Chapter 4 Cells: The Basic Units of Life The Big Idea All organisms are composed of one or more cells. Section 1 The Characteristics of Cells Key Concept Cells function similarly in all living organisms. Cells and Cell Discovery Cells function similarly in all living things. A cell is the smallest structural and functional unit of living things. In 1665, Robert Hooke built a microscope and and observed box-like structures in a sample of cork. He called the structures cells. Because animal cells lack cell walls, Hooke could not see them. He believed that only plants and fungi were made of cells. In 1673, Dutch merchant Anton van Leeuwenhoek made a microscope and observed swimming animacules in a sample of pond scum. Today we call these single-celled organisms protists. Leeuwenhooek also observed blood cells from different animals and was the first person to observe bacteria. Cells and Cell Theory Nearly 100 years later, Matthias Schleiden, a plant scientists, concluded that all plant parts were made of cells. A year later, in 1839, Theodor Schwann concluded that all animal tissues were made of cells. Schwann went on to write the first two parts of modern cell theory: All organisms are made up of one or more cells. The cell is the basic unit of all living things. In 1858, Rudolf Virchow added the third part of the cell theory: All cells come from existing cells. 1
Review Tip Telling a Story Tell a story about the discovery of cells and how the cell theory came into existence. Cell Size The vast majority of cells are too small to be seen without a microscope. If the volume of a cell becomes too large, the surface area of its membrane will not be able to let in enough materials and let out wastes. Cells must remain small to maintain a proper surface-area-to-volume ratio. Cell Membrane Cells come in many shapes and sizes, and may be specialized for different functions. But all cells have some parts in common. All cells have a cell membrane. The cell membrane is a protective layer that covers the cell s surface and acts a barrier. The cell membrane separates the cell s contents from its surroundings. The cell membrane also controls the materials going into and out of the cell. Cytoplasm and Organelles Most of the contents of a cell, including the fluid, is called the cytoplasm. Organelles are part of the cytoplasm. Organelles are structures that have specific jobs inside the cell. DNA and Nucleus All cells have DNA (deoxyribonucleic acid) at some point in their lives. DNA is the genetic material that carries instructions for making new cells. In eukaryotic cells, including plants and animals, the DNA is found within the nucleus of the cell. The nucleus is an organelle specialized to hold the DNA. The nucleus plays a role in growth, metabolism, and reproduction. Bacteria have DNA, but do not have a nucleus. Their DNA floats free in 2
the cytoplasm. Human blood cells have a nucleus and DNA as they are growing. Once mature, they lose their DNA and nucleus. Most cells, however, need DNA throughout their lives. The DNA provides instructions for making proteins. Two Kinds of Cells Cells that do not have a nucleus are called prokaryotes. Bacteria and archaea are prokaryotes. Prokaryotic DNA is a round molecule, twisted like a rubber band. Prokaryotes have cell walls. They lack the membrane-bound organelles found in other organisms. Eukaryotes are organisms made up of cells that have a nucleus enclosed by a membrane. Eukaryotic cells are 10 times as large as prokaryotic cells, although most eukaryotic cells are still microscopic. Eukaryotes have membrane-bound organelles to carry out the functions of the cell. Section 2 Eukaryotic Cells Key Concept Eukaryotic cells have organelles that perform important functions. Cell Wall Eukaryotic cells have many organelles in common to carry out important functions. Plants are eukaryotes that have some structures and organelles not seen in animal cells. Plant cells have a rigid structure that surrounds the cell membrane called the cell wall. Animal cells lack a cell wall. Cell Membrane All cells have a cell membrane made up of proteins and phospholipids. 3
A phospholipid has a water-loving, or hydrophillic, head and a waterfearing, or hydrophobic, tail. The phospholipids form a two-layer protective membrane. The two-layer membrane makes it difficult for materials to pass into or out of the cell. Some materials, like nutrients and wastes, move through the membrane using protein passageways within the membrane. Cytoskeleton The cytoskeleton is a web of proteins in the cytoplasm of some cells. It plays a key role in cell movement, shape, and division. Different cells have different shapes because of the arrangement of their cytoskeletons. Nucleus All eukaryotic cells have a nucleus. The nucleus is the large organelle that contains the cell s DNA. The DNA provides messages for making proteins. The messages are sent out of the nucleus through pores in the nucleus membrane. Some cells have a dark area within the nucleus called the nucleolus. Ribosome assembly begins here. Ribosomes Ribosomes are small organelles that make proteins. There are more ribosomes than any other organelle in the cell. Because all cells need to make proteins, all cells have ribosomes. Unlike most organelles, ribosomes are not covered by a membrane. Endoplasmic Reticulum The endoplasmic reticulum, or ER, is a system of folded membranes in which proteins, lipids, and other materials are made. Rough ER is covered with ribosomes that make proteins near the nucleus. Smooth ER lacks ribosomes. Smooth ER makes lipids and breaks down toxic materials. The ER also 4
