All About Cells

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CELL TYPES

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Cells •Cells are the structural and functional units of life •Discovered by Robert Hooke (1635–1703) in ca. 1665 using a primitive microscope •He observed cork cells and coined the term “cells” for the building blocks of plants that he observed •This started the whole era of using microscopes to study biology

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Outline of Cells Cell Theory Eukaryotes vs. prokaryotes Cell structures Tissues Mitosis

Bo Modern Cell Theory •All living organisms are composed of one or more cells •The smallest life forms capable of self-replication •The chemical reactions of a cell take place within the protoplasm •Cells arise from already existing cells •Cells contain hereditary information that is passed on to the daughter cells

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•Cells or organisms divided into two basic groups, prokaryotes and eukaryotes •Prokaryotes include bacteria (both true bacteria and archaebacteria) •Classified as Monera if 5 kingdom system is used, but Eubacteria and Archaea if 6 kingdom system used

Procaryotes (both true bacteria and archaebacteria) are distinct from the Eukaryotes in the following ways: •They lack a nucleus •Their DNA in a circular strand without a membrane •Most lack organelles (membrane-bound)

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•Eukaryotes include all plants above the bacteria •They have DNA on chromosomes in the nucleus •They have nuclear membrane double layered •They have organelles bound by membranes

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Cell Walls •Distinguishes plant from animal cells •Functions to provide structural support •May have thick or thin walls, depending upon function •Determines size and shape •Protect contents of cell •Basis for most of life on earth, directly or indirectly •Mammals by themselves cannot digest it

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•Between the walls of adjacent cells is the middle lamella, a layer of pectin holding adjacent cells together •Primary cell walls are laid down on either side of it

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Eukarotic cells have structures and organelles: Cell Wall Plasma Membrane Endoplasmic reticulum Ribosomes Dictyosomes Chloroplasts Chromoplasts Leucoplasts Mitocondria Microbodies Vacuoles Cytoskeleton

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•Made of cellulose, a polysaccharide, with 100 to 15,000 monomers of glucose attached end to end to form string-like microfibrils •Also present in the cell wall: Hemicellulose (a glue-like substance that holds cellulose fibrils together) Pectin (used in jelly) cements contiguous walls together Glycoproteins (sugar + protein molecule)

Bo •Secondary walls derived from primary walls by addition of lignin, a complex polymer •Formed from primary wall after it ceases growth •Range in thickness from 5 to 95% of cell volume •Cells that store, make, or process food generally have thin walls •Support walls have thick walls

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•Plasmodesmata (singular, plasmodesma) are strands of cytoplasm that extend through cell wall openings •Serve in translocation and allow certain molecules to pass directly from one cell to another •Important in cellular communication •Lined by plasma membrane and traversed by a desmotubule from the endoplasmic reticulatum

Plasma membrane •Ca. 8-millionth of a mm thick •Composed of two layers of phospholipids •The two layers of phosolipids are arranged with the “heads” on the outside and the “tails” on the inside •Osmosis pushes it up again cell wall

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•Proteins molecules are interspersed, some going all the way to both sides, some not •Proteins passing all the way through aid in passage of certain ions •Molecules of cholesterol are also found imbedded in it •It can form folds that break and drift into cytoplasm, and shrink away from the wall

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Nucleus •Generally the most conspicuous structure in the cell (other than chloroplasts in green cells) •It is the control center of the cell and sends coded messages via DNA •Functions: stores the cell’s hereditary information, and controls all cell activities

Functions of the Plasma Membrane •Serves as an envelope enclosing the cytoplasm •Controls passage of molecules into and out of the cell, preventing some and promoting others •Coordinates the synthesis of cell wall microfibrils

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•Bounded by a double membrane (=nuclear envelope) is sometimes continuous with endoplasmic reticulatum •Separates the cytoplasm outside from the nucleoplasm •Structurally complex pores occupy much of the surface •One nucleus per cell in higher plants, but many in some fungi cells lacking partitions (cenocytic)

