2.1.5 other features of eukaryotic cells

Question 1

which organelles are not covered by membranes

Answer 1

• ribosomes
• cytoskeleton (incl. centrioles)

Question 2

structure of ribosomes

Answer 2

• small, spherical & about 20nm in diameter
• made of ribosomal RNA
• made in nucleolus as 2 separate subunits
• pass through nuclear envelope into cytoplasm & combine
• some remain free in cytoplasm & some attach to RER

Question 3

function of ribosomes

Answer 3

• ribosomes bound to RER are mainly for synthesising proteins that’ll be exported out cell
• free ribosomes (cytoplasm) are primarily site of assembly of proteins that’ll be used in cell

Question 4

structure of centrioles

Answer 4

• consist of 2 bundles of microtubules at right angles to each other
• microtubules made of tubulin protein subunits
• arranged to form cylinder

Question 5

function of centrioles
- cell division

Answer 5

• before cell division = spindle (threads of tubulin) forms from centrioles
• chromosomes attach to middle part of spindle & motor proteins walk along tubulin threads to pull chromosomes to opposite poles

Question 6

function of centrioles
- formation of cilia/undulipodium

Answer 6

• before cilia form, centrioles multiply & line up beneath cell surface membrane
• microtubules then sprout outwards from each centriole

Question 7

are there centrioles in plants

Answer 7

usually absent from (higher) plants
–> may be present in some unicellular green algae

Question 8

structure of cytoskelton

Answer 8

• network of protein structures in cytoplasm

consists of:
- rod-like microfilaments made of subunits of protein actin (polymers of actin & each = abt 7nm)
- intermediate filaments (abt 10nm)
- straight, cylindrical microtubules - made of protein subunits tubulin (abt 18-30nm)
- cytoskeletal motor proteins = molecular motors, enzymes & have site that binds to/allows hydrolysis of ATP as energy source

Question 9

3 cytoskeletal motor proteins

Answer 9

myosins, kinesins & dyneins

Question 10

function of cytoskeleton

Answer 10

• give support & mechanical strength
• keep cell’s shape stable & allow movement
• also provide shape/support to cells
• help substances/organelles move through cytoplasm in cell
• intermediate filaments made of variety of proteins

Question 11

how does the cytoskeleton help substances/organelles move through cytoplasm in cell

Answer 11

• form track along which motor proteins (dynein & kinesin) walk & drag organelles from 1 part of cell to other
• form spindle before cell divides = enable chromosomes to be moved within cell
• microtubules make up cilia, undulipodia & centrioles

Question 12

2 motor proteins

Answer 12

dynein & kinesin

Question 13

what do intermediate filaments do (as part of cytoskeleton)

Answer 13

• anchor nucleus in cytoplasm
• extend between cells in some tissues, between special junctions, enable cell-cell signalling & allow cells to adhere to a basement membrane = stabilising tissues

Question 14

structure of cellulose cell wall

Answer 14

• outside of plasma membrane
• made from bundles of cellulose fibres

Question 15

function of cellulose cell wall

Answer 15

• strong & can prevent plant cells bursting if turgid
• provide strength/support
• maintain cell’s shape
• contribute to strength/support of whole plant
• permeable & allow solutions to pass through

Question 16

what cell walls do fungi have

Answer 16

contain chitin