PLASMODESMATA

Plasmodesmata are microscopic channels which transverse the cell wall of plant cell and some algal cell and enabling transport and communication between them
Neighboring plant cells are therefore separated by a pair of cell wall and the intervening middle lamella, forming an extra cellular domain known as the apoplast.
Cell wall are permeable to small soluble protein and other solute.
Plasmodesmata enable direct regulated symplastic intracellular transport of substance between cells.
There are two forms of plasmodesmata .

#Primary – It is formed during cell division

#Secondary – It is form between mature cell

The plasmodesmata are narrow channels that act as intercellular cytoplasmic bridge to facilitate communication and transport of material between plant cells.
The plasmodesmata serve to connect the symplastic space in the plant and are extremely specialize channel that allow for intracellular movement of water various nutrient and other molecules.
These are located in narrow areas of cell walls called primary pit fields and they are so dense in these area.
They make up one percent of the entire area of cell wall

Structure of Plasmodesmata

plasma membrane is continuous between cell
with in the plasmodesmal pore a tightly wound cylinder of membrane termed the desmotubule runs the length of the plasmodesmata
This structure is studied by Tilney by using plasmolysis, Trion X detergent extraction and protease digestion.
Thus he suggests that desmotubule provide a rigid stability to plasmodesmata and confer a fixed diameter and pore size to plamodesmal canal.
Desmotuble is linked to E.R. in each of the adjacent cells forming a dynamic endomembrane continuum in the symplastic space
A typical plant cell may have between 103 and 105 plasmodesmata connecting it with adjacent cell to between 1-10µm²
Plasmodesmata are approximately 50-60 nm in diameter, at the midpoint are constructed of three main layers: the plasma membrane, cytoplasmic sleeve, and desmotubules.

1 – Plasmodesmatal Plasma membrane

This is continuous extension of the cell membrane or plasma lemma.

2-Cytoplasmic sleeve

It is fluid filled space enclosed by plasma membrane
The larger molecules including protein and RNA pass through the cytoplasmic sleeve diffusively.
The mechanism of regulation of plasmodesmatal transport is the accumulation of the polysaccharide callose that accumulates around the neck region of plasmodesmata to form a collar reducing their diameter and thereby controlling permeability to substance in the cytoplasm.

3- Desmotubules

These are tube of appressed endoplasmic reticulum that run between two adjacent cell.
some molecule are known to be transported through this channel
Around the desmotubule and P.M. area of and electron dense material have been seen often joined together by spoke -like structures that seem to split the plasmodesmata in to smaller channel.
These may be composed of myosin and actin which are part of the cyto-skeleton.

INTERNAL STRUCTURE

In vascular plants the basic plasmodesmal structure is a tube of plasma membrane surrounding a strand of modified ER, with particulate material between them.
interprets the ultrastructure of vascular plants in a cross and longitudinal sections.
Essential features of plasmodesmata is a cell to cell tubule of the plasma membrane that surrounds a cylindrical strand of tightly furled ER, the desmotubule.
A thin darkly stained central rod occupies the center of the desmotubule.
A cytoplasmic sleeve or the cytoplasmic annulus lying between the desmotubule and plasma membrane is considered as a possible pathway for the cell-to-cell water and solute movement.

The desmotubule is essentially a solid strand of lipid, the central rod is composed of the lipid polar groups and a few proteins that can physically occupy the inner core of the tightly furled lipid bilayer.
Much of the cytoplasmic annulus is occupied by proteinaceous material.
These particles are associated with both the outer surface of the desmotubules and the inner surface of the plasma membrane.
In the cross section, 7-to-9 gaps occur between the particles.
The distance across the gaps is about 2-3nm as comparable with the 4nm channel diameter.
These gaps are the physical basis of cell-to-cell transport.
A wide range of plasmodesmal morphologies have been observed, within the same plants.
These include differences in length, branching and size of the central cavity.
The functional significance of the variations is largely unknown.

FUNCTIONS

It plays an important role in cell to cell communication.
To investigate this function microelectrodes have been employed to monitor the flow of electric current from cell to cell in plant tissue (as in case of animal gap junctions.).
Such studies reveal that electric current passes between plant cells linked by plasmodesmata more readily than it does between the cells that are not linked by plasmodesmata.
The magnitude of the current flow is directly related to the number of plasmodesmata present.
This suggests that it plays a role in cell-to-cell communication, comparable to that played by gap junctions in animals.
They serve to connect the symplastic space in the plant and are extremely specialized channels that allow for intercellular movement of water, various nutrients, and other molecules.
They allow the passage of molecules with molecular weights of less than 800 daltons.
Plasmodesmata have been shown to transport proteins (including transcription factors), short interfering RNA, messenger RNA, viroids, and viral genomes from cell to cell.
Plasmodesmata are also used by cells in the phloem, and symplastic transport is used to regulate the sieve-tube cells by the companion cells.

