Topic 7 Photosynthesis

Topic 7: Photosynthesis

h0ps://youtu.be/1XilneV3cJI

During photosynthesis, plants emit what is called fluorescence – light

invisible to the naked eye but detectable by satellites orbiHng

hundreds of miles above Earth. NASA scienHsts have now established a

method to turn this satellite data into global maps of the subtle

phenomenon in more detail than ever before.

Learning Outcomes

1.  Define what is photosynthesis 2.  Explain the light dependent reacHons 3.  Explain the Calvin cycle 4.  Define what is photorespiraHon 5.  Compare the photorespiraHon in C3, C4

and CAM plants

h0ps://www.khanacademy.org/science/biology/photosynthesis-‐in-‐plants

h0ps://youtu.be/joZ1EsA5_NY


and CAM plants

Group tasks

1.  Write your definiHon of photosynthesis 2.  Find an animaHon or video on photosynthesis

that you will recommend

3.  Prepare an infographic on photosynthesis process

Flowering Plants as Photosynthesis Reactor

1.  The green porHons of plants, such as leaves, carry out photosynthesis, using carbon dioxide and water as substrates.

2.  Carbon dioxide enters leaves through stomata.

3.  The carbon dioxide and water diffuse to the chloroplast, the site of photosynthesis.

Chloroplasts

1.  The chloroplast has a double membrane that surrounds the liquid stroma.

2.  The stroma contains numerous flat thylakoid disc arranged in stacks called grana.

3.  The chlorophyll pigments imbedded in the thylakoid membranes absorb solar energy during photosynthesis.

The Photosynthesis Process

CO2 + H2O (CH2O) + O2

oxida>on

reduc>on

gain of hydrogen atoms

loss of hydrogen atoms

Solar energy Produce/genera>ng

glucose

Release/waste product

Two Sets of Reac>ons 1.  The light reacHons 2.  The Calvin cycle reacHons / light independent reacHon

Light Reac>ons Occur in granum

1.  Chlorophyll absorbs solar energy, which energizes electrons.

2.  ATP is produced using an electron transport chain.

3.  NADP+, a coenzyme, accepts electrons to become NADPH.

4.  ATP & NADPH produced in thylakoid membrane are used by the Calvin cycle in the stroma to reduce CO2 to a carbohydrate

Calvin cycle

Occur in the stroma

1.  CO2 is taken up by one of the substrates in the cycle.

2.  ATP and NADPH from the light reacHons reduce CO2 to a carbohydrate.

Calvin Cycle/Light Independent Reaction/Dark reaction


and CAM plants

LIGHT REACTIONS

The light reac>ons

The electron pathway of the light reac>ons •  The light reacHons consist of an electron pathway that produces

ATP and NADPH.

•  The pathway uses 2 photosystems to complete the light reacHons.

–  Photosystem I (PS I)

–  Photosystem II (PS II)

A photosystem (PS I & PS II) consists of 3 major parts.

1.   Pigment complex, as an antenna /light harvesHng antenna: ConsisHng 300 chlorophyll molecules and 40 beta carotenes and other accessory pigments acHng as a light harvesHng antenna

2. Reac>on center; a special chlorophyll pigment

* Pigment complex (light antenna) gathering solar energy and passed photon from one pigment to the other un9l in a reac9on center (chlorophyll a)

3. Electron acceptor molecules:

Photon is absorbed by one of the pigment molecules and,

! transfers that energy to neighboring molecules,

! unHl it reaches the acHon center where,

! the energy is used to transfer an energeHc electron to an electron acceptor.

Photosystem II contains the same kind of chlorophyll a as Photosystem I but in a different protein environment with an absorpHon peak at 680 nm. (It is designated P680).

Light reac>ons: PS II & PS I

Light reac>ons: PS II

1.  When PS II absorbs solar energy, electron in reacHon center of PS II become energized electrons

2.  Energized electron are passed to electron acceptors.

3.  PS II splits a water molecule to recover the electrons passed to the electron acceptors (at electron transport chain), releasing O2 + H2O

4.  The electron acceptors send the energized electrons down an electron transport chain.

5.  Energy is released and stored in the form of H+ gradient in thylakoid lumen

6.  Electron transport chain establishes an energy gradient.

7.  Electron transport chain carries electron, pass electron from one to the other

8.  As the electrons are passed down an electron transport chain, energy is released and stored in the form of a hydrogen ion (H+) gradient.

9.  This H+ gradient is used later in photosynthesis to produce ATP

ATP Produc>on 1.  During photosynthesis, the thylakoid space becomes an H+

reservoir.

2.  The H+ ions that fill this reservoir come from two sources.

1.  The oxidaHon of water by PS II adds H+.

2.  The flow of electrons through the electron transport chain releases energy that pumps H+ into the thylakoid space.

3.  As the H+ are released through an ATP synthase, the H+ flow down their concentraHon gradient and release energy.

4.  The ATP synthase couples that release of energy to the producHon of ATP (ADP + Pi " ATP).

Light reac>ons: PS I 1.  When PS I absorbs solar energy, energized electrons are passed to

different electron acceptors.

2.  Electrons from the end of the electron transport chain (from PS II) replace the electrons from PS I.

3.  The electron acceptors pass the electrons to NADP+ to form NADPH.

4.  Electron from PS II replace those lost by PS I

5.  When PSI absorbs solar energy, electron are energized 6.  Electron energized are passed to electron acceptor. 7.  Electron acceptor pass the electron to series of electron transport

chain, then final electron acceptor (enzyme that accepts H+) ; NADP

8.  NADP+ to form NADPH


and CAM plants

CALVIN CYCLE

•  The Calvin cycle is a series of reacHons that conHnually produce a carbohydrate (glucose) from carbon dioxide during photosynthesis.

