Bio Factsheet

1

Number 153www.curriculum-press.co.uk

The Light Dependent Stage of Photosynthesis

water enters throughroot hairs

lower epidermis

guard cell

stomatal pore

Carbon dioxide enter leaves(through stomatal pore)

CO2

Oxygen given offthrough stomatal pore

O2

wat

er fl

ow b

y tr

ansp

irat

ion

SUNLIGHT

6CO2

+ 6H2O → → → → → C

6H

12O

6 + 6O

2

carbon dioxideenters the leafby diffusionthrough thestomata

water entersthrough roothairs and istransported tothe leaves inthe xylem

glucose - thussunlight energyhas beenconverted intochemicalenergy

oxygen isgiven off

Soil root/root hairs

light

loopof DNA

innermembrane

stroma

lamella

lipidstore

starchgrain

thylakoids

ribosome Structures

Ribosomes in stroma

Starch grain

Grana

Thylakoid membranes

DNA

Functions

Synthesising enzymes – Ribulosebisphosphate carboxylase, for example.

The soluble sugars made in photosynthesiscannot be stored. They can only be stored ifthey are converted into an insoluble form.

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

Compartmentalization - allowing differentenzyme – driven reactions to occur withinthe chloroplasts at the same time.

Codes for some of the chloroplast proteins.

one granum

The reaction is catalysed by light energy absorbed bychlorophyll contained in chloroplasts in leaves and greenstems.

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

Fig 2

Fig 1 summarises what you learned at GCSE.

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

Fig 1

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.

Bio Factsheet

2

153 Light Dependent Stage of Photosynthesiswww.curriculum-press.co.uk

LIRs takeplace in thestroma

thylakoidmembrane

stroma a liquidcontaining

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

sugar

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

Fig 3 b. Photosystem unit

LightLight

Reaction Centre:central chlorophyll molecules

chlorophyll a molecule

"excited" chlorophyll molecule -containing trapped energy

carotenoid molecule (accessory pigment) - passes trapped energy tochlorophyll molecule

energy passed on to the centralpair in reaction centre

thylakoid membrane

light + water

thylakoidmembrane

stroma

light reactionstake place onthe thylakoidmembrane ATP & NADPH

ATP & NADPH passed to stromatake part in the light independentreaction.

oxygen (to atmosphere)

Photosynthesis can be broken down into two stages:

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

• Starch grains• Lipids• Enzymes• DNA and RNA• Ribosomes

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

Both PSI and PSII consist of primary pigments (forms of chlorophyll a molecules) and accessory pigments (other forms of chlorophyll a, along withchlorophyll 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.

PSI

700 nmPSII

680 nm

PSI – is found on the single thylakoids. PSIabsorbs light of wavelength 700 nm mosteffectively.

PSII – is found on the thylakoids which arestacked into grana. PSII absorbs light ofwavelength 680 nm most effectively.

Fig 3a

*

*

phospholipidbilayer

light harvesting unit

Bio Factsheet

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153 Light Dependent Stage of Photosynthesiswww.curriculum-press.co.uk

Chlorophyll a and b and the caratenoids absorb different parts of the visible spectrumYou 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 thewavelengths absorbed by chlorophyll a, chlorophyll b and the caratenoids. Red (650nm) and blue(460nm) are absorbed strongly. Green (550nm) is poorlyabsorbed 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.

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

Typical Exam Questions1. 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 4. Absorption/action spectrum

Rat

e of

Pho

tosy

nthe

sis

Abs

orba

nce/

%

550 700400

Absorption spectrum

Action spectrum

Wavelength (nm)

Wavelength (nm)

violet red

Chlorophyll aChlorophyll bCarotenoids

green

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

blue

red

red and blue stimulate photosynthesis the most

blue green red

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

0

100

low

high

Bio Factsheet

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153 Light Dependent Satge of Photosynthesis

Ele

ctro

nC

arri

er(E

C)

PSI

AT

P

PSI

I

light

ene

rgy

abso

rbed

by

chlo

roph

yll

mol

ecul

es

2e−−−− −

2e−−−− −

2e−−−− −

2e−−−− −

light

ene

rgy

NA

DP

NA

DPH

(us

ed i

n L

ight

inde

pend

ent r

eact

ion)

