Tour of the cell

Question 1

What are the basic features that all cells have in common?

Answer 1

• bounded by a plasma membrane
• contain cytosol
• contain chromosomes
• have ribosomes

Question 2

What is a cell?

Answer 2

the simplest collection of matter than can live

Question 3

what are the characteristics of prokaryotic cells?

Answer 3

-> Bacteria and archaea
- no nucleus
- dna located in nucleoid (=unbound region)
- no membrane-bound organelles
- cytoplasm bound by the plasma membrane

Question 4

What are the characteristics of eukaryotic cells?£

Answer 4

-> protists, fungi, animals and plants

• DNA in nucleus that is bound by nuclear envelope
• membrane-bound organelles
• cytoplasm between plasma membrane and nucleus
• generally larger than prokaryotes

Question 5

plasma membrane

Answer 5

• selective barrier that allows sufficient passage of oxygen, nutrients and waste to service the cell volume
• double layer of phospholipids
• each square micrometer of membrane only limited amount of a particular substance can cross per second

Question 6

nucleus: information center

Answer 6

• most of the genes in eukaryotes
• most noticeable organelle
• nuclear envelope (double membrane) encloses the nucleus
• pores for export and import
• shape regulated by nuclear lamina (protein filaments)
• nuclear matrix (framework of protein fibers throughout the interior)
• chromatin =DNA + proteins -> condenses to chromosomes
• nucleolus = rRNA synthesis
• nucleosome= DNa+ histones

Question 7

Ribosomes: factory machines

Answer 7

• complexes of rRNA and proteins
• free ribosomes in cytosol
• bound ribosomes at rER or nuclear envelope
• small and large subunits

Question 8

Endomembrane system

Answer 8

• regulates protein traffic and performs metabolic functions in the cell
• nuclear membrane
• ER
• Golgi
• lysosomes
• vacuoles
• plasma membrane
• they are either continuous or connected via tiny vesicles

Question 9

Endoplasmatic reticulum : factory floor

Answer 9

• accounts for more than half of the total membrane in eukaryotic cells
• extensive network of membranous tubules and sacs -> cisternae
• inside space: lumen
• membrane is continous with nuclear envelope
• smooth ER: lacks ribosomes
  -> produce steroids, lipids
• rough ER: has ribosomes
  -> produce proteins

Question 10

smooth ER

Answer 10

• synthesis of lipids
• metabolism of carbohydrates
• detoxification of drugs and poisons
  (in liver cells, OH group is added to drug molecules to make them more soluble and easier to flush)
  ( some drugs like barbiturates and alcohol induce the proliferation of sER and detoxifying enzymes -> increases rate of detox -> increases drug tolerance)
• storage of calcium -> stimulates muscle contraction

Question 11

rough ER

Answer 11

• growing polypeptides on ribosomes -> threaded into the ER lumen through a pore formed by protein complex on ER membrane
• polypeptide folds into native shape
• destined for secretion
• carbohydrates are attached to secretory proteins (= glycoproteins)
  -> secretory proteins leave ER by transport vesicles
• membrane factory of the cell
  -> grows by adding membrane proteins and phospholipids to its own membrane, expands and portions of it are transferred to other parts of endomembrane system via transport vesicles

Question 12

Golgi apparatus: shipping departement

Answer 12

• flattened membranous sacs =cisternae
• modification of products of rER
• manufacture of certain macromolecules
• sorts and packages materials into transport vesicles
• has distinct structural directionality -> cisternae membranes on opposite sides of stack differ in thickness and composition
• cis -> receive - trans -> ship
• proteins and lipis from cis to trans golgi are further modified
• carbohydrates on glycoproteins are modified via replacement of sugar monomers
• membrane phospholipids are modified
• addition of molecular tags (eg phosphate groups)
• product refinement
• cisternal maturation model : cisternae progress forward from cis to trans face, carrying and modifying their cargo along the way
• golgi also manufactures some macromolecules

Question 13

lysosomes: clean up crew

Answer 13

• membranous sac of hydrolytic enzmyes
• digest all kinds of macromolecules
• work best in acidic environment
• hydrolytic enzyme and lysosomal membrane are made in rER and transferred in Golgi for further processing
• intercellular digestion by phagocytosis and autophagy

Question 14

lysosome phagocytosis

Answer 14

• engulf another cell creating a food vacuole
• lysosome fuses with food vacuole and digest the molecules
• products pass into the cytosol and become nutrients for the cell
• used by some protists to digest food
• human macrophages use lysosomes as defence mechanism

Question 15

lysosome autophagy

Answer 15

lysosomes use enzymes to recycle the cell’s own organelles and macromolecules
- breaks down damaged organelles to simple sugars, amino acids and fats
- organic monomers are returned to cytosol for reuse

Question 16

Vacuole: storage/maintenance

Answer 16

• in plants and fungi
• derived from rER and golgi
• main repository of inorganic ions, incl potassium and chloride
• plays major role in growth of plant cells
• food vacuole: formed by phagocytosis
• contractile vacuole: found in freshwater protists, pump excess water out of cells
• central vacuole: found in mature plant cells, hold organic compounds and water , store ions, hold dangerous by-products, some contain pigments, store poison as defense

Question 17

mitochondria and chloroplasts

Answer 17

• change energy from one form to another
• double membrane
• not part of endomembrane system
• proteins from free ribosomes
• contain own DNA
• semiautonomous organelles
• chloroplast: found in plants and algae
• sites of photosynthesis - convert solar energy into chemical energy
• mitochondria:
  – sites of cellular respiration
• peroxisomes: oxidative organelles

