Unit 2: The Cell and Its Organelles

Unit 2: The Cell and Its Organelles

Nucleus

• Function & Structure:
  • Stores genetic material (DNA).
  • Surrounded by a double membrane known as the nuclear envelope, which contains pores to allow mRNA to exit.
  • Site of transcription (RNA synthesis).
  • Houses the nucleolus, responsible for assembling ribosomal RNA (rRNA) for ribosome formation.

Endoplasmic Reticulum (ER)

Rough Endoplasmic Reticulum (Rough ER)

• Structure:
  • Composed of flattened sacs that connect to the nucleus and are studded with ribosomes.
• Function:
  • Synthesizes proteins that are either secreted or embedded in membranes.
  • Proteins made by the ribosomes attached to Rough ER travel to the Golgi apparatus for further modifications.

Smooth Endoplasmic Reticulum (Smooth ER)

• Structure:
  • An extension of the Rough ER, but lacks ribosomes.
• Function:
  • Involved in lipid synthesis, detoxification processes, and the storage of calcium ions, especially in muscle cells.

Golgi Apparatus

• Structure:
  • Comprises a series of flattened, stacked membrane sacs.
• Function:
  • Modifies and packages proteins and other molecules for transport to their final destinations, such as the plasma membrane or for secretion.
  • Works in conjunction with Rough ER to ensure proper protein delivery.

Mitochondria

• Structure:
  • Features a double membrane with folds called cristae that enhance surface area.
• Function:
  • Main site of cellular respiration:
  • Krebs cycle (Citric Acid Cycle) occurs in the matrix of the mitochondria.
  • Electron transport chain resides in the inner membrane, crucial for ATP production.
  • Contains its own DNA and ribosomes, supporting the endosymbiotic theory.

Chloroplasts (Plant Cells)

• Structure:
  • Triple membrane system with thylakoid stacks known as granum.
• Function:
  • Major site of photosynthesis:
  • Light-dependent reactions take place in the thylakoid membranes.
  • Calvin cycle occurs in the stroma.
  • Also houses its own genetic material and ribosomes, consistent with the endosymbiotic theory.

Lysosomes

• Function:
  • Responsible for digesting cellular waste and pathogens using hydrolytic enzymes.
  • Plays a critical role in apoptosis (programmed cell death).
  • Fuses with food vacuoles to digest nutrients.

Vacuoles

• Types of Vacuoles:
  • Central Vacuole (Plant Cells): Stores water and nutrients, providing turgor pressure to maintain cell rigidity.
  • Contractile Vacuole (Unicellular Organisms): Pumps excess water out of cells, helping to regulate osmotic pressure.
  • Food Vacuole: Formed through phagocytosis and later digested by lysosomes.

Surface Area-to-Volume Ratio

• Smaller cells possess a higher surface area-to-volume ratio, facilitating more efficient diffusion.
• Larger cells may have inefficient diffusion and require compartmentalization (organelles) to maintain function.
• A higher surface area-to-volume ratio is advantageous for the exchange of nutrients and waste.

Membrane Transport

Plasma Membrane Structure

• Composed of a phospholipid bilayer:
  • Hydrophilic heads face outward towards the water, while hydrophobic tails face inward.
• Membrane proteins facilitate molecular transport (includes channel and carrier proteins).
• Glycoproteins and glycolipids play a role in cell signaling and communication.
• Cholesterol molecules stabilize membrane fluidity.

Passive Transport (No ATP Required)

• Simple Diffusion:

  • Small, non-polar molecules (e.g., O₂, CO₂, steroids) can move freely across the membrane.
  • Water can also slightly diffuse through the membrane.

• Facilitated Diffusion:

  • Involves specific transport proteins:
  • Channel proteins: Provide passageways for molecules.
  • Carrier proteins: Bind to specific molecules and change shape to aid transport.
  • Assisted transport mainly occurs for charged ions (e.g., Na⁺, K⁺, Ca²⁺) and large hydrophilic molecules (e.g., glucose).

Active Transport (ATP Required)

• Transports substances against their concentration gradient (from low to high concentration).
• Involves carrier proteins or pumps (e.g., Sodium-Potassium Pump and Proton Pump for H⁺ ions).
• Requires energy in the form of ATP to function.

Bulk Transport (Endocytosis & Exocytosis)

• Endocytosis: Mechanism that brings large substances into the cell.

  • Phagocytosis: Engulfs large particles (‘cellular eating’).
  • Pinocytosis: Engulfs extracellular fluid (‘cellular drinking’).
  • Receptor-Mediated Endocytosis: Uses receptors on the cell surface for selective uptake of molecules.

• Exocytosis: Releases large molecules outside the cell (e.g., neurotransmitters, hormones).

Osmosis & Tonicity

• Osmosis: The diffusion of water across a semi-permeable membrane from a hypotonic to a hypertonic solution.
• Types of Solutions to Consider:
  • Hypertonic: Higher concentration of solutes, causing water to exit the cell, leading to cell shriveling (crenation in animal cells, plasmolysis in plant cells).
  • Hypotonic: Lower concentration of solutes, causing water to enter the cell, potentially leading to lysis (bursting in animal cells), while making plant cells turgid (preferred state).
  • Isotonic: Equal solute concentration, resulting in no net movement of water.
• Water movement is guided by water potential, from regions of high to low potential.

Experiment Review

Dialysis Bag Experiment

• Demonstrates diffusion based on molecular size:
  • Starch remains inside the bag while glucose moves out.
  • Water influx increases the volume inside the bag.

Contractile Vacuole & Osmoregulation

• Paramecium utilizes a contractile vacuole to eliminate excess water:
  • The contraction rate decreases in hypertonic environments (less water intake) and increases in hypotonic environments (more water intake).

CFTR Protein & Cystic Fibrosis

• CFTR: ATP-gated ion channel that helps in moving Cl⁻ ions across the membrane.
• A mutation in CFTR impedes Cl⁻ transport, adversely affecting water movement and causing thick mucus accumulation in various tissues.
• CFTR protein is synthesized in the Rough ER, modified in the Golgi apparatus, and embedded in the plasma membrane.

Morning Glory Flower & pH Changes

• Proton pumps expel H⁺ ions, resulting in an increased pH.
• Higher pH results in blue petal coloration (lower pH yields red).
• Water influx into vacuoles, driven by hypertonicity, leads to petal swelling.