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•Inside the nucleus is one or more nucleoli (singular, nucleolus) •Roughly spherical and without a membrane (so a “suborganelle” •Made of protein and ribosomal DNA •Serve in the production and assembly of ribosomes components

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ER (Endoplasmic Reticulum) •Three dimensional membrane system within the cell •Network of flattened sacs and tubes that form channels, creating subcompartments •Double membrane with a space (lumen) between them •Continuous with outer nuclear membrane, which is possibly only a specialized type ER

Bo •Many cellular respiration enzymes synthesized on its surface •Site of membrane synthesis •Facilitates cellular communication and channeling of materials •Synthesizing membranes for other organelles, on the surface or inside

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•The rest of the nucleus comprises the granular nucleoplasm packed with chromatin, the resting-cell form of DNA •Chromatin forms chromosomes when cell is dividing •Number of chromosomes unrelated to size or complexity of the species *Adder’s tongue fern has over 1000 * Goldenweed (Asteraceae) have as low as 4

•Two kinds of ER Bo •Rough ER is covered with ribosomes *Functions in synthesis, secretion, and storage of proteins *Predominates in cells storing proteins •Smooth ER is mostly without ribosomes *Functions in making lipids *Predominates in cells that secrete lipids •The two are interconvertible, depending upon cell needs

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Ribosomes •Small ellipsoidal particles 17—23 nm in diameter •About 60% of mass is RNA, the rest mostly protein •Considered an organelle, but not membrane bounded •Located floating in the cytoplasm, on the rough ER, in other organelles, and on the nuclear membrane

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Comprise two subunits made of RNA and proteins made in the nucleolus and exported to the cytoplasm where they are assembled The two parts function in assembling proteins by reading (transcribing) messenger RNA and putting the amino acids in the correct order

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Golgi Apparatus Functions •Like a post office, handling incoming proteins, lipids, etc. and directing their export •Produce and secrete non-cellulose polysaccharides sent to the cell wall •Involved in the transport of lipids around the cell, as well as creating lysosomes

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Golgi Bodies (Dictyosomes) •Golgi apparatus collective term for golgi bodies (dictysomes) •Roundish flattened sacs scattered in cytoplasm •Polarized membrane system with one side facing the nucleus (cis) & the other (trans) the plasma membrane •Often 5 to 8 in a stack, but 30+ in simpler organisms

•Proteins formed by the Bo ER collect in (transport) vesicles that are pinched off •Vesicles move to cis side of Golgi apparatus •Fuse with Golgi apparatus •Proteins modified and coded based on destination •These (secretory) vesicles migrate from trans side to cell membrane, fuse with it, and secrete contents (e.g., cell wall polysaccharides, nectar, essential oils) outside

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Plastids •Found only in plants and algae (not animals) •They are organelles bound by a double membrane •Several kinds concerned with storage and photosynthesis *Chloroplasts—for photosynthesis *Chromoplasts—storage of color pigments *Leucoplasts—synthesis of starch •Believed to have originated when a small prokaryotic cell was engulfed or infected a eukaryotic cell and became established permanently in symbiosis •Theory of endosymbiosis now widely accepted today

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Endosymbiosis theory explains why organelles have double membrane (and the two are different) and some of their own DNA

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•Bounded by delicate, double cell membranes, •Inner one from cell membrane of a cyanobacterium •Colorless liquid matrix called stroma contains enzymes •Has one small circular strand of DNA for synthesizing proteins related to synthesis of proteins used in photosynthesis and other activities

Chloroplast Most of higher plants like two Frisbees attached together on edges Usually 75 to 125 per cell is common, more than that in algae, and up to several hundred in some plants Usually 4–6 (2–10 ) microns in diameter Capture light energy in photosynthesis and convert it to energy used in cell—virtually all of world’s food

•Grana are stacks of discs Bo with double membranes •40 to 60 granum linked together by arms •Each comprise stacks of 2 to 100 thylakoids •Contain chlorophyll that carry out photosynthesis •Starch grains, oil droplets, and enzymes, and sometimes proteins also found in the chloroplast •Grana are made by DNA in chloroplast

