Download Prokaryotic Cells

Cell Structure
Cell Theory
• All organisms are made of cells
• The cell is the simplest collection of matter
that can live
• Cell structure is correlated to cellular
function
• All cells are related by their descent from
earlier cells
2
Organisms and Cells
3
• In 1665, the English scientist Robert Hooke coined
the term “cellulae” for the small box-like structures
he saw while examining a thin slice of cork under a
microscope.
4
Cells were discovered in 1665 by Robert Hooke.
Early studies of cells were conducted by
- Mathias Schleiden (1838)
- Theodor Schwann (1839)
Cell Theory
5
Cell Theory
• A unifying concept in biology
• Originated from the work of biologists Schleiden and
Schwann in 1838-9
• States that:
– All organisms are composed of cells
• German botanist Matthais Schleiden in 1838
• German zoologist Theodor Schwann in 1839
– All cells come only from preexisting cells
• German physician Rudolph Virchow in 1850’s
– Smallest unit of life
6
Cell size is limited.
-As cell size increases, it takes longer for
material to diffuse from the cell membrane
to the interior of the cell.
Surface area-to-volume ratio: as a cell
increases in size, the volume increases
10x faster than the surface area
7
Cell Size
• Most much smaller than one millimeter (mm)
• Some as small as one micrometer (mm)
• Size restricted by Surface/Volume (S/V) ratio
–Surface is membrane, across which cell acquires
nutrients and expels wastes
–Volume is living cytoplasm, which demands nutrients
and produces wastes
–As cell grows, volume increases faster than surface
–Cells specialized in absorption modified to greatly
increase surface area per unit volume
8
Sizes of Living Things
Most cells are microscopic
Length of nerve cell
Largest single cell
5-100 microns
1 meter (m)
Ostrich egg
9
Microscopes are required to visualize cells.
Light microscopes can resolve structures
that are 200nm apart.
Electron microscopes can resolve
structures that are 0.2nm apart.
10
All cells have certain structures in common.
1. genetic material – in a nucleoid or nucleus
2. cytoplasm – a semifluid matrix
3. plasma membrane – a phospholipid bilayer
11
Prokaryotic Cells
• Lack a membrane-bound nucleus
• Structurally simple
• Two domains:
– Bacteria
• Three Shapes
– Bacillus (rod)
– Coccus (spherical)
– Spirilla (spiral)
– Archaea
• Live in extreme habitats
12
Prokaryotic Cells
Prokaryotic cells possess
-genetic material in the nucleoid
-cytoplasm
-plasma membrane
-cell wall
-ribosomes
-no membrane-bound organelles
13
Prokaryotic Cells
Prokaryotic cell walls
-protect the cell and maintain cell shape
Bacterial cell walls
-may be composed of peptidoglycan
-may be Gram positive or Gram negative
Archaean cell walls lack peptidoglycan.
14
Prokaryotic Cells
Flagella
-present in some prokaryotic cells
-used for locomotion
-rotary motion propels the cell
15
Eukaryotic Cells
-possess a membrane-bound nucleus
-are more complex than prokaryotic cells
-compartmentalize many cellular functions
within organelles and the
endomembrane system
-possess a cytoskeleton for support and
to maintain cellular structure
16
Nucleus
-stores the genetic material of the cell in
the form of multiple, linear chromosomes
-surrounded by a nuclear envelope
composed of 2 phospholipid bilayers
-in chromosomes – DNA is organized with
proteins to form chromatin
17
Ribosomes
-the site of protein synthesis in the cell
-composed of ribosomal RNA and
proteins
-found within the cytosol of the cytoplasm
and attached to internal membranes
18
Endomembrane System
• Restrict enzymatic reactions to specific
compartments within cell
• Consists of:
– Nuclear envelope
– Membranes of endoplasmic reticulum
– Golgi apparatus
– Vesicles
• Several types
• Transport materials between organelles of system
19
Rough endoplasmic reticulum (RER)
-membranes that create a network of
channels throughout the cytoplasm
-attachment of ribosomes to the
membrane gives a rough appearance
-synthesis of proteins to be secreted, sent
to lysosomes or plasma membrane
20
Smooth endoplasmic reticulum (SER)
-relatively few ribosomes attached
-functions:
-synthesis of membrane lipids
-calcium storage
-detoxification of foreign substances
21
22
Golgi apparatus
-flattened stacks of interconnected
membranes
-packaging and distribution of materials to
different parts of the cell
-synthesis of cell wall components
23
Lysosomes
-membrane bound vesicles containing
digestive enzymes to break down
macromolecules
-destroy cells or foreign matter that the cell
has engulfed by phagocytosis
24
Vacuoles
-membrane-bound structures with various
functions depending on the cell type
-There are different types of vacuoles:
-central vacuole in plant cells
-contractile vacuole of some protists
-vacuoles for storage
25
Vacuoles
• Membranous sacs that are larger than vesicles
–Store materials that occur in excess
–Others very specialized (contractile vacuole)
• Plants cells typically have a central vacuole
–Up to 90% volume of some cells
–Functions in:
• Storage of water, nutrients, pigments, and waste
products
• Development of turgor pressure
• Some functions performed by lysosomes in other
eukaryotes
26
Microbodies
-membrane bound vesicles
-contain enzymes
-not part of the endomembrane system
-glyoxysomes in plants contain enzymes
for converting fats to carbohydrates
