chapter 6: a tour of the cell. observation is the keystone of science. need: techniques to observe...
TRANSCRIPT
Chapter 6: A Tour of the Cell
Observation
Is the keystone of science. Need: Techniques to observe cells.
Question ?
Can cells be seen with the naked eye? Yes, a few are large enough, but most
require the use of a microscope.
Microscope History
1590 - Janseen Brothers invent the compound microscope.
1665 - Robert Hooke “discovers” cells in cork.
Early 1700’s - von Leeuwenhoek makes many observations of cells including bacteria.
Light Microscope - LM
Uses visible light to illuminate the object.
Relatively inexpensive type of microscope.
Can examine live or dead objects.
Light Microscope
Occular Lens
Objective Lens
Stage with specimen
Light Source
Magnification
Increase in diameter or size.
Resolution
Ability to detect two discrete points as separate from each other.
As Magnification increases, resolution decreases.
LM working limits are 100 - 1000X.
Limitations - LM
Miss many cell structures that are beyond the magnification of the light microscope.
Need other ways to make the observations.
Light Microscope Variations
Fluorescence: uses dyes to make parts of cells “glow”.
Phase-contrast: enhances contrasts in density.
Confocal: uses lasers and special optics to focus only narrow slides of cells.
Electron Microscopes
Use beams of electrons instead of light.
Invented in 1939, but not used much until after WWII.
TEM SEM
Advantages
Much higher magnifications. Magnifications of 50,000X or higher
are possible. Can get down to atomic level in some
cases.
Disadvantages
Need a Vacuum. Specimen must stop the electrons. High cost of equipment. Specimen preparation.
Transmission Electron Microscope - TEM
Sends electrons through thinly sliced and stained specimens.
Gives high magnification of interior views. Many cells structures are now visible.
TEM Limitations
Specimen dead. Specimen preparation uses extreme
chemicals so artifacts are always a concern.
Scanning Electron Microscope - SEM
Excellent views of surfaces. Produces 3-D views. Live specimens possible.
Limitations
Lower magnifications than the TEM.
EM Variations
High Voltage TEM Tunnel SEM Elemental Composition SEM
TEM - interior SEM - surface
Cell Biology or Cytology
Cyto = cell - ology = study of Should use observations from several
types of microscopes to make a total picture of how a cell is put together.
Other Tools for Cytology
Cell Fractionation Chromatography Electrophoresis
Cell Fractionation
Disrupt cells. Separate parts by centrifugation at
different speeds. Result - pure samples of cell structures
for study.
Cell Fractionation
Chromatography
Technique for separating mixtures of chemicals.
Separates chemicals by size or degree of attraction to the materials in the medium.
Ex - paper, gas, column, thin-layer
Electrophoresis
Separates mixtures of chemicals by their movement in an electrical field.
Used for proteins and DNA.
History of Cells
Robert Hooke - Observed cells in cork. Coined the term "cells” in 1665.
History of Cells
1833 - Robert Brown, discovered the nucleus.
1838 - M.J. Schleiden, all plants are made of cells.
1839 - T. Schwann, all animals are made of cells.
1840 - J.E. Purkinje, coined the term “protoplasm”.
Cell Theory
All living matter is composed of one or more cells.
The cell is the structural and functional unit of life.
R. Virchow
“Omnis cellula e cellula” All cells are from other cells.
Types of Cells
Prokaryotic - lack a nucleus and other membrane bounded structures.
Eukaryotic - have a nucleus and other membrane bounded structures.
Prokaryotic Eukaryotic
Nucleus
Prokaryotic
Eukaryotic
How small can a cell be?
Mycoplasmas - bacteria that are .1 to 1.0 m. (1/10 the size of regular bacteria).
Why Are Cells So Small?
Cell volume to surface area ratios favor small size.
Nucleus to cytoplasm consideration (control).
Metabolic requirements.
Basic Cell Organization
Membrane Nucleus Cytoplasm Organelles
Animal Cell
Plant Cell
Membrane
Separates the cell from the environment.
Boundary layer for regulating the movement of materials in/out of a cell.
Cytoplasm
Cell substance between the cell membrane and the nucleus.
The “fluid” part of a cell. Exists in two forms: gel - thick sol - fluid
Organelle
Term means "small organ” Formed body in a cell with a specialized function.
Important in organizational structure of cells.
Organelles - function
Way to form compartments in cells to separate chemical reactions.
Keeps various enzymes separated in space.
