Photosynthesis: Energy Conversion

• Photosynthesis is vital because it allows plants to convert solar energy into food.
• Animals cannot directly use sunshine for energy.
• The original solar energy is passed along the food chain when animals consume plants or other animals.

What is Photosynthesis?

• Photosynthesis converts solar energy into chemical energy used by biological systems.
• Photosynthesis consists of three major events:
• Conversion of sunlight into chemical energy
• Splitting of water (H₂O) into oxygen (O₂)
• Fixing carbon dioxide (CO₂) into sugars (C₆H₁₂O₆)
• Photosynthesis reaction formula: 6CO₂ + 12H₂O + sunlight -> C₆H₁₂O₆ + 6H₂O + 6O₂

Organisms that Photosynthesize

• Photosynthesis is performed by certain bacteria, plants, most algae, cyanobacteria, and phytoplankton.
• These organisms are photoautotrophs or producers, creating their own food and energy from the sun.
• Consumers like herbivores and carnivores rely on the products of photosynthesis made by producers.

What is Glucose?

• Glucose (C₆H₁₂O₆) is a sugar with a specific molecular structure.
• During photosynthesis, plants produce glucose molecules, converting light energy into chemical energy stored in the bonds of glucose.
• Plants use glucose for energy.
• Excess glucose is stored.
• Plants produce much more sugar than they require for survival.
• Biological energy originates from glucose.
• Animals digest plants, breaking down glucose bonds to release stored energy.

Importance of Glucose in Plants

• Glucose molecules can be broken apart for energy in power reactions.
• Plants convert glucose into carbohydrate chains called polysaccharides.
• poly means many
• saccharide means carbohydrate or sugar
• Glucose is a simple sugar as it is a small unit of carbohydrates and a monosaccharide.
• Two polysaccharide chains in plants:
• Cellulose which is a structural component
• Starch which is a long-term energy store

Tracking Atoms in Photosynthesis

• Each atom's movement can be traced through the photosynthesis reaction.
• Sunlight provides the energy for sugar.
• One molecule of glucose has 6 carbon, 12 hydrogen, and 6 oxygen atoms.
• If there are 6 carbon dioxide molecules, then there are 6 carbon atoms and 12 oxygen atoms.
• If there are 12 water molecules, there are 24 hydrogen atoms and 12 oxygen atoms.
• Before and after photosynthesis, every atom balances.

Chloroplasts

• Plant cells, certain bacteria, and some algae have acquired chloroplasts to perform photosynthesis.
• Chloroplasts contain round organelles, flattened discs called thylakoids.
• A stack of thylakoids create a granum.
• Photosynthesis occurs within chloroplasts.
• The stroma is the space inside chloroplasts.

Origin of Chloroplasts

• Long ago, plant cells were ancient eukaryotic cells that enveloped cyanobacteria.
• The cyanobacteria became part of the cell leading to the cell gaining photosynthesis.
• This is the endosymbiotic theory.
• endo means inside
• Chloroplasts have their own unique DNA, which is similar to bacterial DNA but different from plant DNA.

Cyanobacteria in Photosynthesis

• Cyanobacteria are photosynthetic.
• aqua means colored, from the Greek word cyanin.
• Not all bacteria that undergo photosynthesis are cyanobacteria.
• Some bacteria such as purple bacteria were first discovered that can photosynthesize.
• Cyanobacteria undergo photosynthesis in lakes, ponds, and oceans.
• Cyanobacteria don't have chloroplasts.

Photosynthesis in Plants

• Photosynthesis in plants happens in the chloroplasts.
• Chloroplasts have thylakoids that are stacked in granum.
• The thylakoid membranes are lined by chlorophyll and chlorophyll.
• Chlorophyll is green and the most abundant.
• Chlorophyll absorbs all colors of light except green, giving plants their visual appearance.
• Pigments harvest light energy packets called photons when absorbing light.

Photosynthesis Reaction

• Photosynthesis is divided into light and dark reactions.

