Chapter 6: Cell Structures and Their Functions

Chapter 6 Cell Structures and Their Functions

6.1 Cell Study and Technology

6.2 Two Basic Types of Cells

6.3 Prokaryotic Cell Structure

6.4 Eukaryotic Cell Structure

6.5 Cooperation Among Cells

6.6 Division of Labor

6.7 Systems

Chapter Summary

Review Questions

6.1 Cell Study and Technology

Whatever their size, all living things are composed of cells – the basic unit of life.
Many biologists contributed data and ideas that led to the cell theory, which can be stated in two parts:
1. Cells, or products made by cells, are the unit of structure and function in organisms
2. All cells come from preexisting cells
Examples of varieties of cells – Fig. 6.1
Once the cell theory was established, scientists began to study cell structure and function in detail.
Some cell structures are too small to see without the electron microscope, which was developed in the 1930s.
Electron microscopes reveal very tiny cell parts and even some large molecules down to 0.5 nm – a magnification of mare than a million times. (Fig. 6.2)
The major drawback of the electron microscope is that the steps needed to prepare samples for examination kill any living cells before they can be observed.
Scanning tunneling microscopes can be more powerful than electron microscopes and do not require such harsh treatment of samples.
Both the electron and scanning tunneling microscopes can reveal only surface features.
Cells differ in size – note Fig. 6.3.

6.2 Two Basic types of Cells

Living cells can be separated into two groups, prokaryotes and eukaryotes, that differ in structure.
Prokaryotes – the bacteria – are the simplest living cells and are found almost anywhere on Earth. (Fig. 6.5)
Prokaryotic organisms are nearly always unicellular.
Prokaryotes range in size from about 0.3 μm to 5 μm in diameter.
The cells of eukaryotes are larger (10-50μm) and more complex than prokaryotes.
Eukaryotic cells can form multicellular organisms such as plants, animals and fungi.
Eukaryotic cells have many parts, each with a specific function, that gives them the flexibility to develop in to hundreds of specialized cell types.
The membrane-enclosed nucleus is the most obvious difference between prokaryotes and eukaryotes.
Note the differences between the two types of cells – Fig. 6.6.

6.3 Prokaryotic Cell Structure

Nearly all prokaryotic cells have:
1. A rigid cell wall made of lipids, carbohydrates other than cellulose and protein.
2. A plasma membrane that encloses the cell.
3. One chromosome that is attached to the plasma membrane in an area of the cell known as the nuclear region, or nucleoid.
Most prokaryotes – bacteria – are unicellular but can associate in clusters, chains and films.
In addition, bacteria usually contain one or more smaller circular DNA molecules called plasmids.
Some have flagella (singular: flagellum), long, whip-like extensions made of protein that rotate like propellers, enabling cells to swim through water or the body fluids of larger organisms.
Note the structure of a prokaryotic cell – Fig. 6.7
Most bacteria have one of three shapes – rod, sphere or corkscrew. (Fig. 6.8)
Many of the prokaryotic metabolic processes, such as glycolysis, are similar to those of eukaryotes. However, others are unique.
All ecosystems include many types of bacterial decomposers that help recycle nutrients such as carbon, nitrogen and sulfur compounds.
Many prokaryotes are autotrophs and are important primary producers in lakes and oceans.
Although some bacteria can cause human diseases, such as skin infections and strep throat, most are beneficial. Bacteria in your intestines help you digest food.

