Cell Structures

Lesson Objectives

• Describe the structure and function of the plasma membrane.
• Identify the roles of the cytoplasm and exoskeleton.
• Outline the form and function of the nucleus and other organelles.
• List special structures of plant cells, and state what they do.
• Explain how cells are organized in living things.

Vocabulary

Introduction

Your body is made up of trillions of cells, but all of them perform the same basic life functions. They all obtain and use energy, respond to the environment, and reproduce. How do your cells carry out these basic functions and keep themselves—and you—alive? To answer these questions, you need to know more about the structures that make up cells.

Overview of Cell Structures

In some ways, a cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit, the cytoplasm of the cell also contains various structures, such as a nucleus and other organelles. Figure 3.9 shows the structures inside a typical eukaryotic cell, in this case the cell of an animal. Refer to the figure as you read about the structures below. You can also explore the structures of an interactive animal cell at this link: http://www.cellsalive.com/cells/cell_model.htm

Plasma Membrane

The plasma membrane forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It protects and supports the cell and also controls everything that enters and leaves the cell. It allows only certain substances to pass through, while keeping others in or out. To understand how the plasma membrane controls what crosses into or out of the cell, you need to know its composition.

Phospholipid Bilayer

The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. As shown inFigure 3.10, each phospholipid molecule has a head and two tails. The head "loves" water (hydrophilic) and the tails "hate" water (hydrophobic). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell. Molecules that are hydrophobic can easily pass through the plasma membrane, if they are small enough, because they are water-hating like the interior of the membrane. Molecules that are hydrophilic, on the other hand, cannot pass through the plasma membrane—at least not without help—because they are water-loving like the exterior of the membrane.

Other Molecules in the Plasma Membrane

The plasma membrane also contains other molecules, primarily other lipids and proteins. The green molecules in Figure 3.10, for example, are the lipid cholesterol. Molecules of cholesterol help the plasma membrane keep its shape. Many of the proteins in the plasma membrane assist other substances in crossing the membrane.

Extensions of the Plasma Membrane

The plasma membrane may have extensions, such as whip-like flagella or brush-like cilia. In single-celled organisms, like those shown in Figure 3.11 and 3.12, the membrane extensions may help the organisms move. In multicellular organisms, the extensions have other functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose.

Cytoplasm and Exoskeleton

The cytoplasm consists of everything inside the plasma membrane of the cell. It includes the watery, gel-like material called cytosol, as well as various structures. The water in the cytoplasm makes up about two thirds of the cell's weight and gives the cell many of its properties.

Functions of the Cytoplasm

The cytoplasm has several important functions, including

• suspending cell organelles
• pushing against the plasma membrane to help the cell keep its shape
• providing a site for many of the biochemical reactions of the cell

Cytoskeleton

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.

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

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.

Mitochondria

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.

Endoplasmic Reticulum

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

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.

Golgi Apparatus

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

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

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

Cell Wall

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.

Central Vacuole

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.

Plastids

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.

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.

Lesson Summary

• 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

Recall

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.

Apply Concepts

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.

Think Critically

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?