Week 2 - Cells, mitosis/cytokinesis, phragmoplast, preprophase band, cell walls, cell types, tissues, ground tissues Flashcards
What is a cell?
A self-propgating compartment bound by a semi-permeable membrane and filled with protoplasm (nucleus and cytoplasm)
What are characteristics of plant cells that animals don’t have?
- Chloroplasts
- Vacuole
- Cell wall
Describe mitosis and cytokinesis in plants?
During prophase when the nuclear membrane breaks down and the DNA supercoils microtubules form a preprophase band. In metaphase the chromosomes move to the middle. During anaphase the chromosomes are pulled apart by spindle fibres/ microtubles . In telphase and cytokinesis, the phragmoplast begins to form in the place of the spindle fibre equator. The cell then splits as the cell plate grows centrifugally (outwardly)
What is the Preprophase band? What is it’s role?
The Prephrophase band (PPB) is a concentrated ring made of microtubules circiling the nucleus. It is positioned carefully because it determines where the cell divides.
What is the phragmoplast? What is it’s role?
The phragmoplast is formed from parallel microtubles in the plane of the former spindle fibre equator. It helps transport vesicles with components for the cell wall and acts as a framework for the new cell wall.
What are plasmodesmata? How are they formed?
Plasmodesmata are cytoplasmic connections between cells. They allow for transport of water through the plant. They are formed during the formation of the cell wall when endoplasmic reticulum (ER) are trapped between between fusing vesicles.
What are the types of cell walls?
There is a primary cell wall (outside) and a secondary cell wall (inside)
How is the primary cell wall formed?
The primary cell wall (outside) is mainly made of cellulose packed into microfibrils.
Cellulose synthesising enzymes are embedded in the plasma membrane; they take glucose on the inside of the membrane and push it out, forming cellulose chains that weave together and form the primary wall.
What are some characteristics of the primary cell wall?
The cellulose provides strength. Hemicellulose and pectin provide strength as well as flexibility. The cellulose crytalline structure means it is not flexible, although it can slide. As a cell grows, the direction of the microfibrils determines the direction of expansion.
What is the primary pit field? What are some additions?
Occurs where clusters of plasmodesmata form pits in the wall; these pits are used for transport (eg. water). The gaps on the wall are called desmotubules. Desmotubules are formed by endoplasmic reticulum (ER) tubules that pass through the plasmodestmata.
Once there is a secondary cell wall, it forms a pit membrane (actually a wall), some pits form cavities and some have a torus to block water transport.
When are secondary cell walls present? What is the role of the secondary wall? What are some characteristics?
The secondary cell wall occurs once cells have stopped dividing and begin to differentiate. The role of the secondary cell wall is to provide mechanical strength.
The secondary cell wall has more cellulose as well as lignin and less pectin. It does not cover primary pits.
What are the tissue systems? What are the types of meristems involved?
- Ground tissue System (packing and supporting) (ground meristems)
- Dermal tissue system (epidermis) (protoderm meristems)
- Vascular tissue system (xylem, phloem) (procambium meristems)
What is the ground tissue system?
consists of packing and supporting tissues (mesophyll in leaves, cortex and pith in stems and the cortex in roots)
What are the types of tissues in the ground tissue system?
- Parenchyma tissue
- Collenchyma tissue
- Sclerenchyma tissue
What is parenchyma tissue? What are it’s characteristics? What are it’s functions?
Parenchyma tissues are made of unspecialised parenchyma cells (living) and other constituents that make up the cortex, pith, mesophyll, and fruit flesh.
They appear isodiametric (TS) and cylindrical (LS). They begin as circles but squish against each other.
Functions:
- support (tugur pressure)
- storage (oils, starch, sugars, etc..)
- regeneration (unspecialised cells allow cell division)
- photosynethesis (chlorenchyma)
- Boyancy (aerenchyma)
- Movement of solutes
What are chlorenchyma and aerenchyma? - they are part of parenchyma
Chlorenchyma - parenchyma cells with chloroplasts allow light to pass through to chloroplasts and do photosynethesis
Aerenchyma - parenchyma tissues with excessive air spaces, allows boyancy
What is collenchyma? What are its features? What are its functions?
Collenchyma is elongated and thickened ground tissue. Collenchyma is living when mature and has thickened primary walls for support. Collenchyma is flexible, allowing elongation, and is therefore frequently found in growing tissues. The metabolic cost of producing collenchyma is greater than that of parenchyma due to the extra glucose required for thickening primary walls.
What is sclerenchyma? What are its features? What are the types of sclerenchyma? What is its function?
Sclerenchyma is a ground tissue. It has secondary walls, and it is often dead at maturity. It cannot elongate once fully developed due to the lignified secondary walls. The two types of sclerenchyma are sclereids and fibres. Sclerenchyma is used for strength/support and protection.
What are sclereids? What are their characteristics? What is its function?
Sclereids are a short/reduced form of sclerenchyma cells that come in a variety of shapes. Asterosclereids (star), Osteiscereids (bone shape), Macrosclereids (columnar), Brachysclereids/stone cells (isodiametric).
Sclereids are found in all the places ground tissues are found; they can be in groups or singly. Randomly distributed sclereids may give structure/support to parenchyma tissue. Sclereids can also be deterrents to herbivores due to stone cells (brachysclereids).
What are fibres? What are the two types of fibres? What are their features and functions?
Fibres are long, slender sclerenchyma tissues that are usually lignified. The fibres occur in bundles or singly. Extra-xylary fibres are longer than xylary fibres and found where strength and elasticity are important.
Xylary fibres - develop from the same meristematic tissue as xylem (not ground tissues).
Extra-xylary fibres - Occur in places other than the xylem. They form sheaths around vascular bundles in monocots and bundle caps in eudicots. They develop partly from procambium and partly from ground tissues.
Bast fibres - found in phloem and bundle caps.
