Light Dependent Stage of Photosynthesis Factsheet

Bio Factsheet

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Number 153

The Light Dependent Stage of Photosynthesis

This Factsheet focusses on te first stage of photosynthesis: the light dependent stage. It updates Factsheet 2 (The essential guide to photosynthesis) 1994 and reviews the exam questions which have appeared since September 2000 (all speccification)

Fig 1 summarises what you learned at GCSE.

Fig 1

SUNLIGHT

Carbon dioxide enter leaves (through stomatal pore)

CO2 stomatal pore

guard cell

lower epidermis O2 Oxygen given off through stomatal pore

water flow by transpiration

6CO 2

+

6H O 2

light

C H O + 6O

6 12 6

2

carbon dioxide water enters

enters the leaf through root

by diffusion hairs and is

through the transported to

stomata

the leaves in

the xylem

glucose - thus oxygen is sunlight energy given off has been converted into chemical

energy

The reaction is catalysed by light energy absorbed by chlorophyll contained in chloroplasts in leaves and green stems.

water enters through root hairs

Soil

root/root hairs

Photosynthesis occurs in the chloroplast and you've got to know your basic chloroplast structure (Fig 2).

Fig 2 lamella

lipid store

starch grain

thylakoids

one granum

ribosome inner membrane

loop of DNA stroma

Structures

Functions

Ribosomes in stroma Synthesising enzymes ? Ribulose bisphosphate carboxylase, for example.

Starch grain

The soluble sugars made in photosynthesis cannot be stored. They can only be stored if they are converted into an insoluble form.

Grana

Creates large surface area for chlorophyll so lots of light can be absorbed.

Thylakoid membranes Compartmentalization - allowing different enzyme ? driven reactions to occur within the chloroplasts at the same time.

DNA

Codes for some of the chloroplast proteins.

Exam Hint: In the exam you won't be asked to draw the chloroplast but could be asked to label or describe the functions of the parts.

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153 Light Dependent Stage of Photosynthesis

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Photosynthesis can be broken down into two stages:

1. The Light-Dependent Reaction (LDR) which only occurs in the light.

2. The Light- Independent reaction (LIR), which as the name implies, doesn't need light. So it can, in theory, occur day and night.

light + water

ATP + NADPH (from LDR) + CO2 (from atmosphere) sugars

light reactions take place on the thylakoid membrane

*ATP & NADPH

thylakoid membrane

stroma

LIRs take place in the stroma

oxygen (to atmosphere) *ATP & NADPH passed to stroma take part in the light independent

reaction.

sugar

thylakoid membrane

stroma a liquid containing ? Starch grains ? Lipids ? Enzymes ? DNA and RNA ? Ribosomes

Photosystems On the thylakoid membranes chlorophyll molecules are arranged into clusters called photoystems I and II (PSI and PSII) (Fig3a)

Fig 3a

680 nm

PSII

PSII ? is found on the thylakoids which are stacked into grana. PSII absorbs light of wavelength 680 nm most effectively.

700 nm

PSI

PSI ? is found on the single thylakoids. PSI absorbs light of wavelength 700 nm most effectively.

Both PSI and PSII consist of primary pigments (forms of chlorophyll a molecules) and accessory pigments (other forms of chlorophyll a, along with chlorophyll b and caratenoids) (Fig 3 b). The role of the accsesory pigments is to capture light energy and pass it to a chlorophyll a - the primary pigment.

Fig 3 b. Photosystem unit

carotenoid molecule (accessory pigment) - passes trapped energy to chlorophyll molecule

Light

chlorophyll a molecule

Light

"excited" chlorophyll molecule containing trapped energy

thylakoid membrane

phospholipid bilayer energy passed on to the central pair in reaction centre

Reaction Centre: central chlorophyll molecules

light harvesting unit

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153 Light Dependent Stage of Photosynthesis

Bio Factsheet

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Chlorophyll a and b and the caratenoids absorb different parts of the visible spectrum You need to know about two important graphs :

1. The absorption spectrum is a graph that shows which wavelengths of light are absorbed by a pigment. The absorption spectrum below shows the wavelengths absorbed by chlorophyll a, chlorophyll b and the caratenoids. Red (650nm) and blue(460nm) are absorbed strongly. Green (550nm) is poorly absorbed most of it is in fact reflected ? which is why leaves appear green.

