Quick Review of the Key Concepts of Photosynthesis

In plants, photosynthesis is used to convert light energy from sunlight into chemical energy (glucose). Carbon dioxide, water, and light are used

to make glucose and oxygen.

Photosynthesis is not a single chemical reaction, but rather a set of chemical reactions. The overall reaction is:

6CO2 + 6H2O + light → C6H12O6 + 6O2

The reactions of photosynthesis can be categorized as light-dependent reactions and dark reactions.

Chlorophyll is a key molecule for photosynthesis, though other cartenoid pigments also participate. There are four (4) types of chlorophyll: a, b,

c, and d. Although we normally think of plants as having chlorophyll and performing photosynthesis, many microorganisms use this molecule,

including some prokaryotic cells. In plants, chlorophyll is found in a special structure, which is called a chloroplast.

The reactions for photosynthesis take place in different areas of the chloroplast. The chloroplast has three membranes (inner, outer, thylakoid)

and is divided into three compartments (stroma, thylakoid space, inter-membrane space). Dark reactions occur in the stroma. Light reactions

occur the thylakoid membranes.

There is more than one form of photosynthesis. In addition, other organisms convert energy into food using non-photosynthetic reactions (e.g.

lithotroph and methanogen bacteria)

Products of Photosynthesis

Steps of Photosynthesis

Here is a summary of the steps used by plants and other organisms to use solar energy to make chemical energy:

In plants, photosynthesis usually occurs in the leaves. This is where plants can get the raw materials for photosynthesis all in one convenient

location. Carbon dioxide and oxygen enter/exit the leaves through pores called stomata. Water is delivered to the leaves from the roots through a

vascular system. The chlorophyll in the chloroplasts inside leaf cells absorbs sunlight.

The process of photosynthesis is divided into two main parts: light dependent reactions and light independent or dark reactions. The light

dependent reaction happens when solar energy is captured to make a molecule called ATP (adenosine triphosphate). The dark reaction happens

when the ATP is used to make glucose (the Calvin Cycle).

Chlorophyll and other carotenoids form what are called antenna complexes. Antenna complexes transfer light energy to one of two types of

photochemical reaction centers: P700, which is part of Photosystem I, or P680, which is part of Photosystem II. The photochemical reaction

centers are located on the thylakoid membrane of the chloroplast. Excited electrons are transferred to electron acceptors, leaving the reaction

center in an oxidized state.

Photosynthesis Light Reactions

Not all wavelengths of light are absorbed during photosynthesis. Green, the color of most plants, is actually the color that is reflected. The light

that is absorbed splits water into hydrogen and oxygen:

H2O + light energy → ½ O2 + 2H+ + 2 electrons

Excited electrons from Photosystem I can use an electron transport chain to reduce oxidized P700. This sets up a proton gradient, which can

generate ATP. The end result of this looping electron flow, called cyclic phosphorylation, is the generation of ATP and P700.

Excited electrons from Photosystem I could flow down a different electron transport chain to produce NADPH, which is used to synthesize

carbohydratyes. This is a noncyclic pathway in which P700 is reduced by an exicted electron from Photosystem II.

An excited electron from Photosystem II flows down an electron transport chain from excited P680 to the oxidized form of P700, creating a

proton gradient between the stroma and thylakoids that generates ATP. The net result of this reaction is called noncyclic photophosphorylation.

Water contributes the electron that is needed to regenerate the reduced P680. The reduction of each molecule of NADP+ to NADPH uses two

electrons and requires four photons. Two molecules of ATP are formed.

Photosynthesis Dark Reactions

Dark reactions don’t require light, but they aren’t inhibited by it, either. For most plants, the dark reactions take place during daytime. The dark

reaction occurs in the stroma of the chloroplast. This reaction is called carbon fixation or the Calvin cycle. In this reaction, carbon dioxide is

converted to sugar using ATP and NADPH. Carbon dioxide is combined with a 5-carbon sugar to form a 6-carbon sugar. The 6-carbon sugar is

broken into two sugar molecules, glucose and fructose, which can be used to make sucrose. The reaction requires 72 photons of light.

