David Sadava David M Hillis H Craig Heller

David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia. blu Le cellule e i viventi

5 Cells: The Working Units of Life

What Features Make Cells the Fundamental Units of Life? Cells are small because a high surface area-to-volume ratio is essential.

Looking at Cells (Part 1) Most cells are < 200 μm in size. To see them, we use microscopes.

Looking at Cells (Part 2)

The Scale of Life

What Features Characterize Prokaryotic Cells? (Part 1) Two types of cells: Prokaryotic cells and Eukaryotic ones. Bacteria and Archaea are prokaryotes. They have no membrane-enclosed internal compartments.

What Features Characterize Prokaryotic Cells? (Part 2) Characteristics of prokaryotic cells: • Enclosed by a plasma membrane. • Cytoplasm consists of cytosol (liquid component) plus filaments and particles. • DNA is contained in a region called the nucleoid. • Ribosomes: RNA and protein complexes; sites of protein synthesis.

What Features Characterize Prokaryotic Cells? (Part 3) Most prokaryotes have a rigid cell wall outside the plasma membrane. Some bacteria have an additional outer membrane. Some bacteria have a slimy capsule of polysaccharides. Photosynthetic bacteria have an internal membrane system that contains molecules necessary for photosynthesis.

What Features Characterize Prokaryotic Cells? (Part 4) 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. Cytoskeleton: system of protein filaments that maintain cell shape and play roles in cell division.

What Features Characterize Eukaryotic Cells? Eukaryotic cells are up to ten times larger than prokaryotes. Eukaryotic cells have membrane-enclosed compartments called organelles with specific roles in cell functioning.

Eukaryotic Cells: Animal Cell (Part 1)

Eukaryotic Cells: Animal Cell (Part 2)

Eukaryotic Cells: Plant Cell (Part 3)

Eukaryotic Cells: Plant Cell (Part 4)

What Features Characterize Eukaryotic Cells? The Nucleus 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

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 (r. RNA) and more than 50 different protein molecules. 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.

What Features Characterize Eukaryotic Cells? The Endomembrane System The endomembrane system includes the plasma membrane, nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes.

What Features Characterize Eukaryotic Cells? The Endomembrane System

What Features Characterize Eukaryotic Cells? The Endoplasmic reticulum (ER): network of interconnected membranes in the cytoplasm; it 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. Smooth endoplasmic reticulum (SER): more tubular, no ribosomes. It chemically modifies small molecules such as drugs and pesticides and it is the site of lipids and steroids synthesis.

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

What Features Characterize Eukaryotic Cells? Lysosomes originate from the Golgi apparatus. They contain digestive enzymes which hydrolyze macromolecules into monomers.

What Features Characterize Eukaryotic Cells? Peroxisomes and vacuoles Peroxisomes: collect and break down toxic byproducts of metabolism such as H 2 O 2, using specialized enzymes. Plant and protist cells have vacuoles.

What Features Characterize Eukaryotic Cells? Chloroplasts Plastids occur only in plants and some protists. Chloroplasts: Site of photosynthesis— light energy is converted to the energy of chemical bonds. Chloroplasts have a double membrane.

What Features Characterize Eukaryotic Cells? Mitochondria (Part 1) Mitochondria have two membranes. The inner membrane folds inward to form cristae. This creates a large surface area for the proteins involved in cellular respiration reactions. The mitochondrial matrix contains enzymes, DNA, and ribosomes.

What Features Characterize Eukaryotic Cells? Mitochondria (Part 2) In the mitochondria, energy in fuel molecules is transformed to the bonds of energy-rich ATP (cellular respiration). Cells that require a lot of energy have many mitochondria.

What Features Characterize Eukaryotic Cells? The cytoskeleton (Part 1) The cytoskeleton: • Supports and maintains cell shape • Holds organelles in position • Moves organelles • Involved in cytoplasmic streaming • Interacts with extracellular structures to hold cell in place

What Features Characterize Eukaryotic Cells? The cytoskeleton (Part 2) The cytoskeleton has three components:

What Features Characterize Eukaryotic Cells? Microfilaments (Part 1) Microfilaments: • Help a cell or parts of a cell to move (formation of pseudopodia) • Determine cell shape (microvilli that line the human intestine) • Made from the protein actin • Actin has + and – ends and polymerizes to form long helical chains (reversible)

What Features Characterize Eukaryotic Cells? Microfilaments (Part 2)

What Features Characterize Eukaryotic Cells? Intermediate filaments: • 50 different kinds in six molecular classes • Tough, ropelike protein assemblages • Anchor cell structures in place • Resist tension

What Features Characterize Eukaryotic Cells? Microtubules: • Long, hollow cylinders • Form rigid internal skeleton in some cells • Act as a framework for motor proteins • Made from the protein tubulin—a dimer • Cilia and eukaryotic flagella are made of microtubules Cilia are shorter, usually many present, move with stiff power stroke and flexible recovery stroke. Flagella are longer, usually one or two present, movement is snakelike.

What Are the Roles of Extracellular Structures? The Cell wall Plant cell wall: Cellulose fibers are embedded in other complex polysaccharides and proteins. Adjacent plant cells are connected by plasma membrane-lined channels called plasmodesmata.

What Are the Roles of Extracellular Structures? The extracellular Matrix Many animal cells are surrounded by an extracellular matrix, composed of fibrous proteins such as collagen, gel-like proteoglycans (glycoproteins), and other proteins.

How Did Eukaryotic Cells Originate? The endosymbiosis theory proposes that mitochondria and plastids arose when one cell engulfed another cell.

Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved