Cell Structure Animal/Human cell is basic unit of all life; structural and functional if its alive, must contain at least a single cell the function of an organism is the summation of functions of its individual cells cell first described 300 years ago by Robert Hook in human body single fertilized egg 75 100 trillion cells (adult) [>10x s more bacterial cells in body] range in size from: ostrich egg is single cell human egg barely visible to naked eye 3 ft long (humans) 3 meters long (giraffe leg nerve cell) diversity in size and shape less diversity in function Structure Basic Components of Cell: a. cell membrane - boundary b. cytoplasm includes everything except cell membrane c. nucleus -genetic material d. organelles and internal structures - highly organized, specialized structures e. cell inclusions -misc. crystals and globules 1. Cell Membrane defines boundary highly ordered phospholipids, proteins, cholesterol phospholipids have polar and nonpolar ends tend to arrange in two layered films framework for all membranes of cells Microscopy, Cells, Tissues: Cell Structure Animals/Humans, Ziser, 2003 1
proteins float in this bilayer fluid mosaic (1972 Singer & Nicholson) proteins randomly arranged in lipid bilayer not fixed peripheral proteins and integral proteins protein functions: receptors (ie. docking sites) for specific substances carriers: move specific solutes across membrane channel proteins: have pore, act as pumps and gates to selectively move specific substances through c.m. enzymes: energy transforming enzymes anchoring: bound to cytoskeleton recognition: eg many glycoproteins selectively permeable some things enter freely acts as hydrophobic barrier structurally and functionally interconnected to many of the cells organelles including the nucleus =endomembrane system 2. Cytoplasm Endomembrane system: plasma membrane nucleus endoplasmic reticulum golgi bodies lysosome microbodies contains mostly water, ions, organic molecules etc also colloids - too large to completely dissolve too small to settle out sol-gel reversal proteins most water in cell is organized by colloids, hydrohilic/phobic molecules probably little free water inside cells 3. Nucleus Microscopy, Cells, Tissues: Cell Structure Animals/Humans, Ziser, 2003 2
largest of internal cell structures (except in plants) number varies: one to many membrane bound surrounded by nuclear envelope same structure and cell membrane but with larger perforations in membrane inside is nucleoplasm contains genetic material: replication and reproduction daily metabolism genetic material =chromosomes 100 s times length of cell humans ~3 of DNA/cell = DNA with chromatin =highly organized histones 4. Nucleolus densely packed chromosome region within nucleus with proteins and ribosome precursors site of RNA synthesis important in formation of ribosomes Organelles = membrane bound compartments compartmentalization increases efficiency of metabolism 5. Endoplasmic Reticulum membranous tubes, sacs and flattened channels two types: rough ER - with ribosomes attached smooth ER - without ribosomes most cells contain both types (in different proportions) Microscopy, Cells, Tissues: Cell Structure Animals/Humans, Ziser, 2003 3
Rough ER: protein synthesis and transport especially in protein exporting cells esp liver and pancreas Smooth ER: narrow pipes transports lipids & proteins lipid metabolism cholesterol metabolism fat synthesis steroid synthesis breaks down glycogen and fats most common in lipid synthesizing cells esp liver cells adrenal cortex (steroid hormones) testes steroid hormone synthesis plant seeds ER highly modified in skeletal muscle cells =sarcoplasmic reticulum Ca ++ storage 6. Golgi Bodies, Golgi Apparatus, (Desmosomes) usually near nucleus stack of membranes (flattened sacs) functions: synthesize large carbohydrates and combine them with proteins brought by ER to form glycoproteins secretions of glycoproteins makes membrane components makes lysosomes proteins exported from ER move to GB s become modified then enclosed in vessels pinched off and move to surface released by exocytosis few to 25,000/cell highest in secretory cells Microscopy, Cells, Tissues: Cell Structure Animals/Humans, Ziser, 2003 4
7. Lysosomes: also membrane bound size and shape changes used for digestion and disposal: used to destroy invading pathogens; eg WBC s, neuroglial cells gets rid of worn out parts = suicide bags contains up to 40 different enzymes capable of breaking almost every kind of large organic molecule 8. Peroxisomes: contains oxidases Functions: digests fatty acids and amino acids detoxifies poisons (free radicals) eg used to degrade alcohol 9. Mitochondria: about size of small bacterial cell walled by two membranes inner membrane forms invaginations = cristae power plants of cells: contain enzymes for respiration and ATP synthesis produces most of cells ATP (=energy) most eucaryotic cells thrive in O2 use oxygen gas in energy releasing reactions the oxygen dependent energy releasing reactions = aerobic respiration occur in mitochondria typical cell has dozens to 100,000 s Microscopy, Cells, Tissues: Cell Structure Animals/Humans, Ziser, 2003 5
10. Ribosomes number related to cells activity eg: muscle cells (?) 100,000 s/cell liver cells 1000 s/cell sperm cells ~25/cell animal cells have more than plant cells protein factories sites of protein synthesis (translation) some attached to ER -make proteins for export some free in cytoplasm - make proteins for cells own use up to 1000 s per cell ># ribosomes = > amount of protein synthesis often structurally and functionally grouped together = polyribosomes ~2/3 rd RNA and 1/3 rd protein protein synthesis occurs in ribosomes but some synthesis, modification and transport of these proteins occurs in endomembrane system (esp. ER) 11. Cytoskeleton microfilaments and microtubules nonrigid, contractile Functions: cell movement phagocytosis holds organelles in place 12. Centrioles (Centrosome) each cell has a pair at right angles to each other Microscopy, Cells, Tissues: Cell Structure Animals/Humans, Ziser, 2003 6
made of microtubules 9 bundles of 3 tubules functions in cell division: mitosis & meiosis helps form spindle 13. Inclusions everything else storage crystals oil globules 14. Cell Surface Structures (Specializations of Plasma Membrane): a. microvilli greatly increase surface area for absorption (intestine) or reabsorption (kidney) 1000 s/cell in intestine b. membrane junctions: c. cilia & flagella tight junctions proteins of two different cell membranes fuse together form impermeable junction encircling cell eg: keep digestive enzymes in intestine from leaking into blood desmosomes rivet-like couplings of linked proteins guy wires throughout sheet of cells prevents sheets of cells from tearing eg. skin, neck of uterus gap junctions allows direct passage of materials between cells eg. intercalated discs in cardiac muscle cells motility Microscopy, Cells, Tissues: Cell Structure Animals/Humans, Ziser, 2003 7
made from microtubules (9+2) cilia line respiratory system flagella only in sperm cells d. membrane surface receptors diverse molecular sites at which cells chemically recognize and bind extracellular substances proteins, glycoproteins, lipoproteins binding often causes metabolic changes => activates cell to do something also important in cell-to-cell identity eg. surface markers defense against foreign cells and tissues Microscopy, Cells, Tissues: Cell Structure Animals/Humans, Ziser, 2003 8