Organisms Capture & Store Free Energy for Use in Biological

Processes

Photosynthesis & Cellular RespirationAnabolic pathway Catabolic pathway

Heterotrophs-Capture free energy present in carbon compounds

produced by other organisms.

Autotrophs-Capture free energy from physical sources in the environment.

Photosynthesis

• Plants & other photosynthetic organisms produce foods that begin food chains.

• The sun is a constant energy source.– Must be converted into a chemical energy in order

to be useful to all non-photosynthetic organisms.

• Most common chemical energy is glucose which is also the most common fuel organisms use for cellular respiration (more on that later)

How does the CO2 get to the chloroplast?

Inner Membrane

Outer Membrane

Thylakoid

Stroma

Where do plants get the resources they need to make their own food?

The sun, air, and soil

What does each resource offer the plant?

Sun is the energy source used to drive anabolic/endergonic synthesis of glucose.

Air provides the carbon necessary for glucose production.

Soil water and trace elements come from here.

What are the by-products of photosynthesis?

Oxygen and water

If plants, bacteria & other autotrophs did not make glucose from air & sunlight, , how would

the earth’s heterotrophs be affected?

They would all die once everything on earth had been eaten, since only

autotrophs can make food.

ACTION AND ABSORPTION SPECTRA OF PHOTOSYNTHESIS

Various pigments in photosynthesis absorb photons of light from specific wavelengths of the visible spectrum.

Overall Process of Photosynthesis

There are two major stages

The Light-Dependent ReactionThe Light-Independent Reaction

(Calvin Cycle)

Write a one-sentence summary, describe what happens in each

of these phases.

How does a satellite dish bring more TV stations & better reception to your TV?

The larger the parabola, the more signals it can gather & bounce on to a single focus point before it sends the

signal to your TV.

How are the pigments like a satellite dish?Accessory pigments in the thylakoid membranes train the collected energy onto a focal point so that the sum total of its strength is used to excite the electrons on chlorophyll a.

Which pigments are at the focal point? Chlorophyll a

Which pigments are accessory pigments surrounding the chlorophyll a?

Chlorophyll b & carotenoids

What are these central chlorophyll a molecules called?

Photosystems – PS I and PS II

Modern-day plants have 2 photosystems

• Photosystem I– Most efficient at absorbing wavelengths at 700nm

• Photosystem II– Most efficient at absorbing wavelengths at 680nm

• These 2 photosytems work together to bring about a non-cyclic electron transfer.

Light Dependent Reaction• Occurs in the thylakoids or grana of chloroplast.• Light is absorbed in the pigments (chlorophylls and

carotenoids) which are organized on the membranes of the thylakoids.

• The regions of organization are called photosystems which include:

• Chlorophyll a molecules• Accessory pigments• A protein matrix

• The reaction centre is the portion of the photosystem that contains:

• A pair of cholorophyll molecules• A matrix of protein• A primary electron acceptor

Photosystem IICollects photons from the surrounding pigments embedded in the thylakoid membranes & uses that gathered energy to excite two electrons on a chlorophyll a molecule in the reaction center (also embedded in the thylakoid membrane).

Chlorophyll a now as two excited, high energy electrons in PS II & needs to capture their kinesthetic energy to convert it to ATP or NADPH (the 2 energy molecules needed to fuel the Calvin cycle.

When would be the best time to capture

the electron’s energy?

These electrons are captured by the primary acceptor of the reaction center.

Water is split by an enzyme to produce electrons, hydrogen ions, and an oxygen atom. This process is driven by light energy & is called photolysis.

The electrons are supplied one by one to the chlorophyll a molecules of the reaction center.

PHOTOSYSTEM II

Chlorophyll a is a strong oxidizing agent when it has lost its electron. What will the chlorophyll a molecule do now that it is missing an electron from its orbital?

The leftover oxygen will find another broken water molecule & become O2 gas (a by-product of photosynthesis).

The excited electrons pass from the primary acceptor down an electron transport chain (ETC) losing energy at each exchange.

The energy lost from the electrons moving down the ETC drives chemiosmosis to bring about phosphorylation of ADP to produce ATP

Movement of ions down their electrochemical gradient through a selectively permeable membrane.

