Chapter 3: Anatomy of Cells

FUNCTIONAL ANATOMY OF CELLS The typical cell (Figure 3-1) Also called composite cell Vary in size; all are microscopic (Table 3-1) Vary in structure and function (Table 3- 2)

FUNCTIONAL ANATOMY OF CELLS (cont.) Cell structures Plasma membrane: separates the cell from its surrounding environment Cytoplasm: thick, gel-like substance inside the cell composed of numerous organelles suspended in watery cytosol; each type of organelle is suited to perform particular functions (Figure 3-2) Nucleus: large membranous structure near the center of the cell

CELL MEMBRANES Each cell contains a variety of membranes Plasma membrane: outer boundary of cell (Figure 3-3) Membranous organelles: sacs and canals made of the same material as the plasma membrane

CELL MEMBRANES (cont.) Fluid mosaic model: theory explaining how cell membranes are constructed Molecules of the cell membrane are arranged in a sheet The mosaic of molecules is fluid; that is, the molecules are able to float around slowly This model illustrates that the molecules of the cell membrane form a continuous sheet Chemical attraction is the force that holds membranes together

CELL MEMBRANES (cont.) Groupings of membrane molecules form rafts that float as a unit in the membrane (Figure 3-4) Rafts may pinch inward to bring material into the cell or organelle Primary structure of a cell membrane is a double layer of phospholipid molecules Heads are hydrophilic (“water loving”) Tails are hydrophobic (“water fearing”) Arrange themselves in bilayers in water Cholesterol molecules are scattered among the phospholipids to allow the membrane to function properly at body temperature Most of the bilayer is hydrophobic; therefore water and water-soluble molecules do not pass through easily

CELL MEMBRANES (cont.) Membrane proteins (Table 3-4) A cell controls what moves through the membrane by membrane proteins embedded in the phospholipid bilayer Some membrane proteins have carbohydrates attached to them and, as a result, form glycoproteins that act as identification markers Some membrane proteins are receptors that react to specific chemicals, sometimes permitting a process called signal transduction

CYTOPLASM AND ORGANELLES Cytoplasm: gel-like internal substance of cells that includes many organelles suspended in watery intracellular fluid called cytosol Two major groups of organelles (Table 3-3) Membranous organelles are sacs or canals made of cell membranes Nonmembranous organelles are made of microscopic filaments or other nonmembranous materials

CYTOPLASM AND ORGANELLES (cont.) Endoplasmic reticulum (Figure 3-5) Made of membranous, walled canals and flat, curving sacs arranged in parallel rows throughout the cytoplasm; extend from the plasma membrane to the nucleus Proteins move through the canals

CYTOPLASM AND ORGANELLES (cont.) Two types of endoplasmic reticulum Rough endoplasmic reticulum Ribosomes dot the outer surface of the membranous walls Ribosomes synthesize proteins, which move toward the Golgi apparatus and then eventually leave the cell Function in protein synthesis and intracellular transportation Smooth endoplasmic reticulum No ribosomes border the membranous wall Functions are less well established and probably more varied than those of the rough endoplasmic reticulum Synthesizes certain lipids and carbohydrates and creates membranes for use throughout the cell Removes and stores calcium ions from the cell’s interior

CYTOPLASM AND ORGANELLES (cont.) Ribosomes (Figure 3-6) Many are attached to the rough endoplasmic reticulum and many lie free, scattered throughout the cytoplasm Each ribosome is a nonmembranous structure made of two pieces, a large subunit and a small subunit; each subunit is composed of rRNA and protein Ribosomes in the endoplasmic reticulum make proteins for “export,” or to be embedded in the plasma membrane; free ribosomes make proteins for the cell’s domestic use

CYTOPLASM AND ORGANELLES (cont.) Golgi apparatus Membranous organelle consisting of cisternae stacked on one another and located near the nucleus (Figure 3-7) Processes protein molecules from the endoplasmic reticulum (Figure 3-8) Processed proteins leave the final cisterna in a vesicle; contents may then be secreted to outside the cell

CYTOPLASM AND ORGANELLES (cont.) Lysosomes (Figure 3-9) Made of microscopic membranous sacs that have “pinched off” from Golgi apparatus The cell’s own digestive system; enzymes in lysosomes digest the protein structures of defective cell parts, including plasma membrane proteins, and particles that have become trapped in the cell

CYTOPLASM AND ORGANELLES (cont.) Proteasomes (Figure 3-10) Hollow protein cylinders found throughout the cytoplasm Break down abnormal or misfolded proteins and normal proteins no longer needed by the cell (and that may cause disease) Break down protein molecules one at a time by tagging each one with a chain of ubiquitin molecules, unfolding the protein as it enters the proteasome, and then breaking apart peptide bonds

