* 6 Flashcards
3 important parameters in microscopy:
- magnification: the ratio of an object’s image size to its real size
- resolution: measure of the clarity of the image; the minimum distance 2 points can be separated and still be distinguished as 2 points
- contrast: accentuates differences in parts of the sample
electron microscope (EM)
Rather than light, the EM focuses a beam of e- through the specimen or onto its surface. Resolution is inversely related to the wavelength of the radiation a microscope uses for imaging, and electron beams have much shorter wavelengths than visible light.
scanning electron microscope (SEM)
Useful for detailed study of the topography of a specimen. The e- beam scans the surface of the sample, usually coated w/ a thin film of gold. The beam excites e- on the surface, and these secondary e- are detected by a device that translates the pattern of e- onto an electronic signal to a video screen. The result is an image of the specimen’s surface that appears 3D.
transmission electron microscope (TEM)
Useful for studying the internal structure of cells. TEM aims an e- beam through a very thin section of the specimen. The specimen has been stained with atoms of heavy metals, which attach to certain cellular structures, thus enhancing the e- density of some parts of the cell more than others. The e- passing through the specimen are scattered more in the denser regions, so fewer are transmitted. The image displays the pattern of transmitted e-. Instead of using glass lenses, the TEM uses electromagnets as lenses to bend the paths of the e-, ultimately focusing the image on a monitor for viewing.
specimen preparation
In e- microscopy, may kill the cells.
For all microscopy techniques, specimen preparation can introduce artifacts, structural features seen in micrographs that don’t exist in the living cell.
cell fractionation
Takes cells apart and separates major organelles and other subcellular structures from one another. The centrifuge is used; it spins test tubes holding mixtures of disrupted cells at a series of increasing speeds. At each speed, the resulting force causes a fraction of the cell components to settle to the bottom of the tube, forming a pellet. At lower speeds, the pellet consists of larger components, and higher speeds yield a pellet with smaller components.
cell fractionation advantage
Enables researchers to prepare specific cell components in bulk and identify their functions, a task not usually possible w/ intact cells.
cytosol vs cytoplasm
cytosol: semifluid, jellylike portion of the cytoplasm
cytoplasm: the contents of the cell bounded by the plasma membrane; in eukaryotes, the portion exclusive of the nucleus.
all cells share certain basic features:
- bounded by plasma membrane
- cytosol
- chromosomes
- ribosomes
surface area
Cells are small b/c smaller objects have a greater ratio of SA:V.
nuclear membrane and pores
An intricate protein structure called a pore complex lines each pore and regulates the entry and exit of proteins, RNAs, and large complexes of macromolecules.
nuclear lamina
The interior side of the nuclear envelope is lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope.
chromosome
one long DNA molecule + associated proteins, some of which help coil the DNA molecule of each chromosome, reducing its length
nucleolus
- rRNA is synthesized from instructions in DNA
- proteins imported from cytoplasm are assembled w/ rRNA into large and small subunits of ribosomes; these subunits then exit to the cytoplasm, where a large and a small subunit can assemble into a ribosome
ribosomes
- free: suspended in cytosol; make proteins that function in the cytosol
- bound: attached to the outside of ER or nuclear envelope; make proteins that are destined for insertion into membranes, packaging within certain organelles, or secretion
vesicle
A membranous sac in the cytoplasm of a eukaryotic cell. The endomembrane system is related either through physical continuity or by the transfer of membrane segments as tiny vesicles.
endomembrane system: includes
nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles + vacuoles, plasma membrane
endomembrane system: function
- protein synthesis
- transport or proteins into membranes, organelles, or out of the cell
- metabolism and movement of lipids
- detoxification of poisons
endoplasmic reticulum (ER)
An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane. Consists of a network of membranous tubules and sacs called cisternae.
functions of smooth ER
- synthesis of lipids
- metabolism of carbohydrates
- detoxification of drugs and poisons: usually involves adding hydroxyl groups to drug molecules, making them more soluble and easier to flush from the body
- storage of calcium ions
glycoprotein
A protein with one or more covalently attached carbohydrates. Most secretory proteins are glycoproteins.
functions of rough ER
- Ribosomes attached to rough ER produce secretory proteins. As a polypeptide chain grows from a bound ribosome, the chain is threaded into the ER lumen through a pore formed by a protein complex in the ER membrane. As the polypeptide enters the ER lumen, it folds into its native shape. The secretory protein departs from the ER wrapped in the membranes of vesicles that bud like bubbles from a specialized region called transitional ER.
