Cell Structure and Function

Historical Foundations of Cell Biology

• Robert Hooke (1665):

  • Discovered the cell while observing thin slices of cork under a crude microscope.

  • Described a multitude of tiny pores that looked like "tiny rooms."

  • Coined the term Cellulae.

• Antonie van Leeuwenhoek (1670s):

  • Constructed his own advanced microscopes.

  • Was the first person to witness a live cell under a microscope.

  • Known for discovering free-living and parasitic microscopic protists, sperm cells, and blood cells.

  • Acknowledged as the Father of Microbiology.

The Cell Theory

• Original Cell Theory (Schleiden and Schwann, 1838; Virchow, 1858):

  1. All living organisms are made of cells.

  2. The cell is the fundamental unit of structure, function, and organization of life.

  3. All cells come from pre-existing cells (Rudolph Virchow, $1858$).

• Modern Interpretation of Cell Theory:

  • Cells contain hereditary material (DNA).

  • Cells process energy (Metabolism).

Visualizing Cells and Microscopy

• Cell Size:

  • Most cells range between $1$ and $100\,\mu m$ in diameter.

• Microscopy Types:

  • Light Micrograph (LM):

    • Can be used to view both living and dead specimens.

    • Example: Light micrograph of the protist Paramecium at $300 \times$ magnification.

  • Scanning Electron Micrograph (SEM):

    • Provides a view of surface features.

    • Specimens must be killed for this process.

    • Example: Colorized SEM of Paramecium at $300 \times$ magnification.

  • Transmission Electron Micrograph (TEM):

    • Provides a view of internal structures.

    • Specimens must be killed for this process.

    • Example: TEM of Paramecium at $300 \times$ magnification.

Common Components of All Cells

• Regardless of whether a cell is prokaryotic or eukaryotic, all cells share these components:

  • Plasma Membrane: The outer limiting layer.

  • Cytoplasm: The internal fluid environment.

  • DNA: Genetic material located either in a nucleus or a nucleoid region.

  • Ribosomes: Structures responsible for protein synthesis.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells:

  • First evolved approximately $3.5$ billion years ago.

  • Smaller and simpler in structure.

  • Do not possess membrane-bound organelles.

  • Contain a single circular chromosome located in a nucleoid region.

  • Most are surrounded by cell walls; some have capsules, pili, and/or flagella.

  • Cytoplasm occupies the entire interior of the cell.

• Eukaryotic Cells:

  • First evolved approximately $1.8$ billion years ago.

  • Larger and more complex.

  • Contain membrane-bound organelles (e.g., nucleus, Endoplasmic Reticulum).

  • Contain one or more linear chromosomes housed within a nucleus.

  • Cytoplasm occupies only the region between the nucleus and the plasma membrane.

  • Plant cells are surrounded by cell walls; animal cells are surrounded by an extracellular matrix.

Anatomy of a Prokaryotic Cell

• Nucleoid: Region containing the single circular bacterial chromosome.

• Ribosomes: Sites of protein synthesis; structured slightly differently from eukaryotic ribosomes.

• Plasma Membrane: Encloses the cytoplasm.

• Cell Wall: Provides rigidity to the cell.

• Capsule: A sticky outer coating.

• Flagella: Tail-like structures used for propulsion.

• Fimbriae: Surface structures used for attachment.

Anatomy of Eukaryotic Cells

• Common Structures in Animal and Plant Cells:

  • Cytoskeleton

  • Mitochondrion

  • Nucleus

  • Rough Endoplasmic Reticulum (Rough ER)

  • Smooth Endoplasmic Reticulum (Smooth ER)

  • Golgi Apparatus

  • Ribosomes

  • Plasma Membrane

  • Cytoplasm

• Structures Unique to Animal Cells:

  • Centrosome

  • Lysosome

• Structures Unique to Plant Cells:

  • Central Vacuole

  • Cell Wall

  • Chloroplast

  • Channels between cells (Plasmodesmata)

Non-Cellular Entities: Viruses

• Viruses are not considered cells.

• Structure of a Virus:

  • Protein Coat: Protective outer shell.

  • Nucleic Acid: Genetic code consisting of either DNA or RNA.

  • Envelope: A lipid bilayer found in some viruses.

  • Spikes: Used for attaching to specific surfaces on host cells.

The Plasma Membrane

• Composition: Phospholipids are the main components.

• Permeability: It is a semi-permeable or selectively permeable membrane, controlling what enters and exits the cell.

• Fluid Mosaic Model:

  • Mosaic: Describes the various molecules embedded in or attached to the lipid bilayer, including cholesterol, proteins, glycoproteins, and glycolipids.

  • Fluid: Describes how phospholipids are not chemically bonded to one another, allowing movement within the layer.

The Nucleus and Genetic Material

• Function: Protects DNA from the metabolic processes of the cell.

• Anatomy:

  • Nuclear Envelope: The outer boundary of the nucleus consisting of a double membrane; it controls access to DNA.

  • Nuclear Pores: Openings in the envelope that allow for the passage of materials.

  • Nucleoplasm: The viscous fluid enclosed within the nuclear envelope.

  • Nucleolus: A dense, irregularly shaped region where ribosomal subunits are produced.

