8-1 Energy and Life
Energy is the ability to do work, and there is plenty of work to be done at
the cellular level.
Autotrophs and Heterotrophs
• Plants and some other types of organisms are able to use light energy from
the sun to produce food- autotrophs.
• Other organisms, such as animals, cannot use the sun’s energy directly.
These organisms obtain energy from the foods they consume- heterotrophs.
Chemical Energy and ATP
• Energy comes in many forms, including light, heat, and electricity.
• Energy can be stored in chemical compounds.
• One of the principle chemical compounds that cells use to store and release
energy is adenosine triphosphate (ATP).
1. Adenine
2. 5-carbon sugar (ribose)
3. phosphate groups
Storing Energy
• adenosine diphosphate (ADP), has two phosphate groups.
• Adding a phosphate group to ADP = ATP.
Releasing energy
• Breaking of the chemical bond between the second and third phosphates
releases energy.
• The characteristics of ATP make it exceptionally useful as the basic energy
source of all cells
Using Biochemical Energy
• One way cells use the energy provided by ATP is to carry out active transport.
o Many cell membranes contain sodium (Na)-potassium (K) pump, a membrane
protein that pumps Na+ ions out of the cell and K+ ions into it. ATP provides
the energy to keep the pump working.
• ATP provides movement, providing the energy for motor proteins.
• Energy from ATP powers the synthesis of proteins and nucleic acids and
responses to chemical signals at the cell surface.
• Most cells have only a small amount of ATP, enough to last them a few
seconds of activity.
• ATP is a great molecule for transferring energy, not storing large amounts
of energy.
• A single molecule of glucose stores more than 90 times the chemical energy
of a molecule of ATP.
• Cells can regenerate ATP from ADP as needed by using the energy in foods
like glucose.
8-2 Photosynthesis: An Overview
Investigating Photosynthesis
Van Helmont’s Experiment (1643):
• Concludes that trees gain most of their mass from water.
• Explains hydrate portion of carbohydrates, but does not explain
carbo-portion which comes from CO2.
Priestley’s Experiment (1771):
• Using a bell jar, a candle, and a plant, Priestly concludes that the plant
releases oxygen.
Jan Ingenhousz (1779):
• Observes that aquatic plants produce oxygen bubbles in the light but not in
the dark, he concludes that plants need sunlight to produce oxygen.
The Photosynthesis Equation:
light
6CO2 + 6H2O C6H12O6 + 6O2
Light and Pigments
• In addition to water and carbon dioxide, photosynthesis requires light and
chlorophyll, a molecule in chloroplasts.
• Plants gather the sun’s energy with light-absorbing molecules called pigments.
• The principal pigment in plants is chlorophyll.
o Chlorophyll a
o Chlorophyll b
8-3 The Reactions of Photosynthesis
Inside a Chloroplast
• In plants photosynthesis takes place inside the chloroplasts.
• The chloroplasts contain saclike photosynthetic membranes called thylakoids.
• Thylakoids are arranged in stacks known as grana (singular: granum).
• Proteins in the thylakoid membrane organize chlorophyll and other pigments
into clusters known as photosystems.
o Photosystems are the light-collecting units of the chloroplast.
• Photosynthesis occurs in two stages:
1. Light-dependent reactions (photosystems- thylakoid membranes)
Use energy from light to produce ATP and NADPH.
Produce O2 gas and convert ADP and NADP+ into the energy carriers ATP and NADPH
Electron Carriers
• When sunlight excites electrons in chlorophyll, electrons gain energy.
• High-energy electrons need a carrier molecule- NADP+ (nicotinamide adenine
dinucleotide phosphate).
• NADP+ accepts and holds 2 high-energy electrons along with a hydrogen ion
(H+), this converts NADP+ into NADPH.
• NADPH is one way in which some of the energy of sunlight can be trapped in
chemical form.
• NADPH can then carry high-energy electrons produced by light absorption in
chlorophyll to chemical reactions elsewhere in the cell.
2. Light-independent reactions (Calvin cycle –stroma)
• Stroma- region outside the thylakoid membrane
The Calvin Cycle (Light-Independent Reactions)
• During the Calvin Cycle, plants use the energy from that ATP and NADPH
contain to build high-energy compounds that can be stored for later use.
• The Calvin cycle uses ATP and NADPH from the light-dependent reactions to
produce high-energy sugars.
Factors Affecting Photosynthesis
1. Shortage of water can slow or even stop photosynthesis.
2. Temperature- Photosynthesis depends on enzymes that function best between
0C and 35C.
3. Intensity of light – increasing light intensity increases the rate of
photosynthesis. After a certain level the plant reaches a maximum rate of
photosynthesis.
