Photosynthesis is the process where plants and other photosynthesizing organisms use carbon dioxide, water, and sunlight to make glucose and oxygen. This is a process that occurs in the chloroplasts for plant cells and can be broken down into two parts. The first part is called the "light reactions". In the light reactions light energy is converted and stored into two temporary energy-storing compounds called ATP and NADPH. In this process, which occurs in the pigment found in the thylakoid, the water is also used to make oxygen. In part two of the reaction the ATP and NADPH are used in conjunction with carbon dioxide in the Calvin Cycle (also called the dark reactions) to make glucose. This occurs in the stroma of the chloroplast, which is the dense liquid that surrounds the grana stacks. (see pic below)

Please note that this is a simplified overview of the photosynthesis process...

Above: see the parts of the chloroplast that are involved in photosynthesis


Above: this is an overall summary of what happens in photosynthesis

Photosynthesis 2.JPG
Above: this breaks down the process into light reactions and the Calvin cycle. Pay attention to all of the reactants and products involved!

So Why do leaves change colors?
As the Earth makes its 365-day journey around the sun, some parts of the planet will get fewer hours of sunlight at certain times of the year. In those regions, the days become shorter and the nights get longer. The temperature slowly drops. Autumn comes, and then winter.
Trees respond to the decreasing amount of sunlight by producing less and less chlorophyll. Eventually, a tree stops producing chlorophyll. When that happens, the carotenoid pigment already in the leaves can finally show through. The leaves become a bright rainbow of glowing yellows, sparkling oranges and warm browns. What about red leaves? There is actually another pigment involved that we haven't talked about-its called Anthocyanin, and it also starts to show as the chlorophyll breaks down in the leaf. Eventually the trees will shed their leaves in an effort to conserve energy for the winter.

Cellular Respiration

Cellular respiration is a process that plants, animals, and other living things go through to make energy. It is a process that uses glucose from the food we eat to make usable energy in the form of ATP. There are two types of cellular respiration available depending on the organism. Below I will detail each.

Aerobic respiration is when oxygen is used. Glucose and oxygen diffuse into the cells and go through a multi-step process to make ATP, carbon dioxide, and water. The first break down is when the glucose is broken down into 2 pyruvic acids in a process called glycolysis. When this happens a net of 2 ATP are made. The pyruvic acids will then enter the mitochondria with the oxygen to go through the Kreb's Cycle and the Electron Transport Chain, which will convert (through a series of a lot of steps) the pyruvates and oxygen into carbon dioxide and water. By the end of these two processes, 34 additional ATP have been made. This is why the mitochondria is called the powerhouse!


However, not all organisms undergo cellular respiration in this way. Yeast go through a process of anaerobic respiration (without oxygen) called alcoholic fermentation. Glycolysis still occurs (where the glucose is broken down into 2 pyruvic acids and 2 ATP, but then the pyruvic acids are broken down further by the yeast and alcohol and carbon dioxide are produced. No additional ATP are formed! A second type of anaerobic respiration is performed by bacteria cells called lactic acid fermentation. Again, the bacteria will begin with glycolysis, but will then break the pyruvic acids down further into lactic acid. This is the liquid that you often times see in your dairy products, like in yogurt or cottage cheese! Animals have the ability to switch to lactic acid fermentation if their cells run out of oxygen in order to use the glucose and make ATP, but the process is not nearly as beneficial because so few ATP are produced.

Above: the first two show results from lactic acid fermentation while the remaining three show what can be made using alcoholic fermentation


*A fill-in-the-blank practice for photosynthesis

*A review sheet on both processes

*An extra practice worksheet for cellular respiration


-Illuminating Photosynthesis
*A website by nova to help review the concepts of photosynthesis

-Photosynthesis Review
*A great website to review through biology4 kids!