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Thylakoids: Definition & Functions

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  • 0:02 Plant & Animal Cells
  • 0:42 What Are Thylakoids?
  • 1:05 Thylakoid Structure
  • 1:50 Role of Photosynthesis
  • 3:13 Lesson Summary
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Lesson Transcript
Instructor: Jeremy Battista
Inside the leaves of plants exists numerous organelles that work together to collect and create energy. One of the most important places that this occurs is the thylakoid. We will explain what thylakoids are and what they do here.

Plants and Animal Cells

When we learn about living cells, we should have a basic understanding that all cells contain numerous organelles (subunits within cells) with different and specific jobs. In both animal and plant cells, we find very similar organelles, but there are obviously differences between animal and plant cells. One is that animals lack a cell wall; another is that plants contain something called a chloroplast. Inside of the chloroplast, we find an important structure: the thylakoid. It is inside this thylakoid that the magic happens in that we see the creation of food.

What Are Thylakoids?

Thylakoids are tiny compartments found inside of chloroplasts. Their role is to help absorb sunlight in order for photosynthesis to occur. They contain all of the chlorophyll that the plant has which, in turn, allows for the absorption of sunlight. This is why the thylakoid is the site of the light dependent portion of photosynthesis, which is the portion that requires sunlight.

Thylakoid Structure

Thylakoids are bound by a membrane, as most organelles are. They are found inside of the chloroplast in stacks. (They can also be un-stacked, as in some algae.) The stacks of thylakoids are called granum, or grana for multiples. Outside of the thylakoids is stroma, which contains chloroplast DNA as well as some assorted ribosomes and enzymes.

The grana are connected by stromal thylakoids, which are basically structures that connect to the bottom of the thylakoid and resemble a tunnel. The membrane of the thylakoid divides the surrounding chloroplast into two spaces: the thylakoid space and the stroma. It also is the site of some of the important light gathering operations of the thylakoid.

Role of Photosynthesis

Much of the light gathering and light dependent reactions occur here in the thylakoid. In the thylakoid membrane itself, two light reactions occur: photosystem I and II. In each photosystem there are different proteins and slightly different chlorophyll pigments that allow for different light absorption. The chlorophyll molecules act as giant antennas that collect the sun's energy.

The interaction between photosystem I and II is complex, but I will attempt to simplify it. Photosystem II absorbs light energy, exciting (energizing) an electron that takes off into the electron transport chain. It moves along the chain (similar to the one in respiration) and, as it does so, it gives off energy which is harnessed by the thylakoid as ATP (adenosine triphosphate, an energy molecule). It arrives at photosystem I and fills in a hole left by a departing electron that got excited as well. The second electron travels down a similar electron transport chain, except this time it ends as a different energy molecule, NADPH, to be used later.

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