Light Reactions

• Light reactions capture light energy to power photosynthesis and occur during the daytime.
• They occur in the thylakoids.
• Pigments in the thylakoid membranes form protein complexes Photosystem I and Photosystem II.
• Photosystems harvest photons to power energy-carrying molecules for the dark reactions.

Dark Reactions

• Dark reactions, known as "light-independent reactions," do not require light energy and can occur both day and night.
• The light reactions work to power the dark reactions.
• The dark reactions were discovered by three scientists, and are also called the Calvin-Bensen-Bassham cycle.
• Dark reactions occur in the stroma of chloroplasts to fix carbon dioxide.

Energy-Carrying Molecules

• ATP and NADP+ are used in photosynthesis and cellular respiration.
• NADP+ holds excited electrons (e-) from light energy harvested by chlorophyll to become NADPH.
• NADPH drives the dark reactions and reverts back to NADP+.
• ATP, adenosine triphosphate, is the "cellular currency" and powers reactions in all living cells.
• When ATP's third phosphate is broken, energy is released.
• ATP is made when adding a third phosphate group to ADP (adenosine diphosphate)

Light Reactions Process

• Energy absorbed by chlorophyll powers photosystem II which breaks bonds of water absorbed through the plant's roots.
• Oxygen atoms bind, creating the gas O₂ then O₂ leaves.
• The thylakoid is filled with protons.
• When there are more protons inside the thylakoid compared to the stroma outside, the protons leave the crowded thylakoid through the protein, ATP synthase.
• This powers ATP production.
• Light reactions happen in the thylakoid.
• Light energy absorbed by chlorophyll also powers photosystem I.
• Photosystem I charges up the energy carrier molecule NADP+ into NADPH.
• NADPH carries its energy over to the dark reactions or Calvin Cycle.

Summary of Light Reactions

• Photons are absorbed by pigments, powering photosystem I and photosystem II.
• Photosystem II splits water molecules into protons (H+) and oxygen atoms, which are expelled as O₂ gas through the stomata.
• Protons cross the thylakoid membrane, powering ATP synthase to make ATP.
• Photosystem I powers NADP+ into NADPH.
• Light-dependent reactions produce charged NADPH and ATP and release O₂.

Understanding Dark Reactions

• Dark reactions, also known as the Calvin Cycle, cycle from beginning to end with the same products.
• Dark reactions occur in the stroma of the chloroplast.
• The Calvin Cycle starts with RuBP molecules and carbon dioxide molecules.
• An enzyme called Rubisco combines them to form an unstable intermediate.
• Carbon dioxide molecules are used to start the Calvin Cycle.
• Each starting molecule and the cycle ends in glucose.
• Most of the G3P from the Calvin Cycle converts into RuBP.
• These use energy molecules from ATP.
• The process continues to begin again.
• The spent ATP from the reaction leaves ADP and a phosphate group.
• These are reused in the light reactions to make more ATP.

Key Features of Photosynthesis

• The Calvin Cycle converts carbon in carbon dioxide into glucose in the stroma through a process called carbon fixation.
• There are two steps:
• Two light-dependent photosystems
• A light-independent carbon fixation cycle called the Calvin Cycle.
• Plants convert sunlight, water, and CO₂ into glucose and ATP.

Summary of Key Points

• Plants generate oxygen and glucose for most organisms.
• Chloroplasts facilitate photosynthesis in plants.
• Thylakoids within chloroplasts facilitate light reactions.
• Light reactions convert sunlight into ATP and NADPH.
• Dark reactions or the Calvin Cycle convert CO₂ into sugar using ATP and NADPH.
• Together, light and dark reactions convert light energy into chemical energy in glucose.

Photosynthesis and Environment

• Plants help the environment by removing carbon as carbon dioxide from the air, turning it into useful carbon based forms.
• Fossil fuels release harmful carbon dioxide, increasing the amount of carbon in the atmosphere.
• The excessive amount of greenhouse gasess, such as carbon dioxide, trap heat.
• Earth is becomming warmer.
• Burning fossil fuels happens at a rate that is too high for plants and oceans to absorb.
• Deforestation contributes to atmospheric carbon, and the destruction of forests reduces Earth potential to reduce carbon.