chapter 7 photosynthesis: using light to make...

10/28/15

1

Photosynthesis: Using Light to

Make Food

Chapter 7

BIOL 1408 Dr. Chris Doumen

Understanding of the finer details of Photo-synthesis is globally important. Fossil fuels come from plants and the new fuel

resources will come from plants derived resources. (forest plantation, algae cultures, sun energy panels,…)

Introduction

10/28/15

2

Future of bio-fuels

Algae-Based Jet Fuel Research Gets $25 Million Boost

Car completes cross-country trip on algae fuel

Future of bio-fuels

Joule's bioreactors, mimicking solar panels, host photosynthetic bacteria engineered to secrete diesel-like fuel

components

10/28/15

3

A little Photosynthesis History

Aspects of plant science only became know in the last of hunderd years.

The early Greeks thought plants eat soil…… In the 1600’s (1648) Jan Baptist van Helmont devised a

simple experiment that showed how wrong the Greeks were…

He concluded that plants do not grow by consuming soil, but

consuming water…. (not really true either)


In 1771 , Joseph Priestley finds that air which has been made "noxious" by the breathing of animals or burning of a candle can be restored (i.e., made to support breathing or combustion again) by the presence of a green plant.

10/28/15

4


1774 : Antoine Lavoisier begins to investigate and later names oxygen. He recognizes that it is consumed in both animal respiration and combustion. His work discredits the theory of "phlogiston," a hypothetical substance then believed to be emitted during respiration or combustion, and lays the foundations of modern chemistry.

1779: Jan Ingenhousz discovers that only the green parts of plants release

oxygen and that this occurs only when they are illuminated by sunlight. 1782: Jean Senebier demonstrates that green plants take in carbon dioxide

from the air and emit oxygen under the influence of sunlight.


1791: Comparetti observes green granules in plant tissues, later identified as chloroplasts.

1804: Nicolas de Saussure shows that the carbon assimilated

from atmospheric carbon dioxide cannot fully account for the increase in dry weight of a plant. He hypothesized that the additional weight was derived from water. At this point, therefore, the basic equation of photosynthesis was established. It was understood as a process in which a green plant illuminated by sunlight takes in carbon dioxide and water and converts them into organic material and oxygen.

10/28/15

5

PhotoSynthesis

Light energy

PHOTOSYNTHESIS

6 CO2 6 + H2O

Carbon dioxide Water

C6H12O6 6 + O2

Glucose Oxygen gas

Photosynthesis is thus the process where •  light energy is converted to chemical energy •  the chemical energy is made from carbon dioxide and

water. •  that is turned into sugars, where the sun-energy is now

stored in the covalent bonds between the carbon molecules (the chemical form).

Introduction Autotrophs •  make their own food through the process of

photosynthesis, •  sustain themselves, and •  do not usually consume organic molecules derived from

other organisms.

In other words, autotrophs produce their own food and sustain themselves without eating other organisms

Autotrophs are the producers of the biosphere

Definitions

10/28/15

6

There are two kinds of autotrophs •  Chemoautotrophs are prokaryotes (bacteria and such) that

use inorganic chemicals as their energy source.

•  Photoautotrophs use the energy of light to produce organic molecules.

•  Plants, algae, and some bacteria are photo-autotrophs

Definitions

The importance of Photo-autotrophs is that they are the Producers of food consumed by virtually all organisms !

Introduction

10/28/15

7

Organisms that rely on the food sources produced by autotrophs are called heterotrophs

A heterotroph is a consumer that feeds on

•  plants (herbivores) or •  Animals (carnivore) , or •  decompose organic material (decomposers)

Definitions

Food chains

Autotroph Heterotroph

Food chains in terms of autotrophs and heterotrophs (herbivores/carnivores)

10/28/15

8

Photosynthesis Plants execute Photosynthesis in the cellular organelles called chloroplasts !

In plants :

•  Chloroplasts are almost always located in the leaves of a plant

•  Thus Photosynthesis occurs primarily in the leaves

•  The chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf.

The exterior leaf cells have specialized cells that form stomata

Stomata are tiny pores in the leaf that allow •  carbon dioxide to enter and •  oxygen to exit.

The leaves also contain veins that deliver water absorbed by roots.

