• Light energy is absorbed by chlorophyll.

• Absorbed light energy causes the emission of energised electrons.

• Some of these electrons can release their energy in a controlled fashion synthesising ATP.

• Other electrons are carried into the Dark Stage.

• The absorbed light energy is also responsible for the break up of water.

• The break up of water releases oxygen, electrons and protons (H+, hydrogen ions).

• The oxygen passes from the chloroplast, some will be used by the mitochondria for aerobic respiration and the remainder will be passed into the atmosphere.

• The electrons from water run to chlorophyll releasing energy forming ATP.

• The protons are released into the ‘pool of protons’ in the chloroplast.

• In the Dark Stage electrons from chlorophyll, protons from the pool and carbon dioxide react together forming carbohydrate

Detailed Description of Photosynthesis

The Light Phase or Light-Dependent Stage (only takes place in the presence of light)

• Cyclic Electron Transport – Pathway 1.

• Non-cyclic Electron Transport – Pathway 2.

Cyclic Electron Transport

• Light energy is absorbed by chlorophyll in the green internal membranes of the chloroplast.

• An ‘excitable’ electron in chlorophyll absorbs light energy.

• This electron is elevated to a higher energy level.

• The energised electron escapes the chlorophyll molecule.

• An electron acceptor molecule picks up this energised electron.

• The electron is passed along an ‘electron carrier system’ where its ‘excess’ energy is released.

• The excess energy is used to produce ATP by the phosphorylation of ADP.

• The electron then returns to chlorophyll after all the excess energy has been given off.

Non-cyclic Electron Transport

• Light energy is absorbed by chlorophyll in the green internal membranes of the chloroplast.

• An ‘excitable’ electron in chlorophyll absorbs light energy.

• This electron is elevated to a higher energy level.

• The energised excited electron escapes form the chlorophyll molecule.

• An electron acceptor picks up this energised electron.

• The electron acceptor passes this electron to NADP+ becoming NADP (neutral).

• NADP then receives another electron becoming NADP-.

• NADP- now attracts a hydrogen ion (H+) or proton from the proton pool in the chloroplast.

• NADP- now becomes NADPH.

• The loss of electrons from chlorophyll causes the break up of water.

• Water breaks up into oxygen, protons (hydrogen ions) and electrons.

• The oxygen passes out of the chloroplast by diffusion.

• The protons pass into the general pool of protons in the chloroplast.

• The electrons pass to chlorophyll as replacement for those lost to NADP-.

• The electrons from water pass through an electron transport chain resulting in ATP formation.

The Dark Stage

• The Light Independent Phase

• Does not require light.

• Occurs in light and also in darkness as long as the products of the light phase are still available.

• The dark stage runs if ATP, NADPH and carbon dioxide are present.

• Takes place in the liquid portion of the chloroplast.

• Carbohydrate is the crucial product of the dark stage.

• ADP, P and NADP+ are recycled for use in the light stage.

Details of Pathway

• CO₂ diffuses into the chloroplast from the mitochondria or from the external environment.

• NADPH delivers electrons and a proton to CO₂ (NADP ? NADP+ + 2e- + H+)

• Electrons, protons and carbon dioxide combine to form carbohydrate [C_x (H₂O)_y] e.g. glucose.

• The energy needed for this reaction is supplied by the break up of ATP.

• NADP+, ADP and P pass back to the light stage.

NADP+

• NADP+: nicotinamide adenine dinucleotide phosphate

• Function: to transfer energy for the formation of complex organic compounds.

• The energy is in the form of energy-rich electrons.

• In photosynthesis these electrons came from light activated chlorophyll.

• NADP+ becomes NADPH in the light stage.

• NADPH transfer electrons and protons to carbon dioxide reducing it to carbohydrate.

• NADP- is regenerated when NADPH passes on the electrons and proton.

Promoting Crop Growth in Greenhouses

Artificial Lighting

• Increase the light intensity to increase the rate of photosynthesis

• Increase the light duration to increase the total amount of photosynthesis.

Increase Carbon Dioxide Concentration:

• Raises the rate of photosynthesis so more food produced.

• Carbon dioxide enrichment is achieved by gas cylinders or kerosene burners.

Mandatory Activity

To Investigate the Influence of Light Intensity on the Rate of Photosynthesis

Textbook Diagram: set up of the investigation.

• Place a funnel over Elodea, pondweed, in a beaker of pond water at 25°C.

• The funnel is raised off the bottom on pieces of blue-tack. This allows continuous free diffusion of CO₂ to Elodea.

• Invert a test tube full of water over the stem of the funnel to collect any gas from the Elodea.

• Place the beaker on a hot plate at 25°C.

• Maintain and monitor the temperature of the water with a thermometer.

• Excess sodium bicarbonate is placed in the water to give a constant saturated solution of CO₂.

• Place the lamp (the only light source) at a predetermined distance from the plant.

• Use a light meter to measure the light intensity at this distance. Record the light intensity.

• Allow the plant five minutes to adjust to the new conditions.

• Count the number of oxygen bubbles given off by the plant in a five-minute period.

• Repeat the count twice more and calculate the average of the three readings. This is the rate of photosynthesis at that particular light intensity.

• The gas should be checked to prove that it is indeed oxygen — it relights a glowing splint.

• Repeat at different light intensities by moving the lamp to different distances.

• Run a control: identical set up but at a constant light intensity.

• Result: no change in the rate of photosynthesis.

• Conclusion: change in light intensity causes a change in the rate of photosynthesis.

• Graph the results placing light intensity on the x-axis.

Note: make sure you know the shape of this graph and are able to interpret it.

Respiration

Respiration is the enzymatic-controlled release of energy from organic compounds in a living cell.

Definitions  

Aerobic Respiration The enzymatic-controlled release of energy from organic compounds using free molecular oxygen.

Anaerobic Respiration The enzymatic-controlled release of energy from organic compounds in a living cell using substances other than free molecular oxygen as electron acceptors.

Fermentation The enzymatic controlled release of energy form organic compounds yielding simpler organic compounds.

The energy released by respiration is of very little value unless it is used to produce ATP.

ATP   Adenosine triphosphate (ATP) is the most abundant short-term energy store and immediate source of energy for cell work.

ATP: adenine + ribose sugar + three phosphate groups	Adenine + Riboseis called Adenosine
ATP = A + P + P + P	A: adenosine P: phosphate

 

ATP releases energy when the last phosphate is removed
A + P + P + P ATP		A + P + P + energy + P (free) ADP + energy + P (free)	ADP: adenosine diphosphate

 ATP can be remade by the addition of a phosphate onto ADP, i.e., by the phosphorylation of ADP. The phosphorylation of ADP requires energy. Respiration is one source of energy to produce ATP.

Living cells use up ATP at a very fast rate — a human cell needs about 2 million a second. In order to maintain constant energy, a supply of ATP must be replaced as it is used.

 

Note: Light is the energy source to make ATP in the light dependent stage of photosynthesis. ATP is made during pathway 1 and pathway 2 of the light stage. The ATP from the light stage is used to drive the dark phase reactions in the production of glucose.

Aerobic Respiration: Glucose + Oxygen ? Carbon Dioxide + Water + Energy (38 ATP) C6H12O6 + 6O2 ? 6CO2 + 6 H2O + Energy (38 ATP)

Fermentation Plants and Fungi: Glucose ? 2 Ethanol + 2 Carbon Dioxide + Energy (2 ATP) Animals and some Bacteria: Glucose ? 2 Lactic Acid + Energy (2 ATP)

Lactic acid is a colourless liquid miscible with water.