Chapter 5: Photosynthesis

• Process by which plants, algae, and some microorganisms harness solar energy and convert it into chemical energy

• With few exceptions, all life on this planet ultimately depends on photosynthesis

• Specialized pigment molecules in plants capture the sun’s energy

• Enzymes use the energy to build glucose (C6H12O6) from carbon dioxide (CO2)

• Plant uses water in the process releasing oxygen gas (O2)

6CO2 + 6H2O → C6H12O6 + 6O2

Light energy

Photosynthesis

• Redox – Chemical reaction where electrons are moved from H2O to CO2

– Fueled by energy from the sun

• Uses for glucose – about half for plant’s own cellular respiration, manufacture other compounds, store as starch or sucrose, cellulose for cell walls

Photosynthesis

• Heterotrophs – organisms that obtain carbon by consuming preexisting organic molecules

• Autotrophs – organisms that make their own organic compounds from inorganic compounds (water and CO2)

• Rise of photosynthesis radically altered Earth– Decreased CO2, lowered global temperature, added

oxygen gas to atmosphere

Sunlight as energy source

• Electromagnetic spectrum – range of all possible frequencies of radiation

• Consists of photons – discrete packets of kinetic energy

• Shorter wavelengths have more energy• Sunlight

– Ultraviolet (UV) – short wavelength, high energy– Visible – perceived by humans, powers

photosynthesis– Infrared (IR) – long wavelength, low energy, heat

• Chloroplast – organelle of photosynthesis– 2 outside membranes enclose stroma– Stroma – gelatinous with enzymes,

ribosomes, DNA, and grana– Grana – stack of thylakoids– Thylakoid – membrane studded with

photosynthetic pigments enclosing thylakoid space

Photosynthesis occurs inside the cell!

• Pigments– Chlorophyll a

• Most abundant photosynthetic pigment in plants, algae, and cyanobacteria

– Accessory pigments• Chlorophyll b • Carotenoids

– Only absorbed light is photosynthetically active

• Absorb red and blue• Reflect green• Accessory pigments

extend used range of wavelengths used

• 2 stages of photosynthesis1. Light reactions

– Occur in thylakoid membranes– Convert solar energy to chemical energy– Energy from captured photons used to make ATP

(stores energy) and NADPH (carries electrons)– Oxygen produced as byproduct

2. Carbon reactions– ATP and NADPH used to reduce CO2 to glucose– Occur in stroma– Do not require light – light-independent reactions

• Chemiosmotic phosphorylation– Phosphorylation - adding P to ADP to make ATP– Chemiosmosis – movement of protons across

membrane

Carbon reactions

• Calvin cycle assembles CO2 into glucose

1.Carbon fixation

2.PGAL synthesis

3.RuBP regenerated

• ATP and NADPH from light reactions provide potential energy and electrons

C3, C4, and CAM

• All plants use the Calvin cycle (C3 pathway) to make glucose

• C3 plants use only the Calvin cycle– About 95% of plant species (peanuts, spinach, trees)

• Photorespiration is a problem in hot, dry conditions– Rubisco uses O2 as a substrate (instead of CO2)

starting a process that removes already fixed carbon

– If stomata kept closed to prevent water loss, O2 builds up and photorespiration increases

• C4 plants– CO2 combines with a 3 carbon “ferry” to form

4 carbon oxaloacetate in mesophyll– Moves into bundle sheath cells where Calvin

cycle occurs – Costs 2 ATP but lose less carbon to

photorespiration than C3 plants in hot, dry weather

– Significant water savings– 1% of plants – corn, sugarcane

• CAM plant– Crassulacean acid

metabolism– Open stomata to fix

carbon only at night• Store malate in vacuole

– Fix it again in Calvin cycle during the day

• Stored malate moves into chloroplast in the same cell

– 3-4% of plants– Pineapple and cacti– Saves water

Chapter 6: Cellular RespirationCells use energy to make food

• ATP powers every activity that requires energy input in the cell

• Explains constant need for food (potential energy)

• 3 ATP-generating pathways– Aerobic respiration– Anaerobic respiration– Fermentation

• Aerobic cellular respiration takes in oxygen and produces carbon dioxide

• Explains why we must take oxygen into our body and get rid of carbon dioxide

3 main processes of cellular respiration

• Glucose oxidized and oxygen reduced

• Released energy becomes trapped in ATP to be used as needed

1. Glycolysis – in cytoplasm - anaerobic

2. Krebs cycle – in mitochondria - aerobic

3. Electron transport chain – in mitochondria - aerobic

Where does cellular respiration occur?

• Glycolysis occurs in cytoplasm• Prokaryotes – other reactions also happen in

cytoplasm• Eukaryotes – other reactions happen in

mitochondria• Outer and inner membrane• Intermembrane compartment (Cristae)• Matrix enclosed by highly folded inner

membrane• ATP synthase part of inner membrane

Glycolysis

• 10 steps all in the cytoplasm• First 5 steps “activate” glucose – invest 2 ATP to

get the process going• Last 5 steps produce 4 ATP • ATP produced through phosphorylation – donor

molecule transfers P to ADP• Does not require oxygen (anaerobic)• Net gain of 2 ATPs• Results in 2 pyruvate and 2 NADH molecules

per glucose that goes through glycolysis

Krebs Cycle/ Citric Acid Cycle

• Pyruvate moves into mitochondria from cytoplasm• It is converted to Acetyl CoA and enters Krebs

cycle

• Electrons transferred to NADH and FADH2

• ATP produced via phosphorylation• Carbon dioxide produced as waste product• Cell can use intermediate products to produce

other organic molecules

Electron transport chain

• Embedded in inner mitochondrial membrane• In Aerobic respiration, final electron acceptor

is oxygen • oxygen forms water with hydrogen molecules

from the broken-down glucose• Chemiosmotic phosphorylation

– H+ removed from carriers NADH and FADH2 – Pumped into intermembrane compartment

creating proton gradient– Protons moving down gradient through ATP

synthase powers the production of many ATP

• Most ATP generated from chemiosmotic phosphorylation

• Produces about

30 ATP total

• 32% of kilocalories in glucose

• Rest of potential energy lost as heat

Proteins and lipids for energy

• Glucose is not our only food; usually a cell uses amino acids from the diet to make more proteins– May use amino acids as energy source

• Fats broken down into glycerol and fatty acids– Long fatty acid molecules can yield many

acetyl CoA molecules• Feed into Krebs cycle

Life can thrive without oxygen

• Anaerobic respiration– Essentially the same as aerobic respiration

– Final electron acceptor is not O2

– Lower ATP yield

• Fermentation– Stops after glycolysis– Far less efficient than aerobic respiration– Alcoholic or lactic acid fermentation