Question 18

endosymbiont theory

Answer 18

• early ancestor of eukaryotic cells engulfed oxygen-using nonphotosynthetic cell prokaryotic cell
• endosymbiosis
• host cell and endosymbiont merged into a single organism -> eukaryotic cell with mitochondion

Question 19

mitochondria: chemical energy conversion

Answer 19

• found in nearly all eukaryotic cells
• some have only one huge one, but majority have hundreds
• smooth outer membrane and inner membrane is folded into cristae
• inner membrane creates intermembrane space and mitochondrial matrix
• mitochondrial matrix contains enzymes, mDNA and ribosomes
  – some metabolic steps are catalysed there
• cristae to increase surface area
• enzyme for ATP production is built into inner membrane

Question 20

Chloroplasts: capture light energy

Answer 20

• specialised members of closely related plant organelles =plastids
• green pigment chlorophyll and enzymes for photosynthesis
• thylakoids= membranous sacs, stacked to form granum
• stroma= fluid outside the thylakoids

Question 21

peroxisomes: oxidation

Answer 21

• specialised metabolic compartments bounded by a single membrane
• contain enzymes that remove hydrogen atoms from substances and transfer them to oxygen -> produces hydrogen peroxide H2O2 and convert it into water
• oxygen produced is used to break down molecules
• detoxification of alcohol in liver
• glycosomes in fat storing tissues of plant seeds, convert fatty acid to sugar to feed the emerging seedling
• grow larger by taking proteins from cytosol and lipids from ER
• increase in numbers by splitting into 2 when they reach a certain size

Question 22

Cytoskeleton: support, motility and regulation

Answer 22

network of fibers extending throughout the cytoplasm

• organises the cell’s structures and activities, anchoring organelles
• support the cell and maintain its shape
• vesicles can use motor protein feet to travel along the tracks provided by cytoskeleton

Question 23

What are the roles of cytoskeletons

Answer 23

• interacts with motorproteins to produce motility (microtubules, sometimes microfilaments)
• dynamic structure - easily dismantles and reassembles
• vesicles can travel along monorails
• may help regulate biochemical activities

Question 24

3 main types of cytoskeleton

Answer 24

• microtubules
• microfilaments: actin
• intermediate filaments

Question 25

microtubules

Answer 25

• hollow rods of tubulin molecules
• 25nm diameter, 200nm to 25 micrometer long
• grow by adding tubulin dimers
  – plus end: accumulates or releases tubulin dimers faster than other end
• shape and support the cell
• guide movement of organelles
• help separate chromosomes
• centrosome near nucleus -> microtuble-organising center
  – centrosome has a pair of centrioles ( each with 9 triplets of microtubules organised into a ring)
• not essential in all eukaryotes
• cilia and flagella
  differ in beating patterns

Question 26

cilia and flagella

Answer 26

• group of microtubules sheathed in an extension of the plasma membrane
• 9x2 microtubules arranged in a ring with 2 single microtubules in center
• basal body that anchors the cilium or flagellum
• dynein (motor protein) drives the bending movement , uses ATP

Question 27

microfilaments

Answer 27

• actin
• solid rods about 7nm diameter
• twisted double chain of actin subunits
• bears tension, resisting pulling forces in the cell
• 3D network = cortex inside the plasma membrane to help support the shape
• microvilli of intestinal cells
  – increase surface area, transport materials across plasma membrane
• actin + myosin (motor protein)
  – cellular motility
  – muscle cells, parallel arrangement of actin
  – amoeboid movement: actin and myosin
  -> pseudopodia (cellular extension) extend and contract, filaments in the direction of cell movement lengthen and shorten in the opposite direction
• cytosplasmic streaming : circular flow of cytoplasm within cell
  – speeds distribution
  – drive: actinmyosin and sol-gel transformation

Question 28

intermediate filaments

Answer 28

• diameter 8-12 nm
• support cell shape
• fix organelles in place
• more permanent than the others
• composed of protein family incl keratin
• fibrous proteins coiled into cables

Question 29

Extracellular components to help coordinate cellular activities

Answer 29

• cell wall
• ECM of animal cells
• intercellular junctions

Question 30

cell wall

Answer 30

made of cellulose fibers embedded in other polysaccharides and proteins

• primary cell wall: relatively thin and flexible
• middle lamella: thin layer between primary wall and wall of adjacent cells
  – rich in pectin (polysaccharide)
  – strengthens the wall of mature cells
• secondary cell wall: added between plasma membrane and primary wall
  – strong matrix that offers protection and support

Question 31

ECM

Answer 31

• glycoproteins
  – collagen form strong fibers outside cell
  – proteoglycan build fibers
  – fibronectin interacts with integrin
• ECM proteins bind to receptor proteins in plasma membrane -> integrins
• support
• adhesion
• movement
• regulation of cell behaviour
• influence activity of genes in nucleus

Question 32

Intercellular junctions

Answer 32

• neighbouring cells often adhere, interact and communicate through direct physical contact
• plasmodesmata
• tight junctions
• desomosomes
• gap junctions

Question 33

plasmodesmata in plant cells

Answer 33

• channels that perforate plant cell walls
• water and small solutes can pass from cell to cell

Question 34

tight junctions

Answer 34

membranes of neighbouring cells are pressed together, preventing leakage of extracellular fluid

Question 35

desmosomes

Answer 35

anchoring junctions
fasten cells together into strong sheets

Question 36

gap junctions

Answer 36

communication juctions
provide cytoplasmatic channels between adjacent cells through which ions, sugars, amino acids and other small molecules can pass