UNIT 6: PHOTOSYNTHESIS

(PROCESS OF FOOD

PRODUCTION BY PLANTS)

Refer to Campbell and Reece

2010 Chapter 10 p. 187-193

WHAT IS PHOTOSYNTHESIS?The process that occurs in green plants,

whereby solar energy is converted into chemical energy and stored as organic molecules by making use of carbon dioxide, sunlight, and water. Water and Oxygen are formed as byproducts

Photosynthesis can be summarized in the following equation:

6 CO2 + 12 H2O + Light energy

C6H12O6 + 6 O2 + 6 H2O (glucose)

WHO OR WHAT CAN PHOTOSYNTHESIZE?

Photoautotrophs can photosynthesize.Include: Green plants, algae, cyanobacteria

and green protists.

Plants

Green protists

Cyanobacteria

Algae

WHY DO PLANTS PHOTOSYNTHESIZE?

To provide nutrients and oxygen for heterotrophs.

Heterotrophs are dependent on autotrophs, because they cannot produce there own food.

WHAT PART OF THE PLANT IS RESPONSIBLE FOR PHOTOSYNTHESIS?

Photosynthesis occurs in the chloroplasts of plant cells.

The chloroplasts are mainly concentrated in the mesophyll cells of leaves.

Chloroplast contain chlorophyll – green pigment that absorbs sunlight.

Chlorophyll fill the space in the thylakoid membrane.

CHLOROPLAST MESOPHYLL CELL (PALLISADE AND

SPONGY)

1 µm

Thylakoidspace

Chloroplast

GranumIntermembranespace

Innermembrane

Outermembrane

Stroma

Thylakoid

RAW MATERIALS OF PHOTOSYNTHESIS

• The raw materials of photosynthesis are:

water, carbon dioxide and sunlight.

HOW RAW MATERIALS REACH THE

CHLOROPLASTS Water is absorbed through the root hair into the xylem of the roots, into the xylem of the stem, through the xylem of the leaves into the mesophyll cells and finally into the chloroplasts.

Carbon dioxide diffuses from the atmosphere through the stomata, into the intercellular airspaces in the leaves, and finally into the chloroplasts of the mesophyll cells.

The chlorophyll and other pigments in the thylakoid membrane absorb the solar energy to drive photosynthesis

PHOTOSYNTHESIS CONSIST OF 2 STAGES:

LIGHT REACTION PHASE (Dependent on light)

DARK PHASE/ CALVIN CYCLE(Light independent)

LIGHT REACTION PHASETakes place in the thylakoids of the chloroplasts.Chlorophyll absorbs solar energy from the sun.When a chlorophyll pigment absorbs light (photon

of energy), it excites the electrons, which goes from ground state to an excited state, which is unstable, but can be used as potential energy.

When unused excited e- fall back to the ground state, photons and heat are given off.

The electrons are excited in the photosystems fount in the thylakoid membrane.

This potential energy is then used firstly to split water – into hydrogen & oxygen.

2H2O 2H2 + O2

Oxygen is released as a byproduct – diffuse through stomata into atmosphere.

The hydrogen reduces NADP+ to NADPHSome energy is then used to

photophosphorylate ADP to generate ATP.ADP + P ATP

Light

Fig. 10-5-2

H2O

Chloroplast

LightReactions

NADP+

PADP

i+

ATP

NADPH

O2

CALVIN CYCLE Carbon dioxide diffuses through the

stomata of the leave and finally into the stroma of the chloroplast.

The carbon dioxide is accepted by a 5C molecule called ribulose biphosphate (RuBP) which then forms an unstable 6C compound.

6C compound dissociates into 2 x 3C compounds called phosphoglycerate (PGA)

PGA is then reduced to phosphoglyceraldehyde (PGAL/ G3P) by accepting a phosphate from ATP and a hydrogen electron from NADPH.

Thus changing ATP back to ADP and NADPH to NADP.

PGAL are now used for the following reactions: Some PGAL are used to make RuBP again,

so that the cycle can start over again. Some PGAL are used to form hexose sugars

like glucose and fructose. Which combine to form disaccharides and polysaccharides.

* The carbohydrates can then be converted to other biological compounds like proteins or fats by adding mineral salts like nitrates and phoshates.

CO2 +RuBP(5C)

6C compound

2x PGA (3C)

PGAL

ATP = ADP + PNADPH = NADP + H

.

Fig. 10-21

LightReactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

CO2

NADP+

ADPP i+

RuBP 3-PhosphoglycerateCalvinCycle

G3PATP

NADPH Starch(storage)

Sucrose (export)

Chloroplast

Light

H2O

O2

THE NATURE OF SUNLIGHTLight is a form of energy =

ELECTROMAGNETIC ENERGY/ ELECTROMAGNETIC RADIATION

The electromagnetic energy travel in waves.Distance between crests of electromagnetic

waves = WAVELENGTHWavelength range from ≤ 1nm (gamma rays) –

≥ 1 km (radio waves)The entire range of radiation wavelengths =

ELECTROMAGNETIC SPECTRUM

ELECTROMAGNETIC SPECTRUM

UV

Fig. 10-6

Visible light

Infrared Micro-waves

RadiowavesX-raysGamma

rays

103 m1 m

(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm

380 450 500 550 600 650 700 750 nmLonger wavelengthLower energyHigher energy

Shorter wavelength

The most important part for life is the visible light (380nm – 750nm)

We can see this light as various colours.Light consist of particles = PHOTONSPhotons have energy- The shorter the wave

length the greater the energy of the photon.Therefore violet light has more energy than red

light.Photosynthesis are driven by visible light of the

sun.

MAIN PIGMENTS USED DURING PHOTOSYNTHESIS:

Chlorophyll a – Absorb violet, blue and red light. Reflects and transmits green light (that is why plant leaves appear green)

Chlorophyll b – Absorb violet, blue and red light. Reflects and transmits green light (that is why plant leaves appear green).

Carotenoids – Play an accessory role in photosynthesis. They are shades of yellow and orange and able to absorb light in the violet-blue-green range. These pigments become noticeable in the fall when chlorophyll breaks down.

 

HOW A PHOTOSYSTEM HARVESTS LIGHT

The thylakoid membrane of a chroloplast contains several photosystems.

A photosystem consist of a protein complex called a reaction-centre complex surrounded by several light harvesting complexes.

Study the diagram to understand the process of light harvesting.

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Thyl

akoi

d m

embr

ane

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

The Importance of Photosynthesis: A Review

Energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells.

Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits

In addition to food production, photosynthesis produces the O2 in our atmosphere