Chapter 9

Photosynthesis and Cellular Respiration

Energy and Living Things• Energy from the sun enters living systems when

plants, algae, and certain bacteria absorb sunlight. Some of the energy in sunlight is captured and used to make organic compounds that store energy and serves as food for organisms.

• Metabolism involves either using energy to build molecules or breaking down molecules in which energy is stored. Photosynthesis is the process by which light energy is converted to chemical energy by autotrophs (organisms that use energy from the sun or inorganic substances to make organic compounds.

Breaking Down Food for Energy• Organisms that must get energy from food

instead of directly from sunlight or inorganic substances are called heterotrophs.

• Cellular respiration is a metabolic process similar to burning fuel. It releases much of the energy in food to make ATP. This ATP provides cells with the energy they need to carry out the activities of life.

• In cells, chemical energy stored in food molecules is released gradually in a series of enzyme-assisted chemical reactions.

• When cells break down food molecules, some of the energy in the molecules is released as heat. Much of the remaining energy is stored in molecules of ATP. ATP delivers energy wherever energy is needed in a cell.

ATP Stores and Releases Energy• ATP (adenosine triphosphate) is a nucleotide

with two extra energy-storing phosphate groups. The removal of a phosphate group from ATP produces adenosine diphosphate, or ADP. Cells use the energy released by this reaction to power metabolism.

ATP -> ADP + P + energy

5.2, Photosynthesis• Plants, algae, and some bacteria capture about 1%

of the energy in the sunlight that reaches Earth and convert it to chemical energy through the process of photosynthesis.

Overview:1. Energy is captured from the sun2. Light energy is converted to chemical energy, which

is temporarily stored in ATP and NADPH.3. The chemical energy stored in ATP and NADPH

powers the formation of organic compounds, using carbon dioxide, CO2

• Photosynthesis occurs in the chloroplasts of plant cells and algae and in the cell membrane of certain bacteria.

3CO2 + H2O ->light-> C3H6O3 + 3O2

Stage One, Light Energy Absorbed

• 1st and 2nd stages of photosynthesis are sometimes called the “light reactions”. Without the absorption of light, these reactions would not take place.

• Light is a form of radiation, energy in the forms of waves that travel through space. When the sun shines on you, your body is bombarded by many kinds of radiation from the sun. However, you can see only radiation known as visible light.

Pigments Absorb Different Wavelengths of Light

• Pigments (light absorbing) absorb only certain wavelengths and reflect all others. Chlorophyll, the primary pigment involved in photosynthesis, absorbs mostly blue and red light and reflects green and yellow. This makes many plants, especially their leaves, look green.

• Pigments that produce yellow and orange fall leaf colors as well as on many fruits/veggies are called carotenoids. Carotenoids absorb wavelengths of light differently from those absorbed by chlorophyll, so using both pigments enables plants to absorb more light energy during photosynthesis.

Production of Oxygen• Pigments involved in plant photosynthesis are

located in the chloroplasts of leaf cells. Clusters of pigments are embedded in the membranes of disk-shaped structures called thylakoids.

• When light strikes a thylakoid, energy is transferred to electrons in chlorophyll and other pigments. This energy transfer causes the electrons to jump to a higher energy level. Electrons with extra energy are said to be “excited”.

• Excited electrons jump from chlorophyll molecules to other nearby molecules in the thylakoid membrane, where the electrons are used to power the 2nd stage of photosynthesis. The excited electrons that leave chlorophyll molecules must be replaced by other electrons. Plants get these replacements from water molecules. Water molecules are split by an enzyme inside the thylakoid.

Stage Two, Light Energy to Chemical Energy

• Excited electrons that leave chlorophyll molecules are used to produce new molecules that temporarily store chemical energy (like ATP). The electron is passed through a series of molecules along the thylakoid membrane. The series of molecules through which excited electrons are passed along a thylakoid membrane are called electron transport chains.

Action of Electron Transport Chains

• Electron transport chain contains a protein that acts as a membrane pump. Excited electrons lose some of their energy as they each pass through this protein. The energy lost by the electrons is used to pump hydrogen ions, H+, into the thylakoid.

• As H+ pass through the channel portion of the protein, the protein catalyzes a reaction in which a phosphate group is added to a molecule of ADP, making ATP. This creates the ATP needed to complete the 3rd stage of photosynthesis.

• While one electron transport chain provides energy used to make ATP, a second electron transport chain provides energy used to make NADPH. NADPH is an electron carrier that provides the high energy electrons needed to make carbon-hydrogen bonds in the 3rd stage.

• Pigment molecules in the thylakoids of chloroplasts absorb light energy. Electrons in the pigments are excited by light and move through electron transport chains in the thylakoid membranes. These electrons are replaced by electrons from water molecules. The H+ accumulates to provide energy to make ATP and NADPH and the O2 is a byproduct.

Stage Three, Energy is Stored

• Carbon atoms from carbon dioxide in the atmosphere are used to make organic compounds, in which chemical energy is stored. The transfer of carbon dioxide to organic compounds is called carbon dioxide fixation.

