cell wall components Flashcards
cell wall components
- cellulose
- hemicelluloses
- pectins
- other components (proteins, lignin, cutin)
cellulose
-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
hemicelluloses
-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
pectins
- 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
other components may be in or on the wall
- proteins (catalyze)
- lignin (adds rigidity to a wall are non elastic and waterproof)
- cutin, suberin, and waxes to reduce water loss
wall layers
primary and secondary
meristematic
only primary wall
plasmodesmata
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)
primary pit fields
connections between cells
- plasmodesmata concentrated in certain areas
- primary wall is thinner
Pits
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
membrane
- control passage of material into and out of cells
- phospholipid bilayer
- unsaturated fatty acid tails
- more saturated = less fluid
membrane molecules
- transmembrane proteins
- peripheral proteins
- glycoproteins
- sterols
two modes of passive transport
- simple diffusion
- facilitated diffusion
membrane movements
passive or active
passive
transport follows a concentrations gradient
-no energy required
active
requires energy input by the cell
simple diffusion
Small molecules pass through membrane lipids
-route blocked for charged particles
facilitated diffusion
Proteins embedded in membrane permit passage of selected substances
facilitated types
carrier and channel
osmosis
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
carrier
1. Carrier proteins Not open continuously Recognize specific substances Bind substances for transport may be uniport or cotransporter
uniport
unidirectional transport of a single substance
cotransporter
two solutes transported,
symport and antiport
channel proteins
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
active transport
- 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
Primary active transport
Energy generated via an ion pump
A transmembrane protein hydrolyzes ATP
Energy used to pump H+ ions across a membrane
Pumped against their gradient
Secondary active transport
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
ion pumps in plants
proton ATPase pump at PM
proton ATPase pump at tonoplast
Calcium-ATPase or calcium-proton- ATPase pumps
Proton-ATPase pump at PM
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
Proton-ATPase pump at tonoplast
This acidifies the vacuole
May facilitate storage function
Calcium-ATPase or calcium-proton- ATPase pumps
Regulate [Ca+2] in the cytoplasm and ER
Important for some physiological responses
Vesicle-mediated transport
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
