Chloroplasts

there are about half a million chloroplasts in a chunk of a leaf with a surface area of 1 mm^2

they're found in the cells of the mesophyll, the tissue in the interior of the leaf

oxygen exits through microscopic pores called stomata

water is then absorbed by the roots and delivers to the leaves in veins

leaves also use veins to export sugar to the roots and other nonphotosynthetic parts of the plant

a chloroplast has two membranes surrounding a dense fluid called that stroma

suspended within the stroma is a third membrane system, made up of sacs called thylakoids (which segregates the stroma from the thylakoid space inside these sacs

in some places, thylakoid sacs are stacked in columns called grana

chlorophyll (the green pigment in leaves) resides in the thylakoid membranes of the chloroplast

the two stages of photosynthesis are known as the light reactions (the photo part) and the Calvin cycle (the synthesis part)

water is split, giving a source of electrons and protons (H ions) and giving off oxygen as a by-product

light absorbed by chlorophyll drives a transfer of the electrons and H+ ions from water to an acceptor called NADP+ where they are temeporarelly stored

using solar energy, it converts NADP+ to NADPH by adding a pair of electrons along with an H+

these reactions also generate ATO (using chemiosmosis to power the addition of a phosphate group to ADP, a process called phosphophorylation

light energy is initially converted to chemical energy in the form of NADPH and ATP (NADPH acts as 'reducing power' and ATP is the versalitile energy currency of cells

THIS PART DOES NOT CREATE ANY SUGAR

the cycle begins by using carbon dioxide from the air and incorporating it into organic molecules already present in the chloroplast

this incorporation of carbon into organic compounds is called carbon fixation

the Calvin cycle then reduces the fixed carbon to carbohydrate by adding electrons, the reducing power is provided by NADPH (which was acquired during the light reactions)

to convert the carbon dioxide to carbohydrate, this cycle also requires chemical energy in the form of ATP (also made by the light reactions)

THE CALVIN CYCLE MAKES SUGAR BUT ONLY SO BC OF THE NADPH AND ATP PRODUCED FROM THE LIGHT REACTIONS

electromagnetic waves are disturbances of electric and magnetic fields

electromagnetic energy travels in rhythmic waves analogous to those created by dropping a pebble into a pond

wavelength is the distance between the crests of electromagnetic waves

wavelengths range from less than a nanometer (gamma) to more than a kilometer (radio); the entire range of radiation is known as the electromagnetic spectrum

the segment most important to life is about 380 nm to 750 nm in wavelength
the above band description is known as visible light bc it can be detected as various colors by the human eye

photons are not tangible, but they make lights behave as though it consists of discrete particles (the particles are photons)

spectrophotometer = the measurement of the ability of a pigment to absorb various wavelengths of light

absorption spectrum = a graph plotting a pigment's light absorption versus wavelength

chlorophyll a = the key light-capturing pigment that participates directly in the light reactions; suggests that violet-blue and red light work best for photosynthesis (since they are absorbed) and green is the least effective color

chlorophyll b = the accessory pigment

light drives the synthesis of ATP and NADPH by energizing the two types of photosystems embedded in the thylakoid membranes of chloroplasts

the key to the above statement is a flow of electrons through the photosystems and other components built into the thylakoid membrane - this is called linear electron flow

LEF occurs during the light reactions of photosynthesis

STEPS ☑

1. A photon of light hits one of the pigment molecules in a light-harvesting complex, boosting one of its electrons to a higher energy level. When this electron falls back to its original ground state, an electron nearby goes into its excited stat. This process continues until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state
   
   
   

2. This electron is transferred from the excited P680 to the primary electron acceptor
   
   
   

3. An enzyme then catalyzes the splitting of water into two electrons, two hydrogen ions (H+), and an oxygen atom. The electrons are then given one by one to the P680 pair, each electron replacing the one transferred to the PER. The hydrogen ions are released into the thylakoid space. The oxygen atom combines with another oxygen atom generated by the splitting of another water molecule. forming O2

4. Each excited electron passes from the primary electron acceptor of PS II to PS I by an electric transport chain
   This chain is made up of the electron carrier plastoquinone, which is a cytochrome complex, and a protein called plastocyanin. Each component carries out redox reactions as electrons flow down the ETC and releasing free energy that is used to pump H+ protons into the thylakoid space, contributing to a proton gradient across the thylakoid membrane
   
