Module 4 - Eukaryotic Cell Structure and Function

Question 1

Plasma Membrane

Answer 1

• The fluid mosaic model is based on
  eukaryotic membranes
• Eukaryotic membranes contain microdomains
  called lipid rafts
  – Enriched for certain lipids and proteins
  – Participate in a variety of cell processes such as
  cell movement and transduction

Question 2

Cytoplasmic Matrix

Answer 2

• The many organelles of eukaryotic cells lie in the
  cytoplasmic matrix
• Cytoskeleton:
  – Vast network of interconnected filaments within
  the cytoplasmic matrix
  – Filaments that form the cytoskeleton:
  microfilaments, microtubules, and intermediate
  filaments
  – plays role in both cell shape and cell movement

Question 3

Cytoskeleton

Answer 3

Microfilaments
* Minute actin protein filaments, 4 - 7 nm in diameter
* Scattered or organized into networks and parallel arrays
* Involved in cell motion and shape changes
Microtubules
* Shaped like thin cylinders (~25 nm diameter)
* Help maintain cell shape
* Proteins -tubulin and -tubulin
* Work with microfilaments in cell movement
* Participate in intracellular transport processes
Intermediate Filaments
* Heterogeneous
* Role is unclear
– Some form nuclear lamina
– Others help link cells together to form tissues

Question 4

Secretory and endocytic systems

Answer 4

• Endoplasmic reticulum
• Golgi apparatus
• Lysosomes

Question 5

Endoplasmic Reticulum

Answer 5

• Irregular network of branching
  and fusing membranous tubules
  and flattened sacs
• Rough ER have ribosomes
  attached
  – synthesis of secreted
  proteins
• Smooth ER is devoid of
  ribosomes
  – synthesis of lipids by ER-
  associated enzymes
• ER transports proteins, lipids,
  and other materials within cell
• Major site of cell membrane
  synthesis
• Synthesis of lysosomes

Question 6

golgi apparatus

Answer 6

• Membranous organelle
  made of cisternae
  stacked on each other
• Dictyosomes
  – stacks of cisternae
• Involved in modification,
  packaging, and secretion
  of materials

Question 7

Lysosomes

Answer 7

• Membrane-bound vesicles found in most eukaryotes
• Involved in intracellular digestion
• Contain hydrolases, enzymes which hydrolyze
  molecules and function best under slightly acidic
  conditions
• Maintain an acidic environment by pumping protons
  into their interior

Question 8

In the lysosome…

Answer 8

• Digestion occurs without the release of
  lysosome enzymes into the cytoplasmic
  matrix
• As the contents are digested, the products
  leave the lysosome and can be used as
  nutrients
• The resulting lysosome is called a residual
  body
• The residual body can release contents to the
  cell exterior by a process called lysosome
  secretion

Question 9

The biosynthetic-sectretory pathway

Answer 9

• Used to move materials to lysosomes, as well as from
  the inside of the cell to either the cell membrane or cell
  exterior
• Proteins synthesized by ribosomes on rough ER
  released in small vesicles → cis face of Golgi
  apparatus→ trans face of Golgi apparatus
• Transport vesicles released from trans face of Golgi
• After release, some vesicles deliver their contents to
  lysosomes, while others deliver to cell membrane
• Quality assurance mechanism
  – Unfolded or misfolded proteins are secreted into the cytosol, and
  targeted for destruction by ubiquitin polypeptides
  – Proteasomes destroy targeted proteins

Question 10

Endocytic pathway

Answer 10

• Endocytosis is used to bring materials into the cell
  – Solutes or particles taken up and enclosed in vesicles
  pinched from plasma membrane
  – Some cells can engulf larger particles including other cells
  – In many cases, materials are then delivered to lysosome and
  destroyed

