Biology 2010 Lecture Notes

Unit 2. Cell Structure

This page was last updated September 5, 2006

Some key words and phrases:

• organization, metabolism, reproduction, responsiveness
• structural diversity vs metabolic diversity; autotroph vs heterotroph
• microscopy, resolution, magnification, micrometer vs nanometer; cell fractionation, centrifugation, electrophoresis
• cell, multicellular, unicellular, non-cellular; prokaryote cell, eukaryote cell, virus, viroid
• subcellular structures: cell wall, cell membranes, ribosome, endomembrane system, plastid, mitochondrion, cytoskeleton, nucleus, chromosome
• macromolecules: DNA, RNA, protein (histone, actin, myosin, tubulin), phospholipid, polysaccharide, lipopolysaccharide, peptidoglycan
• autogenous origin vs endosymbiotic origin

1. The unity and diversity of life--What are living things? How do they differ from non-living structures?
1. The basic characteristics of living things -- what properties do all organisms share
1. organization
2. metabolism - organisms process chemicals and energy to maintain their structure, create new structures
3. adaptability and responsiveness - organisms can respond to changes in their environment is such a way that a balance (homeostasis) is maintained
4. reproduction - usually with mistakes that can be passed on to subsequent generations; this ability
2. The diversity of life -- over 1 million species of organism have been described, at least 1 million, and possibly another 30 million, species are waiting to be discovered; these have been divided into a number of kingdoms on the basis of structure and metabolism
1. Plantae, Animalia, Fungi -- organisms in these kingdoms have complex, usually macroscopic, structures; plants are capable of photosynthesis, fungi and animals are heterotrophs; plants and fungi have cell walls and reproduce by means of spores, while animals don't have cell walls or spores
2. Protista, Monera (bacteria) -- organisms in these kingdoms have much simpler, usually microscopic, structures; protistans, like plants, animals, and fungi, have eukaryotic cells, bacteria have prokaryotic cells
3. Viruses -- biologists are undecided about whether viruses are alive or not; their structure is so simple that they don't even have cells; they use the metabolic machinery of other organisms to carry out the functions of living things
4. Clay?!? -- a few researchers (notably Cairns-Smith) studying the origin of life have postulated that some clays may possess the fundamental properties of life
5. Aliens from other planets???
3. The cell theory of life
1. humans have long known that animals contain lots of organs and organ systems
2. closer observations, beginning three hundred years ago, revealed that organs are composed of smaller units
1. van Leeuwenhoek (1665) discovered microscopic life forms
2. Hooke discovered that cork was composed of small chambers or cells
3. additional observations led Schleiden, Schwann, and Virchow to propose the cell theory of life
1. all living things are composed of cells
2. cells represent some sort of basic unit of life
1. site of metabolism
2. smaller objects exist, but do not reproduce
3. cells come from pre-existing cells -- cells don't form spontaneously; this was later demonstrated fairly conclusively by Pasteur
2. Basic tools for studying cell structure
1. the implication of the cell theory of life is that cells are important, and that understanding cell structure and function is fundamental to biology; unfortunately, cells are microscopic and require special tools and techniques; a few of these are mentioned below
2. microscopy
1. purpose is to magnify small objects so that details can be seen
2. twin problems
1. how much can the object be magnified--depends on the arrangement of lenses
2. how much detail can be resolved (distinguished)--depends on the size of the particles used
3. basic microscopes
1. light microscopes use visible light and glass lenses; can magnify well, but the resolution is limited by the size of the photons used--visible photons have wavelengths ranging from 0.4 to 0.7 micrometers (400 to 700 nanometers), so the limit of resolution is 0.2 micrometers, about the size of a small bacterium; on the plus side light microscopes are small, relatively inexpensive and easy to use
2. electron microscopes use electrons and magnetic lenses; can magnify well; in addition, the size of the electron is on the order of 0.2 nanometers so the resolution of an electron microscope is about 0.1 nanometers, a thousand times better than that of a light microcope; unfortunately, electron microscopes are big and expensive and the sample preparation is long and laborious--don't count on using one in this class
3. cell fractionation
1. purpose is to separate the components of the cell so that they can be analyzed chemically and their function determined outside of the cell
2. two processes are involved
1. cell disruption to liberate the components without damaging them; a number of techniques can be used, from mechanical grinding to chemical digestion
2. cell fractionation to separate the results of cell disruption; again a number of techniques are available, depending on the size of the object
1. differential centrifugation separates pieces by "mass"; used primarily to separate organelles and large fragments
2. electrophoresis separates pieces by charge and "mass"; used primarily with large molecules (proteins, DNA)
3. chromatography separates pieces by solubility and "mass"; used primarily with smaller molecules
3. Cellular structures
1. basic features of all cells
1. from a structural point of view
1. cell membrane
2. cytoplasm
3. genetic material
2. from a functional point of view
1. boundary
2. metabolic machinery
3. genetic machinery
2. initially, biologist recognized two basic types of cells, prokaryotic cells and eukaryotic cells; in the last 20years, third group has been added os we now recognize three domains of cellular life: eubacteria, archaea, and eukarya; understanding the differences between these cell types not only provides us with information concerning the origins and diversity of life, but also helps us develop treatments for different types of infectious diseases
3. features of eubacterial cells
1. usually small, with simple morphologies and simple internal structure
   
