Key Concepts

1. Key Concepts The structure and function of a cell’s overall shape and composition, as well as individual cell components, are closely related. Molecular “zip codes” aid material transport within a cell. The cell’s cytoskeleton provides a structural framework within the cell, and plays a role in cell division, movement, and transport.

2. Key Concepts Cells are highly dynamic and integrated; within a cell, thousands of chemical reactions occur every second, molecules are constantly moving across the plasma membrane, cell products are transported along protein fibers, and elements of the cell’s internal skeleton grow and shrink.

3. Grouping Cells According to morphology, there are two broad groupings of life: Prokaryotes, which lack a membrane-bound nucleus Eukaryotes, which have such a nucleus According to phylogeny, or evolutionary history, there are three domains: Bacteria Archaea Eukarya – eukaryotic prokaryotic

4. Prokaryotic Cells – Structural Overview All prokaryotes lack a membrane-bound nucleus. Recent advances in microscopy reveal complexity in prokaryotic structure. Archaeal cell structure is relatively poorly understood. Bacterial cells vary greatly in size and shape, but most bacteria contain several structural similarities: Plasma membrane A single chromosome Ribosomes, which synthesize proteins Stiff cell wall

6. Prokaryotic Cells – Genetic Information Most prokaryotic species have one supercoiled circular chromosome found in the nucleoid region of the cell. The chromosome contains a long strand of DNA and a few supportive proteins. In addition to the large chromosome, many bacteria contain plasmids. Small, supercoiled, circular DNA molecules Plasmids usually contain genes that help the cell adapt to unusual environmental conditions.

9. Prokaryotic Cells – Internal Structure In addition to the nucleoid chromosome and plasmids, other structures are contained within the cytoplasm: All prokaryotic cells contain ribosomes, consisting of RNA molecules and protein, for protein synthesis. Many prokaryotes have internal photosynthetic membranes. Cyanobacteria (or Cyanophyta)  Classfication Orders Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales, Stigonematales Some prokaryotes have membrane-enclosed organelles. The inside of many prokaryotic cells is supported by a cytoskeleton of long, thin protein filaments.

10. Bacterial Organelles Recently, internal compartments in many bacterial species were discovered. These compartments qualify as organelles (“little organs”). An organelle is a membrane-bound compartment inside the cell that contains enzymes or structures specialized for a particular function. Organelles are common in eukaryotic cells. Each type of bacterial organelle is found in certain species. Bacterial organelles perform an array of tasks.

11. Prokaryotic Cells – External Structure Some prokaryotes have tail-like flagella on the cell surface that spin around to move the cell. Most prokaryotes have a cell wall. Bacterial and archaeal cell walls are a tough, fibrous layer that surrounds the plasma membrane. Many species have an additional layer outside the cell wall composed of glycolipids.

15. An Introduction to Eukaryotes Eukaryotes range from microscopic algae to 100-meter-tall redwood trees. Many eukaryotes are multicellular, others are unicellular. Most eukaryotic cells are larger than most prokaryotic cells.

16. Eukaryotic Cells The relatively large size of the eukaryotic cell makes it difficult for molecules to diffuse across the entire cell. This problem is partially solved by breaking up the large cell volume into several smaller membrane-bound organelles. The compartmentalization of eukaryotic cells offers two primary advantages: Separation of incompatible chemical reactions Increasing the efficiency of chemical reactions

17. Eukaryotes and Prokaryotes Compared Four key differences between eukaryotic and prokaryotic cells have been identified: Eukaryotic chromosomes are found inside a membrane-bound compartment called a nucleus. Eukaryotic cells are often much larger. Eukaryotic cells contain extensive amounts of internal membrane. Eukaryotic cells feature a diverse and dynamic cytoskeleton.

20. The Nucleus The nucleus is large and highly organized. STRUCTURE: The nucleus is surrounded by a double-membrane nuclear envelope. The nucleus has a distinct region called the nucleolus. FUNCTION: Information storage and processing Contains the cell’s chromosomes (nucleosome complexes) Ribosomal RNA synthesis (in the nucleolus)

21. Rough Endoplasmic Reticulum STRUCTURE: The rough endoplasmic reticulum (rough ER, RER) is a network of membrane-bound tubes and sacs studded with ribosomes. The interior is called the lumen. The rough ER is continuous with the nuclear envelope. FUNCTION: Ribosomes associated with the rough ER synthesize proteins. New proteins are folded and processed in the rough ER lumen.

22. Golgi Apparatus STRUCTURE: The Golgi apparatus is formed by a series of stacked flat membranous sacs called cisternae. FUNCTION: The Golgi apparatus processes, sorts, and ships proteins synthesized in the rough ER. Membranous vesicles carry materials to and from the organelle.

23. Ribosomes STRUCTURE: Ribosomes are non-membranous (they are not considered organelles). Have large and small subunits, both containing RNA molecules and protein Ribosomes can be attached to the rough ER or free in the cytosol, the fluid part of the cytoplasm. FUNCTION: Protein synthesis

25. Peroxisomes STRUCTURE: Peroxisomes areglobular organelles bound by a single membrane. FUNCTION: Center of oxidation reactions Specialized peroxisomes in plants called glyoxysomes are packed with enzymes that oxidize fats to form a compound that can be used to store energy for the cell.

