CHAPTER 7 Photosynthesis Using Light to Make Food

CHAPTER 7 Photosynthesis: Using Light to Make Food Overview: Photosynthesis Light Reactions Calvin Cycle Review of photosynthesis & C 3, C 4, CAM plants Greenhouse effect Ozone Layer

Life in the Sun • 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 • 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

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

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

Chloroplasts: Sites of Photosynthesis • 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 O 2 gas by splitting water • The O 2 liberated by photosynthesis is made from the oxygen in water

Photosynthesis is a redox process, as is cellular respiration • Water molecules are split apart and electrons and H+ ions are removed, leaving O 2 gas – These electrons and H+ ions are transferred to CO 2, producing sugar

Photosynthesis is a two-step process – An Overview: Light Reaction & Calvin cycle • The light reactions convert solar energy to chemical energy & produce O 2 • The Calvin cycle makes sugar from carbon dioxide Calvin cycle

• Light Reactions

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY • Sunlight is a type of energy called radiation, or electromagnetic 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 • Chlorophyll molecules absorb photons – Electrons in the pigment (chlorophyll) gain energy • Light behaves as photons, discrete packets of energy

Chloroplast Pigments • Chloroplasts contain several 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 a 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 P 700 (absorbs light with wavelength 700 nm) • Photosystem II: Chlorophyll a of the reaction center is P 680 (absorbs light with wavelength 680 nm)

• Let’s see how these 2 systems work together to generate ATP and NADPH. • Two types of photosystems cooperate in the light reactions

• An electron transport chain – Connects the two photosystems – Releases energy that the chloroplast uses to make ATP

In the light reactions, electron transport chains generate ATP, NADPH, and O 2 • 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 photosystem I, electrons from the bottom of the cascade pass into its P 700 chlorophyll • Photosystem II regains electrons by splitting water, leaving O 2 gas as a by-product

• The light reactions in the thylakoid membrane To Calvin Cycle 1/ 2

Chemiosmosis powers ATP synthesis in the light reactions • 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

• The production of ATP by chemiosmosis in photosynthesis

• The Calvin Cycle

ATP and NADPH power sugar synthesis in the Calvin cycle • The Calvin cycle occurs in the chloroplast’s stroma – This is where carbon fixation takes place and sugar is manufactured

THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE • The Calvin cycle – Functions like a sugar factory within a chloroplast – Regenerates the starting material with each turn

• The Calvin cycle constructs G 3 P using – carbon from atmospheric CO 2 – electrons and H+ from NADPH – energy from ATP • Energy-rich sugar is then converted into glucose

PHOTOSYNTHESIS REVIEWED AND EXTENDED

Review: Photosynthesis uses light energy to make food molecules • A summary of the chemical processes of photosynthesis

• 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

• C 3 plants – Use CO 2 directly from the air – Are very common and widely distributed • Most plants are C 3 plants, which take CO 2 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 CO 2 and an increase in O 2 in the leaf

C 4 and CAM plants have special adaptations that save water • C 4 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

• Some plants have special adaptations that enable them to save water – Special cells in C 4 plants —corn and sugarcane— incorporate CO 2 into a four-carbon molecule – This molecule can then donate CO 2 to the Calvin cycle

• The CAM plants—pineapples, most cacti — employ a different mechanism – They open their stomata at night and make a fourcarbon compound – It is used as a CO 2 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 • Due to the increased burning of fossil fuels, atmospheric CO 2 is increasing – CO 2 warms Earth’s surface by trapping heat in the atmosphere – This is called the greenhouse Sunlight effect

How Photosynthesis Moderates the Greenhouse Effect • Old-growth forests – 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 CO 2 from the atmosphere, it moderates the greenhouse effect – It swaps O 2 for CO 2 – Unfortunately, deforestation may cause a decline in global photosynthesis

• The O 2 in the atmosphere results from photosynthesis – Solar radiation converts O 2 high in the atmosphere to ozone (O 3) – 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