UNIT 6: PHOTOSYNTHESIS

(PROCESS OF FOOD

PRODUCTION BY PLANTS)

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 PHASE Takes place in the thylakoids of the chloroplasts.

Chlorophyll absorbs solar energy from the sun.

When a chlorophyll pigment absorbs light 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, 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 NADPH Some energy is then used to

photophosphorylate ADP to generate ATP. ADP + P

ATP

[Light

Fig. 10-5-2

H2O

Chloroplast

LightReactions

NADP+

P

ADP

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)

PGALATP = ADP + P

NADPH = NADP + H

.

Fig. 10-21

LightReactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

CO2

NADP+

ADPP i+

RuBP 3-Phosphoglycerate

CalvinCycle

G3PATP

NADPHStarch(storage)

Sucrose (export)

Chloroplast

Light

H2O

O2

THE NATURE OF SUNLIGHT

Light is a form of energy = ELECTROMAGNETIC ENERGY/ ELECTROMAGNETIC RADIATION

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

waves = WAVELENGTH Wavelength 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

InfraredMicro-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 nm

Longer wavelength

Lower 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 = PHOTONS Photons 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

Th

yla

koid

me

mb

ran

e

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