Light and Photosynthesis Notes


Diagram of the visible spectrum

What is light?


Visible light is one component of the electromagnetic spectrum.  Although sunlight appears to be white light, it is actually made up of a variety of different colors. (You can separate White Light into its component colors by passing the light through a prism.)


The resulting array of colors, ranging from red at one end to violet at the other is called the visible spectrum. Each color of light has different wavelengths, and a different amount of energy.

When light strikes an object, its component colors can be Reflected, Transmitted, or Absorbed by an object.


An Object that absorbs all colors appears black.


A pigment is a molecule that absorbs certain wavelengths of light and reflects or transmits others. Objects or organisms vary in color because of their specific combination of pigments.


Wavelengths that are reflected by Pigments are seen as the object's color. For example, grass absorbs all of the colors of the spectrum except green.  Since grass reflects green, green light waves travel to our eyes.  Therefore, we see the grass as green.


Located in the membrane of the thylakoids are a variety of pigments. Chlorophyll is the most common and important pigments in plants and algae. The TWO most common types of Chlorophyll are designated

You can see in the diagram below that the chlorophylls are only slightly different in structure.

Image comparing the chemical structures of chlorophyll a and b

Chlorophyll ABSORBS VIOLET, BLUE AND RED LIGHT.  These are the wavelengths of light that photosynthesis uses. 

Chlorophyll a ABSORBS LESS BLUE light but MORE RED light than Chlorophyll b absorbs.

ONLY Chlorophyll a is directly involved in the light reactions of photosynthesis.  Chlorophyll b assists Chlorophyll a in capturing light energy and is called an accessory pigment.  By absorbing colors that Chlorophyll a CANNOT absorb, the accessory pigments enable plants to capture more of the energy in light. 

Another group of accessory pigments found in the Thylakoid Membranes, called the carotenoids,  includes yellow, red, and orange pigments that color carrots, bananas, squash, flowers, and autumn leaves. The carotenoids in green leaves are usually masked by chlorophyll until autumn when chlorophyll breaks down.


Inside a Chloroplast

A photosynthetic cell contains anywhere from one to several thousand chloroplasts. A chloroplast has two membranes. The inner membrane is folded into many layers.  A chloroplast's inner membrane layers fuse along the edges to form thylakoids. (Thylakoids are disk shaped structures that contain photosynthetic pigments.)

Image depicting the interior of a chloroplast  

Each thylakoid is a closed compartment surrounded by a central space. The thylakoids are surrounded by a gel like material called the stroma. The neatly folded layers of the thylakoids that resemble stacks of pancakes are called grana. The thylakoids are interconnected and are layered on top of one another to form the stacks of grana. (Each chloroplast may contain hundreds or more grana.)



Photosynthesis is the process that provides energy for almost all life. During photosynthesis, autotrophs use the sun's energy to make carbohydrate molecules from water and carbon dioxide, releasing oxygen as a byproduct.
The Process of PHOTOSYNTHESIS can be summarized by the following equation:

Equation showing water plus carbon dioxide plus energy in the form of light yeilds glucose and oxygen

 (Notice that the water is split to release oxygen; its hydrogen becomes part of the glucose; the carbon dioxide re-forms to include the hydrogen and makes glucose!)



Occurs in the thylakoids


Energy is captured from sunlight. Water is split into Hydrogen ions, electrons, and Oxygen (O2). Photolysis is the splitting of a water molecule.


The light reaction traps energy from sunlight and does not involve CO2 and does not make sugars. The O2 diffuses out of the chloroplast (Byproduct).



Occurs in the thylakoids

The light energy is converted to chemical energy, which is temporarily stored in ATP and NADPH.

The electrons that absorbed the energy are passed along the (electron transport chain) ETC and store energy in ATP. NADP picks of the H+ to form NADPH+ and electrons from photolysis and stores it for later use.



Occurs in the stroma


A series of reactions referred to as the CALVIN CYCLE

The chemical energy stored in ATP and NADPH powers the formation of Organic Compounds (Sugars), using carbon dioxide, CO2 .and the H+ from the NADPH+.

Photosynthesis occurs in the chloroplasts of plant cells and algae and in the cell membranes of certain bacteria.


Diagram of the Calvin Cycle

Image source


The rate at which a plant can carry out photosynthesis is affected by the plant's environment. Three things in the plant's environment affect the rate of photosynthesis:

  1. Light Intensity: this is one of the most important factors. As light intensity increases, the rate of photosynthesis initially increases and then levels off to a plateau.
  2. CO2 Levels: Increasing the level of CO2 stimulates photosynthesis until the rate reaches a plateau.
  3. Temperature: Raising the temperature accelerates the chemical reactions involved in photosynthesis. The rate of photosynthesis increases as temperature increases. The rate of photosynthesis generally peaks at a certain temperature, and photosynthesis begins to decrease when the temperature is further increased.


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