Cellular Respiration & Photosynthesis

AP Biology

Photosynthesis• Photosynthesis is the process of converting energy

from sunlight into energy in chemical bonds:

• Autotrophs are able to make their own food (glucose)– Ex: Plants, mosses, some protists, kelp, algae,

cyanobacteria

• The process of photosynthesis occurs in the chloroplast an organelle in autotrophs.

• Chloroplasts have – sac structures called thylakoids. (A stack of thylakoids

is called a granum (grana))– fluid where sugars are made called stroma

• The light reactions of photosynthesis occur on the thylakoid membranes- the dark reactions occur in the stroma.

Light • Light is unique in that it acts as a wave, but also acts as a

particle (called a photon). • Plants utilize blue and red light, but reflect green light (which

is why they appear green to us)

Pigments• A pigment molecule is able to absorb energy from light within a narrow

range of wavelengths. The process of photosynthesis begins with light- absorbing pigments in plant cells.

• Because of this, plants use a variety of pigments to absorb different wavelengths. These include:

Chlorophyll a- absorbs violet and red (reflects green and yellow)

Chlorophyll b- absorbs violet and red (reflects green and blue)

The carotenoids- reflect red, orange and yellow *accessory pigment

Xanthophylls- reflect yellow, brown, blue and purple*accessory pigment

Pigments in action• When light is absorbed into one of these pigments the

energy from the light is incorporated into electrons within the atoms of that pigment molecule.

• These energized/excited electrons are unstable and almost immediately re-emit the absorbed energy.

• This energy bounces from one pigment molecule to another.

• The process ends when the energy is absorbed by one of two special chlorophyll a molecules

• Together with other pigments, these pigments form clusters called photosystems. Photosystem I and Photosystem II.

A Photosystem

http://www.youtube.com/watch?v=hj_WKgnL6MI

Photosynthesis • Photosynthesis is divided into two stages:

1. The light reactions- occur on the thylakoid membranes. Convert solar energy into chemical energy. The light reactions are divided into two processes- cyclic electron flow and noncyclic electron flow.

2. Calvin Cycle- occurs in the stroma and produces sugar.

Light Reaction- Noncyclic Photophosphorylation

• Photophosphorylation- the process of making ATP from ADP and inorganic phosphate (Pi) using energy from light. – Each photosystem is directly next to an electron transport

chain located within the thylakoid membrane

Noncyclic electron flow begins with photosystem II and follows these steps: 1. Photosystem II- electrons trapped are energized by light. 2. Primary electron acceptor- two energized electrons are passed to a

molecule called a primary electron acceptor.3. Electron transport chain- those electrons are passed from one protein to

another in a chain. 4. Phosphorylation- as the electrons move “down” the chain they lose

energy. The energy lost by the electrons is used to phosphorylate (add a phosphate to) ATP molecules

5. Photosystem I- The electron transport chain terminates with photosystem I. The electrons are again energized by sunlight and passed to another primary electron acceptor NADP to make NADPH.

Light Reaction- Noncyclic Photophosphorylation

6. NADPH- the electrons pass through a short electron transport chain. At the end of the chain, the electrons combine with NADP+ and H+ to form NADPH. NADPH is a coenzyme. Since the electrons have a considerable amount of energy left, NADPH is an energy-rich molecule.

7. Photolysis- the electrons that originated in photosystem II have now been incorporated in NADPH. The loss of these two electrons from photosystem II is replaced when water is split into two electrons (2 H+ and 1/2 02) This is why water is needed and oxygen is produced from photosynthesis.

In summary, photophosphorylation take the energy in light and electrons in water to make the energy rich molecules ATP and NADPH:

Water + ADP + phosphate + NADP + light--> ATP + NADPH + oxygen

Light Reaction- Noncyclic Photophosphorylation

Light Reaction- Noncyclic Photophosphorylation

Light Reaction- Cyclic Photophosphorylation

• A second photophosphorylation sequence occurs when the electrons energized in photosystem I are “recycled” due to excess NADPH

• Energized electrons join with protein carriers and generate ATP as they pass through another electron transport chain. The electrons then return to photosystem I (in a circle).

• This process is necessary because the calvin-benson cycle

requires more ATP than NADPH.

Light Independent Reaction-Calvin Benson Cycle

Uses NADPH and ATP to convert carbon dioxide into sugar. Occurs in the stroma of the chloroplasts.

