Download Cells: Beyond the Membrane

Cells: Organelles
Chapter 4
Overview
Eukaryotic cells
Cytoplasm
Organelles
Cytoskeleton
How cells move
Prokaryotic cells
Eukaryotic Cells
Membrane-bound
organelles
= metabolic activities occur
within controlled
environment
DNA contained within true
nucleus
Protein cytoskeleton
Cytoplasm
Cellular material inside cell where cellular
activities are carried out
3 components:
– Cytosol
– Inclusions
– Cytoplasmic organelles
Cytosol
Thick semi-translucent fluid in which
organelles are suspended
Composed of water with variety of solutes
e.g. proteins, salts, sugars, etc.
Has both colloidal & suspension
properties:
= sol-gel transformation
= visible particles that settle out
Inclusions
Chemical substances contained within cell
Vary depending on type of cell
e.g. glycogen in liver & muscle cells
e.g. lipid droplets in adipose cells
e.g. melanin pigment in skin & hair
Organelles
“Little organs”
= cellular compartments
Individual structures within cells that
perform specific functions
Some are non-membranous
Some are membrane-bound
= have own internal environments
Nucleus: A Cell’s Control Centre
Largest cytoplasmic organelle
Contains genetic material & protein-building
instructions in DNA
Keeps DNA separate from cytoplasmic
components
a. Nuclear Envelope & Nucleoplasm
Nuclear envelope:
– Semi-permeable lipid bilayer with membrane
proteins
– Outer membrane: continuous with endoplasmic
reticulum
– Inner membrane: fibrous proteins that help
organize DNA molecules
Nucleoplasm:
– Semi-fluid matrix
b. Nucleolus
Builds ribosomes used for protein
synthesis
Contains proteins & ribosomal RNA
c. Chromosomes & Chromatin
Chromosome
= 1 DNA molecule &
associated proteins
Chromatin
= all DNA molecules &
associated proteins
The Endomembrane System
Consists of ribosomes, endoplasmic
reticulum, Golgi apparatus, & vesicles
Makes, stores, & transports biological
molecules
Destroys potentially harmful materials
All proteins bound for export or plasma
membrane pass through
Ribosomes
Small granules of proteins & ribosomal
RNA
Synthesize all proteins secreted by cells
Can be free-floating or membrane-bound
Endoplasmic Reticulum:
Where Proteins Are Born
Site of protein & lipid synthesis
Tubules & membranes connect to form fluidfilled cavities that adjoin nuclear envelope
a. Rough ER
Surface studded with
ribosomes
Where all proteins are
made, regardless of fate
Cells that make, store, &
secrete proteins have lots
of RER
e.g. liver cells, antibodyproducing plasma cells,
etc.
b. Smooth ER
No ribosomes
No role in protein synthesis
Enzymes catalyze many
reactions
e.g. plasma membrane lipid
synthesis
e.g. detoxification in liver & kidney
e.g. breakdown of glycogen in liver
After proteins have been synthesized in the
ER, they are packaged in transport vesicles
Vesicles bud off & proceed to the Golgi
apparatus for further processing
Golgi Apparatus
“Traffic director” for proteins
Modifies, packages, & transports proteins
& lipids from RER to export vesicles
Transport vesicles from ER fuse with Golgi
apparatus
Proteins & lipids are modified (sugar side
chains added, etc.) & repackaged into
vesicles
Export vesicles transport products away to be:
– Digested
– Incorporated into plasma membrane
– Expelled from cell via exocytosis
Vesicles
Sacs that are used for storage or transport
Bud off from ER, Golgi apparatus, plasma
membrane
• Peroxisomes
• Lysosomes
a. Peroxisomes
Rid cells of toxic substances &
neutralize free radicals
Contain enzymes that digest:
– Fatty acids
– Amino acids
– H2O2
e.g. break-down of alcohol in liver
& kidney cells
b. Lysosomes
Contain acidic digestive enzymes
= can digest almost all biological materials
Abundant in phagocytes
Functions include:
– Degrading old, dying, & non-functional
organelles & tissues
– Digesting foreign materials
– Breaking down bone to release Ca2+ into
blood
Mitochondrion: A Cell’s
Powerplant
Site of aerobic respiration
(found only in aerobic eukaryotic cells)
Allow cell to produce lots of ATP
# differs depending on cell type
Have own DNA & RNA so can replicate
themselves when cellular ATP needs
increase
Mitochondrion Structure
Outer membrane
• Selectively permeable
Inner membrane
• Highly impermeable
• Contains ATP synthase
• Has membrane potential
Cristae
• ↑ surface area of inner
membrane, which ↑ capacity to
generate ATP
Matrix
• Contains 100s of enzymes which
oxidize pyruvate & fatty acids, &
control the Krebs cycle
Possible Endosymbiotic Origins of
Mitochondria
Endosymbiont Theory:
One prokaryotic cell entered
another cell (ingested or
parasitic) & escaped digestion
= able to reproduce inside host
Over generations, evolved to life
on the inside & lost traits
necessary for independent life
Specialized Plant Organelles
Plastids
= Organelles used in photosynthesis or for
storage
Central vacuole
= Fluid-filled compartment
a. Plastids
Chloroplasts
– Store chlorophyll a & b
– Capture light E
– Endosymbiotic origins?
