Cells Chapter 3

Outline  History of Cell Theory  Modern Microscopes  Eukaryotic and Prokaryotic Cells  Cell Structure and Communication  Cell Components  Cellular Reproduction  Higher Plant Cells Versus Animal Cells

History of Cell Theory  Cells discovered by Robert Hooke.  Cell Theory - all living organisms are composed of cells and cells form a unifying structural basis of organization.  Robert Brown discovered nucleus.  Rudolf Virchow argued no spontaneous generation of cells. – Louis Pasteur experimentally disproved in 1862.

Modern Microscopes  Light Microscopes - increase magnification as light passes through series of transparent lenses made of glass or calcium fluoride crystals  Two main types: – Dissecting (Stereomicroscopes) – Compound Microscopes

Modern Microscopes Dissecting Microscopes (Stereomicroscopes) Allow three- dimensional viewing of opaque objects Can magnify up to 30x

Modern Microscopes Compound Microscopes Light passes through thinly sliced material Can distinguish organelles 2 micrometers or larger in diameter Can magnify up to 1500x

Modern Microscopes  Electron Microscopes – use beam of electrons produced when high-voltage electricity is passed through wire  Two main types: – Transmission Electron Microscopes (TEMs) – Scanning Electron Microscopes (SEMs)

Modern Microscopes Transmission Electron Microscopes Up to 200,000x magnification, but material must be sliced extremely thin

Modern Microscopes Scanning Electron Microscopes Up to 10,000x magnification Surface detail can be observed on thick objects.

Modern Microscopes Scanning Tunneling Microscope Uses probe that tunnels electrons upon sample Produces map of sample surface Even atoms can become discernible First picture of DNA segment showing helical structure

Eukaryotic Versus Prokaryotic Cells  Prokaryotic - cells lack a nucleus Bacteria and archaea  Eukaryotic - cells contain a nucleus Plants, animals, fungi and protists Cell walls - Rigid boundary of cells Organelles - Membrane-bound bodies found within eukaryotic cells

Cell Structure and Communication  Cell wall surrounds protoplasm.  Protoplasm consists of all living cell components. – Bound by plasma membrane

Cell Structure and Communication  Cytoplasm - all cellular components between plasma membrane and nucleus Cytosol - fluid within cytoplasm containing organelles Organelles - persistent structures of various shapes and sizes with specialized functions » Most, but not all, bound by membranes

Cell Structure and Communication

Cell Structure and Communication Cell Size Cells of higher plants vary in length between 10 and 100 micrometers Smaller cells have relatively large surface to volume ratios enabling faster and more efficient cellular communication

Cell Structure and Communication Cell Wall  Main structural component of cell walls is cellulose (long chains of glucose monomers) Also contain matrix of: −Hemicellulose - holds cellulose fibrils together −Pectin - gives stiffness (like in fruit jellies) −Glycoproteins - proteins with associated sugars

Cell Structure and Communication Cell Wall  Middle lamella - produced when new cell walls are formed Shared by two adjacent cells  Flexible primary walls laid down on either side of middle lamella

Cell Structure and Communication Cell Wall Secondary walls produced inside primary walls – Derived from primary walls by thickening and inclusion of lignin – Cellulose microfibrils embedded in lignin for strength.

Cell Structure and Communication Communication Between Cells  Fluids and dissolved substances pass through primary walls of adjacent cells via plasmodesmata Plasmodesmata - cytoplasmic strands that extend between cells through minute openings Two adjacent cells connected by plasmodesmata

Cell Components Plasma Membrane  Plasma Membrane - semipermeable outer boundary of living part of cell Regulates movement of substances into and out of cell Model of plasma membrane

Cell Components Nucleus  Nucleus-control center of cell and contains DNA Sends coded messages from DNA to be used in other parts of cell Bound by two membranes constituting nuclear envelope Structurally complex pores occupy up to one-third of total surface area −Permit only certain kinds of molecules to pass between nucleus and cytoplasm

Cell Components Nucleus  Contains fluid nucleoplasm in which are: Nucleoli - composed primarily of RNA Chromatin strands – Composed of DNA and proteins – Coil and become chromosomes

Cell Components Endoplasmic Reticulum  Endoplasmic reticulum - enclosed space consisting of network of flattened sacs and tubes forming channels throughout cytoplasm Facilitates cellular communication and channeling of materials Synthesizes membranes for other organelles and modifies proteins

Cell Components Endoplasmic Reticulum  Rough ER - ribosomes distributed on outer surface of ER Associated with protein synthesis and storage  Smooth ER - devoid of ribosomes and associated with lipid secretion Endoplasmic reticulum and ribosomes

Cell Components Ribosomes  Ribosomes - consist of two subunits composed of RNA and proteins Link amino acids to construct complex proteins Subunits assembled in nucleolus May occur on outside of rough ER, or in cytoplasm, chloroplasts or other organelles No bounding membranes

Cell Components Dictyosomes  Dictyosomes - (Golgi bodies in animals) - stacks of flattened discs or vesicles Dictyosome