functions as a delivery system for the cell. Mitochondria A mitochondrion is the main power source of a cell. Mitochondria are organelles in which sugar is broken down to release energy. Mitochondria have two membranes. The folded inner membrane of a mitochondrion is where ATP is made. ATP is the energy storing molecule used by the cell to carry out its functions. Most eukaryotic cells have mitochondria. They are unusual organelles in that they have their own DNA and divide on their own, like bacteria. Chloroplasts Chloroplasts are organelles in which photosynthesis takes place. Plants, algae, and some prokaryotes have cells with chloroplasts. Chloroplasts are green because they contain chlorophyll, the pigment that traps the energy of sunlight. This energy is used to make sugar during photosynthesis. Chloroplasts, like mitochondria, have their own DNA and two membranes. Photosynthesis takes place in the inner membrane. Golgi Complex The Golgi complex packages and distributes proteins and lipids. The materials may be taken to other parts of the cell or may exit the cell. Materials may be modified in the Golgi complex to do different jobs before they are sent off. The Golgi complex pinches off portions of its membrane to create bubbles. The bubbles contain the materials to be transported. The Golgi complex looks much like the smooth ER. Cell Compartments The bubble that forms from the Gogi complex s membrane is an example of a vesicle. 5
A vesicle is a small sac that surrounds materials to be moved into or out of the cell. Vesicles also move materials within the cell. All eukaryotic cells have vesicles. Lysosomes Lysosomes are vesicles that contain digestive enzymes. They carry out digestion inside a cell. Lysosomes destroy worn-out organelles, engulf foreign invaders, and get rid of wastes. Lysosomes are found mainly in animal cells. Vacuoles A vacuole is another type of vesicle found in cells. They play an important role in plants. Vacuoles in plants and fungi may act like lysosomes. The large central vacuole in a plant cell stores water and other liquids. Some plants wilt when their large central vacuoles lose water. Review Tip Creating Diagrams Create a Venn diagram to categorize the parts of plant and animals cells. Section 3 The Organization of Living Things Key Concept As multicellular organisms develop, their cells differentiate and form levels of organization. Unicellular Organisms Anything that can perform life processes by itself is an organism. Organisms that are made up of only one cell are unicellular. All prokaryotes are unicellular. Some eukaryotes, including yeasts, some algae, and some protists, are unicellular. Unicellular organisms need fewer resources and can live in harsher conditions. 6
Multicellular Organisms Multicellular organisms do not simply have more cells than unicellular organisms. They differ in other ways, too. Larger Size Multicellular organisms grow by making more cells, not be making their cells larger. Larger organisms can eat a wider variety of foods, and are prey to fewer predators. Longer Life Multicellular organisms live longer than unicellular organisms. Specialization Multicellular organisms have cells specialized for different functions. This makes them more efficient than unicellular organisms. Cells: The First Level of Organization Specialized cells allow multicellular organisms to be more organized than unicellular organisms. Cells in a multicellular organism are specialized to perform a specific function. A function of a cell is the activity that the cell performs. The function of a cell relates to the cell s structure. Structure is the arrangement of parts in an organism. Muscle cells include structures that contract. Leaf cells include structures that allow gasses to move in and out of the leaf. Tissues: The Second Level of Organization A tissue is a group of cells that work together to perform a specific job. The four basic types of animal tissue are nerve tissue, muscle tissue, connective tissue, and protective tissue. The three basic types of plant tissues include transport tissue, protective tissue, and ground tissue. Organs: The Third Level of Organization An organ is a structure that is made up of two or more tissues working together to perform a specific function. Your heart is an organ made mostly of cardiac muscle tissue. It also 7
has nerve tissue and blood vessel tissue. The leaf of a plant is an organ that contains ground tissue for photosynthesis. It also has transport tissue for water movement and protective tissue to prevent water loss. Organ Systems: The Fourth Level of Organization A group of organs working together to perform a function is called an organ system. For example, the cardiovascular system includes the heart organ and the blood vessel organs. Their combined function is to transport blood throughout the body. Cells, Tissues, Organs, Organ Systems, and Organisms In multicellular organisms, cells form tissues, tissues form organs, and organs form organ systems. Organ systems then work together to form the organism. Unicellular Organization Some unicellular organisms come together to form a large group or colony. For example, slime mold is made up of many individual cells. Although they may be part of a large group of cells, each cell in a unicellular colony is an individual. The cells in a unicellular colony are not specialized. Review Tip Drawing Flowcharts Create a flowchart that illustrates the organization of an organism from smallest parts to largest parts. Review Tip Wrap- Up Think about the methods you have used to study the concepts in this chapter. Which types of Review Tips are the most helpful to you? What types of concepts do they help you study? Think about review methods you can use when you are studying. 8