Cells: The Working Units of Life

5 Cells: The Working Units of Life 5.1 What Features Make Cells the Fundamental Units of Life? 5.2 What Features Characterize Prokaryotic Cells? 5.3 What Features Characterize Eukaryotic Cells? 5.4 What Are the Roles of Extracellular Structures? 5.5 How Did Eukaryotic Cells Originate?

5.1 What Features Make Cells the Fundamental Units of Life? Cell theory was the first unifying theory of biology. Cells are the fundamental units of life All organisms are composed of cells All cells come from preexisting cells

5.1 What Features Make Cells the Fundamental Units of Life? Implications of cell theory: Functions of all cells are similar Life is continuous Origin of life was the origin of cells

5.1 What Features Make Cells the Fundamental Units of Life? Cells are small (mostly). Exceptions: Bird eggs, neurons, some algae, and bacteria cells

Figure 5.1 The Scale of Life (Part 1)

Figure 5.1 The Scale of Life (Part 2)

5.1 What Features Make Cells the Fundamental Units of Life? Cells are small because a high surface area-to-volume ratio is essential. Volume determines the amount of chemical activity in the cell per unit time. Surface area determines the amount of substances that can pass the cell boundary per unit time.

Figure 5.2 Why Cells Are Small (Part 1)

Figure 5.2 Why Cells Are Small (Part 2)

5.1 What Features Make Cells the Fundamental Units of Life? Most cells are < 200 μm in size. To see them, we use microscopes: Magnification: Increases apparent size Resolution: Clarity of magnified object – minimum distance two objects can be apart and still be seen as two objects.

5.1 What Features Make Cells the Fundamental Units of Life? Two basic types of microscopes: Light microscopes—use glass lenses and light. Resolution = 0.2 μm Electron microscopes—electromagnets focus an electron beam. Resolution = 0.2 nm

Figure 5.3 Looking at Cells (Part 1)

Figure 5.3 Looking at Cells (Part 2)

Figure 5.3 Looking at Cells (Part 3)

5.1 What Features Make Cells the Fundamental Units of Life? Pathology is a branch of medicine that uses microscopy to analyze cells and diagnose diseases. Many methods are used, including phase-contrast microscopy, staining the cells with general or fluorescent dyes, and electron microscopy.

5.1 What Features Make Cells the Fundamental Units of Life? The plasma membrane is the outer surface of every cell, and has more or less the same structure in all cells. It is made of a phospholipid bilayer with proteins and other molecules embedded. It is not rigid, but more like an oily fluid in which the proteins and lipids are in constant motion.

5.1 What Features Make Cells the Fundamental Units of Life? The plasma membrane: Is a selectively permeable barrier Allows cells to maintain a constant internal environment Is important in communication and receiving signals Often has proteins for binding and adhering to adjacent cells

5.1 What Features Make Cells the Fundamental Units of Life? Two types of cells: Prokaryotic and eukaryotic Bacteria and Archaea are prokaryotes. The first cells were probably prokaryotic. Eukarya are eukaryotes—the DNA is in a membrane-enclosed compartment called the nucleus.

5.1 What Features Make Cells the Fundamental Units of Life? Eukaryotes have other membrane- enclosed compartments in which specific chemical reactions occur. This has allowed diversification of functions in eukaryotic cells, and their specialization into tissues.

5.2 What Features Characterize Prokaryotic Cells? Prokaryotic cells are very small. Individuals are single cells, but often found in chains or clusters. Prokaryotes are very successful—they can live on a diversity of energy sources and some can tolerate extreme conditions.

5.2 What Features Characterize Prokaryotic Cells? Are enclosed by a plasma membrane The DNA is contained in the nucleoid Cytoplasm consists of cytosol (water and dissolved material) and suspended particles Ribosomes—sites of protein synthesis

Figure 5.4 A Prokaryotic Cell

5.2 What Features Characterize Prokaryotic Cells? Most prokaryotes have a rigid cell wall outside the plasma membrane. Bacteria cell walls contain peptidoglycan. Some bacteria have an additional outer membrane. Some bacteria have a slimy capsule of polysaccharides.

5.2 What Features Characterize Prokaryotic Cells? Photosynthetic bacteria have an internal membrane system that contains molecules necessary for photosynthesis. Others have internal membrane folds that are attached to the plasma membrane; they may function in cell division or in energy-releasing reactions.