Carbon fixation- CO2 is attached to the protein RuBP- this reaction is catalyzed by the enzyme rubisco. This molecule and others cycle through 5 more times each time adding more CO2 until glucose is complete. – H comes from NADPH– ATP is required – Glucose is stored as starch (and later converted to sucrose and

distributed to stems leaves and roots)

Light Independent Reaction- Calvin Benson Cycle

Calvin Cycle• In summary, the calvin benson cycle takes carbon

dioxide from the atmosphere and energy from ATP and NADPH to create a glucose molecule. The energy in ATP and NADPH was captured from the sun through photophosphorylation (cyclic and noncyclic):

• 6CO2 + 18 ATP + 12 NADPH --> 18 ADP + 12 NADP + 1 glucose

• Each step of the calvin cycle is catalyzed by a specific enzyme.

Chemiosmotic Theory• Chemiosmotic theory describes the process by which ADP is

phosphorylated to ATP:– 1. H+ ions (protons) accumulate inside the thylakoids. The

H+ ions come from photolysis during the light reactions.– 2. An electrical gradient is created across the thylakoid

membrane as H+ is concentrated in the stroma.

– 3. As protons pass through the enzyme ATP synthase (embedded in the thylakoid membrane) the enzyme uses the energy to phosphorylate ATP from ADP.

Chemiosmotic Theory

Photorespiration• Rubisco is the most abundant protein on Earth. • In addition to being able to fix carbon dioxide, it can

also fix oxygen if the oxygen levels are too high in a plant. This is called photorespiration.

• This doesn’t lead to production of useful molecules and the waste products are instead broken down by organelles called peroxisomes.

Photosynthesis in Hot, Arid Climates

• The plants that photosynthesize in the methods we’ve discussed so far are called C3 plants. In these plants, on hot and dry days their stomata close and oxygen builds up. Also, CO2 levels decrease. This causes rubisco to bind to oxygen instead of carbon dioxide-photorespiration.

• Alternative methods of carbon fixation have evolved to prevent excess water loss in hot, arid climates.

C4 Plants• C4 plants place CO2 into a 4 carbon compound (instead of 3)

and have a unique leaf anatomy. They have two types of photosynthetic cells.

• 1. Bundle Sheath cells: are where the calvin cycle occurs. • 2.Mesophyll cells: are where carbon dioxide is fixed. In

Mesophyll cells, CO2 is bound to the molecule PEP by the enzyme PEP carboxylase which has a higher affinity for CO2 than rubisco does. The bound CO2 then moves into Bundle sheath cells through plasmodesmata. This keeps CO2 levels high in Bundle sheath cells so that rubisco accepts it even when stomata are closed.

• Examples: tropic grasses

CAM Plants• Crassulacean acid metabolism (CAM) is used as an

adaptation for arid conditions• Succulents and many cacti are CAM Plants. • They deal with hot, arid environments with a different

strategy. They open their stomata at night and close them during the day. This is just the reverse of how other plants behave. Carbon dioxide is fixed in a variety of molecules at night and then during the day when the light reactions supply ATP and NADPH, carbon dioxide is released and the Calvin cycle takes place.

Classwork

• Generate a 3 circle Venn diagram to compare and contrast C3, C4 and CAM plants.

• With your group of 3 and Bat book– Person 1: read 6.1 p90 and answer Q: How is your

breathing related to your cellular respiration?– Person 2: read 6.2 p90-91 and answer Q: Why are

sweating and other body-cooling mechanisms necessary during vigorous exercise?

– Person 3: read 6.3 p91 and answer Q: Walking at 3 mph, how far would you have to travel to “burn off” a slice of pizza (475 kcal)? How long would it take?

Cellular RespirationCh 6 Bat book Ch 7 Babboon book

Cellular Respiration• In class activity: Biology Flower text

• Group 1: read Overview and Figure 9.1 and summarize (responsible for recording)

• Group 2: read 9.1- Catabolic Pathways and Production of ATP and summarize

• Group 3: read 9.1- Redox Reactions- the Principle of Redox and summarize

• Group 4: read 9.1- Redox Reactions-Oxidation of Organic Fuel and summarize

• Group 5x2: read 9.1- Redox Reactions- Stepwise Energy Harvest and summarize

• Respiration extracts stored energy from glucose to form ATP (from ADP and Pi) in a series of steps: – Chemical Equation: C6H12O6 + 6 O2 6 H2O + 6 CO2 + energy glucose + oxygen water + carbon dioxide + (ATP)

• These steps couple energy-releasing (exergonic) chemical reactions to energy-storing (endergonic) ones. As hydrogen changes places as H+ and e- energy is transferred.