Nucleus
Most conspicuous organelle. usually spherical, but can be lobed or
irregular in shape.
Structure
Nuclear membrane Nuclear pores Nucleolus Chromatin
Nuclear Membrane
Double membrane separated by a 20-40 nm space.
Inner membrane supported by a protein matrix which gives the shape to the nucleus.
Nuclear Pores
Regular “holes” through both membranes.
100 nm in diameter. Protein complex gives shape. Allows materials in/out of nucleus.
Nucleolus
Dark staining area in the nucleus. 0 - 4 per nucleus. Storage area for ribosomes.
Chromatin
Chrom: colored - tin: threads DNA and Protein in a “loose” format.
Will form the cell’s chromosomes.
Nucleus - Function
Control center for the cell. Contains the genetic instructions.
Ribosomes
Structure: 2 subunits made of protein and rRNA. No membrane.
Function: protein synthesis.
Subunits
Large: 45 proteins 3 rRNA molecules
Small: 23 proteins 1 rRNA molecule
Locations
Free in the cytoplasm - make proteins for use in cytosol.
Membrane bound - make proteins that are exported from the cell.
Endomembrane System
Membranes that are related through direct physical continuity or by the transfer of membrane segments called vesicles.
Endomembrane System
Endoplasmic Reticulum
Often referred to as ER. Makes up to 1/2 of the total membrane
in cells. Often continuous with the nuclear
membrane.
Structure of ER
Folded sheets or tubes of membranes. Very “fluid” in structure with the
membranes constantly changing size and shape.
Types of ER
Smooth ER: no ribosomes. Used for lipid synthesis, carbohydrate
storage, detoxification of poisons. Rough ER: with ribosomes. Makes secretory proteins.
Golgi Apparatus or Dictyosomes
Structure: parallel array of flattened cisternae. (looks like a stack of Pita bread)
3 to 20 per cell. Likely an outgrowth of the ER system.
Structure Has 2 Faces
Cis face - side toward the nucleus. Receiving side.
Trans faceface - side away from the - side away from the nucleus. Shipping side.nucleus. Shipping side.
Function of Golgi Bodies
Processing - modification of ER products.
Distribution - packaging of ER products for transport.
Golgi Vesicles
Small sacs of membranes that bud off the Golgi Body.
Transportation vehicle for the modified ER products.
Movie
Lysosome
Single membrane. Made from the Trans face of the Golgi
apparatus.
Movie
Function
Breakdown and degradation of cellular materials.
Contains enzymes for fats, proteins, polysaccharides, and nucleic acids.
Over 40 types known.
Lysosomes
Important in cell death. Missing enzymes may cause various
genetic enzyme diseases. Examples: Tay-Sachs, Pompe’s
Disease
Vacuoles
Structure - single membrane, usually larger than the Golgi vesicles.
Function - depends on the organism.
Protists
Contractile vacuoles - pump out excess water.
Food vacuoles - store newly ingested food until the lysosomes can digest it.
Plants
Large single vacuole when mature making up to 90% of the cell's volume.
Tonoplast - the name for the vacuole membrane.
Function
Water regulation. Storage of ions. Storage of hydrophilic pigments.
(e.g. red and blues in flower petals).
Function: Plant vacuole
Used to enlarge cells and create turgor pressure.
Enzymes (various types). Store toxins. Coloration.
Microbodies
Structure: single membrane. Often have a granular or crystalline
core of enzymes.
Function
Specialized enzymes for specific reactions.
Peroxisomes: use up hydrogen peroxide.
Glyoxysomes: lipid digestion.
Enzymes in a crystal
Mitochondria
Structure: 2 membranes. The inner membrane has more surface area than the outer membrane.
Matrix: inner space. Intermembrane space: area between
the membranes.
Inner Membrane
Folded into cristae. Amount of folding depends on the level
of cell activity. Contains many enzymes. ATP generated here.
Function
Cell Respiration - the release of energy from food.
Major location of ATP generation. “Powerhouse” of the cell.
Mitochondria
Have ribosomes. Have their own DNA. Can reproduce themselves. May have been independent cells at
one time.
Chloroplasts
Structure - two outer membranes. Complex internal membrane. Fluid-like stroma is around the internal
membranes.
Inner or Thylakoid Membranes
Arranged into flattened sacs called thylakoids.
Some regions stacked into layers called grana.
Contain the green pigment chlorophyll.
Function
Photosynthesis - the use of light energy to make food.
