energy and life. the flow of energy in living cells energy is the ability to do work energy is...
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Energy and Life
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The Flow of Energy in Living Cells
Energy is the ability to do work
Energy is considered to exist in two states kinetic energy
the energy of motion potential energy
stored energy that can be used for motion
All the work carried out by living organisms involves the transformation of potential energy to kinetic energy
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The Flow of Energy
Cellular activity requires energy.Energy is defined as the capacity to do work.Kinetic energyPotential energyThe study of energy is called
thermodynamics.
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Figure 6.1 Potential and kinetic energy
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The Flow of Energy in Living Things
There are many forms of energy but all of them can be converted to heat
Heat energy is the most convenient form of energy to measure
Thermodynamics is the study of energy or heat changes
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Laws of Thermodynamics
Laws of thermodynamics govern the energy changes that are involved with any activity by an organism
The First Law of Thermodynamics: Energy cannot be created nor destroyed; it can
undergo conversion from form to another. Energy is lost during the conversion.
The Second Law of Thermodynamics Disorder (entropy) in the universe is increasing. Energy from the sun is converted to heat or random
molecular motion.
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The Flow of Energy in Living Things
Energy from the sun is captured by some types of organisms and is used to build molecules
These molecules then posses potential energy that can be used to do work in the cell
Chemical reactions involve the making and breaking of chemical bonds
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Chemical Reactions
The starting molecules of a chemical reaction are called the reactants or, sometimes, substrates
The molecules at the end of a reaction are called the products
There are two kinds of chemical reactions endergonic reactions have products with more energy
than the reactants these reactions are not spontaneous
exergonic reactions have products with less energy than the reactants these reactions are spontaneous
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Chemical Reactions
All chemical reactions require an initial input of energy called the activation energy the activation energy initiates a chemical reaction
by destabilizing existing chemical bonds
Reactions become more spontaneous if their activation energy is lowered this process is called catalysis catalyzed reactions proceed much faster than non-
catalyzed reactions
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Chemical reactions and activation energy
Figure 6.4 (a) Figure 6.4 (b)
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Chemical Reactions
Reactions that occur on their own are called exogonic and release energy
Reactions that need assistance to start are endogonic and require energy. (Activation energy)
Activation energy is needed by endogonic reactions to destabilize bonds and cause the reaction to occur.
Catalysis is the process of lowering activation energy…helps both exogonic and endogonic reactions.
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(c) Catalyzed reaction
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How Enzymes Work
Enzymes are the catalysts used by cells to perform particular reactions enzymes bind specifically to a molecule and stress the
bonds to make the reaction more likely to proceed active site is a site on the surface of the enzyme that
binds to a reactant the site on the reactant that binds to an enzyme is
called the binding site
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Enzymes
Allosteric sites are the points where signal molecules bind to control the rate of enzyme activity.
Metal ions act as cofactors to aid catalysis.Nonprotein organic molecules called coenzymes
aid catalysis.Coenzymes carry energy-bearing electrons in
biochemical reactions (NAD NADH)Enzymes need optimal temperature and pH to
operate effectively..these are specific to each enzyme.
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How Enzymes Work
The binding of a reactant to an enzyme causes the enzyme’s shape to change slightly this leads to an “induced fit” where the enzyme and
substrate fit tightly together as a complex the enzyme lowers the activation energy for the
reaction while it is bound to the reactant the enzyme is unaffected by the chemical reaction and
be re-used
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Figure 6.6 How Enzymes Work
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Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
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Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
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How Enzymes Work
Catalyzed reactions may occur together in sequence the product of one
reaction is the substrate for the next reaction until a final product is made
the series of reactions is called a biochemical pathway
Figure 6.7
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Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
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How Enzymes Work
Temperature and pH affect enzyme activity enzymes function within an optimum temperature
range when temperature increases, the shape of the enzyme
changes due to unfolding of the protein chains enzymes function within an optimal pH range
the shape of enzymes is also affected by pH most enzymes work best within a pH range of 6 - 8
exceptions are stomach enzymes that function in acidic ranges
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How Cells Regulate Enzymes
Cells can control enzymes by altering their shape allosteric enzymes are affected by the binding of
signal molecules the signal molecules bind on a site on the enzyme called
the allosteric site some signals act as repressors
inhibit the enzyme when bound other signals act as activators
change the shape of the enzyme so that it can bind the substrate
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Figure 6.9 Allosteric enzyme regulation
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Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
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6.5 How Cells Regulate Enzymes
Feedback inhibition is a form of enzyme inhibition where the product of a reaction acts as a repressor competitive inhibition
the inhibitor competes with the substrate for the active site
the inhibitor can block the active site so that it cannot bind substrate
non-competitive inhibition the inhibitor binds to the allosteric site and changes the
shape of the active site so that no substrate can bind
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How enzymes can be inhibited
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ATP: The Energy Currency of the Cell
The energy from the sun or from food sources must be converted to a form that cells can use adenosine triphosphate (ATP) is the energy
currency of the cell
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ATP: The Energy Currency of the Cell
The structure of ATP suits it as an energy carrier
each ATP molecule has three parts1. a sugar2. an adenine nucleotide3. a chain of three phosphate groups
the phosphates are negatively charged and it takes a lot of chemical energy to hold them together
the phosphates are poised to come apart
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ATP: The Energy Currency of the Cell
ATP cycles in the cell with respect to its energy needs photosynthesis
some cells convert energy from the sun into ATP and then use it to make sugar where it is stored as potential energy
