Cell Biology Overview

General Information on Protease Function

• Proteins are chopped into their constituent amino acids by proteases.
• Potential danger: If proteases are active in the cell, they would degrade all cellular proteins leading to cell death.

Control of Protease Activity

• Importance of controlling where proteases are active in the cell.
• Mechanism: Proteases are synthesized in an inactive form, ensuring they do not damage cellular proteins prematurely.

Ribosome Function

• Ribosomes are cellular factories that synthesize proteins, including proteases.
• Once synthesized, proteases fold into a specific shape determined by their amino acid sequence.
• In its folded state, the protease is inert and inactive due to its shape, indicating that it cannot perform its function.

Transport to Lysosome

• The inactive protease is packaged into a transport vesicle.
• This vesicle merges with the lysosome where the internal environment alters its activity.

Role of Lysosome

• Inside the lysosome, special proteins lower the pH, changing the environment to acidic (around pH 2-4).
• The acidic environment causes the protease to fold into its active form and allows it to degrade organelles or food particles via hydrolysis.

Safety Mechanism

• If lysosome ruptures and proteases escape, they return to the neutral pH of the cytosol, reverting to their inactive forms.
• Provides safety for cellular integrity.

Zymogens

• Definition: Enzymes created in an inactive form (zymogens) that become active upon environmental changes.
• Example: Pepsin in the stomach, which is activated by stomach acid but is kept inactive in neutral environments.
• Illustrates a general theme of enzyme regulation and cell safety from enzyme activity.

Mitochondria Overview

• Known as the powerhouse of the cell with two primary functions:
    - Aerobic respiration: Complete oxidation of food to generate energy.
    - Lipid breakdown: Occurs only in the mitochondria, possibly for energy or other purposes.

Anatomy of Mitochondria

• Characterized by two membranes:
    - Outer membrane
    - Inner membrane
• Inner Mitochondrial Membrane:
    - Contains folds known as cristae which increase surface area for biochemical reactions.
• Intermembrane Space: Space between the inner and outer membranes.
• Mitochondrial Matrix: Central compartment containing mitochondrial DNA (circular chromosome) and ribosomes, responsible for synthesizing proteins involved in aerobic respiration.

Mitochondrial DNA and Ribosomes

• Mitochondrial DNA:
    - Small circular chromosome resembling bacterial DNA.
    - Contains genes essential for aerobic respiration.
• Mitochondria contain their own ribosomes for protein synthesis.

Mitochondrial Replication

• Mitochondria replicate through fission (similar to bacteria) when more energy production capacity is needed, such as in muscle growth.
• Mitochondrial DNA is matrilineal inherited - passed from mother to offspring.

Endosymbiotic Theory

• Mitochondria and chloroplasts are believed to have originated from free-living bacteria that entered early eukaryotic cells.
• Evidence based on:
    - Double membrane structure
    - Circular DNA similar to bacteria
    - Unique ribosomes
    - Binary fission for replication

Cytoskeleton

• A network of proteins that provide structural support to the cell.
• Divided into three main components:
    - Microtubules
    - Microfilaments
    - Intermediate filaments

Microtubules

• Hollow tubes made up of polymers of tubulin proteins.
• Provide cell shape and structural integrity; dynamic in nature, can grow or shrink as needed.
• Important for organelle movement, utilizing motor proteins to transport vesicles along microtubule structures.
• Essential for flagella and cilia movements in eukaryotic cells.
  

Microfilaments

• Also known as actin filaments; provide support just beneath the plasma membrane.
• Essential for cellular movement and shape changes (e.g., in amoebas).
• Major component of muscle fibers.

Intermediate Filaments

• Structural proteins that provide mechanical support to the cell.
• Examples include keratin and laminins; involved in reinforcing cell structure.

Extracellular Matrix in Animal Cells

• Composed of proteins supporting the cell, different from the plant cell wall.
• Includes collagen as a key component for flexibility and strength.
• Integrin proteins connect collagen in the extracellular matrix with actin filaments inside the cell for structural integrity and potential signaling.

Extracellular Matrix in Plant Cells

• Plant cells possess a rigid cell wall primarily made of cellulose and pectin that provide structure and support.
• Wood formation introduces lignins which reinforce the rigidity and hydrophobic nature of the cell walls.
  

Summary of First Cell Functions

• Cell structures, organelles, and their respective cellular functions are elaborately interconnected, emphasizing the importance of compartmentalization, structural integrity, and symbiotic relationships in cellular life.