15. Inputs, outputs, and locations of the light dependent and light independent stages of photosynthesis in C3 plants

Photosynthesis as an example of biochemical pathways I

What are autotrophs?

Organisms that manufacture organic material from inorganic material using light (or chemical) energy e.g. plants, algae and cyanobacteria

What are heterotrophs?

Organisms that obtain organic material by consuming other organisms (or their products), e.g. animals, fungi, most bacteria and some protists.

What is photosynthesis?

The process in which autotrophs (photoautotrophs) convert the energy in sunlight into chemical energy and then use this chemical energy to assemble simple inorganic raw materials into complex organic molecules.

Takes place in the chloroplast and occurs in 2 stages (light dependent and light independent)

What is the chemical equation of photosynthesis?

What is the word equation of photosynthesis?

Photosynthesis is …lic and …ic

Anabolic (larger molecules made from smaller molecules) and endergonic (requires energy to proceed)

What are chloroplasts?

The site of photosynthesis in eukaryotes (plants and algae).

Chloroplasts are mostly found in the cells of the mesophyll, the tissue in the interior of the leaf.

Structure/contents of chloroplasts

Chloroplasts are membrane-bound organelles. They have an inner and outer membrane, which together regulate the movement of materials into and out of the organelle.

Gel-like fluid enclosed by membranes = the stroma. Contains chloroplast DNA, starch, ribosomes and enzymes.

In the stroma, small, flattened membrane discs called thylakoids lie on top of one another.

• each stack of thylakoids = grana

• pigment chlorophyll found within the thylakoid discs

• enzymes also embedded in the thylakoid membranes

Why do most plants appear green?

Because chlorophyll absorbs the violet, blue and red wavelengths of the visible light spectrum, while the green wavelengths are predominately reflected back.

What are the stages of photosynthesis?

The light dependent stage and the light independent stage (aka the calvin cycle)

Where does the light dependent stage take place?

In the grana, on the thylakoid membranes.

Outline the light dependent stage

Chlorophyll absorbs light energy, which is used to split 12 water molecules (12H2O) into 24 hydrogen ions (24H+) and 6 oxygen molecules (6O2), and in the formation of 18 ATP molecules.

The hydrogen ions are taken up by NADP+ (a coenzyme) to form NADPH).

The oxygen is released as a waste product.

Light dependent stage (bit more detail)

Chlorophyll molecules in the thylakoids capture the energy of the sunlight. This energy is absorbed by electrons in the chlorophyll, making them high energy electrons.

Water molecules are split to produce electrons, hydrogen ions (H+, aka protons), and oxygen.

The high energy electrons are passed down a chain of electron acceptors during which they release their energy.

The electrons and hydrogen ions are loaded onto NADP+ to form NADPH+ (NADP+ + H+ + 2e− → NADPH)

The energy from the electrons is used to pump protons (H+) from the stroma to inside the thylakoid, creating a proton gradient.

The protons move down the concentration gradient from the thylakoids back into the stroma through ATP synthase, which uses their kinetic energy to produce ATP from ADP and Pi.

(In the edrolo vid he explains it as that half of the hydrogens (12) from the water molecules are loaded into NADP+, while the other 12 move through ATP synthase into the light independent stage)

Inputs and outputs of the light dependent stage of photosynthesis + their roles

Inputs:

Sunlight - initial input of energy to chlorophyll

H2O (x12) - supplier of e- and H+

NADP+ (x12) - unloaded coenzyme and acceptor of hydrogen ions and electrons

ADP + Pi (x18) - unloaded coenzyme

Outputs:

O2 (x6) - waste product

NADPH (x12) - Loaded coenzyme and donor of hydrogen ions and electrons

ATP (x18) - loaded coenzyme and energy supplier

Summarised chemical equation of the light dependent stage of photosynthesis

Where does the light independent stage (aka the Calvin cycle) take place?

In the stroma of the chloroplast

Outline the light independent stage of photosynthesis

Does not require light energy.

Hydrogen (carried by NADPH from the light dependent stage) combines with carbon dioxide.

• NADP+ is once again available to be recycled to NADPH to light dependent stage.

ADP is formed as all the ATP from the light dependent stage (and more) are used.

Glucose (C6H12O6) and 6H2O are formed.

A closer look at the light independent stage of photosynthesis

6CO2 from the atmosphere combines with a 5-carbon acceptor molecule (RuBP)

• This reaction is catalysed by the enzyme RuBisCo

The resultant 6-carbon molecule is then split into two 3-carbon molecules (3-PGA, which then becomes G3P, which is also a 3 carbon molecule)

In the cytoplasm, two G3P molecules combine to produce glucose (C6H12O6), with the remaining GAP molecules recycled into RuBP, allowing the cycle to begin again.

(Note: attached diagram only shows the carbons and coenzymes)

The light independent stage of photosynthesis (more detail, mainly from the edrolo vids and textbook

Edrolo:

The Calvin cycle uses an enzyme called RuBisCo.

6CO2 molecules from the atmosphere are broken into carbons and oxygens (6 carbons, 12 oxygens). Half of the oxygens attach again to the carbons, and half don’t.

Of the 24 hydrogen from the light dependent stage (12 from NADPH, the other 12 having moved through ATP synthase in the light dependent stage), half of those attach to the oxygens to form water, and half attach to the carbons and oxygens to form one glucose molecule (C6H12O6).

Diagram of the calvin cycle (with only the carbons and coenzymes shown)

Jac: (feel like this is much less helpful)

Inorganic CO2 is converted into the carbon in organic molecules, carbon fixation.

Carbon dioxide molecules are accepted into the Calvin cycle by organic 5C acceptor molecules.

Loaded NADPH coenzymes donate hydrogens and electrons as molecules are reduced to higher energy levels.

ATP supplies energy for the anabolic steps of this cycle

Glucose is formed as an output

Inputs and outputs of the light independent stage of photosynthesis + their roles

Inputs:

NADPH (x12) - loaded coenzyme and donor of hydrogens and electrons

ATP (x18) - loaded coenzyme and energy supplier

CO2 (x6) - supplier of carbon and oxygen atoms

Outputs:

NADP+ (x12) - unloaded coenzyme and acceptor of hydrogens and electrons

ADP and Pi (x18) - unloaded coenzyme

Glucose/C6H12O6 (x1) - Final product in photosynthesis

Summarised equation of the light independent stage of photosynthesis

What is ATP and how does it work?

Adenosine triphosphate (ATP) is the energy currency of a cell. ATP is needed for every activity that requires energy.

ATP structure - adenosine attached to a sugar group (ribose), which is bound to a chain of three phosphate groups.

• ATP is a nucleotide (sugar, nitrogen containing base, and a phosphate group)

The energy source lies in the energy rich bond holding the third phosphate to the rest of the molecule.

Outline the ADP-ATP cycle

Cells make their own ATP in a cyclic process.

When energy is used, ATP is broken down into Adenosine diphosphate (ADP).

ATP is then formed during photosynthesis, cellular respiration, etc.

Purpose of NADP+/NADPH

NADP+ and NADPH are involved in the transfer of electrons for different processes. wow great stuff