Chloroplasts
Contain ribosomes. Contain DNA. Can reproduce themselves. Often contain starch. May have been independent cells at
one time.
Plastids
Group of plant organelles. Structure - single membrane. Function - store various materials.
Examples
Amyloplasts/ Leucoplasts - store starch.
Chromoplasts - store hydrophobic plant pigments such as carotene.
Ergastic Materials
General term for other substances produced or stored by plant cells.
Examples: Crystals Tannins Latex Resins
Cytoskeleton
Network of rods and filaments in the cytoplasm.
Functions
Cell structure and shape. Cell movement. Cell division - helps build cell walls and
move the chromosomes apart.
Components
Microtubules Microfilaments Intermediate Filaments
Microtubules
Structure - small hollow tubes made of repeating units of a protein dimer.
Size - 25 nm diameter with a 15 nm lumen. Can be 200 nm to 25 m in length.
Tubulin
Protein in microtubules. Dimer - and tubulin.
Microtubules
Regulate cell shape. Coordinate direction of cellulose fibers
in cell wall formation. Tracks for motor molecules.
Microtubules
Form cilia and flagella. Internal cellular movement. Make up centioles, basal bodies and
spindle fibers.
Cilia and Flagella
Cilia - short, but numerous. Flagella - long, but few. Function - to move cells or to sweep
materials past a cell.
Movie
Cilia and Flagella
Structure - 9+2 arrangement of microtubules, covered by the cell membrane.
Dynein - motor protein that connects the tubules.
Dynein Protein
A contractile protein. Uses ATP. Creates a twisting motion between the
microtubules causing the structure to bend or move.
Centrioles
Usually one pair per cell, located close to the nucleus.
Found in animal cells. 9 sets of triplet microtubules. Help in cell division.
Basal Bodies
Same structure as a centriole. Anchor cilia and flagella.
Basal Body
MTOCs
Microtubule Organizing Centers - sites that microtubules grow from.
Assist in cell division by anchoring spindle fibers.
May be anchored by centrioles.
Microfilaments
5 to 7 nm in diameter. Structure - two intertwined strands of
actin protein.
Microfilaments are stained green.
Functions
Muscle contraction. Cytoplasmic streaming. Pseudopodia. Cleavage furrow formation. Maintenance and changes in cell
shape.
Movie
Intermediate Filaments
Fibrous proteins that are super coiled into thicker cables and filaments 8 - 12 nm in diameter.
Made from several different types of protein.
Functions
Maintenance of cell shape. Hold organelles in place.
Cytoskeleton
Very dynamic; changing in composition and shape frequently.
Cell is not just a "bag" of cytoplasm within a cell membrane.
Cell Wall
Nonliving jacket that surrounds some cells.
Found in: Plants Prokaryotes Fungi Some Protists
Plant Cell Walls
All plant cells have a Primary Cell Wall.
Some cells will develop a Secondary Cell Wall.
Primary Wall
Thin and flexible. Cellulose fibers placed at right angles
to expansion. Placement of fibers guided by
microtubules.
Secondary Wall
Thick and rigid. Added between the cell membrane
and the primary cell wall in laminated layers.
May cover only part of the cell; giving spirals.
Makes up "wood”.
Middle Lamella
Thin layer rich in pectin found between adjacent plant cells.
Glues cells together.
Cell Walls
May be made of other types of polysaccharides and/or silica.
Function as the cell's exoskeleton for support and protection.
Extracellular Matrix - ECM
Fuzzy coat on animal cells. Helps glue cells together. Made of glycoproteins and collagen. Evidence suggests ECM is involved
with cell behavior and cell communication.
Intercellular Juctions
Plants-Plasmodesmata
Plasmodesmata
Channels between cells through adjacent cell walls.
Allows communication between cells. Also allows viruses to travel rapidly
between cells.
Intercellular Juctions
Animals: Tight junctions Desmosomes Gap junctions
Tight Junctions
Very tight fusion of the membranes of adjacent cells.
Seals off areas between the cells. Prevents movement of materials
around cells.
Movie
Desmosomes
Bundles of filaments which anchor junctions between cells.
Does not close off the area between adjacent cells.
Coordination of movement between groups of cells.
Gap Junctions
Open channels between cells, similar to plasmodesmata.
Allows “communication” between cells.
Movie
Summary
Answer: Why is Life cellular and what are the factors that affect cell size?
Be able to identify cellular parts, their structure, and their functions.