Plant Structure (Lecture 1) Flashcards
How are plants fundamentally different from animals?
They are immobile, rooted in the ground - mine the soil via their roots.
They are green - chlorophyll!
They are autotrophic - using sunlight and inorganic nutrients to make food.
They are rigid and tough, structurally reinforced by a cell wall.
They are plastic - they have an open form, with their shape depending on the environment.
They are decentralized - they don’t have a central controller (brain). Instead, they rely upon hormones and mobile RNAs.
They are chemical factories, with their adaptations solving their problems & making myriads of miraculous molecules.
How do plant cells and animal cells differ, in terms of their structure?
- Chloroplasts (plants): Biochemical factories for sugars, amino acids, many compounds
- Vacuole (plants): Central, space-filling storage and disposal dump.
-Plasmodesmata connects the cytoplasm of plant cells.
Thus, all cells in the plant are connected via these ‘pores, cytoplasmic extensions’
The cytosol inside the connected cell is called symplast.
How do plant cells and animal cells differ, in terms of their functions?
- Plant cells are less differentiated.
- Plant cells are often totipotent: can divide and differentiate (generate whole new plants!).
- Plant cells contain lots of carbohydrates, due to the chemical constituents of their cell walls.
What are the two main systems within a plant?
Root system (below ground) and shoot system (above ground).
Define: Root
Organ anchoring a vascular plant in the soil. It absorbs water and minerals while storing carbohydrates and other reserves.
Are the environments for the two systems different?
Yes! Roots must be adapted to a very different environment compared to shoots.
In which two ways can plants be organized?
- Organs: Leaves, roots, stems, flowers, fruit
2. Tissues: Groups of cells of one or more types, performing primarily one specialized function.
What are the three plant tissue systems?
Dermal, vascular, ground systems
Define: Dermal system
Covers the plants. Skin acting as the first line of defense against physical damage, pathogens.
In the shoots = Epidermis
In the roots = Periderm
Define: Vascular system
Transports water and nutrients, via xylem and phloem.
Long-distance transport between root and shoot systems.
Provides mechanical support.
Define: Ground tissue system
Everything between the dermal system and the vascular system, with lots of non-specialized cells.
Why is the tissue type considered a system?
Each tissue type forms a system that connects all of the plant’s organs - it is continuous.
What are the three types of simple tissues?
Parenchyma, collenchyma, sclerenchyma
What is parenchyma tissue?
- Most common, space-filling cells found in various tissues
- Thin cell walls, with there only being a primary wall (no secondary)
- Can differentiate into other types of cells
What are the functions of parenchyma tissue?
- Metabolism, photosynthesis in leaves
- Filling space: Giving plants their shape
- Storage: Starch, fructans, sucrose, proteins
- Wound repair: Reservoir of new cells
What are examples of parenchyma tissue?
- Leaf mesophyll
- Palisade cells
- Root cortex
- Stem pith
What is collenchyma tissue?
- Elongated, thick-walled cells with reinforced corners
- Their primary walls are unevenly thickened
- Not as common as the other types of cells
- Living and flexible at maturity
What is the function of collenchyma tissue?
-Flexible support, in elongating stems and leaves
What is sclerenchyma tissue?
- Very thick-walled cells, often dead at maturity
- Comprise a secondary cell wall, reinforced with lignin
What are the functions of sclerenchyma tissue?
- Protection: Stones in fruit, hard seed coats
- Support: Flax, hemp fibres in stems
Aka – the rigid walls serve as a rigid ‘skeleton’
What are the two types of complex tissue?
Xylem and phloem
What are complex tissues comprised of?
Several cell types in vascular bundles. They are found in leaves, roots, stems, petioles.
What is xylem?
It is the water-conducting tissue of plants.
What types of simple tissues does the xylem contain?
Primarily dead cells - Parenchyma and sclerenchyma.
What are the two sub-types of xylem?
- TRACHEIDS: Spindle-shaped, water-conducting cells, with bordered pits for water flow. Found in all higher plants.
