Chapter 7

PHOTOSYNTHESIS

Photosynthesis •Photosynthesis is the process of harnessing energy

from sunlight to produce sugars.

•Photosynthesis equation:

• Energy + 6 CO2 + 6 H2O C6H12O6 + 6 O2

• C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (Cell Resp.)

• The equations are the exact opposites of each other

•Photosynthesis is carried on in the green portions of plants (leaves, stems, petals, etc)

• (Plant cells also contain mitochondria, so they undergo photosynthesis AND cell respiration)

Chloroplasts •Chloroplasts contain a double membrane surrounding

a fluid called stroma

•Within the stroma are stacks of tiny membranes called thylakoids • The stacks are called “grana”

• Inside the thylakoids are pigments. The main pigment of photosynthesis is chlorophyll

•Chlorophyll is the chemical which reacts to sunlight and triggers the initiation of photosynthesis

•Other chemicals (carotene, xanthophyll, etc) absorb sunlight as well and transfer their energy to chlorophyll

Photosynthetic Pigments •The white light that comes from the sun is actually a

mixture of all colors of light

• “White” is all wavelengths of color, “black” is no wavelength of color.

•When visible light photons enter your eyes, what you are seeing are the reflected wavelengths of light from the surface you are looking at.

• All other wavelengths of color have been absorbed

•Chlorophyll is excellent at absorbing red and blue light, but not green. The green light hits the chlorophyll and is reflected off.

•This is why plants appear mostly green.

Photosynthetic Reactions •Two separate sets of reactions account for

photosynthesis

• Light Reactions, and the Calvin cycle (Dark Reactions)

•The light reactions use an electron transport chain to capture light energy for electron carriers and ATP

• Light reactions take place in thylakoids

•The Calvin cycle uses this energy from the light reactions, and CO2, to build carbohydrate sugars

• The Calvin cycle takes place in the stroma

•Thus, plants harness sunlight to provide the energy to build their own sugars for use in cell respiration.

The Light Reactions •The light reactions are two separate energy-harnessing systems called photosystems (PS) 1 and 2. • PS 1 was discovered first, but it actually comes after PS 2

•Step 1 • Chlorophyll in PS 2 absorbs blue and red wavelengths of light

from the sun.

• The absorption causes a reaction which releases an electron.

• This electron will be used to power an ETC within the thylakoid membrane

• To replace the electron, a molecule of water is split into 2 H+ and ½O2

• This oxygen goes into the atmosphere

The Light Reactions •Step 2

•As the electron powers the enzymes in the ETC, it pulls hydrogen ions across the thylakoid membrane

• This creates an unbalanced concentration of H+ ions.

• The hydrogen ions flow back to equilibrium through an ATP synthase

• The electron that was powering the ATP synthase then enters PS I

• The ATP produced here will be used to power the Calvin cycle

The Light Reactions • Step 3

• PS I begins with wavelengths of light activating chlorophyll

• These wavelengths of light are slightly different than the wavelengths that activate PS II

• The light causes an electron to pass through a second series of enzymes

• The replacement electron comes from the electron for PS II’s ETC

The Light Reactions • Step 4

• The enzymes powered in PS I attach hydrogen ions to electron carriers

• For photosynthesis, it’s NADP+ becoming NADPH

• The NADPH will also be used in the Calvin Cycle

• The purpose of the light reactions is to harness potential energy in sunlight

• This energy is transferred to the Calvin cycle, where plants will use it to make sugars for cellular respiration

ATP Synthase • The ATP synthase for chloroplasts is found embedded in

thylakoid membranes

• Hydrogen ion’s for ATP synthase come from two areas

• H+ ions pulled from the stroma into the inner thylakoid

• Extra H+ ions from the H2O that was split in PS II

• The process of using a concentration gradient to build ATP molecules is called chemiosmosis

The Calvin Cycle

•The Calvin cycle was named for Melvin Calvin who discovered the series of steps of the cycle

•Step 1: Carbon fixation

• 3 CO2 are attached to 3 molecules of Ribulose Bisphosphate (RuBP)

• The molecules produced are six 3-phosphoglycerate (PGA)

• The enzyme used to start the process is called Rubisco

• FYI: Rubisco is about 50% of the enzymes in a chloroplast. It’s unusually slow, only reacting with 5-10 substrates/second rather than 1000’s per second like typical enzymes

The Calvin Cycle •Step 2--Reduction

•Reduction means to gain an electron. Oxidation means to lose an electron.

