Chapter 7: Cell Structure

Section 1: Introduction to Cells Preview •

The Discovery of Cells

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Looking at Cells

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Cell Features

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Summary

The Discovery of Cells • Microscope observations of organisms led to the discovery of the basic characteristics common to all living things. • Scientists first discovered cells in the 1600s using crude microscopes. • Observations made by scientists using more powerful microscopes in the 1800s led to the formation of the cell theory.

Visual Concept: Cell Theory

Looking at Cells • Cells vary greatly in their size and shape. • A cell’s shape reflects its function. Cell size is limited by a cell’s surface area-to-volume ratio. • Cells can be branched, flat, round, or rectangular.

Looking at Cells, continued • All substances that enter or leave a cell must cross the surface of the cell. • A cell’s ability to move substances across its surface can be estimated by finding its surface area-to-volume ratio. • Cells with greater surface area-to-volume ratios can exchange substances more efficiently.

Relationship between Surface Area and Volume

Looking at Cells, continued • When comparing cells of the same shape, small cells have greater surface area-tovolume ratios than large cells. • So, small cells function more efficiently than large cells.

Cell Features • All cells share common structural features, including a cell membrane, cytoplasm, ribosomes, and DNA. • The cell membrane is the outer layer that covers a cell’s surface and acts as a barrier between the outside environment and the inside of the cell. • The cytoplasm is the region of the cell within the cell membrane. The cytoplasm includes the fluid inside the cell called the cytosol.

Cell Features, continued • A ribosome is a cellular structure that makes proteins. • The DNA of a cell provides instructions for making proteins, regulates cellular activities, and enables cells to reproduce.

Features of Prokaryotic and Eukaryotic Cells

Cell Features, continued Features of Prokaryotic Cells • A prokaryote is an organism made of a single prokaryotic cell. • Prokaryotic cells do not have a nucleus or other internal compartments. The genetic material of a prokaryotic cell is a single loop of DNA. • For millions of years, prokaryotes were the only organisms on Earth.

Cell Features, continued Features of Eukaryotic Cells • A eukaryote is an organism made up of one or more eukaryotic cells. All multicellular organisms are made of eukaryotic cells. • The DNA of a eukaryotic cell is found in an internal compartment of the cell called the nucleus. • All eukaryotic cells have membrane-bound organelles. An organelle is a small structure found in the cytoplasm that carries out specific activities inside the cell.

Cell Features, continued • Each organelle in a eukaryotic cell performs distinct functions. • The complex organization of eukaryotic cells enables them to carry out more specialized functions than prokaryotic cells.

Visual Concept: Comparing Prokaryotes and Eukaryotes

Comparing Prokaryotes and Eukaryotes

Summary • Microscope observations of organisms led to the discovery of the basic characteristics common to all living things. • A cell’s shape reflects its function. Cell size is limited by a cell’s surface area-to-volume ratio. • The complex organization of eukaryotic cells enable them to carry out more specialized functions than prokaryotic cells.

Concept Check • How were cells discovered? • Why does cell shape vary? • What enables eukaryotes to perform more specialized functions than prokaryotes?

Test Prep

1. The discovery of cells is linked most directly with A. the development of the microscope. B. early investigations of causes of disease. C. observations of large, unicellular organisms. D.efforts to reproduce organisms in the laboratory.

2. Eukaryotic cells differ from prokaryotic cells in that eukaryotic cells A. have a nucleus. B. lack organelles. C. lack ribosomes. D.have a cell wall.

3. The Dutch scientist Anton van Leewenhoek used a microscope that made objects appear 300 times larger than they were. If a cell appeared to be 6 mm long under this microscope, how long was the cell in real life?

A. 0.02 mm B. 0.20 mm C. 0.05 mm D. 0.50 mm

Section 2: Inside the Eukaryotic Cell Preview • • • • • •

The Framework of the Cell Directing Cellular Activity Protein Processing Storage and Maintenance Energy Production Summary

The Framework of the Cell • Eukaryotic cells have an intricate network of protein fibers called the cytoskeleton which provides the interior framework of the cell. • The cytoskeleton helps the cell move, keep its shape, and organize its parts. • There are three different kinds of cytoskeleton fibers: microfilaments, microtubules, and intermediate fibers.

Cytoskeleton Click to animate the image.

A B C D

Directing Cellular Activity • DNA contains instructions for making proteins which control most of the activity of the cell. • The DNA of eukaryotic cells is stored in the nucleus. • DNA instructions are copied as RNA messages, which leave the nucleus. In the cytoplasm, ribosomes use the RNA messages to assemble proteins.

