VIEWS: 22 PAGES: 27 POSTED ON: 5/10/2012
Endomembrane System • Includes all membrane-enclosed organelles, and is derived from the endoplasmic reticulum • Endoplasmic reticulum, Golgi apparatus, vacuoles, vesicles, lysosomes, microbodies, peroxisomes, glyoxysomes Endoplasmic Reticulum • Structure: Flattened, fluid-filled, membrane enclosed sacs that compartmentalize the cell with a series of channels formed between overlapping membranes. • Two types of Endoplasmic Reticulum: – A. Rough ER – B. Smooth ER – The Nuclear envelope connects to the Rough ER which then connects to the Smooth ER. Rough Endoplasmic Reticulum • Structure: (continuous with nuclear envelope) tubular canals with attached ribosomes • Function: produces proteins for export via smooth ER and Golgi apparatus • Ex. hormones, proteins for the cell membrane, digestive enzymes, mucus • Proteins made in the rough ER are further modified with a “mailing label” (oligosaccharide) which states where the protein should go and helps to sort proteins. • Then proteins move to the smooth ER Smooth Endoplasmic Reticulum • Structure: membrane enclosed sacs that lack ribosomes • Function: membranes are embedded with enzymes that catalyze many reactions and then send products to the Golgi apparatus: – packages proteins into vesicles for transport within the cell, abundant in secretory cells (hormones), muscles and liver cells – produces lipids like phospholipids and produces vesicles to replace the cell membrane – Produces steroid hormones in testes and adrenal cortex – Stores enzymes to break/detoxify drugs and poisons in liver cells, some drugs after being broken down are packaged into vesicles and transported to another organelle for further breaking down (Peroxisomes, Glyoxysomes) – Stores calcium ions in muscle cells, Ca2+ also used in controlling, building, or disassembling cytoskeleton elements. Golgi Apparatus • Discovered in 1898 by Camillo Golgi; Animal cells 10-20, Plant cells 100’s • Structure: stacks of flattened saccules similar to hollow pancakes; inner face directed toward nucleus and ER, outer face directed toward cell membrane • Function: (Post Office) vesicles from the ER move to the inner face of the Golgi body where they are then collected, stored, modified, concentrated as they move from layer to layer, and then repackaged into vesicles and pinched off the outer face of the Golgi body. • Ex. Proteins that leave the Golgi apparatus: hormones, digestive enzymes, glycoproteins, glycolipids, protein fibers that strengthen organs like the skin Golgi Apparatus Functions – Modifies “mailing labels” with polysaccharides, glycoproteins and glycolipids, so that the proteins can be sorted and packaged. If there are errors with mailing labels, then the Golgi body sends the protein back to the ER. – Regulates the surface area of the cell membrane by producing vesicles that can rupture and attach to the cell membrane or vesicles move from the cell membrane back to the Golgi body. – Free ribosomes send their products here for modification and packaging Vacuoles • Structure: large, long-lived membrane enclosed sacs that are relatively static; both plants and animals but greatest development in plants • Function: stores water, sugars, amino acids, salts, pigments (beets), organic acids, wastes, and toxic substances (toxins or tannins that deter herbivores); called cell sap – Vacuoles may start as many small vacuoles that as they fill and mature, combine to form one large Central vacuole that can occupy 90% of the plant cell volume – Pushes cytoplasm to the periphery against the cell wall (turgor pressure); provides structure and stiffness – Contractile vacuoles in protists are used to expel water and waste, propeling the protist (Ex. amoeba) – Food vacuoles in animals and protists store and digest food (Ex. paramecium food vacuoles stained red) Vesicles • Structure: small, short-lived membrane enclosed sacs, which move around cell • Function: vehicles to move substances from one place to another in the cell and to the cell membrane for export. May also bud off of vacuoles. Lysosomes • Structure: Membrane-bounded vesicles formed by the Golgi apparatus • Function: Contains very acidic hydrolytic enzymes (pH 5 vs cytosol pH 7.2) used in the break down of macromolecules Lysosomes: Functions • Contains very acidic hydrolytic enzymes (pH 5 vs cytosol pH 7.2) used in the break down of macromolecules – White blood cells digest bacteria and dead/worn out cells (neutrophils), take in antigens by professional antigen-presenting cells (B-cells) – Digest worn out cell organelles (Ex. mitochondria every 10 days) – Digest food molecules taken in by endocytosis – Digests and releases products obtained from other vesicles – Used to believe involved in autodigestion when lysosome breaks down and enzymes pour into the cell digesting the whole cell. (Ex. tadpoles tail, human embryo’s webbed fingers) Today know that Apoptosis or Programmed cell death involves cells shrinking, mitochondria breaking down, chromatin degrading/breaking into small fragments, and then these engulfed by macrophages – Tay-sachs disease – lysosome is missing a digestive enzyme so nerve cells can not transmit nerve impulses Microbodies • Structure: single membrane-bound sacs containing specific enzymes in a dense crystalline core. Enzymes and proteins used here are made by free ribosomes. – Two types: • Animal/Plant