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 Algae Cyanobacteria

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.

MESOPHYLL CELL (PALLISADE AND SPONGY) CHLOROPLAST MESOPHYLL CELL (PALLISADE AND SPONGY)

1 µm Thylakoid space Chloroplast Granum Intermembrane Inner membrane Outer Stroma

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 (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 NADPH Some energy is then used to photophosphorylate ADP to generate ATP. ADP + P ATP

H2O + P i Light NADP+ ADP Light Reactions ATP NADPH Chloroplast O2 Fig. 10-5-2 H2O Light NADP+ ADP + P i Light Reactions ATP NADPH Chloroplast 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 + P NADPH = NADP + H

Electron transport chain Fig. 10-21 H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Starch (storage) NADPH Chloroplast O2 . Sucrose (export)

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

1 m (109 nm) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 103 m Micro- waves Fig. 10-6 1 m (109 nm) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 103 m Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Higher energy Lower energy

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.

(INTERIOR OF THYLAKOID) Photosystem STROMA Photon Primary electron acceptor Light-harvesting complexes Reaction-center complex e– Thylakoid membrane Pigment molecules Transfer of energy Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

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