AP Biology Unit 2: The Cell

Cell Structure and Function

• Nucleus:
  • Contains DNA, the genetic material.
  • Composed of a double membrane (nuclear envelope) with pores.
  • Pores allow mRNA to exit for translation and proteins/enzymes to enter for transcription.
  • Stores genetic material (DNA/RNA).
  • Site of transcription (RNA synthesis).
  • Assembles ribosomes (using rRNA).
• Rough Endoplasmic Reticulum (Rough ER):
  • Network of flattened sacs connected to the nuclear envelope.
  • Studded with ribosomes, facilitating protein synthesis within a membrane.
  • Synthesizes secreted proteins (exocytosis).
  • Important for cell compartmentalization and mechanical support.
  • Involved in intracellular transport (vesicle movement).
• Smooth Endoplasmic Reticulum (Smooth ER):
  • Extension of Rough ER, no ribosomes.
  • Critical for detoxification of harmful substances.
  • Stores calcium ions (Ca²⁺) in muscle cells, acting as a secondary messenger in cell signaling pathways.
  • Site of lipid synthesis (steroids, other lipids).
• Golgi Apparatus (Golgi):
  • Stacks of flattened sacs.
  • Packages and modifies proteins and other macromolecules (carbohydrates) made by the Rough ER.
  • Involved in protein transport to the plasma membrane for secretion.
• Ribosomes:
  • Composed of a large and small subunit, both made of rRNA and proteins.
  • Synthesize proteins via translation, reading mRNA.
  • Can be free in the cytoplasm or bound to the Rough ER.
• Mitochondria:
  • Double membrane: outer membrane and inner membrane (cristae).
  • Cristae are highly folded (increased surface area) for oxidative phosphorylation.
  • Contains its own DNA and ribosomes (endosymbiotic theory).
  • Site of oxidative phosphorylation (third step of cellular respiration).
  • Matrix (inner space of the cristae) is the site of the Krebs/citric acid cycle (second step of cellular respiration).
  • Glycolysis occurs in the cytoplasm.
• Chloroplasts:
  • Triple membrane (outer, inner, thylakoid).
  • Thylakoids are stacks of sacs containing chlorophyll, which absorbs light energy for photosynthesis.
  • Stroma (fluid within chloroplast) contains the Calvin cycle.
  • Contains its own DNA and ribosomes (endosymbiotic theory).
  • Site of photosynthesis.
• Lysosomes:
  • Membrane-bound sacs containing hydrolytic enzymes.
  • Function in intracellular digestion.
  • Recycles cell's organic material.
  • Involved in programmed cell death (apoptosis).
• Vacuoles:
  • Membrane-bound sacs with diverse functions.
  • Storage of macromolecules and cellular waste products.
  • Central Vacuole (plants): Water retention, turgor pressure for rigidity.
  • Contractile Vacuole (single-celled organisms): Osmoregulation, expelling excess water from cells.
  • Food Vacuole: Formed by phagocytosis; fuses with lysosomes for digestion.

Cellular Transport and Membranes

• Plasma Membrane:

  • Phospholipid bilayer (hydrophilic heads, hydrophobic tails).
  • Cholesterol (buffers fluidity).
  • Proteins (facilitate transport).
  • Glycolipids and glycoproteins (cell communication).

• Membrane Transport (Passive):

  • Simple Diffusion:
    • Movement of small, non-polar molecules (O₂, CO₂, steroids) down a concentration gradient, directly through the membrane.
    • Small amounts of water also move without aid of channels.
  • Facilitated Diffusion:
    • Movement of polar or charged molecules down a concentration gradient.
    • Channel proteins allow molecules to pass through the membrane.
    • Carrier proteins physically bind to molecules and carry them across.

• Membrane Transport (Active):

  • Requires energy (ATP).
  • Moves molecules against their concentration gradient.
  • Often uses carrier proteins (pumps).

• Bulk Transport:

  • Endocytosis: Bringing material into cell.
    • Phagocytosis (cellular eating): Ingesting large particles via pseudopods.
    • Pinocytosis (cellular drinking): Ingesting extracellular fluid.
    • Receptor-mediated endocytosis: Specific molecules bind to receptors to initiate engulfment.
  • Exocytosis: Materials transported out of cell using vesicles.

• Osmosis:

  • Movement of water across a semi-permeable membrane.
  • Hypertonic: High solute concentration, low water potential; water moves out.
  • Hypotonic: Low solute concentration, high water potential; water moves in.
  • Isotonic: Equal solute and water concentrations; water moves in and out equally.

Surface Area-to-Volume Ratio

• Cells with a high surface area-to-volume ratio are more efficient for diffusion and exchange of materials

Cell sizes

• Smaller cells are more efficient for transport within the cell than larger cells