2. cell boundary
1. cell membrane with phospholipids and proteins
2. layer of peptidoglycan that helps keep the cell from exploding (this layer is not formed properly in the presence of penicillin)
3. an outer membrane with lipopolysaccharides; some of the lipopolysaccharides are toxic to humans
3. cytoplasm with ribosomes, but usually with few other distinguishable structures; you should be aware that exceptions exist--some of the chemolithotrophs and the cyanobacteria have a complex internal membrane system
4. nucleoid with genetic material in the form of a single circular molecule of DNA, usually without lots of associated proteins; this molecule of DNA floats free in the cytoplasm
4. features of archaeal cells
1. very similar to eubacteria in size, morphology, and internal structure
2. differ in
   
1. cell walls do not contain peptidoglycan; use some other complex polymer instead
2. cell membranes contain branched hydrocarbons
3. ribosomes not inhibited by chloramphenicol
4. the circular molecules of  DNA have associated histones
   
5. features of eukaryotic cells
1. nucleus
1. about 5 micrometers in diameter
2. surrounded by a nuclear envelope consisting of two membranes with pores and a protein layer (the nuclear lamina) just below the inner membrane
3. contains chromatin
1. genetic material - several molecules of DNA associated with special proteins called histones
2. the chromatin condenses during mitosis and meiosis to form chromosomes
4. also a region called the nucleolus, now known to be the site of ribosome production and RNA processing
2. cytoplasmic structures
1. ribosomes
1. small structures of RNA and protein that make other proteins
1. similar structures are found in prokaryote cells, but the prokaryote version is smaller
2. come in 2 parts: the large and small subunits
3. may be attached or free
2. the endomembrane system
1. endoplasmic reticulum - system of tubes and sacs (cisternae)
1. two distinct regions
1. smooth ER, that functions in lipid & carbohydrate metabolism, alcohol detoxification (more alcohol, more smooth ER), steroid production, calcium ion storage
2. rough ER, with bound ribosomes, that functions in protein synthesis (secretion) & membrane production
2. transport vesicles that carry material to the Golgi apparatus
3. the Golgi apparatus, a series of flattened sacs that performs the final preparation of materials from the rough ER, and processing of material for export
1. two-sided--transport vesicles fuse to form the cis side of the apparatus; new vesicles with processed material bleb off of the trans side
4. new transport vesicles carry the processed material to its final destination
3. special vesicle-like compartments
1. lysosomes contain enzymes that digest large molecules (intracellular digestion after phagocytosis)
2. vacuoles
1. food vacuoles
2. contractile vacuoles
3. central vacuole of plants
1. bounded by tonoplast
2. functions include storage of water soluble materials, maintenance of cell turgidity
3. peroxisomes contain enzymes that add hydrogen to oxygen, creating hydrogen peroxide as a by-product; the toxic hydrogen peroxide is converted to water by other enzymes; peroxisomes are not considered to be part of the endomembrane system
1. example: glyoxysomes oils stored in seeds to provide energy for germination
4. mitochondria
1. special structures bound by two membranes and containing DNA and ribosomes; both the DNA and the ribosomes are similar to prokaryote versions
1. inner membrane folded into cristae
2. functions in the conversion of pyruvate energy to ATP energy
5. plastids (only in plants)
1. special structures bounded by two membranes and containing DNA and ribosomes; the DNA and ribosomes are very similar to prokaryote versions
2. usually contain a complex system of membranes referred to as the thylakoids; these contain chlorophyll and are used in energy capture
3. the stoma contains enzymes for glucose and starch production
4. note that many varieties of plastids can occur in a single plant
1. chloroplasts for photosynthesis
2. chromoplasts with carotenoid pigments for color
3. amyloplasts (leucoplasts) for starch storage
6. the cytoskeleton
1. microtubules
1. hollow rods 25 nm in diameter
2. composed of the protein tubulin; tubulin is composed of two subunits
3. often seem to radiate from a centrosome(=microtubule organizing center (MTOC)) near the nucleus; centrosomes of animal cells contain two centrioles (each equals

