BSC 2010 - Exam I Lectures and Text Pages

1. BSC 2010 - Exam I Lectures and Text Pages • I. Intro to Biology (2-29) • II. Chemistry of Life • Chemistry review (30-46) • Water (47-57) • Carbon (58-67) • Macromolecules (68-91) • III. Cells and Membranes • Cell structure (92-123) • Membranes (124-140) • IV. Introductory Biochemistry • Energy and Metabolism (141-159) • Cellular Respiration (160-180) • Photosynthesis (181-200)

2. Ways of Crossing the Plasma Membrane • Passive Transport vs. Active Transport • Passive transport: pathways that do not involve energy expenditure from the cell (in the form of ATP) • Diffusion : Molecules naturally move from an area of higher concentration to one of lower concentration until equilibrium is reached. Rate can be affected by temperature, molecule size, and charge.

3. (a) Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Molecules of dye Membrane (cross section) Equilibrium Net diffusion Net diffusion Figure 7.11 A PASSIVE TRANSPORT - Diffusion • Is the tendency for molecules of any substance to spread out evenly into the available space

4. (b) Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concen- tration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. Equilibrium Net diffusion Net diffusion Net diffusion Equilibrium Net diffusion Figure 7.11 B Diffusion • Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another

5. Effects of Osmosis on Water Balance • Osmosis :  the diffusionof water through a semi-permeable membrane. Water moves from an area of higher water concentration to an area of lower water concentration to reach equilibrium on both sides of the membrane.

6. Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can Water molecules cluster around sugar molecules More free water molecules (higher concentration) Fewer free water molecules (lower concentration) Osmosis  Water moves from an area of higher free water concentration to an area of lower free water concentration Figure 7.12 Osmosis • Is affected by the concentration gradient of dissolved substances

7. Water Balance of Cells Without Walls • Tonicity • Is the ability of a solution to cause a cell to gain or lose water. • Has a great impact on cells without walls. • Is always a comparison of relative solute concentrations between two solutions.

8. 3 Categories of Relative Concentration (Tonicity) • When comparing two solutions (A and B), solution A is … * Hypotonic  to solution B if solution A has a lower concentration of solutes than B. * Hypertonic to solution B if solution A has a higher concentration of solutes than B. * Isotonic to solution B if the concentration of solutes is equal.

9. 3 Categories of Relative Concentration (Tonicity) • Example:  A cell with a concentration of sodium at 1 g/L is placed in beaker of water with a sodium concentration of 10 g/L. How do we describe this? ---We can say that the cell is hypotonic to the water in the beaker OR we can say that the water is hypertonic to the cell. Most cells are bathed in an isotonic solution, so there is no net osmosis occurring. • Be sure you understand these terms and read this section of your text.

10. Isotonicity • If a solution is isotonic to the cell. • The concentration of solutes is the same as it is inside the cell. • Therefore, the concentration of water is the same between the two solutions. • There will be no net movement of water.

11. Hypertonicity • If a solution is hypertonic to the cell • The concentration of solutes is greater in the external solution than it is inside the cell. • Therefore the concentration of water is greater inside the cell than outside. • The cell will lose water to the external solution.

12. Hypotonicity • If a solution is hypotonic • The concentration of solutes in the external solution is less than it is inside the cell. • Therefore, the concentration of water is greater outside the cell. • The cell will gain water from the external solution.

13. Hypotonic solution Hypertonic solution Isotonic solution (a) Animal cell. An animal cell fares best in an isotonic environ- ment unless it has special adaptations to offset the osmotic uptake or loss of water. H2O H2O H2O Normal Shriveled Lysed Figure 7.13 Water balance in cells without walls • Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments • Must have special adaptations for osmoregulation H2O

14. Water Balance of Cells with Walls • Cell walls • Help maintain water balance

15. Turgidity • If a plant cell is turgid • It is in a hypotonic environment • It is very firm, a healthy state in most plants

16. Flaccidity • If a plant cell is flaccid • It is in an isotonic or hypertonic environment • In a hypertonic environment, the cell may even become separated from the cell wall.