   
   

5. The potential energy stored in the proton gradient is used to make ATP in a process called chemiosmosis
   
   
   

6. Light energy has been transferred by light-harvesting complex, exciting an electron of the P700 pair of chlorophyll a molecules. The excited electron is then transferred to PS I's PEA and creates an electron "hole" in the P700. Basically, now P700 can act as an electron acceptor, accepting an electron that reaches the bottom of the ETC from PS II

7. Excited electrons are passed through redox reactions from the primary electron acceptor of PS I down a second ETC through the protein ferredoxin (Fd)
   
   
   

8. The enzyme NADP reductase catalyzes the transfer of electrons from Fd to NADP. Two electrons are required for its reduction to NADPH. Electrons in NADPH are at a higher energy level than they are in water so they're more ready for the reactions of the Calvin cycle

a second photosynthetic adaptation to arid conditions has evolved in many succulent plants, cati, pineapples, and some other plant families

these plants open their stomata during the night and close them during the day (reverse of other plants)

closing the stomata helps them conserve water, but it also helps prevent CO2 from entering the leaves

during the night when their stomata are open, the plants take CO2 and incorperate it into different organic acids. This mode of carbon fixation is called crassulacean acid metabolism (CAM)

the mesophyll cells of these CAM plants store the organic acids in their vacuoles until morning when the stomata closes

during the day (when light reactions can supply ATP and NADPH for the Calvin cycle) CO2 is released form the organic acids to become incorporated into the sugar in the chloroplasts

excited electrons can take this path; the difference is that this path allows the electrons to use photosystem I but not photosystem II

chloroplasts and mitochondria make ATP the same way: through chemiosmosis

an electron transport chain pumps H+ protons across a membrane as electrons are passed through a series of carriers that are progressively more electronegative

ETC transform redox energy to proton-motive force, potential energy stored in the form of an H+ gradient across a membrane

an ATP sythase complex in the same membrane couples the diffusion of H ions down their gradient to the phosphorylation of ADP (forming ATP)

both work by the way of chemiosmosis, but chloroplasts, the high-energy electrons dropped down the transport chain come from water while in mitochondria, they are extracted from organic molecules (which are then oxidized)

chloroplasts do not need molecules from food to make ATP (photosystems), but mitochondria transfer chemical energy from food molecules to make ATP. chloroplasts use it to transform light energy into chemical energy into ATP

the carbohydrate produced from the Calvin cycle is not glucose

it is a three-carbon sugar called glyceradhyde 3-phosphate (G3P)
for the synthesis of one molecule of G3P the cycle has to go three times (obe per turn of the cycle)

the Calvin cycle incorperates each CO2 molecule by attaching it to a 5-carbon sugar named ribulose bisphosphate (RuBP)

the enzyme rubisco (carboxylase-oxygenase) catalyzes this first step

the product of this reaction is a six-carbon intermediate that is so unstable that it immediately split in half, forming two molecules of 3-phosphoglycerate (for each CO2 fixed)

each molecule of 3-phosphoglycerate gets an extra phosphate group from ATP, becoming 1,3-bisphosphoglycerate

a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate (which loses a phosphate group in the process) becomes glyceraldehyde 3-phosphate (G3P)

the electrons from NADPH reduce a carboxyl group on 1,3-bisphosphoglycerate to the aldehyde group of G3P, which stores more potential energy

for every 3 molecules of carbon dioxide that enter the cycle, there are 6 molecules of G3P formed

final result is 18 carbons' worth of carbohydrate in the form of six molecules of G3P

one molecule exits the cycle to be used by the plant, but the other 5 molecules are recycled to regenerate 3 molecules of RuBP

the carbon skeletons of 5 molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP

the cycle spends three more molecules of ATP

the RuBP is ready to receive CO2 again and the cycle continues

C3 plants = basically 'normal' plants that don't have photosynthetic adaptations to reduce photorespiration

these plants fixate carbon dioxide by rubisco

about 85% of all plants on earth are C3 plants

C4 plants = the light-dependent reactions and the Calvin cycle are physically seperated, with light-dependent happening in the mesophyll cells and the Calvin cycle occuring in special cells around the leaf veins

CAM plants = came up with a way to minimize photorespiration

instead of separating light and Calvin, CAM plants seperate by time

CAM open their stomata at night instead of during the day, they don't open their stomata during the day but can still photosynthesize