Cell Energy: Photosynthesis and Respiration Section 1: Photosynthesis: Capturing and Converting Energy

Photosynthesis In the process of photosynthesis, plants convert the energy of sunlight into the energy in the chemical bonds of carbohydrates – sugars, and starches Put more simply, plants use the energy of sunlight to produce carbohydrates in a process called photosynthesis

Requirements for Photosynthesis Experiments reveal that in the presence of light, plants transform carbon dioxide and water into carbohydrates and release oxygen Usually produces the sugar glucose 6CO2 + 6H2O C6H12O6 + 6O2 light

Sunlight Nearly all organisms on Earth depend on the sun for energy Autotroph – organisms that are able to use a source of energy, such as sunlight, to produce food directly from simple inorganic substances in the environment Heterotroph – organisms that obtain energy from the foods they eat The sun bathes the Earth in a steady stream of light We see colorless “white” light but it is actually a mixture of different wavelengths of light Visible spectrum

Pigments Process of photosynthesis begins when light is absorbed by pigments in the plant cell Colored substances that absorb or reflect light Principal pigment in green plants is chlorophyll Absorbs red and blue light but does not absorb light in the middle region of the spectrum very well These wavelengths are reflected

Energy-Storing Compounds In a green plant, the energy of sunlight is transferred to electrons, raising them to a higher energy level The electrons belong to the pigment chlorophyll High-energy electrons are trapped in chemical bonds Two ways in which energy of sunlight is trapped in chemical bonds

Energy-Storing Compounds First way sunlight is trapped in chemical bonds Simpler of the two A pair of high-energy electrons are passed directly to an electron carrier A molecule that can accept a pair of electrons and later transfer them along with most of their energy to another compound Plants use the electron carrier NADP+ When NADP+ accepts a pair of high-energy electrons, it is converted to NADPH ONE WAY IN WHICH SOME OF THE ENERGY OF SUNLIGHT CAN BE TRAPPED IN CHEMICAL FORM

Energy-Storing Compounds Second way sunlight is trapped in chemical bonds Involves adenosine triphosphate (ATP) Consists of adenine, a 5-carbon sugar called ribose, and three phosphate groups During photosynthesis, green plants produce ATP, which is an energy-storing compound used by every living cell

As one might suspect, there are 3 phosphate groups. There is a high E bond between the 2nd and 3rd P group. When cells need E this high E bond is broken and E is released. It’s not ATP anymore. What is the new molecule formed?? ADP Notice the other two components of the the ATP molecule. Adenine and Ribose

E from the food a cell takes in is used to convert ADP back to ATP. ADP + phosphate    ATP by the enzyme ATP synthetase

Chapter 6: Cell Energy: Photosynthesis and Respiration Section 2: Photosynthesis: The Light and Dark Reactions

Photosynthesis: The Light and Dark Reactions The production of NADPH and ATP requires sunlight Light reactions – the energy of sunlight is captured and used to make energy-storing compounds Another set of reactions called the dark reactions uses the energy stored in NADPH and ATP to produce glucose Do not require light However, they can and do occur in the light also

The Light Reactions Photosynthesis takes place in the chloroplast Within the chloroplast are saclike photosynthetic membranes that contain chlorophyll Light reactions take place in these membranes Can be divided into four basic processes: light absorption, electron transport, oxygen production, and ATP formation

Light Absorption Photosynthetic membranes contain clusters of pigment molecules, or photosystems, that are able to capture the energy of sunlight Two photosystems in plants Photosystem I Photosystem II Each contains several hundred chlorophyll molecules as well as other accessory pigments Absorb light in the regions of the spectrum where chlorophyll does not

Light Absorption After light energy is absorbed by one of the pigment molecules in a photosystem, the energy is passed from one pigment molecule to the next until it reaches a special pair of chlorophyll molecules in the reaction center of the photosystem In the reaction center, high-energy electrons are released and are passed to the first of many electron carriers

Electron Transport High-energy electrons are transferred along a series of electron carriers Electron transportthe electron carriers themselves are known as the electron transport chain At the end of the chain, the electrons are passed to NADP+, converting it to NADPH

Oxygen Production The photosynthetic membrane contains a system that provides new electrons to chlorophyll to replace the ones that wound up in NADPH Four electrons are removed from two water molecules 4 H+ ions 2 O atoms Form a single molecule of oxygen gas Released into the air

ATP Formation H+ ions are released inside the photosynthetic membrane as well as being pumped across the membrane The inside of the membrane fills up with H+ ions Makes the outside negatively charged and the inside positively charged Forms ATP

