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)

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.

PASSIVE TRANSPORT - Diffusion Is the tendency for molecules of any substance to spread out evenly into the available space Figure 7.11 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) Net diffusion Equilibrium (a)

Diffusion Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another Figure 7.11 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. (b) Net diffusion Equilibrium

Effects of Osmosis on Water Balance Osmosis :  the diffusion of 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.

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

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.

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.

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.

If a solution is isotonic to the cell. 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.

If a solution is hypertonic to the cell 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.

If a solution is hypotonic 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.

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 Figure 7.13 Hypotonic solution Isotonic solution Hypertonic solution 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. (a) H2O Lysed Normal Shriveled H2O

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

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

If a plant cell is flaccid 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.

Water balance in cells with walls Hyptotonic Solution Isotonic Solution Hypertonic Solution 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. (b) H2O Turgid (normal) Flaccid Plasmolyzed Figure 7.13

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.

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

Carrier proteins Carrier proteins Undergo a subtle change in shape that translocates the solute-binding site across the membrane Figure 7.15 Carrier protein Solute 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. (b)

ACTIVE TRANSPORT Active transport uses 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.

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

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

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

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

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

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

Three Types of Endocytosis Phagocytosis, pinocytosis, receptor-mediated endocytosis 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. EXTRACELLULAR FLUID Pseudopodium CYTOPLASM “Food” or other particle Food vacuole 1 µm of amoeba Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). PINOCYTOSIS Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). 0.5 µm 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, pinocytosis is nonspecific in the substances it transports. Plasma membrane Vesicle PHAGOCYTOSIS Figure 7.20

RECEPTOR-MEDIATED ENDOCYTOSIS Ligand Coat protein Coated pit vesicle A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Plasma membrane Coat protein 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.

Review: Passive and active transport compared 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. 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. ATP Figure 7.17