The efficiency of photosynthesis is limited by environmental factors, including light, water, and carbon dioxide. In hot or dry weather, plants may

close their stomata to conserve water. When the stomata are closed, the plants may start photorespiration. Plants called C4 plants maintain high

levels of carbon dioxide inside cells that make glucose, to help avoid photorespiration. C4 plants produce carbohydrates more efficiently than

normal C3 plants, provided the carbon dioxide is limiting and sufficient light is available to support the reaction. In moderate temperatures, too

much of an energy burden is placed on the plants to make the C4 strategy worthwhile (named 3 and 4 because of the number of carbons in the

intermediate reaction). C4 plants thrive in hot, dry climates.Study Questions

Here are some questions you can ask yourself, to help you determine if you really understand the basics of how photosynthesis works.
Photosynthesis is the process by which plants and certain other organisms make glucose from carbon dioxide and water. In order to understand

and remember how photosynthesis works, it helps to know the terminology. Use this list of photosynthesis terms and definitions for review or to

make flashcards to help you learn important photosynthesis concepts.

ADP – ADP stands for adenosine diphosphate, a product of the Calvin cycle that is used in the light-dependent reactions.

ATP – ATP stands for adenosine triphosphate. ATP is a major energy molecule in cells. ATP and NADPH are products of the light-dependent

reactions in plants. ATP is used in reduction and regeneration of RuBP.

Autotrophs – Autotrophs are photosynthetic organisms which convert light energy into the chemical energy they need to develop, grow, and

reproduce.

Calvin cycle – The Calvin cycle is the name given to the set of chemical reactions of photosynthesis that does not necessarily require light. The

Calvin cycle takes place in the stroma of the chloroplast. It involves the fixing of carbon dioxide into glucose using NADPH and ATP.

Carbon dioxide (CO2) – Carbon dioxide is a gas naturally found in the atmosphere that is a reactant for the Calvin Cycle.

Carbon fixation – ATP and NADPH are used to fix CO2 into carbohydrates. Carbon fixation takes place in the chloroplast stroma.

Chemical equation of photosynthesis – 6 CO2 + 6 H2O → C6H12O6 + 6 O2

Chlorophyll – Chlorophyll is the primary pigment used in photosynthesis. Plants contain two main forms of chlorophyll: a & b. Chlorophyll has a

hydrocarbon tail that anchors it to an integral protein in the thylakoid membrane of the chloroplast. Chlorophyll is the source of the green color of

plants and certain other autotrophs.

Chloroplast – A chloroplast is the organelle in a plant cell where photosynthesis occurs.

G3P – G3P stands for glucose-3-phosphate. G3P is an isomer of PGA formed during the Calvin cycle

Glucose (C6H12O6) – Glucose is the sugar that is the product of photosynthesis. Glucose is formed from 2 PGAL’s.

Granum – A granum is a stack of thylakoids (plural: grana)

Light – Light is a form of electromagnetic radiation; the shorter the wavelength the greater amount of energy. Light supplies the energy for the

light reactions of photosynthesis.

Light harvesting complexes (photosystems complexes) - A photosystem (PS) complex is a multi-protein unit in the thylakoid membrane that

absorbed light to serve as energy for reactions

Light reactions (light dependent reactions) – The light dependent reactions are chemical reactions requiring electromagnetic energy (light) that

occur in the thylakoid membrane of the chloroplast to convert light energy into chemical forms ATP and NAPDH.

Lumen – The lumen is the region within the thylakoid membrane where water is split to obtain oxygen. The oxygen diffuses out of the cell, while

the protons remain inside to build positive electrical charge inside the thylakoid.
Mesophyll cell – A mesophyll cell is a type of plant cell located between the upper and lower epidermis that is the site for photosynthesis

NADPH – NADPH is a high-energy electron carrier used in reduction

Oxidation – Oxidation refers to the loss of electrons

Oxygen (O2) – Oxygen is a gas that is a product of the light-dependent reactions

Palisade mesophyll – The palisade meophyill is the area of the mesophyll cell without many air spaces

PGAL – PGAL is an isomer of PGA formed during the Calvin cycle.

Photosynthesis – Photosynthesis is the process by which organisms convert light energy into chemical energy (glucose).

Photosystem – A photosystem (PS) is a cluster of chlorophyll and other molecules in a thylakoid that harvest the energy of light for

photosynthesis

Pigment – A pigment is a colored molecule. A pigment absorbs specific wavelengths of light. Chlorophyll absorbs blue and red light and reflects

green light, so it appears green.

Reduction – Reduction refers to the gain of electrons. It often occurs in conjunction with oxidation.

Rubisco – Rubisco is an enzyme that bonds carbon dioxide with RuBP

Thylakoid – The thylakoid is a disc-shaped portion of chloroplast, found in stacks called grana.