1. Chapter 5 The Working Cell
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.
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2. MEMBRANE STRUCTURE AND FUNCTION
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3. 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.
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4. 5.1 Membranes are fluid mosaics of lipids and proteins with many functions
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.
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5. Cytoplasmic side of membrane Extracellular side of membrane
Figure 5.1
Cytoplasmic side of membrane
Extracellular side of membrane O2
CO2
Fibers of extracellular matrices (ECM)
Phospholipid
Cholesterol
Membrane proteins
Figure 5.1 Some functions of membrane proteins
Microfilaments of cytoskeleton 5

6. Diffusion of small nonpolar molecules
Figure 5.1-1 O2
CO2
Diffusion of small nonpolar molecules
Enzyme
Enzyme
Receptor protein
Attachment protein
Junction protein
Channel protein
Junction protein
Active transport protein
ATP
Glyco- protein

7. 5.1 Membranes are fluid mosaics of lipids and proteins with many functions
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.
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8. 5.1 Membranes are fluid mosaics of lipids and proteins with many functions
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.
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9. 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.
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10. Figure 5.2 Artificial membrane-bounded sacs
Water-filled bubble made of phospholipids 10

11. 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.
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12. 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.
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13. Figure 5.3A 5.3A Passive transport of one type of molecule
Molecules of dye
Membrane
Pores
Figure 5.3A Passive transport of one type of molecule
Net diffusion
Net diffusion
Equilibrium 13

14. Figure 5.3B Passive transport of two types of molecules
Net diffusion
Net diffusion
Equilibrium
Figure 5.3B Passive transport of two types of molecules
Net diffusion
Net diffusion
Equilibrium 14

15. 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.
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16. 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.
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17. 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 17

18. 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.
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19. 5.5 Water balance between cells and their surroundings is crucial to organisms
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.
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20. 5.5 Water balance between cells and their surroundings is crucial to organisms
For an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation, the control of water balance.
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21. 5.5 Water balance between cells and their surroundings is crucial to organisms
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.
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22. Figure 5.5 How animal and plant cells react to changes in tonicity
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) 22

23. 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.
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24. 5.6 Transport proteins can facilitate diffusion across membranes
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.
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25. Solute molecule Transport protein
Figure 5.6 Transport protein providing a channel for the diffusion of a specific solute across a membrane
Solute molecule
Figure 5.6 Transport protein providing a channel for the diffusion of a specific solute across a membrane
Transport protein 25

26. 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.
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27. Solute binds to transport protein.
Figure 5.8
Transport protein
ATP
Solute
Figure 5.8_s4 Active transport of a solute across a membrane (step 4) 1
Solute binds to transport protein. 2
ATP provides energy for change in protein shape. 3
Protein returns to original shape and more solute can bind. 27

28. 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.
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29. 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.
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30. “Food” or other particle
Figure 5.9
Phagocytosis
EXTRACELLULAR FLUID
CYTOPLASM
Pseudopodium
“Food” or other particle
Food vacuole
Receptor-mediated endocytosis
Coat protein
Figure 5.9 Three kinds of endocytosis
Coated vesicle
Receptor
Coated pit
Specific molecule 30

31. ENERGY AND THE CELL
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32. 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.
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33. 5.10 Cells transform energy as they perform work
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.
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34. 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 34

35. 5.10 Cells transform energy as they perform work
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.
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36. 5.10 Cells transform energy as they perform work
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.
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37. 5.10 Cells transform energy as they perform work
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.
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38. 5.10 Cells transform energy as they perform work
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.
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39. 5.11 Chemical reactions either release or store energy
release energy (exergonic reactions) or
require an input of energy and store energy (endergonic reactions).
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40. 5.11 Chemical reactions either release or store energy
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.
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41. Figure 5.11A Exergonic reaction, energy released
Reactants
Amount of energy released
Potential energy of molecules
Energy
Products
Figure 5.11A Exergonic reaction, energy released 41

42. 5.11 Chemical reactions either release or store energy
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.
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43. Figure 5.11B Endergonic reaction, energy required
Products
Amount of energy required
Potential energy of molecules
Energy
Reactants
Figure 5.11B Endergonic reaction, energy required 43

44. 5.11 Chemical reactions either release or store energy
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.
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45. 5.11 Chemical reactions either release or store energy
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.
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46. 5.11 Chemical reactions either release or store energy
Energy coupling uses the
energy released from exergonic reactions to drive
essential endergonic reactions,
usually using the energy stored in ATP molecules.
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47. 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.
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48. 5.12 ATP drives cellular work by coupling exergonic and endergonic reactions
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.
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49. Triphosphate Adenosine P P P ATP H2O Diphosphate Adenosine P P P
Figure 5.12A
Triphosphate
Adenosine P P P
ATP
H2O
Diphosphate
Adenosine P P P
Energy
Figure 5.12A_s2 The structure and hydrolysis of ATP (step 2)
Phosphate
ADP 49

50. 5.12 ATP drives cellular work by coupling exergonic and endergonic reactions
There are three main types of cellular work:
chemical,
mechanical, and
transport.
ATP drives all three of these types of work.
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51. Figure 5.12B How ATP powers cellular work
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 51

52. 5.12 ATP drives cellular work by coupling exergonic and endergonic reactions
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.
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53. Energy from exergonic reactions Energy for endergonic reactions
Figure 5.12C The ATP cycle
ATP
Phosphorylation
Hydrolysis
Energy from exergonic reactions
Energy for endergonic reactions
Figure 5.12C The ATP cycle
ADP P 53

54. HOW ENZYMES FUNCTION
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55. 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).
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56. 5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
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.
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57. Progress of the reaction
Figure 5.13 a b
Reactants
Energy c
Products
Progress of the reaction
Activation energy barrier
Enzyme
Activation energy barrier reduced by enzyme
Figure 5.13A The effect of an enzyme in lowering EA
Reactant
Reactant
Energy
Energy
Products
Products
Without enzyme
With enzyme 57

58. 5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
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.
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59. 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.
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60. Enzyme available with empty active site
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 60

61. 5.14 A specific enzyme catalyzes each cellular reaction
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.
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62. 5.14 A specific enzyme catalyzes each cellular reaction
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.
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63. 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.
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64. 5.15 Enzyme inhibitors can regulate enzyme activity in a cell
Noncompetitive inhibitors
bind to the enzyme somewhere other than the active site,
change the shape of the active site, and
prevent the substrate from binding.
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65. Figure 5.15A How inhibitors interfere with substrate binding
Active site
Enzyme
Allosteric site
Normal binding of substrate
Competitive inhibitor
Noncompetitive inhibitor
Figure 5.15A How inhibitors interfere with substrate binding
Enzyme inhibition 65

66. 5.15 Enzyme inhibitors can regulate enzyme activity in a cell
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.
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67. Figure 5.15B How inhibitors interfere with substrate binding
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 67

68. 5.16 CONNECTION: Many drugs, pesticides, and poisons are enzyme inhibitors
Many beneficial drugs act as enzyme inhibitors, including
Ibuprofen, inhibiting the production of molecules that increase pain (prostaglandins),
some blood pressure medicines,
many antibiotics (penicillin inhibits cell wall enzymes), and
protease inhibitors used to fight HIV and destroy viral proteins.
Enzyme inhibitors have also been developed as pesticides and deadly poisons for chemical warfare.
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69. Figure 5.16 Ibuprofen, an enzyme inhibitor that stops “pain” enzymes69