Biology Chapter 7
Photosynthesis: Using light to make food
Terms in this set (146)
The allergic component of poison ivy sap
Urishiol; binds to clothing/skin/pet fur on contact where it remains active until washed off; even dead leaves or vines retain active urishiol for several years
What percent of the population is allergic to poison ivy
- Found throughout much of North America
- Grows along ground in woods/open areas
- Grows as a vine
- Characteristic shiny leaves
What is contributing to global warming?
Burning fossil fuels and deforestation release CO2 so there is a current rise in atmospheric CO2 (risen by 40% since 1850, the start of the industrial revolution)
How might higher CO2 levels affect plant growth?
Weeds grow faster than crop plants or trees
How far has light energy traveled?
150 million kilometers from the sun
Plants use solar energy from the sun to convert CO2 and H2O to sugars and other organic molecules (chemical energy) and release O2 as by product
"Self-feeders"; make their own food
They feed themselves and are the ultimate source of organic molecules for almost all other organisms
Plants and other photo synthesizers (since they use the energy of light); known as producers of the biosphere because these producers produce its food supply for consumers
Cannot make their own food but must consume plants or animals or decompose organic material
- We are dependent on photographs for raw materials and organic fuel necessary to maintain life and oxygen required to burn fuel in cellular respiration
What do photoautrophs do besides supplying food?
- Clothe us (cotton)
- House us (wood)
- Provide energy for warmth, light, transport and manufacturing (fossil fuels represent stores of sun's energy captured by ancients photoautotrophs in the far distant past; hundreds of millions of years ago)
Photoautotrophs/photographs on land
Photoautotrophs/photographs in aquatic environments
Unicellular and multicellular algae and photosynthetic prokaryotes
Large alga that forms extensive underwater forests off coast of California
Important producers in freshwater and marine ecosystems
Where does photosynthesis occur in plant cells?
How does form/structure fit function in chloroplasts?
Photosynthetic pigments and enzymes are grouped together in membranes/compartments facilitating the complex series of chemical rxns in photosynthesis
Photosynthesis in bacteria
Infolded regions of the plasma membrane containing clusters of pigments and enzymes
Chloroplasts originated from photosynthetic prokaryote that took up residence in a eukaryotic cell
What do self feeding photoautotrophs require from the environment to make their own food?
What indicates the presence of chloroplasts in plant cells?
All green parts of a plant
Where are most chloroplasts located? Where is the major site of photosynthesis?
Leaves of a plant
Why is a leaf green?
Green color comes from clorophyll
Light absorbing pigment in the chloroplasts that plays central role in converting solar energy to chemical energy
Leaf cross section
Chloroplasts are concentrated in the cells of the mesophyll
Green tissue in the interior of the leaf
Tiny pores through which CO2 enters a leaf and O2 exits
Water absorbed by the roots is delivered to the leaves in veins; leaves also use veins to export sugar to roots and other parts of the plant
How many chloroplasts does a typical mesophyll cell hold?
30 to 40
What is the importance of the membranes of the chloroplast?
Membranes form the framework within which many of the reactions of photosynthesis occur (like mitochondria and cellular respiration)
Membranes of the chloroplast
An envelope of two membranes enclose an inner compartant which is filled with stroma
Interconnected membranous sacks suspended in stroma
Enclosed by thylakoids (acts like the intermembrane space of mitochondrion does in ATP synthesis)
In many places, thylakoids are concentrated in stacks
What is built into the thylakoid membranes
- Chlorophyll is built into these membranes to capture light energy
- Also house much of the machinery that converts light energy to chemical energy which is used in the stroma to make sugar
Which plants have bubbles and why?