- Membrane factory for the cell; it grows in place by adding membrane proteins and phospholipids to its own membrane.
- Produce hydrolytic enzymes (used in lysosomes) and lysosomal membrane, which are transferred to the Golgi for further processing.
Golgi apparatus
- consists of cisternae, flattened membranous sacs
- cis face: near ER; receiving department
- trans face: shipping department
- The Golgi adds molecular identification tags, such as phosphate groups. The trans face gives rise to vesicles that pinch off and travel to “docking sites,” external molecules on other organelles.
lysosome
Membranous sac of hydrolytic enzymes that an animal cell uses to digest (hydrolyze) macromolecules. Work best in the acidic environment found in lysosomes. (Ineffective in cytosol, which has neutral pH.)
phagocytosis
- Amoebas and many other protists eat by engulfing smaller organisms or food particles, a process called phagocytosis. This forms a food vacuole.
- The food vacuole fuses w/ a lysosome, whose enzymes digest the food. Digestion products pass into the cysotol and become nutrients for the cell.
autophagy
- Process whereby lysosomes use their hydrolytic enzymes to recycle the cell’s own organic material.
- During autophagy, a damaged organelle or small amount of cytosol becomes surrounded by a double membrane, and a lysosome fuses w/ the outer membrane. The lysosomal enzymes dismantle the enclosed material, and the organic monomers are returned to the cytosol for reuse.
vacuole
Large membrane-bound vesicle derived from the ER and Golgi apparatus.
contractile vacuoles
Many freshwater protists have contractile vacuoles that pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell.
central vacuole
- Mature plant cells generally contain a large central vacuole, which develops by the coalescence of smaller vacuoles.
- The solution inside the central vacuole, called cell sap, is the plant cell’s main repository of inorganic ions.
- Plays a major role in the growth of plant cells, which enlarge as the vacuole absorbs water, enabling the cell to become larger w/ a minimal investment in new cysotplasm (maintains sufficient SA:V ratio).
mitochondrion membranes
- 2 membranes, each a phospholipid bilayer w/ a unique collection of embedded proteins.
- outer membrane: smooth
- inner membrane: convoluted, with infoldings called cristae
- inner membrane divides the mitochondrion into two internal compartments: intermembrane space and mitochondrial matrix
cristae
Infoldings of the inner membrane of a mitochondrion. The inner membrane houses electron transport chains and molecules of the enzyme catalyzing the synthesis of ATP (ATP synthase). Highly folded –> enhances productivity of cellular respiration
plastid
One of a family of closely related organelles that includes chloroplasts, chromoplasts, and amyloplasts. They are found in cells of photosynthetic eukaryotes.
amyloplast
Colorless organelle that stores starch (amylose), particularly in roots and tuber.
chromoplast
Has pigments that give fruits and flowers their orange and yellow hues.
peroxisome
- Specialized metabolic compartment bounded by a single membrane.
- Contain enzymes that remove H atoms from various substrates and transfer them to oxygen (O2), thus producing hydrogen peroxide (H2O2) as a by-product.
- H2O2 is toxic, but peroxisomes contain an enzyme that converts H2O2 to water.
- The enzymes that produce H2O2 and those that dispose of it are sequestered away from other cellular components that could be damaged.
glyoxysomes
- Specialized peroxisomes found in the fat-storing tissues of plant seeds.
- Contain enzymes that initiate the conversion of fatty acids to sugar, which the emerging seedling uses as a source of energy and carbon until it can produce its own sugar by photosynthesis.
cytoskeleton
A network of microtubules, microfilaments, and intermediate filaments that extend throughout the cytoplasm and serve a variety of mechanical, transport, and signaling functions.
motor protein
A protein that interacts with cytoskeletal elements and other cell components, producing movement of the whole cell or parts of the cell.
microtubules
- Hollow tubes. Wall is constructed from a globular protein called TUBULIN. Each tubulin protein is a dimer: molecule made of 2 subunits, alpha-tubulin and beta-tubulin.
