Photosynthesis and Cellular Respiration

Photosynthesis is essential to all life on Earth. It is the only biological process that can capture the
energy that originates in outer space and convert it into chemical compounds that every organism
uses to power its metabolism.

Main Structure of Photosynthesis

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide, and water as
substrates. After the process is complete, it releases oxygen and produces GA3P, simple
carbohydrate molecules that can subsequently be converted into glucose, sucrose, or any of dozens
of other sugar molecules.

Two parts of Photosynthesis:

● Light Independent Reaction (LDR)

● Light Dependent Reaction (LIR)

Photosynthesis takes place in two sequential

stages:

In light-dependent reactions, energy from sunlight is

absorbed by chlorophyll and that energy is converted
into stored chemical energy.

In light-independent reactions, the chemical energy harvested during the light-dependent

reactions drives the assembly of sugar molecules from carbon dioxide.

CHLOROPLAST
Surrounding the chloroplast is a double membrane.

Interior to the chloroplast’s inner membrane and

surrounding the thylakoids is a fluid called the
 stroma.

The interior of the chloroplast contains another

membrane—the thylakoid membrane—which is
folded to form numerous connected stacks of
discs.

LIGHT-DEPENDENT REACTION
The overall function is to convert solar energy
into chemical energy in the form of NADPH and ATP.

Protein complexes and pigment molecules work together to produce NADPH and ATP.

The actual step that converts light energy into chemical energy takes place in a multiprotein complex
called a photosystem, two types of which are found embedded in the thylakoid membrane,
photosystem I and II

LIGHT-INDEPENDENT REACTION

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and
NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy
storage.

The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of
millionths of seconds, whereas the products of the light-independent reactions (carbohydrates
and other forms of reduced carbon) can survive for hundreds of millions of years.
The carbohydrate molecules made will have a backbone of carbon atoms.
 OTHER NAMES OF LIGHT
INDEPENDENT REACTION

LIGHT-INDEPENDENT REACTION

In plants, carbon dioxide (CO2) enters the

leaves through stomata

Once in the mesophyll cells, CO2 diffuses

into the stroma of the chloroplast

Another term, the Calvin cycle, is named for the

man who discovered it, and because these
reactions function as a cycle.

Others call it the Calvin-Benson cycle to include the name of another scientist involved in its
discovery.
The most outdated name is dark reactions because the light is not directly required.

The light-independent reactions of the Calvin cycle can be organized into three basic stages:
fixation, reduction, and regeneration.

STAGES OF CALVIN CYCLE

STAGE 1: FIXATION

In the stroma, in addition to CO2, two other molecules are present to initiate the Calvin cycle
RuBP has five atoms of carbon and a phosphate group on each end.
RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is
immediately converted into two three-carbon compounds.

STAGES OF CALVIN CYCLE

STAGE 1: FIXATION

This process is called carbon fixation,

because CO2 is “fixed” from its inorganic
form into organic molecules. Because of
this very important role in photosynthesis,
RuBisCO is probably the most abundant
enzyme on earth.

This process is called carbon fixation,

because CO2 is “fixed” from its
inorganic form into organic molecules.
Because of this very important role in photosynthesis, RuBisCO is probably
the most abundant enzyme on earth.

STAGE 2: REDUCTION

ATP and NADPH are used to convert the

six molecules of 3-PGA into six molecules
of a chemical called glyceraldehyde 3-
phosphate (G3P).

Recall that a reduction in the gain of an

electron by an atom or molecule. Six
molecules of both ATP and NADPH are
used.

For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for
NADPH, both energy and a hydrogen atom are lost, converting it into NADP+.
STAGE 2.

STAGE 3: REGENERATION

Interestingly, at this point, only

one of the G3P molecules
leaves the Calvin cycle and is
sent to the cytoplasm to
contribute to the formation of
other compounds needed by
the
plant.

STAGE 3.
Because the G3P exported from the
chloroplast has three carbon atoms, it takes
three "turns" of the Calvin cycle to fix
enough net carbon to export one G3P. But
each turn makes two G3PS, thus three
turns make six G3PS.

One is exported while the remaining five

G3P molecules remain in the cycle and are
used to regenerate RuBP, which enables
the system to prepare for more CO2 to be fixed. Three more molecules of ATP are used in these
regeneration reactions.

CELLULAR RESPIRATION
Cellular respiration is a process that all living things use to convert glucose into energy.

STAGES OF CELLULAR RESPIRATION

GLYCOLYSIS, KREBS CYCLE, ELECTRON TRANSPORT CHAIN

STAGE 1: GLYCOLYSIS

Glycolysis is the first pathway in cellular respiration. This pathway is anaerobic and takes place in the
cytoplasm of the cell.

This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules.

There are two halves of glycolysis,

with five steps in each half. The first
half is known as the “energy
requiring” steps. This half splits
glucose, and uses up 2 ATP. If the
concentration of pyruvate kinase is
high enough, the second half of
glycolysis can proceed. In the
second half, the “energy releasing”
steps, 4 molecules of ATP and 2
NADH are released.
Glycolysis has a net gain of 2 ATP molecules and 2 NADH.
INTERMEDIATE STEP: PYRUVATE OXIDATION
In eukaryotes, pyruvate oxidation takes place in the mitochondria. Pyruvate oxidation can only
happen if oxygen is available.
In this process, the pyruvate created by glycolysis is oxidized. In this oxidation process, a carboxyl
group is removed from pyruvate, creating acetyl groups, which compound with CoA to form acetyl
CoA. This process also releases CO2

STAGE 2: KREBS
CYCLE

The citric acid cycle is

the second pathway in
cellular respiration, and
it also takes place in the
mitochondria. The rate
of the cycle is controlled
by ATP concentration.

When there is more ATP

available, the rate slows down; when there is less ATP the rate increases. This pathway is a closed
loop.

The citric acid cycle is considered an aerobic pathway because the NADH and FADH2 it produces
act as temporary electron storage compounds, transferring their electrons to the next pathway, which
uses atmospheric oxygen.

Each turn of the citric acid cycle provides a net gain of CO2, 1 GTP or ATP, and 3 NADH and 1
FADH2.
STAGE 3: ETC

Most ATP from glucose is generated in the electron transport chain. It is the only part of cellular
respiration that directly consumes oxygen

In eukaryotes, this pathway takes place in the inner mitochondrial membrane. In prokaryotes it
occurs in the plasma membrane.

In chemiosmosis, a proton pump takes hydrogens from inside mitochondria to the outside; this spins
the “motor” and the phosphate groups attach to that. The movement changes from ADP to ATP,
creating 90% of ATP obtained from aerobic glucose catabolism.