LECTURE 9
The Anatomy of Cells (continued)
Rough ER and Protein Synthesis
Proteins destined for secretion are synthesized by ER-associated ribosomes (e.g., antibodies (IgG) secreted by white blood cells)
As protein is made, it is threaded through the ER membrane into the lumen. Secreted proteins are typically glycoproteins. The (CH2O)n units of these glycoproteins are attached once the protein is in the ER lumen.
Proteins destined for secretion have an N-terminal signal sequence of about 20 amino acids which signals"I am to be exported".
This N-terminal signal sequence is first recognized by a cytoplasmic signal recognition particle (SRP) which then interacts with an SRP receptor located on the ER. This interaction attaches the ribosome making the secretory protein to the ER, where the protein is threaded through the SRP receptor into the lumen.
Once the secretory protein is within the lumen, the signal sequence is clipped off by a peptidase.
Signal sequences are like ZIPCODES
They target a protein undergoing synthesis to the proper compartment: ER lumen; mitochondrion; chloroplast
The Golgi Complex (sometimes referred to as the dictysome in plants)
Function: manufacturing, warehousing, shipping of ER products; plentiful in secretory cells. Puts"finishing touches" on transported molecules synthesized in ER. Also, synthesis of some cell-surface polymers like hyaluronic acid, an acid mucopolysaccharide.
Flattened (pancake-like) sacs. Overall Golgi structure has a distinct polarity:
cis face: near smooth ER
trans face: maturing face; releases vesicles which pinch off and move away to the plasma membrane or other destination
Target of vesicle contents is determined by"address labels" such as an oligosaccharide"tag" attached to a glycoprotein. The Golgi typically phosphorylates this oligosaccharide, sorting of"stamping it" as confirmation of proper delivery, for example, to a lysosome.
Lysosomes
Function: principal sites of intracellular digestion; membrane-enclosed bag of hydrolytic enzymes for digestion of proteins, fats, polysaccharides, nucleic acids; acidic compartment - pH » 5 (the pH optimum for its hydrolytic enzymes)
Lysosomal enzymes are made by the rough ER and transferred via the Golgi. Lysosomes arise by budding from the Golgi.
Lysosomes fuse with phagosomes (vesicles arising from invaginations in the plasma membrane), allowing their hydrolytic enzymes to digest the ingested contents of the phagosome.
Lysosomes also function in programmed cell death (apoptosis) to carry out the programmed destruction of cells (as in tadpole tail resorption).
Peroxisomes: an example of a group of small organelles called microbodies: compartmentalized small vesicles with a highly specialized metabolic purpose.
Peroxisomes are specialized for peroxide degradation.
Glyoxisomes are plant organelles that convert fat to sugar in the germinating seeds of plants.
Vacuoles: membrane-bounded intracellular sacs
food vacuoles (phagosomes) from phagocytosis
contractile vacuole in Protists (water excretion)
central vacuole of plants:
Surrounded by tonoplast (membrane derived from and part of endomembrane system). Central vacuole functions as an internal storage depot, dump, and processing center:
The storage of organic compounds, inorganic ions, and hydrolytic enzymes (like a lysosome)
It also functions as a disposal site and in water absorption to maintain cell turgor.
Mitochondria and Chloroplasts: Energy-Transducing Organelles
Neither is part of the endomembrane system.
Both are semi-autonomous in their growth & division:
Both have DNA and ribosomes and make some of their proteins.
However, most of their proteins are synthesized on cytoplasmic ribosomes from mRNA copied from nuclear genes.
Both are enclosed by double membranes, as appropriate for their endosymbiotic origin (Figure 4.15)
Mitochondrion: (in both animal and plant cells): Figure 4.11
Food energy ® cellular respiration ® ATP
((CH2O)n; fats; amino acids)
one or many (>1,000) per cell
1 m m diameter x 1-10 m m length
2 membranes:
outer mitochondrial membrane: rather permeable, sieve-like. Allows small molecules to pass, excludes macromolecules
inner mitochondrial membrane: highly impermeable; infolded to form cristae. Increases surface area; encloses inner soluble phase (the matrix)
cristae and matrix are the sites for the enzymes of cellular respiration and ATP synthesis.
Chloroplast: (in plant cells only): Figure 4.12
Light energy ® photosynthesis ® ATP
Most prominent plastid (plant-specific organelles)
Similar in some ways to mitochondria, BUT 3 compartments rather than 2:
1. space between the outer membrane and inner membrane
2. inner membrane (thylakoid membrane)-enclosed soluble phase (stroma). The reactions that convert CO2 to carbohydrate (the CO2-fixation reactions) are localized in the stroma
3. thylakoid lumen: space enclosed by the thylakoid vesicles (flattened sacs derived from the inner membrane), also called grana. Enzyme systems localized in the thylakoid vesicles convert light energy into chemical energy for ATP synthesis.
Cytoskeleton:
A network of protein fibers throughout the cell giving structural support to the cell; a scaffold that can be dis-assembled and re-assembled.
The cytoskeleton is also associated with cell motility and cell division
The 'muscle and bones' of the cell
3 Types of Fibers Contribute to the Cytoskeleton
1. microtubules - 25 nm diameter; hollow tubes composed of dimers of the a -tubulin and b -tubulin. The microtubule has polarity: a + end and a - end. Tubulin dimers can add to or dissociate from the + end, lengthening or shortening the microtubule, as appropriate.
A dynamic."girder-like" framework, which can readily polymerize or de-polymerize.
Involved in:
the mitotic spindle
centrioles of animal cells (structures involved in cell division)
eukaryotic cilia (short) & flagella (long)
250 nm core of microtubules arranged as 9 doublets in a ring around 2 central microtubules. ("9 + 2" arrangement) Figure 4.25
Motor proteins: dynein & kinesin - ATP-dependent"motors" that travel along microtubules.
Dynein travels along a microtubule towards the - end; kinesin travels along a microtubule towards the + end. These motor proteins can attach to organelles, vesicles, & other structures and move them about the cell.
2. intermediate filaments - 8-12 nm diameter; 5 distinct types of fibers, all of which are composed of proteins of the keratin family. (The lamins of the nuclear lamina are one form of intermediate filament.)
The cell"superstructure": a less dynamic, more stable cytoskeletal element.
The nucleus is fixed within an intermediate filament"cage".
3. microfilaments - 7 nm diameter; composed of 1 or more strands of actin filaments, each filament consisting of a pair of actin chains (F actin) twisted about each other; the protein monomer is called G actin.
Microfilaments are involved in:
cell support
muscle contraction (with myosin)
amoeboid movement of cells
cell division (cleavage furrow)
Cell Surface Structures (Extracellular Structures)
structures exterior to the plasma membrane, typically forming a protective coat of some kind
Plants, fungi, bacteria: cell walls - rigid, laminar structures, typically composed of polysaccharide polymers (e. g., cellulose). Constructed like re-inforced concrete (cellulose rods in a complex matrix of other substances)
Animal cells: extracellular matrix or glycocalyx;"fuzzy coat";"cell glue"
chemically varied: includes oligosaccharides extending from the surface of plasma membrane, membrane glycolipids, membrane glycoproteins, as well as secreted glycoproteins, proteoglycans (Figure 4.29), and collagen.
Functions:
protective layer
cell-cell recognition
cell-cell adhesion