cells called Peroxisomes • Plant cells called Glyoxysomes • Function: Conducts enzymatic reactions that build up and break down specific molecules often used for respiration, therefore they often lie next to mitochondria or chloroplasts. • Some sources do not include microbodies in the endomembrane system because they do not bud from the ER, instead they increase in number by splitting in two and grow in size by incorporating proteins from free ribosomes, lipids from the ER, and lipids made within themselves. Peroxisomes (animal cells and plant cells) • Contain many enzymes used to rid the cell of toxic substances by transferring hydrogen from the poisons to molecules of oxygen (oxidation) to form H2O2. Hydrogen peroxide is highly reactive so catalase is used to break down hydrogen peroxide into water and oxygen. • Liver peroxisomes – detoxify alcohol and other harmful compounds, remove amino group from amino acids, break down fatty acids. These reactions make molecules used in respiration and other biochemical pathways so they are transferred to a mitochondrion. • Some peroxisomes produce phospholipids, cholesterol, bile acids, and lipids that make myelin around neurons. • Kidney peroxisomes - breaks down excess purines to yield uric acid • Converts fat to carbohydrates. • Able to self-replicate but has no internal DNA and must import proteins it needs to make copies of itself Glyoxysomes (plant cells) • Contain specific enzymes that make molecules for photosynthesis, respiration, and other biochemical pathways. • Breaks down fats and converts fatty acids to carbohydrates, so often found in the fat-storing tissues of plant seeds. • Plays key role during germination and growth because a lot of carbohydrates are needed to produce cell walls (cellulose) and the emerging seedling can use the sugars produced as a source of energy and carbon until it can produce its own sugar by photosynthesis. Energy Related Organelles: • 1. Mitochondrion • 2. Chloroplast • Endosymbiotic hypothesis – how these organelles were derived • Not part of the endomembrane system because they are composed of a double membrane, their proteins are made by free ribosomes or themselves and not by the ER, and they can grow and self-replicate. Mitochondria • Found in both plant and animal cells, can number into the hundreds or thousands per cell. • Structure: double membrane-bound organelle with inner membrane forming shelves / infoldings called “cristae” that project into a matrix (inner space) and contain embedded enzymes in cristae, enzymes in the matrix, energy-rich reactants, DNA, and ribosomes. Cristae provide large surface area to enhance productivity. (1-10 µm long, move around, change shapes, and divide in two) • Function: “Power house” of the cell, produces ATP for cellular energy by aerobic respiration = – Glucose + Oxygen → Carbon dioxide + Water + ATP – C6H12O6 + 6O2 → 6CO2 + 6H20 + 36ATP • Extract energy from food to make little packets of energy that is just right for cellular activities. Chloroplasts • Found only in plant cells and photosynthetic protists (algae) • Structure: double membrane-bound organelle that contains thylakoids (flattened sacs) piled up into stacks called grana and the fluid-filled space around the grana is called stroma, containing DNA, ribosomes, and enzymes. Grana are joined together by lamella. (2-5 µm long, move around, change shapes, and divide in two) Chloroplasts: Function • Function: Carries out photosynthesis which captures sunlight energy and transforms it into food energy (glucose). • Sunlight + CO2 + H20 → C6H12O6 + O2 • Chlorophyll is a pigment on the surfaces of thylakoid membranes that absorbs solar energy and makes leaves appear green. Endosymbiotic Hypothesis • Contends that some early prokaryotes were engulfed by larger bacterial cells and became mitochondria and chloroplasts found in modern eukaryotic cells. (Proposed by Lynn Margulis) • On prehistoric earth, cyanobacteria began to produce oxygen after 3.5 bya which made it hard for anaerobic bacteria to survive. Perhaps around 1.5 bya, anaerobic bacteria engulfed bacteria that could use and survive in oxygen. The new engulfed bacteria resisted digestion, and began to live symbiotically with the new host and helped the host to carry out metabolic processes to make ATP or synthesize food. (mitochondria were aerobic heterotrophic bacteria and chloroplasts were cyanobacteria) • There are many symbiotic associations between Prokaryotes and Eukaryotes like Coral which is cyanobacterium living inside an animal cell. • Some believe that peroxisomes, basal bodies, centrioles, and nematocyts, flagella (spirochete prokaryote) also have endosymbiotic origins. • Ch 25 pg 412 Mader Evidence for Endosymbiosis – Size similar to bacterium (1-3 µm) – Double membrane came from engulfing a bacterium so that the inner membrane = bacteria’s membrane and outer membrane = plasma membrane of host – Own DNA in a circular loop like a bacterium’s DNA; can be copied but once incorporated most bacterial genes must have been transferred into the host cell’s DNA (for metabolism). – Own Ribosomes similar in size and RNA base sequence to bacterial ribosomes; can copy its DNA and use ribosomes to make proteins for use within the mitochondrion or chloroplast. – Able to Reproduce / Divide by fission similar to the way in which bacteria divide into two. This is the only way to transfer