AP Biology Unit 2 — Cell Organelles & Subcellular Components (Functions)

Plasma (Cell) Membrane

• Phospholipid bilayer with embedded proteins (fluid mosaic).

• Selectively permeable: controls movement of ions, polar, and nonpolar molecules.

• Functions in cell communication, recognition (glycoproteins), and maintaining homeostasis.

Cytosol

• Aqueous interior of the cell (mostly water).

• Site of many metabolic reactions (e.g., glycolysis).

• Contains enzymes, metabolites, and cytoskeleton components.

Nucleus

• Stores genetic information (DNA).

• Controls gene expression and cell activities.

• Surrounded by a double membrane (nuclear envelope).

Nuclear Envelope

• Contains nuclear pores that regulate RNA and protein transport.

Nucleolus

• Synthesizes rRNA and assembles ribosomal subunits.

Ribosomes

• Sites of protein synthesis (translation).

• Free ribosomes: synthesize cytosolic proteins.

• Bound ribosomes (on RER): synthesize proteins for secretion, membranes, or lysosomes.

• No, mRNA and tRNA don't come from ribosomes; rather, they are essential partners that work with ribosomes during protein synthesis (translation). Ribosomes, made of rRNA and proteins, provide the factory where mRNA (the blueprint) is read and tRNA (the delivery trucks) bring the specific amino acids needed to build a protein chain

Rough Endoplasmic Reticulum (RER)

• Studded with ribosomes.

• Synthesizes, folds, and modifies proteins (e.g., adding initial carbohydrates).

• Transports proteins to the Golgi apparatus.

Smooth Endoplasmic Reticulum (SER)

• Lacks ribosomes.

• Functions:
  

  • Lipid and steroid synthesis

  • Phospholipid production

  • Detoxification of drugs and poisons

  • Calcium ion storage (important in muscle cells)

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids.

• Adds carbohydrates → glycoproteins & glycolipids.

• Ships products in vesicles to final destinations (secretion, membrane, lysosome).

Vesicles (little taxi of the cell)

• Membrane-bound sacs for transport and storage.

• Move materials between ER, Golgi, plasma membrane, and lysosomes.

• Small sac fluid filled

Lysosomes

(mostly animal cells)

• Contain hydrolytic enzymes for intracellular digestion.

• Functions:
  

  • Breakdown of macromolecules

  • Recycling of damaged organelles (autophagy)

  • Defense against pathogens

  • Programmed cell death or cellular suicide by apoptosis

Peroxisomes

• Break down fatty acids and toxins.

• Contain enzymes that produce and degrade hydrogen peroxide (H₂O₂).

• Important for detoxification and metabolism.

Mitochondria

• Site of cellular respiration and ATP production.

• Double membrane:
  

  • Inner membrane folds → cristae (increase surface area).

• Contains its own DNA and ribosomes (supports endosymbiotic theory).

Chloroplasts

(plant cells)

• Site of photosynthesis.

• Thylakoid membranes contain chlorophyll (light reactions).

• Stroma contains enzymes for the Calvin cycle.

• Double membrane + internal membrane system.

Central Vacuole

(plant cells)

• Maintains turgor pressure (the water pressure acting upon the plasma membrane from inside the cell, usually caused by water) which provides structure, support, stability, strength, and rigidity to the plant

• Stores water, ions, pigments, and wastes.

• Helps regulate cell size and homeostasis.

Cell Wall

(plants, fungi, prokaryotes)

• Provides structural support and protection.

• Plant cell walls made of cellulose.

• Bacteria cell walls made of peptidoglycan (carbohydrates/polysaccharide)

• Archaea cell walls made of psuedomurein

• Fungi cell walls is made of chitin

• Prevents excessive water uptake.

• Most cell walls are made up of polysaccharides or carbohydrates

Cytoskeleton

Provides structural support and enables movement.

Microfilaments (Actin)

• Maintain cell shape.

• Muscle contraction and cell motility.

Intermediate Filaments

• Provide tensile strength.

• Stabilize cell structure.

Microtubules

• Made of tubulin.

• Form mitotic spindle, cilia, and flagella.

• Serve as tracks for vesicle transport.

Centrosome & Centrioles

(animal cells)

• Microtubule-organizing center.

• Important for spindle formation during cell division.

• Organized in a 9+0 arrangements

Cilia & Flagella

• Motility and movement of substances across cell surface.

• Composed of microtubules in a 9+2 arrangement.

AP Exam Tip (Very Important)

College Board focuses on:

• Structure → function relationships

• Membrane-bound vs non-membrane-bound organelles

• Energy processing (mitochondria & chloroplasts)

• Protein trafficking pathway:
  RER → Golgi → vesicle → destination