Photosynthesis

Photosynthesis is the process by which organisms use the energy of the sun to synthesize organic compounds (sugars) from inorganic compounds (CO 2 and water) Photosynthesis transforms the energy of the sun into chemical energy (glucose) Provides the oxygen we breath, removes CO 2, and provides food, energy and shelter!

Photosynthesis Many organisms participate in photosynthesis: – Nearly all plants – Some Protists (Euglena, kelp) – Some bacteria = cyanobacteria

Carbon dioxide C 6 H 12 O 6 Photosynthesis H2OH2O CO 2 O2O2 Water + 66 Light energy Oxygen gas Glucose + 6 Photosynthesis Carbon dioxide and water are waste products of cellular respiration! Photosynthesis takes these products and converts them to the glucose and O 2 necessary for cellular respiration

Photosynthesis Photosynthesis occurs on a cellular level Chloroplasts are organelles which carry out photosynthesis Chloroplasts contain chlorophyll, a light- absorbing pigment which give autotrophs their distinctive color

The Chloroplast A chloroplast contains two membranes (as do mitochondria) A thick fluid called the stroma fills the inner compartment of the chloroplast Suspended in the stroma are the thylakoids, a system of interconnected membranous sacs, which enclose another compartment known as the thylakoid space

Chloroplast Outer and inner membranes Intermembrane space Granum Stroma Thylakoid space Thylakoid

The Chloroplast Built into the thylakoid membranes are the chlorophyll molecules that capture light energy Photosynthesis occurs throughout a plant (all the green parts), but is concentrated in the leaves A plant will invest much of its energy into the production of its leaves in order to capture as much light energy as possible

Make like a tree and… Leaves are designed to capture light and increase the absorption of carbon dioxide Carbon dioxide enters the leaf (and oxygen exits the leaf) via the stomata, tiny pores protected by guard cells Water is supplied to the tree via its roots, but may exit the leaves when the stomata are open (a catch 22!); why stomata open at night in many plants

Pigments Pigments are light-absorbing molecules built into the thylakoid membranes Pigments absorb some wavelengths of light, but reflect others We do not see the absorbed wavelengths (because their energy has been absorbed by the pigment molecules); we see the wavelengths that the pigment reflects!

Pigments A leaf is green because chlorophyll absorbs colors other than green; absorbs light most strongly in the blue and red, but poorly in the green Different pigments absorb different wavelengths Chloroplasts contain different types of pigments

Wavelength (nm) 10 –5 nm Increasing energy Visible light 650 nm 10 –3 nm 1 nm10 3 nm10 6 nm 1 m 10 3 m Radio waves Micro- waves Infrared X-rays UV Gamma rays

Pigments Chlorophyll a (a type of chlorophyll pigment) absorbs light mainly in the blue-violet (high energy) and red (low energy) wavelengths Light Chloroplast Thylakoid Absorbed light Transmitted light Reflected light

Autumn color change Autumn leaf color is a phenomenon that affects the normally green leaves of deciduous trees and shrubs, changing to reds and yellows (and various shades in between) In late summer, the veins that carry fluids into and out of the leaf are gradually closed off, and chlorophyll decreases

Autumn color change In addition to chlorophyll, other pigments, known as accessory pigments are present in plants; these include carotene, and cyanins When chlorophyll concentrations decrease at the end of summer, some of these other pigments – which are usually masked by chlorophyll – reveal their colors Carotene, for example, is especially long- lasting

Photosynthesis Photosynthesis occurs in 2 stages, each with multiple steps 1. Light reactions convert light energy into chemical energy, and produce O Light-independent reactions (the Calvin Cycle) assembles glucose molecules using CO 2 (carbon fixation) and the energy-rich products of the light reactions

1. Light (dependent) Reactions Light reactions occur in the thylakoid membranes of the chloroplast Water is split, providing a source of electrons and giving off O 2 as a by-product H 2 O  2H + + 1/2 O 2 + 2e - Light energy absorbed by chlorophyll molecules is used to drive the transfer of electrons and H + from water to NADP + and generate ATP

1. Light (dependent) Reactions Light reactions absorb solar energy and convert it to chemical energy (NADPH) Produces O 2 as a by-product No sugar is produced in light reactions

2. Light-independent Reactions Light-independent reactions occur in the stroma of chloroplasts Consist of the Calvin Cycle, which assembles sugar molecules using CO 2 and NADPH The incorporation of Carbon from CO 2 into organic compounds is called carbon fixation

