Understanding the Cell: Structure, Theory, and Types

• The cell is the basic structural and functional unit of all known living organisms
• It is the smallest unit of an organism that is considered a living thing
• Cells can exist as single entities (like bacteria and yeast) or as building blocks of multicellular organisms (like animals and plants)

Cell Theory

• All living organisms are composed of one or more cells
• The cell is the basic unit of structure and organisation in organisms
• Cells arise from pre-existing cells

Cell Types

• There are two primary types of cells: prokaryotic and eukaryotic
• Prokaryotic cells:
  • Do not have a nucleus or other membrane-bound organelles
  • Their DNA is located in the cytoplasm
  • Generally smaller and simpler than eukaryotic cells
  • Examples: bacteria and archaea
• Eukaryotic cells:
  • Have a nucleus and other membrane-bound organelles, such as mitochondria, endoplasmic reticulum, and Golgi apparatus
  • Their DNA is housed within the nucleus
  • Generally larger and more complex than prokaryotic cells
  • Examples: protists, fungi, plants, and animal cells

Cell Structure: Common Components

• Despite the differences between prokaryotic and eukaryotic cells, some components are common to both:
  • Plasma Membrane:
    • Outer boundary of the cell that separates it from the external environment
    • Composed of a phospholipid bilayer with embedded proteins
    • Regulates the movement of substances into and out of the cell
  • Cytoplasm:
    • Gel-like substance within the cell
    • Contains water, ions, enzymes, nutrients, and organelles
    • Site of many metabolic reactions
  • DNA (Deoxyribonucleic Acid):
    • Genetic material of the cell
    • Contains the instructions for cell function and reproduction
  • Ribosomes:
    • Responsible for protein synthesis
    • Found in both prokaryotic and eukaryotic cells

Cell Structure: Eukaryotic Cell Organelles

• Nucleus:
  • Control center of the cell
  • Contains the cell's DNA, organized into chromosomes
  • Surrounded by a nuclear envelope with pores that regulate the movement of materials in and out
• Endoplasmic Reticulum (ER):
  • Network of membranes involved in protein and lipid synthesis
  • Rough ER: Studded with ribosomes and involved in protein synthesis and modification
  • Smooth ER: Involved in lipid synthesis, detoxification, and calcium storage
• Golgi Apparatus:
  • Modifies, sorts, and packages proteins and lipids synthesized in the ER
  • Forms vesicles to transport these molecules to other destinations
• Mitochondria:
  • Powerhouse of the cell
  • Site of cellular respiration, which generates ATP (energy)
  • Have a double membrane structure
• Lysosomes:
  • Contain enzymes that break down cellular waste and debris
  • Involved in recycling cellular components
• Peroxisomes:
  • Involved in detoxification and lipid metabolism
  • Contain enzymes that break down hydrogen peroxide (H2O2)
• Cytoskeleton:
  • Network of protein filaments that provide structural support and facilitate cell movement
  • Includes microtubules, intermediate filaments, and actin filaments
• Cell Wall (in plant cells):
  • Rigid outer layer that provides support and protection
  • Composed of cellulose
• Chloroplasts (in plant cells):
  • Site of photosynthesis, which converts light energy into chemical energy (glucose)
  • Contain chlorophyll, the pigment that captures light energy
  • Have a double membrane structure

Plasma Membrane Details

• The plasma membrane is a phospholipid bilayer:
  • Phospholipids have a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails
  • Arranged in a bilayer with the hydrophilic heads facing outward and the hydrophobic tails facing inward
• Membrane proteins:
  • Embedded in the phospholipid bilayer
  • Perform various functions, including transport, signaling, and cell recognition
• Fluid Mosaic Model:
  • Describes the plasma membrane as a fluid structure with a mosaic of proteins embedded in it
  • The components of the membrane are constantly moving and changing

Cell Size

• Cells vary in size
• Prokaryotic cells are generally smaller than eukaryotic cells
• Cell size is limited by the surface area to volume ratio:
  • As a cell increases in size, its volume increases faster than its surface area
  • A smaller surface area to volume ratio makes it difficult for the cell to efficiently exchange materials with its environment
• Cells can overcome these limitations by:
  • Being long and thin
  • Developing folds or other structures to increase their surface area

Cell Specialization

• In multicellular organisms, cells become specialized to perform specific functions
• This process is called cell differentiation
• Different cell types express different genes, which determines their structure and function
• Examples of specialized cells include:
  • Nerve cells (neurons)
  • Muscle cells
  • Red blood cells
  • Epithelial cells