Cell Components Dictyosomes  Dictyosomes function: To modify carbohydrates attached to proteins synthesized and packaged in ER To assemble polysaccharides and collect them in small vesicles −Vesicles pinched off from margins of dictyosomes −Vesicles migrate to plasma membrane, fuse with it, and secrete contents to outside of cell – Contents may include cell wall polysaccharides, floral nectars, and essential oils in herbs

Cell Components Plastids  Chloroplasts - most conspicuous plastids Bound by double membrane and contain: −Grana made up of thylakoids – Thylakoid membranes contain chlorophyll – First steps of photosynthesis occur in thylakoid membranes −Stroma - matrix of enzymes involved in photosynthesis −Small circular DNA molecule – Encodes for production of certain proteins for photosynthesis

Cell Components Plastids TEM and drawing of chloroplast structure

Cell Components Plastids  Other types of plastids include: Chromoplasts −Synthesize and accumulate carotenoids (yellow, orange, red) Leucoplasts −Colorless −May synthesize starches (amyloplasts) −Or oils (elaioplasts) Chromoplasts in red pepper cells

 Mitochondria - release energy produced from cellular respiration Bound by two membranes Inward membrane forms numerous folds = cristae Increase surface area available to enzymes in matrix Matrix also includes DNA and RNA Cell Components Mitochondria

Cell Components Microbodies  Microbodies - small, spherical bodies distributed throughout cytoplasm containing specialized enzymes Bound by a single membrane Peroxisomes - serve in photorespiration Glyoxisomes - aid in conversion of fat to carbohydrates

Cell Components Vacuoles  In mature cells, 90% of volume may be taken up by central vacuoles Bounded by vacuolar membranes, tonoplasts Filled with cell sap - helps maintain pressure within cell −Contains dissolved substances i.e., salts, sugars, organic acids, and small proteins −Frequently contains water-soluble pigments called anthocyanins (red, blue, purple)

Cell Components Cytoskeleton  Cytoskeleton - involved in movement within cell and in cell’s architecture Network of microtubules and microfilaments Microtubules: −Control addition of cellulose to cell wall −Involved in movement of flagella and cilia −Found in fibers of spindles and phragmoplasts in dividing cells −Are thin, hollow, tubelike and composed of tubulins (proteins) Microfilaments - role in cytoplasmic streaming

Cellular Reproduction Cell Cycle  Cell cycle - orderly series of events when cells divide Divided into interphase and mitosis  Interphase Occupies up to 90% of cell cycle Period when cells are not dividing −G 1 - cell increases in size −S - DNA replication takes place −G 2 - mitochondria and other organelles divide, and microtubules produced

Cellular Reproduction Mitosis  Mitosis - process of cellular division Produces two daughter cells with equal amounts of DNA and other substances duplicated during interphase Each daughter cell exact copy of parent cell Occurs in meristems Although a continuous process, divided into 4 phases: prophase, metaphase, anaphase and telophase

Cellular Reproduction Mitosis  Prophase Chromosomes condense by coiling and tightening to become shorter and thicker. −Chromosomes made of two identical chromatids held together by centromeres – Kinetochore (protein complex) located on outer surface of each centromere – Spindle fibers (microtubules) attach to kinetochore and anchored to two poles of cell Nuclear envelope fragments and nucleolus disintegrates

Cellular Reproduction Mitosis  Metaphase Chromosomes align between poles around circumference of spindle at cell’s equator −Spindle fibers collectively referred to as spindle −At end of metaphase, centromeres holding each sister chromatid separate lengthwise. Prophase Metaphase

Cellular Reproduction Mitosis  Anaphase Sister chromatids separate and pulled to opposite poles, with centromeres leading the way −Spindle fibers shorten as material is continuously removed from polar ends −Chromatids after separation are called daughter chromosomes Anaphase

Cellular Reproduction Mitosis  Telophase Each group of daughter chromosomes become surrounded by nuclear envelope Daughter chromosomes become longer and thinner and eventually, indistinguishable Nucleoli reappear Spindle fibers disintegrate Phragmoplast and cell plate form at equator Telophase

Cellular Reproduction Mitosis  Cell Plate Formation: Phragmoplast develops between daughter cell nuclei −Phragmoplast - complex of microtubules and ER Microtubules trap dictyosome- derived vesicles Vesicles fuse to form cell plate −Cell plate grows outward toward mother cell walls Portions of ER trapped between vesicles, forming plasmodesmata Cell plate formation

Higher Plant Cells Versus Animal Cells  Plants: Cell walls Cell plate and plasmodesmata Plastids and vacuoles  Animals: Internal or external skeletons; no cell walls Plasma membrane called cell membrane Divide by pinching in two; no cell plate nor plasmodesmata Centrioles present during cell division No plastids nor vacuoles

Review  History of Cell Theory  Modern Microscopes  Eukaryotic and Prokaryotic Cells  Cell Structure and Communication  Cell Components  Cellular Reproduction  Higher Plant Cells Versus Animal Cells