1. What are plasmodesmata?

A) Microscopic channels in the cell membrane of animal cells
B) Microscopic channels in the cell wall of plant and algal cells
C) Protein channels in bacteria
D) Ion channels in fungi

✅ Answer: B) Microscopic channels in the cell wall of plant and algal cells

2. What is the primary function of plasmodesmata?

A) To transport oxygen in plants
B) To facilitate communication and transport of substances between cells
C) To store genetic information
D) To break down waste material

✅ Answer: B) To facilitate communication and transport of substances between cells

3. What is the extracellular domain that separates neighboring plant cells?

A) Symplast
B) Cytoplasm
C) Middle lamella
D) Vacuole

✅ Answer: C) Middle lamella

4. What type of transport do plasmodesmata enable?

A) Symplastic intracellular transport
B) Passive diffusion through apoplast
C) Active transport through cell membrane
D) Endocytosis

✅ Answer: A) Symplastic intracellular transport

5. What are the two types of plasmodesmata?

A) Simple and complex
B) Primary and secondary
C) Open and closed
D) Small and large

✅ Answer: B) Primary and Secondary

6. When are primary plasmodesmata formed?

A) During cell division
B) During photosynthesis
C) During respiration
D) During cell death

✅ Answer: A) During cell division

7. When are secondary plasmodesmata formed?

A) Between mature cells
B) During mitosis
C) During seed germination
D) Only in roots

✅ Answer: A) Between mature cells

8. Where are plasmodesmata located?

A) In the vacuole
B) In the mitochondria
C) In the primary pit fields of the cell wall
D) In the nucleus

✅ Answer: C) In the primary pit fields of the cell wall

9. What percentage of the total plant cell wall area is occupied by plasmodesmata?

A) 0.1%
B) 1%
C) 10%
D) 50%

✅ Answer: B) 1%

10. What is the approximate diameter of plasmodesmata?

A) 5-10 nm
B) 50-60 nm
C) 100-200 nm
D) 500 nm

✅ Answer: B) 50-60 nm

11. What is the role of the plasma membrane in plasmodesmata?

A) It is a separate structure unrelated to plasmodesmata
B) It forms a continuous extension between cells
C) It creates vesicles for transport
D) It produces ATP

✅ Answer: B) It forms a continuous extension between cells

12. What is the function of the desmotubule in plasmodesmata?

A) Provides a rigid structure and connects the endoplasmic reticulum of adjacent cells
B) Synthesizes proteins
C) Stores calcium ions
D) Transports oxygen molecules

✅ Answer: A) Provides a rigid structure and connects the endoplasmic reticulum of adjacent cells

13. What scientist studied plasmodesmata using plasmolysis and detergent extraction?

A) Charles Darwin
B) Tilney
C) Robert Hooke
D) Gregor Mendel

✅ Answer: B) Tilney

14. Which structure is responsible for regulating plasmodesmatal transport?

A) Cellulose
B) Lignin
C) Callose
D) Phospholipids

✅ Answer: C) Callose

15. What happens when callose accumulates around plasmodesmata?

A) Increases the size of the pore
B) Reduces the diameter and controls permeability
C) Breaks down the cell wall
D) Enhances active transport

✅ Answer: B) Reduces the diameter and controls permeability

16. Which molecules can pass through the cytoplasmic sleeve of plasmodesmata?

A) Proteins and RNA
B) DNA only
C) Only ions
D) Only gases

✅ Answer: A) Proteins and RNA

17. What is the role of myosin and actin in plasmodesmata?

A) They help in transport by forming spoke-like structures
B) They break down ATP
C) They synthesize cell wall components
D) They store nutrients

✅ Answer: A) They help in transport by forming spoke-like structures

18. What is the function of the cytoplasmic sleeve?

A) It prevents molecules from passing
B) It acts as a fluid-filled space for molecular transport
C) It produces enzymes
D) It absorbs light for photosynthesis

✅ Answer: B) It acts as a fluid-filled space for molecular transport

19. What is the significance of plasmodesmata in vascular plants?

A) They allow rapid transport of solutes between cells
B) They break down old cell walls
C) They produce photosynthetic pigments
D) They store starch

✅ Answer: A) They allow rapid transport of solutes between cells

20. What is the molecular weight limit for passage through plasmodesmata?

A) 200 Daltons
B) 400 Daltons
C) 800 Daltons
D) 2000 Daltons

✅ Answer: C) 800 Daltons

21. What is the function of plasmodesmata in phloem cells?

A) Regulating sieve-tube cells through companion cells
B) Breaking down sugars
C) Synthesizing proteins
D) Transporting oxygen

✅ Answer: A) Regulating sieve-tube cells through companion cells

22. Which of the following molecules can be transported through plasmodesmata?

A) Viroids
B) Viral genomes
C) mRNA
D) All of the above

✅ Answer: D) All of the above

23. What type of junction in animal cells is functionally similar to plasmodesmata in plants?

A) Tight junctions
B) Desmosomes
C) Gap junctions
D) Hemidesmosomes

✅ Answer: C) Gap junctions

24. How do scientists study plasmodesmatal function?

A) By using microelectrodes to monitor electric current flow between cells
B) By staining the nucleus
C) By freezing the cell membrane
D) By blocking photosynthesis

✅ Answer: A) By using microelectrodes to monitor electric current flow between cells

25. Why do plasmodesmata have varying morphologies within the same plant?

A) Due to differences in function, branching, and size of the central cavity
B) Because they are randomly distributed
C) Because they only exist in roots
D) Because they dissolve over time

✅ Answer: A) Due to differences in function, branching, and size of the central cavity