Eukaryote Organelles
BIO 224
Intro to Molecular and Cell Biology
Endoplasmic Reticulum
• A network of cisternae (enclosed tubular sacs)
extending from the nuclear membrane
throughout the cytoplasm
• The largest organelle of most eukaryotic cells
• Two general types
– Rough ER has ribosomes on its surface
• Transitional ER buds off vesicles that exit to the Golgi
apparatus
– Smooth is involved in lipid metabolism
Rough ER
• Surface covered in ribosomes
• Site of synthesis of proteins
• Proteins travel to Golgi in vesicles for modification
and secretion
• Proteins to be secreted or incorporated into ER,
Golgi, lysosomes, or plasma membrane targeted to
ER for synthesis
• Proteins to remain in cytosol or be incorporated into
nucleus, mitochondria, chloroplasts, or peroxisomes
made on free ribosomes
Smooth ER
• Major site of membrane lipid production
– Synthesized in association with existing membranes
– Most made in ER, some in other membranes
• Transported to destination by vesicles or carrier
proteins
• Most phospholipids made on cytosol side of ER
– Enzymes called flippases allow half of the phospholipids to
pass through to even out the bilayer
• Ceramide products go to Golgi for conversion to
glycolipids or sphingomyelin
• Cholesterol and steroid hormones made in ER
• Smooth ER of the liver detoxifies drugs
Golgi Apparatus
• Factory that receives protein products from the ER to
process and sort for transport to their destination
• Site of glycolipid and sphingomyelin synthesis
• Site of cell wall polysaccharide synthesis in plants
• Made of flattened membrane enclosed sacs (cisternae)
and associated vesicles
• Contains 4 discrete compartments
– cis Golgi network, medial and trans Golgi stacks, trans Golgi
network
• Proteins enter cis face and exit trans face of Golgi
Golgi Organization
• Proteins enter at cis Golgi network
• Move to medial and trans compartments for
most metabolic processes like modification
• trans Golgi network receives modified
products to be sorted and distributed to final
destinations
• Mechanism of protein movement through
Golgi not entirely understood
Golgi Processes
• Site of glycosylation of glycoproteins
– Glycosyltransferases add sugar groups and
glycosidases remove them
• Site of glycolipid and sphingomyelin
production using ceramide from ER as
precursor
• Plant cells use Golgi to produce
polysaccharide hemicelluloses and pectins of
cell wall
Transport in the Golgi
• Proteins, polysaccharides and lipid transported from Golgi to
final destination by the secretory pathway
– Proteins sorted into different types of transport vesicles that bud from
trans Golgi network
• Some proteins carried by constitutive pathway to be incorporated into
plasma membrane or continuously secreted from cell
• Others transported to cell surface through regulated secretion or
specifically targeted to other intracellular destinations like lysosomes or
vacuoles
• Proteins with functions internal to Golgi remain there
– Golgi proteins associated with the membrane
– Constitutive pathway allows for their continued unregulated secretion
• Some cells use regulated secretion pathway with proteins
secreted in response to environmental signals
– Proteins made and packaged into vesicles to be stored until needed
Lysosomes
• Membrane enclosed organelles containing enzymes
for degradation of polymers
• Act as cellular digestion system
• Breaks down internalized materials and recycles old
cell components
• Contain 50 different enzymes to break down
molecules
– Most enzymes are acid hydrolases
• Activity at pH near 5, nonfunctional in cytoplasm
• Use a proton pump to increase H+ ion concentration for
maintaining pH
• Mutation in genes for those enzymes cause lysosomal storage
disease in humans
Lysosome Functions
• Destroy material taken in by endocytosis
– Extracellular material is “eaten” by cell
– Transport vesicles fuse with endosomes for digestion
• Phagocytosis: type of endocytosis by macrophages
and other cells to ingest large particles (bacteria) for
digestion
• Autophagy: process used to recycle cell components
– Occurs in all cells, important in embryonic development
Mitochondria
• Major site of energy production in eukaryotic cells
• Provide most of the usable energy from carbohydrate
and FA breakdown via oxidative phosphorylation
• Most of their associated proteins translated on free
cytosolic ribosomes and imported into the organelle
• Have their own DNA coding for tRNAs, rRNAs, and
some mitochondrial proteins
• Assembled from proteins internally coded for and
translated along with proteins encoded for by nuclear
genome and imported after translation
Mitochondrial Organization
• Surrounded by double membrane
