biology chapter 10 - nov 2025

photosynthesis

transforms the energy of sunlight into chemical energy stored in sugars and other organic molecules

6CO2 + 12H2O + Light Energy —> C6H12O6 + 6O2 + 6H2O

glucose used to simplify, direct product is a three carbon sugar used to make glucose

-trophs

autotrophs (water, minerals from the soil, co2 from the air)

photoautotrophs

heterotrophs

• decomposers

origin of photosynthesis

group of bacteria that had infolded regions of the plasma membrane containing clusters of such molecules, infolded photosynthetic membranes function similarly to the internal membranes of the chloroplast 

endosymbiont theory — original chloroplast was a photosynthetic prokaryote that lived inside an ancestor of eukaryotic cells

how many chloroplasts are in a chunk of leaf with a top surface area of 1 mm²

half a million 

anatomy of photosynthesis

mesophyll — tissue of the interior of the leaf, where chloroplasts are mainly found (30-40 chloroplasts)

stomata (stoma) — pores that CO2 enters from

veins — water absorbed by roots travels in

stroma — dense fluids in chloroplast, calvin cycle site

membranes — 2 in chloroplast

thylakoids — third membrane system, made up of sacs (segregates the stroma from the thylakoid space), sites of light reactions

grana — columns of thylakoids

chlorophyll — green pigment

what is the O2 given off by plants derived from?

H2O not CO2

Van Niel investigated bacteria that make their carbohydrate from CO2 but do not release O2, and bacteria that used hydrogen sulfide instead of water - formed sulfur as a waist product

later, confirmed theory by using different isotopes of oxygen in water and CO2 to track the path of oxygen

H2O goes into O2

CO2 goes into CH2O + H2O

redox in photosynthesis

water is split, and its electrons are transferred along with hydrogen ions H+ from the water to carbon dioxide, reducing it to sugar
C2 reduced
H2O oxidized

electrons increase in potential energy as they move from water to sugar, endergonic

light reactions

solar energy to chemical energy

water is split (source of electrons and protons), giving off O2

light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+

NADP+ reduced to NADPH, with solar energy

generate ATP

chemiosmosis to power the addition of a phosphate group to ADP (photophosphorylation)

light energy —> NADPH + ATP
NADPH acts as reducing power that can be passed along to an electron acceptor, reducing it

no sugar produced!

on the outsides of thylakoids, NADP+ and ADP pick up electrons and phosphate respectively, NADPH & ATP are released into the stroma

calvin cycle

CO2 from the air into organic molecules — carbon fixation

reduces the fixed carbon to carbohydrate by adding electrons (provided by NADPH)

requires ATP

needs the molecules produced by light reactions

“dark reactions” none of the steps require light directly, but still occur in the daylight

electromagnetic

wavelength

electromagnetic spectrum (380 nm to 740 nm, visible)

photons

pigments — absorb visible light (the wavelengths absorbed disappear so we do not see them)

spectrophotometer — measures ability of a pigment to absorb various wavelengths

absorption spectrum — graph plotting a pigment’s light absorption versus wavelength

chlorophyll a — key pigment (violet-blue & red = best, green least) — red fluorescence & gives off heat

action spectrum — effectiveness of wavelengths in driving the process

chlorophyll b — accessory pigment (blue & orange are the best, green worst)

carotenoids — accessory pigments (blue and purple are the best, yellow/orange worst); photoprotection (absorb and dissipate excessive light energy that would otherwise damage the chlorophyll) — anti-oxidants!

chlorophyll d & f — absorb higher wavelengths of light (higher wavelength = less energy, the lower limit of energy needed for photosynthesis to occur is extended)

only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and the excited state (fluorescence given off)

photosystem

reaction-center complex surrounded by light-harvesting complexes

reaction-center complex — organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor

light-harvesting complex — various pigment molecules bound to proteins (act as antenna for the reaction-center complex)

• pigment molecules absorbs photon, energy is transferred from pigment to pigment within a light-harvesting complex, passed to the pair of chlorophyll a molecules in the reaction-center complex, they boost one of their electrons to a higher energy level & transfer it to the primary electron acceptor

primary electron acceptor — molecule capable of accepting electrons and becoming reduced

1st step of the light reactions

isolated chlorophyll fluoresces because there is no electron acceptor

PS II & PS I

each has a characteristic reaction-center complex

reaction-center chlorophyll a of PS II is P680 (red)

chlorophyll a of PS I is called P700 (wavelength 700)

several groups of bacteria have one or the other

linear electron flow

occurs during the light reactions of photosynthesis

electrons also pass from PS II to PS I via electron transport chain (Pq ,a cytochrome complex, and Pc)

production of NADPH & release of O2

cyclic electron flow

uses PS I but NOT PSII

short circuit, electrons cycle back from Fd to the cytochrome complex, then via Pc to a P700 in the PSI

generates ATP , no NADPH & O2

chemiosmosis

proton-motive force of redox energy something something electron transport chain

ATP synthase complex couples the diffusion w/ phosphorylation of ADP forming ATP

electrons come from water!!! (in oxidative phosphorylation it comes from the organic molecules)

calvin cycle part 2

uses ATP & NADPH to reduce CO2 to sugar

stroma

anabolic, building carbohydrates

produces glyceraldehyde 3-phosphate G3P

must take place three times for 1 G3P, fixing three molecules of CO2

rubisco — catalyzes first step, enzyme

plant evolution stuff

stomata close to not lose water, but this decreases CO2 levels, O2 released from the light reactions increases

photorespiration - wasteful process, consumes O2 while producing CO2, uses ATP & does not produce a sugar

C3 plants - first organic product of carbon fixation is a 3-C compound (rice, wheat, soybeans)

C4 plants - sugarcane & corn, grass, partially closes its stomata (sugar still made because they use a multistep process even under low Co2 conditions)

mesophyll cells begin photosynthesis but it is completed in bundle-sheath cells (arranged into tightly packed sheaths around the veins of the leaf) PEP carboxylase

CAM plants — pineapple, cacti, succulents, open stomata at night & close them during the day cressulacean acid metabolism, mesophyll cells store the organic acids they made during the night in their vacuoles until mornin

molecular oxygen is produced during the…

cyclic electron flow during light reactions, photosystem II

a plot of photosynthetic activity versus wavelength of light is referred to as 

an action spectrum

where do the electrons entering photosystem II come from?

water

what happens to CO2 in the calvin cycle?

it gets reduced
ATP (hydrolyzed)
NADPH (oxidized)