2. The action spectrum is a graph which shows the wavelengths of light that are actually used in photosynthesis. Fig 4. Absorption/action spectrum

100

Absorption spectrum

green (550 nm) poorly absorbed - is reflected, hence leaves appear green

Chlorophyll a Chlorophyll b Carotenoids

Absorbance/ %

0 high

blue

green

Wavelength (nm)

Action spectrum

red and blue stimulate photosynthesis the most

red

Rate of Photosynthesis

low blue

400

violet

green

550 Wavelength (nm)

red 700

red

As you might expect, the absorption and action spectrums look similar ie the wavelengths that are absorbed most strongly are the ones that stimulate photosynthesis the most.

Typical Exam Questions 1. What is the advantage of having more than one type of pigment?

Answer : because each type absorbs a different part of the visible spectrum. So, having several pigments means that more light can be absorbed. The job of the accessory pigments is to absorb light energy and pass it to the primary pigment molecules.

2. Define absorbtion spectrum/action spectrum

Fig 5. Overleaf describes what happens in the light dependent stage. Start at 1 and follow the numbers round.

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153 Light Dependent Satge of Photosynthesis

electron at higher energy level

Route B: electrons are cycled back through the electron carriers to generate ATP. This process is called cyclic photophosphorylation(CCP) It`s called CPP because: 1. electrons "cycle" : PSI ECs PSI ECs..... 2. "photo" - light is the energy source 3. "phosphorylation" - energy released from the

electrons is used to phosphorylate (add a P) to ADP

Photosystems involved PSI Products: ATP

EC

EC

EC

ATP

EC

ATP

PSI

ATP

What happens to the electrons emitted from PSI? They can go in two directions: Route A or B

3 electrons passed to an electron carrier

Route B

Electron Route A

Carrier (EC)

electron energy level

Electron Carrier (EC)

2e-

2e-

ADP + iP

AT P

"excited" electrons 2 released & boosted to higher energy level

4 The electrons released from PSII pass through a series of electron carriers. The electrons lose energy as they pass between ECs - this energy is used to make ATP

Electron Carrier (EC)

2e-

ADP + iP AT P

Electron Carrier (EC)

2e-

2e-

6 PSI absorbs light. Electrons again become excited & emitted. They pass to an electron carrier at an even higher energy level

light energy

Route A : electrons are used as part of a reaction to make NADPH. This is part of non-cyclic photophosphorylation (NCP).

EC

EC

EC

ATP

EC

ATP

PSI

ATP

+ NADP H+ NADPH

water

PSII

It's called NCP because:

O2

1. "non cyclic" - electrons don't cycle (they don't end

up where they started. PSII ECs PSI NADP

2. "photo" - light is the energy source

3. "phosphorylation" - energy released from the

electrons is used to phosphorylate (add a P) to

ADP

Photosystems involved PSI & PSII

Products: ATP, reduced NADP (NADPH), oxygen

electrons combine with H+ released from photolysis of H O and are used to reduced NADP

2

+ NADP

NADPH (used in Light independent reaction)

ADP + iP

AT P 1 light energy absorbed by chlorophyll

molecules

PSI 5 electrons pass into PSI

2H+ given off through stomatal pores

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Bio Factsheet

electron at lower energy level

PSII 2e-

7 loss of electrons from PSII makes it unstable. PSII stimulates (splitting / photolysis of water) H2O 2H+ + ?O2 + 2e- The electrons then pass to PSII making it stable again. The H+ are used

(along with electrons from PSI) to reduce NADP

HO 2

2H+ + ?O + 2

2e-

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