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Chromoplast •Found in complex plants, not algae and lower plants •Similar in size and shape to chloroplasts, and sometimes derived from them after loss of chlorophyll •A type of plastid that contains carotenoid pigments •They synthesize and store these red, yellow or orange pigments

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Mitochondria •Like a scooped out watermelon inside with incomplete partitions from walls •Double membrane organelle, thought to be formed during endosymbiosis •Form from mature mitochondria (divide independently) using its own DNA About 1–3 or more microns long

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Perioxisomes •Several other kinds of minor structures (“microbodies”) are found in cells •Peroxisomes have enzymes involved in photorespiration •Floating in cytoplasm and bound by a single membrane •Can reproduce themselves, but no DNA or ribosomes •A type called a glyoxisome contains enzymes that act on fats in seed germination

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Leucoplasts •Common in higher plants, and two common types Amyloplast, which synthesize and store starches Elaioplasts, which synthesize and store lipids •Plastids develop from protoplastids or division of mature plastids •Sometimes inter-convertible in light to chloroplasts •Simpler than plastids and lack elaborate system of internal membranes

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•Inner membrane forms cristae (partitions) •These increase surface area for enzymes to work •Matrix also contains DNA, RNA, ribosomes, proteins, and dissolved substances •Common in cell and move about cytoplasm to where they are needed •Energy released in cellular respiration inside them

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Vacuoles •Along with cell wall and plastids, it distinguishes plant cells from animal cells •May take up to 90% of cell contents •The cytoplasm often a thin layer between it and cell wall •Bounded by vacuolar membranes (tonoplast) single, and similar to cell membrane •Filled with cell sap, slightly to moderately acidic

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•Contains inorganic ions, sugars, acids, and amino acids •Sometimes crystals (often calcium oxalate derived from other parts of the cell •Immature cells have small vacuoles that coalesce in growth •Vacuolar growth accounts for most cell growth

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Cytoskeleton •Unbranched, thin, tube-like structures like tiny straws •Made of proteins called tubulins 15 to 25 micrometers long •Three kinds: microtubules, actin filaments (microfilaments), and intermediate filaments

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Microfilaments (Actin Filaments) •A quarter or third as big as microtubules, composed of proteins •Often in bundles and appear to play a role in cytoplasmic streaming •Thought to facilitate exchanges of materials in the cell

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Vacuole Functions •Accumulate substances, and also aid in breakdown and digestion of organelles Remove toxic secondary metabolites •Primary site for pigment deposition *Sometimes contains water-soluble pigments called anthocyanins that cause the colors of aging leaves and flowers *These differ from carotenoid pigments of chromoplasts, also part of leaf fall •Maintains pressure in cell •Increases surface area by expanding surface of cytoplasm that is pushed outward and to cell wall

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Microtubules •Control addition of cellulose to cell wall •Control movement of vesicles from dictysomes •Found in spindle filaments and phragmoplasts •Also in flagella and cilia •Made of the protein tubulin

Bo` •Cells are associated with each other in various ways to form functional and structural units •Tissues are groups of cells that are functionally and/or structurally distinct •Simple tissues are comprised of a single cell type, e.g., ground tissues (collenchyma, parenychyma, and sclerenchyma) •Complex tissues are those composed of two or more cell types, e.g., xylem, phloem, periderm, epidermis

TISSUES

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•Meristematic tissues comprised regions of permanent growth called meristems *Not found in animals, only plants •Two types of meristem *Apical meristem found at tips of roots and stems *Lateral meristem that increase girth of roots and stems

•Apical meristem comprises the meristematic tissues found at the tips of roots and stems •The tissue increases in length as the cells divide •It produces three kinds of primary meristem in primary growth *Protoderm, ground meristem, and procambium These three meristems develop into primary tissues *Epidermis, ground tissue, and the cambium

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•Lateral meristem increases the girth (secondary growth) *Vascular cambium *Cork cambium •New cells small and typically six-sided, but grow to different shapes depending on function •A maturity, up to 90% of cell is the vacuole

