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Transcript
Cell Theory • All organisms are composed of one or more cells • Cells are the smallest living things • Cells arise only by division of a previously existing cell 4.1 Cells Fig. 4.1 The size of cells and their contents 20 mm 2 mm 0.2 mm 20 µm 2 µm 0.2 µm 20 nm 2 nm 0.2 nm Natural laws limit cell size • At minimum, a cell must be large enough to house the parts it needs to survive and reproduce • The maximum size of a cell is limited by the amount of surface needed to obtain nutrients from the environment and dispose of wastes • A small cell has a greater ratio of surface area to volume than a large cell of the same shape 30 µm Figure 4.3 Surface area of one large cube = 5,400 µm2 10 µm Total surface area of 27 small cubes = 16,200 µm2 4.3 Prokaryotic Cells • There are two major kinds of cells – Prokaryotes – Eukaryotes Fig. 4.9 Rod • Prokaryotes include bacteria and archaea – Over 5,000 species are recognized • Prokaryotes come in three main shapes Spherical Spiral • Prokaryotes have a very simple architecture – They lack a nucleus and organelles – But they make all their own vitaminsand us complicated humans can’t!! Fig. 4.8 Pilus Found in all prokaryotes 4.4 Eukaryotic Cells • Appeared about 1.5 billion years ago • Include all cells alive today except bacteria and archaea • Are larger than prokaryotic cells • Can be single cells (yeast) or multicellular • Have a much more complex architecture – Possess nucleus and a variety of organelles Fig. 4.10 Structure of a plant cell Fig. 4.11 Structure of an animal cell 4.2 The Plasma Membrane • Surrounds all living cells • Made of phospholipids + proteins • Membranes are selectively permeable -They control the flow of substances into and out of a cell • Membranes organize the chemical reactions making up metabolism Membranes contain phospholipids • Phospholipids each have a hydrophilic head and two hydrophobic tails Head Symbol Tails Fig. 4.4 Membrane phospholipids form a bilayer • In water, phospholipids form a stable bilayer – The heads face outward and the tails face inward Hydrophilic heads Water Hydrophobic tails Fig. 4.5 Water Fig. 4.6 Proteins are embedded within the lipid bilayer •The fluid-mosaic model –Phospholipid bilayer with embedded proteins Proteins Within the Membrane • Two main types – Cell-surface proteins • Project from the surface of the membrane • Act as markers or receptors – Transmembrane proteins • Extend all the way across the bilayer • Provide channels in and out of the cell • The plasma (outside) membrane controls the cell’s contact with the environment • The cytoplasm contains organelles most with membranes as boundaries -This allows the cell to carry out a variety of activities simultaneously 4.5 The Nucleus: The Cell’s Control Center • Usually the largest organelle • The nucleus is separated from the cytoplasm by the nuclear envelope • The nucleus contains the DNA – – – – – It directs all of the cell’s activities The DNA is associated with proteins During cell division, it condenses into chromosomes Messenger RNA (mRNA) is made in the nucleus Rbosomal RNA (rRNA) is made in the nucleolus Passage for RNA and proteins Site of assembly of ribosome subunits Fig. 4.12 The nucleus Free ribosomes • Ribosomes are made of ribosomal RNA (rRNA) and protein and are part of the machinery that make proteins • Free ribosomes make proteins in the cytosol – Enzymes that work in the cytosol, – Stuctural proteins like microtubules – Nuclear proteins that transcription factors, histones, ribosomal proteins – Mitochondrial proteins 4.6 The Endomembrane System • More than half of the membranes in a eukaryote are within the cell (endo) • The Endomembrane System is a collection of organelles surrounded by membranes – – – – Endoplasmic reticulum Golgi complex Lysosomes Peroxisomes • These organelles divide the cell into compartments, manufacture and distribute cell products Endoplasmic Reticulum (ER) • Consists of two distinct regions – Rough ER • Studded with ribosomes • Involved in protein synthesis • Also makes membranes – Smooth ER • no ribosomes • Embedded with enzymes • Involved in lipid and carbohydrate synthesis Rough ER makes proteins and membrane Fig. 4.13 The endoplasmic reticulum Rough ER makes proteins and membrane • When ribosomes encounter an ER signal sequence on the mRNA they bind the surface of the ER • Makes and modifies proteins that are inserted in membranes • Makes and modifies proteins that are secreted (i.e. insulin) -most secreted proteins are glycoproteins, covalently bonded to carbohydrates • Transport vesicles bud off Transport vesicle buds off 4 Ribosome Sugar chain 1 Figure 4.8 Polypeptide 3 Secretory (glyco-) protein inside transport vesicle Glycoprotein 2 ROUGH ER Smooth endoplasmic reticulum has a variety of functions • Smooth ER no ribosomes on surface • Smooth ER makes lipids (fatty acids, phospholipids and steroids) • In liver cells, it regulates carbohydrate metabolism and breaks down toxins and drugs • In muscle cells it stores calcium needed for muscle contraction The Golgi Complex – The Golgi receives transport vesicles from the ER – Finishes, sorts, and then ships cell products to other organelles OR to the plasma membrane for secretion Fig. 4.14 • Golgi bodies are flattened stack of membranes that are scattered throughout the cytoplasm – These are collectively referred to as the Golgi complex Lysosomes • Bud off from the Golgi complex • They contain enzymes that break down macromolecules • Function in intracellular digestion of – Worn-out cellular components – Substances taken into cells – The digested material is then recycled Peroxisomes •Bud off from the ER –In animals peroxisomes have enzymes that detoxifies various harmful molecules Vacuoles • In plants – Store dissolved substances – Can increase the cell’s surface area • In protists – Contractile vacuoles are used to pump excess water Fig. 4.23 Fig. 4.15 The various organelles of the endomembrane system are interconnected structurally and functionally Path of Protein Production