The leaves of plants that live in lakes and ponds are often covered in bubbles of oxygen gas produced during photosynthesis
Basic equation for photosynthesis
6CO2 + 6H2O --> C6H12O6 + 6O2
Scientists thought photosynthesis splits CO2 (CO2 --> C + O2) releasing oxygen and adds water to carbon to produce sugar
Idea challenged by C. B. Van Niel
Working with photosynthesizing bacteria that produce sugar from CO2 but do not release O2 in the process
- Hypothesized that in plants H2O is split, with hydrogen becoming incorporated into sugar and O2 released as gas
Scientists traced the process of photosynthesis using isotopes and confirmed Niel's theory
- Used heavy isotope of oxygen (O-18) to follow fate of oxygen atoms during photosynthesis
Atoms with differing number of neutrons
- O-18 has two more neutrons in nucleus of its atom than the more common isotope O-16
+ 12H2O -->
Experiment 2: 6CO2 +
--> C6H12O6 + 6H2O +
What did the experiments show?
The O2 released during photosynthesis comes from water and not from CO2
The oxygen atoms from CO2 and the hydrogen atoms from H2O end up in both the sugar molecule and the H2O molecule that are formed as products
Melvin Calvin's experiment using radioactive isatopes
Used radioactive C-14 to trace the sequence of intermediates formed in the cyclic pathway that produces sugar from CO2 (called the Calvin Cycle)
Photosynthesis produces billions of tons of carbohydrates a year. Where does most of the mass of this huge amount of organic matter come from?
CO2 produces both the carbon and the oxygen in carbohydrates while H2O supplies only the hydrogen
Photosynthesis is what kind of reaction?
A redox reaction
What is oxidized and what is reduced in photosynthesis?
CO2 becomes reduced to sugar as electrons and hydrogen ions (H+) from water are added to it to produce glucose
Water molecules are oxidized, losing electrons along with hydrogen ions to produce oxygen
How is photosynthesis process different from cellular respiration process
Food-producing redox reactions of photosynthesis require energy; potential energy of electrons increase as they move from H2O to CO2; energy boost provided by light energy captured by chlorophyll molecules
Where does photosynthesis store energy?
Convert light energy to chemical energy stored in chemical bonds of sugar molecules
Is photosynthesis endergonic or exergonic?
Photosynthesis occurs in many steps
Two stages of photosynthesis
1.) Light reactions
2.) Calvin cycle
Linked by ATP and NADPH
Where: in thylakoids
Input: Light and H2O
Water is split providing a source of electrons and giving off O2 as byproduct. Light energy absorbed by chlorophyll molecules built into thylakoid membranes drive transfer of electrons and H+ from water to the electron acceptor NADP+ reducing it to NADPH. The light reactions also generate (chemical energy) ATP from ADP and a phosphate group
Cousin of NADH; differs only in extra phosphate group
- Temporarily stores electrons and hydrogen ions and provides reducing power to Calvin Cycle
Summary of light reactions
Absorb solar energy and convert to chemical energy stored in ATP and NADPH and produce oxygen
*Do not produce sugar
Where: in stroma
Cyclic series of rxns that assembles sugar molecules using CO2 and energy rich products of light reactions
Incorporation of carbon from CO2 into organic compounds; afterwards, carbon compounds are reduced to sugars
What is NADPH from light reactions used for?
Provides the electrons for reducing carbon compounds in the Calvin Cycle
What is ATP from light reactions used for?
Chemical energy that powers several steps of the Calvin cycle
Another name for the calvin cycle because the steps don't require light directly; however, in most plants the calvin cycle occurs during daylight when light reactions power the cycle's sugar assembly line by supplying it with NADPH and ATP
For chloroplasts to produce sugar from CO2 in the dark, they would need to be supplied with:
ATP and NADPH
Photosynthesis (root of word)
Photo (light) - light reactions
Synthesis (putting together) - sugar construction in Calvin cycle
What drives light reactions? What do we mean when say photosynthesis is powered by the sun?
Visible radiation absorbed by pigments
What type of energy is sunlight? In what form does light travel?
Electromagnetic energy or radiation; travels in space as rhythmic waves
Distance between the crests of electromagnetic waves
The full range of electromagnetic wavelengths from the very short gamma rays to the very long radio waves
Small fraction of the spectrum; from 380 nm to 750 nm
The wave nature of light
The model of light as waves explains many of its properties
What is a photon? What is the relationship between photon length and energy?
