- List the organelles of the cell and their functions.
- Distinguish between plant and animal cells.
Check Your Understanding
- What is a cell?
- How do we visualize cells?
Do brain cells have the same internal structures as your other cells?
Yes. Although brain cells look quite different from your other cells, they have the same internal structures as other cells. They need the same structures because they need to perform the same tasks, such as making proteins and obtaining energy.
Eukaryotic cells have many specific functions, so it can be said that a cell is like a factory. A factory has many machines and people, and each has a specific role. Just like a factory, the cell is made up of many different parts. Each part has a special role. The different parts of the cell are called organelles, which means "small organs." All organelles are found in eukaryotic cells. Prokaryotic cells are "simpler" than eukaryotic cells. Though prokaryotic cells still have many functions, they are not as specialized as eukaryotic cells. Thus, most organelles are NOT found in prokaryotic cells.
Below are the main organelles found in eukaryotic cells (Figure 2.1):
- The nucleus of a cell is like a safe containing the factory's trade secrets. The nucleus contains the genetic material-the information about how to build thousands of proteins.
- The mitochondria are the powerhouses of the cell; they provide the energy needed to power chemical reactions. This energy is in the form of ATP (adenosine triphosphate). Cells that use a lot of energy may have thousands of mitochondria.
- Vesicles are small membrane bound sacs that transport materials around the cell and to the cell membrane.
- The vacuoles are like storage centers. Plant cells have larger vacuoles than animal cells. Plants store water and nutrients in their large central vacuoles.
- Lysosomes are like the recycling trucks that carry waste away from the factory. Lysosomes have digestive enzymes that break down old molecules into parts that can be recycled.
- In both eukaryotes and prokaryotes, ribosomes are the non-membrane bound organelles where proteins are made. Ribosomes are like the machines in the factory that produce the factory's main product. Proteins are the main product of the cell.
- Some ribosomes can be found on folded membranes called the endoplasmic reticulum (ER), others float freely in the cytoplasm. If the ER is covered with ribosomes, it looks bumpy like sandpaper, and is called the rough endoplasmic reticulum. If the ER does not contain ribosomes, it is smooth and called the smooth endoplasmic reticulum. Many proteins are made on the ribosomes on the rough ER. These proteins immediately enter the ER, where they are modified, packaged into vesicles and sent to the Golgi apparatus. Lipids are made in the smooth ER.
- The Golgi apparatus works like a mail room. The Golgi apparatus receives proteins from the rough ER and puts "shipping addresses" on them. The Golgi then packages the proteins into vesicles and sends them to the right place in the cell or to the cell membrane. Some of these proteins are secreted from the cell (they exit the cell); others are placed into the cell membrane.
- The cytoskeleton gives the cell its shape, and the flagella helps the cell to move. Prokaryotic cells may also have flagella.
Figure 2.1 Eukaryotic cells contain special compartments surrounded by membranes, called organelles. For example, notice in this image the mitochondria, lysosomes, and Golgi apparatus.
Do plants have cells like yours?
Yes, your cells are actually very similar to a plant's cells. For example, they are both eukaryotic cells, both contain DNA in a nucleus, and both make proteins in ribosomes. However, plant cells also differ in some crucial ways from your own cells.
Even though plants and animals are both eukaryotes, plant cells differ in some ways from animal cells (Figure2.2) Plant cells have a large central vacuole, are surrounded by a cell wall, and have chloroplasts, which are the organelles of photosynthesis.
Figure 2.2 The Plant Cell
First, plant cells have a large central vacuole that holds a mixture of water, nutrients, and wastes. A plant cell's vacuole can make up 90% of the cell’s volume. The large central vacuole essentially stores water. What happens when a plant does not get enough water? In animal cells, vacuoles are much smaller.
Figure 2.3 A cell wall gives the plant cell strength and protection.
A third difference between plant and animal cells is that plants have several kinds of organelles called plastids. And there are several different kinds of plastids in plant cells. For example, Chloroplasts are needed for photosynthesis, leucoplasts can store starch or oil, and brightly colored chromoplasts give some flowers and fruits their yellow, orange, or red color. It is the presence of chloroplasts and the ability to photosynthesize, that is one of the defining features of a plant. No animal or fungi can photosynthesize, and only some protists are able to. The photosynthetic protists are the plantlike protists, represented mainly by the unicellular algae.
View the Plant and Animal Cell Animation - Cells alive
1. Select "Animal Cell". Click on the following organelles: nucleus, cell membrane, mitochondrion, ribosomes, and nucleolus and record a fact about each organelle in your SLJ.
2. Select "Plant Cell". Click on the following organelles: cell wall, chloroplast, and vacuole. Record a fact about each organelle in your SLJ.
- The nucleus stores the genetic information.
- The vacuoles are needed for storage.
- The lysosomes recycle waste.
- The cytoskeleton provides the shape of the cell.
- The ribosomes produce proteins.
- The rough ER is covered with ribosomes and makes proteins, while the smooth ER makes lipids.
- The Golgi apparatus packages proteins.
- Plant and animal cells differ in that plants have a large central vacuole, while animals have smaller vacuoles.
- Plant cells also have cell walls and plastids, while animal cells do not.
Points to Consider
- What do you think could happen if your cells divide uncontrollably?
- When new life is formed, do you think it receives all the DNA of the mother and the father?
- Why do you think you might need new cells throughout your life?