Plant cells have three kinds of structures that are not found in animal cells and that are extremely important to plant survival: plastids, central vacuoles, and cell walls.
PLANT CELLS
Most of the organelles and other parts of the cell just described are common to all eukaryotic cells. However, plant cells have three additional kinds of structures that are extremely important to plant function: cell walls, large central vacuoles, and plastids. To understand why plant cells have structures not found in animal cells, consider how a plant’s lifestyle differs from an animal’s. Plants make their own carbon-containing molecules directly from carbon taken in from the environment. Plant cells take carbon dioxide gas from the air, and in a process called photosynthesis, they convert carbon dioxide and water into sugars. The organelles and structures in plant cells are important.
In addition to containing almost all of the types of organelles that animal cells contain, plant cells contain three unique features. Those features are the cell wall, the central vacuole, and plastids, such as chloroplasts.
CELL WALL
The cell wall is a rigid layer that lies outside the cell’s plasma membrane. Plant cell walls contain a carbohydrate called cellulose. Cellulose is embedded in a matrix of proteins and other carbohydrates that form a stiff box around each cell. Pores in the cell wall allow water, ions, and some molecules to enter and exit the cell.
Primary and Secondary Cell Walls
The main component of the cell wall, cellulose, is made directly on the surface of the plasma membrane by enzymes that travel along the membrane. These enzymes are guided by microtubules inside the plasma membrane. Growth of the primary cell wall occurs in one direction, based on the orientation of the microtubules. Other components of the cell wall are made in the ER. These materials move in vesicles to the Golgi and then to the cell surface. Some plants also produce a secondary cell wall. When the cell stops growing, it secretes the secondary cell wall between the plasma membrane and the primary cell wall. The secondary cell wall is very strong but can no longer expand. The wood in desks and tabletops is made of billions of secondary cell walls. The cells inside the walls have died and disintegrated.
CENTRAL VACUOLE
Plant cells may contain a reservoir that stores large amounts of water. The central vacuole is a large, fluid-filled organelle that stores not only water but also enzymes, metabolic wastes, and other materials. The central vacuole forms as other smaller vacuoles fuse together. Central vacuoles can make up 90 percent of the plant cell’s volume and can push all of the other organelles into a thin layer against the plasma membrane. When water is plentiful, it fills a plant’s vacuoles. The cells expand and the plant stands upright. In a dry period, the vacuoles lose water, the cells shrink, and the plant wilts.
The central vacuole occupies up to 90 percent of the volume of some plant cells. The central vacuole stores water and helps keep plant tissue firm.
Other Vacuoles
Some vacuoles store toxic materials. The vacuoles of acacia trees, for example, store poisons that provide a defense against plant-eating animals. Tobacco plant cells store the toxin nicotine in a storage vacuole. Other vacuoles store plant pigments, such as the colorful pigments found in rose petals.
PLASTIDS
Plastids are another unique feature of plant cells. Plastids are organelles that, like mitochondria, are surrounded by a double membrane and contain their own DNA. There are several types of plastids, including chloroplasts, chromoplasts, and leucoplasts.
Chloroplasts
A chloroplast captures energy from sunlight and uses that energy to convert carbon dioxide and water into sugar and other carbohydrates.
Chloroplasts use light energy to make carbohydrates from carbon dioxide and water. Each chloroplast contains a system of flattened, membranous sacs called thylakoids. Thylakoids contain the green pigment chlorophyll, the main molecule that absorbs light and captures light energy for the cell. Chloroplasts can be found not only in plant cells but also in a wide variety of eukaryotic algae, such as seaweed.
Chloroplast DNA is very similar to the DNA of certain photosynthetic bacteria. Plant cell chloroplasts can arise only by the division of preexisting chloroplasts. These facts may suggest that chloroplasts are descendants of ancient prokaryotic cells. Like mitochondria, chloroplasts are also thought to be the descendants of ancient prokaryotic cells that were incorporated into plant cells through a process called endosymbiosis.
Chromoplasts
Chromoplasts are plastids that contain colorful pigments and that may or may not take part in photosynthesis. Carrot root cells, for example, contain chromoplasts filled with the orange pigment carotene. Chromoplasts in flower petal cells contain red, purple, yellow, or white pigments.
Other Plastids
Several other types of plastids share the general features of chloroplasts but differ in content. For example, amyloplasts store starch. Chloroplasts, chromoplasts, and amyloplasts arise from a common precursor, called a proplastid.
COMPARING CELLS
All cells share common features, such as a cell membrane, cytoplasm, ribosomes, and genetic material. But there is a high level of diversity among cells. There are significant differences between prokaryotes and eukaryotes. In addition, plant cells have features that are not found in animal cells.
Prokaryotes Versus Eukaryotes
Prokaryotes differ from eukaryotes in that prokaryotes lack a nucleus and membrane-bound organelles. Prokaryotes have a region, called a nucleoid, in which their genetic material is concentrated. However, prokaryotes lack an internal membrane system.
Plant Cells Versus Animal Cells
Three unique features distinguish plant cells from animal cells. One is the production of a cell wall by plant cells. Plant cells contain a large central vacuole. Third, plant cells contain a variety of plastids, which are not found in animal cells. Cell walls, central vacuoles, and plastids are unique features that are important to plant function.
Prokaryotes can be distinguished from eukaryotes in that prokaryotes lack a nucleus and membrane-bound organelles. Plant cells have the same organelles that animal cells do and have a cell wall, a central vacuole, and plastids.
