How Cells Work

1
How Cells Work

• Chapter 4

2
Beer, Enzymes and Your Liver

• Alcohol is toxic
• Cells in liver break down alcohol to nontoxic
  compounds
• Breakdown is accelerated by enzymes
• Heavy drinking damages liver and other organs

3
Impacts, Issues Video
Alcohol, Enzymes, and Your Liver
4
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5
Energy Laws

• Energy the capacity to do work
• Total amount of energy in the universe is
  constant
• Energy flows from higher to lower energy forms

6
ENERGY LOST Energy continually flows from the sun.
ENERGY GAINED Sunlight energy reaches
environments on Earth. Producers secure some and
convert it to stored forms of energy. They and
all other organisms convert stored energy to
forms that can drive cellular work.
ENERGY LOST With each conversion, there is a
one-way flow of a bit of energy back to the
environment.
Fig. 4-1, p.59
7
One-way energy flow and materials cycling
8
ATP

• Main energy carrier in cells
• Can give up phosphate group to another molecule
• Phosphorylation energizes molecules to react

nucleotide base (adenine)
sugar (ribose)
3 phosphate groups
9
The Cells Energy Currency

• ATP couples energy inputs and outputs
• ATP/ADP cycle regenerates ATP

ATP
energy output
energy input
ADP Pi
10
ATP
base
three phosphate groups
cellular work (e.g., synthesis, breakdown, or
rearrangement of substances contraction of
muscle cells active transport across a cell
membrane)
sugar
ATP
reactions that release energy
reactions that require energy
ADP Pi
Fig. 4-2, p.59
11
The Role of ATP

• Structure of ATP

12
Energy Changes

• Endergonic reactions require energy
• Synthesis of glucose from carbon dioxide and
  water during photosynthesis
• Exergonic reactions release energy
• Breakdown of glucose to carbon dioxide and water
  by aerobic respiration

13
Energy in Glucose

glucose (product)
6O2
energy in
starting substances
6
6
Fig. 4-3a, p.60
14
Energy in Glucose
glucose 6O2 (starting substances)
energy out
6
products
6
Fig. 4-3b, p.60
15
Exergonic Reactions

• Energy changes in chemical work.

16
Electron Transfers

• Oxidation loss of an electron
• Reduction gain of an electron
• Electron transfer chains are vital to the
  formation of ATP during photosynthesis and
  aerobic respiration

17
Electron Transfer Chain
H2
1/2 O2
H2
1/2 O2
1) Energy input splits hydrogen into protons (H)
and electrons
2H
2e-
3) Some released energy is harnessed for
cellular work (e.g., making ATP)
electric spark
Explosive release of energy as heat that cannot
be harnessed for cellular work
2) Electrons transferred through electron
transfer chain
2e-
2H
1/2 O2
4) Spent electrons and free oxygen form water.
H2O
H2O
Fig. 4-4, p.60
18
Uncontrolled vs. Controlled Energy Release

• Controlling energy release

19
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20
Reactions Forward and Reverse

• Most chemical reactionsare reversible
• Direction of reaction depends upon
• Energy content of participants
• Reactant-to-product ratio

21
Chemical Equilibrium

• Reaction rate is the same in both directions
• Conversions continue, but proportions of reactant
  and product do not change
• Usually amounts of reactant and product are not
  equal

22
Chemical Equilibrium
highly spontaneous
equilibrium
highly spontaneous
Fig. 4-5, p.61
23
Chemical Equilibrium

• Chemical equilibrium

24
Metabolic Pathways

• Biosynthetic (anabolic) pathways
• Require energy inputs
• Assemble large molecules from subunits
• Photosynthesis
• Degradative (catabolic) pathways
• Release energy
• Breakdown large molecules to subunits
• Aerobic respiration

25
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26
Enzymes

• Catalyze (speed up) reactions
• Recognize and bind specific substrates
• Act repeatedly
• Most are proteins

27
Activation Energy

• For a reaction to occur, an energy barrier must
  be surmounted
• Enzymes make the energy barrier smaller

activation energy without enzyme
starting substance
activation energy with enzyme
energy released by the reaction
products
28
Activation Energy
reactants
energy barrier with no enzyme to promote reaction
energy barrier with an enzymes participation
products
Fig. 4-6, p.62
29
Activation Energy

• Activation energy

30
Factors Influencing Enzyme Activity

• Coenzymes and cofactors
• Allosteric regulators
• Temperature
• pH
• Salt concentration

31
Help From Cofactors
32
How Catalase Works

• How catalase works

33
Feedback Inhibition

• Product of pathway binds to and inhibits enzyme
  in the pathway

enzyme 2
enzyme 3
enzyme 4
enzyme 5
Excess molecules of end product bind to
molecules of an enzyme that catalyzes this
pathways first step. The greater the excess, the
more enzyme molecules are inhibited, and the
less tryptophan is synthesized.
enzyme 1
end product (tryptophan)
substrate
34
Feedback Inhibition

