AP Bio Unit 2:Cell Structure and Function Notes

Introduction to Cells

• Cells are the basic units of life, both structurally and functionally.
• Cell structure includes:
  • A membrane separating the cytoplasm from the exterior.
  • Genetic information in the form of DNA.
  • Systems to maintain and replicate the cell and its DNA.
  • Transcription of DNA into messenger RNA and translation by ribosomes into proteins.
• Proteins, especially enzymes, are crucial for cell metabolism and various cell functions.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells:
  • Small and simple, lacking a nucleus.
  • Circular chromosome and extra chromosomal DNA called plasmids.
  • Found in archaea and bacteria domains.
• Eukaryotic Cells:
  • Larger and more complex, found in the eukarya domain.
  • Have a nucleus and multiple linear chromosomes with DNA associated with proteins.
  • Contain mitochondria and membrane-bound organelles.

Cell Size and Surface Area to Volume Ratio

• Cells need sufficient membrane surface area for diffusion of substances.
• Small cells have a higher surface area to volume ratio than large cells.
• Example:
  • A cell with a 1 micrometer side has a surface area of 6 μm^2, a volume of 1 μm^3, and a surface area to volume ratio of 6:1.
  • A cell with a 10 micrometer side has a surface area of 600 μm^2, a volume of 1000 μm^3, and a surface area to volume ratio of 0.6:1.
• Larger objects have a decreased surface area to volume ratio, limiting efficient diffusion.

Increasing Surface Area in Organisms

• Organisms increase surface area for diffusion of molecules or heat.
• Methods include:
  • Thin sheets of tissue (e.g., gills for oxygen intake).
  • Large, flat tissues (e.g., elephant ears for heat loss).
  • Highly folded surfaces (e.g., internal mitochondrial membranes, villi in the intestine).

Marine Mammals and Surface Area to Volume Ratio

• Small mouse-sized marine mammals don't exist due to heat stress in cold ocean water.
• Increased size decreases the surface area to volume ratio, reducing heat loss.
• Small mammals like mice have a high surface area to volume ratio, leading to excessive heat loss in water.
• Whales have evolved to be larger, reducing heat loss to the environment due to a lower surface area to volume ratio.

Cellular Compartmentalization

• Compartmentalization is the internal division of a cell into sections.
• Advantages:
  • Allows distinct internal chemistry in different regions.
  • Provides internal surface area for membrane-bound enzymes.
  • Example: Lysosomes contain hydrolytic enzymes separated from the cytoplasm.

Compartmentalization in Cells

• Prokaryotic cells have fewer compartments but specialized regions.
• Eukaryotic cells are highly compartmentalized with many internal membranes.
• Examples: Lysosomes, endoplasmic reticulum, Golgi complex, vacuoles.

Endomembrane System

• The endomembrane system is a dynamic, interconnected system of internal membranes and compartments.
• Includes the nuclear membrane, rough ER, smooth ER, Golgi, lysosomes, and vesicles.
• Membrane and materials flow from one compartment to the next.

Origin of Eukaryotes and Endosymbiosis

• Eukaryotes arose around 1.8 billion years ago through mutualistic endosymbiosis.
• An archaeal cell engulfed a bacterial cell, which evolved into a mitochondrion.
• The mitochondrion secreted vesicles that became the nuclear membrane and endoplasmic reticulum.
• A second endosymbiotic event led to chloroplasts.

Evidence for Endosymbiosis

• Mitochondria and chloroplasts have their own circular DNA and replicate through binary fission.
• They use their own ribosomes to produce some proteins.
• These ribosomes resemble bacterial ribosomes.
• They have two membranes, with the outer one being a vestige of an endocytotic vesicle.

Eukaryotic Cell Parts and Functions

• Nucleus:
  • Stores and protects genetic information in the form of DNA.
  • DNA is wrapped around proteins to form chromosomes or chromatin.
  • The nucleolus assembles ribosomes.
  • The nuclear membrane has pores for molecules to enter and leave.
• Ribosomes:
  • Particles composed of ribosomal RNA and protein.
  • Consist of large and small subunits.
  • Translate messenger RNA into a sequence of amino acids to form proteins.
  • Can be free in the cytoplasm or bound to the rough ER.
• Mitochondria:
  • Convert food energy into ATP.
  • Key structures: chromosome with DNA, ribosomes, inner membrane, matrix, intermembrane space, outer membrane.
  • Evidence of endosymbiosis.
• Endoplasmic Reticulum (ER):
  • Series of interconnected channels between the nuclear membrane and Golgi body.
  • Two forms: rough ER (studded with ribosomes) and smooth ER.
  • Rough ER synthesizes proteins.
  • Smooth ER synthesizes lipids, converts toxins, breaks down carbohydrates.
• Golgi Complex:
  • Series of membrane-bound flattened sacs.
  • Receives vesicles from the ER, modifies their contents, and packages them into vesicles.
  • Vesicles are sent to organelles, the cell membrane, or outside the cell.
• Lysosomes:
  • Membrane-bound organelles containing hydrolytic enzymes.
  • Carry out intracellular digestion.
  • Recycle worn-out organelles and molecules.
  • Play a role in apoptosis (programmed cell death).
• Cytoskeleton:
  • Network of protein fibers that provide structure and enable cell movement.
  • Enables internal movement of materials and organelles.
  • Allows cells to move and change shape (e.g., endocytosis).
• Centrosomes and Centrioles:
  • Centrosomes contain two centrioles.
  • Create spindle fibers that separate chromosomes during mitosis and meiosis.
• Central Vacuole:
  • Found in plant cells.
  • Functions: water storage, storing macromolecules, sequestering waste, maintaining turgor pressure.
• Chloroplasts:
  • Found in plant cells.
  • Endosymbiotic descendants of free-living photosynthetic bacteria.
  • Create carbohydrates through photosynthesis.
• Plant Cell Wall:
  • Composed primarily of cellulose.
  • Acts as a pressure vessel, preventing over-expansion due to water inflow.

