plasmodesmata, porins, ion channels, membrane potential
1. PLANT PHYSIOLOGY
SUBMITTED TO,
DR. AGHIL SOORYA. A
ASSISTANT PROFESSOR
DEPT OF BOTANY
ST TERESA’S COLLEGE
SUBMITTED BY,
SILPA SELVARAJ
ROLL NO: 13
I MSC BOTANY
ST TERESA’S COLLEGE
1
2. INTRODUCTION
Membranes participate in many aspects of cell structure and function.
They separate intracellular and extracellular environments.
Membranes also define the intracellular organelles like mitochondria, chloroplast,
nucleus, lysosome etc.
These bio membranes have the same basic architecture.
By preventing the unassisted movement of water soluble molecules, the
phospholipid bilayer act as a permeability barrier.
The embedded proteins in the membrane helps in the regulated transport of
substances .
2
4. PLASMODESMATA
Singular : plasmodesma.
It is the microscopic cytoplasmic linkage between two adjacent cells.
Helps in the communication between cells.
Regulation of passage of molecules between the plant cells.
2 types;
Primary plasmodesmata : forms during cell division.
Secondary plasmodesmata : between mature cells.
4
5. Primary plasmodesmata are formed when fractions of ER are trapped across the
middle lamella as new cell wall is synthesized between two newly dividing cells.
They eventually becomes cytoplasmic connections between the cells.
5
6. STRUCTURE OF PLASMODESMATA
A plant cell contains 1000-1,00,000 plasmodesmata.
Diameter is approximately ranged from 50-60-nm.
3 main components;
Plasma membrane
Desmotubule
Cytoplasmic sleeve
6
7. 1. Plasma membrane
The plasma membrane portion of the plasmodesmata is continuous extension of the
cell membrane and has a similar phospholipid bilayer structure.
2. Cytoplasmic sleeve
It is a fluid filled space and it is the continuous extension of the cytosol.
Movement of molecules occur through this space.
3. Desmotubule
It is a tube of appressed ER.
Some molecules are known to be transported through desmotubule.
7
8. In some cases, around the desmotubule, certain structures are present that may
split the plasmodesma into small channels.
These structures may be made of myosin or actin .
Smaller molecules and ions can easily pass through the plasmodesmata by
diffusion without the need of additional energy.
Larger molecules like proteins, RNA etc can also pass through the cytoplasmic
sleeve diffusively.
8
9. Mechanism of pore size regulation
Accumulation of polysaccharide callose around the neck region to form a collar.
This reduces the diameter of the pore which is available for the transport.
Through dilation, active gating or structural remodelling the permeability can be
increased.
This increase in the size of the pore allows larger molecules like signalling
molecules, RNA protein complexes etc to be transported to the adjacent cells.
Gene responsible for callus synthesis and deposition is Cals3.
9
10. PORINS
Porins are beta barrel proteins that act as a pore through
which molecules can diffuse through.
They are quite larger in size.
Involved in passive transport.
Present on the outer membrane of gram negative bacteria,
outer membrane of mitochondria, outer membrane of
chloroplast etc.
Eg : aquaporins
10
11. Structure
Porins are composed of beta sheets which are made of beta strands .
Beta strands are linked together by beta turns on the cytoplasmic side and amino acid
loops on the other side.
The beta strands lie in antiparallel direction and they form a cylindrical structure called
beta barrel.
The amino acids of the beta strands bear polar and nonpolar residues.
Non polar residues face outwards to interact with the non polar lipids.
Polar residues face inwards to the centre to create the aqueous channel.
Majority : monomers, but dimeric to octameric porins have been discovered.
PORINS
11
12. MEMBRANE
POTENTIAL
• It is the difference in the
electrical charge between
the inside and outside of the
membrane.
• Denoted by millivolt. (Mv)
12
13. Electrical potential difference across the cell membrane is called membrane
potential.
Inside the cell is negative and outside is positive. (how? Lets see !)
On the basis of charge : outer side – positively charged
inner side negatively charged.
On the basis of ions : outside - Na+ concentration – high.
inside – K+ concentration - high.
13
+++++++++++++++++++++++
+++
- - - - - - - - - - - - - - - - - - - - - - -
- -
Cell membrane
15. Sodium has few leakage (non gated) channels, it can mostly move through gated channels.
Potassium has leakage channels as well as voltage channels.
As the concentration of potassium ions are more inside the cell and less outside the cell, they will
move to the area of lower concentration via the leakage channels.
As a result the outer side of the cell membrane becomes more positive.
Inner side of the cell membrane becomes more negative.
As K+ leaves the cell, negativity increases on the inside of the cell membrane and electrostatically
attracts K+ ions that have gone to outside.
This electrostatic force prevents the potassium ions from leaving the cell.
Passive efflux of K+ > passive influx of Na+ ; helps in establishing and maintaining membrane
potential.
15
16. ION CHANNELS
Ion channels are proteins present in the membrane that helps in the transport of
ions across the membrane.
Highly selective in the type of ion transported.
Passive mechanism.
2 types ;
16
17. Ligand gated ion channels
Ligand gated ion channels open or close in response to the binding of a ligand.
Binding of the ligand causes the opening of the closed channel.
Ligand – neurotransmitters.
17
18. Voltage gated ion channels
Open and close in response to membrane potential.
Conformational changes in response to the potential gradient.
Enables the passage of selected ions.
E.g. ; potassium ion channels, sodium ion channels etc.
18
19. POTASSIUM ION CHANNELS
( kv channels)
Found in almost all species except in some parasites.
No other ions can pass through this channel except
potassium.
Consist of 6 transmembrane alpha helices (S1-S6) .
S1-S4 constitute of voltage sensor .
S4 act as the major voltage sensor. It contain positively
charged residues which are responsible for the voltage
sensing mechanism.
S5-S6 constitute the pore domain.
In between S5 and S6 selectivity filter is present. This
filter ensures that only K+ ions pass through this channel.
19
21. There are two alternative states ;
Closed state : voltage sensor is at the lower portion.
Open state : voltage sensor is pushed upwards.
So in the open state, the pore is open and K+ ions can pass
through this pore.
21
22. 22
-80 -40 +40 +80
0
Voltage (mV)
Current
(A)
• At a –ve membrane voltage, the
inside of the membrane is highly -
ve.
• So there will an attraction between
the +ve charges in the voltage
sensor and the –ve charge at inside
of the membrane.
• So, the pore will remain in a closed
state.
----------
--
----
-
• At a +ve membrane voltage, the
inside of the membrane would be
positive.
• So there will be a net repulsion
between the +ve charges of the
voltage sensor and the +ve charges
of the inside of the membrane.
• It will push the voltage sensor to
outwards and the pore opens.
+++++++ ++++++
+
