Chapter 4: Cell Structure and Function Cytoskeleton • The cytoskeleton is a network of fibers that organizes structures and activities in the cell. Microtubules (the largest) Intermediate fibers Microfilaments (the smallest) Think tube vs filaments – tubes are always larger Provide shape and support for the cell MTs MFs IFs 1 Other functions of the cytoskeleton • Provide movement if the organism has flagellum or cilia • Intracellular movement Transports organelles, proteins and other cell components along microtubule “rails”. Nucleus • Only present in eukaryotic cells • Location of the organism’s genome DNA • Nuclear envelope Membrane that surrounds the nucleus Semipermeable does not allow DNA out, but messenger RNA does leave the nucleus. Nucleolus • Darkly stained regions within the nucleus • Ribosome assembly takes place in this organelle. 2 Ribosomes • Ribosomes Protein factories Synthesizes most proteins. • Two types Free ribosomes Found in all cells Ribosomes attached to the endoplasmic reticulum (ER) Endoplasmic reticulum (ER) • Rough endoplasmic reticulum Place where most ribosomes attach Protein synthesis & transportation • Smooth endoplasmic reticulum Lipid production, including steroids. In liver cells, it detoxify drugs and alcohol Also in liver cells, it breaks down Golgi apparatus • The warehouse of the cell Stores, sorts, and transports proteins and other molecules within the cell Directs transportation of proteins and other molecules to the outside of the cell. Also synthesizes cellulose and pectin (plants) 3 Lysosomes • Made by the golgi apparatus • Highly acidic environments Due to digestive enzymes. • Digest waste particles & malfunctioning organelles. Vacuoles • Food vacuoles Transport food from cell membrane to other parts of the cell. This process is known as phagocytosis. Vacuoles 4 Contractile vacuoles • Freshwater protists need these vacuoles to pump excess water out of the cell. Otherwise the cell continues to expand until it explodes. Paramecium Central vacuoles • Found in plant cells • Storage organelle Water May store proteins and carbohydrates Provides turgor, giving plants strength Wilted plants have little turgor due to lack of water Plant turgor Plant needs water to fill Normal plant central vacuoles. 5 Mitochondria & chloroplasts • Mitochondria are the batteries of the cell. Produces a lot of ATP, the energy currency of most metabolic functions. • Chloroplasts Found only in plants and some protists Site of photosynthesis. Chapter 5: Membrane Structure and Function Plasma Membrane: Thin barrier separating inside of cell (cytoplasm) from outside environment Function: 1) Isolate cell’s contents from outside environment 2) Regulate exchange of substances between inside and outside of cell 3) Communicate with other cells Note : Membranes also exist within cells forming various compartments where different biochemical processes occur 6 The Fluid Mosaic Model of Cellular Membranes: Phospholipid Bilayer: Double layer of phospholipids • Hydrophilic ends form outer border • Hydrophobic tails form inner layer • Lipid tails of phospholipids are unsaturated (C = C) Cell Membrane Proteins: 1) Transport Proteins: • Regulate movement of hydrophilic molecules through membrane A) Channel Proteins (form pores; e.g., Na + channels) B) Carrier Proteins (binding sites; e.g., glucose transporter) 2) Receptor Proteins: • Trigger cell activity when molecule from outside environment binds to protein 3) Recognition Proteins: • Allow cells to recognize / attach to one another • Glycoproteins: Proteins with attached carbohydrate groups 7 How are Substances Transported Across Membranes? Answer: Concentration Gradients Concentration = Number of molecules in a given unit of volume (e.g., grams / liter; moles / liter) Gradient = Difference between two regions of space such that molecules move from one region to the other Diffusion: Movement of molecules from an area of high concentration to an area of low concentration • Greater the concentration gradient, the faster diffusion occurs • Diffusion will continue until gradient eliminated (dynamic equilibrium) • Diffusion cannot move molecules rapidly over long distances Types of Movement Across Membranes (Table 4.1): 1) Passive Transport • Requires no energy • Substances move