Chloroplasts, Light, and Pigments

Inside a Chloroplast

A plant or algae cell contains anywhere from one to several thousand chloroplasts. Each chloroplast’s role is to capture light and process it so that it can store the light energy in glucose. Let’s take a look at a chloroplast’s internal structure so that we can better understand how it captures light and performs photosynthesis.

ChloroplastLabel

Notice that the chloroplast has two membranes. The inner membrane is folded into many layers. The inner membrane layers fuse along the edges to form thylakoids. Thylakoids are disk shaped structures that contain photosynthetic pigments. Each thylakoid is a single, closed compartment.

The neatly folded layers of the thylakoids that resemble stacks of pancakes, or coins, 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.

The thylakoids are surrounded by a gel like material called the stroma.

Visible Light is Used in Photosynthesis

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 individual colors by using a prism.

Look at the picture below that shows the visible spectrum, or the resulting array of colors, ranging from red at one end to violet at the other. Each color of light has different wavelengths, and a different amount of energy.

chloroplastspectrum

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.

chloroplastStopAndThink

What are pigments?

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.

Chlorophyll

Chlorophyll is a type of pigment used by plants to trap the energy in sunlight for use in photosynthesis.

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 as

  1. Chlorophyll a and
  2. Chlorophyll b

You can see in the diagram below that the chlorophylls are only slightly different in structure. Each also has a long hydrocarbon tail that easily sits in the membranes of the chloroplast so that the larger portion can collect light like a solar cell:

chloroplastChlAandB

Both chlorophyll absorb blue, violet and red light. Look at the diagram below. The peaks show us the wavelengths of light that chlorophyll a and b can use:

chloroplastabsorbspectrum

You should have noticed that 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.

Accessory Pigments

By absorbing colors that chlorophyll a cannot absorb, the accessory pigments enable plants to capture more of the energy in light.

The green we see in many plant leaves is actually a combination of a number of pigments, with the majority of those pigments being chlorophyll. The amount and variety of those pigments determine the color of the leaf and result in the different shades of green that we see in nature. Remember that the color of the pigment we see reflected is not absorbed by the plant for use in photosynthesis.

Plant Pigments and Colors That They Reflect

Pigment Name Color(s) Pigment Reflects
Cholorophyll a, b Green
Carotene Orange
Xanthophyll Yellow
Anthocyanin Red, Purple
Phaeophytin Gray-Brown, Yellow-Brown

Plants with other colors of leaves have more or less of the pigments deposited in their leaves. There are also plants that change their colors seasonally due to environmental triggers (leaves changing color in the fall).

Paper Chromatography

Scientists use paper chromatography to find out which pigments are present in plant leaves. This involves dissolving the pigments in a solvent that carries them up absorbent paper at different rates.

 

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