Photosynthesis Details

Photosynthesis: How do plants make food?

Objectives:

• Illustrate the cycling of matter and the flow of energy through photosynthesis and respiration
• Measure the production of one or more products of either photosynthesis or respiration

Introduction

Almost all life on Earth depends on photosynthesis.

• Recall that photosynthesis is the process by which plants use the sun's energy to make their own “food” from carbon dioxide and water.
  • For example, animals, such as caterpillars, eat plants and therefore rely on the plants to obtain energy.
  • If a bird eats a caterpillar, then the bird is obtaining the energy that the caterpillar gained from the plants.
  • So the bird is indirectly getting energy that began with the “food” formed through photosynthesis.
  • Almost all organisms obtain their energy from photosynthetic organisms, either directly, by eating photosynthetic organisms, or indirectly by eating other organisms that ultimately obtained their energy from photosynthetic organisms.
• Therefore, the process of photosynthesis is central to sustaining life on Earth.

Section 1: Overview of Photosynthesis: Where does the energy come from?

Photosynthesis is the process that converts the energy of the sun, or solar energy, into carbohydrates, a type of chemical energy.

• During photosynthesis, carbon dioxide and water combine with solar energy, yielding glucose (the carbohydrate) and oxygen.
• As mentioned previously, plants can photosynthesize, but plants are not the only organisms with this ability.
• Algae, which are plant-like protists, and cyanobacteria (certain bacteria which are also known as blue-green bacteria, or blue-green algae) can also photosynthesize.
• Algae and cyanobacteria are important in aquatic environments as sources of food for larger organisms.

Photosynthesis mostly takes place in the leaves of a plant.

• The green pigment in leaves, chlorophyll, helps to capture solar energy.
• The veins within a leaf carry water which originates from the roots, and carbon dioxide enters the leaf from the air through special pores called stomata. 

 Stomata are special pores that allow gasses to enter and exit the leaf.

 The water and carbon dioxide are transported within the leaf to the chloroplast, the organelle in which photosynthesis takes place.

• The chloroplast has two distinct membrane systems; an outer membrane surrounds the chloroplast and an inner membrane system forms flattened sacs called thylakoids.
  • As a result, there are two separate spaces within the chloroplast.
  • The interior space that surrounds the thylakoids is filled with a fluid called stroma.
  • The inner compartments formed by the thylakoid membranes are called the thylakoid space.

The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (CO2) and 6 molecules of water (H20), with the addition of solar energy, yields 1 molecule of glucose (C6H12O6) and 6 molecules of oxygen (O2).

• Using chemical symbols the equation is represented as follows:

Oxygen: An Essential Byproduct

Oxygen is a byproduct of the process of photosynthesis and is released to the atmosphere through the stomata.

• Therefore, plants and other photosynthetic organisms play an important ecological role in converting carbon dioxide into oxygen.
• Animals need oxygen to carry out the energy-producing reactions of their cells.
• Without photosynthetic organisms, many other organisms would not have enough oxygen in the atmosphere to survive.
• Oxygen is also used as a reactant in cellular respiration, so essentially, oxygen cycles through the processes of photosynthesis and cellular respiration.

THE LIGHT REACTIONS AND THE CALVIN CYCLE

The overall process of photosynthesis does not happen in one step.

• The chemical equation of photosynthesis shows the results of many chemical reactions.
• The chemical reactions that make up the process of photosynthesis can be divided into two groups:
  • the light reactions (also known as the light-dependent reactions, because these reactions only occur during daylight hours)
    • During the light reactions, sunlight splits water to release oxygen and captures it’s energy
  • the Calvin Cycle, or the light-independent reactions.
    • during the Calvin Cycle, carbon dioxide is converted into glucose, which is a type of sugar.

The light reactions:

• include the movement of electrons down the electric transport chain
• splitting water and releasing hydrogen ions into the thylakoid space
  • 1st: light hits chlorophyll
  • 2nd: electrons are excited
  • 3rd: Water is split into Oxygen & Hydrogen ions
  • 4th: Hydrogen ion gradient produced
  • 5th: Hydrogens move through ATP synthase producing ATP
  • 6th: Hydrogens from ATP synthase combines with NADP to form NADPH.

The Calvin Cycle:

• begins with carbon dioxide attaching to the carbon molecule RuBP
• forming a 6-carbon molecule
• splitting immediately in to two 3-carbon molecules
• After 2 cycles -- Glucose is the final product.

The 3-carbon product of the Calvin Cycle can be converted into many types of organic molecules.

• Glucose, the energy source of plants and animals
• cellulose, a structural carbohydrate
• starch, a long-term storage carbohydrate.

Photosynthesis is crucial to most ecosystems since animals obtain energy by eating other animals, or plants and seeds that contain these organic molecules.

• In fact, it is the process of photosynthesis that supplies almost all the energy to an ecosystem.

Lesson Summary

• The net reaction for photosynthesis is that carbon dioxide and water, together with energy from the sun, produce glucose and oxygen.
• During the light reactions of photosynthesis, solar energy is converted into the chemical energy of ATP and NADPH, and releases oxygen.
• During the Calvin Cycle, the chemical energy of ATP and NADPH is used to convert carbon dioxide into glucose.
• Links for more understanding:  

  https://www.youtube.com/watch?v=fTXh7A7Uc2M