Lecture 5- A Tour of the Cell 1
In this lecture Prokaryotes vs. eukaryotes The organelles of the eukaryotic cell The cytoskeleton Extracellular components 2
What are cells? Cells are the fundamental unit of life Basic features of all cells Plasma membrane Semifluid substance called cytosol Contain DNA (carry genes) Ribosomes (make proteins) 3
Eukaryotic Two types of cells Plants, animals, fungi, protists Prokaryotic Bacteria, archaea 4
Prokaryotes Prokaryotic cells have: No nucleus DNA in an unbound region called the nucleoid No membrane-bound organelles Cytoplasm bound by the plasma membrane 5
Eukaryotes Eukaryotic cells have: DNA in a nucleus that is bounded by a membranous nuclear envelope Membrane-bound organelles Cytoplasm in the region between the plasma membrane and nucleus Eukaryotic cells are generally much larger than prokaryotic cells 6
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The organelles of the eukaryotic cell (Plasma membrane) Nucleus Ribosome Mitochondria Endomembrane System Endoplasmic reticulum Golgi apparatus Lysosomes Vacuoles Plasma membrane Nuclear membrane Cytoskeleton Extracellular Matrix 8
The plasma membrane The Immigration and Customs Enforcement of the cell A selective barrier that allows passage of oxygen, nutrients, and waste Double layer of phospholipids Present in both eukaryotic and prokaryotic cells 9
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Upper limit of cell size Metabolic requirements set upper limits on the size of cells The surface area to volume ratio of a cell is critical As the surface area increases by a factor of n 2, the volume increases by a factor of n 3 Small cells have a greater surface area relative to volume 11
Figure 6.7 Surface area increases while total volume remains constant 1 5 1 Total surface area [sum of the surface areas (height width) of all box sides number of boxes] 6 150 750 Total volume [height width length number of boxes] 1 125 125 Surface-to-volume (S-to-V) ratio [surface area volume] 6 1.2 6 12
The nucleus The library of the cell The nucleus contains most of the DNA in a eukaryotic cell Mitochondria/chloroplasts contain the rest DNA is wound up into coils called chromatin The nucleolus is where ribosomes are manufactured The nuclear envelope encloses the nucleus, separating it from the cytoplasm Pores within the envelope regulate the entry and exit of molecules from the nucleus The nuclear membrane is a double membrane; each membrane consists of a lipid bilayer 13
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1 m 0.25 m 1 m Figure 6.9 Nucleus Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Rough ER Surface of nuclear envelope Ribosome Pore complex Close-up of nuclear envelope Chromatin Pore complexes (TEM) Nuclear lamina (TEM) 15
Structurally identical Protein factories Ribosomes Made of ribosomal RNA (rrna) and proteins Two subunits: large and small Ribosomes carry out protein synthesis in two locations In the cytoplasm (as free ribosomes) On the outside of the endoplasmic reticulum or the nuclear envelope (as bound ribosomes) 16
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Mitochondria Powerhouse of the cells Break down incoming sugars to produce ATP using oxygen Enclosed by a double lipid bilayer, like the nucleus Contain fragments of their own DNA Found in all eukaryotic cells 18
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10 m Mitochondria Mitochondrial DNA Nuclear DNA (b) Network of mitochondria in a protist cell (LM) Dyamic Mitochondrial Networks in Cancer 20
Powerhouse of plant cells Chloroplasts Plant cells contain BOTH chloroplasts and mitochondria Found in plant and algae cells Build up glucose through photosynthesis Also contain fragments of DNA Chloroplast structure includes Thylakoids, membranous sacs, stacked to form a granum Stroma, the internal fluid The chloroplast is one of a group of plant organelles, called plastids 21
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Evolutionary origins of mitochondria and chloroplasts Mitochondria probably came from an ancient bacteria that was swallowed by our ancestor cells Why there is a double lipid bilayer the original plus our own Called the endosymbiont theory Chloroplasts came about the same way at a later date 23
