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General Biology (Bio107) Chapter 4 – Cells Cells are the smallest living units of life • Two types of cells exist on earth: 1. Prokaryotic cells 2. Eukaryotic cells • All cells are surrounded by a phospholipid bilayermade barrier called the plasma membrane. • The semifluid substance within the membrane is the cytosol, containing the organelles. • All cells contain chromosomes which have genes in the form of DNA. • All cells also have ribosomes, tiny organelles that make proteins using the instructions contained in genes. • Besides showing a difference in size (prokaryotic cells are small), a major difference between prokaryotic and eukaryotic cells is the location of chromosomes. • In eukaryotic cells, chromosomes are contained in a membraneenclosed organelle, the nucleus. • In much smaller prokaryotic cells, the DNA is concentrated in the nucleoid without a membrane separating it from the rest of the cell. The prokaryotic cell is much simpler in structure, lacking a nucleus and the other membrane-enclosed organelles of the eukaryotic cell. • Eukaryotic cells are generally much bigger than prokaryotic cells. • The logistics of carrying out metabolism set limits on cell size. – At the lower limit, the smallest bacteria, mycoplasmas, are between 0.1 to 1.0 micron. – Most bacteria are 1-10 microns in diameter. – Eukaryotic cells are typically 10-100 microns in diameter. • Metabolic requirements also set an upper limit to the size of a single cell. • As a cell increases in size its volume increases faster than its surface area. – Smaller objects have a greater ratio of surface area to volume. Fig. 7.5 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • The plasma membrane functions as a selective barrier that allows passage of oxygen, nutrients, and wastes for the whole volume of the cell. Fig. 7.6 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • The volume of cytoplasm determines the need for this exchange. • Rates of chemical exchange may be inadequate to maintain a cell with a very large cytoplasm. • The need for a surface sufficiently large to accommodate the volume explains the microscopic size of most cells. • Larger organisms do not generally have larger cells than smaller organisms - simply more cells. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Eukaryotic cell • A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell into compartments and membraneous organelles. • These membranes also participate in metabolism as many enzymes are built into membranes. • The barriers created by membranes provide different local environments that facilitate specific metabolic functions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Different membranous organelles can be found in eukaryotic cells: 1. Rough endoplasmic reticulum (rER) 2. Smooth endoplasmic reticulum 3. Golgi apparatus 4. Mitochondrion 5. Lysosomes 6. Peroxisomes • Each type of membranous organelle has a unique combination of lipids, proteins and enzymes for its specific functions. – For example, those in the membranes of mitochondria function in cellular respiration. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 7.7 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Cells of eukaryotic photosynthesizing life forms, e.g. algae and plants, contain unique sets of organelles, most namely: 1. Tonoplast (central vacuoles) and 2. Chloroplasts • Their plasma membrane is further surrounded by a thick, protective cell wall. Fig. 7.8 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Plasma membrane • Phospholipid bilayer-made barrier between inside and outside of cell; also contains cholesterol • Semi-permeable; only water, gases and lipophilic molecules can freely cross • Controls entry of materials with the help of selective transport proteins, e.g. carrier, porins, channels • Receives chemical and mechanical signals with the help of receptor proteins • Transmits signals between intra- and extracellular spaces Facilitated Diffusion Active Transport Solutes are transported across plasma membranes with the use of ATP-derived energy; capable to transport from an area of lower concentration to an area of higher concentration Example: Sodium-potassium pump Na+ gradient Extracellular fluid Na+/K+ ATPase Cytosol K+ gradient Cytosol K+ gradient 3 Na+ expelled 2K+ 3 Na+ 1 P 3 Na+ 1 ATP 2 ADP 3 P + 4 2K imported 17 Nucleus • The nucleus contains chromosomal DNA and most of the genes in a eukaryotic cell. – The nucleus of each human cell contains 46 chromosomes. – Some genes are located in mitochondrial and chloroplast DNA. • The nucleus averages about 5 microns in diameter. • The nucleus is separated from the cytoplasm by a double membrane. – These are separated by 20-40 nm. • Where the double membranes are fused, a nuclear pore allows gene-regulating proteins, large macromolecules and particles to pass through. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • The nuclear side of the envelope is lined by the nuclear lamina, a network of intermediate filaments that maintain the shape of the nucleus. Fig. 7.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Within the nucleus, the DNA and associated proteins (histones) are organized into fibrous material, called chromatin. • In a normal cell they appear as diffuse mass. • However when the cell prepares to divide, the chromatin fibers coil up to be seen as separate structures, chromosomes. • Each eukaryotic species has a characteristic number of chromosomes. – A typical human cell has 46 chromosomes, but sex cells (eggs and sperm) have only 23 chromosomes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the nucleolus. – There, ribosomal RNA (rRNA) is synthesized and assembled with proteins from the cytoplasm to form ribosomal subunits. – The subunits pass from the nuclear pores to the cytoplasm where they combine to form ribosomes. • The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA). – mRNA travels to the cytoplasm and combines with ribosomes to translate its genetic message into the primary structure of a specific polypeptide. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Ribosomes • Ribosomes which contain rRNA and protein are responsible for synthesis of new proteins in cells. • A ribosome is composed of two subunits that combine to carry out protein synthesis. