Photosynthesis: Energy from the Sun

• Lecture 7 is about Photosynthesis
• Associate Professor Tina Summerfield from the Department of Botany has prepared this lecture

Lecture 7 Objectives

• Describe the structure and function of the chloroplast.
• Outline how cells capture light energy and transduce it to cellular energy in the two stages of photosynthesis.
• Summarize the main inputs and outputs of photosynthesis.
• Explain the process of energy supply in both plant and animal cells.
• Explain the origin of chloroplasts and mitochondria (endosymbiosis).

Core Concepts

• The primary energy-carrying molecule in cells is made by photosynthesis.
• The starting molecule comes from the sun.
• Life on earth depends on energy from the sun.
• Photosynthesis occurs only in plants
• Cellular respiration occurs in plants and animals.
• Photosynthesis is responsible for almost all the planet's energy resources

Overall Photosynthesis Reaction

• 6CO2 + 6H2O, with plant and light yields C6H12O6 + 6O2

Chloroplasts

• Chloroplasts are the site of photosynthesis.
• Chloroplasts have an outer membrane, inner membrane, and stroma.
• Chloroplasts also contain DNA, ribosomes, thylakoid, thylakoid space, and intermembrane space.
• The thylakoid is functionally important.
• The stroma is functionally important.
• The thylakoid space is very functionally important.
• Light reactions occur on the thylakoid membrane
• Carbon fixation occurs in the stroma.

Light Reactions

• Photosystems capture light energy and convert it to chemical energy in the thylakoid membrane
• Protein complexes contain Photosystems and chlorophyll
• Chlorophyll absorbs light energy
• Light energy is passed to electrons, and high-energy electrons leave the photosystem
• High-energy electrons pass through protein complexes in the thylakoid membrane.
• This is the Photosynthetic electron transport chain.
• The Photosynthetic electron transport produces NADPH, oxygen, and a H+ gradient
• The H+ gradient produces ATP
• Light energy is converted into chemical energy
• Outputs are ATP and NADPH (high energy molecules) and O2
• Water is split to give oxygen, also ATP and NADPH

Calvin Cycle or Carbon Fixation

• ATP and NADPH produced in the light reactions are used to fix CO2 and produce carbohydrate
• Carbon fixation (Calvin cycle) occurs in the chloroplast stroma

Calvin Cycle Steps

• It has three steps to go from carbon dioxide to glucose

1. Fixation: 3 x 5-carbon molecules (Ribulose-1,5-bisphosphate) + 3 x CO2 --> 6 x 3-carbon molecules (3-phosphoglycerate)
2. Reduction: 6 x 3-carbon molecules (3-phosphoglycerate) are converted into a different type of 3-carbon molecule (Glyceraldehyde-3-phosphate)
3. Regeneration: converting the remaining 5 x 3-carbon molecules (Glyceraldehyde-3-phosphate), to 3 x 5-carbon molecules (Ribulose-1,5-bisphosphate).

• 1 x 3-carbon molecule leaves the cycle, this molecule combines with another 3-carbon molecule to make glucose.

Final Results of Calvin Cycle

• The output is a high energy 3-carbon molecule. Two of these 3-carbon molecules convert to glucose.
• ATP and NADPH produced in the light reactions are used to fix CO2 and produce carbohydrate
• Plant cells break down glucose from photosynthesis in the mitochondria using cellular respiration.

Photosynthesis: Inputs and Outputs

• Inputs: Light, Water, CO2
• Outputs: Glucose, Oxygen
• The ATP and NADPH produced in the light reactions are only used in the Calvin Cycle

Energy Supply in Plants & Animals

• Both plants and animals breakdown glucose in cellular respiration to generate ATP
• Animals must have an external source of glucose
• Plants generate glucose during photosynthesis and then break this down during respiration
• Almost all glucose on Earth has directly or indirectly come from photosynthesis
• ATP is generated in both respiration and photosynthesis
• ATP synthase is responsible for ATP generation in both processes
• They both require a proton gradient across a membrane in both the chloroplast and mitochondrion

Origin of Chloroplasts & Mitochondria

• Mitochondria and chloroplasts both have DNA, ribosomes and are able to make some proteins
• Both have an outer and inner membrane (chloroplasts have a third membrane system too)

Endosymbiotic Theory

• Engulfed aerobic bacterium becomes mitochondrion
• Engulfed photosynthetic bacterium becomes plastid.

Lecture 7 Summary

• Chloroplast has a three-membrane structure that houses the different stages of photosynthesis.
• The light reactions convert light energy into chemical energy.
• The carbon fixation reactions use the products of the light reactions to fix CO2 into energy-rich sugar molecules.
• Animals and plants both break down sugars in cellular respiration.
• Photosynthesis and cellular respiration have common features.
• The endosymbiotic theory supports chloroplasts and mitochondria originating from independent organisms.