basal body of flagellum), each of 9 triplets of microtubules; plant centrosomes don't have centrioles in center
4. functions
1. basic shape of cell, hold parts in place, move by pushing and pulling (spindle)
2. forms a railroad in conjunction with motor molecules such as dynein
3. also basis of flagella & cilia
1. have 9+2 skeleton with side-arms of dynein between pairs for movement
2. microfilaments
1. actin (& myosin) rods about 7-8 nanometers in diameter
2. function in support, intracellular movement (cytoplasmic streaming), and the transition from sol to gel in seen in pseudopodia
3. intermediate filaments
1. diverse group of protein filaments about 8 to 10 nanometers in diameter; proteins involved usually belong to the family of proteins called keratins
2. function in cell structure (nuclear lamina, for example)
3. cell surface
1. cell wall
1. absent in animal cells, but found in most other eukaryotes
1. animals have an extracellular matrix of glycoproteins including collagen and fibronectins that connect to internal proteins (microfilaments) through membrane-bound proteins (integrins)
2. composition varies, depending on the phylum of the organisms
1. in plants consists of cellulose fibers embedded in polysaccharide matrix in several layers
1. outside is a thin and flexible primary wall joined to the primary walls of other cells by a middle lamella of pectins
2. inside of the primary wall and formed later is a thicker, more durable, secondary wall
2. in fungi, the wall contains lots of chitin
2. cell membrane inside of the cell wall and attached to the cytoplasm
1. similar in structure to the cell membrane of prokaryotes (phospholipids and proteins)
2. responsible for junctions with neighboring cells
1. plasmodesmata--complete cytoplasmic union of two cells; especially common in multicellular plants
2. tight junctions--complete protein belts around cells, firmly gluing two cells together
3. desmosomes--protein rivets or buttons between cells; attach interiorly to the intermediate filaments
4. gap junctions--protein gates that allow direct communication between cytoplasms when appropriate
4. Origin and diversity of cells
1. first cellular organisms appear about 3,500 million years ago
1. exact structure unknown, but assumed to be simple
2. how cells first developed a matter of speculation and debate
2. first cells went through a period of relatively quick evolution leading to a number of different cell lines; three major lines of evolution can still be seen
1. Eubacteria--typical prokaryotic structures described above
2. Archaea--another group of prokaryotes, distinguished most easily from Eubacteria by differences in ribosome structure; other differences exist, including differences in cell walls, cell membranes lipids, and metabolic machinery
3. Proto-Eukaryotes--precursors to eukaryotes; eventually with a well-developed endomembrane system and nucleus, but lacking mitochondria and plastids
3. long period of evolution, primarily of metabolic pathways, but with parallel evolution of subcellular structures--this is when the endomembrane system, cytoskeleton, and nucleus of eukaryotes developed, as well as the internal membranes in advanced eubacteria and archaea; these developed autogenously
1. flagella?
2. different types of endomembrane systems?
4. development of endosymbiotic relationships between some eubacteria and some proto-eukaryotes, beginning about 2,500 million years ago
1. possibly triggered by the evolution of oxygen-producing photosynthesis in the eubacteria
2. aerobic bacteria were engulfed by anaerobic proto-eukaryotes and gradually evolved into the mitochondria of modern eukaryotes
3. oxygen-producing photosynthetic eubacteria were engulfed and gradually evolved into the plastids of modern photosynthetic eukaryotes
4. these endosymbiotic events may have occurred many times with different eukaryote hosts and bacterial symbionts, creating some of the diversity of eukaryote cell-lines seen today
5. Non-cellular life-forms
1. virus particles
1. simple structure
1. genetic material of either RNA or DNA
2. protein capsid
3. in some forms a phospholipid envelope
2. metabolic machinery almost completely absent, must make use of a host cell's metabolic machinery (ribosomes, etc.) to reproduce
3. some disagreement about whether virus particles are alive or not
2. viroids
1. very simple structure (naked RNA), but capable of causing formation of more viroids
2. alive?
3. prions
1. simple proteins capable of converting other proteins into infectious prions
2. below the cutoff between living and not living

Continue with next unit
Return to Dr. Nienow's HomePage