Photosynthesis

How is energy being obtained by the organisms in this picture?

Autotrophs & Heterotrophs (Review)  Plants are able to use light energy from the sun to produce food. - Autotrophs  Organisms that obtain energy from the foods they eat. - Heterotrophs

Photosynthesis  Definition:  Photosynthesis is the process by which light energy is converted to chemical energy in form of glucose.

Energy  Energy is the ability to do work and there’s always work to be done by the cell

First Law of Thermodynamics  Energy cannot be created or destroyed—it can only be changed in form. Energy is changed from potential energy to kinetic energy (the energy of motion) Light energy is changed to chemical energy

Jan Van Helmont’s Experiment-1600s  Question: When a seedling with a mass of only a few grams grows into a tree with a mass of several tons, where does the tree’s increase in mass come from? From the soil? The air? The water?

Jan Van Helmont’s Experiment  Hypothesis: Since a plant’s roots are in the soil, perhaps the tree is eating the soil and therefore, its’ mass comes from the soil. He imagined tiny faces on the ends of roots “eating” the soil.

Jan Van Helmont’s Experiment  Procedure: Van Helmont carefully found the mass of a pot of dry soil and a small seedling.  He planted the seedling in the pot of soil, watering it regularly for 5 years.

Jan Van Helmont’s Experiment  Results: At the end of 5 years, the seedling had gained 75 kilograms.  The soil and the pot had only decreased a few grams.

Jan Van Helmont’s Experiment  Van Helmont’s Conclusion: He concluded that most of the mass of the tree must have come from the water because that was the only thing that was added to the pot. Trees are made of water??????

Van Helmont was incorrect!  Van Helmont’s experiment accounts for the hydrate or water portion of the carbohydrate produced by photosynthesis. But where does the carbon of the carbo portion come from? Glucose—the most common and important carbohydrate/sugar made by plants in photosynthesis

The Rest of the Story that Van Helmont didn’t know: CO 2 !  Although Van Helmont did not realize it, carbon dioxide in the air made a major contribution to the mass of his tree. It is the carbon in CO 2 that is used to make carbohydrates. Carbon Dioxide

Joseph Priestley’s Experiment-1780  Priestly took a candle, placed a glass jar over it, and watched as the flame gradually died out.  Something in the air was necessary to keep the candle burning.  Today we call that substance oxygen.

Plants Produce Oxygen  Priestley than found out that if he put a sprig of mint under the jar and allowed a few days to pass, the candle could be re-lighted and would remain lighted longer.  The plant produced the substance needed for the candle to burn.  Later it was shown that this would only occur if the plant was placed in the light.

Requirements for Photosynthesis  Photosynthesis requires:  Carbon dioxide  Water  Light  Pigments such as chlorophyll to absorb the light energy

Formula For Photosynthesis:  6 CO H 2 O + Sunlight → C 6 H 12 O O 2  Carbon Dioxide + Water + Light→ Glucose + Oxygen

Sunlight  Nearly all organisms on Earth depend on the sun for energy.  Sunlight contains wavelengths of energy that can be absorbed by plants.

Electromagnetic Spectrum  Energy exists in different wavelengths.  Shorter wavelengths have more energy

Why are leaves green?  Leaves contain the pigment chlorophyll.  This pigment is green because it absorbs all other colors of light except green.  It reflects green light.

When light hits a pigment, what happens?  Light can be:  Absorbed (wavelengths that are absorbed disappear to the viewer)  Reflected  Transmitted

Pigments Absorb Light Energy  The process of photosynthesis begins when light is absorbed by pigments in the plant cell.  The most important pigment is chlorophyll-a green pigment.  There are 2 types of chlorophyll: a and b Chlorophyll

Chlorophyll a and b  Chlorophyll a and b absorb wavelengths of light that are used for photosynthesis.  This energy will eventually be transferred to energy storage compounds such as ATP.

Energy-Storing Compounds  Light energy that is absorbed by chlorophyll is transferred to ATP molecules.

NADPH  Another energy storage compound is NADPH. During photosynthesis, energy is transferred to NADP+ in the form of high-energy electrons, forming NADPH

Photosynthesis: The Light and Light-Independent Reactions  Photosynthesis occurs in 2 sets of reactions:  Light Reactions-trap sunlight energy and use it to make ATP (The “photo” part of photosynthesis)  Light-Independent Reactions-use energy from ATP plus CO 2 and H 2 O to build glucose. (the “synthesis” part of photosynthesis).

Chloroplast  Photosynthesis takes place in the chloroplast.  Light energy is trapped by chlorophyll molecules embedded in the membranes of the chloroplast.

Overview of Photosynthesis

Light Reactions  In the light reactions, light energy is transformed into ATP and NADPH. This occurs in the thylakoid membranes of the chloroplast.

Light Reactions  When light hits a chlorophyll molecule, it excites an electron that then travels down an Electron Transport Chain, making ATP and NADPH  Water is split into H+ ions, electrons, and oxygen atoms. O2 is released to the atmosphere.

Light-Independent Reactions  Also called the Calvin Cycle (named for the scientist who discovered it: Melvin Calvin)  In these reactions CO 2 is used to build glucose.  This takes place in the stroma of the chloroplast.

The Calvin Cycle (The Light- Independent Reactions)  In the Calvin Cycle, CO2 is taken in by the plant.  It binds to a molecule called RUBP.  Through a series of reactions, glucose is made and RUBP is regenerated. ATP & NADPH are used in the Calvin Cycle for energy to make glucose.

Glucose

Overview of Photosynthesis  In summary, light energy is changed to chemical energy that is stored in a molecule of glucose.  Plants and almost all other organisms use glucose to provide them with energy for cellular work.