mitochondria/chloroplasts into new cells. – Able to build their own membranes Cytoskeleton • Network of protein fibers crisscrossing the cell and functions to help maintain the cell’s shape, provide mechanical strength (gelatin-like consistency of cytosol), anchor organelles and enzymes, allows cells and its organelles to move, and helps to regulate the activities of the cell by transmitting mechanical forces through the cytoskeleton to influence cell activity. Can be broken down and reassembled in a new location to change the shape of a cell. (The muscles and skeleton of the cell) (Discovered in mid 1970’s) • Three types: – Actin filaments / microfilaments – Intermediate filaments – Microtubules Actin Filaments / Microfilaments • Structure: are long, thin fibers composed of two chains of the subunit protein actin twisted together (7 nm diameter). • Functions: Attached to cell membrane to provide mechanical strength and maintain shape (mostly concentrated near the plasma membrane), links plasma membrane proteins to cytoplasmic proteins, anchors centrosomes to opposite poles during mitosis (cell division), allow microvili to shorten and extend into the intestine, aid in muscle contraction with myosin motor protein walking along the actin, help pseudopods to change shape for cell motility, involved in cytoplasmic streaming, forms cleavage furrow / pinches cell inward in a dividing animal cell by forming a constriction ring with myosin. Intermediate Filaments • Structure: Composed of 5 types of protein subunits (such as vimentin and keratin). Two Fibrous polypeptides / chains wrap themselves together in a rope-like fashion to form dimer. Then 2 dimers join with other dimers head to tail forming strands. Eight strands bind in a hollow tube, or coil together to make thicker cables (8-12 nm diameter, different depending on the protein). More permanent structures, unlike actin and microtubules which assemble and disassemble. • Functions: Provides structure and tensile strength for the cell to prevent excessive stretching, forms nuclear lamina providing structure to nucleus as well as holding the nucleus in place in a basket-like network of filaments, attaches ribosomes and other organelles within the cytoplasm, aids in cell- to-cell junctions by joining plasma membranes, makes up keratin in epithelial cells even after cell death (hair and nails), anchors the actin and myosin filaments together in muscle cells, and provides mechanical strength to long axons in neuron cells. Microtubules • Structure: Hollow cylinders composed of 13 helical rows of tubulin dimers forming a circle (2 tubulin proteins, alpha and beta, make a dimer). (25 nm diameter) Assembly of microtubules is under the control of the Centrosome, area located just outside the nucleus, and in animal cells there is a pair of centrioles within the centrosome which microtubules grow out of. Grows by adding tubulin and shrinks by taking away tubulin. Microtubule: Functions • Functions: Plays a role in cell structure, helps stretch out ER, but mainly involved in movement of cell parts; moves vesicles and other organelles on “tracks” or “conveyor belts”, moves chromosomes apart (spindle) during cell division, and involved in cell motility by moving cilia and flagella • Associated with two types of motor molecules: – Kinesin – moves vesicles and organelles (vesicles out toward plasma membrane) by walking along microtubules. – Dynein – moves cilia and flagella (vesicles in toward the nucleus) Centrioles, Cilia, and Flagella • All made from microtubules but differ in their arrangement of microtubules. • All involved in movement Centrioles • Structure: Ring of nine sets of three microtubules and no microtubules in the center = 9 + 0 pattern. Only found in animal cells. Two centrioles lie at right angles to one another near the nucleus and replicate before cell division with one pair lying on each side of the nucleus. • Functions: During cell division, centrioles (within the centrosome) sends out microtubule fibers to attach to chromosomes (spindle). Chromosomes move along the microtubules to divide genetic material while at the ends the microtubules disassemble. (Not necessary for cell division because plants lack centrioles.) Also produce basal bodies which anchor cilia and flagella. Cilia and Flagella • Structure: Hair-like extensions of the cell made of nine microtubule doublets arranged in a circle around two central microtubules = 9 + 2 pattern. Plasma membrane extends to cover the cilia and flagella. • Cilia are 2-20 µm in length and often found in large numbers on the cell surface. • Flagella are 10-200 µm in length and there is usually one to a few per cell. • Anchored by basal bodies with a 9 + 0 pattern. • Function: Provide movement across the cell or for the entire cell. • Cilia beat in unison in oar-like fashion to move some protists (paramecia), sweep debris trapped in mucus out of the lungs back up into the throat in the respiratory tract, in a woman’s fallopian tubes moves the egg to the uterus, and are the sensory hairs in cochlea of ear. • Flagella move in an undulating motion to propel sperm. (Sperm’s basal bodies of the flagella become the centrioles in the new fertilized egg) • Movement caused when microtubule doublets slide past one another with the aid of bending dynein molecules attached as side-arms to the microtubule doublets and ATP.
"The cell Part II"