Download Chapter 5 - Ellis Benjamin

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
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)
Light energy
6CO2 + 6H2O → C6H12O6 + 6O2
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
• 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
Photosynthesis occurs inside
the cell!
• 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
• 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 photosynthesis
1. 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 Respiration
Cells 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