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 & NucleoplasmNuclear 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 SystemConsists 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

RibosomesSmall granules of proteins & ribosomal

RNA

Synthesize all proteins secreted by cells

Can be free-floating or membrane-bound

Endoplasmic Reticulum: Where Proteins Are BornSite of protein & lipid synthesis

Tubules & membranes connect to form fluid-filled 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 RERe.g. liver cells, antibody-producing plasma cells,

etc.

b. Smooth ERNo 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. LysosomesContain 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

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

Anchor / move organelles & cell structures

2. MicrofilamentsStrengthen 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 WallsPlants, 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 proteinse.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 CellsOutwardly 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/videoplayer/?src=harvard/harvard.swf&width=640&height=520

Narrated version:

http://multimedia.mcb.harvard.edu/media.html