Chapter 3 Anatomy of Cells
Introduction
•In the 1830s, two German scientists, Matthais Schleiden and Theodore Schwann, advanced one of the most important and unifying concepts in biology- the cell theory
•It states simply that the cell is the fundamental organizational unit of life
•They were the first to suggest that all living things are composed of cells
Functional Anatomy of Cells
•The typical cell (Figure 3-1)
Also called composite cell
Varies in size; all are microscopic
Varies in structure and function
Functional Anatomy of Cells
•Cell structures
Plasma membrane—separates the cell from its surrounding environment
Cytoplasm—thick gel-like substance inside of 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 (Figure 3-3), functions:
•Selective barrier, maintains cells integrity
•Serves as markers that self-identify each individual cell
•Molecule receptor site for certain hormones and other molecules
•Transport mechinism
Membranous organelles—sacs and canals made of the same material as the plasma membrane
•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
•Primary structure of a cell membrane is a double layer of phospholipid molecules
Heads are hydrophilic (water-loving)
Tails are hydrophobic (water-fearing)
Molecules 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 or water-soluble molecules do not pass through easily
•Chemical attractions are the forces that hold membranes together
Groupings of membrane molecules form rafts, each of which float as a unit in the membrane (Figure 3-4)
Rafts help organize the various components of a membrane
Rafts may pinch inward, bringing material into the cell or organelle
•Membrane proteins (Table 3-4)
A cell controls what moves through the membrane by means of membrane proteins embedded in the phospholipid bilayer
Some membrane proteins have carbohydrates attached to them, forming 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 specialized sacs or canals made of cell membranes
Nonmembranous organelles are made of microscopic filaments or other nonmembranous materials
•Endoplasmic reticulum (Figure 3-5)
Made of canals with membranous walls and flat, curving sacs arranged in parallel rows throughout the cytoplasm; extend from the plasma membrane to the nucleus
Proteins move through the canals
Two types of endoplasmic reticulum:
•Rough endoplasmic reticulum
Ribosomes dot the outer surface of the membranous walls giving it the “rough” appearance
Ribosomes synthesize proteins, which move toward the Golgi apparatus and then eventually leave the cell
Function in protein synthesis and intracellular transportation
Cytoplasm and Organelles
Two types of endoplasmic reticulum (cont.)
•Smooth endoplasmic reticulum
No ribosomes border membranous wall
Functions are less well established and probably more varied than for rough endoplasmic reticulum
Synthesizes certain lipids and carbohydrates and creates membranes for use throughout cell
Removes and stores Ca++ (calcium ions) from cell’s interior.
•Ribosomes (Figure 3-6)
Many are attached to the rough endoplasmic reticulum and many lie free, scattered through 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 (ribosomal RNA)
Their function is protein synthesis
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
•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
•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
•Proteasomes (Figure 3-10)
Hollow, protein cylinders found throughout the cytoplasm
Break down abnormal/misfolded proteins and normal proteins no longer needed by the cell
Break down protein molecules one at a time by tagging each one with a chain of ubiquitin molecules and unfolding it as it enters the proteasome, then breaking apart peptide bonds
Cytoplasm and Organelles
•Peroxisomes
Small membranous sacs containing enzymes that detoxify harmful substances that enter the cells
Often seen in kidney and liver cells
•Mitochondria (Figure 3-11)
Made up 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 about 95% of cell’s energy supply
Each mitochondrion has a DNA molecule, allowing 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
•One of the largest cell structures
•Structure
Consists of nuclear envelope (composed of two membranes each with essentially the same molecular structure as plasma membrane) surrounding nucleoplasm; nuclear envelope has holes called nuclear pores (Figure 3-12)
Contains DNA (heredity molecules), which appear as the following:
•Chromatin threads or granules in nondividing cells
•Chromosomes in early stages of cell division
•Functions of nucleus are functions of DNA molecules; DNA determines both structure and function of cells and heredity
Cytoskeleton
•The cell’s internal supporting framework made up of rigid, rod-like pieces that provide support and allow movement and mechanisms that can move the cell or its parts (Figure 3-13)
•Cell fibers
Intricately arranged fibers of varying lengths that form a three-dimensional, irregularly shaped lattice
Fibers appear to support the endoplasmic reticulum, mitochondria, and “free” ribosomes
Cytoskeleton
•Cell fibers (cont.)
Microfilaments (Figure 3-14)
•Smallest cell fibers
•Often serve as “Cellular muscles”
•Made of thin, twisted strands of protein molecules that lie parallel to the long axis of the cell
•Microfilaments can slide past each other, causing shortening of the cell; occurs most in muscle cells
Intermediate filaments
•Slightly thicker than microfilaments
•Form much of the supporting framework in many types of cells; such as the outer layer of skin
•Microtubules
Thickest of cell fibers
Made up of protein subunits arranged in a spiral fashion
Called “engines” because they move things around in a cell or even the entire cell
•Centrosome
An area of the cytoplasm near the nucleus that coordinates the building and breaking of microtubules in the cell
Nonmembranous structure also called the microtubule-organizing center (MTOC)
Plays an important role during cell division
The general location of the centrosome is identified by the centrioles
•Cell extensions
Cytoskeleton forms projections that extend the plasma membrane outward to form tiny, fingerlike processes
There are three types of these processes; each has specific functions (Figure 3-15):
•Microvilli—found in epithelial cells that line the intestines and other areas where absorption is important; they help to increase the surface area manyfold
•Cilia and flagella—cell processes that have cylinders made of microtubules at their core; cilia are shorter and more numerous than flagella; flagella are found only on human sperm cells
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
•There are three types of direct cell connections (Figure 3-16)
•Desmosome
Fibers on the outer surface of each desmosome interlock with each other; anchored internally by intermediate filaments of the cytoskeleton
Spot desmosomes, connecting adjacent membranes, are like “spot welds” at various points
Belt desmosomes encircle the entire cell like a collar
•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 it is important to control what gets through a sheet of cells