Unit 2: Cell Structure and Function

Cells: The Basic Units of Life

  • Cells are the fundamental structural and functional units of all organisms.

  • All cells are bound by a plasma membrane, contain cytosol, chromosomes, and ribosomes.

Types of Cells

  • Prokaryotic cells: Bacteria and Archaea.

    • DNA is located in the nucleoid region (no nucleus).

    • Generally smaller than eukaryotic cells.

  • Eukaryotic cells: Protists, fungi, animals, and plants.

    • DNA is housed in the nucleus.

    • Contain membrane-bound organelles.

Cell Size and Metabolism

  • Cellular metabolism is dependent on cell size.

  • Cells must regulate the intake of nutrients/resources and the removal of waste.

  • Surface Area-to-Volume Ratio:

    • Cells require a high surface area-to-volume ratio to optimize the exchange of materials across the plasma membrane.

Formulas for Surface Area (SA) and Volume (V)

  • Cuboidal Cells:

    • Total SA = height x width x number of sides x number of boxes

    • Total V = height x width x length x number of boxes

    • SA to V ratio = SA/V

  • Spherical Cells:

    • SA=4πr2SA = 4πr^2

    • V=(4/3)πr3V = (4/3)πr^3

    • SA:V ratio = SA/V

Implications of SA:V Ratio

  • Small cells have a higher SA:V ratio, which optimizes the exchange of materials at the plasma membrane.

  • Larger cells have a lower SA:V ratio, reducing the efficiency of material exchange and increasing the demand for resources.

Organelles in Eukaryotic Cells

  • Membrane-bound structures with specific functions.

    • Endomembrane System Organelles:

    • Nuclear envelope

    • Endoplasmic reticulum (ER)

    • Golgi complex

    • Lysosomes

    • Vesicles/vacuoles

    • Plasma membrane

    • Energy Organelles:

    • Mitochondria

    • Chloroplasts

Nucleus

  • Contains chromosomes (genetic information).

  • Enclosed by the nuclear envelope (double membrane with pores).

    • Pores regulate entry and exit of materials.

    • Nuclear lamina maintains shape.

Nucleolus

  • Dense region in the nucleus where ribosomal RNA (rRNA) is synthesized.

    • rRNA combines with proteins to form ribosome subunits.

    • Subunits exit via nuclear pores and assemble into ribosomes.

Ribosomes

  • Composed of ribosomal RNA and protein.

  • Function: synthesize proteins.

    • Locations:

    • Cytosol (free ribosomes): Proteins function within the cytosol.

    • Bound to ER or nuclear envelope: Proteins can be secreted from the cell via transport vesicles.

Endoplasmic Reticulum (ER)

  • Smooth ER:

    • Synthesizes lipids, oils, phospholipids, steroids, and hormones.

    • Hydrolyzes glycogen into glucose (in the liver).

    • Detoxifies drugs and poisons (in the liver).

  • Rough ER:

    • Has bound ribosomes, which secrete glycoproteins.

    • Distributes transport vesicles.

Golgi Complex

  • Contains flattened membranous sacs called cisternae (not connected).

    • Cis face: receives vesicles from the ER.

    • Trans face: sends vesicles out to other locations or the plasma membrane.

  • Function: Modifies, sorts, tags, and packages materials from the ER into new transport vesicles.

Lysosomes

  • Membranous sac with hydrolytic enzymes.

  • Function: Hydrolyzes macromolecules.

    • Autophagy: recycles the cell’s organic materials.

  • Lysosomal storage diseases: result from non-digestion of biomolecules, which disrupt cell and organ function (e.g., Tay-Sachs disease).

Peroxisomes

  • Digestive enzyme sacs found in both animal & plants

  • Breakdown fatty acids and detoxify the cell (e.g., alcohol).

  • Produce hydrogen peroxide (H2O2), which are broken down into water.

Vacuoles

  • Large vesicles derived from the ER and Golgi.

    • Types:

    • Food vacuole: formed by phagocytosis.

    • Contractile vacuole: maintains water levels in cells (freshwater protists).

    • Central vacuole: found in plants. Contains inorganic ions and water; important for turgor pressure.

Mitochondria and Chloroplasts

  • Not part of the endomembrane system.

  • Have a double membrane.

  • Contain proteins made by free ribosomes.

  • Contain their own DNA.

Endosymbiont Theory

  • Explains the similarities between mitochondria/chloroplasts and prokaryotes.

    • An early eukaryotic cell engulfed a prokaryotic cell.

    • The prokaryotic cell became an endosymbiont.

  • Evidence:

    • Double membrane

    • Circular DNA and ribosomes

    • Capable of functioning on their own

Mitochondria

  • Site of cellular respiration.

    • Structure: Double membrane.

    • Outer membrane is smooth.

    • Inner membrane has folds called cristae (increasing surface area for ATP synthesis).

    • Intermembrane space: space between inner and outer membranes.

