Cell Structure & Function

• Cells as the Basic Structural Unit of All Living Organisms


  The concept of cells being the fundamental building blocks of life is a cornerstone in biology. This idea forms part of the cell theory, which is one of the most important principles in biological sciences.
• The cell theory states that:


  1. All living organisms are composed of one or more cells
  2. The cell is the basic unit of structure, function, and organization in all organisms
  3. All cells come from pre-existing cells
• Structure of Cells


  Cells are microscopic units that contain all the machinery necessary for life. They are enclosed by a cell membrane and contain various organelles, each with specific functions.
• Universality of Cells


  From the smallest bacteria to the largest animals, all living organisms are made up of cells. This universality suggests a common origin for all life on Earth.
• Cellular Diversity

  While all cells share some basic features, there's also incredible diversity:

  • Prokaryotic cells (like bacteria) are simpler and lack a nucleus
  • Eukaryotic cells (found in plants, animals, fungi, and protists) are more complex with membrane-bound organelles
• Cells and the Nature of Science

  The development of cell theory is a great example of how scientific knowledge progresses. It involved:

  • Observations (using microscopes)
  • Hypothesis formation
  • Experimentation
  • Deductive reasoning
• Structures Common to Cells in All Living Organisms

  All living cells, despite their diversity, share some fundamental structures that are essential for life. These common structures include DNA, cytoplasm, and a plasma membrane.
• DNA as Genetic Material

  DNA (Deoxyribonucleic Acid) is the hereditary material in all living organisms.
• Reasons for DNA:

  • Stores genetic information
  • Allows for inheritance of traits
  • Provides instructions for protein synthesis
  • Enables evolution through mutations
• Cytoplasm


  Cytoplasm is the gel-like substance within cells, composed mainly of water
• Reasons for Cytoplasm:

  • Provides a medium for cellular reactions
  • Contains dissolved nutrients and waste products
  • Houses organelles in eukaryotic cells
  • Facilitates movement of molecules within the cell
• The high water content of cytoplasm (about 70-90%) is crucial because:


  • Water is an excellent solvent for biological molecules
  • It provides a medium for diffusion of substances
  • It's essential for many biochemical reactions
• The consistency of cytoplasm can change from fluid to more gel-like, which is important for cellular processes like cytoplasmic streaming.
• Plasma Membrane


  The plasma membrane, composed mainly of lipids (phospholipids), encloses the cell contents.
• Reasons for Plasma Membrane:

  • Defines the cell boundary
  • Controls what enters and exits the cell (selective permeability)
  • Maintains cell shape
  • Involved in cell signaling and recognition
• The lipid bilayer structure of the plasma membrane is crucial because:


  • It's impermeable to most water-soluble molecules, allowing the cell to control its internal environment
  • It's flexible, allowing for cell movement and shape changes
  • It can self-seal small tears, maintaining cell integrity
• Prokaryote Cell Structure


  Prokaryotes are single-celled organisms that lack membrane-bound organelles. The focus here is on Gram-positive eubacteria, such as Bacillus and Staphylococcus.
• Cell Wall

  The cell wall in Gram-positive bacteria is thick and composed primarily of peptidoglycan.Function:

  • Provides structural support
  • Maintains cell shape
  • Protects against osmotic lysis
• The thick peptidoglycan layer is responsible for retaining the crystal violet dye in the Gram staining procedure, hence the term "Gram-positive".
• Plasma Membrane in Prokaryotic Cell Structure

  A phospholipid bilayer that encloses the cell contents.

  Function:

  • Controls movement of substances in and out of the cell
  • Site of many enzymatic reactions
• These common structures reflect the shared evolutionary history of all life on Earth and the basic requirements for cellular life:

  1. A way to store and transmit genetic information (DNA)
  2. A medium for biochemical reactions (cytoplasm)
  3. A barrier to separate the cell from its environment (plasma membrane)

  The presence of these structures in all cells supports the cell theory, one of the fundamental principles of biology.
• Function of Cytoplasm:

  • Medium for cellular reactions
  • Contains ribosomes, plasmids, and other cellular components
• Naked DNA in a Loop

  Prokaryotes have a single, circular chromosome located in a region called the nucleoid.
• Characteristics of Naked DNA in a Loop:

  • Not enclosed by a nuclear membrane
  • Often called "naked" DNA
  • May be associated with proteins but not organized into chromatin
• 70S Ribosomes


  Prokaryotic ribosomes are smaller than eukaryotic ones.

  • Composed of 30S and 50S subunits
  • Site of protein synthesis
• Additional Structuresin Prokaryotic Cell Structure (not always present):

  • Pili: Hair-like appendages used for attachment or DNA transfer
  • Flagella: Whip-like structures for movement
  • Capsule: Extra layer outside the cell wall in some bacteria
• Variation in Prokaryotic Cell Structure

  While this description focuses on Gram-positive bacteria, it's important to appreciate that prokaryotic cell structure can vary:

  • Some prokaryotes lack cell walls (e.g., Mycoplasma)
  • Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane
  • Some bacteria have different shapes (cocci, bacilli, spirilla)
• The simplicity of prokaryotic cells compared to eukaryotic cells reflects their evolutionary history as the earliest forms of life on Earth.
• Plasma Membrane in Eukaryotic Cell Structure


  A phospholipid bilayer that encloses the cell contents.

  Function:

  • Controls movement of substances in and out of the cell
  • Involved in cell signaling
• Compartmentalized Cytoplasm

  The gel-like substance within the cell, divided into various compartments by organelle membranes.

  Function:

  • Provides a medium for cellular reactions
  • Houses organelles and cytoskeleton
• 80S Ribosomes


  Larger than prokaryotic ribosomes, composed of 60S and 40S subunits.

  Function:

  • Site of protein synthesis
• Components of Nucleus:

  • Chromosomes (DNA bound to histones)
  • Nuclear envelope (double membrane with pores)
  • Nucleolus (site of ribosome production)
• Nucleus


  The control center of the cell, enclosed by a double membrane with pores.

  Function:

  • Houses and protects genetic material
  • Controls gene expression
• The nuclear pores allow for selective transport between the nucleus and cytoplasm.
• Membrane-bound Organelles

  a) Mitochondria

  b) Endoplasmic Reticulum (ER)

  c) Golgi Apparatus

  d) Vesicles and Vacuoles
• Vesicles and Vacuoles

  • Include lysosomes (contain digestive enzymes)
  • Function: Storage, transport, digestion
• Endoplasmic Reticulum (ER)


  • Rough ER: Studded with ribosomes, involved in protein synthesis
  • Smooth ER: Lipid synthesis, detoxification
• Golgi Apparatus

  • Function: Modification, packaging, and distribution of proteins
• Mitochondria


  • Function: Energy production through cellular respiration
  • Often called the "powerhouse" of the cell
• Cytoskeleton


  A network of protein filaments that provides structure and facilitates movement within the cell.

  Function:

  • Maintains cell shape
  • Enables cell movement
  • Facilitates intracellular transport