A TOUR OF THE CELL Chapter 6 KEY CONCEPTS: Eukaryotic cells have internal membranes that compartmentalize their functions The eukaryotic cell s genetic instructions are housed in the nucleus and carried out by the ribosomes The endomembrane system regulates protein traffic and performs metabolic functions in the cell Mitochondria and chloroplasts change energy from one form to another The cytoskeleton is a network of fibers that organizes structures and activities in the cell Extracellular components and connections between cells help coordinate cellular activities PROKARYOTIC CELLS CELL SIMILARITIES Membrane bound plasma (cell) membrane Cytosol semifluid in which organelles are found Chromosomes carrying genes in form of DNA Ribosomes protein synthesis Lacks nucleus and membrane bound organelles Nucleoid region where DNA is concentrated ; no membrane separates this region from the rest of the cell EUKARYOTIC CELLS True nucleus bound by nuclear envelope Cytoplasm region between nucleus and plasma membrane Organelles membrane bound structures that carry out specific functions CELL SIZE Eukaryotic generally larger than prokaryotic Mycoplasmas smallest cell known; bacteria As size increases, its volume grows proportionately more than its surface area Smaller objects have a greater ratio of surface area to volume Rate of chemical exchange may be inadequate for cell size Thus cell size remains small 1
NUCLEUS Contains genes of eukaryotic cell (few in mitochondria & chloroplast) ; directs protein synthesis by making messenger RNA Nuclear envelope double membrane separating its contents from cytoplasm Perforated with pores Chromosomes structures that carry genetic information ; composed of chromatin (proteins & DNA) Nucleolus contains ribosomal RNA which synthesizes ribosomes RIBOSOMES Particles made of ribosomal RNA and protein & carry out protein synthesis Free ribosomes in cytosol; protein remains in cell Bound ribosomes attached to ER; protein secreted from the cell 2
ENDOPLASMIC RETICULUM - ER Network of tubules & sacs called cristernae ER lumen internal compartment of ER ER is continuous with nuclear envelope SMOOTH ER Lacks ribosomes, Synthesis of lipids (oils, phospholipids, steroids) Detoxification of drugs (adds hydroxyls making drugs soluble & easier to flush from body) Stores calcium (muscle contraction) ROUGH ER Studded with ribosomes Synthesis of secretory proteins (glycoproteins carbs attached) Keeps secretory proteins separate from free proteins GOLGI APPARATUS Modifies, stores & ships ER products Consists of flattened membranous sacs (cristernae) Cis face receiving end Trans face shipping end Also makes other polysaccharides (pectin) LYSOSOME Membranous sac of hydrolytic enzymes that animal cells use to digest macromolecules Prefer acidic environment Phagocytosis engulfing of single organisms or food particles Digested materials pass to cytosol and become nutrients Autophagy cell recylcing of own organic material 3
VACUOLES Membrane bound sacs (larger than vesicles) Food vacuoles phagocytosis Contractile vacuole removes excess water in freshwater organisms Central vacuoles (plants) Tonoplast membrane Reserves important organic compounds Stores K+ and Cl- Disposes biproducts Pigments Protective poisons growth OTHER MEMBRANE ORGANELLES Mitochondria, chloroplast, peroxisomes Differ from endomembrane structures: 2 layers separating them from cytosol Proteins made by free ribosomes Small amounts of DNA MITOCHONDRI A Site of cellular respiration Quantity correlates with metabolic activity of cell 2 layers of phospholipids Outer is smooth Inner convoluted cristae ; increase surface area Intermembrane space region between inner & outer membranes Mitochondrial matrix region enclosed by inner membrane ; contains enzymes needed to metabolize compounds in cellular respiration 4
CHLOROPLAST Site of photosynthesis Thylakoids flattened sacs containing chlorophyll Granum stacks of thylakoids Stroma fluid surrounding thylakoid stacks containing DNA, ribosomes & enzymes Other plastids: amyloplast store starch and chromoplast orange & yellow pigments PEROXISOMES Transfer hydrogen from various substances to oxygen to form H 2 O 2 H 2 O 2 toxic peroxisome converts it to water within another compartment Can also Break down fatty acids Detoxify alcohol CYTOSKELETON Network of fibers extending through cytoplasm Support & shape to cell Cell motility interaction of motor proteins 3 types: microtubules, microfilaments & intermediate filaments MICROTUBULES Thickest Hollow tubes constructed of globular protein tubulin Maintain shape & support, transport & chromosome separation Serve as the structure of centrioles, cilia & flagella 5
CENTROSOMES & CENTRIOLES Centrosome region near nucleus where microtubules are organized Centrioles 9 sets of triplet rings used to separate chromosomes during mitosis CILIA & FLAGELLA Locomotive appendages Cilia beating hair like structures Flagella undulating tail like structures 9 + 2 pattern 9 doublets of microtubules in a ring 2 single microtubules in enter Connected by radial spokes Anchored to cell by basal body Dynein proteins large protein; join doublets MICROFILIAMENTS Thinnest Composed of globular protein actin Cell motility contractile apparatus Play a role in: Muscle contraction interaction with myosin protein Ameoboid movement interactions force interior fluid into pseudopodia (false foot) Cytoplasmic streaming cytoplasm cycling over parallel actin filaments 6
INTERMEDIATE FILAMENTS Size in between microfilaments & microtubules Permanent fixtures not disassembled & reassembled like microfilaments & microtubules Can exist after cell death eg. Keratin in skin EXTRACELLULAR COMPONENTS Cell Wall Maintains shape Prevents excessive uptake of water Protects plant cell Structure: Primary cell wall develops as cell grows Middle lamella sticky substances that glues adjacent cells together ; pectin Secondary cell wall forms between primary cell wall & plasma membrane after growth stops EXTRACELLULAR MATRIX Extracellular Matrix (ECM) Glycoproteins proteins covalently bonded to carbohydrates Collagen embedded in proteoglycan (another glycoprotein) Fibronectins bind to receptor proteins in plasma membrane ; these integrin proteins serve as a means of communication between ECM & cytoplasm 7
INTERCELLULAR JUNCTIONS STRUCTURES THAT SERVE AS A MEANS OF JOINING CELLS IN TISSUES, ORGANS & SYSTEMS PLANTS: Plasmodermata channels that perforate plant cell walls cytosol, water, solutes, proteins & RNA can freely pass from cell to cell ANIMALS: Tight junctions proteins that form a seal between adjacent cells; prevent leakage of extracellular fluid Desmosomes (anchoring junctions) function as rivets, fastening cells together Gap junctions (communicating junctions) provide cytoplasmic channels from cell to cell Necessary for communication 8