Plant Structure (Lecture 1)

Question 1

How are plants fundamentally different from animals?

Answer 1

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.

Question 2

How do plant cells and animal cells differ, in terms of their structure?

Answer 2

• 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.

Question 3

How do plant cells and animal cells differ, in terms of their functions?

Answer 3

• 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.

Question 4

What are the two main systems within a plant?

Answer 4

Root system (below ground) and shoot system (above ground).

Question 5

Define: Root

Answer 5

Organ anchoring a vascular plant in the soil. It absorbs water and minerals while storing carbohydrates and other reserves.

Question 6

Are the environments for the two systems different?

Answer 6

Yes! Roots must be adapted to a very different environment compared to shoots.

Question 7

In which two ways can plants be organized?

Answer 7

1. Organs: Leaves, roots, stems, flowers, fruit

2. Tissues: Groups of cells of one or more types, performing primarily one specialized function.

Question 8

What are the three plant tissue systems?

Answer 8

Dermal, vascular, ground systems

Question 9

Define: Dermal system

Answer 9

Covers the plants. Skin acting as the first line of defense against physical damage, pathogens.

In the shoots = Epidermis
In the roots = Periderm

Question 10

Define: Vascular system

Answer 10

Transports water and nutrients, via xylem and phloem.
Long-distance transport between root and shoot systems.
Provides mechanical support.

Question 11

Define: Ground tissue system

Answer 11

Everything between the dermal system and the vascular system, with lots of non-specialized cells.

Question 12

Why is the tissue type considered a system?

Answer 12

Each tissue type forms a system that connects all of the plant’s organs - it is continuous.

Question 13

What are the three types of simple tissues?

Answer 13

Parenchyma, collenchyma, sclerenchyma

Question 14

What is parenchyma tissue?

Answer 14

• 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

Question 15

What are the functions of parenchyma tissue?

Answer 15

• Metabolism, photosynthesis in leaves
• Filling space: Giving plants their shape
• Storage: Starch, fructans, sucrose, proteins
• Wound repair: Reservoir of new cells

Question 16

What are examples of parenchyma tissue?

Answer 16

• Leaf mesophyll
• Palisade cells
• Root cortex
• Stem pith

Question 17

What is collenchyma tissue?

Answer 17

• 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

Question 18

What is the function of collenchyma tissue?

Answer 18

-Flexible support, in elongating stems and leaves

Question 19

What is sclerenchyma tissue?

Answer 19

• Very thick-walled cells, often dead at maturity

- Comprise a secondary cell wall, reinforced with lignin

Question 20

What are the functions of sclerenchyma tissue?

Answer 20

• Protection: Stones in fruit, hard seed coats
• Support: Flax, hemp fibres in stems

Aka – the rigid walls serve as a rigid ‘skeleton’

Question 21

What are the two types of complex tissue?

Answer 21

Xylem and phloem

Question 22

What are complex tissues comprised of?

Answer 22

Several cell types in vascular bundles. They are found in leaves, roots, stems, petioles.

Question 23

What is xylem?

Answer 23

It is the water-conducting tissue of plants.

Question 24

What types of simple tissues does the xylem contain?

Answer 24

Primarily dead cells - Parenchyma and sclerenchyma.

Question 25

What are the two sub-types of xylem?

Answer 25

1. TRACHEIDS: Spindle-shaped, water-conducting cells, with bordered pits for water flow. Found in all higher plants.
2. VESSEL ELEMENTS: Barrel-shaped and line up end to end, with perforation plates for water flow. Found only in angiosperms (flowering plants).

Question 26

How do the bordered pits of tracheids work?

Answer 26

The pits allow the passage of water, without the need to cross a thick secondary wall.

Question 27

What is the key characteristic of xylem?

Answer 27

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.

Question 28

What is the role of a perforation plate in a vessel element?

Answer 28

It allows the free flow of water.

Question 29

What is phloem?

Answer 29

Nutrient-conducting tissue (sugars, organic molecules), alive at maturity

Question 30

What are the components of phloem?

Answer 30

Sieve tubes, companion cells, fibers

Question 31

Define: Sieve elements

Answer 31

• 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

Question 32

What is the function of companion cells?

Answer 32

They have nuclei, thus producing RNA and proteins for the sieve element cells.

Question 33

In vascular bundles of dicot plants, where is the xylem? Where is phloem?

Answer 33

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.

Question 34

What are meristems?

Answer 34

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.

Question 35

Where/how do roots develop?

Answer 35

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.

Question 36

What are apical meristems?

Answer 36

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.

Question 37

How do branch roots form?

Answer 37

From the pericycle, a layer of cells around the vascular cylinder.

Question 38

Where do shoots develop? How?

Answer 38

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.

Question 39

What are axillary meristems?

Answer 39

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).

Question 40

What are the two types of lateral meristems?

Answer 40

The vascular cambium and the cork cambium, which are responsible for secondary growth - growth in thickness.

Question 41

In which type of plants is secondary growth important?

Answer 41

In woody plants, aka trees.

Question 42

What is the vascular cambium?

Answer 42

A cylinder of meristematic cells, which gives rise to xylem cells to the inside, phloem cells to the outside.

Question 43

What is the result of vascular cambium?

Answer 43

Secondary xylem: Wood, in annual rings, due to the seasonal pattern of xylem formation.

Question 44

What do older trees develop that younger trees don’t?

Answer 44

They develop heartwood, which is darker than sapwood and full of anti-fungal chemicals to prevent rotting.

Question 45

What happens to the older layers of secondary xylem, as a woody plant age?

Answer 45

They no longer transport water and minerals, therefore become dead at maturity = Xylem sap

Question 46

What is the cork cambium?

Answer 46

The meristematic (aka dividing) layer of cells in the outer cortex, which gives rise to the periderm.

Question 47

What is the periderm?

Answer 47

The suberized (waxy) protective tissue on the outside of perennial roots and stems.

Question 48

What does bark include?

Answer 48

Periderm and secondary phloem.

Question 49

Where is secondary growth found?

Answer 49

In all gymnosperms and many flowering plants, but is rare in monocots.

Question 50

In what type of tissue would you find the phenolic substance called lignin?

Answer 50

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.

Question 51

Which cells are dead at maturity but still functional?

Answer 51

Xylem cells, notably those within the tracheid and vessel elements.

The lignin is needed to prevent cell lapsing in moving water upwards.