- VESSEL ELEMENTS: Barrel-shaped and line up end to end, with perforation plates for water flow. Found only in angiosperms (flowering plants).
How do the bordered pits of tracheids work?
The pits allow the passage of water, without the need to cross a thick secondary wall.
What is the key characteristic of xylem?
Lignin: A complex polymer of cross-linked phenolics.
It prevents the collapse of the cell due to the pressures of water transport, while also providing support.
What is the role of a perforation plate in a vessel element?
It allows the free flow of water.
What is phloem?
Nutrient-conducting tissue (sugars, organic molecules), alive at maturity
What are the components of phloem?
Sieve tubes, companion cells, fibers
Define: Sieve elements
- Chains of cells connecting via sieve plates.
- SEs have continuous cytoplasm
- Anucleate
- Have specialized sieve plates (end walls between SEs), with pores to facilitate the flow of fluid from cell to cell, along the sieve tubes
What is the function of companion cells?
They have nuclei, thus producing RNA and proteins for the sieve element cells.
In vascular bundles of dicot plants, where is the xylem? Where is phloem?
Xylem is generally on the inside of the stem, phloem is generally on the outside of the stem.
In grass and monocots, it is more mixed.
What are meristems?
Growth region in plants found within the root tips and the tips of new shoots and leaves.
It is the tissue in which growth occurs = Cell division growth zones.
Where/how do roots develop?
At the root meristems!
A zone of cell division (mitosis) is just behind the root cap. This zone includes the apical meristem - with the root cap protecting it.
Then, there is the zone of elongation - where root cells elongate and where most of the growth occurs.
Finally, the zone of differentiation is where the epidermal root hairs form. The cells complete their differentiation and become distinct.
What are apical meristems?
Those responsible for primary growth: Elongation of roots, production of new stems and leaves.
They are unspecialized, continuously dividing, some into more meristematic cells, others into structural or vascular cells.
How do branch roots form?
From the pericycle, a layer of cells around the vascular cylinder.
Where do shoots develop? How?
At the shoot apical meristems = Tips of plants.
These are dome-shaped apical meristems, covered by tiny leaves.
The leaf primordial form periodically from the meristem.
What are axillary meristems?
They are also known as axillary buds. Each bud has the potential to form shoots. It remains dormant when there is strong apical dominance (inhibited by the auxin produced by the apical meristem).
What are the two types of lateral meristems?
The vascular cambium and the cork cambium, which are responsible for secondary growth - growth in thickness.
In which type of plants is secondary growth important?
In woody plants, aka trees.
What is the vascular cambium?
A cylinder of meristematic cells, which gives rise to xylem cells to the inside, phloem cells to the outside.
What is the result of vascular cambium?
Secondary xylem: Wood, in annual rings, due to the seasonal pattern of xylem formation.
What do older trees develop that younger trees don’t?
They develop heartwood, which is darker than sapwood and full of anti-fungal chemicals to prevent rotting.
What happens to the older layers of secondary xylem, as a woody plant age?
They no longer transport water and minerals, therefore become dead at maturity = Xylem sap
What is the cork cambium?
The meristematic (aka dividing) layer of cells in the outer cortex, which gives rise to the periderm.
What is the periderm?
The suberized (waxy) protective tissue on the outside of perennial roots and stems.
What does bark include?
Periderm and secondary phloem.
Where is secondary growth found?
In all gymnosperms and many flowering plants, but is rare in monocots.
In what type of tissue would you find the phenolic substance called lignin?
Simple tissue: Sclerenchyma cells that have thick walls (including secondary walls), reinforced with lignin
Complex tissue (vascular): Xylem cells, which require the lignin to harden and prevent the collapse of the cells during the water transport from the roots to the shoots.
Which cells are dead at maturity but still functional?
Xylem cells, notably those within the tracheid and vessel elements.
The lignin is needed to prevent cell lapsing in moving water upwards.
