Chapter 6: Cell Energy: Photosynthesis and Respiration

Chapter 6: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

Photosynthesis • An understanding of photosynthesis was developed from studies of plant growth • Dutch physician Jan Van Helmont devised an experiment to determine how plant growth actually works • Found the mass of a pot of dry soil and a small seedling • Planted the seedling in the pot of soil • Watered regularly for 5 years • Gained about 75 kilograms • Mass of soil was unchanged • Concluded that most of the mass must come from water because that is all he added to the pot

Photosynthesis • Although Van Helmont did not realize it, carbon dioxide in the air made a major contribution to the mass of his tree • It is the carbon in carbon dioxide that is used to make carbohydrates in photosynthesis

Photosynthesis • Almost 100 years after Van Helmont’s experiment, Joseph Priestley performed an experiment that would give another insight into the process of photosynthesis • Took a candle, placed a glass jar over it, and watched as the flame gradually died out • Something in the air was necessary to keep a candle burning • When that substance was used up, the candle went out • oxygen

Photosynthesis • Priestley then placed a spring of mint under the jar and allowed a few days to pass, the candle could be relighted and would remain lighted for awhile • The mint had produced the substance required for burning • oxygen • Later, Dutch scientist Jan Ingenhousz showed that this only occurred when the plant was exposed to light

Requirements for Photosynthesis • These 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

Chapter 6: Cell Energy: Photosynthesis and Respiration