CYTOPLASM AND ORGANELLES (cont.) Peroxisomes Small membranous sacs containing enzymes that detoxify harmful substances that enter the cells Often seen in kidney and liver cells

CYTOPLASM AND ORGANELLES (cont.) Mitochondria (Figure 3-11) Composed of microscopic sacs; wall composed of inner and outer membranes separated by fluid; thousands of particles make up enzyme molecules attached to both membranes The “power plants” of cells; mitochondrial enzymes catalyze series of oxidation reactions that provide most of a cell’s energy supply Each mitochondrion has a DNA molecule, which allows it to produce its own enzymes and replicate copies of itself

NUCLEUS Definition: spherical body in center of cell enclosed by an envelope with many pores Structure (Figure 3-12) Consists of a nuclear envelope (made of two membranes, each with essentially the same molecular structure as the plasma membrane) surrounding nucleoplasm The nuclear envelope has holes called nuclear pores Nuclear pore complexes are elaborate gateways in and out of the nucleus (Figure 3-13)

NUCLEUS (cont.) Structure (cont.) Contains DNA (heredity molecules), which appear as: Chromatin threads or granules in nondividing cells Chromosomes in early stages of cell division Functions of the nucleus are functions of DNA molecules; DNA determines the structure and function of cells as well as heredity

CYTOSKELETON The cell’s internal supporting framework; made of rigid, rodlike pieces that provide support and allow movement and mechanisms that can move the cell or its parts (Figure 3-14)

CYTOSKELETON (cont.) Cell fibers Intricately arranged fibers of varying length that form a three-dimensional, irregularly shaped lattice Fibers appear to support the endoplasmic reticulum, mitochondria, and “free” ribosomes Microfilaments: smallest cell fibers (Figure 3-15) Serve as “cellular muscles” Made of thin, twisted strands of protein molecules that lie parallel to the long axis of the cell Can slide past each other and cause shortening of the cell

CYTOSKELETON (cont.) Intermediate filaments: twisted protein strands slightly thicker than microfilaments; form much of the supporting framework in many types of cells Microtubules: tiny, hollow tubes that are the thickest of the cell fibers Made of protein subunits arranged in a spiral fashion Function to move things around inside the cell

CYTOSKELETON (cont.) Centrosome (Figure 3-16) An area of the cytoplasm near the nucleus that coordinates the building and breaking apart of microtubules in the cell Nonmembranous structure also called the microtubule organizing center Plays an important role during cell division General location of the centrosome is identified by the centrioles

CYTOSKELETON (cont.) Molecular motors Motor proteins include dynein, myosin, and kinesin (Figure 3-17) Molecular motors can pull larger structures along microtubules and microfilaments as if along a track, providing intracellular transport and movements of the entire cell

CYTOSKELETON (cont.) Cell extensions Cytoskeleton forms projections that extend the plasma membrane outward to form tiny, fingerlike processes Three types of these processes; each has specific functions (Figure 3-18)

CYTOSKELETON (cont.) Microvilli: found in epithelial cells that line the intestines and other areas where absorption is important; help increase the surface area manyfold Cilia and flagella: cell processes that have cylinders made of microtubules and molecular motors at their core (Figure 3-19) Cilia are shorter and more numerous than flagella; cilia have coordinated oarlike movements that brush material past the cell’s surface Flagella are found only on human sperm cells; flagella move with a tail-like movement that propels the sperm cell forward

CELL CONNECTIONS Cells are held together by fibrous nets that surround groups of cells (e.g., muscle cells), or cells have direct connections to each other Three types of direct cell connections (Figure 3-20)

CELL CONNECTIONS: DIRECT Desmosome Fibers on the outer surface of each desmosome interlock with each other; anchored internally by intermediate filaments of the cytoskeleton Spot desmosomes are like “spot welds” at various points connecting adjacent membranes Belt desmosomes encircle the entire cell Gap junctions: membrane channels of adjacent plasma membranes adhere to each other; have two effects Form gaps or “tunnels” that join the cytoplasm of two cells Fuse two plasma membranes into a single structure Tight junctions Occur in cells that are joined by “collars” of tightly fused material Molecules cannot permeate the cracks of tight junctions Occur in the lining of the intestines and other parts of the body where controlling what gets through a sheet of cells is important

Key points for chapter 3 There are several intricate parts that work together in a cell to sustain life.