Membrane Structure and Function

• Cell Membrane:
  • Separates the cell's contents from the environment.
  • Selectively permeable boundary controlling substance passage.
• Phospholipids:
  • Structure: hydrophobic nonpolar tail region and hydrophilic polar head.
  • Form a bilayer in water, with heads interacting with water and tails forming a water-free zone.
  • The bilayer is stabilized by weak van der Waals bonds between tails.
• Fluid Mosaic Model:
  • The membrane is composed of phospholipids, proteins, and cholesterol in motion.
  • Fluid because components move laterally.
  • Mosaic because it's composed of a variety of pieces.
• Proteins in the Cell Membrane:
  • Transmembrane proteins: hydrophobic core fits into the nonpolar membrane portion, hydrophilic regions extend into the cytoplasm.
  • Integral proteins: nonpolar region extends into the hydrophobic membrane middle, hydrophilic region juts into the cytoplasm or cell exterior.
  • Peripheral proteins: attach to phospholipid heads on the cytoplasmic side of the membrane or in the cell exterior.

Membrane Transport

• Diffusion:
  • Molecules spread out from high to low concentration.
  • Spontaneous and driven by kinetic energy.
  • Molecules flow down a concentration gradient.
• Passive Transport:
  • Requires no cellular energy.
  • Simple diffusion: small nonpolar molecules (oxygen, carbon dioxide) and nonpolar substances (steroid hormones, fats) diffuse directly across the bilayer.
  • Facilitated diffusion: polar molecules and ions require protein channels.
• Active Transport:
  • Pumps molecules or ions up their concentration gradient (from low to high concentration).
  • Requires energy expenditure (usually ATP).
• Endocytosis and Exocytosis:
  • Both are forms of bulk transport and require energy.
  • Endocytosis: the membrane pinches in to surround particles or extracellular fluid, creating a vesicle.
  • Exocytosis: a vesicle fuses with the membrane, dumping its contents outside the cell.
• Membrane Potential:
  • Electrical charge across a membrane that creates a voltage difference.
  • Created by cells expending energy to pump ions across membranes.
  • Example: Mitochondria pump protons from the matrix to the intermembrane space, creating a charge gradient that drives ATP synthesis.

Tonicity and Osmoregulation

• Osmosis: The diffusion of water from high to low concentration.
• Water flows from hypotonic (more water, less solute) to hypertonic (less water, more solute) solutions.
• Osmotic pressure: The force of water being pushed up on one side due to osmosis.
• Osmosis in Plant Cells:
  • Hypotonic environment: water leaves the cell, causing plasmolysis and wilting.
  • Isotonic environment: water enters and leaves at the same rate.
  • Hypertonic environment: water flows into the cell, causing turgor pressure and making the cell firm.
• Osmosis in Animal Cells:
  • Hypotonic environment: water leaves the cell, causing it to shrivel.
  • Isotonic environment: water enters and leaves at the same rate (important for animal cells).
  • Hypertonic environment: water flows into the cell, causing it to burst.
• Contractile Vacuole:
  • Freshwater protists use contractile vacuoles to osmoregulate.
  • The vacuole fills with water and contracts to expel water from the cell.
• Leaf Stomata:
  • Pores on the underside of a leaf formed by two guard cells.
  • When water is sufficient, guard cells buckle outward, creating a pore for gas exchange.
  • Stomata can close in response to environmental cues, including water stress.
  • Guard cells are regulated by pumping potassium ions in (opening) and out (closing) in response to water availability.
• Water Potential:
  • A measure of water's tendency to move from one area to another.
  • Adding solute decreases water potential, adding pressure increases it.
  • Water flows from areas of higher water potential to areas of lower water potential.
  • Formula: \Psi = \Psis + \Psip where \Psi is water potential, \Psis is solute potential, and \Psip is pressure potential.