Photosynthesis
Autotrophs & Heterotrophs
• Autotrophs – organisms use can make their
own food
– Some autotrophs capture light energy from the
sun in the process of photosynthesis
• Heterotrophs – obtain energy from the foods
they consume
ATP & Energy
Structure of ATP
A. ATP (Adenosine Triphosphate) – shuttles energy
for cells
B. ATP is composed of ribose (a sugar), adenine (a
nitrogenous base), and three phosphate groups
ATP & Energy
A. The bond between the terminal phosphate
groups of ATP’s can be broken, releasing organic
phosphate and leaving ADP (adenosine
diphosphate).
B. Energy is released from ATP when the terminal
phosphate bond is broken.
C. This release of energy comes from the chemical
change to a state of lower free energy
(stabilizing), not from the phosphate bonds
themselves.
ATP & Glucose
• ATP is not good for storing energy for a long
time.
• Cells only have a small amount of ATP.
• One Glucose can store more than 90 times
more energy than one ATP.
• Cells store glucose, and use glucose to
regenerate ATP as needed (cellular
respiration).
*Photosynthesis – process of capturing light
energy from the sun to convert water & CO2
into oxygen and high energy carbohydrates
(food, ex: glucose, starch, & other sugars)
Equation:
Energy (light) + 6CO2 + 6H2O  6O2 + C6H12O6
Investigating Photosynthesis
1. Van Helmont’s Experiment – do plant’s grow
by taking material from the soil?
• Found mass of dry soil
• Planted a seedling, watered it at regular intervals
until it grew to a mass of 75kg.
• Found mass of soil to be unchanged
• Concluded the mass the plant gained came from
the water he added.
• Partially correct, but did not determine where the
carbon in the carbohydrate comes from
2. Priestley’s Experiment – oxygen is produced
by plants
• Determined that oxygen was required to keep a
flame lit/burning.
• Removed oxygen from a jar by placing a lit candle
under it until the flame went out.
• Then placed a sprig of mint in the jar (empty of
oxygen)
• After a few days, he found he could relight a
candle in this jar and it would remain lit for a
while!
3. Jan Ingenhousz – light is essential to
photosynthesis!
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•
Showed the effect observed by Priestley
occurred only when the plant is exposed to light!
Together, Priestly and Ingenhousz showed the
plants need light and water to produce oxygen.
I. Photosynthesis Basics – occurs in the
chloroplasts of plants, protists, and
some bacteria cells.
A. Chloroplast – organelle where
photosynthesis occurs
1. Surrounded by 2 membranes.
2. Thylakoid – flattened sac made of membrane
inside the chloroplast
3. Granum – stack of multiple thylakoids
4. Stroma – fluid that surrounds the grana and
fills the chloroplast
B. Pigments – compound that absorbs light
1. Chlorophyll – pigment on thylakoid membrane
that absorbs light for photosynthesis
• Chlorophyll a – absorbs less blue and more
red light; directly absorbs sunlight
• Chlorophyll b – absorbs more blue and less
red light; helps chlorophyll a absorb light
• Both chlorophyll a and b reflect green light
• Caretenoid – another pigment that absorbs
blue and green light, but not orange; also
helps chlorophyl a absorb light.
C. Photosynthesis is chemically the opposite of
Respiration.
Respiration
Photosynthesis
1st converts light to ATP
2nd uses ATP to make
glucose
**Equations are also the reverse!
Uses glucose to make ATP
D. NAPDH
1. As chlorophyll absorb sunlight, their electrons
become excited (gain a lot of energy).
2. These high energy electrons require a special
carrier called NADP+
3. NADP+ holds and carries 2 high energy electrons,
along with a H+ ion to become NADPH
II. Light-Dependent Reactions – first step of
photosynthesis, converts sunlight to ATP
1. Occurs on the thylakoid membrane.
2. Light is absorbed by a chlorophyll.
3. The light energy provides electrons for the
Electron Transport Chain (chain of proteins).
•
•
•
The ETC splits water (H+ & O2 are released).
Some H+ is added to NADP+ and produces
NADPH.
The O2 is released to the atmosphere.
4. Chemiosmosis
•
•
Also happens on the membrane of the
thylakoids.
Rest of the H+ (from the splitting of water)
drives ATP Synthase proteins to make ATP.
*Chemiosmosis and the ETC happen at the
same time!!!
III. Calvin Cycle – the 2nd step of photosynthesis.
Also called the Light-independent Reactions
(used to be called Dark Reactions), as light
does not play any direct role.
Uses ATP to make Glucose
Steps of the Calvin Cycle
1. RuBP (carbohydrate in plants) reacts with
NADPH, CO2 (from the atmosphere), and ATP
to make Glucose.
2. In the final step, RuBP is remade so the cycle
can occur again.
Factors Affecting Photosynthesis
1. Water – is a needed raw material; shortage
of water slows or even stops photosynthesis
2. Temperature – photosynthesis relies on
enzymes, which only function between 0oC
and 35oC
3. Intensity of Light – up to a specific level, as
light intensity increases, so does the rate of
photosynthesis