Plastids were in a symbiotic relationship with

Plastids are found in plants and some algae. They are necessary for essential life processes, like photosynthesis and food storage. Read this lesson to learn about major types of plastids and what they do to sustain life.

What Are Plastids?

Did you ever wonder how plants breathe, drink, eat, and grow? They carry out these functions just like us, yet in different ways.

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One category of specialized organelles that plants use in order to survive is plastids.Plastids are double membrane-bound organelles found inside plants and some algae, which are primarily responsible for activities related to making and storing food. Many plastids are photosynthetic, but some are not.Some of the most common plastids include:

  • Chloroplasts
  • Chromoplasts
  • Gerontoplasts
  • and Leucoplasts
Some of the major types of plastids
Types of plastids

Types and Functions of Plastids

The chloroplasts are probably the most-known of the plastids.

These are responsible for photosynthesis. The chloroplast is filled with thylakoids, which is where photosynthesis occurs, and chlorophyll.

The basic structure of the chloroplast

Chromoplasts are what the name describes, a place for the pigments to be stored and synthesized in the plant. These are found in flowering plants, fruits, and aging leaves. The chloroplasts actually convert over to chromoplasts. There are carotenoid pigments here that allow for the different colors you see in fruits and the fall leaves.

One of the main reasons for these structures and the colors is to attract pollinators.Gerontoplasts are basically chloroplasts that are going through the aging process. These are chloroplasts of the leaves that are beginning to convert into different organelles or are being re-purposed, since the leaf is no longer utilizing photosynthesis (such as in the fall months).Leucoplasts are the non-pigmented organelles. Unlike the others we have talked about, leucoplasts have no color at all.

They are found in the non-photosynthetic parts of the plant, such as the roots. Depending on what the plant needs, they may become essentially just storage sheds for starches, lipids, and proteins. They are more readily used for synthesizing amino acids and fatty acids.Leucoplasts are further subdivided into three different plastids:

  • Amyloplasts
  • Proteinoplasts
  • and Elaioplasts

Amyloplasts are the largest of the three and are charged with storing starch. Then there are the proteinoplasts that help to store the proteins that a plant needs and are typically found in seeds.

Finally, the elaioplasts are used to store fats and oils that are needed by the plant, specifically in seeds.

Evolution of Plastids

Plastids arose from what scientists believe were small prokaryotic organisms living inside of other prokaryotic organisms. They believe that these organelles, along with mitochondria, started a symbiotic, or mutually beneficial relationship, with the larger prokaryotic cell before eventually becoming part of the cell.Scientists were even able to determine that mitochondria and chloroplasts were the approximate size of some ancient bacteria and even had membranes that were similar to those bacteria.

These organelles replicate in similar ways to ancient bacteria and carry some of the same enzymes in their membranes as well.In short, scientists have accepted that mitochondria and the plastids evolved from ancient bacteria that were in a symbiotic relationship with a larger prokaryotic cell. Eventually, these bacteria evolved into eukaryotic cells with the organelles that we see and learn about today. The plastids evolved along with them.

Lesson Summary

Plastids are double membrane-bound organelles found in plants.

They arose from tiny ancient bacteria that were consumed by other large prokaryotic cells. They developed a symbiotic relationship and, as evolution continued, they evolved into plastids that exist in our eukaryotic plant cells.There are four main types of plastids:

  • Chloroplasts
  • Chromoplasts
  • Gerontoplasts
  • and Leucoplasts

Chloroplasts are found in the green portions of the plant and are the epicenters for photosynthesis. Chromoplasts are found in the colorful flowers and fruits of the plant.

They contain other color pigments meant to attract pollinators. Gerontoplasts are aging chloroplasts as the leaf dies preparing for winter.Leucoplasts are broken into three subgroups:

  • Amyloplasts
  • Proteinoplasts
  • and Elaioplasts

All are found in the roots of the plant. The amyloplasts store starch, while proteinoplasts store protein. The elaioplasts store other fats and oils. Plastids are essential for plant cell function.

Plastids – Main Terms

  • Plastids: double membrane-bound organelles found in plants
  • Chloroplasts: responsible for photosynthesis
  • photosynthesis: process by which plants breathe, eat and drink
  • Chromoplasts : found in flowering plants, fruits and aging leaves
  • Gerontoplasts: chloroplasts that are aging
  • Leucoplasts: non-photosynthetic plant parts like roots, amyloplasts, proteeinoplasts & elaioplasts
  • Symbiotic: a mutually reliant relationship

Learning Outcomes

Once the above concepts and facts have been memorized, you could be able to:

  • Define plastids and recognize the more common ones
  • Emphasize the types and responsibilities of plastids
  • Discuss the evolution of plastids

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