6.4 Eukaryotic Cell Structure

Eukaryotic cells are divided into small functional parts called organelles.
Any part of eukaryotic cells that has its own structure and function can be considered an organelle.
Compartmentation makes eukaryotic cells more efficient by separating specific processes and enabling a division of labor within the cell.
A plasma membrane encloses the contents of both eukaryotic cells and prokaryotic cells. (Fig. 6.9)
A cell wall surrounds the plasma membrane of plant and fungal cells, as well as some unicellular eukaryotes. (Fig. 6.9)
Animal cells lack a rigid cell wall. (Fig. 6.9)
The nucleus contains the chromosomes and is a cell’s genetic control center. (Fig. 6.9)
A double layer of membranes forms the nuclear envelope, or nuclear membrane, that surrounds the chromosomes. (Fig. 6.9)
One or more drops of concentrated RNA are usually visible in the nucleus, in bodies called nucleoli (singular; nucleolus). (Fig. 6.9)
The nucleoli are the sites where types of RNA are synthesized.
Within the plasma membrane, but outside the nucleus, is the cellular material, or cytoplasm. (Fig. 6.9)
The cytosol is the protein-rich, semifluid material in the cell that surrounds and bathes the organelles. (Fig. 6.9)
The cytoplasm includes the cytosol and the organelles.
A network of several types of very fine protein fibers, known as the cytoskeleton, helps to shape the cell and organize the cytoplasm. (Fig. 6.9)
The cytoskeleton includes hollow microtubules and connecting intermediate filaments. (Fig. 6.20)
Many small bodies composed of RNA and protein, called ribosomes, are scattered throughout the cytoplasm. (Fig. 6.9)
Ribosomes catalyze the synthesis of a cell’s proteins.
In eukaryotes, some ribosomes are attached to a system of membranes called the endoplasmic reticulum (ER). (Fig. 6.9)
The ER membranes form tubes and channels throughout the cytoplasm connecting many of the organelles in the cell.
Proteins are synthesized at the ribosomes attached to the ER.
Proteins and other substances are transported through the ER to their final destinations in the cell.
Many substances that are exported from the cell pass through the ER to the Golgi apparatus. (Fig. 6.9)
Material passing through the Golgi apparatus is packaged in vesicles that appear to pinch off the Golgi membranes. (Fig. 6.9)
Together the ER, Golgi apparatus, and vesicles form a connected internal membrane system.
The structure of the system enables it to direct proteins to target points inside the cell and to the plasma membrane for passage out of the cell. (Fig. 6.15)
Lysosomes are special vesicles in animal cells and some other eukaryotes that contain enzymes that break down the cell’s old macromolecules for recycling. (Fig. 6.9)
Lysosomes can also fuse with vesicles formed by endocytosis, digesting the food particles within.
Some animal cells have lysosomes that fuse with the plasma membrane, releasing digestive enzymes outside the cell.
The vacuoles present in most plant cells are vesicles that enlarge as the cells mature. (Fig. 6.9)
Vacuoles contain water, organic acids, digestive enzymes, salts and pigments.
Up to 90% of the volume of a mature plant cell may consist of its vacuole.
Chloroplasts and mitochondria are double-membrane organelles involved in energy reactions. (Fig. 6.9)
Photosynthesis occurs in chloroplasts.
Mitochondria are the major sites of ATP synthesis in most eukaryotic cells.
Centioles are tubular structures in the cells of animals and some fungi and algae that participate in cell reproduction. (Fig. 6.9)
Centrioles consist of a pair of cylindrical bundles of microtubules.
Some eukaryotic cells have flagella that are covered by the plasma membrane and consist to long bundles of microtubles. (Fig. 6.18)
Enzymes associated with these microtubles provide energy for the motion of the flagellum by breaking down ATP.
Cilia are short flagella. (Fig. 6.19)
Eukaryotic flagella and cilia move cells along by whipping in an oar-like motion against the fluid surrounding a cell.
Cilia can also help move material along a cell or tissue.
Note some differences between prokaryotic and eukaryotic cells – Table 6.1

6.5 Cooperation Among Cells

When one-celled organisms divide, some new cells may remain together in a cluster. (Fig. 6.21)
In a cluster of cells, each cell has an individual life and may break away from the cluster at some point.
Some unicellular microorganisms live in groups called colonies. (Fig. 6.23)
Members of a colony may interact in ways that give them advantages over isolated organisms, but each member is still s separate organism.
In some colonies, such as that of Volvox, a colonial algae, individual cells take on specialized roles. (Fig. 6.22)
Some types of Volvox have delicate strands of cytoplasm connecting the cells and can coordinate their motions.
Volvox has some characteristics of a colony, but it also has some specialized reproductive cells, and the two ends of the colony are different.
Volvox could be considered just barely a multicellular organism.
Organisms must have enough surface area for the living cells within to exchange food, wastes and other substances with their environment.
Large plants and animals use structures, such as blood vessels, lungs and leaves, that add internal surface area.
Structures that enable large organisms to survive require a number of specialized cell types.
All cells must carry on the basic activities of life, but each type of cell often takes on a special job as well.
1. A gland cell is specialized for making certain types of chemicals.
2. A nerve cell is efficient in conducting electric signals.
3. A muscle cell is specialized for movement.
4. The cells that form an organism’s outer covering, or epidermis, may be specialized.
Hydra is a small, threadlike freshwater animal with a ring of tentacles at one end. (Fig. 6.24b)
The animal’s cells look slightly different and are different in specialization. (Fig. 6.24a)
Cells of larger organisms are much more distinctive in appearance. (Fig. 6.25)

6.6 Division of Labor

In multicellular organisms, a group of cells with the same specialization usually work together.
Each specialized mass or layer of cells is called a tissue.
Different tissues may be organized into organs.
Organs may be incorporated into systems of organs.
Note diagram – Fig. 6.26

6.7 Systems

In most multicellular organisms, the inner cells cannot obtain nutrients directly from the outside environment or pass their wastes directly to the outside environment.
Specialized systems are required to handle deliveries between the environment and the cells.
Most specialized systems are necessary for three reasons:
1. A division of labor occurs among cells
2. Many individual cells cannot work together without regulation and coordination, and
3. Most cells are not in direct contact with the outside environment.
In many organisms, additional specializations have developed within the organ systems.
Specialized systems are required to handle deliveries between the environment and the cells.
Cells are the lowest level of organization that truly can be considered living.
Note Levels of Structure in the Biosphere – Fig. 6.29

Summary –

Prokaryotic cells are smaller and less specialized than eukaryotic cells.
The most distinguishing characteristic of eukaryotic cells is the presence of organelles, which include the nucleus, mitochondria, chloroplasts, ribosomes, vacuoles, endoplasmic reticulum and other compartments with specific functions in the eukaryotic cell.
Cells may exist alone as unicellular organisms.
Cells may be clustered and form multicellular organisms.
Tissues, organs and systems become more complex in larger multicellular organisms.

Reviewing Ideas –

What is the major drawback of the electron microscope?
What is the lowest level of organization that truly can be considered living? Where does this fall on the continuum of biological organization?
Why are prokaryotes a vital part of every ecosystem?
How does a Hydra survive without a circulation system?
What are the basic requirements to be considered a multicellular organism?