Chapter 5 The Working Cell.

Chapter 5 The Working Cell

Introduction Some organisms use energy-converting reactions to produce light in a process called bioluminescence. Many marine invertebrates and fishes use bioluminescence to hide themselves from predators. Scientists estimate that 90% of deep-sea marine life produces bioluminescence. The light is produced from chemical reactions that convert chemical energy into visible light. © 2012 Pearson Education, Inc. 2

Membrane Structure and Function
Figure 5.0_1 Chapter 5: Big Ideas Cellular respiration Membrane Structure and Function Energy and the Cell Figure 5.0_1 Chapter 5: Big Ideas How Enzymes Function 3

Figure 5.0_2 Figure 5.0_2 Bioluminescent Squid 4

Introduction Bioluminescence is an example of the multitude of energy conversions that a cell can perform. Many of a cell’s reactions take place in organelles and use enzymes embedded in the membranes of these organelles. This chapter addresses how working cells use membranes, energy, and enzymes. © 2012 Pearson Education, Inc. 5

MEMBRANE STRUCTURE AND FUNCTION
© 2012 Pearson Education, Inc. 6

5.1 Membranes are fluid mosaics of lipids and proteins with many functions
Membranes are composed of a bilayer of phospholipids with embedded and attached proteins, in a structure biologists call a fluid mosaic. © 2012 Pearson Education, Inc. 7

Many phospholipids are made from unsaturated fatty acids that have kinks in their tails. These kinks prevent phospholipids from packing tightly together, keeping them in liquid form. In animal cell membranes, cholesterol helps stabilize membranes at warmer temperatures and keep the membrane fluid at lower temperatures. © 2012 Pearson Education, Inc. 8

Fibers of extracellular matrix (ECM)
Figure 5.1 CYTOPLASM Enzymatic activity Fibers of extracellular matrix (ECM) Phospholipid Cell-cell recognition Cholesterol Receptor Signaling molecule Figure 5.1 Some functions of membrane proteins Intercellular junctions ATP Transport Glycoprotein Signal transduction Attachment to the cytoskeleton and extracellular matrix (ECM) Microfilaments of cytoskeleton CYTOPLASM 9

Membrane proteins perform many functions. Some proteins help maintain cell shape and coordinate changes inside and outside the cell through their attachment to the cytoskeleton and extracellular matrix. Some proteins function as receptors for chemical messengers from other cells. Some membrane proteins function as enzymes. © 2012 Pearson Education, Inc. 10

Some membrane glycoproteins are involved in cell-cell recognition. Membrane proteins may participate in the intercellular junctions that attach adjacent cells to each other. Membranes may exhibit selective permeability, allowing some substances to cross more easily than others. © 2012 Pearson Education, Inc. 11

5.2 EVOLUTION CONNECTION: Membranes form spontaneously, a critical step in the origin of life
Phospholipids, the key ingredient of biological membranes, spontaneously self-assemble into simple membranes. The formation of membrane-enclosed collections of molecules was a critical step in the evolution of the first cells. © 2012 Pearson Education, Inc. 12

Water-filled bubble made of phospholipids
Figure 5.2 Figure 5.2 Artificial membrane-bounded sacs Water-filled bubble made of phospholipids 13

Figure 5.2Q Water Figure 5.2Q Structure of membrane sacs Water 14

5.3 Passive transport is diffusion across a membrane with no energy investment
Diffusion is the tendency of particles to spread out evenly in an available space. Particles move from an area of more concentrated particles to an area where they are less concentrated. This means that particles diffuse down their concentration gradient. Eventually, the particles reach equilibrium where the concentration of particles is the same throughout. © 2012 Pearson Education, Inc. 15

5.3 Passive transport is diffusion across a membrane with no energy investment
Diffusion across a cell membrane does not require energy, so it is called passive transport. The concentration gradient itself represents potential energy for diffusion. © 2012 Pearson Education, Inc. 16

Molecules of dye Membrane Pores Net diffusion Net diffusion
Figure 5.3A Molecules of dye Membrane Pores Figure 5.3A Passive transport of one type of molecule Net diffusion Net diffusion Equilibrium 17

Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion
Figure 5.3B Net diffusion Net diffusion Equilibrium Figure 5.3B Passive transport of two types of molecules Net diffusion Net diffusion Equilibrium 18