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•Vascular cambium is a thin cylinder of brick-shaped cells •When cell divides, the girth of the stem increases •Xylem is formed from the daughter cells inside the cambium, and phloem from those outside of it •Individual cells are called initials, those produced are called derivatives

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•Cork cambium is also a cylinder the length of the stem and root •Lies outside vascular cambium and produces bark, a secondary tissue •Intercalary meristem are found in grass stems near nodes to lengthen stem

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•Simple tissues comprise a single cell type, e.g., ground tissues (parenychyma, collenchyma, and sclerenchyma) •Most abundant type of cell in plant (e.g. cortex, pith) •More or less spherical in shape and 14-sided •Have large vacuoles often used for storage of water & food

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•Found in all major parts of higher plants •Several types of parenchyma *Aerenchyma of aquatic plants have small spaces between them for aeration and/or floatation *Chlorenchyma have numerous chloroplasts and function mainly in photosynthesis •May live a long time and can divide on their own and produce things like scar tissue upon injury to plant

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•Collenchyma is another simple ground tissue •Typically underneath epidermis •Thicker walls, pliable and strong, to provide support (e.g. celery strings) for organs •Long-lived and contain cytoplasm

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•Sclerenchyma is a third type of simple ground tissue •Thick walls generally impregnated with lignin •Function in support for the plant •Two types, sclerids and fibers •Sclerids mostly as long as wide an randomly distributed in other tissue, e.g. stone cells in pears •Also form hard layer of stone fruit pits •Congregated in places rather than random

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Fibers are found in association with other types of tissues •Longer than wide, with a lumen (cavity) in the center •Many plant fibers used commercially (up to 40 species are used for this) •Like sclerids, they usually lack cytoplasm at maturity

Bo •Complex tissues comprise two or more kinds of tissue •Three most important types—xylem, phloem, & epidermis •Most produced by vascular cambium and called vascular tissue •Xylem is important for transport of water and minerals from roots to rest of plant •Comprises parenchyma cells, fibers, vessels, tracheids, and ray cells

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•Vessel elements serve in water •Open at each end, often with bar-like plates extending across the opening •Joined end to end to form the vessels •Found in angiosperms and only a few gymnosperms •Non-living at maturity and lacking a nucleus

•Tracheids have thick secondary walls and tapered at one end, which overlap others •No openings at ends but pairs of pits where they contact another •Pairs of pits where they contact each other and serve in water transport •Tracheids are non-living and lack nucleus at maturity

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Have a pit membrane with a disc called a torus When water flows one way, disc can block hole slowing down movement Often have visible spiral thickenings Lateral transport occurs in rays made of long-lived parenchyma cells between the xylem like spokes of a wheel in woody stems

•Phloem mostly conducts dissolved food from leaves •Two main types of cells, sieve tube elements and companion cells •Sieve tube members are long and tubular and form sieve tubes when laid end to end •Unlike vessel elements, they have end plates (sieve plates) with small pores

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•Cytoplasm continuous thru pores between elements •No nuclei at maturity •Perhaps controlled by adjacent companion cells •Contain callose in solution when under pressure •When pressure drops, callose precipitates, combines with a protein to produce callose that plugs up the end

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•Companion cells are living with nucleus at maturity •They are narrower and more tapered that sieve cells •They are thought to regulate the non-living sieve tube elements •In gymnosperms, sieve cells are associated with albuminous cells, that are derived differently

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•Epidermis tissue is made mostly of parenchyma or similar cells, but other specialized ones as well •Mostly one cell layer thick •Most cells produce a fatty substance called cutin on both sides of outer wall •Forms the cuticle on the outside •Often look like jigsaw puzzle on surface •Form root hairs on roots

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Stomata are opening in the epidermal surface that allow for the exchange of gasses •Bordered by pairs of guard cells that open and close •Shaped differently than other epidermal cells, and have chloroplasts •Usually found on the lower leaf surface