Calvin Cycle Reac>ons The Calvin cycle has 3 steps. 1.  Carbon dioxide fixaHon

2.  Carbon dioxide reducHon

3.  RegeneraHon of ribulose-‐1,5-‐bisphosphate (RuBP)

Step 1: Fixa>on of Carbon Dioxide 1.  During the first step of

the Calvin cycle, CO2 from the air is a0ached (fixed) to RuBP.

2.  The enzyme for this reacHon is RuBP carboxylase oxygenase (rubisco).

3.  Rubisco splits the resulHng 6-‐carbon molecule to form two 3-‐carbon molecules (PGA).

Step 2: Reduc>on of Carbon Dioxide 4.  ReducHon of CO2

is a series of reacHons that uses NADPH and ATP from the light reacHons to form the carbohydrate.

–  NADPH provides electrons for the reducHon.

–  ATP provides the energy.

Step 3: Regenera>on of RuBP 5.  The product of the

Calvin cycle is glyceraldehyde-‐3-‐phosphate (G3P).

6.  About 1/6 of the G3P is used to make glucose.

7.  About 5/6 of the glucose is used to regenerate the RuBP required for the fixaHon of carbon dioxide.

The Importance of the Calvin Cycle •  The G3P molecules

produced by plants can be used to make a wide variety of chemicals :

Eg : glucose, fructose, sucrose, starch, alkaloid


and CAM plants

Photorespira>on (also known as the oxidaHve photosyntheHc carbon cycle, or C2 photosynthesis)

! a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted.

! Consumes O2 and organic fuel and releases CO2 ! Uses ATP but not making ATP ! Doesn’t make sugar ! Decreases photosynthesis output

Types of Photosynthesis : Carbon metabolism

•  Plants are physically adapted to their environment

•  Plants have metabolically adapted photosynthesis to different climates. 1.   C3 plants 2.   C4 plants 3.   CAM plants

1. C3 Photosynthesis •  In areas with moderate temperature

•  plants carry out C3 photosynthesis, •  the first detectable molecule from the Calvin cycle is a 3-‐carbon

compound.

•  FixaHon by RuBP carboxylase (RUBISCO) in C3 plants: with CO2 & O2.

•  The process of RuBP binding with O2 is known as photorespiraHon.

•  PhotorespiraHon reduce photosynthesis product yield

If the weather is hot & dry, C3 plant:

o  stomata closed o  PrevenHng the loss of H2O

o  Also prevents CO2 from entering the leaf o  Traps O2 (by-‐product of photosynthesis within the leaf space in between spongy mesophyll)

o C3 photosynthesis are necessary adapta>on to minimize photorespira>on effect (O2 competes with CO2 for the binding site on Rubisco, decreasing the efficiency of photosynthesis/yield decrease)

C3

•  C4 have an adapta>on that allow them to be successful in hot dry climates

•  These plants carry out/perform C4 photosynthesis

2. C4 Photosynthesis

The anatomy of a C4 plant is different from that of a C3 plant.

General leaf/C3 leaf C4 leaf

Keong BP Cross sec>on of leaf : General Versus C4 Leaf

C4

•  C4 photosynthesis forming a 4-‐carbon compound.

•  PEP carboxylase fix PEP (3C) with CO2 in bundle sheath cell.

•  the first detectable molecule from the Calvin cycle is a 4-‐carbon compound (OAA)

•  C4 are able to avoid the uptake of O2 by Rubisco

C4

•  The anatomy of a C4 plant is different from a C3 plant.

•  Although chloroplasts are found in both the mesophyll and bundle sheath cells, the Calvin cycle occurs primarily in the bundle sheath cells.

•  CO2 taken in by the mesophyll cells is combined with a 3-‐carbon compound (PEP @ phosphoenol phyruvate) to form a 4-‐carbon compound (oxaloacetate).

C4

The C4 pathway

CO2 is fixed by enzyme PEP carboxylase, forming a four-‐carbon product oxaloacetate (4C)

The C4 pathway

CO2 is transported into bundle sheath, released and fixed by rubisco (RuBP) to allowing Calvin cycle to proceed.

Therefore rubisco have to react with CO2 instead of O2 because O2 is absent. So no photorespira>on occurs

•  Crassulacean Acid Metabolism (CAM), found commonly in desert plants, in warm, arid regions.

3. CAM Photosynthesis

CAM •  Similar to C4 photosynthesis, CAM plants separate CO2 fixaHon from

the Calvin cycle reacHon to minimize compeHHon from O2.

•  However CAM plants separate these events by Hme. •  CO2 is fixed during the night. •  The Calvin cycle reacHons occur during the day.

The advantage of CAM photosynthesis involves the conserva>on of water.

• When CAM plants open their stomata at night to obtain CO2, water loss is minimized.

• Calvin cycle reacHon occur during the day * H2O is conserve, but CO2 cannot enter the plant

CAM


and CAM plants

Evolu>onary Trends

•  C4 plants most likely involved in areas with high light, high temperature, and low rainfall.

•  C3 plants survive be0er than C4 plants in temperatures less than 25˚C.

•  CAM plants compete well with both C3 and C4 plants, parHcularly in arid environments.

PEP carboxylase PEP carboxylase

Summary of C3, C4, And CAM Reac>on

Topic 7 Photosynthesis

Education

Transcript of Topic 7 Photosynthesis