H2O

→→→→ → 2

H+ +

½O

2 +

2e−−−− −

2e−−−− −

give

n of

f thr

ough

stom

atal

por

es

electron energy level

elec

tron

at

high

eren

ergy

lev

el

elec

tron

at

low

er e

nerg

yle

vel

“exc

ited”

elec

tron

sre

leas

ed &

boos

ted

toh

igh

eren

ergy

leve

l

loss

of

elec

tron

s fr

om P

SII

mak

es it

uns

tabl

e. P

SII

stim

ulat

es(s

plitt

ing

/ ph

otol

ysis

of

wat

er)

H2O

→→→→ →

2H+ +

½O

2 +

2e−−−− −

The

ele

ctro

ns th

en p

ass

to P

SII

mak

ing

it st

able

aga

in. T

he H

+ a

re u

sed

(alo

ng w

ith e

lect

rons

fro

m P

SI)

to r

educ

e N

AD

P

ADP + iP

ADP + iP

Ele

ctro

nC

arri

er(E

C)

Ele

ctro

nC

arri

er(E

C)

Ele

ctro

nC

arri

er(E

C)

The

ele

ctro

ns r

elea

sed

from

PSI

I pa

ssth

roug

h a

seri

es o

f ele

ctro

n ca

rrie

rs.

The

ele

ctro

ns lo

se e

nerg

y as

they

pas

sbe

twee

n E

Cs

- thi

s en

ergy

is u

sed

tom

ake

AT

P

ADP + iP

+

AT

P

AT

P

Rou

te A

Rou

te B

Rou

te A

: el

ectr

ons

are

used

as

part

of a

reac

tion

tom

ake

NA

DP

H.

Th

is i

s p

art

of

non

-cyc

lic

phot

opho

spho

ryla

tion

(N

CP

).

Wha

t hap

pens

to th

eel

ectr

ons e

mit

ted

from

PSI

? T

hey

can

go in

two

dire

ctio

ns: R

oute

A o

r B

Rou

te B

: ele

ctro

ns a

re c

ycle

d ba

ck th

roug

h th

e el

ectr

onca

rrie

rs to

gen

erat

e A

TP.

Thi

s pr

oces

s is

cal

led

cycl

icph

otop

hosp

hory

lati

on(C

CP

)It

`s c

alle

d C

PP

bec

ause

:1

.el

ectr

ons

“cyc

le”

: P

SI

→ E

Cs

→ P

SI

→ E

Cs.

....

2.

“pho

to”

- li

ght

is t

he e

nerg

y so

urce

3.

“ph

osp

ho

ryla

tio

n”

- en

erg

y

rele

ased

fr

om

th

eel

ectr

ons

is u

sed

to p

hosp

hory

late

(ad

d a

P)

to A

DP

EC

EC

EC

AT

P

AT

P

AT

PPS

I

EC

Pho

tosy

stem

s in

volv

ed P

SI &

PSI

IP

rodu

cts:

AT

P, r

educ

ed N

AD

P (N

AD

PH

), o

xyge

n2

e−−−− −

Pho

tosy

stem

s in

volv

ed P

SI

Pro

duct

s: A

TP

elec

tron

s pa

ss in

to P

SI

PSI a

bsor

bs li

ght.

Ele

ctro

ns a

gain

bec

ome

exci

ted

& e

mitt

ed. T

hey

pass

to a

n el

ectr

on c

arri

erat

an

even

hig

her e

nerg

yle

vel

7

1

2

2H+

4

3el

ectr

ons

pass

ed t

oan

ele

ctro

n ca

rrie

r

5

6

elec

tron

s co

mbi

ne w

ith H

+ re

leas

ed f

rom

pho

toly

sis

of H

2O a

nd a

re u

sed

to r

educ

ed N

AD

P

wat

erPS

II

EC

EC

EC

AT

PA

TP

AT

PPS

I

EC

H+

+ N

AD

P NA

DP

H

O2

It’s

cal

led

NC

P be

caus

e:1.

“no

n cy

clic

” -

elec

tron

s do

n’t

cycl

e (t

hey

don’

t en

dup

whe

re t

hey

star

ted.

PSII

→ E

Cs

→PS

I →

NA

DP

2.

“pho

to”

- li

ght

is t

he e

nerg

y so

urce

3.

“ph

osp

ho

ryla

tio

n”

- en

erg

y r

elea

sed

fro

m t

he

elec

tron

s is

use

d to

pho

spho

ryla

te (

add

a P

) to

AD

P