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This lesson will focus on the major organelles that are found inside of eukaryotic cells. It will discuss their structures and functions. The differences in organelles found in plant and animal cells will also be covered.

Definition of an Organelle

Living things are made of cells, or biological units also referred to as the building blocks of life. Some organisms consist of just one cell, while others are made of trillions of cells.

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Each one of these cells functions as a tiny factory, with individual parts that work together to keep the cell alive and, in turn, keep the organism going. These parts are called organelles.

Organelles and Their Functions

The nucleus is arguably the most important organelle in the cell. It is the control center, telling all of the other organelles what to do and when to do it.The nucleus also contains all of the cell’s genetic material, or its DNA. This material has all the instructions the cell needs for making proteins and many other important molecules.

The nucleus is surrounded by two membranes. These membranes have many openings in them, which allow for the transport of materials into and out of the nucleus. RNA, proteins, and other molecules move out of the nucleus into the rest of the cell.

The cell’s chromosomes are also found in the nucleus. These condensed strands of DNA are what carry all of the cell’s genetic information.When the cell is in its growing phase, the DNA is elongated into very thin, hair-like structures. When the cell is getting ready to divide, the DNA condenses into chromosomes and the material gets duplicated.Within the nucleus of eukaryotic cells is a structure called the nucleolus. This is the site of ribosome formation.Prokaryotic cells lack a nucleus.

In these organisms (which include the bacteria), the genetic material is free-floating within the cell membrane. The genetic material of prokaryotes is a different shape than that of eukaryotes, but it serves the same function.Outside of the nucleus but within the cell membrane is a gel-like substance called cytoplasm (also called cytosol). It is made mostly of water and dissolved salts. It bathes the organelles and keeps them healthy. It is also the medium through which materials move about the cell.

Within the cytoplasm is a network of tiny tubes called the cytoskeleton. These tubes are used to give the cell structure and also to support the organelles by holding them into place. Some parts of the cytoskeleton also work to transport certain things between different parts of the cell.The cytoskeleton is made from two different components: microtubules and microfilaments. Microtubules are hollow and made of protein. They are very important in helping to maintain the cell’s shape. They are also important during cell division.

When this occurs, the microtubules form structures called spindle fibers, which allow the chromosomes to separate. Microtubules are also part of the different projections coming off of the cell surface. These structures, called cilia and flagella, assist with cell locomotion.Microfilaments are very thin threads made of protein. Like the microtubules, they help the cell keep its shape.Making proteins is a very important job for a cell.

Ribosomes are small pieces of RNA found throughout the cytoplasm and on some other organelles. Their only job is to assemble proteins.DNA coding tells them which proteins to make. Prokaryotic cells can have tens of thousands of ribosomes. Eukaryotic cells can have hundreds of thousands, if not millions of them, all making proteins.

Once proteins are made, they need to be moved to different parts of the cell. The endoplasmic reticulum (E.R.) is a collection of lipid membranes that work to move the proteins from one area of the cell to another.The E.R.

comes in two different forms – rough and smooth. Rough E.R. has ribosomes attached to it. These ribosomes make proteins that are scheduled to leave the cell. The E.

R. transports these proteins to another organelle that will package them up and ship them out. The smooth E.

R. does not have any ribosomes on its surface. This is where the lipid part of the cell membrane is assembled.Once the ribosomes on the E.

R. have made the proteins for exporting from the cell, they need to be packaged in such a way that they can leave the cell and be taken in by the parts of the body that need them. The organelle responsible for this is called the Golgi apparatus (or Golgi bodies), which looks like a stack of pancakes. The Golgi apparatus changes, sorts and packages the proteins as they leave the E.

R. so they are ready to leave the cell. Because it is so important to shipping things out of the cell, the Golgi apparatus is always found near the cell membrane.Cell parts wear out and die. These pieces of ‘garbage’ need to be disposed of somehow. That is the job of the lysosomes.

These structures are filled with digestive enzymes that break down those items that would become toxic if they were left in the cell. Lysosomes also break down proteins, lipids and carbohydrates into more usable forms for the cell.Living things need energy in order to perform daily functions. The mighty mitochondria are the organelles that make energy by converting food into more usable forms for the cell.The mitochondria have a double membrane, just like the nucleus. The inner membrane is folded into many twists and turns that increase the surface area where energy can be created.

In a cell, storage of materials is carried out by vacuoles and vesicles. These structures are large, sac-like and enclosed by membranes.Vacuoles store water, proteins, carbohydrates and salts. In those creatures that live in the water, the vacuole is the way the amount of water coming and going from its body is controlled.Vesicles are smaller structures found in eukaryotic cells.

They store and move things between the different organelles, as well as between the cell and its outside environment.

How Plant Cells Are Different

Plants are eukaryotic, just like animals. In this regard, they have all the aforementioned organelles. However, because plants have some different characteristics and processes than other eukaryotes, their cells contain a few additional organelles.

Plant Organelles

Plants are usually green in color. This is because they contain a green pigment called chlorophyll, which is used to collect the light energy from the sun. Plants use this pigment, along with water and carbon dioxide, to make food for themselves. The plastids that contain chlorophyll are called chloroplasts.

Just like in animal cells, plant cell vacuoles are used for storage of water, salts, and other molecules. The difference between plant and animal vacuoles is that plant cells have much larger ones. This is because plants need a lot more water than animal cells.When plant cell vacuoles are filled with water, they create something called turgor pressure, which prevents wilting. When the vacuoles are filled with water, they are putting pressure on the plant cell membrane.

This forces the cells to be rigid, which keeps the plant upright. When the vacuoles lose water, the pressure decreases and the cells become flaccid. At this time, the plant will wilt.

Plants are well adapted to collect sunlight. In addition to having wide leaves, they also have stems that hold the leaves as high up in the air as they can. To reach these heights, plants have a very rigid covering on each of their cells called a cell wall. Cells walls support the plant cell and give it structure. Plant cells are usually square in shape (compared to animals cells, which are amorphic).

Cell walls are made of a large carbohydrate called cellulose. They are porous to allow water, oxygen and carbon dioxide to pass easily through them.

Lesson Summary

Cells can be thought of as tiny factories. There is a main office, a mailroom, a power plant, an internal delivery system and many workers. Each of these parts has its own job to do, but they all contribute to the total functioning of the company. Without one part, the entire machine would stop working.

Key Terms: The Organelles

Cells are made up of organelles that have specific functions to support life
Inside of a cell
  • Nucleus: control center of the cell, contains the condensed strands of DNA called chromosomes. In eukaryotic cells, also contains the nucleolus
  • Cytoplasm: gel-like substance inside the cell that keeps the cell healthy
  • Cytoskeleton: network of tiny tubes in the cytoplasm that give the cell structure; composed of microtubules and microfilaments
  • Ribosomes: small pieces of RNA found in the cytoplasm that assemble proteins
  • Endoplasmic Reticulum: (E.R.) moves proteins from one area of the cell to another
  • Gogli aparatus: packages proteins destined to leave the cell
  • Lysosomes: break down cell waste
  • Mitochondria: make energy for the cell
  • Vacuoles: storage containers for the cell
  • Vesicles: move things between organelles and outside the cell
  • Cell wall: found in plant cells; the outside border of the cell
  • Chloroplasts: found in plant cells; contain chlorophyll, which absorbs the light for photosynthesis

Learning Outcome

Once you are finished, you should be able to name the organelles of eukaryotic cells and explain their functions.

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