Directing Cellular Activity,

continued Nucleus • A double membrane called the nuclear envelope surrounds the nucleus. • Nuclear pores located on the nuclear envelope act as channels to allow certain molecules to move in and out of the nucleus. • The nucleolus is a structure within the nucleus where ribosome parts are made. • These ribosome parts are transported out of the nucleus into the cytoplasm where they are assembled to form a complete ribosome.

The nucleus and the nuclear envelope

Click to animate the image. The Nucleus

B C

D A

Directing Cellular Activity,

continued Ribosomes • Each ribosome in a cell is made of RNA and many different proteins. • Ribosomes that are suspended in the cytosol are called “free” ribosomes. • Free ribosomes make proteins that remain inside the cell.

Directing Cellular Activity,

continued Ribosomes • Ribosomes that are attached to the membrane of another organelle are called “bound” ribosomes. • Bound ribosomes make proteins that are exported from the cell. • Ribosomes can switch between being bound or free, depending on what proteins the cell needs to make.

Protein Processing • Some proteins that a cell manufactures are needed outside the cell that makes them. • Proteins that are sent outside the cell are packaged in vesicles. Vesicles are small, membrane-bound sacs that enclose the proteins and keep them separate from the rest of the cytoplasm. • The endoplasmic reticulum and Golgi apparatus are organelles involved in preparing proteins for extracellular export.

Protein Processing, continued Endoplasmic Reticulum • The endoplasmic reticulum, or ER, is an extensive system of internal membranes that moves proteins and other substances through the cell. • The membranes of the ER are connected to the outer membrane of the nuclear envelope. • The endoplasmic reticulum is divided into two portions: rough ER and smooth ER.

Endoplasmic Reticulum (ER),

Protein Processing, continued Endoplasmic Reticulum • The portion of the ER with attached ribosomes is called rough ER because it has a rough appearance when viewed with an electron microscope. • The portion of the ER with no attached ribosomes is called smooth ER because it has a smooth appearance when viewed with an electron microscope. • The ribosomes on the rough ER make proteins that are packaged into vesicles. Enzymes of the smooth ER make lipids and break down toxic substances.

Visual Concept: Endoplasmic Reticulum (ER) and Ribosomes

Protein Processing, continued Golgi Apparatus • The Golgi apparatus is a set of flattened, membrane-bound sacs. • The Golgi apparatus helps modify, sort, and package cell products for distribution.

Golgi apparatus

Protein Processing, continued Making and Exporting Proteins • The ribosomes located on the rough ER make proteins which then cross into the membranes of the ER. The ER membrane then pinches off and forms a vesicle around the proteins. • Vesicles transport the proteins from the rough ER to the Golgi apparatus, where they are modified by enzymes and repackaged in new vesicles. • These new vesicles transport the modified proteins to the cell membrane to be released outside the cell.

Storage and Maintenance Lysosomes • Vesicles help maintain homeostasis by storing and releasing a variety of substances as the cell needs them. • A lysosome is a vesicle produced by the Golgi apparatus that contains enzymes that break down large molecules. • Lysosomes recycle old or damaged organelles and digest food particles to provide nutrients for the cell.

Visual Concept: Lysosomes

Storage and Maintenance,

continued Vacuoles • A vacuole is a fluid-filled vesicle found in the cytoplasm of many plant cells. • Plant cells contain a large compartment called the central vacuole, which stores water, ions, nutrients, and wastes. • When water fills the central vacuole, the cell becomes rigid, allowing the plant to stand up. When the vacuole loses water, the cell shrinks, and the plant wilts.

Visual Concept: Vacuoles

Storage and Maintenance,

continued Other Vacuoles • Some protists have contractile vacuoles which pump excess water out of the cell in order to control the concentration of salts and other substances. • A food vacuole is another type of vacuole. It is formed when the cell membrane surrounds food particles outside the cell and pinches off to form a vesicle inside the cell.

Energy Production • Cells need a constant source of energy. • The energy for cellular functions is produced by chemical reactions that occur in the mitochondria and chloroplasts. • In both organelles, chemical reactions produce adenosine triphosphate (ATP), the form of energy that fuels almost all cell processes.

Energy Production, continued Chloroplasts • A chloroplast is an organelle found in plant and algae cells that uses light energy to make carbohydrates from carbon dioxide and water. • Chloroplasts are surrounded by two membranes and have several stacks of flattened sacs where energy production takes place. • Plant cells may have several chloroplasts.

Energy Production, continued Mitochondria • Mitochondria are cell organelles that use energy from organic compounds to make ATP. • Most of the ATP needed by a cell is produced inside mitochondria. Both animal and plant cells contain mitochondria. • A smooth outer membrane and a folded inner membrane surround a mitochondrion. ATP is produced by enzymes on the folds of the inner membrane.

