Energy and Life. 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 and Life

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

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.

Figure 6.1 Potential and kinetic energy

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

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.

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

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

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

Chemical reactions and activation energy

Figure 6.4 (a) Figure 6.4 (b)

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.

(c) Catalyzed reaction

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

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.

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

Figure 6.6 How Enzymes Work

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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


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

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

Figure 6.9 Allosteric enzyme regulation


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

How enzymes can be inhibited

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


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


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

Chapter 7

Photosynthesis

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


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


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


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


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

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

Figure 7.1 Photons of different energy: the electromagnetic spectrum

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

Absorption spectra of chlorophylls and carotenoids

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

Figure 7.7 How a photosystem works

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.

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.


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.


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.


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.


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.

Figure 7.6 Plants use two photosystems


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

Plant response in hot weather


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

Figure 7.12 Carbon fixation in C4 plants


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

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.

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Energy and Life. 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