Transcript Chapter 7: Cells What 17th century invention led to the discovery of

Chapter 6: Cells
• In 1665, Robert Hooke first described cells
using the light microscope.
• Two classifications of cells:
Prokaryotes
Eukaryotes
Prokaryotic
Found only in Eubacteria and Archaebacteria
(formerly Monera)
No true nucleus; lacks nuclear envelope
Genetic material in the nucleiod region
No membrane bound organelles
Eukaryotic
• Found in Kingdoms Protista, Fungi, Plantae,
and Animalia
• True nucleus; bound by nuclear envelope
• Genetic material within the nucleus
• Contains cytoplasm with cytosol and
membrane-bound organelles
• Cytoplasm – entire region between the nucleus
and cell membrane(eukaryotic)
• Cytosol – Semi-fluid medium found in the
cytoplasm(all cells)
Size ranges of cells:
 Most bacteria are 1.0 – 10.0 m
 Most eukaryotic cells are 10.0 – 100.0 m
• The importance of compartmental
organization:
The average eukaryotic cell has a thousand
times more volume but only a hundred times
more surface area than prokaryotic cells.
Eukaryotic cells compensate for the small
surface area to volume ratio using internal
membranes.
Organelle – a structure within a cell
that serves a specific function
Organelles include:
1. Nucleus
2. Ribosomes
3. Endoplasmic reticulum
4. Golgi apparatus
5. Lysosomes
6. Vacuoles
7. Peroxisomes
8. Mitochondria
9. Chloroplasts
Nucleus –contains most of the genes
that control the entire cell.
1) enclosed by a nuclear envelope
two lipid bilayers, each with their own specific
proteins.
2) contains most of the cell’s DNA, which is
organized with histone proteins into
complexes called chromatin.
 Each species has a characteristic chromosome
number
Humans= 46 chromosomes (except gametes)
The nucleus controls protein synthesis in the
cytoplasm:
Nucleolus dark spot(s) in a nucleus
 It synthesizes and assembles rRNA, used to form
ribosomes out in the cytoplasm.
Ribosome – a cytoplasmic organelle with no
membrane which is the site of protein synthesis.
 They are complexes of RNA and proteins
* Cells with high rates of protein synthesis have a
particularly high number of ribosomes.
human liver cell has a few million
Ribosomes function either free in the cytosol or bound
to rough endoplasmic reticulum.
 Bound and free ribosomes are structurally identical
and interchangeable.
 Most proteins made by free ribosomes will remain and
function in the cytosol.
 Most proteins made by bound ribosomes are destined
for membrane export.
Endomembrane System:
• Biologists consider many membranes of the eukaryotes to be
interrelated either directly (through physical contact) or indirectly
(through vesicles).
• Vesicles are membrane-enclosed sacs that are pinched off portions
of membranes moving from one membrane to another.
The endomembrane system includes:
 nuclear envelope
 endoplasmic reticulum
 golgi apparatus
 vacuoles
 plasma membrane (not actually an endomembrane, but
related to the endomembrane system)
Endoplasmic reticulum (ER)
– Extensive membranous network of tubules
and sacs (cisternae) which separate its
internal lumen (cisternal space) from the
cytosol.
 There are two distinct regions: smooth
ER(lacks ribosomes) & rough ER
Functions of Smooth ER:
• Synthesizes lipids, phospholipids, and steroids
Mammalian sex hormones and steroids secreted by
adrenal medulla.
• Participates in carbohydrate metabolism
Smooth ER in liver contains an embedded enzyme that
catalyzes the final steps in the conversion of glycogen
to glucose
• Detoxifies drugs and poisons
• Stores calcium ions necessary for muscle contraction
example: Sarcoplamic Reticulums
Rough ER and Protein Synthesis:
• cytoplasmic side is studded with ribosomes.
• continuous with the outer membrane of the
nuclear envelope
• Manufactures secretory proteins and
phospholipid membrane
• Ribosomes attached to rough ER synthesize secretory proteins

• Growing polypeptide is threaded through ER membrane into the
lumen (cisternal space)

• Protein folds into its native conformation

• If destined to be a glycoprotein, enzymes localized in the ER
membrane catalyze the covalent bonding of an oligosaccharide to
the secretory protein