Cellular RespirationCh 6 Bat book Ch 7 Babboon book

Hydrogen Transfer and Carriers• Oxidation-Reduction reaction (redox rxn)-

Movement of electrons from one molecule to another.– Oxidation/oxidized: the loss of electrons

– Reduction/reduced: the addition of electrons

In cellular respiration

glucose loses e- while

oxygen gains e-

(all transported by H)

– Examples of e- carriers• NAD+ and NADH (nicotinamide adenine dinucleotide)• FAD+ and FADH2

LEO the lion says GER

• Cellular respiration occurs in the mitochondria. The mitochondria has two membranes. The inner membrane is highly folded- the folds are called christae. The inside of the mitochondria is called the matrix and the space between the two membranes is called the intermembrane space.

Respiration in the presence of oxygen is called aerobic respiration. It’s divided into three steps:

1. Glycolosis2. Krebs Cycle3. Oxidative Phosphorylation/Electron Transport Chain

Cellular Respiration

Glycolysis• Glycolysis is breaking down glucose into the molecule

pyruvate. Nine intermediate products are formed and enzymes catalyze each step.

• Glucose--> 2 pyruvate molecules• Glucose--> (net) 2 ATP + 2 NADH molecules• Releases < 25% of the energy in glucose- rest is stored

in pyruvate molecules

• No CO2 is released / No O2 is required• Takes place in the cytosol of the cell- outside the

mitochondria

• VIDEO http://www.youtube.com/watch?v=3GTjQTqUuOw

• Go to page 97 Bat book and look at steps 1-9 to understand where ATP, NADH and Pyruvate/Pyruvic acid are generated.

• Summarize your findings in a picture/words/poetry/3-D figure.

Prep Step• If oxygen is present- pyruvate enters the

mitochondrion and is converted to Acetyl CoA by coenzyme A.– Pyruvate is oxidized (NAD+ = NADH)– A carbon atom is released as CO2 – Coenzyme A joins with the remaining 2 carbon

atoms = Acetyl CoA

• Now ready for the Krebs Cycle

The Krebs Cycle/ Citric Acid Cycle• Takes place in the mitochondrial matrix.• Krebs cycle has 8 steps, each catalyzed by a specific enzyme.

Each acetylCoA molecule that enters produces 3 molecules of NADH. Another molecule is also reduced (gains electrons) FADH2. Both of these molecules will donate their electrons in the 3rd step. Carbon dioxide (that you will exhale) is produced as a biproduct of this step.

Oxidative Phosphorylation/ Electron Transport Chain

• Oxidative Phosphorylation is the process of extracting ATP form NADH and FADH2.

• Occurs on the inner membrane of the mitochondria. • Electrons from NADH and FADH2 pass along an electron transport

chain from one protein to another, losing energy at each step• The last electron acceptor is oxygen. When it accepts the two electron,

with two H+, it forms water.

Overview of Cellular Respiration• Glycolysis- makes 2 ATP • Krebs Cycle- makes 2 ATP • Electron Transport Chain- makes approx. 34 ATP

• Total- 36 ATP made for each glucose molecule

Anaerobic Respiration• If oxygen’s not present there is no electron acceptor to

accept the electrons at the end of the electron transport chain. If this occurs, NADH accumulates. This causes the krebs cycle and glycolosis to both stop. If this happens, the cells soon dies as no ATP is made.

• Anaerobic respiration is a method cells use to escape this fate. The pathways in plants and animals, alcoholic and lactate fermentation, respectively, are slightly different but the objective is the same: to replenish NAD+ so that glycolysis can proceed again.

• Anaerobic respiration occurs in the cytoplasm.

Anaerobic Respiration• Alcoholic Fermentation- Occurs in plants, fungi

(such as yeast) and bacteria. Produces alcohol as a biproduct. This process is used to commercially produce alcoholic beverages.

• Lactate Fermentation- occurs in animals. Pyruvate is converted to lactate (or lactic acid). This molecule causes muscle cramps after strenuous exercise.