BIo 161 - Cell Structure

Which cells are larger, eukaryotic or prokaryotic?

Eukaryotic !!

Why are eukaryotic cells large?

• have nucleus

• many organelles & membranous structures

• Purpose: keep reactions separate & localize reactions (enzymes, reactants) —> increase speed/efficiency

Compartmentalization in eukaryotic cells offers two primary, distinct advantages for cellular function:

• Keeps reactions separate:

Specialized compartments, such as lysosomes, maintain unique internal environments—like a low pH and specific hydrolytic enzymes—to perform distinct functions (e.g., waste digestion) without damaging the rest of the cell

Compartmentalization in eukaryotic cells offers two primary, distinct advantages for cellular function: Localizes reactions

By confining specific enzymes and their substrates together, compartments like mitochondria create high local concentrations of reactants, which significantly increases the efficiency and speed of metabolic processes like aerobic respiration

Organelles are what keep things separate…

compartmentalize using lipids to separate functions

The bilayer phospholipid membrane of a eukaryotic cell….

encircles/encloses cytoplasm, regulates transport

What features do all cells have in common?

All cells have:

• Plasma membrane → phospholipid bilayer; regulates transport

• Cytoplasm → fluid interior (cytosol + structures)

• DNA → genetic information

• Ribosomes → protein synthesis

Both prokaryotes and eukaryotes have these.

All cells have: Plasma membrane

• Plasma membrane → phospholipid bilayer; regulates transport

All Cells have: Cytoplasm

Cytoplasm → fluid interior (cytosol + structures)

All cells have: DNA

• DNA → genetic information

All cells have: ribosomes

• Ribosomes → protein synthesis

Eukaryotic Cells: Cytoskeleton

• microfilaments, intermediate filaments & microtubules

• any movement in cell takes place along some component of cytoskeleton

Protein fibers for:

• Shape

• Transport

• Movement

• Division

Component of cytoskeleton: What are microfilaments?

• very thin filaments (4-7 nm in diameter)

• 2 types: actin & myosin

• function: movement, endocytosis (bringing things into cell), cytokinesis (breaking cell apart), & secretion (vesicles that may be released from cell)

Brief description: endocytosis

bringing things into cell

brief description: cytokinesis

breaking cell apart

brief description: secretion

vesicles that may be released from cell

Pathogens can take advantage of microfilaments in that ….

there are certain bacteria that can usr filaments to move through cell

Type of microfilament: Actin

globular protein that polymerizes into filaments

• think “road”

Type of microfilament: Myosin

motor protein that engages with actin

• think “motor that drives on that road”

Example of actin-myosin movement: Cell division in animals

cytokinesis —> formation & contraction of a contractile ring at the cell equator

—> ring, made of actin filaments & myosin-II motor proteins, shrinks, pinching the cytoplasm to divide one cell into two daughter cells

Component of Cytoskeleton: Intermediate filaments

• structural filaments (~10 nm diamerer)

• STRUCTURAL in function (NO motor proteins associated w/these filaments)

Intermediate filaments are…

• true cytoskeleton filaments

• points of attachment for organelles

• HIGHLY STABLE (pretty static, good for binding)

Examples of intermediate filaments

• Keratin

• Vimentin

NO MOTOR associated w/intermediate filaments

Component of cytoskeleton: Microtubules

• helical shaped polar cylinders (~25nm diameter)

• alternating tubulin subunits (protein) forms filaments

—> alpha & beta tubulin

Think of microtubules as…

“freeways” of filaments, largest!!

• good for movememnt, dynamic

Associated motor proteins of microtubules

• Dynein & Kinesin motor proteins

• ATP-driven motor proteins —> transport intracellular cargo by walking along microtubule tracks, convert chemical energy from ATP hydrolysis into mechanical work, typically moving in opposite direction

• —> kinesin moves toward + end (cell periphery), while dynein moves toward the - end (cell center)

Components of cytoskeleton: microfilaments, intermediate filaments, & microtubules: Which have no motor associated with them?

• Intermediate filaments!

Why none for intermediate filaments?: They are stable support cables, not directional transport tracks.

How did an archea change to develop into a eukaryote?

Key events in eukaryotic evolution!

Key event in Eukaryotic Evolution: 1

• Acquisition of a Nucleus !!

