics:

      • Can be unicellular or multicellular.

      • Possess organelles like mitochondria and chloroplasts.

  • Prokaryotic Cells: Cells without a nucleus (contain a nucleoid region).

    • Example Organisms: Bacteria and archaea.

    • Characteristics:

      • Typically single-celled.

      • Simpler, smaller structure than eukaryotic cells.

Structure of Prokaryotic Cells

  • Regions: Three primary regions exist within a prokaryotic cell:

    1. Cell Envelope:

    • Composed of a plasma membrane and a cell wall; some have an additional layer (capsule).

    • Function: Protects the cell and maintains structure.

    1. Cytoplasmic Region:

    • Contains DNA (nucleoid), ribosomes, and various inclusions.

    • DNA exists as a single circular chromosome and may include plasmids for additional genetic functions.

    1. Surface Structures:

    • Features such as flagella for movement and pili for adherence.

Structure of Eukaryotic Cells

  • Main Components:

    • Nucleus: Contains the cell's DNA; organizes genetic information.

    • Organelles: Specialized structures within eukaryotes, including:

      • Mitochondria: Energy production through aerobic respiration.

      • Chloroplasts: Photosynthesis in plant cells.

      • Endoplasmic Reticulum (ER):

        • Rough ER (with ribosomes) for protein synthesis.

        • Smooth ER (without ribosomes) for lipid synthesis.

      • Golgi Apparatus: Processes and packages macromolecules.

      • Lysosomes: Contain enzymes for digestion.

      • Cytoskeleton: Provides cell shape and aids in movement and intracellular transport.

Comparison of Prokaryotic and Eukaryotic Cells



Feature

Prokaryotes

Eukaryotes

Typical Organisms

Bacteria, Archaea

Protists, Fungi, Plants, Animals

Typical Size

0.5 - 2.0 μm

10 - 100 μm

DNA Location

Nucleoid region

In nucleus

Chromosomes

Single, circular

Multiple, linear with histones

Ribosomes

Smaller (70S)

Larger (80S)

Cell Division

Binary fission

Mitosis/Meiosis

Cellular Processes

  • Cell Division: Prokaryotes divide by binary fission; eukaryotes undergo mitosis or meiosis.

  • Metabolism: Involves catabolism (energy production) and anabolism (building complex molecules).

  • Protein Synthesis: Involves transcription (DNA to mRNA) and translation (mRNA to protein) within ribosomes.

  • Motility: Involves cilia or flagella in unicellular organisms for movement. Cells can also migrate in multicellular organisms for processes like wound healing.

Origin of Cells

  • Heterotrophs: First cells likely consumed organic materials available in their environment.

  • Evolutionary Timeline: Cells emerged around 4 billion years ago; eukaryotic cells developed through symbiogenesis about 2.2 billion years ago.

Multicellularity

  • Advantages include specialization of cells for various functions, leading to greater efficiency and increased complexity in organisms.

  • Types of Multicellular Organisms: From simple colonies to complex animals and plants.

  • Evolution: Multicellularity has evolved independently in various organisms, stemming from symbiotic relationships or cellular aggregation.

Review Questions and Answers

  1. What is the fundamental difference between prokaryotic and eukaryotic cells?

    • Prokaryotic cells lack a nucleus and membrane-bound organelles, typically have a simpler structure, and are usually smaller in size (0.5 - 2.0 μm). In contrast, eukaryotic cells have a true nucleus and various organelles (e.g., mitochondria, endoplasmic reticulum) that compartmentalize cellular functions, tend to be larger (10 - 100 μm), and can be multicellular organisms.

  2. Describe the function of the mitochondria and chloroplasts.

    • Mitochondria are known as the powerhouse of the cell; they convert energy from nutrients into adenosine triphosphate (ATP) through aerobic respiration, using oxygen to release energy stored in glucose and other molecules.

    • Chloroplasts are found in the plant cells and algae, and they capture sunlight to perform photosynthesis, converting light energy into chemical energy stored in glucose, while releasing oxygen as a byproduct.

  3. What is the significance of plasmids in prokaryotic cells?

    • Plasmids are small, circular pieces of DNA that exist independently of the chromosomal DNA in prokaryotic cells. They often carry genes that confer advantageous traits, such as antibiotic resistance. This ability allows bacteria to adapt rapidly to environmental pressures and can be transferred between cells through processes like conjugation.

  4. Illustrate the process of protein synthesis in eukaryotic cells.

    • Protein synthesis occurs in two major steps:

      1. Transcription: The process begins in the nucleus where the DNA sequence of a gene is copied into messenger RNA (mRNA). This mRNA then undergoes processing, such as splicing, to remove introns before exiting the nucleus.

      2. Translation: Once in the cytoplasm, the mRNA binds to a ribosome, which reads the genetic code in the mRNA and uses transfer RNA (tRNA) to bring the corresponding amino acids. The ribosome assembles the amino acids into polypeptide chains, which fold into functional proteins.

  5. Discuss the evolutionary significance of multicellularity.

    • Multicellularity allows for complex structures and functions that single-celled organisms cannot achieve. It fosters cell specialization and differentiation, enabling organisms to perform various tasks more efficiently. This complexity has led to the development of diverse life forms, evolutionary innovations, and advanced interactions among cells, allowing multicellular organisms to adapt better to their environments and ecological niches.