Biology (Holt)

How to Use Your Textbook xxiii

Structures of Plant Cells

The organelles described in this section are found in both animalcells and plant cells. However, plant cells have three additional

structures that are not found in animal cells, shown in Figure 17.

Unique Features of Plant CellsCell wallThe cell membrane of a plant cell is surrounded by a thick

cell wall, composed of proteins and carbohydrates, including thepolysaccharide cellulose. The cell wall helps support and maintain the

shape of the cell, protects the cell from damage, and connects it withadjacent cells.

Chloroplastsmore Plant cells contain one or. Chloroplasts are

organelles that use light energy to makecarbohydrates from carbon dioxide and

water. Chloroplasts are found not only inplants but also in a wide variety of eukary-

otic algae, such as seaweed. Chloroplasts,along with mitochondria, supply much of

the energy needed to power the activitiesof plant cells. Like mitochondria, chloro-

plasts are surrounded by two membranes,contain their own DNA, and are thought

to be the descendents of ancient prokary-otic cells.

Central vacuolemuch of a plant cell’s volume is taken upAs shown in Figure 17,

by a large, membrane-bound space calledthe (VAK yoo ohl).The

central vacuole stores water and maycontain many substances, including ions,

nutrients, and wastes. When the centralvacuole is full, it makes the cell rigid. This

rigidity enables a plant to stand upright.

central vacuole

chloroplasts

66 CHAPTER 3Cell Structure

Describeactivities. the role of the nucleus in cell4B

Sequencefrom the rough ER to the outside of the cell. the course of newly made proteins4B

Describeolism of eukaryotic cells. the role of mitochondria in the metab-4B

Explaincell wall help make the cell rigid. how a plant cell’s central vacuole and4A

Critical ThinkingWhat is the importance of a cell enclosing itsInferring Relationships

digestive enzymes inside lysosomes? 4A

as the packaging and distribution center of aWhich organelle serves

eukaryotic cell? Anucleus 4A 4BCmitochondrion

Blysosome DGolgi apparatus

TAKS Test PrepTAKS Test Prep

Section 3 Review

Chloroplast

Central vacuole

Cell wall

Figure 17wall, chloroplasts, and a large central vacuolePlant cell.Plant cells have a cell

(shown in blue).

Packaging and Distribution of ProteinsVesicles that contain newly made proteins move through the cytoplasm

from the ER to an organelle called the Golgi apparatus. The (GOHL jee) is a set of flattened, membrane-bound sacs that

serves as the packaging and distribution center of the cell. Enzymesinside the Golgi apparatus modify the proteins that are received in

vesicles from the ER. The modified proteins are then enclosed in newvesicles that bud from the surface of the Golgi apparatus. Some of

these vesicles include spherical organelles that contain the cell’s digestive enzymes. The ER,(LIE seh sohms), which are small,

the Golgi apparatus, and lysosomes work together in the production,packaging, and distribution of proteins, as summarized in Figure 15.

Step Ribosomes make proteins on the rough ER. The proteinsare packaged into vesicles.

Step The vesicles transport the newly made proteins from therough ER to the Golgi apparatus.

Step In the Golgi apparatus, proteins are processed and thenpackaged into new vesicles.

Step Many of these vesicles move to the cell membrane andrelease their contents outside the cell.

Step Other vesicles, including lysosomes, remain within thecytoplasm. Lysosomes digest and recycle the cell’s used

components by breaking down proteins, nucleic acids,lipids, and carbohydrates.

lysosomes

apparatus Golgi

64 CHAPTER 3Cell Structure

BIOgraphic

Proteins are made by ribosomes on the

rough ER.

Processing of Proteins

1

Vesicles carry proteins from the rough ER to

(^2) the Golgi apparatus.
Proteins are modified in the Golgi apparatus and
(^3) enter new vesicles.
Some vesicles release their proteins outside
(^4) the cell.
Other vesicles remain in the cell and become
(^5) lysosomes.
Nucleus
Proteins are processed by an internal system of membranes.
Figure 15
Interpreting GraphicsAs you read, use Steps 1–5
in the text, shown in red, tohelp you follow the same
numbered steps shown inFigure 15.
Use the Illustrations and Photos
Art shows complex ideas and processes.
Learn to analyze the art so that you better
understand the material you read in the text.
Tables and graphs display important
information in an organized way to help
you see relationships.
A picture is worth a thousand words. Look
at the photographs to see relevant examples of
science concepts you are reading about.
Answer the Section Reviews
Section Reviews test your knowledge over the
main points of the section. Critical Thinking
items challenge you to think about the mater-
ial in greater depth and to find connections
that you infer from the text. TAKS Test Prep
items help prepare you for the TAKS (Texas
Assessment of Knowledge and Skills) exam.
STUDY TIP When you can’t answer a question,
reread the section. The answer is usually there.
Do Your Homework
Your teacher may assign worksheets to help
you understand and remember the material
in the chapter.
STUDY TIP Don’t try to answer the questions
without reading the text and reviewing your
class notes. A little preparation up front will
make your homework assignments a lot easier.
Answering the items in the Chapter Review
will help prepare you for the chapter test.

Visit Holt Online Learning

If your teacher gives you a special

password to log onto the Holt Online

Learningsite, you’ll find your complete

textbook on the Web. In addition, you’ll find some great
learning tools and practice quizzes. You’ll be able to see
how well you know the material from your textbook.

Visit CNN Student News

You’ll find up-to-date

events in science at

cnnstudentnews.com.