Cells and Cellular Structures PDF

**CELLS AND CELLULAR STRUCTURES** **PARTS OF THE CELL** - **Cytoplasm** (SI¯-to¯-plasm; - plasm formed or molded) consists of all the cellular contents between the plasma membrane and the nucleus. - **Plasma Membrane** This forms the cell's flexible outer surface, separating the cell's inte...

**CELLS AND CELLULAR STRUCTURES** **PARTS OF THE CELL** - **Cytoplasm** (SI¯-to¯-plasm; - plasm formed or molded) consists of all the cellular contents between the plasma membrane and the nucleus. - **Plasma Membrane** This forms the cell's flexible outer surface, separating the cell's internal environment (inside the cell) from the external environment (outside the cell). - **Nucleus** (NOO-kle¯-us nut kernel) is a large organelle that houses most of a cell's DNA. **CYTOPLASM** - **Cytosol** (intracellular fluid) is the fluid portion of the cytoplasm that surrounds organelles and constitutes about 55% of total cell volume. **ORGANELLES** - **Cytoskeleton** The cytoskeleton is a network of protein filaments that extends throughout the cytosol. Three types of filamentous proteins contribute to the cytoskeleton's structure, as well as the structure of other organelles.  **TYPES OF CYTOSKELETONS** 1. **Microtubules** 2. **Microfilaments** 3. **Intermediate Filaments** **Microfilaments** - These are the thinnest elements of the cytoskeleton. They are composed of the protein actin, and are most prevalent at the edge of a cell. **Intermediate Filaments** - These filaments are thicker than microfilaments but thinner than microtubules **Microtubules** - These are the largest of the cytoskeletal components and are long, unbranched hollow tubes composed mainly of the protein tubulin. **ORGANELLES** - **Centrosome** The centrosome, located near the nucleus, consists of two components: a pair of centrioles and pericentriolar material. **Cilia** - Cilia are numerous, short, hairlike projections that extend from the surface of the cell. **Flagella** - Flagella are similar in structure to cilia but are typically much longer. Flagella usually move an entire cell.  **Ribosomes** - Ribosomes are the sites of protein synthesis. **Endoplasmic reticulum** - (en-do¯- PLAS-mik re-TIK-u¯-lum; -plasmic cytoplasm; reticulum network) or ER is a network of membranes in the form of flattened sacs or tubules  **Golgi complex** - The first step in the transport pathway is through an organelle called the Golgi complex. - It consists **of 3 to 20 cisternae**, small, flattened membranous sacs with bulging edges that resemble a stack of pita bread **Lysosomes** - Lysosomes are membrane- enclosed vesicles that form from the Golgi complex. -  **Proteasomes** - Lysosomes degrade proteins delivered to them in vesicles. Cytosolic proteins also require disposal at certain times in the life of a cell. - Continuous destruction of unneeded, damaged, or faulty proteins is the function of tiny barrel-shaped structures consisting of four stacked rings of proteins around a central core called *proteasomes*. **Mitochondria** - Because they generate most of the ATP through aerobic (oxygen-requiring) respiration, - Mitochondria are referred to as the "powerhouses" of the cell. **PLAMSMA MEMBRANE** - The *plasma membrane*, a flexible yet sturdy barrier that surrounds and contains the cytoplasm of a cell, is best described by using a structural model called the fluid mosaic model. - The basic structural framework of the plasma membrane is the lipid bilayer, two back-to-back layers made up of three types of Lipid Molecules: **phospholipids, glycolipids. Cholesterol** - The bilayer arrangement occurs because the lipids are amphipathic molecules, which means that they have both polar and nonpolar parts. - In phospholipids, the polar part is the phosphate containing "head," which is hydrophilic (hydro- water; -philic loving). The nonpolar parts are the two long fatty acid "tails," which are hydrophobic (-phobic fearing) hydrocarbon chains. **MEMBRANE PROTEINS** **Integral proteins** extend into or through the lipid bilayer among the fatty acid tails and are firmly embedded in it. **Most integral proteins are *transmembrane proteins*,** which means that they span the entire lipid bilayer and protrude into both the cytosol and extracellular fluid. ***Peripheral proteins*** are not as firmly embedded in the membrane. They associate more loosely with the polar heads of membrane lipids or with integral proteins at the inner or outer surface of the membrane.  **MEMBRANE PERMEABILITY** - The term permeable means that a structure permits the passage of substances through it, while impermeable means that a structure does not permit the passage of substances through it. - The permeability of the plasma membrane to different substances varies. Plasma membranes permit some substances to pass more readily than others. This property of membranes is termed selective permeability. **NUCLEUS** - The *nucleus* is a spherical or oval-shaped structure that usually is the most prominent feature of a cell. - A double membrane called the *nuclear envelope* separates the nucleus from the cytoplasm. - Many openings called *nuclear pores* extend through the nuclear envelope. Each nuclear pore consists of a circular arrangement of proteins surrounding a large central opening that is about 10 times wider than the pore of a channel protein in the plasma membrane. - Inside the nucleus are one or more spherical bodies called *nucleoli* that function in producing ribosomes. - In cells that are not dividing, the chromatin appears as a diffuse, granular mass. - Electron micrographs reveal that chromatin has a *beads-on-a-string structure*. - Each bead is a nucleosome and consists of double-stranded DNA wrapped twice around a core of eight proteins called *histones*, which help organize the coiling and folding of DNA. - The string between the beads is *linker DNA*, which holds adjacent nucleosomes together. - In cells that are not dividing, another histone promotes coiling of nucleosomes into a larger-diameter chromatin fiber, which then folds into large loops. - Just before cell division takes place, however, the DNA replicates (duplicates) and the loops condense even more, forming a *pair of chromatids*. - Within the nucleus are most of the cell's hereditary units, called *genes*, which control cellular structure and direct cellular activities. - Genes are arranged along chromosomes. - Each chromosome is a long molecule of DNA that is coiled together with several proteins. - This complex of DNA, proteins, and some RNA is called chromatin.