27. Lysosomes STRUCTURE: Lysosomes are single-membrane-bound structures containing approximately 40 different digestive enzymes. Lysosomes are found in animal cells. FUNCTION: Lysosomes are used for digestion and waste processing.

29. How Are Materials Delivered to Lysosomes? Materials are delivered to the lysosomes by three processes: Phagocytosis – use of plasma membrane to engulf large particles Autophagy (aka autophagocytosis) – remove unnecessary cellular components Receptor-mediated endocytosis – mechanism which specific molecules are brought into the cell Endocytosis is a process by which the cell membrane can pinch off a vesicle to bring outside material into the cell. In addition to phagocytosis and receptor-mediated endocytosis, a third type of endocytosis called pinocytosis brings fluid into the cell.

31. Mitochondria STRUCTURE: Mitochondria have two membranes; the inner one is folded into a series of sac-like cristae. The solution inside the cristae is called the mitochondrial matrix. Mitochondria have their own DNA and manufacture their own ribosomes. FUNCTION: ATP production is a mitochondrion’s core function.

33. Cytoskeleton The cytoskeleton, composed of protein fibers, gives the cell shape and structural stability, and aids cell movement and transport of materials within the cell. In essence, the cytoskeleton organizes all of the organelles and other cellular structures into a cohesive whole.

34. Structure and Function at the Whole-Cell Level An organelle’s membrane and its enzymes correlate with its function, and cell structure (e.g., the type, size, and number of organelles) correlates with cell function. Cells are dynamic living things with interacting parts and constantly moving molecules.

35. How Dynamic Are Eukaryotic Cells? Your body’s cells use, and synthesize, approximately 10 million ATP molecules per second. Cellular enzymes can catalyze >25,000 reactions per second. Each membrane phospholipid can travel the breadth of its organelle or cell in under a minute. The hundreds of trillions of mitochondria inside you are replaced about every 10 days, for as long as you live. The fluid plasma membrane’s composition is constantly changing.

36. The Endomembrane System The endomembrane system is composed of the smooth and rough ER and the Golgi apparatus, and is the primary system for protein and lipid synthesis. Ions, ATP, amino acids, and other small molecules diffuse randomly throughout the cell, but the movement of proteins and other large molecules is energy demanding and tightly regulated.

37. The Dynamic Cytoskeleton The cytoskeleton is a complex network of fibers that helps maintain cell shape by providing structural support. The cytoskeleton is dynamic; it changes to alter the cell’s shape, to transport materials in the cell, or to move the cell itself. There are three types of cytoskeletal elements: Actin filaments (microfilaments) Intermediate filaments Microtubules

38. Actin Filaments Actin filaments are the smallest cytoskeletal elements. Actin filaments form by polymerization of individual actin molecules. Actin filaments are grouped together into long bundles or dense networks that are usually found just inside the plasma membrane and help define the cell’s shape.

39. Actin-Myosin Interactions Actin filaments can also be involved in movement by interacting with the motor protein myosin. Actin-myosin interactions can cause cell movements such as cell crawling, cytokinesis, and cytoplasmic streaming.

41. Intermediate Filaments Intermediate filaments are defined by size rather than composition. Many types of intermediate filaments exist, each consisting of a different protein. Intermediate filaments provide structural support for the cell. They are not involved in movement. Intermediate filaments form a flexible skeleton that helps shape the cell surface and hold the nucleus in place.

42. Microtubule Structure Microtubules are large, hollow tubes made of tubulindimers (two-part compounds). Microtubules have polarity, are dynamic, and usually grow at their plus ends. Microtubules originate from the microtubule organizing center and grow outward, radiating throughout the cell. Animal cells have just one microtubule organizing center (MOC) called the centrosome. Centrosomes contain two bundles of microtubules called centrioles.

44. Microtubule Function Microtubules provide stability and are involved in movement; they may also provide a structural framework for organelles. Microtubules can act as “railroad tracks”; transport vesicles move through the cell along these microtubule tracks in an energy-dependent process. Microtubules require ATP and kinesin for vesicle transport to occur. Kinesin is a motor protein that converts chemical energy in ATP into mechanical work.

47. Cilia and Flagella: Moving the Entire Cell Flagella are long, hair-like projections from the cell surface that move cells. Bacterial flagella are made of flagellin and rotate like a propeller. Eukaryotic flagella are made of microtubules and wave back and forth. Closely related to eukaryotic flagella are cilia, which are short, filament-like projections. Cells generally have just one or two flagella but may have many cilia.

48. Cilia and Flagella Structure The axoneme of cilia and flagella is a complex “9 + 2” arrangement of microtubules connected by links and spokes. The axoneme attaches to the cell at a structure called the basal body.

49. A Motor Protein in the Axoneme The motor protein dynein forms the arms between doublets and changes shape when ATP is hydrolyzed to “walk” up the microtubule. When the dynein arms on just one side of the axoneme move, cilia and flagella bend instead of elongating because the links and bridges constrain movement of the microtubule doublets.