Photosynthesis: An Overview

Photosynthesis: An Overview The key cellular process identified with

energy production is photosynthesis. Photosynthesis is the process in which

green plants use the energy of sunlight to convert water and carbon dioxide into high-energy carbohydrates and oxygen.

Investigating Photosynthesis

◦Belgian Jan van Helmont conducted an experiment in the late 1600’s to better understand where a plants mass comes from.  

◦Planted a small willow tree in a container filled with soil. 

◦After 5 years of watering it, he found that the tree gained 160 pounds while the soil lost only two ounces. 

◦Concluded that most of the plants mass came from water, not the soil.

Investigating PhotosynthesisVan Helmont’s experiment accounts for the

“hydrate,” or water, portion of the carbohydrate produced by photosynthesis.

But where does the carbon of the “carbo” portion come from?

We now know that carbon dioxide in the air is also a major contributor of plant mass.

In photosynthesis, the carbon in carbon dioxide is used to make sugars and other carbohydrates.

In 1772, Joseph Priestley made another important discovery. 

Placed a shoot of a green plant into a container of water. 

He then covered the container and lit a candle in it until it completely burned out. 

Later, Priestley was able to both burn the candle again and keep mice alive in the air.

Priestley became the first person ever to observe the respiration of plants - the fact that they take in carbon dioxide and release oxygen. 

Also discovered how to dissolve CO2 in water and invented the first soda pop.

Investigating Photosynthesis

Investigating Photosynthesis

Jan Ingenhouz laid the groundwork for understanding photosynthesis when he demonstrated that green leaves absorb carbon dioxide and release oxygen only in sunlight.

In darkness, he showed, leaves absorb oxygen and give off carbon dioxide, just like animals and non-green plant parts.

Photosynthesis

Light and Pigments◦What is the role of light and chlorophyll

in photosynthesis? How do plants capture the energy of sunlight?

In addition to water and carbon dioxide, photosynthesis requires light and chlorophyll.

Plants gather the sun's energy with light-absorbing molecules called pigments.

The main pigment in plants is chlorophyll. There are two main types of chlorophyll:

chlorophyll a chlorophyll b

LightVisible spectrum – ROYGBIVRed = longest wavelengthViolet = shortest wavelengthColor of an object is determined by the color the object reflectsPlants gather light using pigments

Light and Pigments Chlorophyll absorbs light well in the blue-

violet and red regions of the visible spectrum.

Wavelength (nm)

Estim

ated

Abs

orpt

ion

(%) 100

80

60

40

20

0400 450 500 550 600 650 700 750

Chlorophyll b

Chlorophyll a

Wavelength (nm)

Light and Pigments Chlorophyll does not absorb light well in

the green region of the spectrum. Green light is reflected by leaves, which is why plants look green.

Estim

ated

Abs

orpt

ion

(%) 100

80

60

40

20

0 400 450 500 550 600 650 700 750

Chlorophyll b

Chlorophyll a

Wavelength (nm)

Lights and PigmentsCarotenoids are accessory

pigments that allow plants to absorb light in mainly the blue and green regions of the spectrum and reflect red, yellow, and orange.

These pigments are more prominent in the fall.

Light and Pigments Light is a form of energy, so any

compound that absorbs light also absorbs energy from that light.

When chlorophyll absorbs light, much of the energy is transferred directly to electrons in the chlorophyll molecule, raising the energy levels of these electrons.

These high-energy electrons are what make photosynthesis work.

Light and Pigments

Plant pigments can be separated by a process called chromatography.

Chromatography is used to separate mixtures of substances

The Photosynthesis EquationThe Photosynthesis Equation

The equation for photosynthesis is:

6CO2 + 6H2O C6H12O6 + 6O2

carbon dioxide + water sugars + oxygen

◦Glucose is theprimary sugarproduced byphotosynthesis

Light

Light

The Photosynthesis Equation

◦Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen.

Inside a Chloroplastthylakoids –

saclike photosynthetic membranes

grana – stacks of thylakoids

stroma – space outside the thylakoids

Overview of Photosynthesis

photosynthesis occurs only in producers

endergonic reaction – a chemical reaction that uses or stores energy

Two steps of photosynthesis◦Light Reactions◦Calvin Cycle

Light Reactionsrequire lightuse energy from

light to produce ATP and NADPH

occurs in the grana

energy from “excited electrons” is used to bond ADP and Pi to form ATP

Light Reactions3 main events

◦Light energy is absorbed by the chlorophyll and ATP is made

◦Water is split into hydrogen ions, oxygen and electrons

◦The hydrogen ions are used to make NADPH

Light ReactionsInputs:

◦Light◦Water

Outputs:◦ATP ◦NADPH◦Oxygen (O2)

Calvin CycleAlso called the dark reactions or

light-independent reactionsprocess of synthesizing sugarsoccurs in the stromadoes not require lightconverts CO2 to sugar

Calvin Cycle3-carbon sugars are produced

◦Some used as energy sources◦Some converted to lipids, amino

acids, parts of nucleic acids◦Two can combine to form 6-carbon

sugars

Calvin CycleInputs:

◦ATP◦NADPH◦CO2

Outputs:◦Sugars

Photosynthesis Overview

O2

CO2

+H20

Sugar

ADPNADP+

Light-Dependent Reactions

(thylakoids)H2O

ATPNADPH

Calvin Cycle (stroma)

Light energy

In the end…a single 6-carbon sugar is formed

from 6 molecules of carbon dioxide

plants use the sugar for energy and to build more complex carbohydrates such as starches and cellulose

other organisms eat the plants and use the energy stored in carbohydrates

Photosynthesis Overview

◦In van Helmont's experiment, most of the added mass of the tree came from soil and carbon dioxide. water and carbon dioxide. oxygen and carbon dioxide. soil and oxygen.

Quiz

◦Plants use the sugars produced in photosynthesis to make oxygen. starches. carbon dioxide. protein.

Quiz

◦The raw materials required for plants to carry out photosynthesis are carbon dioxide and oxygen. oxygen and sugars. carbon dioxide and water. oxygen and water.

Quiz

◦The principal pigment in plants is chloroplast. chlorophyll. carotene. carbohydrate.

Quiz

◦The colors of light that are absorbed by chlorophylls are green and yellow. green, blue, and violet. blue, violet, and red. red and yellow.

Quiz