Chromoplasts
– Store carotenoids
– Capture light E
Amyloplasts
– Starch storage
b. Central Vacuole
50%-90% of cell interior
Stores amino acids, sugars, ions, toxins
Expansion of vacuole prompts cell SA to ↑
= ↑ absorption of H2O & nutrients
Cytoskeleton: A Cell’s Support
System
Supportive protein filaments between
nucleus & cell membrane
Form cell skeleton that supports cellular
structure & allows for cell movement
(like skeletal & muscular systems in humans)
Found in eukaryotic cells
(prokaryotic cells have poorly-developed
cytoskeletons, although reinforcing filaments are
similar)
Elements include:
– Microfilaments
– Microtubules
– Intermediate
filaments
Centrosomes & Centrioles
Centrosome
= where microtubules are produced
= each contains a pair of centrioles
Centriole
= barrel-shaped organelle important
in cell division
More on these later!
1. Microtubules
Anchor / move organelles & cell structures
Globular tubulin monomers
– “Minus” end anchored in
centrosomes
– “Plus” end grows as tubulin added
Not permanently stable
– Fall apart in controlled ways
– Plant toxins & animal microtubules
2. Microfilaments
Strengthen cell structure, anchor
membrane proteins, & aid in
muscle contraction
2 polypeptide chains of globular actin
monomers
– Form bundles & networks
Form & fall apart in controlled ways
Responsible for cytoplasmic
streaming
Cytoplasmic Streaming
Microfilaments loosen up
Allows fluid motion of cytoplasm
Rearranges substances & structures within
cell interior
3. Intermediate Filaments
Very stable fibrous cytoskeletal
elements
Types of filaments vary depending on cell
type
e.g. lamins: support nucleus, aid in muscle
contraction
e.g. desmins & vimentins: anchor contractile
units
e.g. cytokeratins: strengthen cells that
produce nails, claws, horns, etc.
Eukaryotic Cell Walls
Plants, some protists & fungi
Protective/supportive layer around plasma
membrane
Porous
= allows free movement of H2O & solutes
between external environment & plasma
membrane
Waxes build up at surfaces exposed to air
= ↓ H2O loss
Plants often have 2 cell walls
1° cell wall:
= flexible
= adheres adjoining cells
2° cell wall:
= provides rigidity & structural
support
= very abundant in woody plants
How Cells Move
Cells can move:
– Elements within themselves
– Themselves (parts or whole self)
Internal Cellular Movement
Movement of organelles, etc.
Uses motor proteins
e.g. kinesins, dyneins, myosins,
etc.
Move along cytoskeletal
elements
Fueled by ATP
How Cells Get Around
Cilia
Flagella
Pseudopodia
Cilia
Hair-like extensions of cell
surface
Beat in synchrony
Move substances or cell itself
e.g. Paramecium
e.g. cells in respiratory tract that
move mucus away from lungs
Flagella
Singular = flagellum
Whip-like extension of cell
Propels cell along
e.g. sperm cell, many
protists
Cilium & Flagellum Structure
9+2 array down length
= 9 pairs of microtubules form ring around
central pair
Centriole sits beneath array as basal body
Flagellum / cilium moves by sliding
mechanism
Powered by ATP
Dynein arms in 1 pair of microtubules grab pair
in front, tilt downward, & let go
As 1 pair is being grabbed, it grabs pair in front
Creates bending motion in each cilium /
flagellum
Pseudopodia
“False feet”
Temporary extensions of
cytoplasm
Microfilaments elongate in 1
direction
Attached motor proteins drag
plasma membrane along in that
direction
e.g. amoebas, macrophages
Prokaryotic Cells
Outwardly similar in appearance to eukaryotic
cells
Archaea
– All polypeptide chains begin with methionine
– Make histones to stabilize DNA
• Bacteria
– All polypeptide chains begin with
formylmethionine
– Make histone-like proteins to stabilize DNA
Prokaryotic Cell Membranes
Semi-permeable cell membrane
Many have cell wall
(structurally different from eukaryotic cell
walls)
Sticky polysaccharides on outer surface of
cell wall
Contain no membrane bound-organelles
Photosynthetic bacteria harbour pigments
within inpouchings of plasma membrane
DNA is free-floating in cytoplasm
DNA is circular
Some have plasmids
=small circular DNA molecules with few
genes
Many have ≥ 1 flagellum
Many have pili
= protein filaments
= allow adhesion to surfaces
= allow transfer of genetic material to other
cells
Harvard cell video
Musical version:
http://aimediaserver.com/studiodaily/videopla
yer/?src=harvard/harvard.swf&width=640&h
eight=520
Narrated version:
http://multimedia.mcb.harvard.edu/media.html