down concentration gradients A) Simple Diffusion • Small molecules pass directly through the phospholipid bilayer (e.g., CO 2, H 2O, O 2) Rate depends on: 1) Molecule size 2) Concentration gradient 3) Lipid solubility (Figure 4.3a) 8 Types of Movement Across Membranes: 1) Passive Transport • Requires no energy • Substances move down concentration gradients B) Facilitated Diffusion • Molecules need assistance of channel proteins or carrier proteins (e.g. ions, amino acids, sugars) Channel Proteins (Figure 4.3b) Carrier Proteins (Figure 4.3c) Channel and Carrier proteins Types of Movement Across Membranes: 1) Passive Transport A) Simple Diffusion • Requires no energy B) Facilitated Diffusion • Substances move down concentration gradients C) Osmosis • Movement of water from an area of high [water] to area of low [water] across semi-permeable membrane water 9 Osmosis: water In which direction will there be a net osmotic movement of water? out of the cell into the cell neither Permeable 0.02 M glucose 0.05 M glucose only to water 0.01 M sucrose In which direction will there be a net osmotic movement of water? out of the cell into the cell neither Permeable 0.02 M glucose 0.05 M glucose only to water 0.03 M sucrose 10 Osmosis and Living Cells: Isotonic Solution: • Outside of cell has same [solute] as inside of cell Iso = same. Isotonic = Same solute concentration inside and outside of the cell. (no net water movement) Osmosis and Living Cells: Isotonic Solution: • Outside of cell has same [solute] as inside of cell Hypertonic Solution: • Outside of cell has higher [solute] than inside of cell • Hyper = higher (net water movement out of cell) Osmosis and Living Cells: Isotonic Solution: • Outside of cell has same [solute] as inside of cell Hypertonic Solution: • Outside of cell has higher [solute] than inside of cell Hypotonic Solution: • Inside of cell has higher [solute] than outside of cell Hypo = lower Tonicity is relative to the inside of the cell (net water movement into cell) 11 Water balance in cells without walls Solution is: hypotonic isotonic hypertonic lyse normal shrivel Water balance in cells with walls hypotonic isotonic hypertonic turgid flaccid shriveled Types of Movement Across Membranes: 1) Passive Transport 2) Active Transport (aka pumps) • Requires energy (ATP) • Substances move against concentration gradients 12 (Figure 4.7) Types of Movement Across Membranes: 1) Passive Transport 2) Active Transport 3) Endocytosis • Movement of large particles into cells (vesicle formation) 1) Pinocytosis (“cell drinking”) • Uptake of fluid droplets 2) Receptor-mediated Endocytosis • Uptake of specific molecules via coated pits 3) Phagocytosis (“cell eating”) • Uptake of large particles (e.g. bacteria) Types of Movement Across Membranes: 1) Passive Transport 2) Active Transport 3) Endocytosis 4) Exocytosis • Movement of large particles out of cells (e.g., hormones) (Figure 4.9) Exocytosis 13 How are Cell Surfaces Specialized? Answer: Junctions allow cells to connect and communicate 1) Connection Junctions: A) Desmosomes: Hold cells together via protein filaments (Figure 4.10a) How are Cell Surfaces Specialized? Answer: Junctions allow cells to connect and communicate 1) Connection Junctions: B) Tight Junctions: Protein “seals” prevent leakage (cell →cell) (Figure 4.10b) How are Cell Surfaces Specialized? Answer: Junctions allow cells to connect and communicate 2) Communication Junctions: A) Gap Junctions: Protein channels allowing for signals to pass between cells (animals) (Figure 4.11a) 14 How are Cell Surfaces Specialized? Answer: Junctions allow cells to connect and communicate 2) Communication Junctions: A) Plasmodesmata: Cytoplasmic bridges allowing for signals to pass between cells (plants) (Figure 4.11b) How are Cell Surfaces Specialized? Answer: Cell walls offer support and protection Cell Walls: • Found in bacteria, plants, fungi, & some protists • Composed of carbohydrates (e.g. cellulose, chitin) , proteins, or inorganic molecules (e.g. silica) • Produced by the cell it protects/supports Exam on Thursday! • Bring scantrons and 2 pencils in case one breaks • Covers Chapters 1 – 5. • Will be multiple choice, short answer, and 1 or 2 essay questions. • Study Chapter 3 particularly carefully, it’s the biggest chapter! 15.