Question 11

types of endosytosis

Answer 11

• Phagocytosis
  – Use of cell surface protrusions to surround and engulf particles
  – Resulting vesicles called phagosomes
• Clathrin-dependent endocytosis
  – Involves membrane regions coated on cytoplasmic side with the
  protein clathrin (coated pits)
  – Coated pits have external receptors that specifically bind
  macromolecules
  – Pinching off of coated pits forms coated vesicles
  – Called receptor-mediated endocytosis
• Caveolae–dependent endocytosis
  – Enriched in cholesterol and the membrane protein caveolin
  – When the caveolae pinch off the membrane, they are called
  caveolar vesicles
  – Do not deliver their contents to lysosomes
  – May play role in signal transduction, transport of small molecules,
  as well as macromolecules

Question 12

Eukaryotic Ribosomes

Answer 12

• Protein and RNA
  – 80S in size (60S + 40S subunits)
• May be attached to ER or free in
  the cytoplasm
• Proteins made on ribosomes of
  RER:
  – Often secreted or inserted into
  ER membrane as integral
  membrane proteins
• Proteins made by free ribosomes:
  – Nonsecretory and
  nonmembrane proteins
  – Some proteins are inserted into
  organelles

Question 13

Mitochondria

Answer 13

• Site of tricarboxylic acid cycle
• Site where ATP is generated by
  electron transport and oxidative
  phosphorylation
• About the size of a prokaryotic cell
• Some cells have one, and others
  have 1,000 or more
• Contain ribosomes, DNA, and
  calcium phosphate granules
  – Mitochondrial ribosomes are
  more the size of bacterial
  ribosomes
  – Mitochondrial DNA is often
  closed circle
• reproduce by binary fission

Question 14

Mitochondria Structure

Answer 14

• Outer membrane
  – Contains porins similar to the outer
  membrane of gram negative bacteria
• Inner membrane
  – Highly folded to form cristae
  – Location of enzymes and electron carriers
  for electron transport and oxidative
  phosphorylation
• Matrix
  – Contains ribosomes, mitochondrial DNA,
  and large calcium phosphate granules
  – Contains enzymes of the tricarboxylic acid
  cycle and enzymes involved in catabolism
  of fatty acids

Question 15

nucleus and cell division

Answer 15

• Membrane-bound organelle
  containing genetic material
• Chromatin
  – Dense fibrous material within the
  nucleus
  – Contains DNA
  – Condenses to form chromosomes
  during cell division
• Nuclear envelope
  – Double membrane
  – Penetrated by nuclear pores which
  allow passage of materials

Question 16

nucleolus

Answer 16

• 1 or more per nucleus
• Not membrane enclosed
• Important in ribosome
  synthesis
  – directs synthesis and processing
  of rRNA
  – assembles rRNA and ribosomal
  proteins to form ribosomes

Question 17

mitosis and meiosis

Answer 17

• Mitosis
  – One component of the cell cycle
  – Distributes DNA to 2 new nuclei
  – Ploidy (number of chromosomes) of progeny cells
  is the same as the parent
• thus, after mitosis, a diploid organism remains diploid
• Meiosis
  – Sexual phase of the life cycle
  – Complex, two-stage process of nuclear division
  – Number of chromosomes in the resulting progeny
  cells is reduced by ½
  – diploid → haploid
  – Haploid cells can act as gametes

Question 18

External Cell Coverings

Answer 18

• Vary greatly from prokaryotes
  – Many eukaryotes lack a cell wall
  – Eukaryotic membranes contain sterols, such as
  cholesterol, which make them mechanically
  stronger, reducing the need for a cell wall
• Many do have an external cell wall
  – Rigid, variable make-up
• algae – cellulose and pectin
• diatoms – silica
• fungi – chitin, cellulose, and glucan
  – Chemically simpler than bacterial peptidoglycan
• Many protists have a supportive mechanism
  below the plasma membrane called a pellicle

Question 19

Cilia and flagella

Answer 19

• Cilia (cilium)
  – 5-20 μm long
  – beat with two phases,
  working like oars
• Flagella (flagellum)
  – 100-200 μm long
  – move in undulating fashion

Question 20

Molecular Unity

Answer 20

• Same basic chemical composition
• Same genetic code
• Same basic metabolic processes