• Feedback inhibition

35
Effect of Temperature

• Small increase in temperature increases molecular
  collisions, reaction rates
• High temperatures disrupt bonds and destroy the
  shape of active site

36
Enzymes and the Environment
37
Effect of Temperature

• Enzymes and temperature

38
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39
Concentration Gradient

• Different numbers of molecules or ions in
  different regions
• Substances tend to move down gradient - from
  higher to lower concentration

40
Selective Permeability
oxygen, carbon dioxide, and other small,
nonpolar molecules some water molecules
glucose and other large, polar, water-soluble
molecules ions (e.g.,H, Na, K, Ca, Cl)
41
Cell Membranes Show Selective Permeability
Selective permeability
42
Diffusion

• Net movement of molecules or ions down a
  concentration gradient

dye
dye
water
Fig. 4-12, p.65
43
Diffusion
Diffusion of dye in water
44
Factors Affecting Diffusion Rate

• Steepness of concentration gradient
• Steeper gradient, faster diffusion
• Molecular size
• Smaller molecules, faster diffusion
• Temperature
• Higher temperature, faster diffusion
• Electrical or pressure gradients

45
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46
Transport Proteins

• Span the lipid bilayer
• Interior can open to either side
• Change shape when they interact with solute
• Move water-soluble substances across a membrane

47
Passive and Active Transport
Passive Transport
Active Transport

• Doesnt require energy inputs
• Solutes diffuse through a channel inside the
  proteins interior
• Net movement is down concentration gradient

• Requires ATP
• Protein is an ATPase pump
• Pumps solute against its concentration gradient

48
Passive Transport
glucose, more concentrated outside cell than
inside
glucose transporter
a Glucose binds to a vacant site inside the
channel through the transport protein.
d When the glucose binding site is again vacant,
the protein resumes its original shape.
c Glucose becomes exposed to fluid on other side
of the membrane. It detaches from the binding
site and diffuses out of the channel.
b Bound glucose makes the protein change shape.
Part of the channel closes behind the solute.
Another part opens in front of it.
49
Passive Transport
Passive transport
50
Active Transport
higher concentration of calcium ions outside cell
compared to inside
calcium pump
ATP
e The shape of the pump returns to its resting
position.
a An ATP molecule binds to a calcium pump.
Pi
ADP
b Calcium enters a tunnel through the pump, binds
to functional groups inside.
d The shape change permits calcium to be
released at opposite membrane surface. A
phosphate group and ADP are released.
c The ATP transfers a phosphate group to pump.
The energy input will cause pumps shape to
change.
51
Active Transport
Active transport
52
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53
Which Way Will Water Move?

• Water diffuses across cell membranes

54
Osmosis

• Diffusion of water molecules across a selectively
  permeable membrane, down the water concentration
  gradient
• Higher solute concentration lower water
  concentration

55
Osmosis
semipermeable membrane between two compartments
water molecules
protein molecules
Fig. 4-15, p.68
56
Solute concentration and osmosis
57
Tonicity

• Relative solute concentrations
• Hypertonic higher concentration
• Hypotonic lower concentration

58
Tonicity
2 sucrose solution
1 liter of distilled water
1 liter of 10 sucrose solution
1 liter of 2 sucrose solution
a
b
c
Hypotonic Solution
Hypertonic Solution
Isotonic Solution
Fig. 4-16, p.69
59
Tonicity
Tonicity and water movement
60
Hydrostatic Pressure

• Pressure that a fluid exerts against structure
  enclosing it
• Increases with increased solute concentration
• Influences the osmotic movement of water

61
Pressure and Osmosis
first compartment
second compartment
hypertonic solution
hypotonic solution
membrane permeable to water but not to solutes
fluid volume rises in second compartment
Fig. 4-17, p.69
62
Osmosis experiment
63
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64
Membrane Traffic

• Exocytosis
• Vesicle fuses with membrane, releasing substance
  into intracellular fluid

65
Membrane Traffic

• Endocytosis
• Membrane forms vesicle, bringing substance into
  cell

66
Endocytosis (vesicles in)
Exocytosis (vesicles out)
Fig. 4-20, p.71
67
Types of Endocytosis

• Bulk-phase endocytosis
• Receptor-mediated endocytosis
• Phagocytosis

68
Phagocytosis
69
Phagocytosis
70
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71
exocytosis (out from cytoplasm)
a
endocytosis (into cytoplasm)
b
Fig. 4-18, p.70
72
High
Concentration gradient across cell membrane
ATP
Low
Diffusion of lipid-soluble substances across
bilayer
Passive transport of water- soluble
substances through channel protein no energy
input needed
Active transport through ATPase requires
energy input from ATP
Fig. 4-20, p.71