cell wall components

Question 1

cell wall components

Answer 1

• cellulose
• hemicelluloses
• pectins
• other components (proteins, lignin, cutin)

Question 2

cellulose

Answer 2

-repeating monomers of b-glubose attached with 1-4 bonds
-hydrophillic
-long microfibrils
microfibrils wind together into macrofibrils (high tensile strength)
-oriented same direction as the underlying microtubule
-means a cell will expand more easily in one direction than another

Question 3

hemicelluloses

Answer 3

-shorter polysaccharides
-not very hydrophilic
-wide variety of types (page 38)
-cross link cellulose microfibrils by H bonds
limits cell wall extensibility - regulates cell enlargement

Question 4

pectins

Answer 4

• hydrophilic and form gels
• impart some plasticity to a cell wall - can stretch as cell expands
• cross links with cellulose by Mg and Ca complexes
• also form the middle lamella which cements two adjacent cells together

Question 5

other components may be in or on the wall

Answer 5

• proteins (catalyze)
• lignin (adds rigidity to a wall are non elastic and waterproof)
• cutin, suberin, and waxes to reduce water loss

Question 6

wall layers

Answer 6

primary and secondary

Question 7

meristematic

Answer 7

only primary wall

Question 8

plasmodesmata

Answer 8

narrow channels b/w cells

• traversed by a tubule called desmotubule
• form as strands of ER are trapped within a developing cell late
• cytoplasmic connection b/w cells
• may be involved in electrical signaling
• size limits due to diameter of ER fragment and spokes of actin (some viruses manipulate spokes)

Question 9

primary pit fields

Answer 9

connections between cells

• plasmodesmata concentrated in certain areas
• primary wall is thinner

Question 10

Pits

Answer 10

located on walls b/w 2 adjacent cells

• cells may have only primary walls
• if secondary wall deposition, then there are specific sites with no secondary wall deposition called pits
• primary wall is thinner in these locations = pit membrane
• pits may be simple or bordered

Question 11

membrane

Answer 11

• control passage of material into and out of cells
• phospholipid bilayer
• unsaturated fatty acid tails
• more saturated = less fluid

Question 12

membrane molecules

Answer 12

• transmembrane proteins
• peripheral proteins
• glycoproteins
• sterols

Question 13

two modes of passive transport

Answer 13

• simple diffusion

- facilitated diffusion

Question 14

membrane movements

Answer 14

passive or active

Question 15

passive

Answer 15

transport follows a concentrations gradient

-no energy required

Question 16

active

Answer 16

requires energy input by the cell

Question 17

simple diffusion

Answer 17

Small molecules pass through membrane lipids

-route blocked for charged particles

Question 18

facilitated diffusion

Answer 18

Proteins embedded in membrane permit passage of selected substances

Question 19

facilitated types

Answer 19

carrier and channel

Question 20

osmosis

Answer 20

A special case of simple diffusion involving water
Passive, but speed of movement dependent on
the gradient
Water actually goes through the lipid bilayer (not aquaporins)
Cells can actively establish a gradient to facilitate osmosis

Is affected by pressure
Important in plant cells because they depend on
turgor pressure for support
Plants wilt when they lose too much turgor pressure ie. Lose water
Cells may plasmolyze and even die

Question 21

carrier

Answer 21

1. Carrier proteins
  Not open continuously
Recognize specific substances
Bind substances for transport
may be uniport or cotransporter

Question 22

uniport

Answer 22

unidirectional transport of a single substance

Question 23

cotransporter

Answer 23

two solutes transported,

symport and antiport

Question 24

channel proteins

Answer 24

Can be always open or gated
If gated, usually a signal to open the channel
But, whether open or gated, only specific
substances allowed through
A specific type of channel protein important in plants are aquaporins
These allow only water to pass
Aquaporins greatly enhance the speed of water movements into and out of cells

Question 25

active transport

Answer 25

• primary and secondary
  Moves substances against their concentration gradient
  This requires energy input from the cell
  Energy can be supplied as chemical (ATP),
  electrical or light
  Most often in plants this is powered by a proton (H+ ions) pump
  eg. Chloroplast ion pumps use light energy to pump H+ ions and this powers ATP synthesis

Question 26

Primary active transport

Answer 26

Energy generated via an ion pump
A transmembrane protein hydrolyzes ATP
Energy used to pump H+ ions across a membrane
Pumped against their gradient

Question 27

Secondary active transport

Answer 27

H ions follow their gradient back into the cell
Are only certain locations where they can do so
Another molecule is co-transported with the H+ ions eg. Sucrose
H+ ions are only allowed if the other substance is present
Again can be symports eg. Sucrose into phloem or antiports eg. Heavy metals into vacuole

Question 28

ion pumps in plants

Answer 28

proton ATPase pump at PM
proton ATPase pump at tonoplast
Calcium-ATPase or calcium-proton- ATPase pumps

Question 29

Proton-ATPase pump at PM

Answer 29

Generates a negative membrane charge
Important for facilitated diffusion and 2o active transport
Also acidifies the wall
This is important for cell expansion
Leads to an alkaline cytosol
This promotes the activity of cytoplasmic enzymes

Question 30

Proton-ATPase pump at tonoplast

Answer 30

This acidifies the vacuole

May facilitate storage function

Question 31

Calcium-ATPase or calcium-proton- ATPase pumps

Answer 31

Regulate [Ca+2] in the cytoplasm and ER

Important for some physiological responses

Question 32

Vesicle-mediated transport

Answer 32

or molecules too large for transporters
eg. Proteins, debris, polysaccharides
Vesicles bud off PM or fuse with PM— where from?
Exocytosis releases from cytoplasm or into vacuole
Endocytosis takes into cytoplasm