Packet of light; Fixed quantity of energy; the shorter the wavelength of light the greater the energy of photons
When photons are shorter than those of visible light they have enough energy to damage molecules like proteins and nucleic acids (explains ultraviolet [UV] radiation causing sunburns and skin cancer)
What do photosynthetic pigments do? What happens to visible light in the chloroplast?
Light absorbing molecules called pigments built into the thylakoid membranes absorb some wavelengths of light and transmit or reflect other wave lengths
- We do not see absorbed wavelengths
- What we see when we look at a leaf are the green wavelengths that are transmitted and reflected
Which wavelengths are absorbed?
Different pigments absorb light of different wavelengths b/c it can absorb the specific amounts of energy in those photons; chloroplasts contain more than one type of pigment
- Participates directly in light reactions
- Absorbs mainly blue-violet and red light
- It looks blue green b/c reflects mainly green light
- Absorbs mainly blue and orange light
- It looks olive green b/c reflects green light
- Broadens range of light that a plant can use by conveying absorbed energy to chlorophyll a (which uses energy for light reactions)
Pigments in the chloroplast that are various shades of yellow and orange
- During fall, when green chlorophyll breaks down, longer lasting carotenoids show through and cause fall foliage to change colors
Function of carotenoids
1.) Broaden spectrum of colors that drive photosynthesis
Some carotenoids absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules that can damage cell molecules
Carotenoids from carrots (vegetables/fruits)
Have photo protective role in our eyes
What color of light is least effective at driving photosynthesis?
Green because it is mostly transmitted and reflected - not absorbed - by photosynthetic pigments
Changing energy levels
When a pigment molecule absorbs a photon of light, one of the pigment's electrons jumps to an energy level farther from the nucleus; at this location, the electron has more potential energy (ground state to excited state; excited state is unstable); when electron returns to ground state it releases excess energy as heat
How long does it take an electron to return to ground state?
Generally when isolated pigment molecules absorb light, their excited electrons drop back down to ground state in a billionth of a second
Why are black items hot on a sunny day
Black pigments absorb all wavelengths of light (so much heat produced by movement of electrons)
What else can pigments emit?
Pigments emit heat though some isolated pigments (including chlorophyll) emit light/fluorescence
Chlorophyll behaves differently in an intact chloroplast than in a solution
In thylakoid membrane, chlorophyll and other pigments absorb photons, transfer energy to other pigment molecules, eventually to a special pair of chlorophyll molecules which passes off an excited electron to a neighboring molecule before it has a chance to drop back down to ground state
Consist of a number of light harvesting complexes surrounding a reaction center complex
What is the light harvesting complex? What is a metaphor to describe it?
Contains various pigment molecules bound to proteins; collectively function as light gathering antenna
- Pigments absorb photons and pass energy from molecule to molecule until it reaches the reaction center
Reaction center complex
Contains a pair of special chlorophyll a molecules and a molecule called the primary electron acceptor, capable of accepting electrons and becoming reduced
- Transfer of electron from reaction-center chlorophyll a to the primary electron acceptor is first step in transformation of light energy to chemical energy in light reactions
Two types of photosystems in light reactions
1.) Photosystem II (first)
2.) Photosystem I (second)
- Each have characteristic reaction-center complex with special pair of chlorophyll a molecules associated with a particular primary electron acceptor
Compared with a solution of isolated chlorophyll, why do intact chloroplasts release less heat and fluorescence when illuminated?
