Download Chapter 1 • Lesson 7

Chapter 1 • Lesson 7
Objective 4.2.1
Energy in Cells
Key terms
• ATP • ATP-ADP cycle • photosynthesis • chloroplast • chlorophyll • Calvin cycle
• cellular respiration • aerobic • anaerobic • fermentation
Getting the Idea
Chemical energy is stored in the chemical bonds that hold carbohydrates and other organic
compounds together. Cells release this energy through cellular respiration. Organisms then use
the energy to carry out a variety of activities.
Cells Need Energy
All cells need energy to carry out their life processes. For example, cells use energy to make
new molecules such as enzymes and other proteins. Cells also use energy to build and repair
their organelles and plasma membranes. Many processes involved in maintaining homeostasis,
such as the movement of materials into and out of a cell, also require energy. In animals, nerve
cells use energy to transmit impulses that direct activities in other parts of the body. Some of
these impulses are sent to muscle cells, which need energy for movement.
How do cells get and store all the energy they need to function? Recall from Lesson 1 that
mitochondria in eukaryotic cells release the energy the cell needs. When a cell needs energy,
chemical energy stored in glucose is released and used to produce adenosine triphosphate, or
ATP. ATP is an organic molecule that is used for short-term energy storage and transport in the
cell. Energy is transferred from glucose to ATP. The ATP then delivers the energy to the places
in the cell where it is needed.
ATP and Energy
ATP is a nucleotide with two extra phosphate groups. Recall from Lesson 3 that a nucleotide
consists of a nitrogenous base, a sugar molecule, and a phosphate group. In ATP, the
phosphate group is a molecule of phosphoric acid. The nitrogenous base in ATP is adenine.
The sugar is ribose. A total of three phosphate groups are bound to the ribose in a chain. The
phosphate tail of the ATP molecule holds the usable energy.
To release the stored energy, bonds between the phosphates in ATP must be broken. To break
the bonds, a water molecule is added in a process called hydrolysis. In most cases, only one
phosphate is removed from the ATP molecule to release energy. The ATP turns into a molecule
called adenosine diphosphate, or ADP, which contains only two phosphates. ADP can be
recombined with a free phosphate to form a new molecule of ATP. The process of combining
ADP with a free phosphate is called phosphorylation.
ATP cannot be stored for later use. Instead, ADP is constantly recombined with phosphates to
form new molecules of ATP, which give cells the energy they need. This continuous process,
which is illustrated below, is called the ATP-ADP cycle.
A good way to understand ATP is to think about a rechargeable battery. A rechargeable battery
may start out filled with chemical energy. As the battery is used, it gives up some of this energy,
usually as electrical energy or heat. The depleted battery is then recharged by giving it more
energy, making the battery ready to be used again. ATP is the higher-energy form in this cycle
and is like the recharged battery. ADP is the lower-energy form, much like the used battery.
When the phosphate bond is broken, releasing stored energy, the ATP becomes ADP. Energy
is then added when the ADP picks up a free phosphate, recharging itself and turning back into
ATP.
Photosynthesis—Trapping the Energy Needed for Life
Most of the energy needed for life on Earth is gathered through photosynthesis.
Photosynthesis is the process by which some organisms capture the energy of sunlight and
use it to make food in the form of glucose. Plants, algae, and some bacteria carry out
photosynthesis. Organisms that do not carry out photosynthesis (animals, fungi, and some
protists and bacteria) get their energy from photosynthetic organisms. You will learn more about
this flow of energy in Lesson 14.
Photosynthetic organisms trap the energy in sunlight and use it to convert carbon dioxide (CO2)
and water (H2O) into glucose (C6H12O6) and oxygen gas (O2). Recall from Lesson 4 that
photosynthesis can be summarized in the following equations:
Recall that photosynthesis takes place in cell organelles called chloroplasts. Chloroplasts
contain pigments that absorb light energy. The main pigment involved in photosynthesis is
chlorophyll, the green pigment that gives plants their color.
Photosynthesis involves two different series of reactions, light-dependent and light-independent.