5.4 Osmosis is the diffusion of water across a membrane
One of the most important substances that crosses membranes is water. The diffusion of water across a selectively permeable membrane is called osmosis. © 2012 Pearson Education, Inc. 19

5.4 Osmosis is the diffusion of water across a membrane
If a membrane permeable to water but not a solute separates two solutions with different concentrations of solute, water will cross the membrane, moving down its own concentration gradient, until the solute concentration on both sides is equal. © 2012 Pearson Education, Inc. 20

Lower concentration of solute Higher concentration of solute
Figure 5.4 Lower concentration of solute Higher concentration of solute Equal concentrations of solute H2O Solute molecule Selectively permeable membrane Water molecule Figure 5.4 Osmosis, the diffusion of water across a membrane Solute molecule with cluster of water molecules Osmosis 21

5.5 Water balance between cells and their surroundings is crucial to organisms
Tonicity is a term that describes the ability of a solution to cause a cell to gain or lose water. Tonicity mostly depends on the concentration of a solute on both sides of the membrane. © 2012 Pearson Education, Inc. 22

How will animal cells be affected when placed into solutions of various tonicities? When an animal cell is placed into an isotonic solution, the concentration of solute is the same on both sides of a membrane, and the cell volume will not change, a hypotonic solution, the solute concentration is lower outside the cell, water molecules move into the cell, and the cell will expand and may burst, or a hypertonic solution, the solute concentration is higher outside the cell, water molecules move out of the cell, and the cell will shrink. © 2012 Pearson Education, Inc. 23

The cell walls of plant cells, prokaryotes, and fungi make water balance issues somewhat different. The cell wall of a plant cell exerts pressure that prevents the cell from taking in too much water and bursting when placed in a hypotonic environment. But in a hypertonic environment, plant and animal cells both shrivel. © 2012 Pearson Education, Inc. 25

Shriveled (plasmolyzed)
Figure 5.5 Hypotonic solution Isotonic solution Hypertonic solution H2O H2O H2O H2O Animal cell Lysed Normal Shriveled H2O H2O Plasma membrane H2O Plant cell Figure 5.5 How animal and plant cells react to changes in tonicity Turgid (normal) Flaccid Shriveled (plasmolyzed) 26

5.6 Transport proteins can facilitate diffusion across membranes
Hydrophobic substances easily diffuse across a cell membrane. However, polar or charged substances do not easily cross cell membranes and, instead, move across membranes with the help of specific transport proteins in a process called facilitated diffusion, which does not require energy and relies on the concentration gradient. © 2012 Pearson Education, Inc. 27 27

Some proteins function by becoming a hydrophilic tunnel for passage of ions or other molecules. Other proteins bind their passenger, change shape, and release their passenger on the other side. In both of these situations, the protein is specific for the substrate, which can be sugars, amino acids, ions, and even water. © 2012 Pearson Education, Inc. 28 28

Because water is polar, its diffusion through a membrane’s hydrophobic interior is relatively slow. The very rapid diffusion of water into and out of certain cells is made possible by a protein channel called an aquaporin. © 2012 Pearson Education, Inc. 29 29

Solute molecule Transport protein Figure 5.6
Figure 5.6 Transport protein providing a channel for the diffusion of a specific solute across a membrane Transport protein 30

5.7 SCIENTIFIC DISCOVERY: Research on another membrane protein led to the discovery of aquaporins
Dr. Peter Agre received the 2003 Nobel Prize in chemistry for his discovery of aquaporins. His research on the Rh protein used in blood typing led to this discovery. © 2012 Pearson Education, Inc. 31

Figure 5.7 Figure 5.7 Aquaporin in action 32

5.8 Cells expend energy in the active transport of a solute
In active transport, a cell must expend energy to move a solute against its concentration gradient. The following figures show the four main stages of active transport. © 2012 Pearson Education, Inc. 33

Transport protein Solute Solute binding 1 Figure 5.8_s1
Figure 5.8_s1 Active transport of a solute across a membrane (step 1) 1 Solute binding 34

Transport protein P ATP Solute ADP Solute binding Phosphate attaching
Figure 5.8_s2 Transport protein P ATP Solute ADP Figure 5.8_s2 Active transport of a solute across a membrane (step 2) 1 Solute binding 2 Phosphate attaching 35

