AP Biology: Cell Structure and Function

Unit 2: Cell Structure and Function

• AP Biology goes more in-depth than introductory biology courses, it focuses on organelle function and transport processes.
• Understanding individual concepts is important, Unit 2 study guide can help fill in any blanks.

Macromolecules and Membranes

• All living things are made of the same macromolecules: proteins, lipids, nucleic acids, and carbohydrates.
• Phospholipids have polar heads and non-polar fatty acid tails, creating a bilayer membrane.
• This bilayer structure is due to the aqueous intracellular and extracellular environments.
• Compartmentalization of organelles and selective permeability of the plasma membrane are results of this chemical orientation.

Subcellular Components

• Living things are organized into subcellular components which all contribute to the function of the cell.
• Ribosomes are found in both prokaryotes and eukaryotes and are made of rRNA and protein.
• Ribosomes can be free-floating in the cytoplasm or embedded in the rough ER.
• Ribosomes produce proteins during translation

Endomembrane System

• The endomembrane system includes the nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, Golgi apparatus, and plasma membrane.
• Products are packaged, processed, and transported throughout the system.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum is a series of interconnected folded membranous sacs that is continuous with the nuclear envelope.
• Rough ER is studded with ribosomes, involved in protein production and initial folding, compartmentalizes the cell, and aids in intracellular transport.
• Smooth ER is primarily involved in detoxification and lipid synthesis.

Golgi Complex

• The Golgi Complex is a membrane-bound organelle that packages, processes, sorts, and ships lipids and proteins.
• The cis face is the receiving side, while the trans face is where secretory vesicles bud off.

Lysosomes and Vacuoles

• Lysosomes contain hydrolytic enzymes and are involved in cell recycling and apoptosis.
• Vacuoles have a variety of roles depending on the cell type.
• In plant cells, a large central vacuole holds extra water and dissolved solutes, contributing to turgor pressure.
• Paramecia have contractile vacuoles to regulate osmotic pressure. If empty in a plant cell, the plant will wilt.

Mitochondria

• Mitochondria perform cellular respiration, producing ATP for the cell.
• It has a smooth outer layer, an inner layer folded into cristae, and a fluid matrix.
• The greater folding of the inner membrane creates a greater surface area which means more ATP can be produced.
• The Krebs cycle oxidizes sugars in the matrix, while the electron transport chain and ATP synthesis occur in the cristae.

Chloroplasts

• The purpose of the chloroplast is to perform photosynthesis, which harnesses energy from the sun and stores it in sugars.
• It has inner thylakoid membranes stacked into grana, surrounded by liquid stroma.
• The compartmentalization of the chloroplast divides this process into light and dark reactions.
• The light reaction occurs in the thylakoid membrane with chlorophyll and the electron transport chain, while the Calvin cycle forms sugars in the stroma.

Endosymbiosis

• Mitochondria and chloroplasts are theorized to have formed from independent prokaryotes through endosymbiosis.
• They have a double membrane, their own DNA, ribosomes, replicate independently, and are approximately the size of existing prokaryotic cells.

Surface Area to Volume Ratio

• Cells function best with the greatest surface area to volume ratio.
• Cells function best with the greatest surface area to volume ratio, as it facilitates efficient metabolism.
• Membranes may achieve this through folding (e.g., microvilli) or branching (e.g., root hairs).

Plasma Membrane - Fluid Mosaic Model

• Plasma membranes establish and maintain internal environments that are different from external environments through molecular interactions.
• The plasma membrane is referred to as a "fluid mosaic model" with multiple components constantly in motion.
• It is composed of a bilayer of amphipathic phospholipids with embedded proteins, cholesterol, and carbohydrate chains.
• The plasma membrane is very dynamic.
• The plasma membrane components have unique chemistry and classify as hydrophobic or hydrophilic, the molecules that it interacts with for transport will also be chemically distinct.
• Selective permeability is important for maintaining constant internal conditions.

Selective Permeability and Transport

• Small, non-polar molecules pass freely through the phospholipid membrane, while large or charged molecules pass through embedded proteins.
• Cell walls provide permeability barriers and structural support, composed of different polysaccharides in plants, prokaryotes, and fungi.
• Passive transport moves molecules from high to low concentration without ATP like small, uncharged, non-polar molecules diffusing directly or large/charged molecules using proteins.
• Active transport moves molecules from low to high concentration, requiring ATP.

Types of Active Transport

• Membrane pumps, endocytosis, and exocytosis are three main types of active transport.
• Membrane pumps are transmembrane carrier proteins are solute specific.
• ATP hydrolysis causes phosphorylation, changing the protein's shape and shuttling solutes across the membrane.
• The sodium-potassium pump and proton pump accumulate ions on one side of a membrane for later use during cotransport or with ATP synthase.
• Vesicle formation facilitates endocytosis (bringing substances in) and exocytosis (expelling substances out).

Endocytosis and Exocytosis

• Endocytosis brings substances into the cell in larger quantities and has three types - Phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis, where ligands bind to cell surface receptors, causing vesicle formation.
• Exocytosis involves the fusion of a vesicle with the plasma membrane for molecules to exit the cell.

Facilitated Diffusion and Osmosis

• Facilitated diffusion moves molecules from high to low concentration through a transport protein and does not require ATP.
• Carrier proteins bind specific solutes and undergo a physical change to move the molecule.
• Channel proteins have a pore (sometimes gated) to allow specific ions to cross.
• Aquaporins regulate the movement of water.
• Membranes may become polarized from the movement of ions.
• Osmosis is the passive diffusion of water from high to low concentration.

Tonicity and Water Potential

• Hypertonic solutions have a greater concentration of solutes and a lower water concentration and water potential.
• Hypotonic solutions have a lesser concentration of solutes.
• Water flows from hypotonic to hypertonic solutions until isotonic (equal water concentrations) is achieved.
• Water potential calculations predict water movement due to osmosis, pressure, surface tension, or gravity.

Cell Specialization and Types of Cells

• Cells can become highly specialized and combine to form tissues, organs, and organ systems.
• Prokaryotes are smaller, evolved earlier, lack a nucleus and membrane-bound organelles, and have circular DNA.
• Eukaryotes are larger, evolved later, have a nucleus and membrane-bound organelles, and have linear DNA.
• Cell types that have ribosomes, DNA, cytoplasm and a plasma membrane: Prokaryotes (bacteria and archaea) include protists, fungi, plants and animals.

Unit 2 Recap

• Cells make life possible and more efficient by separating processes and compartmentalizing functions with membranes.
• Organelles integrate seamlessly through the endomembrane system, while others stand alone, but all have unique structures and utilize a large surface area to volume ratio.
• Membrane chemistry controls the rate and exchange of materials by active and passive transport to maintain homeostasis.