The Calvin cycle needs 18ATP molecules and 12 NADPH molecules for every molecule of glucose

produced.

NADPH is an energy storage/shuttle molecule.

We have just discussed how ATP is generated. How do you think NADPH is generated?

• PS I captures light energy (nearly the same manner PS II captured light to generate ATP) & generates an NADPH molecule.

• Chlorophyll a molecule from PS I replaces its missing electrons with the electrons that came from the electron transport chain following PS II.

• NADPH is not made from a chemiosmotic gradient in the thylakoids, but instead the electron pair is given to NADP+ directly to be used in the form of NADPH.

What does the Light Dependent Reactions Do Overall?

• The production of:– NADPH (Nicotinamide Adenine Dinucleotide Phosphate Hydrogen)

– ATP (Adenosine Tri-Phosphate)

• Oxygen is given off as a waste product (lucky for us ).NADPH & ATP supply the chemical energy for

the light independent reactions.

It’s time for a simulation!!!!

I need 6 volunteers

• Summarize the simulation.

• What is limiting the Calvin cycle?

• What is produced in excess?

• How can the stroma accumulate more ATP?

The amount of ATP produced.

NADPH

By running through the 1st electron transport chain of the light reactions more often, rather than running through both electron transport chains an equal number of times.

Cyclic Photophosphorylation (Cyclic Electron Flow)• Another way that light dependent reactions produce ATP

– Light is not a limiting factor– An accumulation of NADPH in the chloroplast

• The additional ATP’s are sent to the Calvin Cycle so it can proceed more rapidly

Now for the Light Independent Reactions

AKA Calvin CycleOccurs within the stroma of the

chloroplast

Light Independent ReactionsAKA: Calvin Cycle

• This reaction uses the ATP and NADPH produced by the light dependent reaction.

• We are synthesizing sugar in this reaction.

What are the starting molecules and the ending molecules?

The process begins with carbon dioxide binding to ribulose bisphosphate

After three turns of the Calvin cycle, half a glucose molecule, called G3P, is produced.

How much energy is used to fuel this anabolic process?

Sunlight, 9 ATP molecules and 6 NADPH molecules are used to make one molecule of G3P

What Happens in the Calvin Cycle?

• Ribulose bisphosphate (RbBP), a 5 carbon compound, binds to an incoming CO2 molecule in a process called carbon fixation.

– RuBP carboxylase (rubisco) catalyzes this fixation.• An unstable 6 carbon compound is made & then breaksdown

to two 3 carbon compounds known as glycerate-3-phosphate.• These molecules are acted upon by ATP & NADPH (remember

these guys?) to form 2 more compounds called glyceraldehyde 3 phosphate (triose phosphate).

• These molecules may then go in one of two directions.– Leave the cycle to become sugars– Continue in the cycle to help reproduce the originating compound RuBP

with the help of ATP

2 of these are made

Each reaction in this multi-step process is catalyzed by a reactant-specific enzyme. The 1st enzyme

performs a critical of capturing CO2 & “fixing” it so that it’s committed

to entering the Calvin cycle. Name this 1st enzyme:

Rubisco (aka RuBP carboxylase) is the enzyme that binds carbon to ribulose

biphosphate.

How does the Calvin cycle regenerate the starting molecule

ribulose bisphosphate (RuBP)?

The cycle uses a series of reactions and 3 molecules of ATP to regenerate RuBP.

Light-dependent Light-independent

Occurs in the thylakoids Occurs in the stroma

Uses light energy to form ATP & NADPH Uses ATP & NADPH to form glyceraldehyde 3 phosphate (triose

phosphate).

Splits water (photolysis) to provide replacement electrons and H+ , & to

release oxygen

Returns ADP, inorganic phosphate & NADP to the light-dependent reaction.

Includes 2 ETC’s & photosystems I & II Involves the Calvin cycle

Chloroplast structure Function allowed

Extensive membrane surface area of the thylakoids

Small space (lumen) within the thylakoids

Stroma region similar to cytosol of the cell

Double membrane on the outside

CHLOROPLAST STRUCTURE & FUNCTIONS

Chloroplast structure Function allowed

Extensive membrane surface area of the thylakoids

Allows greater absorption of light by photosystems

Small space (lumen) within the thylakoids Allows faster accumulation of protons to create a concentration gradient

Stroma region similar to cytosol of the cell Allows an area form the enzymes necessary for the Calvin cycle to work

Double membrane on the outside Isolates the working pars & enzymes of the chloroplast from the surrounding cytosol.

