Crisscrossing the cytoplasm is a structure called the cytoskeleton that consists of thread-like filaments and tubules. You can see these filaments and tubules in the cells in Figure 3.13. As its name suggests, the cytoskeleton is like a cellular "skeleton." It helps the cell maintain its shape and also holds cell organelles in place within the cytoplasm.
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Cytoskeleton. The cytoskeleton gives the cell an internal framework, like the frame of a house. In this photograph, filaments and tubules of the cytoskeleton are green and red, respectively. The blue dots are cell nuclei.
The Nucleus and Other Organelles
Eukaryotic cells contain a nucleus and several other types of organelles. These structures are involved in many vital cell functions.
The nucleus is the largest organelle in a eukaryotic cell and is often considered to be the cell's control center. This is because the nucleus controls which proteins the cell makes. The nucleus of a eukaryotic cell contains most of the cell's DNA, which makes up chromosomes and is encoded with genetic instructions for making proteins.
The mitochondrion (plural, mitochondria) is an organelle that makes energy available to the cell. This is why mitochondria are sometimes referred to as the power plants of the cell. They use energy from organic compounds such as glucose to make molecules of ATP (adenosine triphosphate), an energy-carrying molecule that is used almost universally inside cells for energy. Scientists think that mitochondria were once free-living organisms because they contain their own DNA. They theorize that ancient prokaryotes infected primitive eukaryotic cells, and the two organisms evolved a symbiotic relationship that benefited both of them. The eukaryotes provided the prokaryotes with a place to live, and the prokaryotes provided the eukaryotes with extra energy. Eventually, the prokaryotes became permanent "guests" of the eukaryotes, as organelles inside their cells. This theory is called the endosymbiotic theory and is widely accepted by biologists today.
The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. There are two types of endoplasmic reticulum: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Both types are shown in Figure 3.14.
- RER looks rough because it is studded with ribosomes. It provides a framework for the ribosomes, which make proteins.
- SER looks smooth because it does not have ribosomes. Bits of its membrane pinch off to form tiny sacs called vesicles, which carry proteins away from the ER. SER also makes lipids, stores substances, and plays other roles.
Ribosomes are small organelles where proteins are made. They contain the nucleic acid RNA, which assembles and joins amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm as well as on the RER.
The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is shown in Figure 3.14. The Golgi apparatus is somewhat like a post office. It receives items (proteins from the ER), packages and labels them, and then sends them on to their destinations (to different parts of the cell or to the cell membrane for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell. At the link below, you can watch an animation showing how the Golgi apparatus does all these jobs. http://www.johnkyrk.com/golgiAlone.html
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This drawing includes the nucleus, RER, SER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.
Vesicles and Vacuoles
Both vesicles and vacuoles are sac-like organelles that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus (see Figure 3.14) store and transport protein and lipid molecules. Some vesicles are used as chambers for biochemical reactions. Other vesicles include
- lysosomes, which use enzymes to break down foreign matter and dead cells
- peroxisomes, which use oxygen to break down poisons
Centrioles are organelles involved in cell division. They help organize the chromosomes before cell division so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells and are located near the nucleus (see Figure 3.9).
Special Structures in Plant Cells
Plant cells have several structures that are not found in animal cells, including a cell wall, a large central vacuole, and organelles called plastids. You can see each of these structures in Figure 3.15. You can also view them in an interactive plant cell at the link below. http://www.cellsalive.com/cells/cell_model.htm
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Plant Cell. In addition to the organelles and other structures found inside animal cells, plant cells also have a cell wall, a large central vacuole, and plastids such as chloroplasts. Can you find each of these structures in the figure?
The cell wall is a rigid layer that surrounds the plasma membrane of a plant cell. It supports and protects the cell. Tiny holes, or pores, in the cell wall allow water, nutrients, and other substances to move into and out of the cell. The cell wall is made up mainly of complex carbohydrates, including cellulose.
Most mature plant cells have a large central vacuole. This vacuole can make up as much as 90% of the cell's volume. The central vacuole has a number of functions, including storing substances such as water, enzymes, and salts. It also helps plant tissues, such as stems and leaves, stay rigid and hold their shape. It even helps give flowers, like the ones in Figure 3.16, their beautiful colors.
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These flowers are red because of red pigment molecules in the central vacuoles of their cells. The bright colors are an important adaptation. They help the flowers attract pollinators such as hummingbirds so the plants can reproduce.
Plastids are organelles in plant cells that carry out a variety of different functions. The main types of plastids and their functions are described below.
- Chloroplasts are plastids that contain the green pigment chlorophyll. They capture light energy from the sun and use it to make food. A chloroplast is shown in Figure 3.15.
- Chromoplasts are plastids that make and store pigments. The red pigment that colors the flower petals inFigure 3.16 was made by chromoplasts.
- Leucoplasts are plastids that store substances such as starch or make small molecules such as amino acids.