-peroxisomes contain oxidative enzymes
and catalase
27
Peroxisomes
• Similar to lysosomes
–Membrane-bounded vesicles
–Enclose enzymes
• However
–Enzymes synthesized by free ribosomes in cytoplasm
(instead of ER)
–Active in lipid metabolism
–Catalyze reactions that produce hydrogen peroxide
H2O2
• Toxic
• Broken down to water & O2 by catalase
28
Mitochondria
-organelles present in all types of eukaryotic
cells
-contain oxidative metabolism enzymes for
transferring the energy within macromolecules to
ATP
-surrounded by 2 membranes
-smooth outer membrane
-folded inner membrane with layers
called cristae
29
-matrix is within the inner membrane
-intermembrane space is located
between the two membranes
-contain their own DNA
30
Chloroplasts
-organelles present in cells of plants and
some other eukaryotes
-contain chlorophyll for photosynthesis
-surrounded by 2 membranes
-thylakoids are membranous sacs within
the inner membrane
-grana are stacks of thylakoids
31
Cytoskeleton
-network of protein fibers found in all eukaryotic cells
-supports the shape of the cell
-keeps organelles in fixed locations
-helps move materials within the cell
Cytoskeleton fibers include
-actin filaments – responsible for cellular contractions,
crawling, “pinching”
-microtubules – provide organization to the cell and
move materials within the cell
-intermediate filaments – provide structural stability
32
The Cytoskeleton: Microtubules
• Hollow cylinders made of two globular proteins called a and b tubulin
• Spontaneous pairing of a and b tubulin molecules form structures called
dimers
• Dimers then arrange themselves into tubular spirals of 13 dimers around
• Assembly:
– Under control of Microtubule Organizing Center (MTOC)
– Most important MTOC is centrosome
• Interacts with proteins kinesin and dynein to cause movement of organelles
33
Cell Movement
Cell movement takes different forms.
-Crawling is accomplished via actin filaments and the
protein myosin.
-Flagella undulate to move a cell.
-Cilia can be arranged in rows on the surface of a
eukaryotic cell to propel a cell forward.
The cilia and flagella of eukaryotic cells have a similar
structure:
-9+2 structure: 9 pairs of microtubules surrounded by a
2 central microtubules
-Cilia are usually more numerous than flagella on a cell.
34
Extracellular Structures
Extracellular structures include:
-cell walls of plants, fungi, some protists
-extracellular matrix surrounding animal
cells
35
Cell Wall
-present surrounding the cells of plants,
fungi, and some protists
-the carbohydrates present in the cell wall
vary depending on the cell type:
-plant and protist cell walls - cellulose
-fungal cell walls - chitin
36
Extracellular matrix (ECM)
-surrounds animal cells
-composed of glycoproteins and fibrous
proteins such as collagen
-may be connected to the cytoplasm via
integrin proteins present in the plasma
membrane
37
38
Electron Microscope
• Resolves fine structure of the cell
– Uses an electron beam rather than a light beam
– Relationship between limit of resolution and wavelength
applies for any form of radiation
• Resolving power of electron microscope is 0.1 nm
• Source of illumination is a filament (cathode) that emits
electrons at the top of the column
– Since electrons are scattered by collisions with air
molecules, column must be under a vacuum
39
Under vacuum
40
•How is contrast achieved in the electron microscope?
•Specimen is stained with an electron dense material
•Some of the electrons passing through the specimen are
scattered by structures stained with electron dense material
•Others pass through parts of the cell not stained to form
an image on a phosphorescent screen
•Because the scattered electrons are lost from the beam,
the stained regions show up as dark
•Thus the image is a montage of light (non stained) and
dark (stained) regions
41
Electron micrograph of a cell in a root tip
42
• Preparation of Specimens
– Preserved by fixation
• 1st, glutaraldehyde
– Covalently cross-links proteins
• 2nd, osmium tetroxide
– Binds to and stabilizes lipid bilayers and proteins
– Tissue dehydrated, permeated with a
polymerizing resin & sectioned into ultra-thin
sections
• 50 – 100 nm thick (1/200 thickness of a cell)
43
• Sections stained with electron-dense
material (e.g uranyl acetate) to achieve
contrast
• How does this work?
– Tissue composed of atoms of low atomic
number (e.g. carbon, oxygen, nitrogen,
hydrogen)
– To make them visible impregnated with salts
of heavy metals.
44
Immunogold Electron
Microscopy
• Used to visualize specific proteins
– Incubate thin section with primary antibody
• Then incubate with secondary antibody to which
colloidal gold has been attached
– Gold is electron dense and shows up as black dots
45
Electron Microscopyof
Metal-Shadowed Samples
• The transmission electron microscope
(TEM) can be used to resolve individual
macromolecules on the surface of the
specimen
– Thin film of heavy metal (e.g. platinum) is
evaporated onto the dried specimen
46
Electron Microscopyof
Metal-Shadowed Samples
• For thick samples (e.g. cells), the organic
material is dissolved away after shadowing
– Only the thin metal replica of the surface is left
• This is thin enough for the electron beam to
penetrate.
47
Metal-shadowed electron micrograph of a
freeze-fractured membrane
48