Prokaryotes and Eukaryotes

Chapter 1 : Cellular Biology Introduction to Pathophysiology Prokaryotes and Eukaryotes ● Living cells are divided into eukaryotes and prokaryotes ● Eukaryotes include cells of higher animals, plants, fungi, protozoa, and most algae ● Prokaryotes include cyanobacteria, bacteria, and rickettsiae ● Eukaryotic cells are larger and have more extensive intracellular anatomy and organization than prokaryotic cells ● Eukaryotes have membrane-bound organelles and a well-defined nucleus, while prokaryotes lack organelles and a nuclear membrane Cellular Functions ● Cells become specialized through differentiation ● Eight chief cellular functions are: movement, conductivity, metabolic absorption, secretion, excretion, respiration, reproduction ● Muscle cells generate motion, nerve cells conduct electrical signals, all cells can absorb and use nutrients, certain cells synthesize and secrete substances, all cells can excrete waste products, cells absorb oxygen for respiration, tissue growth occurs through reproduction Communication in Cells ● Communication is vital for cells to survive as a society of cells ● Appropriate communication allows the maintenance of a dynamic steady state Components of a Eukaryotic Cell ● Eukaryotic cells consist of three components: the plasma membrane, cytoplasm, and intracellular organelles ● The nucleus is the largest membrane-bound organelle and is surrounded by the cytoplasm ● The nuclear envelope, composed of two membranes, contains nuclear pores for the exchange of chemical messages The Nucleus ● The nucleus contains the nucleolus, cellular DNA, and DNA-binding proteins (histones) ● Histones help DNA fold into chromosomes, decreasing the risk of breakage and essential for cell division ● The nucleus is involved in cell division, control of genetic information, DNA replication and repair, and RNA transcription Cytoplasmic Organelles ● Cytoplasm is an aqueous solution called cytosol that fills the cytoplasmic matrix ● There are various organelles in the cytoplasm, including mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus, and vesicles Cytosol ● The cytosol represents about half the volume of a eukaryotic cell ● It contains thousands of enzymes involved in intermediate metabolism ● It is crowded with ribosomes making proteins ● Newly synthesized proteins remain in the cytosol if they lack a signal for transport to a cell organelle Organelles in the cytoplasm ● Organelles in the cytoplasm are enclosed in biologic membranes ● They can carry out functions requiring different biochemical environments simultaneously ● Functions include protein and hormone synthesis, transport out of the cell, waste elimination, metabolic processes, breakdown and disposal of cellular debris and foreign proteins, maintenance of cellular structure and motility ● The cytosol is a storage unit for fat, carbohydrates, and secretory vesicles Principal cytoplasmic organelles ● Ribosomes: RNA-protein complexes synthesized in the nucleolus, provide sites for cellular protein synthesis ● Endoplasmic reticulum: network of tubular channels for synthesis, folding, and transport of protein and lipid components of most organelles ● Golgi: network of smooth membranes and vesicles responsible for processing and packaging proteins onto secretory vesicles ● Lysosomes: sac-like structures containing enzymes for digestion of cellular substances ● Peroxisomes: similar to lysosomes, containing oxidative enzymes for detoxification ● Mitochondria: contain metabolic machinery for cellular energy metabolism ● Cell membrane: outer boundary of the cell, controls the passage of substances in and out of the cell ● Cytoskeleton: network of protein filaments, including microtubules, actin filaments, and intermediate filaments, provides structural support and forms cell extensions Functions of Plasma Membranes ● Plasma membranes control the composition of the space they enclose ● They allow or exclude various molecules through selective transport systems ● Membranes exert a powerful influence on metabolic pathways ● Cell polarity, the direction of cellular transport, maintains normal cell and tissue structure ● The plasma membrane also plays a role in cell-to-cell recognition ● Other functions include cellular mobility and maintenance of cellular shape Membrane Composition ● The basic structure of cell membranes is the lipid bilayer consisting of two apposing leaflets and proteins ● Lipid bilayers are not uniformly distributed and may separate into microdomains ● Cell membranes are composed of lipids, proteins, and carbohydrates Phases of Membrane Organization ● Membranes have different physical states called phases, including solid gel, fluid liquid-crystalline, and liquid ordered ● These phases can change under physiologic factors such as temperature and pressure fluctuations