1. Identify three unique features of plant cells.
2. List the differences between the plasma membrane, the primary cell wall, and the secondary cell wall.
3. Identify three functions of plastids.
4. Name three things that may be stored in vacuoles.
5. Describe the features that distinguish prokaryotes from eukaryotes and plant cells from animal cells.
6. Evaluating Viewpoints One student says vacuoles keep plants from wilting. Another says cell walls do this.Who is right? Explain.
7. Making Comparisons If you discovered a new cell, what characteristics would you use to determine which kind of cell it is? Explain.
8. Analyzing Information Tobacco plant cells contain a toxic chemical.Why don’t tobacco plant cells poison themselves? Explain.
The History of Cell Biology
● All living things are made up of one or more cells. Robert Hooke discovered cells. Anton van Leeuwenhoek was the first to observe living cells.
● The cell theory states all living organisms are made of one or more cells, cells are the basic units of structure and function, and cells come only from pre-existing cells.
● All living things are made of organized parts, obtain energy from their surroundings, perform chemical reactions, change with time, respond to their environment, and reproduce.
Introduction to Cells
● A cell’s shape reflects its function.
● Cell size is limited by a cell’s surface area–to-volume ratio.
● The three basic parts of a cell are the plasma membrane, the cytoplasm, and the nucleus.
● Prokaryotes are organisms that lack a nucleus or membrane-bound organelles.
● In multicellular eukaryotes, cells organize into tissues, organs, organ systems, and finally organisms.
Cell Organelles and Features
● Cell membranes are made of two phospholipid layers and proteins.
● The nucleus directs the cell’s activities and stores DNA.
● Mitochondria harvest energy from organic compounds and transfer it to ATP.
● Ribosomes are either free or attached to the rough ER and play a role in protein synthesis.
● The rough ER prepares proteins for export or insertion into the cell membrane. The smooth ER builds lipids and participates in detoxification of toxins.
● The Golgi processes and packages proteins.
● Vesicles are classified by their contents.
● The cytoskeleton is made of protein fibers that help cells move and maintain their shape.
Unique Features of Plant Cells ● Plant cells have cell walls, central vacuoles, and plastids.
● In plant cells, a rigid cell wall covers the cell membrane and provides support and protection.
● Large central vacuoles store water, enzymes, and waste products and provide support for plant tissue.
● Plastids store starch and pigments. The chloroplast converts light energy into chemical energy by photosynthesis.
● Prokaryotes, animal cells, and plant cells can be distinguished from each other by their unique features.
USING VOCABULARY
1. For each pair of terms, explain how the meanings
of the terms differ.
a. nucleolus and nucleus
b. cell wall and cell membrane
c. ribosomes and endoplasmic reticulum
d. chromatin and chromosomes
e. mitochondria and chloroplast
2. Explain the relationship between cilia and
flagella.
3. Use the following terms in the same sentence:
rough ER, smooth ER, Golgi apparatus, vesicle,
and ribosome.
4. Word Roots and Origins The word root eu means
“true,” pro means “before,” and kary means
“nucleus.” Using this information, explain what
the terms eukaryote and prokaryote suggest
about their evolution.
UNDERSTANDING KEY CONCEPTS
5. Name the scientist that first observed nonliving
cells.
6. Discuss the roles of Schleiden, Schwann, and
Virchow in the development of the cell theory.
7. Analyze the three parts of the cell theory.
8. Identify the characteristics shared by all living
organisms.
9. State the relationship between cell shape and cell
function.
10. Identify the factors that limit the growth of cells.
11. Draw the three major parts of a eukaryotic cell.
12. Compare the structure of a prokaryotic cell with
that of a eukaryotic cell.
13. Sequence the relationship between organs, cells,
organ systems, and tissues.
14. Explain why a cell membrane is called a fluid
mosaic.
15. Describe the parts of a nucleus.
16. Propose why muscle cells have more mitochondria
than other kinds of eukaryotic cells have.
17. Describe the role of ribosomes in cells.
18. Compare the functions of the Golgi apparatus
with those of the ER.
19. Discuss the structure and function of vesicles.
20. Describe the structures that make up the
cytoskeleton.
21. Summarize the differences between plant cells
and animal cells.
22. Propose how the cell wall helps give support to a
plant cell.
23. Compare primary and secondary cell walls.
24. Predict what would happen to a plant with a
genetic defect that produced no central vacuole.
25. Compare mitochondria and chloroplasts.
26. CONCEPT MAPPING Use the following
terms to create a concept map that
compares animal cells with plant cells: cell
membrane, cell wall, central vacuole,
chloroplasts, and mitochondria.
CRITICAL THINKING
27. Interpreting Graphics Answer the following questions
based on the figure below.
a. Identify the structures labeled A in the
micrograph.
b. Explain the significance of the shape of these
structures.
28. Applying Concepts Using your knowledge of the
relationship between surface area and volume,
explain why small pieces of a food cook faster
than larger pieces of the same food.
29. Making Comparisons The coils of a radiator provide
a large surface area from which heat is radiated
into a room. Which cell organelles have a
structure similar to that of a radiator? How is
their structure related to their function?
30. Evaluating Differences Explain why colonial
organisms are not considered multicellular. What
features make colonial organisms different from
multicellular organisms?
31. Applying Information Write a
report summarizing the roles of
different types of cell-membrane
proteins in the preservation of body organs
donated for transplant.
Tags: cell wall, central vacuole, chlorophyll, chloroplast, Chloroplast DNA, photosynthetic bacteria, Plant Cells Animal Cells, plastid, Primary and Secondary Cell Walls, Prokaryotes Eukaryotes, thylakoid