Chapter 8

• Objectives:
– Explain where plants get the energy needed to produce food
– Describe the role of ATP in cellular activities

• Energy comes from food
• Autotrophs
– Auto: self troph: feed
– Organisms that produce their own food
– Using sun = photoautotrophs
• Heterotrophs
– Hetero: different
– Organisms that must consume food for energy

• Light, heat, electricity
• Living things rely on energy from chemical compounds
– ATP is the basic energy source of all cells
– Used for active transport, movement, protein and nucleic acid synthesis, cell responses, bioluminescence, etc.

• ATP – adenosine triphosphate
– Primary compound used by all organisms as energy currency
Adenine Ribose 3 Phosphate groups

• Energy can be released from ATP by removing a phosphate
– This makes ADP (adenosine diphosphate)
• Energy is stored by adding a phosphate to ADP, making ATP
• One glucose molecule holds 90X the energy of one ATP
– As a result, cells keep small amounts of
ATP on hand

• Objectives:
– Explain what the experiments of van
Helmont, Priestly, and Ingenhousz reveal about how plants grow
– State the overall equation for photosynthesis
– Describe the role of light and chlorophyll in photosynthesis

• van Helmont – 1600s
– Discovered that the mass of a plant increases without a decrease in the mass of the soil
– This means that the plant is getting the materials for growth from elsewhere
– CO
2
in air, H
2
O in soil

• Priestley – 1700s
– Found that a plant produces a gas
(oxygen) that allows a candle to remain lit in a sealed container

• Jan Ingenhousz (1779)-observes bubble on the leaves of aquatic plants exposed to light, but not in dark and concludes plants need sunlight to produce oxygen

light
6 CO
2
6 H
2
O C
6
H
12
O
6
• Light energy from sun is used to turn carbon dioxide and water into sugar and oxygen
6 O
2

• Leaves are primary site of photosynthesis
– Mesophyll (middle-plant) cells contain greatest concentration of chlorophyll
– Stomata- opening on underside of leaf
• Flanked by 2 guard cells that control open and close
• Let in CO
2
• Release O
2
SEM of a STOMA
(aka stomate; stomata is plural)

• Pigments (chlorophyll a and b) absorb light energy
• Chlorophyll a is involved directly in photosynthesis
• Accessory pigments absorb different colors of light
– Carotenoids, xanthophyll, etc.

• Absorbs:
– Blue
– Violet
– Red
• Reflects:
– Green
– Yellow

8-3 Reactions of Photosynthesis
• Objectives:
– Describe the structure and function of the chloroplast
– Describe what happens in the lightdependent reactions
– Explain what the Calvin Cycle is
– Identify factors that affect the rate of photosynthesis

• Photosynthesis takes place inside chloroplasts
• Thylakoids stacks of membrane sacs
– site of light-dependent reaction
• Stroma - space between thylakoids and inner membrane
– site of light-independent (Calvin Cycle) reaction

Photosystems, clusters of pigment and protein that absorb light energy, are found in these saclike membranes

Light-Dependent Reactions
(in the Thylakoids)

Overview:
• Stage where light energy converted to chemical energy
• Produce O
2
and convert ADP and NADP + to ATP and NADPH

• It is an electron carrier
– Transports high energy electrons around during photosynthesis
• Like a school bus – picks up at point A, drops off at point B
• Formed when 2 e- and H+ bind to
NADP+

• Light energy (photon) excites and e causing it to leave the chlorophyll reaction center in Photosystem II
• Lost e- is replaced by the splitting of a water molecule
– Process called photolysis

• “photo”- light “lysis” split
• Water is split into 2 H + , 2 e , and O
– O
2
is a waste product (so, 2 water molecules are actually used to produce one O
2
)
– Electrons from split water are given to chlorophyll to replace lost electron
• H + will play an important role too

• e from PS II passes down an electron transport chain
– Energy is released is used to pump H + into inner thylakoid space
• Creating a H + gradient to be used later
• e “lands” in PS I
– More light = more energy to e -
– e jumps off PS I into NADP + with H + from water

• H + inside thylakoid space exits through a protein called ATP synthase
– Energy stored in H + gradient makes ATP
• NADPH and ATP go to Calvin cycle

Calvin Cycle AKA – Light Independent
Reactions
Overview:
• ATP, e , and H + produced in light rxns are used to create high energy sugar from CO
2

• CO
2
enters Calvin cycle – carbon fixation
– Added to a 5C molecule (RuBP), splits into two 3C molecules (3-PGA)
– Several reactions later (using ATP and
NADPH), two 3C molecules (G3P aka PGAL) leave the cycle to make glucose
– Some 3C molecules (G3P aka PGAL) recycled
– And using more ATP, are converted into the original 5C molecule RuBP

1. CO
2
enters cycle
2. Energy Input
4. 5C molecules
regenerated
3. 6C sugar produced

1. Photosynthesis is divided into two stages. In which stage is CO
2
used?
2. In which stage is water used?
3. What is the process in which sunlight splits water called?
4. Which comes first in the thylakoid membrane, photosystem I or photosystem II?
5. What replaces the e- lost in PSII?
6. What are the two types of chemical energy produced during the first stage in photosynthesis?
7. In which stage is O
2
produced?
8. Aside from O
2
, what is the main product of photosynthesis?