A light excited electron from the reaction center chlorophyll molecules is trapped by a primary electron acceptor (cannot return to ground state) rather than giving up its energy as heat and light
What leads to ATP/NADPH production
1.) Photosystems II and I in the thylakoid membrane are connected by an electron transport chain
2.) Flow of electrons removed from H2O through these components to NADPH
3.) ATP synthesis is linked to electron transport chain pumping H+ into a membrane compartment from which ions flow through ATP synthase embedded in membrane
Process: How the coupling of two photosystems and an electron transport chain transform energy of light to chemical energy of ATP and NADH (not necessary)
Photon provides energy to boost an electron from photosystem II to a higher energy level where it is caught by primary electron acceptor; this is shuttled to an electron transport chain leading to photosystem I; as electrons travel down transport chain, they release energy used for ATP production; when electron reaches photosystem 2, another photon pumps it up to a higher energy level where it is caught by a primary electron acceptor in photosystem I; photo excited electrons are used from there to produce NADPH
What is the source of the electrons that are moving through the photosystems to NADPH? Where is O2 produced? (not necessary)
An enzyme in the thylakoid space splits water into 2 electrons, 2 hydrogen ions and 1 oxygen atom (1/2 O2). The H+ stay in the thylakoid space while the oxygen immediately joins with another oxygen to form O2; O2 molecules diffuse out of thylakoids, chloroplast and plant cell exit the leaf thru its stomata; electrons are passed one by one to reaction center chlorophyll a molecules in photosystem II replacing photo excited electron that was just captured by primary electron acceptor; electrons then pass thru transport chain to reaction center chlorophyll a molecules in photosystem I where they again replace photo excited electrons that had been captured by its primary electron acceptor; travel thru chain to NADP+ reducing it to NADPH
How does flow of electrons down electron transport chain produce ATP
Involves transport chain and chemiosmosis; potential energy of concentration gradient of H+ across membrane powers ATP synthesis; gradient created when electron transport chain uses energy released as it passes electrons down the chain to pump H+ across a membrane; the energy of concentration gradient drives H+ back across the membrane thru ATP synthase, spinning this rotary motor and phosphorylating ADP to produce ATP
Explain why two photons of light are required in the movement of electrons from water to NADPH
One photon excites an electron from photosystem II, which is then passed down an electron transport chain to photosystem I; a second photon excites an electron from photosystem I, which is then used in the reduction of NADP+ to NADPH
Diagram: the light reactions take place within the thylakoid membrane (not necessary)
*All components are present in numerous copies within each thylakoid
1.) Thylakoid (high H+ concentration)
Photosystem = primary electron acceptor, pigment molecules, reaction center pair of chlorophyll a molecules
a. Pigment molecule absorbs light and passes energy to reaction center of photosystem II.
b. Water is split and its electrons are passed to photosystem II. The oxygen atom combines with another forming O2 (H+ from water also contribute to high H+ concentration in thylakoid space)
b. An excited electron is captured by the primary e acceptor
c. As electrons pass down an electron transport chain, H+ is pumped from stroma into thylakoid space
d. Light excites an electron from photosystem I which is passed to a primary electron acceptor
e. Electrons are passed to NADP+ reducing it to NADPH
f. Flow of H+ thru ATP synthase drives phosphorylation of ADP to ATP
2. Stroma (low H+ concentration)
g. NADPH and ATP travel to Calvin cycle
Electron transport chain produces concentration gradient of H+ across thylakoid membrane which drives H+ thru ATP synthase producing ATP; because initial energy input is light, this chemiosmotic production of ATP is called photophosphorylation
What is the advantage of the light reactions producing NADPH and ATP on the stroma side of the thylakoid membrane?
The Calvin cycle, which uses the NADPH and ATP, occurs in the stroma
Purpose/Input of Calvin Cycle
Reduces CO2 (from air) to sugar using ATP and NADPH (from light reactions)
- ATP used as energy source
- NADPH provides electrons for reducing CO2
Output of Calvin cycle
Energy rich three carbon sugar, glyceraldehyde 3-phosphate (G3P) to make glucose, sucrose and other organic molecules
Cycle because starting material is regenerated
Starting material is five carbon carbon sugar RuBP (ribulose biphosphate).