Like mitochondria, plastids contain their own DNA. Therefore, according to endosymbiotic theory, plastids may also have evolved from ancient, free-living prokaryotes that invaded primitive eukaryotic cells. If so, they allowed early eukaryotes to make food and produce oxygen.
Organization of Cells
Cells can exist as individual cells or as groups of cells. Cells in groups can be organized at several levels.
From One Cell to Many
The simplest level of cell organization is a single-celled organism, and the most complex level is a multicellular organism. In between these two levels are biofilms and colonies.
- A single-celled organism floats freely and lives independently. Its single cell is able to carry out all the processes of life without any help from other cells.
- A biofilm is a thin layer of bacteria that sticks to a surface. Cells in a biofilm are all alike, but they may play different roles, such as taking in nutrients or making the "glue" that sticks the biofilm to the surface. The sticky plaque that forms on teeth is a biofilm of bacterial cells.
- Some single-celled organisms, such as algae, live in colonies. A colony is an organized structure composed of many cells, like the Volvox sphere in Figure 3.17. Volvox are algae that live in colonies of hundreds of cells. All of the cells in the colony live and work cooperatively. For example, they can coordinate the movement of their flagella, allowing them to swim together through the water as though they were part of a single organism.
- A multicellular organism consists of many cells and has different types of cells that are specialized for various functions. All the cells work together and depend on each other to carry out the life processes of the organism. Individual cells are unable to survive on their own.
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Volvox Colony. Volvox cells live in a colony shaped like a hollow ball. The cells of the colony may be connected by strands of cytoplasm and can function together. For example, the whole colony can swim from one place to another as one.
Levels of Organization in Multicellular Organisms
Scientists think that multicellular organisms evolved when many single-celled organisms of the same species started to work together and benefited from the relationship. The oldest known multicellular organisms are algae that lived 1.2 billion years ago. As multicellular organisms continued to evolve, they developed increasingly complex levels of organization. Today there are multicellular organisms at all levels of organization, from the simplest, cell level of organization to the most complex, organ-system level of organization. Consider these examples:
- Sponges have cell-level organization, in which different cells are specialized for different functions, but each cell works alone. For example, some cells digest food, while other cells let water pass through the sponge.
- Jellyfish have tissue-level organization, in which groups of cells of the same kind that do the same job form tissues. For example, jellyfish have some tissues that digest food and other tissues that sense the environment.
- Roundworms have organ-level organization, in which two or more types of tissues work together to perform a particular function as an organ. For example, a roundworm has a primitive brain that controls how the organism responds to the environment.
- Human beings have organ system-level organization, in which groups of organs work together to do a certain job, with each organ doing part of the overall task. An example is the human digestive system. Each digestive system organ—from the mouth to the small intestine—does part of the overall task of breaking down food and absorbing nutrients.
- The plasma membrane is a phospholipid bilayer that supports and protects a cell and controls what enters and leaves it.
- The cytoplasm consists of everything inside the plasma membrane, including watery cytosol and organelles. The cytoplasm suspends the organelles and does other jobs. The cytoskeleton crisscrosses the cytoplasm and gives the cell an internal framework.
- The nucleus is the largest organelle in a eukaryotic cell and contains most of the cell's DNA. Other organelles in eukaryotic cells include the mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus, vesicles, vacuoles, and centrioles (in animal cells only). Each type of organelle has important functions in the cell.
- Plant cells have special structures that are not found in animal cells, including a cell wall, large central vacuole, and organelles called plastids.
- Cells can exist independently as single-celled organisms or with other cells as multicellular organisms. Cells of a multicellular organism can be organized at the level of cells, tissues, organs, and organ systems.
Lesson Review Questions
1. Describe the composition of the plasma membrane.
2. List functions of the cytoplasm and exoskeleton.
3. What is the role of the nucleus of a eukaryotic cell?
4. List three structures that are found in plant cells but not in animal cells.
5. Outline the levels of organization of cells in living things, starting with the simplest level, that of single-celled organisms.
6. Create a diagram to show how the cells of multicellular organisms may be organized at different levels, from the level of the cell to the level of the organ system. Give an example of a multicellular organism at each level of organization.
7. Explain why hydrophobic ("water-hating") molecules can easily cross the plasma membrane, while hydrophilic (water-loving") molecules cannot.
8. What is endosymbiotic theory? How does it explain the presence of certain organelles in eukaryotic cells?
9. Explain how the following organelles ensure that a cell has the proteins it needs: nucleus, rough and smooth ER, vesicles, and Golgi apparatus.
Points to Consider
Cells carry out all the functions of life, and they use nutrients and oxygen and produce wastes. These substances must cross the plasma membrane.
- How do you think substances cross the plasma membrane to enter or leave the cell? Does the membrane have tiny holes in it like a sieve?
- What if the substances are large? Protein molecules, for example, are very large. How do they enter or leave the cell?