Transport protein P P Protein changes shape. ATP Solute ADP
Figure 5.8_s3 Transport protein P P Protein changes shape. ATP Solute ADP Figure 5.8_s3 Active transport of a solute across a membrane (step 3) 1 Solute binding 2 Phosphate attaching 3 Transport 36

Transport protein P P Protein changes shape. Phosphate detaches. P ATP
Figure 5.8_s4 Transport protein P P Protein changes shape. Phosphate detaches. P ATP Solute ADP Figure 5.8_s4 Active transport of a solute across a membrane (step 4) 1 Solute binding 2 Phosphate attaching 3 Transport 4 Protein reversion 37

5.9 Exocytosis and endocytosis transport large molecules across membranes
A cell uses two mechanisms to move large molecules across membranes. Exocytosis is used to export bulky molecules, such as proteins or polysaccharides. Endocytosis is used to import substances useful to the livelihood of the cell. In both cases, material to be transported is packaged within a vesicle that fuses with the membrane. © 2012 Pearson Education, Inc. 38

5.9 Exocytosis and endocytosis transport large molecules across membranes
There are three kinds of endocytosis. Phagocytosis is the engulfment of a particle by wrapping cell membrane around it, forming a vacuole. Pinocytosis is the same thing except that fluids are taken into small vesicles. Receptor-mediated endocytosis uses receptors in a receptor-coated pit to interact with a specific protein, initiating the formation of a vesicle. 39

“Food” or other particle
Figure 5.9_1 Phagocytosis EXTRACELLULAR FLUID CYTOPLASM Food being ingested Pseudopodium “Food” or other particle Figure 5.9_1 Three kinds of endocytosis (part 1) Food vacuole 40

Pinocytosis Plasma membrane Vesicle Plasma membrane Figure 5.9_2
Figure 5.9_2 Three kinds of endocytosis (part 2) Plasma membrane 41

Receptor-mediated endocytosis Coat protein
Figure 5.9_3 Plasma membrane Receptor-mediated endocytosis Coat protein Coated vesicle Receptor Coated pit Coated pit Specific molecule Figure 5.9_3 Three kinds of endocytosis (part 3) Material bound to receptor proteins 42

5.10 Cells transform energy as they perform work
Cells are small units, a chemical factory, housing thousands of chemical reactions. Cells use these chemical reactions for cell maintenance, manufacture of cellular parts, and cell replication. © 2012 Pearson Education, Inc. 44

Energy is the capacity to cause change or to perform work. There are two kinds of energy. Kinetic energy is the energy of motion. Potential energy is energy that matter possesses as a result of its location or structure. © 2012 Pearson Education, Inc. 45

Kinetic energy of movement
Figure 5.10 Fuel Energy conversion Waste products Heat energy Carbon dioxide Gasoline   Combustion Kinetic energy of movement Oxygen Water Energy conversion in a car Heat energy Figure 5.10 Energy transformations in a car and a cell Cellular respiration Glucose Carbon dioxide   ATP ATP Oxygen Energy for cellular work Water Energy conversion in a cell 46

Heat, or thermal energy, is a type of kinetic energy associated with the random movement of atoms or molecules. Light is also a type of kinetic energy, and can be harnessed to power photosynthesis. © 2012 Pearson Education, Inc. 47

Chemical energy is the potential energy available for release in a chemical reaction. It is the most important type of energy for living organisms to power the work of the cell. © 2012 Pearson Education, Inc. 48

Thermodynamics is the study of energy transformations that occur in a collection of matter. Scientists use the word system for the matter under study and surroundings for the rest of the universe. © 2012 Pearson Education, Inc. 49

Two laws govern energy transformations in organisms. According to the first law of thermodynamics, energy in the universe is constant, and second law of thermodynamics, energy conversions increase the disorder of the universe. Entropy is the measure of disorder, or randomness. © 2012 Pearson Education, Inc. 50

Cells use oxygen in reactions that release energy from fuel molecules. In cellular respiration, the chemical energy stored in organic molecules is converted to a form that the cell can use to perform work. © 2012 Pearson Education, Inc. 51