—> protect genetic material

—> taking genetic material & compartmentalizing it (prokaryotes don”t)

Eukaryotic Nucleus

• Protected repository of genetic information

• Chromosomes within nucleus

Eukaryotic Nucleus: Chromosomes

• Chromosomes: composed of condensed chromatin

• Chromatin = complex of DNA, RNA, & proteins

Chromatin that make up chromosomes are

a complex of DNA, RNA, & proteins

Basic repeating structural units that make up chromatin (which make up chromosomes..)

multiple nucleosomes: DNA wrapped around histones (protein) in a ‘bead-on-a-string’ format

Eukaryotic Nuclear Envelope

• two membranes (Inner & outer) —> each membrane are phospholipid bilayers

• separates genetic material of cell from cytoplasm

The eukaryotic nuclear envelope has…

nuclear pore complexes!! —> highly regulated process that allows thing into nucleus

An inside out origin for the eukaryotic cell

• blebs extend out —> engulf other organisms

• perhaps how nuclear pore complex developed…still unknown

The Nuclear Pore Complex….

regulates entry & exit out of the nucleus

• depends on nuclear localization signals & nuclear export signals

• made up of proteins that form gates!!

Nuclear Pore Complex: Nuclear Localization Signals (NLS) =

Into the nucleus

Nuclear Pore Complex: Nuclear Export Signals (NES) =

out of the nucleus

Nuclear Pore Complex: Both NLS and NES are

primarily amino acid/peptide based

• short peptide sequences that may get cleaved from the protein upon entry/exit

Nuclear Pore Complex “Classical” Nuclear localization Signals

proteins w/lysines will be signal to bind to gates

Key event in Eukaryotic Evolution: 2

Endomembrane system!

• membranes within cell

Eukaryotic Plasma Membrane

Plasma membrane

• contains high proportion of sphingolipids & sterols (cholesterol) for strength & rigidity

• selectively permeable barrier

The eukaryotic plasma membrane…

..creates domain

• enable cells to organize lipids & proteins into functional, non-uniform regions

• membrane domains like lipid rafts = highly dynamic, ordered regions w/cholesterol & sphingolipids —> regulate protein-protein interactions, cell signaling, & membrane protein turnover

Brief Description: Microfilament protein

actin

Brief Description: Microfilament motor?

myosin

Brief description: microfilaments main function?

movement, cytokinesis

Brief Description: Intermediate Filaments Protein

Keratin, Vimentin

Brief Description: Intermediate Filaments Motor?

none!!!

Brief Description: Intermediate Filaments Main Function

structural stability

Brief Description: Microtubules Protein

Tubulin subunits

Brief Description: Microtubules Motor?

Dynein & Kinesin

Brief Description: Microtubules Main Function

intracellular transport & movement

Brief Description: What is the function of the endocytic pathway?

Brings materials into the cell from outside.

Brief Description: What is the function of the secretory pathway?

Moves materials through cell and to plasma membrane/exterior.

Functions of microfilaments?

Movement, endocytosis, cytokinesis, secretion.

The endocytic pathway…

..is observed in all eukaryotic cells

• function = to bring materials into the cell (from outside)

Endocytic Pathway: Phagocytosis

• generally mediated by receptor binding (trigger) —> blebs extend out —> engulf/eat whole thing/larger components

(not constitutive) —> particle fuses with lysosome

Endocytic Pathway: Macropinocytosis

• drinking small fluids, constant, constitutive

• Non-specific uptake of extracellular fluid and dissolved materials into large vesicles.

—> go to early endosome & transition to late endosome —> (compartment becomes acidified, breaks components brought in) fuses w/lysosome which degrades components

Endocytic Pathway: Endocytosis

regulated! receptor mediated, smaller components

• whatever is brought in go to early endosome, transition to late endosome—> (compartment becomes acidified, breaks components brought in) fuse w/lysosome which degrades components

Endocytic Pathway: Function of the early endosome?

First sorting station for material brought into cell

Endocytic Pathway: Function of the late endosome?

Further processes and transports materials toward lysosomes for digestion.

Endocytic Pathway: Function of lysosome

Digests macromolecules using hydrolytic enzymes at low pH.

Order of the endocytic pathway?

• Endocytosis → Early endosome → Late endosome → Lysosome

• Macropinocytosis —> early endosome —> late endosome —> lysosome

• Phagocytosis —> particle —> lysosome

Lysosomes

• stomach of the (animal) cell!!