• The reactions that “fix” carbon dioxide are sometimes called “dark reactions” or light-independent reactions. (This does NOT occur only at night. It means it doesn’t need light).

The Calvin CycleThe most common method of carbon dioxide fixation is the

Calvin cycle. The Calvin cycle is a series of enzyme-assisted chemical reactions that produces a 3-carbon sugar (3 CO2s to run one cycle).

• In carbon dioxide fixation, each molecule of CO2 is added to a 5-carbon compound by an enzyme.

• The resulting 6-carbon compound splits into two 3-carbon compounds. Phosphate groups from ATP and electrons from NADPH are added to the 3-carbon compounds, forming 3-carbon sugars.

• One of the resulting 3-carbon sugars is used to make organic compounds in which energy is stored for later use.

• The other 3-carbon sugars are used to regenerate the initial 5-carbon compound, thereby completing the cycle.

5.3,Cellular Respiration• Like in most organisms, your cells transfer

the energy in organic compounds, especially glucose, to ATP through a process called cellular respiration.

• Oxygen makes the production of ATP more efficient, although some ATP is made without oxygen. Metabolic processes that require oxygen are called aerobic. Metabolic processes that do not require oxygen are called anaerobic (means “without air”).

C6H1206 +6O2 -> 6CO2 + 6H2O + ATP

• Glucose is converted to pyruvate creating a small amount of ATP and NADPH.

• Aerobic respiration (produces the most ATP) occurs in the mitochondria of eukaryotic cells and in the cell membrane of prokaryotic cells. When oxygen is not present, pyruvate is converted to either lactate or ethanol and CO2.

Stage One, Glucose is Broken by Glycolysis

• In the 1st stage of cellular respiration, glucose is broken down in the cytoplasm, during a process called glycolysis. Glycolysis is an enzyme-assisted anaerobic process that breaks down one 6-carbon molecule of glucose to two 3-carbon pyruvates.

• Glycolysis uses two ATP molecules but produces four ATP molecules, yielding a net gain of two ATP molecules.

Stage Two, More ATP is Made• When oxygen is present, pyruvate produced during

glycolysis enters a mitochondrion and is converted to a 2-carbon compound. This produces a carbon dioxide molecule, one NADH molecule, and a 2-carbon acetyl group. The acetyl group attaches to a coenzyme A (CoA) forming the compound acetyl-CoA.

• Acetyl-CoA enters a series of enzyme-assisted reactions called the Krebs cycle.

•When the Krebs cycle is completed, the 4-carbon compound that began the cycle has been recycled, and acetyl-CoA can enter the cycle again.

•This is also known as the Citric Acid Cycle.

Electron Transport Chain• In aerobic respiration, electrons donated by

NADH and FADH2 pass through an electron transport chain.

• In eukaryotes, the electron transport chain occurs in the inner membranes of mitochondria. The energy of these electrons is used to pump hydrogen ions out of the inner mitochondrial compartment.

Fermentation Follows Glycolysis in the Absence of Oxygen

• When oxygen is not present, NAD+ is recycled another way. Under anaerobic conditions, electrons carried by NADH are transferred to pyruvate produced during glycolysis.

• The recycling of NAD+ using an organic hydrogen acceptor is called fermentation. Two important types of fermentation are lactic acid fermentation and alcoholic fermentation.

Lactic Acid Fermentation•A 3-carbon pyruvate is converted to a three carbon lactate through lactic acid fermentation.•Fermentation enables glycolysis to continue producing ATP in muscles as long as the glucose supply lasts. Blood removes excess lactate from muscles. Lactate can build up in muscle cells if it is not removed quickly enough, sometimes causing muscle soreness.

Alcoholic Fermentation• 3-carbon pyruvate is broken down to ethanol, a 2-carbon

compound, through alcoholic fermentation. (Carbon dioxide is released).

• First, pyruvate is converted to a 2-carbon compound, releasing carbon dioxide. Second, electrons are transferred from a molecule of NADH to the 2-carbon compound, producing ethanol. (NAD+ is recycled)

• Alcoholic fermentation by yeast, a fungus, has been used in the preparation of many foods and beverages. Wine and beer contain ethanol made during alcoholic fermentation by yeast. Bread dough rising and the carbonation of some alcoholic beverages is actually caused by the releasing of CO2.

• *Just for Fun* Ethanol is actually toxic to yeast. Ethanol kills yeast at a concentration of about 12%. Thus, naturally fermented wine contains about 12% ethanol

Comparing Anaerobic Processes with Aerobic Respiration

• The total amount of ATP that a cell is able to harvest from each glucose molecule that enters glycolysis depends on the presence of absence of oxygen.

• There is a total of 38 ATP made from cellular respiration, none are produced in fermentation.

• When oxygen is present, respiration occurs. When oxygen is not present, fermentation occurs instead.

• Most of a cells ATP is made during aerobic respiration.

End of Chapter 5

• Know your key terms.

• Know the steps to photosynthesis (light-dependent and light-independent)

• Know the steps in aerobic respiration (Krebs cycle)

• Know what happens if O2 is not present.

• Be able to describe ATP