Cycle must turn 3 times to make molecule of G3P incorporation three molecules of CO2
Calvin cycle: step #1
(Carbon fixation) Enzyme rubisco combines CO2 with five carbon sugar RuBP. The unstable 6 carbon molecule splits into two three carbon molecules (organic acid 3-phosphoglycerate [3-PGA]; for 3 CO2 entering, six 3PGA result
Calvin cycle: step #2
(Reduction) A series of two chemical reactions uses energy from 6 ATP molecules and electrons from 6 NADPH molecules to reduce six molecules of 3-PGA to six molecules of the energy rich 3 carbon sugar G3P
Calvin cycle: step #3
(Release of one molecule of G3P) For every three CO2 that enter the cycle, the net output is one G3P sugar molecule; five of the G3PS from step 2 remain in the cycle
Calvin cycle: step #4
(Regeneration of RuBP) a series of chemical reactions uses energy from ATP to rearrange the atoms in the five G3P molecules (15 carbons total) forming three RuBP molecules (15 carbons total)
How much ATP/NADPH is used for net output of 1 molecule of G3P
Calvin cycle consumes 9 ATP and 6 NADPH molecules provided by the light reactions
Why can neither light reactions or calvin cycle function alone?
Photosynthesis is an emergent property of the structural organization of a chloroplast which integrates the two stages of photosynthesis
To synthesize one glucose molecule, the Calvin cycle uses...
6 CO2, 18 ATP and 12 NADPH molecules
Glucose is a highly reduced molecule, storing lots of potential energy in its electrons; the more energy a molecule stores, the more energy and reducing power required to produce that molecule
Most plants use CO2 directly from the air, and carbon fixation occurs when rubisco adds CO2 to RuBP
First product of carbon fixation is 3C compound 3-PGA
Examples of C3 plants
Widely distributed; important agricultural crops
- Soybeans, oats, wheat and rice
Problems farmers face in growing C3 plants
Hot, dry weather decreases crop yield b/c plants close their stomata, the pores in their leaves; this adaptation reduces water loss and helps prevent dehydration but it also prevents CO2 from entering the leaf and O2 from exiting; CO2 levels get very low in leaf and photosynthesis slows and O2 accumulating from light reactions creates a problem
Why is it problematic that O2 builds up in a leaf
As O2 builds up, rubisco adds O2 instead of CO2 to RuBP
After O2 is added to RuBP, the two carbon product of this reaction is broken down in the cell; this process occurs in the light and like respiration it consumes O2 and releases CO2 but unlike cellular respiration, it consumes ATP instead of producing it and unlike photosynthesis it yields no sugar
- Drains away as much as 50% of C fixed by Calvin cycle
Hypothesis regarding photorespiration as evolutionary relic
From when atmosphere had less O2 than it does today
- When rubisco first evolved, the inability of the enzyme's active site to exclude O2 would have made little difference
- Only b/c O2 has become so concentrated in atmosphere that "sloppiness" of rubisco presented a problem
Other possible role of photorespiration
Plays protective role when the products of the light reactions build up in a cell (as occurs when Calvin cycle slows due to lack of CO2)
Evolution of carbon fixation methods
In some plant species found in hot/dry climates, alternate modes of carbon fixation have evolved that minimize photorespiration and optimize the Calvin cycle
First fix CO2 into a four carbon compound; when weather is hot and dry, C4 plant keeps stomata mostly closed to conserve water but continues making sugars by photosynthesis using 2 types of plant cells
- Enzyme in mesophyll cells has high affinity for CO2 and can fix carbon even when CO2 concentration in leaf is low
- The resulting 4C compound acts as carbon shuttle
- It moves into bundle-sheath cells which are packed around the veins of the leaf and releases CO2
- The CO2 concentration in these cells remains high enough for Calvin cycle to make sugars and avoid photorespiration
Examples of agriculturally important C4 plants
Sugarcane and corn
A second photosynthetic adaptation has evolved allowing species to be adapted to very dry climates; CAM plants conserve water by opening stomata and admitting CO2 only at night; CO2 is fixed into a four carbon compound which banks CO2 at night and releases it during the day so Calvin cycle can operate even with stomata closed during the day
Examples of CAM plants
Pineapples, cacti and other succulent (water storing) plants like aloe and jade plants
What is special about C4 plants
Carbon fixation and Calvin cycle occurs in different types of cells (mesophyll/bundle sheath)
What is special about CAM plants
CO2 enters at night
What do plants use the sugar molecules they produce for?