5.2 What Features Characterize Prokaryotic Cells? Some prokaryotes swim by means of flagella, made of the protein flagellin. Some bacteria have pili—hairlike structures projecting from the surface. They help bacteria adhere to other cells. Some rod-shaped bacteria have a cytoskeleton made of the protein actin.

Figure 5.5 Prokaryotic Flagella (Part 1)

Figure 5.5 Prokaryotic Flagella (Part 2)

5.3 What Features Characterize Eukaryotic Cells? Eukaryotic cells are up to ten times larger than prokaryotes. Eukaryotic cells have membrane- enclosed compartments called organelles. Each organelle has a specific role in cell functioning.

5.3 What Features Characterize Eukaryotic Cells? Compartmentalization allowed eukaryotic cells to specialize and form the tissues and organs of multicellular organisms.

Figure 5.7 Eukaryotic Cells (Part 1)

Figure 5.7 Eukaryotic Cells (Part 2)

Figure 5.7 Eukaryotic Cells (Part 3)

Figure 5.7 Eukaryotic Cells (Part 4)

5.3 What Features Characterize Eukaryotic Cells? Organelles were first studied using light microscopy. Cell fractionation separates organelles for study by chemical methods.

Figure 5.6 Cell Fractionation

5.3 What Features Characterize Eukaryotic Cells? Ribosomes—sites of protein synthesis. Occur in both prokaryotic and eukaryotic cells and have similar structure. Ribosomes consist of ribosomal RNA (rRNA) and more than 50 different protein molecules.

5.3 What Features Characterize Eukaryotic Cells? In eukaryotes, ribosomes are free in the cytoplasm, attached to the endoplasmic reticulum, or inside mitochondria and chloroplasts. In prokaryotic cells, ribosomes float freely in the cytoplasm.

5.3 What Features Characterize Eukaryotic Cells? The nucleus is usually the largest organelle. Contains the DNA Site of DNA replication Site where gene transcription is turned on or off Assembly of ribosomes begins in a region called the nucleolus

Figure 5.8 The Nucleus Is Enclosed by a Double Membrane (Part 1)

5.3 What Features Characterize Eukaryotic Cells? The nucleus is surrounded by two membranes—the nuclear envelope. Nuclear pores in the envelope control movement of molecules between nucleus and cytoplasm.

Figure 5.8 The Nucleus Is Enclosed by a Double Membrane (Part 2)

5.3 What Features Characterize Eukaryotic Cells? Some large molecules (e.g., proteins) must have a certain amino acid sequence known as a nuclear localization signal (NLS) to cross the nuclear envelope.

5.3 What Features Characterize Eukaryotic Cells? In the nucleus, DNA combines with proteins to form chromatin in long, thin threads called chromosomes. Before cell division, chromatin condenses, and individual chromosomes are visible in the light microscope.

Figure 5.9 Chromatin and Chromosomes

5.3 What Features Characterize Eukaryotic Cells? Nucleoplasm surrounds the chromatin, and a network of structural proteins (nuclear matrix) helps organize the chromatin. The nuclear lamina attaches to both the chromatin and the nuclear envelope and maintains nuclear shape.

5.3 What Features Characterize Eukaryotic Cells? The endomembrane system includes the plasma membrane, nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes. Tiny, membrane-surrounded vesicles shuttle substances between the various components.

5.3 What Features Characterize Eukaryotic Cells? Endoplasmic reticulum (ER): network of interconnected membranes in the cytoplasm; has large surface area. Rough endoplasmic reticulum (RER): ribosomes are attached. Newly made proteins enter the RER lumen where they are modified, folded, and transported to other regions.

Figure 5.10 The Endomembrane System (Part 1)

Figure 5.10 The Endomembrane System (Part 2)

5.3 What Features Characterize Eukaryotic Cells? Smooth endoplasmic reticulum (SER): more tubular, no ribosomes Chemically modifies small molecules such as drugs and pesticides Hydrolysis of glycogen in animal cells Synthesis of lipids and steroids

5.3 What Features Characterize Eukaryotic Cells? The Golgi apparatus is composed of flattened sacs (cisternae) and small membrane-enclosed vesicles. Receives proteins from the RER—can further modify them Concentrates, packages, sorts proteins In plant cells, polysaccharides for cell walls are synthesized here

5.3 What Features Characterize Eukaryotic Cells? The cis region receives vesicles (a piece of the ER that “buds” off) from the ER. At the trans region, vesicles bud off from the Golgi apparatus and are moved to the plasma membrane or other organelles.

Figure 5.10 The Endomembrane System (Part 1)

Figure 5.10 The Endomembrane System (Part 2)