The light-dependent reactions use the energy absorbed by chlorophyll and other pigments to
produce ATP and another energy carrier, NADPH. These molecules are then used in the lightindependent reactions.
The light-independent reactions of photosynthesis are called the Calvin cycle. The Calvin cycle
is a series of reactions that form the simple sugar glucose from carbon dioxide and water.
During the Calvin cycle, a cell uses ATP and NADPH to rearrange the atoms of water and
carbon dioxide to form glucose and oxygen. This is the "synthesis" part of photosynthesis.
Cellular Respiration
Photosynthesis is one of the most important chemical processes for life on Earth. A second,
equally important process is cellular respiration. Cellular respiration is a process by which cells
release the energy stored in the chemical bonds of food molecules. Cellular respiration releases
energy from sugars, fats, amino acids, and nucleotides. However, the sugar glucose is the most
important source of energy in cells.
The equation below summarizes cellular respiration.
The equation shows that during cellular respiration, organisms use glucose and oxygen to
produce carbon dioxide, water, and energy in the form of ATP. If you compare the equations for
cellular respiration and photosynthesis, you will see that they are inverse processes. The
products of each reaction serve as the reactants for the other.
Anaerobic Respiration
Cellular respiration is an aerobic process. An aerobic process is one that requires oxygen. By
contrast, an anaerobic process is one that does not require oxygen. Fermentation is a process
that enables cells to release energy in the absence of oxygen. There are two important types of
fermentation: lactic acid fermentation and alcohol fermentation.
Before either type of fermentation begins, enzymes break down a glucose molecule, producing
two molecules of pyruvic acid. A cell needs two ATP molecules to start this process, which
occurs in the cytoplasm and does not require oxygen. By the end of this process, the cell
produces four ATP molecules, for a net gain of two ATP molecules.
During lactic acid fermentation, an enzyme converts pyruvic acid into a compound called lactic
acid. Most cells cannot use lactic acid and excrete it as a waste. Lactic acid fermentation is
used to manufacture certain foods, such as yogurt and cheese. The process also occurs in
muscle cells during periods of strenuous exercise, when not enough oxygen may be available
for these cells to carry out aerobic respiration. For example, if you are jogging, your leg muscles
need more ATP than when you are at rest. If your muscles do not have enough oxygen, they
produce a form of lactic acid called lactate. The lactate is later converted back to pyruvate or
glucose.
During alcohol fermentation, some single-celled organisms convert pyruvic acid into ethyl
alcohol and carbon dioxide. The ethyl alcohol and carbon dioxide are excreted as wastes. This
type of fermentation, which occurs in yeast, is used to produce wine and beer, as well as bread.
Comparing Aerobic and Anaerobic Respiration
All organisms rely on respiration to meet their energy needs. Many prokaryotes rely on
anaerobic respiration—either lactic acid fermentation or alcohol fermentation—for their energy.
Fermentation is much less efficient than cellular respiration. During anaerobic respiration, only
two ATP molecules are produced from each molecule of glucose—those produced when
glucose is broken down to form pyruvic acid. The fermentation reactions themselves do not
produce any more ATP.
Aerobic cellular respiration is more efficient than anaerobic respiration. Most eukaryotes use
aerobic respiration to meet their energy needs when oxygen is available. Like anaerobic
respiration, this process begins in the cytoplasm, as enzymes break down glucose to form two
molecules of pyruvic acid. When oxygen is present, the pyruvic acid then moves into the
mitochondria, and cellular respiration continues. Unlike fermentation, which nets only two
molecules of ATP for each molecule of glucose, aerobic cellular respiration produces a net 36 to
38 molecules of ATP for each molecule of glucose.
Cellular respiration, fermentation, and photosynthesis are all chemical processes. Whether
these reactions are possible and the rates at which they occur are affected by factors such as
pH, temperature, light, and the availability of reactants. For example, you have read that muscle
cells obtain energy from lactic acid fermentation rather than cellular respiration when oxygen is
not available in sufficient amounts. Similarly, photosynthesis is slower when a plant receives
less sunlight.