A Summary of the Light Reactions Use water, ADP, NADP+ Produce O2, ATP and NADPH The dark reactions will convert these energy-storing molecules to a more convenient form

The Dark Reactions Light does not play a role in the dark reactions The series of chemical changes that make up the dark reactions is critical to living things Carbon dioxide is used to make organic compounds The dark reactions form a cycle called the Calvin cycle

The Calvin Cycle 5 carbon sugar (C5) combines with CO2 to form two 3 carbon compounds (C3) Relatively slow Uses the enzyme rubisco to speed up the process Using ATP and NADPH, the 3 carbon compounds are converted to PGAL (phosphoglyceraldehyde) 6 turns of the cycle to make one molecule of glucose

Chapter 6: Cell Energy: Photosynthesis and Respiration Section 3: Glycolysis and Respiration

Glycolysis – Breaking Down Glucose C6H12O6 + 6O2  6CO2 + 6H2O Gives off 3811 calories Glycolysis takes place in the cytoplasm of a cell In glycolysis, a series of enzymes catalyzes chemical reactions that change glucose, one step at a time, into different molecules

Respiration If oxygen is available, respiration can take place Aerobic process Respiration is the process that involves oxygen and breaks down food molecules to release energy Uses the pyruvic acid formed in glycolysis Often used as a synonym for breathing Takes place in the cell’s mitochondria

The Krebs Cycle First set of reactions in respiration Krebs cycle 2 carbon atoms added (from the breakdown of pyruvic acid) 2 carbon atoms removed (in 2 molecules of CO2) 3 molecules of NAD+ converted to NADH 1 molecule of FAD converted to FADH2 1 molecule of GDP converted to GTP

Electron Transport in the Mitochondrion High energy electrons from NADH and FADH2 are passed to electron transport enzymes in the mitochondrion Form an ETC along which electrons are passed Enzyme at the end of the chain combines e- from ETC, H+ ions from fluid inside the cell, and O2 to form H2O Oxygen is the final electron acceptor in respiration Is essential for obtaining energy from both NADH and FADH2

ATP Formation Electron transport involves the movement of hydrogen ions As enzymes accept electrons, they pump a hydrogen from the inside to the outside This movement powers the formation of ATP On average, the movement of a pair of electrons down the ETC produces enough energy to form 3 ATP from ADP More H+ ions outside This imbalance supplies the energy to make ATP from ADP

The Totals Glycolysis and respiration together produce a total of 36 ATP molecules

Obtaining Energy From Food Complex carbohydrates are broken down into simple sugars that are then converted into glucose The pathways we have discussed can be used to produce energy The cell can generate chemical energy in the form of ATP from just about any source

Breathing and Respiration Final acceptor for all electrons in respiration is oxygen Without oxygen, electron transport cannot operate, Krebs cycle stops, and ATP production stops With each breath we take, air flows into our lungs Oxygen has a critical role to play in the mitochondria of every cell

Energy in Balance Photosynthesis and respiration can be thought of as opposite processes Photosynthesis deposits energy Respiration withdraws energy

Chapter 6: Cell Energy: Photosynthesis and Respiration Section 4: Fermentation

Fermentation Fermentation is a process that enables cells to carry out energy production in the absence of oxygen Breakdown of glucose and release of energy in which organic substances are the final electron acceptors Fermentation is anaerobic—it does not require oxygen Fermentation enables cells to carry out energy production in the absence of oxygen Produces 2 ATP

Lactic Acid Fermentation In many cells, the pyruvic acid that accumulates as a result of glycolysis can be converted to lactic acid Lactic acid fermentation Pyruvic acid + NADH  lactic acid + NAD+ Lactic acid is produced in muscles during rapid exercise when the body cannot supply enough oxygen to tissues to produce all of the ATP that is required Causes a burning, painful sensation Large muscles quickly run out of oxygen Muscle cells begin to rapidly produce ATP by fermentation

Alcoholic Fermentation Another type of fermentation occurs in yeasts and a few other microorganisms Pyruvic acid is broken down to produce a 2 carbon alcohol and carbon dioxide Alcoholic fermentation Pyruvic acid + NADH  alcohol + CO2 + NAD+

Alcoholic Fermentation Particularly important to bakers and brewers Causes dough to rise and forms bubbles in beer and wine To brewers, alcohol is a welcomed byproduct of fermentation However, it is not desirable from a yeast cell’s point of view Alcohol is toxic When the level of alcohol reaches about 12 percent, yeast cells die Thus alcoholic beverages must be processed if higher concentrations of alcohol are desired