5.11 Chemical reactions either release or store energy
release energy (exergonic reactions) or require an input of energy and store energy (endergonic reactions). © 2012 Pearson Education, Inc. 52

Exergonic reactions release energy. These reactions release the energy in covalent bonds of the reactants. Burning wood releases the energy in glucose as heat and light. Cellular respiration involves many steps, releases energy slowly, and uses some of the released energy to produce ATP. © 2012 Pearson Education, Inc. 53

Amount of energy released
Figure 5.11A Reactants Amount of energy released Potential energy of molecules Energy Products Figure 5.11A Exergonic reaction, energy released 54

An endergonic reaction requires an input of energy and yields products rich in potential energy. Endergonic reactions begin with reactant molecules that contain relatively little potential energy but end with products that contain more chemical energy. © 2012 Pearson Education, Inc. 55

Amount of energy required
Figure 5.11B Products Amount of energy required Potential energy of molecules Energy Reactants Figure 5.11B Endergonic reaction, energy required 56

Photosynthesis is a type of endergonic process. Energy-poor reactants, carbon dioxide, and water are used. Energy is absorbed from sunlight. Energy-rich sugar molecules are produced. © 2012 Pearson Education, Inc. 57

A living organism carries out thousands of endergonic and exergonic chemical reactions. The total of an organism’s chemical reactions is called metabolism. A metabolic pathway is a series of chemical reactions that either builds a complex molecule or breaks down a complex molecule into simpler compounds. © 2012 Pearson Education, Inc. 58

5.12 ATP drives cellular work by coupling exergonic and endergonic reactions
ATP, adenosine triphosphate, powers nearly all forms of cellular work. ATP consists of the nitrogenous base adenine, the five-carbon sugar ribose, and three phosphate groups. © 2012 Pearson Education, Inc. 60

Hydrolysis of ATP releases energy by transferring its third phosphate from ATP to some other molecule in a process called phosphorylation. Most cellular work depends on ATP energizing molecules by phosphorylating them. © 2012 Pearson Education, Inc. 61

ATP: Adenosine Triphosphate Phosphate group P P P Adenine Ribose
Figure 5.12A_s1 ATP: Adenosine Triphosphate Phosphate group P P P Adenine Ribose Figure 5.12A_s1 The structure and hydrolysis of ATP (step 1) 62

ATP: Adenosine Triphosphate Phosphate group P P P Adenine Ribose H2O
Figure 5.12A_s2 ATP: Adenosine Triphosphate Phosphate group P P P Adenine Ribose H2O Hydrolysis Figure 5.12A_s2 The structure and hydrolysis of ATP (step 2) P P P Energy ADP: Adenosine Diphosphate 63

Protein filament moved Solute transported
Figure 5.12B Chemical work Mechanical work Transport work ATP ATP ATP Solute P Motor protein P P Reactants Membrane protein P Figure 5.12B How ATP powers cellular work P P Product Molecule formed Protein filament moved Solute transported ADP P ADP P ADP P 65

ATP is a renewable source of energy for the cell. In the ATP cycle, energy released in an exergonic reaction, such as the breakdown of glucose,is used in an endergonic reaction to generate ATP. © 2012 Pearson Education, Inc. 66

Energy from exergonic reactions Energy for endergonic reactions
Figure 5.12C ATP Phosphorylation Hydrolysis Energy from exergonic reactions Energy for endergonic reactions Figure 5.12C The ATP cycle ADP P 67

5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
Although biological molecules possess much potential energy, it is not released spontaneously. An energy barrier must be overcome before a chemical reaction can begin. This energy is called the activation energy (EA). © 2012 Pearson Education, Inc. 69

We can think of EA as the amount of energy needed for a reactant molecule to move “uphill” to a higher energy but an unstable state so that the “downhill” part of the reaction can begin. One way to speed up a reaction is to add heat, which agitates atoms so that bonds break more easily and reactions can proceed but could kill a cell. © 2012 Pearson Education, Inc. 70

Activation energy barrier
Figure 5.13A Activation energy barrier Enzyme Activation energy barrier reduced by enzyme Reactant Reactant Energy Energy Figure 5.13A The effect of an enzyme in lowering EA Products Products Without enzyme With enzyme 71

