1
Lecture notes: Photosynthesis and Respiration
All organisms require energy for
 Active transport
 Cell division
 Movement
 Production of proteins
Energy is stored in the ATP molecule
 ATP: adenosine triphosphate
 Made up of adenosine + 3 phosphate groups
Adenosine
Phosphate
ATP
Adenosine
ADP
Phosphate
Phosphate
High-energy bond: Stores much energy
*This energy is released when the bond is broken*
Phosphate
Phosphate
Energy
2
 ADP: Adenosine diphosphate
 Cells recycle the ADP to make new ATP to store more energy for future use
 Many proteins have spots where ATP attaches to provide energy for the protein to
do its job, then the ADP is released for recycling
Photosynthesis
 Process that uses the sun’s energy to make glucose
 Carried out by green plants and some bacteria
 Purpose is to trap sun’s energy and store it in glucose (food for the plant)
 Photosynthesis occurs in the chloroplast
 Structure of the chloroplast:
Stroma: Space inside the
chloroplast
Thylakoid: Green disk in the
chloroplast
Granum stack: Stack of
green thylakoids
3
 Thylakoids are green because they contain chlorophyll
 Chlorophyll: green pigment in plants that absorbs light energy
 Pigment: light-absorbing compound
Chemical equation for photosynthesis
 6CO2 + 6H2O + light energy  C6H12O6 + 6O2
 carbon dioxide + water + sunlight  glucose + oxygen
Steps of photosynthesis
1. Light reaction (Light-dependent reaction)
 First step of photosynthesis that traps sunlight and makes electrons and ATP to
run the dark reaction
2. Dark reaction (Light-independent reaction)
3. Second step of photosynthesis that uses ATP and electrons from the light reaction
and carbon dioxide from the air to make glucose
Photosynthesis Step 1: Light Reaction – occurs in the thylakoids inside the chloroplast
-
e
1b
1a
Electron
transport
chain
1c
Chlorophyll in
the thylakoids
e- - ee
1d
Water
1a. Light energy
is absorbed by
the chlorophyll
e1b. Electrons
jump out of
the chlorophyll
atoms
NADPH
ATP
Oxygen
1c. Electrons move down
the electron transport
chain (series of proteins
that pass the electrons
along). Electrons are
caught by the NADPH
compound. ATP is made.
1d. Photolysis: Water is
broken down into oxygen
(given off by plant) and
electrons (replace lost
electrons in chlorophyll).
These electrons recharge
the system so the light
reaction can happen
again.
6
Photosynthesis Step 2: Dark Reaction (Calvin Cycle) – Occurs in the stroma
NADPH -
ee
e-
-
e
ATP
e-
Carbon
dioxide
from the air
Calvin Cycle
PGAL
2 PGAL = 1 glucose
2a. Electrons and ATP
from light reaction get
dumped into the Calvin
Cycle to run it
2b. Calvin Cycle: Series
of steps that build up
compounds using carbon
dioxide from the air
2c. PGAL compound
sometimes leaves the
cycle. 2 PGAL
compounds added
together make 1 glucose.
7
Cellular Respiration
 The process by which mitochondria break down glucose to make ATP
 Two types
o Aerobic respiration: requires oxygen and carried out by plants, animals, and some
bacteria
o Anaerobic respiration: requires no oxygen and carried out by yeast, some
bacteria, and sometimes animals
Chemical equation for aerobic respiration
 C6H12O6 + 6O2  6CO2 + 6H2O + chemical energy
 glucose + oxygen  carbon dioxide + water + ATP
Some of aerobic respiration occurs in the mitochondria (plural of mitochondrion)
 Makes energy for the cell through aerobic respiration
 Structure of a mitochondrion
Inner membrane
Outer membrane
Cristae: Fold in the
inner membrane
8
Steps of aerobic respiration
1. Glycolysis: First step breaks down glucose into pyruvate
(Intermediate step: Change pyruvate to acetyl CoA)
2. Citric Acid Cycle: Second step uses the acetyl CoA to make electrons for the last
step
3. Electron transport chain: Third step uses the electrons to make a lot of ATP
Step 1: Glycolysis – Occurs in the cytoplasm
ATP
Enzymes
Glucose
Pyruvate
Pyruvate
ATP
Glucose breaks down into 2 pyruvate (2 ATP are also made)
9
Intermediate step: Pyruvate becomes acetyl CoA (Occurs in the cytoplasm)
Pyruvate
Acetyl CoA
Step 2: Citric Acid Cycle (Krebs Cycle) – Occurs in the mitochondria
ATP
Acetyl CoA
Citric Acid
Cycle
ATP
Citric
acid
e-
e- - ee
NADH
Step 3: Electron Transport Chain – Occurs in the mitochondria
NADH -
ee
e-
Made in Step 2
10
ee-
Electrons move down the
electron transport chain
(series of proteins that
pass the electrons along).
Electrons are caught by
oxygen to make water.
32 ATP are made.
Electron
transport
chain
-
e
34
ATP
Oxygen we breathe
Water
We use
for
energy
11
ATP Totals for aerobic respiration:
Glycolysis – 2 ATP
Citric Acid Cycle – 2 ATP
Electron Transport Chain – 32 ATP
1 Glucose = 36 ATP in all for aerobic respiration
Photosynthesis and respiration are the opposite of each other
Photosynthesis
Plants
* use sunlight to make glucose
* take in carbon dioxide
* give off oxygen
*carbon dioxide + water + sunlight  glucose +
oxygen
Respiration
Animals and plants
* eat plants to get glucose
* take in oxygen
* give off carbon dioxide
*glucose + oxygen  carbon dioxide +
water + ATP
12
Anaerobic respiration (requires no oxygen)
 Also called fermentation
 2 types: alcoholic fermentation and lactic acid fermentation
o Both begin with glycolysis
o No citric acid cycle or electron transport chain
Glycolysis
Glucose  pyruvate + 2 ATP
Alcoholic fermentation
Pyruvate  ethyl alcohol +
carbon dioxide


Carried out by yeast and some
bacteria
Used in brewing beer, making
wine, and baking bread and
cakes
Only 2 ATP are made
in anaerobic
respiration so it is
not as good as
aerobic respiration
(36 ATP)
Lactic acid fermentation
Pyruvate  lactic acid


Carried out by your muscles
when you’re exercising hard
(need ATP) and can’t get
oxygen into you fast enough
(can’t do aerobic respiration)
Causes muscle cramps and
soreness
13
C3 Photosynthesis : C3 plants.






Called C3 because the CO2 is first incorporated into a 3-carbon compound.
Stomata are open during the day.
RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2.
Photosynthesis takes place throughout the leaf.
Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because
requires less machinery (fewer enzymes and no specialized anatomy)..
Most plants are C3.
C4 Photosynthesis : C4 plants.






Called C4 because the CO2 is first incorporated into a 4-carbon compound.
Stomata are open during the day.
Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into
the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis.
Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)
Adaptive Value:
o Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered
directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.
o Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata
open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.
C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and
many of our summer annual plants.
CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism



Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in
the form of an acid before use in photosynthesis.
Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The
CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is
released to RUBISCO for photosynthesis
Adaptive Value:
14
o

Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration
rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
o May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day.
Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis.
This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration
and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells,
and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and
twigs and go dormant during dry spells).
CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads