CHAPTER 7Photosynthesis:

Using Light to Make FoodOverview:

PhotosynthesisLight ReactionsCalvin Cycle

Review of photosynthesis & C3, C4, CAM plantsGreenhouse effectOzone Layer

• Light is central to the life of a plant

• Photosynthesis is the most important chemical process on Earth– It provides food for

virtually all organisms

• Plant cells convert light into chemical signals that affect a plant’s life cycle

Life in the Sun

• All of the food consumed by humans can be traced back to photosynthetic plants

• Light can influence the architecture of a plant

– Plants that get adequate light are often bushy, with deep green leaves

– Without enough light, plants become tall and spindly with small pale leaves

• Too much sunlight can damage a plant– Chloroplasts and carotenoids

help to prevent such damage

• Almost all plants are photosynthetic autotrophs (=self-feeders; they make their own food), as are some bacteria and protists

THE BASICS OF PHOTOSYNTHESIS

– They generate their own organic matter through photosynthesis

All organisms that produce org molecules from inorg molecules using light: photoautotrophs

• On land, plants such as oak trees and cacti are the predominant producers (produce their food supply)

• In aquatic environments, algae and photosynthetic bacteria are the main food producers

• Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water

Overview of Photosynthesis

• Photosynthesis

Chloroplasts: Sites of Photosynthesis

– Occurs in chloroplasts

• Chloroplasts:– Are found in the cells of the mesophyll (green tissue

in the interior of the leaf)– Contain stroma, a thick fluid– Contain thylakoids, membranous sacs (The

thylakoids contain chlorophyll - Chlorophyll is the green pigment that captures light for photosynthesis)

– (grana- singular granum-, stacks of thylakoids)

Plants produce O2 gas by splitting water

• The O2 liberated by photosynthesis is made from the oxygen in water

• Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas– These electrons and H+ ions are transferred to CO2,

producing sugar

Photosynthesis is a redox process, as is cellular respiration

Photosynthesis is a two-step process – An Overview: Light Reaction & Calvin cycle

• The light reactions convert solar energy to chemical energy & produce O2

• The Calvin cycle makes sugar from carbon dioxide

Calvincycle

• Light Reactions

• Sunlight is a type of energy called radiation, or electromagnetic energy

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY

• The full range of radiation is called the electro-magnetic spectrum

• Certain wavelengths of visible light drive the light reactions of photosynthesis

• Chloroplasts absorb light energy and convert it to chemical energy

• Light behaves as photons, discrete packets of energy

• Chlorophyll molecules absorb photons– Electrons in the

pigment (chlorophyll) gain energy

• Chloroplasts contain several pigments

Chloroplast Pigments

– Chlorophyll a – Chlorophyll b – Carotenoids

Photosystems capture solar power• Photosystems

consist of many light-harvesting complexes (containing chlorophyll a, b, and carotenoid that function as a light-gathering antenna) surrounding a reaction center.

•Reaction center: A protein complex that contains a chlorophyll and a molecule called primary electron acceptor

- a primary electron acceptor receives excited electrons from the reaction-center chlorophyll

Photosystems are 2 types that corporate in the light reaction; (photosystem II functions first)

•Photosystem I:Chlorophyll a of the reaction center is P700 (absorbs light with wavelength 700 nm)

•Photosystem II: Chlorophyll a of the reaction center is P680 (absorbs light with wavelength 680 nm)

• Two types of photosystems cooperate in the light reactions

• Let’s see how these 2 systems work together to generate ATP and NADPH.

• An electron transport chain

– Connects the two photosystems– Releases energy that the chloroplast uses to make

ATP

• Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons

• The excited electrons are passed from the primary electron acceptor to electron transport chains– Their energy ends up in ATP and NADPH

In the light reactions, electron transport chains generate ATP, NADPH, and O2

• Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product

• In photosystem I, electrons from the bottom of the cascade pass into its P700 chlorophyll

To Calvin Cycle

1/2

• The light reactions in the thylakoid membrane

• The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane– The flow of H+ back through the membrane is

harnessed by ATP synthase to make ATP– In the stroma, the H+ ions combine with NADP+ to

form NADPH

Chemiosmosis powers ATP synthesis in the light reactions

• The production of ATP by chemiosmosis in photosynthesis

• The Calvin Cycle

• The Calvin cycle occurs in the chloroplast’s stroma – This is where carbon

fixation takes place and sugar is manufactured

ATP and NADPH power sugar synthesis in the Calvin cycle

• The Calvin cycle

– Functions like a sugar factory within a chloroplast– Regenerates the starting material with each turn

THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE

• The Calvin cycle constructs G3P using

– carbon from atmospheric CO2

– electrons and H+ from NADPH – energy from ATP

• Energy-rich sugar is then converted into glucose

PHOTOSYNTHESIS REVIEWED AND EXTENDED

• A summary of the chemical processes of photo-synthesis

Review: Photosynthesis uses light energy to make food molecules

• Many plants make more sugar than they need

– The excess is stored in roots, tuber, and fruits– These are a major source of food for animals

• Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle

– In these types of plants, stomata on the leaf surface close when the weather is hot

– This causes a drop in CO2 and an increase in O2 in the leaf

• C3 plants

– Use CO2 directly from the air

– Are very common and widely distributed

• C4 plants – Close their stomata to save water during hot and dry

weather– Can still carry out photosynthesis

• CAM plants– Open their stomata only at night to conserve water

C4 and CAM plants have special adaptations that save water

• Some plants have special adaptations that enable them to save water

– Special cells in C4 plants—corn and sugarcane—incorporate CO2 into a four-carbon molecule

– This molecule can then donate CO2 to the Calvin cycle

• The CAM plants—pineapples, most cacti —employ a different mechanism

– They open their stomata at night and make a four-carbon compound

– It is used as a CO2 source by the same cell during the day

Night

PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S

ATMOSPHERE

Human activity is causing global warming; photosynthesis moderates it

Sunlight

• Due to the increased burning of fossil fuels, atmospheric CO2 is increasing– CO2 warms Earth’s surface by

trapping heat in the atmosphere– This is called the greenhouse

effect

• Old-growth forests

How Photosynthesis Moderates the Greenhouse Effect

– Are important for lumber

– Are important for moderating world climates

• Greenhouse gases are the most likely cause of global warming, a slow but steady rise in the Earth’s surface temperature– Destruction of forests may be increasing this effect

• Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect

– Unfortunately, deforestation may cause a decline in global photosynthesis

– It swaps O2 for CO2

• The O2 in the atmosphere results from photosynthesis

– Solar radiation converts O2 high in the atmosphere to ozone (O3)

– Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

• Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer

Sunlight

• International restrictions on these chemicals are allowing recovery