• digestion of macromolecules

• maintains lower pH (pH 5)

Lysosomes contain

Enzymes that break down macromolecules

• DNases (DNA)

• Proteases (Protein)

• Lipases (Lipids)

• Glycosidases (sugars)

Secretory Pathway

• Function: move materials throughout cell to the plasma membrane or to the cell exterior

Secretory Pathway: Two Types of Vesicle Transport

unregulated (constitutive) vs. regulated

The Endoplasmic Reticulum

• complex of membranes derived from outer membrane of nucleus

• where “building blocks” are made

Rough Endoplasmic Reticulum

• studded w/ribosomes

• membrane/organelle specific

Function of rough ER

protein synthesis & packaging for secretion/membranes

Smooth endoplasmic reticulum

no studding w/ribosomes

• lipid synthesis/breakdown/modification

Function of Ribosomes

• Protein synthesis; forms peptide bonds between amino acids

Ribosomes can be..

bound or unbound

• free in cytoplasm or on rough ER

What are ribosomes made of?

ribosomal RNA (rRNA) (60%) & proteins (40%)

What is the size of eukaryotic ribosomes?

80S (40S + 60S subunits)

(small subunit + large subunit)

* subunits are not additive

Difference between free and ER-bound ribosomes?

Free ribosomes make proteins for cytosol; bound ribosomes make proteins for secretion/membranes.

Ribosomes can be used for

for looking at evolutionary changes of chromosomes (bacterial ribosomes vs eukaryotic ribosome)

The secretory pathway: Step 1

nascent protein (nascent protein = protein currently being synthesized by a ribosome, emerging from its exit tunnel…) —> ENTERS lumen of ER

• in lumen, protein spontaneously folds (chaperone proteins allow for proper folding in ER)

The secretory pathway: Step 2

• protein may be tagged w/post translational modification (red dots), exits ER

the secretory pathway: step 3

protein enters golgi apparatus (enters cis face, faces nucleus)

• goes through maturation process/sorting

the secretory pathway: step 4

protein exits golgi apparatus (exits from trans face)

• (could have been tethered to membrane)

• OR enclosed in vesicle

the secretory pathway: step 5 (if in vesicle)

protein is secreted from cell

Cis face of golgi apparatus

interior portion (faces nucleus)

Trans face of golgi apparatus

outer portion

Free vs ER-bound ribosomes

Determined by signal sequence on growing protein.

If signal sequence present:

→ ribosome binds ER

If absent:

→ ribosome stays free

Free ribosomes make proteins for:

• Cytosol

• Nucleus

• Mitochondria

• Peroxisomes

• Chloroplasts

Why organelle abundance varies by cell type

Cells specialize.

Examples:

• Muscle cells → many mitochondria (need ATP)

• Pancreas cells → lots rough ER/Golgi (protein secretion)

• Liver cells → lots smooth ER (detox)

• White blood cells → many lysosomes (digestion)

Structure matches function.

Why protein sorting matters

Proteins must reach the correct location.

Wrong location = nonfunctional or harmful.

Secretory pathway:

Nucleus → Rough ER → Vesicles → Golgi → Vesicles → membrane / lysosome / secretion

Golgi Apparatus Main Functions

Protein/lipid processing, packaging, and delivery.

What enters the cis face of the Golgi?

Materials from the ER.

What exits the trans face of the Golgi?

Processed materials in vesicles.

Key Event in Eukaryotic Evolution 3

Acquisition of Mitochondria

The history of Eukaryotes

• 2 billion years ago

• evolution from prokaryotic organisms by symbiosis

• Organelles from prokaryotic cells trapped inside them

Mitochondria

• semi-independent organelle

• Matrix: DNA (circular) & ribosomes 70s

• reproduce by binary fission (asexual reproduction)

Mitochondria Function

energy production & synthesis

Why are mitochondria considered semi-independent?

Have their own circular DNA, 70S ribosomes, and reproduce by binary fission (asexual reproduction).

Evidence mitochondria evolved from bacteria?

Own DNA, own ribosomes, binary fission (asexual reproduction), double membrane.

Function of chloroplasts?

Photosynthesis and energy transformation.

What features do chloroplasts share with mitochondria?

Own DNA, own ribosomes, high membrane surface area, semi-independent.

Mitochondria & Chloroplasts function for

• energy transformation (produce ATP)

• Mitochondria in ALL eukaryotes

• Chloroplasts in plants & algae