Photosynthesis

10/28/15

9

Introduction Photosynthesis occurs in chloroplasts

Leaf Cross Section

Mesophyll

CO2 O2

Vein

Leaf

Stoma

Mesophyll Cell

Chloroplast

Chloroplasts Chloroplasts consist of an envelope of two phospholipid

membranes

•  The inner membrane encloses an inner compartment filled with a thick fluid called stroma and

•  In this stroma is a system of interconnected

membranous sacs called thylakoids.

•  Thylakoids are stacked like pancakes into a structure called a granum (plural = grana).

10/28/15

10

Figure 7.2_2

Chloroplast

Thylakoid

Thylakoid space

Stroma

Granum

Inner and outer membranes

Thylakoids •  have an internal compartment called the thylakoid

space, which has functions analogous to the inter-membrane space of a mitochondrion.

•  Thylakoid membranes also house much of the machinery that converts light energy to chemical energy.

Chlorophyll molecules •  are built into the thylakoid membrane and •  capture light energy. •  are at the basis of the green color of leaves

Chloroplasts

10/28/15

11

Photosynthesis Reactions Scientists thus eventually determined the chemical equation

for Photosynthesis shown below (compare with respiration) But does this oxygen come from carbon dioxide or water?

For many years, it was assumed that oxygen was extracted

from CO2 taken into the plant.

Light energy

PHOTOSYNTHESIS

6 CO2 6 + H2O

Carbon dioxide Water

C6H12O6 6 + O2

Glucose Oxygen gas

However, later research using a heavy isotope of oxygen, 18O, confirmed that oxygen produced by photosynthesis comes from H2O.

Plants thus produce O2 gas by splitting water ; the O2 liberated by photosynthesis is made from the oxygen atom in water (H2O)

Photosynthesis Reactions

10/28/15

12

Although we write the reaction of photosynthesis as follows

6 CO2 + 6 H2O + Light Energy → C6H12O6 + 6 O2

The actual reaction of photosynthesis is really

6 CO2 + 12 H2O + Light Energy → C6H12O6 + 6 H2O + 6 O2


Isotopes can be very instrumental in un-raveling metabolic reactions. The experiment with oxygen isotope, 18O, was constructed as follows. •  In experiment 1, the labeled oxygen isotope was only present in

carbon dioxide ( labelled with *) •  In experiment 1, the labeled oxygen isotope was only present in

water ( labelled with *) Exp.1: 6 CO2 + 12 H2O + Light → C6H12O6 + 6 H2O + 6 O2 Exp.2: 6 CO2 + 12 H2O + Light → C6H12O6 + 6 H2O + 6 O2

By analyzing the labeled oxygen in the products , one can figure out details of the overall process


* * *

* *

10/28/15

13

Exp.1: 6 CO2 + 12 H2O + Light → C6H12O6 + 6 H2O + 6 O2 Exp.2: 6 CO2 + 12 H2O + Light → C6H12O6 + 6 H2O + 6 O2

In experiment 1, no oxygen became labeled but glucose and water did.

In experiment 2 only the released oxygen was labeled


* * *

* *

•  Thus, all the carbons of carbon dioxide end up in glucose and the oxygens from carbon dioxide end up in glucose andwater.

•  All the hydrogens in glucose and water come from water •  AND the released oxygen comes from the splitting of

water.

Reactants:

Products:


10/28/15

14

Photosynthesis, like respiration, is a redox (oxidation-reduction) process. In this case

•  CO2 becomes reduced to sugar as electrons along with

hydrogen ions from water are added to it. •  Water molecules are oxidized when they lose electrons

along with hydrogen ions.

Becomes reduced

Becomes oxidized


The following once again compares the basic redox reaction that occur in plants during photosynthesis and what happens when mitochondria containing cells use these sugars to generate energy.

Reduction

Oxidation

6 O2

6 H2O

Reduction

Oxidation

6 O2 6 CO2 + 6 H2O C6H12O6 +

C6H12O6 + 6 CO2 +

Photosynthesis vs Respiration

Photosynthesis: the reduction requires energy (= light

energy) ( in chloroplasts)

Cellular respiration: the oxidation releases energy,

captured in ATP ( in mitochondria)

chapter 7 photosynthesis: using light to make...

Documents