cellular respiration cells break down the
potential energy in sugars and convert it ATP
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Chapter 7
Photosynthesis
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An Overview of Photosynthesis
Most of the energy used by almost all living cells ultimately comes from the sun plants, algae, and some bacteria capture the sunlight
energy by a process called photosynthesis only about 1% of the available energy in sunlight is
captured
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An Overview of Photosynthesis
The leaf cells of plants contain chloroplasts the chloroplast contains internal membranes called
thylakoids the thylakoids are stacked together in columns called
grana the stroma is a semiliquid substance that surrounds
the thylakloids
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An Overview of Photosynthesis
The photosystem is the starting point of photosynthesis it is a network of pigments in the membrane of the
thylakoid the primary pigment of a photosystem is chlorophyll the pigments act as an antenna to capture energy from
sunlight individual chlorophyll pigments pass the captured energy
between them
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An Overview of Photosynthesis
Photosynthesis takes places in three stages
1. capturing energy from sunlight
2. using the captured energy to produce ATP and NADPH
3. using the ATP and NADPH to make carbohydrates from CO2 in the atmosphere
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An Overview of Photosynthesis
The overall reaction for photosynthesis may be summarized by this equation
6 CO2 + 12 H2O + light C6H12O6 + 6 H2O + 6 O2
The process of photosynthesis is divided into two types of reactions
light-dependent reactions take place only in the presence of light and produce ATP and NADPH
light-independent reactions do not need light to occur and result in the formation of organic
molecules more commonly known as the Calvin cycle
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How Plants Capture Energy from Sunlight
Light is comprised of packets of energy called photons sunlight has photons of varying energy levels
○ the possible range of energy levels is represented by an electromagnetic spectrum
human eyes only perceive photons of intermediate energy levels○ this range of the spectrum is known as visible light
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Figure 7.1 Photons of different energy: the electromagnetic spectrum
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How Plants Capture Energy from Sunlight
Pigments are molecules that absorb light energy the main pigment in plants is chlorophyll
○ chlorophyll absorbs light at the ends of the visible spectrum, mainly blue and red light
plants also contain other pigments, called accessory pigments, that absorb light levels that chlorophyll does not○ these pigments give color to flowers, fruits, and
vegetables○ they are present in leaves too but are masked by
chlorophyll, until the fall when the chlorophyll is broken down
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Absorption spectra of chlorophylls and carotenoids
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Organizing Pigments into Photosystems
In plants, the light-dependent reactions occur within a complex of proteins and pigments called a photosystem light energy is first captured by any one of the
chlorophyll pigments the energy is passed along to other pigments
until it reaches the reaction center chlorophyll molecule
the reaction center then releases an excited electron, which is then transferred to an electron acceptor
the excited electron that is lost is then replaced by an electron donor
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Figure 7.7 How a photosystem works
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Organizing Pigments into Photosystems
Plants use two photosystems in series to generate power to reduce NADP+ to NADPH with enough energy left over to generate ATP.
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Photosystem Conversion of Light to Chemical Energy
Plants use two photosystems in a two-stage process called noncyclic photophosphorylation.
For every pair of electrons obtained from water, one molecule of NADPH and a little over one molecule of ATP are produced.
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Photosystem Conversion of Light to Chemical Energy
Photosystem II Reaction center is called P680
Oxygen atoms of two water molecules bind to magnesium, causing water to split
Oxygen is released while electrons from water are used to replace those that are boosted from the reaction center by sunlight.
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Photosystem Conversion of Light to Chemical Energy
Path to Photosystem I: The electron taken from the Photosystem II is carried
to photosystem I by several intermediates.Making ATP: Chemiosmosis
Protons cross the thylakoid membranes at embedded proton pumps causing ADP to be phosphorylated to ATP.
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Photosystem Conversion of Light to Chemical Energy
Photosystem I: The reaction center is P700
The electron from photosystem II is boosted to an even higher energy level as light strikes Photosystem I.
The electron is passed to another carrier.
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Photosystem Conversion of Light to Chemical Energy
Making NADPH Electrons transported from Photosystem I are
used to reduce NADP+ to NADPH.Making More ATP
While the electron is passed from water to NADPH, one molecule of NADPH and more than one molecule of ATP are generated.
If more ATP is needed, some plants can by-pass Photosystem I and switch to cyclic photophosphorylation.
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Figure 7.6 Plants use two photosystems
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Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
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Photorespiration: Putting the Brakes on Photosynthesis
Many plants have trouble carrying out C3 photosynthesis when it is hot plants close openings in their leaves, called
stomata (singular, stoma), in order to prevent water loss
the closed stoma also prevent gas exchange O2 levels build up inside the leaves while the
concentrations of CO2 fall the enzyme rubisco fixes oxygen instead of carbon this process is called photorespiration and short-
circuits the Calvin cycle and photosynthesis
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Plant response in hot weather
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Photorespiration: Putting the Brakes on Photosynthesis
Some plants have adapted to hot climates by performing C4 photosynthesis these C4 plants include sugarcane, corn, and many
grasses they fix carbon using different types of cells and
reactions than C3 plants and do not run out of CO2 even in hot weather CO2 becomes trapped in cells called bundle-sheath
cells
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Figure 7.12 Carbon fixation in C4 plants
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Photorespiration: Putting the Brakes on Photosynthesis
Another strategy to avoid a reduction in photosynthesis in hot weather occurs in many succulent (water-storing) plants, such as cacti and pineapples these plants undergo crassulacean acid
metabolism (CAM) photosynthesis occurs via the C4 pathway at night and
the C3 pathway during the day
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A Review of Cellular Respiration
Through respiration, one molecule of glucose generates a total of 36 ATP.
The control of the process works through a system of feedback inhibition in which key enzymes in the Krebs Cycle become stuck.
The binding of the ATP causes the enzyme to change its form and not function as an enzyme.