cell structure

Topic 3: Cell Structure

Concepts 6.2 – 6.7, 27.1

Cell Types

• Prokaryotes

  • Characteristics:
  • Simple structure
  • The earliest cell forms
  • Examples:
  • Bacteria
  • Archaea

• Eukaryotes

  • Characteristics:
  • Complex structure
  • Possess a true nucleus and organelles
  • Examples:
  • Plants
  • Animals
  • Fungi
  • Protists

The Prokaryotic Nucleoid

• Definition: A region of the cell where the genetic material is located; not a true nucleus

• Structure:

  • Contains a single, circular chromosome
  • Approximate size: ~1 mm of supercoiled DNA
  • Anchored to the plasma membrane

• Plasmids

  • Definition: Small, circular DNA molecules independent from the main chromosome
  • Contain nonessential but beneficial genes, such as:
  • Genes for antibiotic resistance
  • Genes for pili production, which facilitate gene transfer

Bacterial Cell Walls

• Distinct structures found in two fundamental groups of bacteria:

  • Gram-positive bacteria
  • Gram-negative bacteria

• Structure: Contains a plasma membrane and peptidoglycan layers.

Bacterial Flagella

• Description: Spiral-shaped protein complexes
• Characteristics:
  • Not covered by a membrane
  • Function: Rotates to propel the bacterium

Bacterial Fimbriae & Pili

• Description: Hair-like appendages used for attachment

• Characteristics:

  • Shorter and thinner than cilia
  • Found only on Gram-negative bacteria

• Pili:

  • Create a conjugation tube between cells
  • Facilitates horizontal gene transfer
  • Only present in F+ bacteria
  • Pilus gene is located on the F plasmid

Draw and Label a Typical Bacterium

• Components to include:
  • Plasma membrane
  • Cell wall
  • Nucleoid (region containing DNA)
  • Ribosomes
  • Pili
  • Flagella

The Eukaryotic Nucleus

• Structure:

  • Nuclear envelope: A double membrane
  • Nuclear pores: Connect nucleoplasm to the cytoplasm
  • Nuclear lamina: Maintains the shape of the nucleus

• DNA Packaging:

  • Chromatin:
  • Euchromatin: Unwound DNA that is available for transcription
  • Heterochromatin: Supercoiled DNA primarily used for storage

Nucleolus

• Description: A distinct region within the nucleus
• Characteristics:
  • Cells may contain one or two nucleoli
  • Function: Site of ribosome subunit assembly which includes:
  • Ribosomal RNA (rRNA)
  • Ribosomal proteins

Ribosomes

• Function: Machines responsible for protein synthesis

• Structure:

  • Composed of large and small subunits made of:
  • rRNA
  • Proteins

• Locations within the cell:

  • Bound to endoplasmic reticulum: Synthesizing proteins for secretion
  • Free in cytosol: Synthesizing proteins for other functions

Endoplasmic Reticulum (ER)

• Primary Role: Biosynthesis of macromolecules

  • Rough ER:

  • Associated with ribosomes

  • Site for synthesizing secreted proteins

  • Transports vesicles to the Golgi Apparatus

  • Smooth ER:

  • Lacks ribosomes

  • Site for lipid and carbohydrate synthesis

Golgi Apparatus

• Definition: The shipping and receiving center of the cell
• Structure: Comprises multiple layers of cisterae
• Functionality:
  • Receives products from the rough ER via transport vesicles at the cis face
  • Modifies ER products during Golgi processing
  • Ships products via secretory vesicles from the trans face to designated locations

Animal Cell Vacuoles

• Description: Mainly consist of lysosomes
• Functions:
  • Phagocytosis: Internalizes food into a vacuole that fuses with a lysosome for digestion
  • Autophagy: Forms a membrane around worn-out organelles, fusing with lysosomes to recycle components

Plant Cell Vacuoles

• Functions:
  • Digestion: Lysosome-like vesicles
  • Storage: Holds defensive compounds such as alkaloids and terpenoids
  • Turgor Pressure: Central vacuole containing an aqueous solution of inorganic ions is essential for plant cell growth

Cell Walls

• Nature and Functions:

  • Rigid yet dynamic structures affecting:
  • 3-D shape of the cell
  • Providing protection from hypotonic environments

• Structural Carbohydrates:

