Lecture #14 - Cell wall Flashcards
Explain the formation of cell wall
- A young plant cell first secretes a relatively thin and flexible wall called the PRIMARY CELL WALL.
- Bw the primary walls of adjacent cells is the MIDDLE LAMELLA, a thin layer rich in sticky polysaccharides called PECTINS. The middle lamella glues adjacent cells together. When the cell matures and stops growing, it strengthens its walls.
- Some plant cells do this simply by secreting hardening substances into the primary wall.
- Other cells add a SECONDARY CELL WALL bw plasma and primary. It’s deposited in several laminated layers and has a strong and durable matrix that offers the cell protection and support
What’s so special about cellulose - the major component of cell wall? (4)
- The most abundant organic macromolecule on Earth
- A glucose poymer
- Highly ordered (crystalline)
- Long, ribbon-like structures
Note: the O bonds make cellulose inaccessible to some organisms eg us AND PLANTS! They can’t metabolise it after it’s built
Celulose forms _____
These highly organised structures are……….
Celulose forms MICROFIBRILS
These highly organised structures are strong and form a major component of both primary and secondary cell walls
Cellulose microfibrils real strong - if there was a FE microfibril and a cellulose microfibril, cellulose would win.
The primary cell wall is…..(3)
The percentages of the cell wall composition
Crystalline microfibrillar phase and
Noncrystalline matrix
- Produced by young cells
- Relatively thin and flexible
- Cells are still able to grow (support comes from cell wall - grow by expanding so need to be thin and flexible)
25 - 30% Cellulose
15 - 25% Hemicellulose
35% Pectin
5 - 10% Protein (extensin) - cellulose is not a passive structure i.e. it can detect when a pathogen attacks
Crystalline microfibrillar phase
-Cellulose
Noncrystalline matrix
- Pectin polysaccharides and
- Hemicellulosic polysaccharides
Cell wall composition: Noncrystalline matrix-hemicellulose and pectin - explain ‘em (3 points each)
Hemicellulose:
- A heterogeneous group of polysaccharides
- Long chain of one type of sugar and short side chains
- Forms a rigid structure
Pectin:
- Branched
- Negatively charged polysaccharides
- Bind water and have gel-like properties (like setting agent in jam)
Primary cell wall proteins are called e____ and explain ‘em
- EXTENSINS are associated with cell wall extensibility (ablility to extend - the ability of the protoplasm to push against cell wall and cause expansion/extension).
- Extensins cross-linking of pectin and cellulose dehydrates the cell wall, reduces extensibility and increases strength
- Secondary cell wall is not this dynamic - cell can regulate (by enzymes) which kinda extensin arrangement to have.
- The cross-linking (cellulose + extensins) causes dehydration in cell wall so makes it stronger - less flexible.
Synthesis of primary cell wall (3 steps)
Ish is being made inside and being dumped out
Co-ordinated synthesis and delivery of:
- Cellulose microfibrils made at plasma membrane (made in plasma membrane and secreted ot in the outside space)
- Polysaccharides (pectin and hemicellulose) made in the Golgi apparatus and transported to the wall in vesicles (secretory vesicles)
- Cell wall proteins (extensins) made from the rough ER and travel to golgi (transported via golgi - same way as #2 - via secreotry vesicles).
The vesicles fuse to the plasma membrane.
Cellulose synthesis - explain
Model of cellulose synthesis by a multisubunit complex containing the enzyme cellulose synthase. It makes the cellulose and spans all the way through the plasma membrane
Basically, sucrose is made of glucose + fructose and cellulose is made of glucose so sucrose is cleaved and the Cellu Synth binds glucose to glucose and excretes em out.
Sucrose cleaved and makes long chains of cellulose which then interact with extensin, pectin etc
Why is it important that the cellulose synthase spans the whole membrane?
It can interact with the microtubules inside the cell and travel along them forming cellulose on the outside .
The cellulose-producing rosettes move parallel to the cortical (relating to the cortex - outer layer) microtubules
The cellulose location is heavily monitiored bc it helps cells have different shapes
Appreciate this image
Long cellulose microfibrils - crosslinked by red hemicellulose chains and also got crosslinking by pectin too. Reasonably structured but still got room for expansion.
Cell wall function is (3)
- Structural support and influences cell morphology (the way cellulose laid down is imp)
- Protection (eg we can’t eat cell wall)
- Prevents excessive water uptake (relates to turgor pressure).
Cell morphology
Orientation of the cellulose microfibrils influences cell morphology
(a) Randomly oriented: the cell will…..
(b) Right angles to the ultimate long axis of the cell: the cell will……
Orientation of the cellulose microfibrils influences cell morphology
(a) Randomly oriented: the cell will expand equally in all directions (air space in this so cells like these are spongy mesophyll etc)
(b) Right angles to the ultimate long axis of the cell: the cell will expand longitudinally along that axis (harvest most light by packing together heaps of these cells)
Cell wall offers protection - tell me more
Cell wall acts as a protec&ve barrier Not just passive protec&on
E.g., this Sorghum plant is responding to fungal infection.
The infected cell produces fungicide-containing inclusion bodies (IB), the neighbouring cell secrete fungicide into the cell walls (black arrows)
Neighbouring cells can also detect and put something in cell wall for protection
Cell wall function: limiting water uptake - tell me more
The plant vacuole has a negatuve osmotic potential and water will enter the cell by osmosis
The plant cell wall limits the amount of water taken up – due to turgor pressure
Turgor pressure results in rigid cells
– this also contributes to structural support
(stops water taking up too much water if protoplasm pushing hard against cell wall - could burst)
The secondary cell wall (4 points)
- Not all plant cells have a secondary cell wall
- Produced only after cell growth has stopped
- Thicker and stronger than primary cell walls
- Provides more structural support than primary cell wall
Secondary cell wall structure - tell me more
- Made of multiple layers
- Microfibrils in each layer have different orientations
- This strengthens the secondary wall.
These are xylem cells who have a hollow thing in middle.
Tell me 3 things about Lignin
- Lignin is the second most abundant organic macromolecule
- Lignin is a complex phenolic polymer
- Confers strength, rigidity and hydrophobicity to the secondary cell wall
Resistant to being broken down - hard to get water or other ish in too
