Chapter 6 Photosynthesis

Energy from the sun:Photosynthetic organisms are vital to

the survival of all life on Earth.For this slide show, Diagrams & information are from Holt biology text

Photosynthesis- the beginning of almost all food chains

• Name 3 foods you ate in the last 2 hours & think about how this food is related to plants.

• What is the difference between an organic compound and an inorganic compound?

• What is a carbohydrate? • How does photosynthesis cause inorganic

compounds to become organic?

Organic Compounds- contain Carbon!

Remember: Carbohydrates:

such as glucose- source of energy • General formula [CH2O]n - where n is a

number between 3 and 6. Glucose is C6H12O6

Section 1 objectives

• Explain why almost all organisms depend on photosynthesis.

• Describe the role of chlorophylls and other pigments in photosynthesis.

• Summarize the main events of the light reactions.

• Explain how ATP is made during the light reactions.

I. The Light Reactions• All organisms need energy to carry out the functions

of life.

• Where does this energy come from-1. Directly from the sun-

•autotrophic organisms- make sugar from sunlight, CO2 & H2O (examples- all plants, algae, cyanobacteria, plant-like protists)

2. Indirectly from the sun-

* heterotrophic organisms – need to eat autotrophs )

A. Obtaining Energy

• Photosynthesis converts light energy from the sun into chemical energy in the form of organic compounds through a series

of reactions : biochemical pathways.

There are 2 parts to photosynthesis

• Light reactions – Light energy is absorbed form the sun and is converted to chemical energy- temporarily stored in the bonds of ATP and NADPH

• Calvin cycle – organic compounds are formed using CO2 (now using the chemical energy stored from the light reactions)

The equation for photosynthesis:

6CO2 + 6H2O + light energy C6H12O6 + 6O2

Carbon Dioxide + Water + sunlight –make- Organic compounds (sugar) + Oxygen

Photosynthesis & Cellular Respiration are related:

• The oxygen (O2) and some of the organic compounds produced by photosynthesis are used by cells in a process called cellular respiration.

Cellular Respiration is basically the opposite of Photosynthesis

• Photosynthesis creates biomass (organic compounds) by converting light energy into chemical energy (stored as carbohydrate, ATP or other high energy molecule)

• Cellular respiration is the process by which cells break down organic compounds to produce ATP (energy).

Cellular respiration is essentially photosynthesis in reverse

B. Capturing Light Energy

• The light reactions – begin with the absorption of light in-

Chloroplasts• organelles found in the cells of plants, some

bacteria, and algae.

• Inside chloroplasts are Thylakoids, a system of membranes inside the chloroplast that look like flattened sacs

Light and Pigments

– White light from the sun is composed of an array of colors called the visible spectrum.

– Pigments absorb certain colors of light and reflect or transmit the other colors.

• Chlorophyll a & b

• Carotenoids

• Chloroplast Pigments– Located in the membrane of the thylakoids

of chloroplasts are several pigments, including chlorophylls (such as chlorophyll a and chlorophyll b) and carotenoids.

How light is absorbed

Light Energy into Chemical Energy

• Photosystems- In the thykaloid membranes of chloroplasts- are the clusters of pigment molecules that harvest light energy for photosynthesis

• There are 2 photosystems: – Photosystem II – Photosystem I

The 2 photosystems have similar pigments but different jobs in the chloroplast:

Photosystem II-is actually first

• Electrons in chlorophyll a are excited by the energy of sunlight.

• The electrons are so excited they LEAVE the chlorophyll a molecule (it is OXIDIZED) and go to the Primary Electron Acceptor (this molecule is reduced)

• A series of molecules in the thykaloid membranes called the Electron Transport Chain transfer the electrons.

• Ultimately- Protons are added to NADPH molecules which store energy as chemical Energy

Photosystem II-

• right after Photosystem II is Photosystem I

• In photosystem I, light is absorbed again

• Excited electrons leave the chlorophyll a molecule, travel along the electron transport chain & produce NADPH

Summary both photosystems II & I:

• Light Energy is absorbed by chlorophyll a molecules.

• “Excited electrons” in this higher energy level have enough energy to leave the chlorophyll a molecules.

• the primary electron acceptor donates the electrons to the electron transport chain.

• NADPH is produced. (now Energy is stored as chemical energy!)

Also- in both photosystems:

• The electrons are replaced by breaking down water– The Hydrogen is used to

replace the H+ and the e- used in the light reactions

– Oxygen is a waste product.

Making ATP in Light Reactions

– An important part of the light reactions is the

synthesis of ATP. –Chemiosmosis is the movement of protons

through ATP synthase (an enzyme) & then into the stroma

– This causes a concentration gradient.– It releases energy, which is used to produce ATP.

– Stroma -the solution that surrounds the thykaloid membrane in chloroplasts.

Your assignment: (will add to your folder)This can be confusing- the help you understand the light reactions- do the

following:1. Draw the diagram on page 116 fig 6-6• Label all parts.2. Summarize all the steps and explanations of the light reactions on

pages 116, 117 & 1183. Answer these questions:

1. How is NADPH made?2. How is ATP made?3. Where does the water molecule go & what is the waste product in the light

reactions?4. Answer questions 1- page 1185. Reading assignment page 119 & 2 questions.

Objectives section 6-2

• Summarize the main events of the Calvin cycle.• Describe what happens to the compounds that are

made in the Calvin cycle.

• Distinguish between C3, C4, and CAM plants.

• Summarize how the light reactions and the Calvin cycle work together to create the continuous cycle of photosynthesis.

II. Calvin Cycle (The dark reactions)

II. Calvin Cycle (The dark reactions)

Carbon Fixation:• The ATP and NADPH produced in the

light reactions drive the second stage of photosynthesis, the Calvin cycle.

• In the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation.

The Calvin cycle is the most common way that plants fix carbon

• Occurs in the stroma of the chloroplast• Is a series of enzyme-assisted chemical reactions

that produces a three-carbon sugar called G3P

• G3P is usually converted to a five-carbon sugar (RuBP) to keep the cycle going, but some of the three-carbon sugars leave the Calvin cycle and are used to make organic compounds, where energy is stored for later use.

Alternative Pathways

• The C4 Pathway– Some plants that evolved in hot, dry climates fix

carbon through the C4 pathway. These plants have their stomata partially closed during the hottest part of the day.

– Certain cells in these plants have an enzyme that can fix CO2 into four-carbon compounds even when the CO2 level is low and the O2 level is high. These compounds are then transported to other cells, where the Calvin cycle ensues.

Alternative Pathways

• The CAM Pathway– Some other plants that evolved in hot, dry

climates fix carbon through the CAM pathway. These plants carry out carbon fixation at night and the Calvin cycle during the day to minimize water loss

Summary photosynthesis

Factors affecting rate of photosynthesis

• Light intensity

• Carbon dioxide levels

• Temperature

Your assignment for part II The Calvin Cycle (put in your folder)• Draw diagram on page 120 Fig 6-9 Calvin

Cycle & Label all parts

• Summarize steps discussed on pages 120-121

• Draw Diagram on page 123 to summarize photosynthesis.

• Answer questions 1-6 on page 124.