• Protein departs in a transport vesicle pinched off from transitional
ER adjacent to the rough ER
• Golgi Apparatus: an organelle made of
stacked, flattened membranous sacs
(cisternae) that modifies, stores, and routes
products of the ER
many transport vesicles leave the ER and travel
to the Golgi apparatus.
• The Golgi apparatus has two poles (cis and trans face):
1) The cis face receives products by accepting transport
vesicles from the ER
2) The trans face pinches off vesicles from the Golgi
and transports molecules to other sites.
• cisternae between the cis and trans face contains
unique combinations of enzymes.
Golgi products in transit from one cisternae to the next
are carried in transport vesicles.
• Lysosome: an organelle which is a membrane-enclosed
bag of hydrolytic enzymes that digest all major classes of
macromolecules.
Enzymes include lipases, carbohydrases, proteases,
and nucleases
• Lysosomal membrane performs two important functions:
a)Sequesters potentially destructive hydrolytic enzymes
from the cytosol.
b)Maintains the optimal acidic environment for enzyme
activity by pumping H+’s inward from the cytosol to the
lumen
• Lysosomes probably pinch off from the trans
face of the Golgi apparatus.
Functions of Lysosomes:
1) Intracellular digestion
Phagocytosis
2)Recycle cell’s own organic material
3)Programmed cell destruction
Vacuole: organelle which is a membrane-enclosed sac
that is larger than a vesicle (transport or lysosome)
Vacuole types and functions:
• Food vacuole – site of intracellular digestion in some protists and
macrophages
• Contractile vacuole – pumps excess water from cell (found in some freshwater protozoa)
• Central vacuole – large vacuole found in most mature plant cells.
- Stores organic compounds (e.g. protein storage in seeds)
- Collects dangerous metabolic by-products from the
cytoplasm(acts as lysosome)
- Contains soluble pigments in some cells (e.g. red and
blue pigments in flowers)
- May protect the plant from predators by containing
poisonous or unpalatable compounds
Peroxisomes: membrane-bound organelles in
most eukaryotic cells that contain specialized
teams of enzymes
ex) breakdown of fatty acids into (acetyl
CoA) which enters Kreb’s Cycle
(mitochondria)
H2O2 is produced as a side product-but
catalase is present in peroxisomes too
Also involved in drug detox
• Mitochondria and Chloroplasts
Similarities
• Enclosed by double membranes
• not part of the endomembrane system
• Contain ribosomes and some DNA that
program a small portion of their own protein
synthesis (rest are imported from cytosol and
under nuclear control)
• Mitochondria – organelles which are the sites
of cellular respiration(#varies per cell)
Structure of mitochondria:
• The smooth outer membrane is highly
permeable to small solutes
• The convoluted inner membrane contains
embedded enzymes that are involved in
cellular respiration.
Intermembrane space
• between inner and outer membranes
with a solute composition like the cytosol,
Mitochondrial matrix
• enclosed by the inner mitochondrial
membrane
both contain enzymes needed for cellular
respiration
Chloroplast – a chlorophyll-containing organelle
which is the site of photosynthesis.
• Found in eukaryotic algae, leaves, and
other green plant organs.
• Are dynamic structures that change shape,
move, and divide.
Structure of Chloroplast:
1) Intermembrane space
2) Thylakoids (grana)
 The thylakoid membrane contains
chlorophyll and separates the thylakoid space
(inside) from the stroma(fluid-outside)
Plant Cell Walls
 Thick
 includes cellulose fibers embedded in a
matrix of other polysaccharides and
proteins.
 Has membrane-lined channels called
plasmodesmata which connect the cytoplasm
of neighboring cells.
Plant cell walls function to:
protect plant cells
maintain their shape
prevent excess water intake
(maintain turgor pressure)
Primary vs. secondary cell walls
1)primary – first-formed
- flexible, external
2) secondary – deposited last
-inside of primary wall
-durable, less flexible
-can be more than one
*Between two adjacent cells, the middle lamella
is found and consists of pectins
Cytoskeleton – a network of fibers throughout
the cytoplasm that forms a dynamic framework
for support and movement.
 Provides mechanical support and helps
maintain cell shape
 Enables a cell to change shape
 three types of fibers: microtubules
(thickest), microfilaments (thinnest), and
intermediate filaments (intermediate in
diameter).
Microtubules are hollow fibers found in
the cytoplasm of all eukaryotic cells.
 Constructed from globular proteins called
tubulin that consist of one -tubulin and tubulin molecule.
Functions of microtubules:
1) Cellular support
2) Tracks for organelle movement
3) Separation of chromosomes (division)
4) Make up centrioles in animal cells
5) Make up cilia and flagella(locomomotor)
Microfilaments – solid rods built from globular protein
monomers called G-actin. (helical formation)
Function of microfilaments:
1) Participate in muscle contraction
 Actin(thin filaments) interacts with thick
myosin filaments (involves ATP)
2) Provide cellular support
3) Responsible for cytokinesis in animal cells
and pseudopodia in ameobas
Intermediate filaments – intermediate diameter
 Specialized for bearing tension (framework for
cytoskeleton)
 May help fix organelle position (e.g.
nucleus)
 Keratin proteins
Extracellular matrix – meshwork of
macromolecules outside the plasma
membrane of animal cells.
 locally secreted by cells
 composed mostly of glycoproteins
(a lot of collagen!)
 provides support and anchorage for cells
Intercellular Junctions
Intercellular junctions in plants:
Plasmodesmata
Intercellular junctions in animals:
Tight junctions(prevent leakage)
Desmosomes(prevent separation)
Gap junctions(rapid ion exchange)