Cell Theory and Cell Types

Cell Theory

• All living organisms consist of one or more cells.
• The cell is the fundamental structural and organizational unit of organisms.
• Cells originate from pre-existing cells.

Cell Types

• Prokaryotic cells lack a nucleus and other membrane-bound organelles.
• Eukaryotic cells possess a nucleus and other membrane-bound organelles.

Prokaryotic Cells

• These cells are typically smaller and simpler compared to eukaryotic cells.
• Genetic material (DNA) resides in the cytoplasm within a region termed the nucleoid, without a membrane-bound nucleus.
• They lack membrane-bound organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus.
• A cell wall offers structural support and protection.
• Ribosomes are present but smaller (70S) than those in eukaryotic cells (80S).
• Bacteria and Archaea are examples.

Eukaryotic Cells

• Eukaryotic cells tend to be larger and more complex
• A membrane-bound nucleus houses the cell's DNA.
• They contain diverse membrane-bound organelles with specialized roles.
• Organelles include mitochondria for energy production, endoplasmic reticulum for protein and lipid synthesis, Golgi apparatus for protein modification, lysosomes for waste disposal, and chloroplasts for photosynthesis in plant cells.
• A cell wall may or may not be present; it exists in plant cells and fungi but not in animal cells.
• Ribosomes are larger (80S) than those in prokaryotic cells (70S).
• Protists, fungi, plants, and animals are examples.

Cell Structures and Functions

• Cell Membrane:
  • This selectively permeable barrier encloses the cell, separating its internal from the external environment.
  • It is made up of a phospholipid bilayer with embedded proteins and cholesterol.
  • It regulates substance transport into and out of the cell.
  • It participates in cell signaling and communication.
• Nucleus:
  • This contains the cell's genetic material (DNA), organized as chromosomes.
  • It regulates cell growth, metabolism, and reproduction.
  • A double membrane, the nuclear envelope, surrounds it and contains nuclear pores for transport.
  • The nucleolus, where ribosomes are assembled, is located within.
• Cytoplasm:
  • This is the region between the nucleus and the cell membrane.
  • It contains the cytosol (a gel-like substance) and various organelles.
  • Many cellular processes like protein synthesis, metabolism, and signaling occur here.
• Ribosomes:
  • These are responsible for protein synthesis.
  • They are present in both prokaryotic and eukaryotic cells.
  • Ribosomal RNA (rRNA) and proteins make up ribosomes.
  • They can be free in the cytoplasm or bound to the endoplasmic reticulum.
• Endoplasmic Reticulum (ER):
  • This network of interconnected membranes spans the cytoplasm.
  • It participates in protein and lipid synthesis, calcium storage, and detoxification.
  • Rough ER is studded with ribosomes, aiding in protein synthesis and modification.
  • Smooth ER lacks ribosomes and is involved in lipid synthesis, carbohydrate metabolism, and detoxification.
• Golgi Apparatus:
  • It modifies, sorts, and packages proteins and lipids from the ER.
  • It is composed of flattened membranous sacs called cisternae.
  • It receives transport vesicles from the ER and modifies their contents.
  • Modified proteins and lipids are packaged into new vesicles for transport to other locations.
• Lysosomes:
  • These membrane-bound organelles contain enzymes for breaking down cellular waste.
  • They digest macromolecules, damaged organelles, and pathogens.
  • They are involved in apoptosis (programmed cell death).
• Mitochondria:
  • These generate energy (ATP) through cellular respiration.
  • Their structure includes an outer membrane and a highly folded inner membrane called cristae.
  • Mitochondria contain their own DNA and ribosomes, enabling self-replication.
• Chloroplasts:
  • These are found in plant cells and algae.
  • They facilitate photosynthesis, converting light energy into chemical energy (glucose).
  • Chlorophyll, a pigment that captures light energy, is present.
  • Their structure includes a double membrane and internal stacks of thylakoids called grana.
  • Similar to mitochondria, chloroplasts have their own DNA and ribosomes.
• Cytoskeleton:
  • It provides structural support, shape, and organization through a network of protein fibers.
  • It is involved in cell movement, division, and intracellular transport.
  • Three main types are present:
    • Microfilaments (actin filaments): involved in cell movement, muscle contraction, and cell division
    • Intermediate filaments: provide structural support and stability
    • Microtubules: involved in cell division, intracellular transport, and cell motility
• Cell Wall:
  • A rigid outer layer provides structural support and protection.
  • It exists in plant cells (cellulose), fungi (chitin), and bacteria (peptidoglycan).

Cell Communication

• Cells use chemical signals to communicate.
• Reception: A signaling molecule attaches to a receptor protein on or inside the cell.
• Transduction: The signaling molecule's binding initiates a sequence of intracellular events that amplify and convert the signal.
• Response: The transduced signal results in a specific cellular response, such as altered gene expression or enzyme activity.

Cell Division

• This process involves a cell dividing into two or more daughter cells.
• It is important for growth, development, and repair.
• Two primary types exist:
  • Mitosis: Occurs in somatic cells, producing two identical daughter cells (cell copies).
  • Meiosis: Occurs in germ cells, producing four genetically unique daughter cells (sexual reproduction).

Cellular Metabolism

• This encompasses all chemical reactions within a cell.
• Catabolism: Complex molecules break down into simpler ones, releasing energy.
• Anabolism: Complex molecules are synthesized from simpler ones, requiring energy.
• Enzymes: Biological catalysts accelerate chemical reactions in cells.