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Cell types that synthesize large quantities of proteins (e.g., pancreas) have large numbers of ribosomes and prominent nuclei. • Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol. • Other ribosomes, bound ribosomes, are attached to the outside of the rough endoplasmic reticulum. – These synthesize proteins that are either included into membranes or for export from the cell. • Ribosomes can shift between roles depending on the polypeptides they are synthesizing. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Endoplasmic Reticulum (ER) • Structure: network of folded membranes • Functions: synthesis, intracellular transport • Types of E.R. – Rough E.R.: studded with ribosomes (sites of protein synthesis & protein folding) – Smooth E.R. lacks ribosomes. Functions: • • • • lipid synthesis release of glucose in liver cells into bloodstream drug detoxification (especially in liver cells) storage and release of Ca2+ in muscle cells (where smooth E.R. is known as sarcoplasmic reticulum or SR) Endoplasmic Reticulum (E.R.) Copyright 2010, John Wiley & Sons, Inc. Golgi Complex • Structure: – Flattened membranes (cisterns) with bulging edges (like stacks of pita bread) • Functions: – Receive protein from rER, modify and sort proteins glycoproteins and lipoproteins that: • Become parts of plasma membranes • Are stored in lysosomes, or • Are exported by exocytosis Golgi Complex Copyright 2010, John Wiley & Sons, Inc. Small cell organelles • Lysosomes: contain digestive enzymes – Help in final processes of digestion within cells – Carry out autophagy (destruction of worn out parts of cell) and death of old cells (autolysis) – Important for phagocytotic cells (e.g. macrophages) – Tay-Sachs: hereditary disorder; one missing lysosomal enzyme leads to nerve destruction • Peroxisomes: special forms of metabolism, detoxify; abundant in liver; produce hydrogen peroxide • Proteasomes: digest unneeded or faulty proteins – Faulty proteins accumulate in brain cells in persons with Parkinson or Alzheimer disease. Lysosomes • Membrane-bounded sacs which contain many hydrolytic enzymes that recycle macromolecules, e.g. DNA, lipids, proteins, back into their monomers. • Lysosomal enzymes work best at acidic pH of 5. – Proteins in the lysosomal membrane pump hydrogen ions from the cytosol to the lumen of the lysosomes. • While rupturing one or a few lysosomes has little impact on a cell, but massive leakage from lysosomes can destroy an cell by autodigestion. • Lysosomes fuse with phagosomes which space allows the cell to digest macromolecules safely. • Lysosomal enzymes and membranes are synthesized by rough ER and then transferred to the Golgi where they bud off. • Lysosomes play crucial role in following cell processes: 1. Food digestion 2. Autophagy (“organelle recycling”) 3. Phagocytosis + bacterial kill • Lysosomes also play a critical role in the programmed destruction of cells (“apoptosis”) in multicellular organisms. – This process allows reconstruction during the developmental process. • Several inherited diseases affect lysosomal metabolism (“lysosomal storage disorders”). – These individuals lack a functioning version of a normal hydrolytic enzyme. – Lysosomes are engorged with indigestable substrates. – These diseases include Pompe’s disease in the liver and Tay-Sachs disease in the brain. Mitochondria • Structure: – Sausage-shaped with many folded membranes (cristae) and liquid matrix containing enzymes – Have some DNA, ribosomes (can make proteins) • Function: – Nutrient energy is released and trapped in ATP; so known as “power houses of cell” – Chemical reactions require oxygen • Abundant in muscle, liver, and kidney cells – These cells require much ATP Mitochondria Chloroplast • Structure: – Oval-shaped with many stacked phospholipid sacs (= thylacoids) and liquid stroma containing enzymes – Have large ring-formed DNA, ribosomes (can make proteins) • Function: – Convert solar energy into ATP and nutrient energy (glucose) by a process called “photosynthesis” – Chemical reactions require carbon dioxide • Abundant in cells of green algae and plants Chloroplast Inner Chloroplast Membrane Thylacoid Stroma Centrosome • Structure: – Two centrioles arranged perpendicular to each other • Composed of microtubules: 9 clusters of 3 (triplets) – Pericentriolar material • Composed of tubulin that grows the mitotic spindle • Function: important for microtubule formation and assembly; important for movement of chromosomes to ends of cell during cell division (mitosis) and for vesicular transport, e.g. in neurons Centrosome Copyright 2010, John Wiley & Sons, Inc. Cytoskeleton • Maintains shape of cell • Positions organelles • Changes cell shape • Includes: microfilments, intermediate filaments, microtubules Copyright 2010, John Wiley & Sons, Inc. Microtubules • Build 9+2 protein core of cilia and flagella. • Associated with ATP-consuming motor proteins. • Important for movement of: 1. Cilia 2. Flagella (sperm) 3. Vesicles (axons) 4. Chromosomes (mitosis & meiosis) • Build from polymerized tubulin monomers. • Monomer: Tubulin • Microtubules play a major role in cell motility. – This involves limited movements of parts of the cell. • The microtubules interacts with ATP-consuming motor proteins, e.g. dynein. – In cilia and flagella motor proteins pull components of the cytoskeleton past each other. – This is also true in muscle cells. • A flagellum of a sperm has an undulatory movement. • Cilia move more like oars with alternating power and recovery strokes. – They generate force perpendicular to the cilia’s axis. Microfilaments • Build protein core of microvilli of epithelial cells. • Builds cortical network underneath cell membrane. • Important for pseudopodia formation and movement of cells, e.g. white blood cells. • Build from polymerized protein monomers. • Monomer: Actin Intermediate Filaments • Are intermediate in size with 8 - 12 nm diameter. • They are specialized for bearing tension. – Intermediate filaments are built from a diverse class of subunits from a family of proteins called keratins. • Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes. • They reinforce cell shape and also fix organelle location.