Mitochondrion

Summary • The cytoskeleton helps the cell move, keep its shape, and organize its parts • DNA instructions are copied as RNA messages, which leave the nucleus. In the cytoplasm, ribosomes use the RNA messages to assemble proteins. • The endoplasmic reticulum and Golgi apparatus are organelles involved in preparing proteins for extracellular export.

Summary, continued • Vesicles help maintain homeostasis by storing and releasing a variety of substances as the cell needs them. • The energy for cellular functions is produced by chemical reactions that occur in the mitochondria and chloroplasts.

Concept Check • What does the cytoskeleton do? • How does DNA direct activity in the cytoplasm? • What organelles are involved in protein production? • What are vesicles and vacuoles? • How does the cell get energy?

Test Prep

4. Which organelle produces proteins that are exported from the cell? A. nucleolus B. rough ER C. free ribosome D.bound ribosome

5. A. B. C. D.

Which structure helps a plant stand upright? lysosome chloroplast central vacuole contractile vacuole

6. A. B. C. D.

Ribosome : protein synthesis :: mitochondria : cell support energy release nutrient storage protein transport

The graph shows the amount of ATP in the muscles of a squid after it had been exposed to low oxygen concentrations. 7. At what time during the experiment were the mitochondria in the squid’s muscles producing the most energy? A. 0 minutes B. 15 minutes C. 30 minutes D. 45 minutes

Section 3: From Cell to Organism Preview • • • •

Diversity in Cells Levels of Organization Body Types Summary

Diversity in Cells • Both prokaryotic and eukaryotic cells can have a variety of shapes and structures. • The function of a cell is determined by its shape and the organelles found in the cell. • The different organelles and features of cells enable organisms to function in unique ways in different environments.

Diversity in Cells, continued Diversity in Prokaryotes • Prokaryotes can vary in shape, the way they obtain and use energy, and their ability to move. • Many prokaryotes have a flagellum, a long, hair-like structure that grows out of the cell and enables the cell to move through its environment. • Prokaryotes may also have pili, short outgrowths that allow the cell to attach to surfaces or other cells.

Flagella On Prokaryotic Cells B

Click to animate the image.

C D

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Diversity in Cells, continued Eukaryotic Cell Specialization • Eukaryotic cells can vary in shape and external features. • Depending on their function, eukaryotic cells can also vary in their internal organelles. For example, muscle cells, which use large amounts of energy, contain many mitochondria. • Animal and plant cells are two types of eukaryotic cells. Both have many of the same organelles, but plant cells also have chloroplasts, a large central vacuole, and a cell wall.

Eukaryotic Click Cells to animate the image. I J

C D

A B E A

M L J

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H G D C B F

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Levels of Organization • Plants and animals have many highly specialized cells that are arranged into tissues, organs, and organ systems. • A tissue is a distinct group of similar cells that perform a common function. • An organ is a collection of tissues that work together to form a structure which performs a specific function. • An organ system is composed of a group of organs that work together to perform major body functions.

Body Types • Unicellular organisms can thrive independently or live together in groups. • Cells that are permanently associated but do not work together or integrate cell activities are called colonial organisms. • A multicellular organism is composed of many individual, permanently associated cells that coordinate their activities with each other. True multicellularity occurs only in eukaryotes.

Visual Concept: Comparing Organisms that are Unicellular and Multicellular

Body Types, continued • In a multicellular body, cells are interdependent. Distinct types of cells have specialized functions to help the organism survive. • The individual cells in a multicellular organism cannot survive alone and are dependent on the other cells of the organism. • Must multicellular organisms begin as a single cell, which divides to form more cells. These cells then grow and become specialized in a process called differentiation.

Visual Concept: Differentiation

Summary • The different organelles and features of cells enable organisms to function in unique ways in different environments. • Plants and animals have many highly specialized cells that are arranged into tissues, organs, and organ systems. • A multicellular organism is composed of many individual, permanently associated cells that coordinate their activities with each other.

Concept Check • What makes cells and organisms different? • How are cells organized in a complex multicellular organism? • What makes an organism truly multicellular?

Test Prep

8. The process by which cells become specialized in form and function during development is called A. association. B. aggregation. C. coordination. D.differentiation.

Use the figure shown below to answer the next two questions.

9. What is Structure 1? A. pilum B. cilium C. flagellum D.mitochondrion

10. What is the function of Structure 1? A. to make ATP B. to grab food C. to store energy D.to move the cell