Energy and 4 Metabolism - University of Sistan and Baluchestan

Energy and Metabolism

4.1 A Toast to Alcohol Dehydrogenase 64

4.2 Life Runs on Energy 65

4.3 Energy in the Molecules of Life 66

4.4 How Enzymes Work 68

4.5 Diffusion and Membranes 71

4.6 Membrane Transport Mechanisms 73

4.7 A Toast to ADH (revisited) 77

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Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.

78   Unit One How Cells Work

SummarySection 4.1 Currently the most serious drug prob-lem on college campuses is binge drinking, which is associated with alcoholism. Drinking more alco-hol than the body’s enzymes can detoxify can be lethal in both the short term and the long term.

Section 4.2 energy is the capacity to do work. Energy cannot be created or destroyed (first law of thermodynamics), but it can be converted from one form to another and transferred between objects or systems. Energy tends to disperse spontaneously (sec-ond law of thermodynamics). A bit disperses at each energy transfer, usually in the form of heat.

Living things can maintain their organization only as long as they harvest energy from someplace else. Energy flows in one direc-tion through the biosphere, starting mainly from the sun, then into and out of ecosystems. Producers and then consumers use the energy to assemble, rearrange, and break down organic molecules that cycle among organisms throughout ecosystems.

Section 4.3 Cells store and retrieve energy by making and breaking chemical bonds in chemical reactions, in which reactants are con verted to products. Some reactions require a net energy input; others end with a net energy release. Activation energy is the minimum energy input required to start a reaction.

Section 4.4 Enzymes greatly enhance the rate of a chemical reaction. Each has an active site and works on a particular substrate within a charac-teristic range of temperature, salt concentration, and pH. Many enzymes require assistance from coenzymes or other cofactors.

ATP functions as an energy carrier between reaction sites in cells. It has three phosphate groups; when one of them is transferred to another molecule, energy is transferred along with it. Phosphate-group transfers (phosphorylations) to and from ATP couple reactions that release energy with reac-tions that require energy.

Cells build, convert, and break down substances in enzyme-mediated reaction sequences called metabolic pathways. Controls

over enzymes and metabolic pathways allow cells to con-serve energy and resources by producing only what they

require. Feedback inhibition is an example of meta-bolic control. electron transfer chains in some

pathways harvest electron energy in small, man-ageable increments.

Section 4.5 Molecules or ions tend to spread spon-taneously (diffuse), with the eventual result being a gradual and complete mixing. A concentration gradi-ent is a difference in the concentration of a solute between adjoining regions of solution. The steepness of the gradient, temperature, solute size, charge, and pressure influence the diffusion rate.

Osmosis is the diffusion of water across a selectively perme-able membrane, from the region with a lower solute concentration

(hypotonic) toward the region with a higher solute concentration (hypertonic). There is no net movement of water between isotonic solutions. Osmotic pressure is the amount of turgor (fluid pressure against a cell membrane or wall) that stops osmosis.

Section 4.6 Gases, water, and small nonpolar molecules can diffuse across a lipid bilayer. Most other molecules, and ions in particular, cross only with the help of transport proteins.

Transport proteins allow a cell or membrane-enclosed organelle to control which substances enter and exit. The types of transport proteins in a membrane determine which substances can cross it. Calcium pumps and other active transport proteins use energy, such as a phosphate transfer from ATP, to pump a solute against its concentration gradient. Passive trans-port proteins work without an energy input; a solute’s movement is driven by its concentration gradient.

Substances in bulk and large particles are moved across plasma membranes by processes of exocytosis and endocytosis. In exocy-tosis, a cytoplasmic vesicle fuses with the plasma membrane, and its contents are released to the outside of the cell. The vesicle’s mem-brane lipids and proteins become part of the plasma membrane. In endocytosis, a patch of plasma membrane balloons into the cell, and forms a vesicle that sinks into the cytoplasm. Some cells engulf large particles such as prey or cell debris by the endocytic pathway of phagocytosis.

Self-Quiz AnswersinAppendixI

1. is life’s primary source of energy.a. Food b. Water c. Sunlight d. ATP

2. Which of the following statements is not correct?a. Energy cannot be created or destroyed.b. Energy cannot change from one form to another.c. Energy tends to disperse spontaneously.

3. If we liken a chemical reaction to an energy hill, then an reaction is an uphill run.

a. energy-requiring c. ATP-assistedb. energy-releasing d. both a and c

4. are always changed by participating in a reaction. (Choose all that are correct.)

a. Enzymes c. Reactantsb. Cofactors d. Active sites

5. Enzymes .a. are proteins, except for a few RNAsb. lower the activation energy of a reactionc. are changed by participating in a reactiond. a and b

6. One environmental factor that influences enzyme function is .

a. temperature c. lightb. wind d. radioactivity

7. A metabolic pathway .a. may build or break down moleculesb. generates heatc. can include an electron transfer chaind. all of the above

ATP

glucose

Cytoplasm

ExtracellularFluid

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Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.