17. (b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environ- ment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. H2O H2O H2O H2O Turgid (normal) Flaccid Plasmolyzed Figure 7.13 Water balance in cells with walls Hyptotonic Solution Isotonic Solution Hypertonic Solution

18. Facilitated Diffusion: Passive Transport Aided by Proteins • In facilitated diffusion • Transport proteins speed the movement of molecules across the plasma membrane • Facilitated diffusion: Normal diffusion occurs, but through a protein channel in the membrane, not through the phospholipid bilayer. This also takes no added energy from the cell.

19. EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 7.15 Facilitated Diffusion • Channel proteins • Provide corridors that allow a specific molecule or ion to cross the membrane

20. Solute Carrier protein (b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute. Figure 7.15 Carrier proteins • Carrier proteins • Undergo a subtle change in shape that translocates the solute-binding site across the membrane

21. ACTIVE TRANSPORT • Active transportuses energy (ATP) to move solutes against their gradients • Uses carrier proteins to help molecules across membrane 1. usually pumps one thing out of cell while pumping another into the cell. e.g. Na-K pump in nerve cells - pumps Na+ out and K+ in. 2. used to keep the concentration of a certain substance higher inside the cell than outside.  Very important in keeping trace element levels high enough in the cell though they may be low outside.

22. 6 2 1 5 3 4 [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ ATP [Na+] low [K+] high P Na+ ADP CYTOPLASM Na+ binding stimulates phosphorylation by ATP. Cytoplasmic Na+ binds to the sodium-potassium pump. Na+ Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. Na+ Na+ K+ P K+ K+ is released and Na+ sites are receptive again; the cycle repeats. K+ K+ K+ K+ Loss of the phosphate restores the protein’s original conformation. Extracellular K+ binds to the protein, triggering release of the Phosphate group. The sodium-potassium pump • The sodium-potassium pump • Is one type of active transport system EXTRACELLULAR FLUID P P i Figure 7.16

23. Maintenance of Membrane Potential by Ion Pumps • Membrane potential • Is the voltage difference across a membrane • An electrochemical gradient • Is caused by the difference in concentration of ions across a membrane

24. – EXTRACELLULAR FLUID + – ATP + H+ H+ Proton pump H+ + – H+ H+ + – CYTOPLASM + H+ + – Electrogenic pumps • An electrogenic pump • Is a transport protein that generates the voltage across a membrane Figure 7.18

25. – + H+ ATP H+ + – H+ Proton pump H+ – + H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ – + – Sucrose + Figure 7.19 Cotransport: Coupled Transport by a Membrane Protein • Cotransport: • active transport driven by a concentration gradient • occurs when active transport of a specific solute indirectly drives the active transport of another solute

26. Bulk Transport • Bulk transport across the plasma membrane occurs by exocytosis and endocytosis • Large proteins and polysaccharides • Cross the membrane in vesicles

27. Exocytosis and Endocytosis • In exocytosis, transport vesicles migrate to the plasma membrane and fuse with it, becoming part of the plasma membrane. Then, vesicle contents are released to the outside of the cell = secretion • In endocytosis, the cell takes in macromolecules. New vesicles bud inward from the plasma membrane

28. EXTRACELLULAR FLUID 1 µm CYTOPLASM Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). In pinocytosis (cell drinking), the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports. PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Vesicle Three Types of Endocytosis • Phagocytosis, pinocytosis, receptor-mediated endocytosis PHAGOCYTOSIS In phagocytosis (cell eating), a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane- enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. Figure 7.20

29. Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle. RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Coat protein Plasma membrane 0.25 µm

30. Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. ATP Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. Review: Passive and active transport compared • Review: Passive and active transport compared Figure 7.17