  • Plants & some protists: Composed of Cellulose
  • Bacteria: Composed of Peptidoglycan
  • Archaea: Composed of Pseudopeptidoglycan
  • Fungi: Composed of Chitin
  • Animals and other protists do not have a cell wall

Eukaryotic Cilia and Flagella

• Structure: Feature a complex 9+2 arrangement

• Description:

  • Surrounded by the plasma membrane
  • Basal body anchors flagellum within the cytoplasm

• Dynein motor proteins:

  • Located at the base of each doublet
  • Produce movement through a ‘walking’ mechanism on neighboring doublets

Evolution of Eukaryotes

Step 1: Endomembrane Theory

• Timeline: Occurred about 2 billion years ago
• Process: Involves an Archaean ancestor that becomes heterotrophic and undergoes invagination, leading to the separation of organelles from the cellular membrane

Step 2: Endosymbiont Theory (Mitochondria)

• Description: Explains the evolution of mitochondria in eukaryotes through the interaction between an early heterotrophic cell with an endomembrane system and a heterotrophic bacterium.
• Outcome: The bacterium survives as an endosymbiont, creating a mutualistic relationship where the bacterium evolves into mitochondria

Step 3: Endosymbiont Theory (Chloroplasts)

• Context: Similar process for the evolution of chloroplasts, involving the engulfment of an autotrophic bacterium.
• Outcome: Development of mutualism results in the bacterium evolving into chloroplasts

Serial Endosymbiosis

• Definition: A continuing process where bacteria are engulfed; both mitochondria and chloroplasts are examples
• First Chloroplasts: Arise from endosymbiosis of red or green algae

Chloroplasts

• Description: Double membrane organelle
• Structure:
  • Outer membrane
  • Inner membrane: Folded into stacks of thylakoids (grana)
• Function: Site of photosynthesis and related processes in photosynthetic cyanobacteria, which feature highly folded plasma membranes

Cytoskeleton

• Definition: Composed of protein filaments
• Characteristics: Grow and shrink at one end, originating from the centrosome (which includes two centrioles)
• Functions:
  • Support cell shape
  • Facilitate locomotion (e.g., flagella, cilia)
  • Assist in cellular trafficking via dynein motor proteins that help vesicles move along filaments

Cytoskeleton Protein Subunits

• Functions include:
  • Supporting cell shape in animals
  • Enabling cytoplasmic streaming in plants
  • Facilitating cell movement in amoebae
  • Various fibrous proteins contribute, including those that reinforce cell shape and anchor organelles

Cellular Junctions

• Description: No cell functions in isolation; cells are interconnected through junctions.
• Types of Junctions:
  • Adhesion Junctions:
  • Tight junctions (prevent leakage)
  • Anchoring junctions (mechanically attach cells)
  • Communication Junctions:
  • Gap junctions in animal cells (allow chemical and electrical signals)
  • Plasmodesmata in plant cells (connect adjacent cells)

Extracellular Matrix (ECM)

• Definition: Located outside the plasma membrane and serves multiple functions; holds cells together, supports structure.
• Key Components:
  • Collagen: Major component providing tensile strength.
  • Glycoproteins: Proteins with branched polysaccharides that connect fibers within the ECM.
  • Integrins: Transmembrane proteins that link the ECM to the cytoskeleton.

Size Matters

• Discussion about the size trade-off for cells:

  • Cells need to be large enough to house their components.
  • Cells must remain small enough to efficiently exchange materials with their environment.

• Surface Area to Volume Ratio: A critical factor in cell function.

  • Formula for calculating the surface area of a cube: $SA = 6s^2$
  • Formula for calculating the volume of a cube: $V = s^3$

Draw and Label a Typical Plant Cell

• Components to include:
  • Plasma membrane
  • Cell wall
  • Nucleus
  • Nucleolus
  • Ribosomes
  • Endoplasmic reticulum
  • Golgi apparatus
  • Vacuoles
  • Cytoskeleton
  • Flagella
  • Mitochondria
  • Chloroplasts

Draw and Label a Typical Animal Cell

• Components to include:
  • Plasma membrane
  • Nucleus
  • Nucleolus
  • Ribosomes
  • Endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
  • Vacuoles
  • Cytoskeleton
  • Flagella
  • Mitochondria

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