Chapter 7Biological Pathways-

Photosynthesis

Biology 100

Energy Pathways• Ultimately, the source of all

energy in almost all ecosystems is the energy of fusion in the sun.

• Primary Producers (autotrophs) capture this energy in photosynthesis and use it to create organic molecules.

• Consumers and decomposers ingest these organic molecules, rework them to build their own tissues, break some of them down to support their own metabolism.

Primary Production in Other Ecosystems• While the open ocean is the largest ecosystem (65% of the

Earth’s surface), it contributes only 24% of the net primary production.

• In contrast, tropical rain forests are a small ecosystem (3.3%), but contribute over 21% of the net primary production.

Fig. 54.3

Photosynthesis

• Photosynthetic autotrophs use light as an energy source– Include some cyanobacteria prokaryotes, some

protozoa, all algae and green plants– 99.9% of life on Earth relies on photosynthesis for

energy needs• Light is a good source of energy, since everyday

we have the equivalent of 1 million Hiroshima-sized atomic bombs of radiant energy that reaches the Earth

Photosynthesis

• Photosynthesis is essentially the reverse of aerobic respiration– Energy + 6CO2 + 6 H2O → C6H12O6 + 6O2 – This is an endergonic reaction– In plants, photosynthesis takes place in the leaves.

Photosynthesis

• Photosynthesis will occur in the chloroplast.•The light-capturing event will occur in the thylakoids (membranous sacs, stacked as the grana)•The light-dependent reactions will occur in the thylakoids•The light-independent reaction will occur in the stroma

Photosynthesis

• Photosynthesis takes place in three stages:– Light-capturing events

• Capture energy from the sunlight

– Light-dependent reactions• Using energy to make ATP

– Light-independent reactions• Using ATP to power the synthesis of plant molecules

from CO2 in the air.

Light-Capturing Event• In the light-capturing event, photosynthetic

pigments, especially chlorophyll, absorb photons of light

• This excites some electrons in chlorophyll to the point where they are passed to molecules in the light-dependent reactions

Photosynthesis• In the light-dependent reactions, the energy of some of these

excited electrons is used to generate ATP.

• Some of the energy is used to split H2O into O2 and H+.• Oxygen is released as a waste product• The H+ and excited electrons are loaded onto a carrier, NADP+.

Photosynthesis• The light-independent reaction, also known as the Calvin

Cycle is where CO2 is used to synthesize plant molecules

• The end product is sugar, a food source for the plant. This makes the plant an autotroph.

What is Light?

• Light is actually tiny packet of energy, known as photons.

• Some of these photons in light carry more energy than others. Light, is a form of electromagnetic energy, and is thought of as a wave. The range of the electromagnetic energy is represented by the electromagnetic spectrum.

Electromagnetic Spectrum

Notice the highest-energy photons(gamma rays) have the shortest wavelengths.

Electromagnetic SpectrumOnly a narrow range of the electromagnetic radiation (EMR) reaches the Earth’s surface. They are in the form of visible light from 380 nm to 750 nm range.Our eyes/brain interpret these wavelengths as colors.•400 nm = blue•550 nm = green•670 nm = red

What is Color?• Sunlight (and “white” light) is actually composed of photons

travelling at many different wavelengths.• When photons reach an object, some are absorbed, while

other are reflected. – This is due to the characteristics of molecules on the

surface.• If red photons are reflected and all others are absorbed, the

object will look red to us.

Chlorophyll

• Chlorophyll is the main pigment in plants that absorb light.– Chlorophyll exists in two forms

• Chlorophyll a• Chlorophyll b

• Carotenoids are another group of pigments that capture light of wavelengths not efficiently absorbed by chlorophylls.

Pigments• Chlorophyll will absorb

red and blue portions of the electromagnetic spectrum, while reflecting green light.

• Carotenoids will absorb blue and blue-green light, while reflecting oranges and yellows.– When chlorophyll

disentegrates in the fall, this pigment causes the red, yellow and orange colors to show through.

Anatomy of a Leaf• The leaf of a plant is

covered by a waxy cuticle over a layer of epidermal cells

• Gases, such as CO2 in and O2 out, pass through special pores, called stoma on the underside of the leaf.

• The mesophyll cells conduct most of the photosynthesis for the plant.

Photosynthesis• Photosynthesis begins when

photosynthetic pigments intercept photons with the appropriate

• The energy of the absorbed photons excites some of the electrons of the chlorophyll wavelengths.

• The excited electrons are passed to an electron carrier.

Photosynthesis: Light-dependent Rxn• Ultimately, the energy from excited electrons is used to

generate ATP or carried off with NADP+ to form NADPH.• Water is split into O2, H+, and electrons• The electrons replace the ones carried off by NADPH.• The O2 is released to the atmosphere

Light-independent Reaction• The light independent

reactions (Calvin Cycle) use ATP and high energy electrons from the light reactions to add one carbon, from CO2, to a 5C organic molecule.

• Each time around the cycle adds one more carbon

• To build a glucose molecule would require 6 cycles

Fig. 7.8, pg. 138.

Photosynthesis: Light Dependent Rxns• The key enzyme is the light-independent phase of

photosynthesis is RuBisCO• Full name: ribulose bis phosphate carboxylase

oxygenase.• Molecules of this enzyme account for 16% of the

protein content of chloroplasts• It is probably the most abundant enzyme on the planet.• It begins the process of fixing carbon by adding carbon

dioxide to a 5C sugar.• By enzyme standards, RuBisCO is slow, adding only a

few carbon dioxide molecules per second