Activation energy barrier
Figure 5.13A_1 Activation energy barrier Reactant Energy Figure 5.13A_1 The effect of an enzyme in lowering EA (part 1) Products Without enzyme 72

Activation energy barrier reduced by enzyme
Figure 5.13A_2 Enzyme Activation energy barrier reduced by enzyme Reactant Energy Figure 5.13A_2 The effect of an enzyme in lowering EA (part 2) Products With enzyme 73

Progress of the reaction
Figure 5.13Q a b Energy Reactants c Figure 5.13Q Activation energy with and without an enzyme Products Progress of the reaction 74

function as biological catalysts by lowering the EA needed for a reaction to begin, increase the rate of a reaction without being consumed by the reaction, and are usually proteins, although some RNA molecules can function as enzymes. © 2012 Pearson Education, Inc. 75

5.14 A specific enzyme catalyzes each cellular reaction
An enzyme is very selective in the reaction it catalyzes and has a shape that determines the enzyme’s specificity. The specific reactant that an enzyme acts on is called the enzyme’s substrate. A substrate fits into a region of the enzyme called the active site. Enzymes are specific because their active site fits only specific substrate molecules. © 2012 Pearson Education, Inc. 76

Enzyme available with empty active site
Figure 5.14_s1 1 Enzyme available with empty active site Active site Enzyme (sucrase) Figure 5.14_s1 The catalytic cycle of an enzyme (step 1) 78

Figure 5.14_s2 1 Enzyme available with empty active site Active site Substrate (sucrose) 2 Substrate binds to enzyme with induced fit Enzyme (sucrase) Figure 5.14_s2 The catalytic cycle of an enzyme (step 2) 79

Figure 5.14_s3 1 Enzyme available with empty active site Active site Substrate (sucrose) 2 Substrate binds to enzyme with induced fit Enzyme (sucrase) H2O Figure 5.14_s3 The catalytic cycle of an enzyme (step 3) 3 Substrate is converted to products 80

Figure 5.14_s4 1 Enzyme available with empty active site Active site Substrate (sucrose) 2 Substrate binds to enzyme with induced fit Enzyme (sucrase) Glucose Fructose H2O Figure 5.14_s4 The catalytic cycle of an enzyme (step 4) 4 Products are released 3 Substrate is converted to products 81

For every enzyme, there are optimal conditions under which it is most effective. Temperature affects molecular motion. An enzyme’s optimal temperature produces the highest rate of contact between the reactants and the enzyme’s active site. Most human enzymes work best at 35–40ºC. The optimal pH for most enzymes is near neutrality. © 2012 Pearson Education, Inc. 82

Many enzymes require nonprotein helpers called cofactors, which bind to the active site and function in catalysis. Some cofactors are inorganic, such as zinc, iron, or copper. If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme. © 2012 Pearson Education, Inc. 83

5.15 Enzyme inhibitors can regulate enzyme activity in a cell
A chemical that interferes with an enzyme’s activity is called an inhibitor. Competitive inhibitors block substrates from entering the active site and reduce an enzyme’s productivity. © 2012 Pearson Education, Inc. 84

Normal binding of substrate
Figure 5.15A Substrate Active site Enzyme Allosteric site Normal binding of substrate Competitive inhibitor Noncompetitive inhibitor Figure 5.15A How inhibitors interfere with substrate binding Enzyme inhibition 86

Enzyme inhibitors are important in regulating cell metabolism. In some reactions, the product may act as an inhibitor of one of the enzymes in the pathway that produced it. This is called feedback inhibition. © 2012 Pearson Education, Inc. 87

Feedback inhibition Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1
Figure 5.15B Feedback inhibition Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting molecule Product Figure 5.15B Feedback inhibition of a biosynthetic pathway 88

5.16 CONNECTION: Many drugs, pesticides, and poisons are enzyme inhibitors
Many beneficial drugs act as enzyme inhibitors, including Ibuprofen, inhibiting the production of prostaglandins, some blood pressure medicines, some antidepressants, many antibiotics, and protease inhibitors used to fight HIV. Enzyme inhibitors have also been developed as pesticides and deadly poisons for chemical warfare. © 2012 Pearson Education, Inc. 89

Figure 5.16 Figure 5.16 Ibuprofen, an enzyme inhibitor 90

Chapter 5 The Working Cell.

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