•Periderm replaces epidermis after secondary growth Formed by the phellogen (cork cambium) layer •Comprises mostly rectangular box-like cork (phellem) cells •Fatty substance, suberin, secreted into walls as waterproofing and protection from drying and freezing •Cork from oaks harvested commercially

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•Lenticels made of loosely packed cells protruding above surface of the bark •Gas exchange function and lack suberin that would inhibit gas exchange •Often found in bark fissure bases

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Secretory Cells and Tissues •Use to secret waste products or substances harmful to cytoplasm, often into cavities (e.g., citrus oils) •May function as secretory cells or as part of secretory tissue •Often derived from parenchyma •Flower nectar, citrus oils, mint aromatics, sundew mucilage, latex, resins

Bo •Mitosis is occurs until organism dies •Strictly speaking, nuclear division, but cytokinesis usually follows immediately •In most tracheophytes it occurs in meristem (root, stem tip, and vascular cambium, and cork cambium of woody plants •Five minutes to several hours start to finish, mostly half to 2 or 3 hours •Initiated by ring-like pre-prophase band of microtubules just below the cell membrane

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Cell Division •The division of a cell into two daughter cells identical to the parent is called mitosis •Differs from meiosis with is a pair of divisions (the second of which is mitiotic) that result in haploid cells •The life of a cell or “cell cycle” is divided between interphase and mitosis •Most of cycle spent in interphase (non-dividing phase) *No longer thought of as resting phase *G1, Gap or Growth 1 period is the lengthy time after nucleus divides, during with cell grows in size Mitosis divided into four stages *Prophase *Metaphase *Anaphase *Telophase

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•During interphase, the cell increases in size •Production of substances that effect the next phase •S period—DNA replication •G2 period—Organelles divide and microtubules and other structures produced

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Prophase •Chromatin condenses to form chromosomes, which then become shorter, thicker, and visibly two-stranded •Two chromatids for each chromosome become evident •Independently coiled and identical to each other •Held together by centromere •Sometimes constrictions (satellites) occur, mostly near ends, but they have no known function

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Metaphase •Marked by the alignment of chromosomes in a circle (plate) between two poles, with the centromeres aligned perpendicular to axis of spindle •These are attached to the spindle fibers made of microtubules •At the end of metaphase, the centromeres separate from each other

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•Nuclear envelope fragments and is absorbed into ER •Nucleolus disintegrates gradually •About half of all mitosis time •Spindle fibers form by end of prophase and arc between two invisible poles •Centrioles in animals and some algae and fungi

Anaphase •Briefest of phases •Sister chromatids pulled apart and to the invisible poles •Spindles lose material from polar ends •Centromeres lead with trailing arms of chromatids, assuming a V shape •All separate and move at same time

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Telophase •Two nuclear envelopes form around daughter chromosomes •Daughter chromosomes lengthen and revert to chromatin •Nucleoli reappear •Many of spindle fibers disintegrate •Microtubules form the phragmoplast on the equator •Cell plate forms

•Microtubules apparently trap dictyosome-derived vesicles to fuse & form flattened, hollow cell plate •Middle lamella is synthesized from carbohydrates and shared between the two daughter cells •Plasmodesmata formed as ER parts are trapped between fusing vesicles of cell plate •New plasma membranes & materials added to cell walls

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Bo Differences Between Animal and Plant Cells •Animal cells have no cell walls •Plant cells have cell walls with plasmodesmata connecting cells through the cell walls •Animal cells do not form a cell plate, but instead pinch apart rather than forming cell plate •Most plant cells lack centrioles •Animal cells have to plastids •Vacuoles are small or absent in animal cells

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Differences Between Mitosis and Meiosis

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•Mitosis is involved in tissue growth, meiosis in the making of gametes •Mitosis produces two equal daughter cells that are identical with the mother cell, while meiosis produces two daughter cells with different genetic make-up and only half as many chromosomes as the mother cell •Mitosis is a single division, meiosis involves two •In mitosis, the centromere splits and the two chromatids separate, but in meiosis, identical chromosomes (sister chromosomes) rather than chromatids move apart

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