Chapter 4 Energy and Metabolism     79

8. Which of the following statements is incorrect?a. Some metabolic pathways are cyclic.b. Glucose can diffuse through a lipid bilayer. c. Feedback inhibition controls some metabolic pathways. d. All coenzymes are cofactors. e. Osmosis is a case of diffusion.

9. Ions or molecules tend to diffuse from a region where they are (more/less) concentrated to another where they are (more/less) concentrated.

10. cannot easily diffuse across a lipid bilayer.a. Water c. Ionsb. Gases d. all of the above

11. Transporters that require an energy boost help sodium ions across a cell membrane. This is a case of .

a. passive transport c. facilitated diffusionb. active transport d. a and c

12. If you immerse a red blood cell in a hypotonic solution, water will .

a. diffuse into the cell c. show no net movementb. diffuse out of the cell d. move in by endocytosis

13. Fluid pressure against a wall or cell membrane is called .a. osmosis c. diffusionb. turgor d. osmotic pressure

14. Vesicles form in .a. endocytosis c. phagocytosis b. exocytosis d. a and c

15. Match each term with its most suitable description. reactant a. assists some enzymes phagocytosis b. forms at reaction’s end first law c. enters a reaction of thermodynamics d. requires energy boost product e. one cell engulfs another cofactor f. energy cannot be created diffusion or destroyed passive transport g. faster with a gradient active transport h. no energy boost required ATP i. currency in an energy economy

Critical Thinking1. Beginning physics students are often taught the basic concepts of thermodynamics with two phrases: First, you can’t win. Second, you can’t break even. Explain.

2. Water molecules tend to diffuse in response to their own concen-tration gradient. How can water be more or less concentrated?

3. Dixie Bee wanted to make JELL-O shots for her next party, but felt guilty about encouraging her guests to consume alcohol. She tried to compensate for the toxicity of the alcohol by adding pieces of healthy fresh pineapple to the shots, but when she did, the JELL-O never solidified. What happened? Hint: JELL-O is mainly sugar and a gelatinous mixture of proteins.

4. The enzyme trypsin is sold as a dietary enzyme supplement. Explain what happens to trypsin taken with food.

5. The enzyme catalase combines two hydrogen peroxide molecules (H2O2 + H2O2) to make two molecules of water (2H2O). A gas also forms. What is the gas?

Figure 4.21   pH anomaly of Ferroplasma enzymes. (a) Courtesy of Dr. Katrina J. Edwards; (b) From Golyshina et al., Environmental Microbiology, 8(3): 416–425. © 2006 John Wiley and Sons. Used with permission of the publisher.

Digging into DataOne Tough Bug

Ferroplasma acidarmanus is a species of archaea discovered in an abandoned California copper mine (Figure 4.21A). These cells use an energy-harvesting pathway that combines oxygen with iron–sulfur compounds in minerals such as pyrite. The reaction dissolves the minerals, so groundwater that seeps into the mine ends up accu-mulating high concentrations of metal ions such as copper, zinc, cadmium, and arsenic. Another reaction product, sulfuric acid, low-ers the pH of the resulting solution to zero.

Unwalled F. acidarmanus cells maintain their internal pH at a cozy 5.0 despite living in an environment with a composition essen-tially the same as hot battery acid. Thus, researchers investigating Ferroplasma metabolic enzymes were surprised to discover that most of the cells’ enzymes function best at very low pH (Figure 4.21B).

1. What does the dashed line in the graph signify?

2. Of the four enzymes profiled in the graph, how many function optimally at a pH lower than 5? How many retain significant func-tion at pH 5?

3. What is the optimal pH for Ferroplasma carboxylesterase?

Enz

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activ

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pH pH pH pH3 4 5 60 1 2 3 4 5 60 1 2 3 4 5 6 70 1 2

α-glucosidase GlyFa1 GlyFa2carboxylesterase

3 4 5 6 70 1 2

A Deep inside one of the most toxic sites in the United States: Iron Mountain Mine, in California. The water in this stream, which is about one meter (3 feet) wide, is hot (around 40°C, or 104°F), heavily laden with arsenic and other toxic metals, and has a pH of zero. The slime streamers growing in it are a biofilm dominated by a species of archaea, Ferroplasma acidarmanus.

B pH profiles of four enzymes isolated from Ferroplasma. Researchers had expected these enzymes to function best at the cells’ cytoplasmic pH (5.0).

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Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.