Cell Structure & Function
A cell is a living unit greater than the sum of its parts.
A. Prokaryotic Cell
1. Does not have a nucleus
2. Does not have membrane bound organelles.
3. Small in size.
B. Eukaryotic Animal Cell
1. Has nucleus
2. Large cells
3. Animal, Plant, Fungi, Protist
4. Have many membrane bound organelles
Ex. Animal Cell
a. plasma membrane- selectively semipermeable barrier which defines the periphery of the cell. Composed of amphipathic phospholipids
a. Cell Membrane- a selective semipermeable membrane that surrounds the cell It is composed of amphipathic phospholipids bilayer. Separates the intracellular and extracellular environments.
b. Cytosol- The intracellular area of the cell
c. Nucleus- This organelle is bound by a double membrane (nuclear envelope). It houses the Chromosomes (DNA genes). Chromosomes are only visible when the cell gets ready to divide. The remainder of the time the DNA is in the form of Chromatin. The nucleus is the site of DNA Replication and translation into messenger RNA (mRNA).
d. Nucleolus- organelle within the nucleus. Ribosomal RNA (rRNA) is synthesized and assembled with proteins from the cytoplasm to form ribosomal subunits within the nucleolus. The subunits pass from the nuclear pores to the cytoplasm where they combine to form ribosomes.
e. Ribosomes- Organelles used for protein synthesis. Composed of rRNA and proteins. Found in prokaryotes and eukaryotes.
•Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol.
•Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic reticulum. These synthesize proteins that are either included into membranes or for export from the cell.
f. Endoplasmic Reticulum (ER)- The ER is part of the endomembrane system which includes: the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.
•The ER includes membranous tubules and internal, fluid-filled spaces, the cisternae.
•The ER membrane is continuous with the nuclear envelope and the cisternal space of the ER is continuous with the space between the two membranes of the nuclear envelope.
• Smooth ER looks smooth because it lacks ribosome. Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.
•Rough ER looks rough because ribosomes (bound ribosomes) are attached to the outside, including the outside of the nuclear envelope.
As a polypeptide is synthesized by the ribosome, it is threaded into the cisternal space through a pore formed by a protein in the ER membrane.
g. Golgi Body- The Golgi is a center of manufacturing, warehousing, sorting, and shipping. Many transport vesicles from the ER travel to the Golgi for modification of their contents.
•The Golgi apparatus consists of flattened membranous sacs - cisternae - looking like a sac of pita bread.
•The membrane of each cisterna separates its internal space from the cytosol
•One side of the Golgi, the cis side, receives material by fusing with vesicles, while the other side, the trans side, buds off vesicles that travel to other sites.
h. Lysosomes- a membrane-bounded sac of hydrolytic enzymes that digests macromolecules.
•Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids. These enzymes work best at pH 5. This is a space where the cell can digest macromolecules safely.
•Lysosomes can fuse with food vacuoles, formed when a food item is brought into the cell by phagocytosis.
•Lysosomes can also fuse with another organelle or part of the cytosol. This recycling, this process of autophagy renews the cell.
i. Vesicles and vacuoles (larger versions) are membrane-bound sacs with varied functions.
•Food vacuoles, from phagocytosis, fuse with lysosomes.
•Contractile vacuoles, found in freshwater protists, pump excess water out of the cell.
The endomembrane system plays a key role in the synthesis (and hydrolysis) of macromolecules in the cell and their transport.
J. Mitochondria are the sites of intracellular respiration, generating ATP from the catabolism of sugars, fats, and other fuels in the presence of oxygen.
• have small quantities of DNA that direct the synthesis of the polypeptides produced internal ribosomes. Mitochondria grow and reproduce as semiautonomous organelles. (Endosymbiotic)
•Mitochondria have a smooth outer membrane and a highly folded inner membrane, the cristae. The cristae present ample surface area for the enzymes that synthesize ATP during the Electron Transport Chain.
•The inner membrane encloses the mitochondrial matrix, a fluid-filled space with DNA, ribosomes, and enzymes. Here is where the krebs cycle of Cellular Respiration occurs.
• Mitochondria are mobile and move around the cell along tracks in the cytoskeleton.
k. cytoskeleton is a network of fibers extending throughout the cytoplasm.
•The cytoskeleton provides mechanical support and maintains shape of the cell. There are three main types of fibers in the cytoskeleton: microtubules, microfilaments, and intermediate filaments.
•The cytoskeleton provides anchorage for many organelles and cytosolic enzymes.
•The cytoskeleton is dynamic, dismantling in one part and reassembling in another to change cell shape.
•The cytoskeleton also plays a major role in cell motility.
l. Extracellular matrix (ECM), found in Animals cells, consits of glycoproteins, especially collagen fibers, embedded in a network of proteoglycans.
•In many cells, fibronectins in the ECM connect to integrins, intrinsic membrane proteins.
•The integrins connect the ECM to the cytoskeleton.
•The interconnections from the ECM to the cytoskeleton via the fibronectin-integrin link permit the interaction of changes inside and outside the cell.
•The ECM can regulate cell behavior. This may coordinate all the cells within a tissue. The extracellular matrix can influence the activity of genes in the nucleus via a combination of chemical and mechanical signaling pathways.
•Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact.
•Animal have 3 main types of intercellular links: tight junctions, desmosomes, and gap junctions.
•In tight junctions, membranes of adjacent cells are fused, forming continuous belts around cells.This prevents leakage of extracellular fluid.
•Desmosomes (or anchoring junctions) fasten cells together into strong sheets, much like rivets. Intermediate filaments of keratin reinforce desmosomes.
•Gap junctions (or communicating junctions) provide cytoplasmic channels between adjacent cells.