Plants Lecture 1 – The Light Reaction of Photosynthesis

Question 1

Light energy → ______ + ______ (this lecture)
which needs to happen to then allow
CO2 + H2O → ________ (next lecture)

Answer 1

ATP + NADPH

Carbohydrates

Question 2

What is the chromophore?

Answer 2

The absorbing molecule

Question 3

The energy in a quantum of light must = the energy required ________

Answer 3

to excite the chromophore

Question 4

If the chromophore loses energy it falls back down to its _____ state and gives off 4 things. What are they?

Answer 4

Ground state

Heat, fluorescence, exciton transfer, charge transfer

Question 5

What are the 2 sources of energy that drive ATP synthesis?

Answer 5

1. Chemical bonds of nutrients (food)

2. Sunlight

Question 6

Photosynthesis can be divided into 2 types/phases of reactions. What are they? Where do each of them occur in the plant?

Answer 6

1. Light-dependent reactions - thylakoid membranes (this lecture)
2. Carbon-assimilated reactions - stroma (next lecture)

A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen.

Question 7

Thylakoids stack to form _____?

Answer 7

Grana

Question 8

Structure of chlorophyll = a _______ cage + a _____ atom

Answer 8

Tetrapyrrole cage + a Mg atom (in heam it is Fe though)

Question 9

What is the main pigment used in red algae and cyanobacteria?

Answer 9

Phycobilins

Question 10

Carotenoids are accessory pigments. What are 2 of their roles?

Answer 10

1. Capture more light at wavelengths that chlorophyll can’t

2. Protect against ROS (particularly singlet oxygen - a type of ROS)

Question 11

What does a pigment’s action spectra help to tell us?

Answer 11

It tells us the wavelength at which a pigment most strongly absorbs and hence is most active at.
They help us deduce which pigment is responsible for a particular light-induced biological effect.

Question 12

What part of thylakoid membranes do the first photosynthetic steps occur? What is it made up of?
What happens here starting from when light is absorbed?

Answer 12

Photosystems which are found in the thylakoid membranes. Spinach ones comprise ~200 chlorophyll + ~50 carotenoid molecules.
2 chlorophyll molecules form the REACTION CENTRE in the middle and the leftover chloropgyll + carotenoids form ANTENNA MOLECULES.
The AMs absorb light and pass it into the middle to the RC.

Question 13

In the Reaction Centre, _____ energy is converted to ____ energy.

Answer 13

Light –> chemical

Question 14

The Antenna Molecules surrounding the Reaction Centre in Photosystems can bind to specific proteins to form specific structures. What are 2 of these structures?

Answer 14

1. CORE COMPLEX around the RC
2. LIGHT-HARVESTING COMPLEXES (LHC) around the outside of the CC e.g. LHCII which comprises 36 chlorophyll + 6 lutein molecules

Question 15

Once light reaches one of the Chls in the RC, an electron is promoted to a higher/lower energy level. This electron is then _____ to a nearby ____ ____ (which is part of the chloroplast electron transport chain and is negatively charged) and is replaced by an electron from a neighbouring ____ ____.

This continuously happens (electron passed out, one comes in to take its place, setting up a series of ____ ____. This is known as ____ ______.

Answer 15

higher, donated, electron acceptor
electron donor

redox reactions, charge separation

Question 16

Photosynthetic electron transport allows ______ + ______.

Answer 16

Proton pumping (across the thylakoid membrane) + ATP synthesis

Question 17

The cytochrome bc1 complex is analogous to _____ in the mitochondrial electron transport chain. This complex acts as a pump pumping protons across the thylakoid membrane to create a proton gradient which is required to form ____.

Answer 17

Complex III

ATP

Question 18

If we precisely orientate our photosystems (most importantly the chromophores ) we can prevent _________ by the process of ____ _____.

Answer 18

the excited state of the RC decaying to its ground state.

internal conversion.

Question 19

The reactions that occur in charge separation are _____ and ____ ____. This makes them irreversible.

Answer 19

Fast and thermodynamically downhill (negative delta g)

Question 20

Does each photosystem have the same wavelength of light that is absorbs or different?

Answer 20

Different

Question 21

What wavelengths do PSI and PSII absorb at?

Answer 21

PSI: 700nm
PSII: 680nm

** Both in red light **

Question 22

Is it PSI or PSII that rips electrons from water?

Where does this electron go?

Answer 22

PSII
Goes through PSII then the electron goes out into the electron donor and then into the Cytochrom b6f complex and into PSI. It then moves through PSI and onto another electron acceptor which then moves it onto NADP+ converting it to NADPH.
*** This is the Z-scheme

Question 23

Often photosystems work in parallel to maximise efficiency. PSI and PSII work together in a Z-_____

Answer 23

Z-scheme

“Electrons are transferred from H20 to NADP+ in a ‘Z-scheme’.”

Question 24

__ electron is transferred from H2O to NADP+ for every __ photons absorbed.
__ molecule of O2 is formed for every __ photons absorbed.

Answer 24

1, 2.

1, 8.

Question 25

Cyclic electron transfer occurs when electrons from the ______ move back to the ______ complex, resulting in the production of more ___ and less ___.

Answer 25

Ferrodoxin, cytochrome b6f complex, more ATP, less NADPH.

Question 26

PSII is a dimer/trimer. Each monomer in the dimer/trimer consists of ___ proteins. The ability to split water by this complex is dependent on the complex _____. This mechanism is similar to that found in the photo system of purple _____.

PSI is a dimer/trimer. Each monomer in the dimer/trimer consists of ___ proteins.

Answer 26

Dimer, 19, Mn4CaO5, purple bacteria

Trimer, 16