1.) 50% of carbohydrate made by photosynthesis is consumed as food source/fuel for cellular respiration in mitochondria
2.) Sugars also serve as starting material for making other organic molecules; proteins, lipids and cellulose
3.) Most plants make excess food which they store in roots, tubers, seeds and fruits
- Linked glucose molecules
- Main component of cell walls
- Most abundant organic molecule in a plant and the world
Global significance of photosynthesis
Photosynthesis provides food and O2 for almost all living organisms
*Even energy in meat was originally captured by photosynthesis
How much carbohydrate does photosynthesis produce per year?
Around 150 billion metric tons (165 billion tons)
Used to grow plants when the weather outside is too cold; solar radiation passes thru transparent walls and much of heat that accumulates inside is trapped
Global scale; solar radiation passes through atmosphere and warms Earth's surface; heat radiating from warmed planet is absorbed by greenhouse gases including CO2, water vapor and methane which then reflect some of the heat back to earth
What would earth be like without natural heating
Average air temperature would be cold (-18˚C/-.4˚F) and most of life as we know it could not exist
Is too much heating a bad thing
Yes; greenhouse gases are starting to warm the earth too much
Global climate change
A major aspect of global warming caused by increasing concentrations of greenhouse gases
Predicted consequences of global climate change
- Melting of polar ice
- Rising sea levels
- Extreme weather patterns
- Increased extinction rates
- Spread of tropical diseases
How does global climate change affect plants?
It seems that increasing CO2 levels would increase photosynthesis in plants; this has been documented, though growth rates of weeds increase more than those of crop plants and trees
How do scientists study the effects of increasing CO2 on plants?
- Many different experiments
1.) Laboratory growth chambers
- Variables carefully controlled
- Availability of facilities/resources limits scope/length
2.) Field studies in areas that naturally vary in CO2 levels
- Urban, suburban and country locations
3.) Long term and large scale field studies in which CO2 levels are manipulated
Free-Air CO2 Enrichment (FACE) Experiment
By Duke University; scientists monitored effects of elevated CO2 levels on forest ecosystems over 15 years
- 6 study sites; 30 m in diameter and ringed by 16 small towers that release CO2 concentrations; central tower adjusts distribution of CO2 so stable concentration
- Other factors (temperature/precipitation/wind patterns) varied normally b/w experiment + adjacent control plots
- Experiment showed that trees only produce 15% more wood in experimental plots than control plots
Poison Ivy Study
Compares growth of poison ivy in experimental plots with elevated CO2 and control plots; elevated CO2 plots experienced annual growth increase of 149%; elevated CO2 plots created more toxic and potent poison ivy
What is the importance of O2
1.) Used by nearly all organisms in cellular respiration
2.) High in atmosphere, high energy solar radiation converts it to ozone (O3)
Shields earth from potentially harmful ultraviolet radiation before it can reach the planet's surface
- The balance b/w ozone formation and natural destruction has been disturbed by human actions
Chemicals developed in 1930s widely used in aerosol sprays, refrigerators and styrofoam production
- Accumulating in atmosphere are not broken down in lower atmosphere but instead by solar radiation in upper atmosphere, releasing chlorine atoms
- Chlorine reacts w/ ozone reducing it to O2
- Other reactions release more chlorine destroying more O3
Findings of British Antarctic Study/National Aeronautics and Space Administration (NASA)
Ozone level above Antartica had decreased drastically; gigantic hole appears every spring
What explains the drastic depletion of the ozone in Antartica?
Susan Solomon and National Oceanic and Atmospheric Administration (NOAA) proved that ice clouds when hit by sun in the spring speed up reactions that destroy ozone; once clouds warm and disperse, reactions slow
First treaty to address earth's environment (1987); 2 dozen nations agreed to phase out CFCs; by 1990, nearly 200 nations participated
When will ozone be healed?
CFC emission is nearly zero but stability of these compounds means recovery is not expected until 2060
What problems will result from unblock UV rays?
- More skin cancer
- More cataracts
- Damaged crops
- Damage phytoplankton in oceans
Besides destroying the ozone, why are CFC harmful?
They also act as greenhouse gases; phaseout of CFCs avoided what would have been the equivalent of adding 10 gigatons of CO2 to the atmosphere