What happens if there is a lack of available CO2?

• Photorespiration occurs.– O2 binds to RuBP (which has a greater affinity to

oxygen) and enters the Calvin cycle.– The oxygen splits carbon chains but neither ATP

nor glucose are produced by this process.• ATP is actually consumed.

– Plants can lose as much as 50% of their fixed carbon through photorespiration.

– It is most likely to occur on hot, dry days when the stomata close to conserve water and the concentration of CO2 inside the leaf drops.

How can you explain the evolution of photorespiration when this

process appears to be expensive and counterproductive to the

survival of the plant?

Develop a supporting hypothesis to this question.

What type of challenges do you think this plant might face in its

native habitat?

dehydration

What do plants lose when their stomata are open,

collecting CO2 ?water

What part of photosynthesis would stop if water were unavailable?

• Chlorophyll a would not have an electron donor, so ATP would not be made and the Calvin cycle, in turn, would stop.

How has this plant evolved to conserve water?

• It has a thicker, waxier cuticle

• It has leaves modified to be spines so that its surface to volume ratio is reduced.

• Many cacti have clear hairs on their surfaces to reflect sunlight and make an insulated layer of humidity around the plant.

• Cacti are able to expand greatly when it

rains in order to store water for times of drought.

CAM plants & photosynthetic adaptations

• CAM (crassulacean acid metabolism): a temporal separation of the light (occurring only at light) and dark reactions (occur anytime it’s convenient).

• Open their stomata at night and keep them closed during the day.– Helps plants conserve water.– Take up CO2 & makes malic acid

– Store the organic acid in vacuoles until morning – CO2 is taken out of the malic acid & sent to the Calvin cycle.

C4: a spatial separation of light and dark reactions.

Use 2 different cells

Fixes carbon with the help of PEP

carboxylase which has a higher affinity towards

CO2

Makes oxaloacetate

C4 plants & photosynthetic adaptations

Determine the answers to the following questions.

• Why are C4 and CAM photosynthesis considered to be coping mechanisms used by plants living in arid climates?

• Describe 3 specific differences in the processes of C4 and CAM compared to the processes that occur in C3 photosynthesis.

• Do you think C4 and CAM plants photorespirate? Support your opinion with a scientific argument.

Why are C4 and CAM photosynthesis considered to be coping mechanisms used by plants living in arid

climates?

C4 plants use PEP to fix carbon, which has a much higher affinity to carbon dioxide than rubisco. This allows C4 plants to keep their stomata closed or partially closed without losing the ability to fix carbon.

CAM plants keep their stomata closed during the day to minimize water loss when the sun is hottest.

Describe 3 specific differences in the processes of C4 and CAM compared to the processes that occur in C3

photosynthesis.

C4 plants use PEP rather than rubisco to fix carbon.C4 plants have a spatial separation of carbon fixation & the Calvin cycle.C4 plants use 2 distinct types of mesophyll cells- mesophyll cells for carbon fixation and bundle sheath cells for the Calvin cycle.C4 plants store carbon as oxaloacetate.

CAM plants store carbon as an organic acid until it is needed by the Calvin cycle.CAM plants have a temporal separation of carbon fixation and the Calvin cycle.CAM plants open their stomata during the night and close them during the day.

Do you think C4 and CAM plants photorespirate? Support your opinion with a scientific argument.

C4 plants are less likely to photorespirate because photorespiration takes place when rubisco is in the presence of higher concentrations of oxygen & low concentrations of carbon dioxide. In C4 plants the Calvin Cycle occurs in bundle-sheath cells where the carbon dioxide levels are kept high.

CAM plants are unlikely to lose much of their energy to photorespiration because these plants maintain a high level of carbon dioxide by fixing adequate amounts of carbon in organic acids during the night.

Because we see C4 plants and CAM plants dominating arid environments where photorespiration would normally be very high, it can be assumed that these plants have more successfully adapted to this particular type of environmental stress.