    • Mitochondrial matrix: location for the Krebs cycle. Contains enzymes, mitochondrial DNA, and ribosomes.

  • Number of mitochondria correlates with metabolic activity.

Chloroplasts

  • Site of photosynthesis in photosynthetic organisms.

    • Structure: Double membrane.

    • Thylakoids: Membranous sacs that form stacks called grana (light-dependent reactions).

    • Stroma: Fluid around thylakoids (Calvin cycle). Contains chloroplast DNA, ribosomes, and enzymes.

Cytoskeleton

  • Function: Structural support, maintain cell shape, anchorage for organelles, cell motility.

    • Structure: Network of fibers throughout the cytoplasm.

Three Main Protein Fibers

  • Microtubules: thickest; cell structure and motility; tubulin.

  • Microfilaments: thinnest; internal movements; actin, myosin.

  • Intermediate filaments: intermediate size; more permanent fixtures; keratin.

Centrioles

  • Cell division (animal cells): organize microtubules to guide chromosomes.

Cilia and Flagella

  • Extensions of the eukaryotic cytoskeleton.

  • Cilia: numerous and short.

  • Flagella: 1-2 per cell and longer.

Structure of Cilia and Flagella

  • 9 pairs of microtubules around 2 single microtubules in the center.

  • Bending is driven by dynein (motor protein).

  • Basal body anchors the cilium or flagellum; requires ATP.

Microfilaments (Actin Filaments)

  • Structure: Thinnest fibers; solid rods of protein, actin; twisted double chain of actin subunits.

Intercellular Junctions

  • Plant Cells:

  • Plasmodesmata: channels allowing cytosol components to pass between cells.

Intercellular Junctions (Animal Cells)

  • Tight junctions: membranes of adjacent cells fuse, forming a barrier.

  • Gap junctions: communicating junctions; allow cytoplasmic movement.

  • Desmosomes: anchoring junctions; fasten cells together in strong sheets.

Unique Cell Components

  • Plants: Chloroplasts, central vacuole, cell wall, plasmodesmata.

  • Animals: Lysosomes, centrosomes, flagella.

Cell Compartmentalization

  • Internal membranes divide the cell into compartments.

  • Organelles: membrane-bound structures in eukaryotes with specific roles.

  • Compartmentalization: maintains internal chemistry different from the rest of the cell.

  • Internal membranes facilitate processes by minimizing interactions and localizing metabolic processes.

Origins of Cell Compartmentalization

  • Eukaryotic organelles evolved from free-living prokaryotes via endosymbiosis.

Endosymbiotic Theory

  • Larger cells engulfed smaller cells that provided energy.

  • Structures of modern cells (mitochondria and chloroplasts) support this theory.

Plasma Membrane

  • Separates internal cell environment from external environment.

  • Comprised primarily of phospholipids (amphipathic).

Fluid Mosaic Model

  • Unsaturated hydrocarbon tails maintain fluidity at low temps.

  • Cholesterol helps maintain fluidity at high and low temps.

Membrane Proteins

  • Integral proteins: embedded in the lipid bilayer (transmembrane).

  • Peripheral proteins: not embedded in the lipid bilayer; loosely bonded to the surface.

Membrane Carbohydrates

  • Important for cell-to-cell recognition.

    • Glycolipids: carbohydrates bonded to lipids.

    • Glycoproteins: carbohydrates bonded to proteins (most abundant).

Selective Permeability

  • Some substances cross the membrane more easily than others.

    • Easy passage: Small nonpolar, hydrophobic molecules (hydrocarbons, CO2, O2, N2)

    • Difficult/assisted passage: Hydrophilic, polar molecules, large molecules, ions (sugars, water).

Passive Transport

  • No energy required; moves down the concentration gradient.

    • Diffusion: Movement from high to low concentration.

    • Facilitated diffusion: Diffusion via transport proteins (channel and carrier).

    • Osmosis: Diffusion of water across a selectively permeable membrane.

Active Transport

  • Requires energy; moves against the concentration gradient.

    • Pumps: Use ATP to move molecules.

    • Electrogenic pumps: Generate voltage across membranes.

    • Sodium-potassium pump: Regulates Na+ and K+ concentrations.

    • Proton pump: Pumps H+ out of the cell (plants, fungi, bacteria).

    • Cotransport: Couples favorable movement of one substance with unfavorable movement of another substance.

Transport of Large Molecules

  • Exocytosis: Secretion of molecules via vesicles that fuse to the plasma membrane.

  • Endocytosis: Uptake of molecules from vesicles fused from the plasma membrane.

    • Phagocytosis: Cell engulfs particles.

    • Pinocytosis: Nonspecific uptake of extracellular fluid.

    • Receptor-mediated endocytosis: Specific uptake of molecules via solute binding to receptors.

Tonicity and Osmoregulation

  • Concentration gradients affect movement across membranes.

    • Hypertonic: higher solute concentration.

    • Hypotonic: lower solute concentration.

    • Isotonic: equal concentrations.