2. Light-independent Reactions The Calvin Cycle does not require light, but occurs during daylight hours when the light reactions power the cycle by supplying NADPH and ATP Often called “dark reactions” NADPH provides the electrons to reduce Carbon from CO 2 into glucose, and ATP powers the cycle

H2OH2O ADP P LIGHT REACTIONS (in thylakoids) Light Chloroplast NADPH ATP O2O2 CALVIN CYCLE (in stroma) Sugar CO 2  NADP +

How do photosystems capture solar energy? In the thylakoid membrane, chlorophyll molecules are organized into clusters (with other pigments and proteins) called photosystems A photosystems consists of a number of light- harvesting pigments bound to proteins surrounded by a reaction center complex

How do photosystems capture solar energy? The pigments absorb light energy and pass the electrons (energy) from molecule to molecule until it reaches the reaction center The reaction center complex contains a pair of chlorophyll a molecules and an electron acceptor There are 2 photosystems: Photosystem II and Photosystem I

Reaction center complex e–e– Primary electron acceptor Light-harvesting complexes Photon Photosystem Transfer of energy Pigment molecules Pair of Chlorophyll a molecules Thylakoid membrane

Photosystems So how does this work? When light is absorbed by the pigments, energy passes from pigment to pigment molecules until it reaches the reaction center of the photosystem where it excites an electron of chlorophyll to a higher energy state that is captured by the primary electron acceptor

Photosystems Water is split, supplying its electrons to the chlorophyll molecule which lost its electrons to the primary electron acceptor, releasing O 2 as a by-product Photosystems I and II were named in order of their discovery, but Photosystem II functions first in the sequence of steps that make up the light reactions

A mechanical analogy of the light reactions NADPH Photosystem II e–e– Mill makes ATP Photon Photosystem I ATP e–e– e–e– e–e– e–e– e–e– e–e– Photon

Photosynthesis: a review Light reactions occur in the thylakoid membrane – 2 photosystems capture solar energy which energizes electrons – Photosystems transfer these excited electrons through electron transport chains which produces ATP and NADPH – Water is split and O 2 is released

Photosynthesis: a review In the stroma of the chloroplast, sugars are produced via the Calvin Cycle (light- independent reactions) CO 2 is reduced to form glucose

Photosynthesis: a review The sugar produced by plants during photosynthesis provides the starting materials to make structural components such as cellulose 50% of this sugar goes toward cellular respiration (plants respire!)

Photosynthesis: a review Most plants make considerably more food each day than they need They stockpile this sugar as _____? storing it in roots, tubers, and fruits (sound familiar?) Plants not only produce fuel for themselves, but ultimately provide food for virtually all other organisms (heterotrophs)

Global warming and the greenhouse effect

CO 2 is an important greenhouse gas Greenhouse gases are gases in the atmosphere that absorb heat radiation When solar radiation reaches the atmosphere, visible light passes is absorbed by the planet’s surface warming it

Global warming and the greenhouse effect This heat is radiated back as longer, infrared wavelengths, which are absorbed by gases in the atmosphere reflecting some of the heat back to earth Very important! Without these greenhouse gases, the Earth’s surface temperature would be ~33 ̊C (59 ̊F) cooler than at present This process is called the greenhouse effect

Global warming and the greenhouse effect CO 2, water vapor, and methane are naturally- occurring greenhouse gases Photosynthetic organisms absorb billions of tons of CO 2 every year! Most of this carbon returns to the atmosphere via cellular respiration, but much of it remains locked in large tracts of forests and in long- term storage as fossil fuels buried deep under the Earth’s surface

Global warming Since 1850 (the start of the Industrial Revolution), the concentration of CO 2 in the atmosphere has increased ~40% Increasing concentrations of CO has been linked to global warming, a steady rise in Earth’s surface temperature Earth’s average surface temperature has increased by ̊F in the last 100 years!

Annual Average Global Surface Temperature Anomalies

Year Carbon Dioxide (CO 2 ) Methane (CH 4 ) Nitrous Oxide (N 2 O) CO 2 (ppm), N 2 O (ppb) CH 4 (ppb)

Concentration of CO

Global warming Rapid warming is changing the global climate Sea level rise, melting ice & permafrost, extreme weather events, changing weather patterns

Global climate change Global climate change affects biomes, ecosystems, communities and populations The earlier arrival of warm temperatures in the spring is disrupting ecological communities – What would happen if plants bloomed before their pollinators arrived? – What will happen to coral under increasing temperature?