- Grow in length by adding tubulin dimers; can be disassembled and their tubulin used to build microtubules elsewhere in the cell.
- B/c of the orientation of tubulin dimers, the 2 ends of a microtubule are slightly different. One end, the “plus end”, can accumulate or release tubulin dimers at a much higher rate than the other.
flagella vs cilia
- Flagella are longer and are limited to just 1 or a few per cell.
- Flagella: undulating motion that generates force in the same direction as the flagellum’s axis
- Cilia: work like oars, w/ alternating power and recovery strokes generating force in a direction perpendicular to the cilium’s axis
primary cilium
- Membrane proteins on primary cilia transmit molecular signals from the cell’s environment to its interior, triggering signaling pathways that may lead to changes in the cell’s activities
- Only one per cell
- Nonmotile
- “9 + 0” pattern; no central 2 microtubules
motile cilia and flagella share a common structure:
- Each has a group of microtubules sheathed in an extension of the plasma membrane
- “9 + 2” pattern: 9 pairs of microtubules are arranged in a ring; in the center of the ring are 2 single microtubules
microfilaments
- Twisted double chain of ACTIN subunits; actin is a globular protein
- Can form structural networks when certain proteins bind along the side of an actin filament and allow a new filament to extend as a branch
structural role of microtubules vs microfilaments
tubules: compression-resisting
filaments: bear tension (pulling forces)
cortex
The outer region of cytoplasm in a eukaryotic cell, lying just under the plasma membrane, that has a more gel-like consistency than the inner regions due to the presence of multiple cortical microfilaments.
cytoplasmic streaming
A circular flow of cytoplasm, involving actin-myosin interactions and sol-gel transformations, that speeds the distribution of materials within cells. Especially common in large plant cells.
intermediate filaments
- Diameter: microfilaments < intermediate filaments < microtubules
- Like microfilaments, specialized for bearing tension
- Very diverse; each type is constructed from a molecular protein subunit (including the keratins)
- More permanent fixtures than microfilaments and microtubules; especially sturdy – remain even after cells die
cell wall
Protects plant cell, maintains its shape, prevents excessive water uptake. Hold the plant up against the force of gravity.
middle lamella
- Glues primary walls of adjacent cells together
- Thin layer rich in sticky polysaccharides called pectins
plant cell maturation
- Some plant cells strengthen their cell walls by secreting hardening substances into the primary wall.
- Others add a SECONDARY CELL WALL btwn the plasma membrane and the primary wall. Secondary cell wall, often deposited in several laminated layers, has a strong and durable matrix.
extracellular matrix (ECM)
The meshwork surrounding animal cells, consisting of glycoproteins, polysaccharides, and proteoglycans synthesized and secreted by the cells.
collagen
- A glycoprotein in the extracellular matrix of animal cells that forms strong fibers, found extensively in connective tissue and bone; the most abundant protein in the animal kingdom.
- In the ECM, collagen fibers are embedded in a network woven out of proteoglycans
proteoglycan
- A large molecule consisting of a small core protein with many carbohydrate chains attached, found in the extracellular matrix of animal cells. They may consist of up to 95% carbohydrate.
- Large proteoglycan complexes can form when hundreds of proteoglycan molecules become noncovalently attached to a single long polysaccharide molecule.
plasmodesmata
An open, membrane-lined channel through the cell wall that connects the cytoplasm of adjacent plant cells, allowing water, small solutes, and some larger molecules to pass between the cells. Unify the plant into one living continuum – similar to gap junctions.
cell junction
- 3 types: tight junctions, gap junctions, desmosomes.
- Especially common in epithelial tissue
tight junction
- Plasma membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins.
- Prevent leakage of extracellular fluid across a layer of epithelial cells.
desmosome
- AKA anchoring junction
- Function like rivets, fastening cells together into strong sheets
- Anchored in cytoplasm by intermediate filaments made of sturdy keratin proteins
- Attach muscle cells to each other
gap junction
- AKA communicating junctions
- Provide cytoplasmic channels from one cell to an adjacent cell – similar to plasmodesmata
- Consist of membrane proteins that surround a pore through which small molecules may pass
- Necessary for intercellular communication
fimbriae
attachment structures on the surface of some prokaryotes
capsule
jellylike outer coating of many prokaryotes