Figure 6.16 Engulfing of oxygenusing nonphotosynthetic prokaryote, which becomes a mitochondrion Mitochondrion Endoplasmic reticulum Nuclear envelope Nucleus Ancestor of eukaryotic cells (host cell) Nonphotosynthetic eukaryote At least one cell Engulfing of photosynthetic prokaryote Chloroplast Mitochondrion Photosynthetic eukaryote 24
The Endomembrane System The Government employees Responsible for protein trafficking and some metabolic functions Includes Nuclear envelope Endoplasmic reticulum Golgi apparatus Lysosomes/perixisomes Vacuoles Plasma membrane 25
Endoplasmic reticulum The factory The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells The ER membrane is continuous with the nuclear envelope There are two distinct regions of ER Smooth ER, which lacks ribosomes Rough ER, surface is studded with ribosomes 26
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Smooth vs. Rough ER Smooth ER: Synthesizes lipids Metabolizes carbohydrates Detoxifies drugs and poisons Stores calcium ions Rough ER: Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates) Distributes transport vesicles, proteins surrounded by membranes Is a membrane factory for the cell 28
In what kinds of cells would you expect to see lots of mitochondria? An extensive smooth ER? 29
The Golgi Apparatus The post office of the cell Consists of flattened membranous sacs called cisternae (vs. cistae in mitochondria) Functions of the Golgi apparatus Modifies products of the ER Manufactures certain macromolecules Sorts and packages materials into transport vesicles Tags proteins for certain destinations 30
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Lysosymes The garbage trucks of the cell A membranous sac of hydrolytic enzymes that can digest macromolecules Hydrolytic = breaking down through consuming water Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids Lysosomal enzymes work best in the acidic environment inside the lysosome 32
Some types of cell can engulf another cell by phagocytosis This forms a food vacuole A lysosome fuses with the food vacuole and digests the molecules Lysosomes also use enzymes to recycle the cell s own organelles and macromolecules, a process called autophagy A human liver cell recycles half of its macromolecules per week through its lysosomes 33
Figure 6.13 Nucleus 1 m Vesicle containing two damaged organelles 1 m Mitochondrion fragment Lysosome Peroxisome fragment Digestive enzymes Lysosome Lysosome Plasma membrane Digestion Peroxisome Food vacuole Vesicle Mitochondrion Digestion (a) Phagocytosis (b) Autophagy 34
Also garbage men Peroxisomes Peroxisomes are specialized metabolic compartments bounded by a single membrane Produce hydrogen peroxide and convert it to water Perform reactions with many different functions How peroxisomes are related to other organelles is still unknown 35
Figure 6.19 1 m Chloroplast Peroxisome Mitochondrion 36
Vacuoles The Public Storage of the cell Storage compartments for food, enzymes, and water 37
How the endomembrane system all fits together 38
The Cytoskeleton The infrastructure, roads, freeways of the cell The cytoskeleton is a network of fibers extending throughout the cytoplasm It organizes the cell s structures and activities, anchoring many organelles It is composed of three types of molecular structures Microtubules Microfilaments Intermediate filaments 39
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Table 6.1 10 m 10 m 5 m Column of tubulin dimers 25 nm Actin subunit Keratin proteins Fibrous subunit (keratins coiled together) Tubulin dimer 7 nm 8 12 nm 41
Microtubules Microtubules are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long Compression-resistant Functions of microtubules Shaping the cell Guiding movement of organelles Separating chromosomes during cell division In animal cells, grow out from a centrosome The centrosome is a microtubule-organizing center Form cilia and flagella 42
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Microfilaments Solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits Bear tension, resisting pulling forces within the cell Form a 3-D network called the cortex just inside the plasma membrane to help support the cell s shape Actin protein structure 45