• Inner and outer membranes separated by
mitochondrial space
• Inner membrane has cristae (folds) extending
into matrix
– Internal fluid material
• Matrix and inner membrane are major
working compartments
Mitochondrial Organization
• Matrix contains genetic system and enzymes used for
oxidative phosphorylation
– Citric Acid Cycle takes place here
• Inner Membrane has electron transport chain embedded
into it
– Primary site of ATP production using chemiosmosis
– Surface area increased for reaction by folding into cristae
– High protein concentration for oxidative phosphorylation and
metabolite transport between cytosol and mitochondria
– Impermeable to most ions and small molecules
• Outer membrane permeable to small molecules because
of porins
– Proteins that form channels allowing diffusion of small
molecules
Mitochondrial Genetic System
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Separate and distinct from nuclear genome
Similar to free-living bacteria
Circular DNA molecules in multiple copies per organelle
Size of genome varies by species
Most code for small proteins used in oxidative phosphorylation
– Code for all rRNAs and most tRNAs needed for internal translation of protein
coding sequences
• Other mitochondrial proteins coded for by nuclear genes
– Human mitochondrial DNA codes for 13 proteins, 2 rRNAs and 22tRNAs
• Uses slightly different genetic code due to latitude in recognition of codons
• Can be altered by mutation
– Germline mutation can be passed to next generation mainly via mother
– Mutation associated with disease and aging in humans
• Proteins not well understood, only half mammalian mitochondrial proteins
identified
– Mitochondria in different tissues produce different proteins based on function of
specialized cells
Protein Import in Mitochondria
• Proteins needed for DNA replication, transcription,
and translation of mitochondrial genome coded for
by nuclear genome
• Nuclear genome codes for most proteins needed for
oxidative phosphorylation and all enzymes of
mitochondrial membrane
• >95% mitochondrial proteins made on cytosolic
ribosomes and imported as complete chains
Chloroplasts
• Share similarities to mitochondria
• Generate metabolic energy, evolved through
endosymbiosis, separate genetic system,
replicate by division
• Convert CO2 to carbohydrates, synthesize
amino acids, fatty acids and membrane lipids,
reduce nitrate to ammonia
Chloroplast Structure
• Large organelle (5-10um)
• Double membrane chloroplast envelope (outer and inner) and
internal thylakoid membrane system
– Network of flattened thylakoids arranged in stacks called grana
• Chloroplasts divided into compartments by the three membranes
– Intermembrane space between the envelope membranes
• Outer membrane contains porins permeable to small molecules
• Inner membrane impermeable to ions and metabolites
• Stroma inside chloroplast envelope, outside thylakoid membrane
– Stroma contains genetic system and metabolic enzymes
• Thylakoid lumen inside the thylakoid
– Thylakoid membrane contains electron transport system and site of
chemiosomtic production of ATP
Chloroplast Genome
• Circular DNA molecules in multiple copies per
organelle
• Larger and more complex than mitochondrial
genome
• Genes code for RNAs (rRNAs, tRNAs) and
proteins involved in gene expression and
proteins for photosynthesis
• Uses standard genetic code
• Codes for 1/3 of ribosome proteins for
chloroplast
Chloroplast Import and Sorting
• 95% of chloroplast proteins coded for by
nuclear genes
• Made on cytosolic ribosomes and imported as
complete chains into chloroplast
Peroxisomes
• Small membrane enclosed organelles
• Contain 50 different enzymes involved in various
biochemical pathways in different cell types
• Human cells have about 500
• Do not have their own genome
• All proteins (peroxins) made from nuclear genome
– Most peroxins made on free ribosomes and imported as
complete chains
• Replicate by division
• Can be rapidly regenerated if lost
Peroxisome Function
• Oxidative reactions lead to production of H2O2
– Catalase degrades H2O2 for protection of cellular components
• Various substrates metabolized down by peroxisomes
– Involved in production of lipids and amino acids
– Some animal cells make cholesterol and dolichol in peroxisomes in
addition to ER
• Contain enzymes for production of plasmalogens
– Found in membranes of brain and heart tissue
• Important in fatty acid breakdown in plants and yeasts
– Convert stored FAs in plants to carbohydrates for energy for growth of
germinating plants
– Peroxisomes in leaves metabolize side products of photosynthesis
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