  • Chromatin: Thin fibers of DNA molecule wrapped around proteins.

  • Chromosome: Thickened, condensed structures of chromatin formed during cell division.

Ribosomes and Protein Synthesis

• Function: Responsible for protein synthesis by forming peptide bonds between different amino acids.

• Location: Can be free-floating in the cytoplasm or attached to the Endoplasmic Reticulum (visible as tiny dark blue dots at $50,000 \times$ magnification under TEM).

• Involvement: mRNA carries the genetic instruction to the ribosome for translation into a protein.

The Endomembrane System

• A series of interacting organelles between the nucleus and the plasma membrane involved in manufacturing and distributing cellular products.

• Rough Endoplasmic Reticulum (Rough ER):

  • Ribosomes are attached to the surface.

  • Proteins enter the Rough ER to be folded into their tertiary structure.

  • Some proteins produced here become part of the ER membrane.

• Smooth Endoplasmic Reticulum (Smooth ER):

  • Lacks ribosomes.

  • Contains diverse enzymes for various functions:

    • Production of lipids (phospholipids and steroid hormones).

    • Breakdown of carbohydrates (like glycogen) and fatty acids.

    • Detoxification of drugs, alcohol, and metabolic waste.

    • Involved in muscle contraction.

• Golgi Apparatus:

  • Consists of a stack of flattened sacs.

  • Functions as a "molecular warehouse and finishing factory."

  • Receives and ships products; finishes proteins and lipids received from the ER.

• Vesicles:

  • Small, membrane-enclosed organelles.

  • Used for transporting and storing substances.

  • Lysosomes: Special vesicles containing digestive enzymes to break down cellular waste and debris.

• Vacuoles:

  • Large vesicles with varied functions.

  • Contractile Vacuole: Found in organisms like Paramecium; fills with water and contracts to pump it out.

  • Central Vacuole: Often the largest organelle in a mature plant cell; stores cell products and water.

Energy-Converting Organelles

• Mitochondria (The Power House):

  • Perform cellular respiration: converting the chemical energy of food into ATP (the energy currency of the cell).

  • Structure: Outer membrane, inner membrane, cristae (folds), and matrix.

  • Contain their own DNA and ribosomes.

• Chloroplasts:

  • Perform photosynthesis: converting light energy into chemical energy.

  • Structure: Two outer membranes, stroma (fluid), thylakoids (discs), and granum (stacks of thylakoids).

  • Inner membrane is much-folded where photosynthesis occurs.

The Cytoskeleton and Extracellular Surface

• Cytoskeleton:

  • A network of protein fibers acting as a skeleton inside the cell.

  • Provides structural support and facilitates cell motility.

  • Composed of three types of protein fibers: Microtubules, microfilaments, and intermediate filaments.

• Extracellular Matrix (ECM):

  • A complex mixture of polysaccharides and proteins found outside the cell.

  • In animals: Provides structural support, tissue organization, and cell signaling.

• External Surface Protections:

  • Cell Walls: Found in plants, fungi, and some protists.

  • Cuticle: A secretion from cells at a body surface. In plants, it provides water retention and protection from insect damage.

Questions & Discussion

• What are the three parts of the original cell theory?

  • 1) All organisms are made of cells; 2) The cell is the fundamental unit of structure/function; 3) All cells come from pre-existing cells.

• Which type of microscopes is used to observe live cells?

  • Light Microscope (LM).

• What do all cells have in common?

  • Plasma membrane, cytoplasm, DNA, and ribosomes.

• Why are viruses not considered cells?

  • They lack the standard components of cells (like cytoplasm and ribosomes) and cannot reproduce outside a host cell.

• What kind of molecules make up the extracellular matrix?

  • Polysaccharides and proteins.

• What are the differences between the smooth and rough ER in structure and function?

  • Structure: Rough ER has ribosomes; Smooth ER does not. Function: Rough ER folds proteins; Smooth ER synthesizes lipids and detoxifies.

• What kind of molecules make up the cytoskeleton?

  • Protein fibers (microtubules, microfilaments, intermediate filaments).

• Do prokaryotic cells have any membrane-bound organelles?

  • No.

• What is the function of fimbriae in a prokaryotic cell?

  • Attachment to surfaces.

• Where is the chromosome found in a prokaryotic cell?

  • The nucleoid region.

• List three differences between plant and animal cells.

  • 1) Plants have cell walls (animals don't); 2) Plants have chloroplasts (animals don't); 3) Animals have lysosomes/centrosomes (plants don't).

• Which organelles are part of the endomembrane system?

  • Nucleus (envelope), ER, Golgi apparatus, vesicles, lysosomes, vacuoles, and plasma membrane.

• Which organelles are energy converting organelles?

  • Mitochondria and chloroplasts.

Multiple Choice Practice

• Question: The maintenance of a constant internal body temperature is ____ and is called ____.

  • Answer: D) a characteristic of all life; homeostasis

• Question: An example of a dehydration synthesis reaction is:

  • Answer: D) glycerol + $3$ fatty acids $\rightarrow$ triglyceride + $H_2O$

• Question: Organisms that are not prokaryotes are in the Domain:

  • Answer: C) Eukarya", "title": "Comprehensive Study Notes on Cell Structure and Function"}