Figure 6.27a Muscle cell Actin filament Myosin filament Myosin head (a) Myosin motors in muscle cell contraction 0.5 m 46
Figure 6.27b Pseudopodia (cellular feet ) extend and contract through the contraction of actin in microfilaments Cortex (outer cytoplasm): gel with actin network Inner cytoplasm: sol with actin subunits 100 m (b) Amoeboid movement Extending pseudopodium BIOL 211 Winter 2012 47
Intermediate filaments Range in diameter from 8 12 nanometers, larger than microfilaments but smaller than microtubules Support cell shape and fix organelles in place More permanent cytoskeleton fixtures than the other two classes Outer layers of skin are composed of keratin, which remains behind when the cell dies 48
This image is on the website of hair salon and is advertised as in a keratin hair treatment. What s wrong with this picture? 49
Motor proteins The public transport of the cell Motor proteins called dyenins and kinesins walk along microtubules by expending ATP They can be attached to lipid bubbles full of molecules, transporting these packages from one side of the cell to the other 50
Figure 6.21 ATP Vesicle Receptor for motor protein (a) Motor protein (ATP powered) Microtubule of cytoskeleton Microtubule Vesicles 0.25 m (b) 51
Extracellular Structures Most cells synthesize and secrete materials that are external to the plasma membrane These extracellular structures include Cell walls of plants The extracellular matrix (ECM) of animal cells Intercellular junctions 52
Plant cell walls Most plants, fungi, and some protists have cell walls Made of cellulose fibers embedded in other polysaccharides and protein May have multiple layers Primary cell wall: relatively thin and flexible Middle lamella: thin layer between primary walls of adjacent cells Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall Plasmodesmata are channels between adjacent plant cells 53
Figure 6.28 Secondary cell wall Primary cell wall Middle lamella 1 m Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata 54
Animals Extracellular Matrix Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM) The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin ECM proteins bind to receptor proteins in the plasma membrane called integrins 55
Figure 6.30 Collagen EXTRACELLULAR FLUID Polysaccharide molecule Proteoglycan complex Carbohydrates Fibronectin Core protein Integrins Plasma membrane Proteoglycan molecule Proteoglycan complex Microfilaments CYTOPLASM 56
Glycoproteins and proteoglycans Glycoprotein a (large) protein covalently bonded to a short polysaccharide Collagen is a glycoprotein, and accounts for about 40% of the proteins in the human body Fibronectin Proteoglycan a small protein covalently bonded to a long polysaccharide 57
Cell Junctions Neighboring cells in tissues, organs, or organ systems often stick, interact, and talk through direct physical contact Intercellular junctions facilitate this contact There are several types of intercellular junctions Plasmodesmata Tight junctions Desmosomes Gap junctions 58
More cell junctions At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid Desmosomes (anchoring junctions) fasten cells together into strong sheets Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells 59
TEM Figure 6.32 Tight junctions prevent fluid from moving across a layer of cells Tight junction TEM 0.5 m Tight junction Intermediate filaments Desmosome Gap junction TEM 1 m Ions or small molecules Plasma membranes of adjacent cells Space between cells Extracellular matrix 0.1 m 60
A macrophage is a type of cell that hunts and eats foreign bacteria through phagocytosis. Describe how each of the following organelles plays a role in its movement and eating. Cytoskeleton Lysosome Vacuole Plasma membrane 61
Vocabulary Eukaryotic, prokaryotic cell Plasma membrane Cell wall Nucleus Nuclear envelope Nucleolus Nuclear pore Chromatin Ribosome Mitochondria Mitochondrial matrix Endoplasmic reticulum Golgi apparatus Cytoskeleton Lysosomes/Peroxisomes Phagocytosis Vacuoles Cytoplasm Microtubules, intermediate filaments, Motor proteins Actin, myosin Centrosome Glycoprotein, proteoglycan Cell junctions Gap junction Tight junctions Plasmodesmata Desmosomes BIOL 211 Winter 2012 62
Questions? 63