• OIL RIG, or LEO the lion goes GER

•ATP and NADPH built in the light reactions release their energy and electrons into the PGA

• In two steps, the molecules are rearranged to form 6 molecules of glyceraldehyde-3-phosphate (PGAL)

•One PGAL will exit the cycle, the remaning five continue.

The Calvin Cycle •Step 3—Regeneration

• The remaining molecules of PGAL are rearranged to form 3 molecules of RuBP

• This process requires more ATP from the light reactions

• The RuBP will fixate with CO2 and rejoin the Calvin cycle

Counting carbons

•RuBP is a 5-carbon molecule; CO2 has one carbon, PGA and PGAL each have 3 carbons

•All carbons must be accounted for

•3 turns x 5-carbon RuBP = 15 carbons

•+ (3 turns x 1 CO2)= 18 carbons

•- 1 PGAL (3 carbons) = 15 carbons

• 15 carbons ÷ 5 = three 5-carbon RuBP

Counting Carbons

•The Calvin cycle takes three full cycles, and the only product is a single PGAL

•This PGAL will eventually bond with another PGAL from the Calvin cycle and form a sugar

•This sugar is the starting point for • Glucose for cell respiration

• Sucrose, for carbohydrate transportation in plants

• Starch, for carbohydrate storage in root systems

• Cellulose, a carbohydrate in cell walls

• Forming fatty acids for plant oils

• It’s an incredibly expensive process in terms of ATP and NADPH, BUT…all the ATP and NADPH was free

ATP Production • In respiration and photosynthesis, ATP has been

produced in two different methods, furthering the efficiency of these systems

• 1) Oxidative Phosphorylation

• Takes place during ETC in both respiration and photosynthesis

• Oxidation refers to the loss of electrons by a molecule

• Since NADH, FADH2, and NADPH all are oxidized for the purpose of creating ATP, they are considered oxidative phosphorylaters

• 2) Substrate-Level Phosphorylation

• Takes place during Glycolysis, and Krebs

• Enzymes directly attach free-floating phosphates to ADP to form ATP

Plant Anatomy

•Photosynthesis is obviously weather-dependent, and plant structures change based on this fact

•Plants have microscopic openings in leaves and stems, called stomata (sing. stoma)

•Stomata are where plants draw in CO2, but unfortunately also release water.

• If the weather is hot and dry, the stomata will close. No water is lost, but no CO2 intake either.

• This is called photorespiration

•Under these circumstances, some plants have alternative forms of photosynthesis

C3 C4 and Cam Plants

• The plants that use the system we’ve been describing so far are referred to as C3 plants, for the 3-carbon molecule that is formed

• C4 plants are able to go through photosynthesis in dry climates

• In C4 plants, CO2 is pumped into the plant through structures other than stomata to reduce water loss.

• Then the CO2 is transported to chloroplasts.

• C3: grasses, wheat, rice, oats

• C4: corn, sugarcane, bermuda grass

• CAM plants, such as cactus, undergo photorespiration by opening their stoma at night, then saving it until daylight

Photosynthesis Summary

•Energy + 6 CO2 + 6 H2O C6H12O6 + 6 O2 • Energy: Sunlight

•6 CO2 : for the Calvin cycle

•6 H2O : for the light reactions

•C6H12O6 : built in the Calvin cycle

•6 O2 : released as waste in the light reactions

Extra Credit Question • This question is worth an extra 5% on your essay exam

• You may check your answers with me ahead of time for a yes or no response as many times as you like.

•Fruits and vegetables are made of carbon compounds. Where does the plant obtain the carbon for these compounds?