Translocation in Plants

(A) Mass-Flow/Pressure Flow Hypothesis:

 1.   The mass-flow/pressure flow hypothesis:

-   postulates that dissolved sugar moves in phloem

-   by mean of pressure gradient

-   which exists between the source and sink.

2.   The photosynthetic cells in:

      -   leaves = common source of sugars
      -   roots = sinks.

3.   At the leaves

-   sucrose is actively transported

-   from mesophyll cells

-   to companion cell

-   into sieve tube

-   against its concentration gradient

-   process = phloem loading.

-   high conc of sucrose↑ à lowers cell water potential (Ψ_w)↓.

-   water - drawn into sieve tube

-   from nearby xylem vessel

-   creating a high hydrostatic pressure (HP)↑

-   forces the bulk/mass flow of the phloem sap

-   towards the sink.

Phloem loading and unloading of sucrose

4.   At the root:

-   sucrose is actively transported

-   from the sieve tube

-   into the companion cell

-   into a root cell.

-   process = phloem unloading.

5.   Loading (at the source) and unloading of sugar (at the sink)

      - require energy derived from ATP.

(B) In Electro-Osmosis Mechanism:

• potential diff develops across sieve plate
• by companion cell (actively transport K⁺ into sieve tube).
• K⁺ accumulate at one end of sieve plate 
• creates a potential diff between sieve plate.
• caused K⁺ speed across sieve plate
• water + dissolved sucrose follow (attracted by +ve charge).
• water in phloem moves by osmosis 
• as accumulation of K⁺ lower Ψ_w in sieve tube (compared to next cell).



K⁺ accumulate at one end of sieve plate creates a potential diff between sieve plate



(C) In Cytoplasmic Streaming Mechanism:

• water + dissolved compounds (in phloem sap) 
• move + circulate together
• in one direction (in sieve tube)
• it’s slow + depends on metabolic energy/due to their kinetic energy.
• circulation slow down at sieve plate
• and forced out from cytoplasmicc streaming (thru pores)
• to cytoplasmic streaming of next sieve tube.



Circulation slow down at sieve plate



(D) In Peristaltic Wave Mechanism:

• sieve tube is filled with fine cytoplasmic filaments
• continuous from sieve tube to the next
• thru pores of sieve plate.
• contain phloem sap tube constrict + relax alternately
• pushing sap from one sieve tube to the next.
• constriction + relaxation/peristaltic movement form a pattern of wave = peristaltic wave
• can be at diff speed + in opposite direction (in sieve tube)
• depends on metabolic energy/ATP.



Phloem sap tube constrict + relax alternately



Macromolecules across the plasma membrane

http://www.slideshare.net/contactmeasif/transport-of-macromolecules-across-cellmembrane

Transport of Water: Mechanism

1.      Two theories to explain water + minerals transport in plants

-   root pressure theory

-   cohesion-tension theory.

2.      Root-pressure theory:

-   accumulation of mineral ions in the xylem

-   enhances water molecules to move into root hairs (by osmosis).

-   water pressure ↑ builds up in the root

-   pressure pushes up water +  dissolved minerals

-   through the xlem

-   toward the top of the plant.

-   but not strong enough to push up water to the top of tall trees.

Root pressure



3.      In small plants:

-   root pressure can build high enough

-   to force water and minerals completely out of the tips of the leaves

-   the process = guttation. 

4.      Cohesion-tension theory suggests that:

-   water inside the xylem is pulled upward

-   by the -ve pressure (or tension)

-   that extends all the way from leaves to roots.

5.      In the leaf xylem:

-   -ve pressure (tension) builds

-   as water evaporates during transpiration.

-   evaporated water is continually replaced

-   thus cohesive bond pull the string of water molecules up

-   to create a transpiration pull.

-   transpiration pull is relayed

-   molecule by molecule

-   down the entire column of water in the xylem.


Transpirational pull in the leaf



6.      In the stem, water molecules:

-   exist as a long unbroken chain in the xylem.

-   are pulled upwards by tensions produced (during transpiration).

-   are held by cohesion + adhesion forces

Cohesion and adhesion forces in the xylem

7.      Transpiration pull:

-   can extend down to the roots

-   only through an unbroken chain of water molecules.

8.      At the cellular level:

-   the gradients of water potential

-   drive the osmostic movement of water

-   from cell to cell

-   within the roots up to the leaves.

Water potential in leaf, stem and root

Transport of Water: Concept

1.      Dissolved substances (inside a plant cell) = contribute to solute potential (ψ_s).

2.      More solute molecules present --> the lower is the water potential (ψ).

3.      When water pontential is lower than> external solution:

-   water molecules move into the cell .

-   pressure inside the cell increases

-   sell contents press against the cell wall

-   create a pressure potential (ψ_s).

4.      Water potential (of a plant cell) = solute potential + pressure potential.

5.      Water potential = free energy of water.

6.      By convention, water potential of pure water = 0 megapascal (MPa).

7.      Water will move:

-   from a region of higher (less -ve) water potential

-   to a region of lower (more -ve) water potential.

8.      In plasmolysed cell:

-   pressure potential = zero

-   water potential = solute potential.

9.      As more water molecules enter a cell

-   pressure potential ↑ increases

-   so is its water potential ↑

-   cell becomes turgid.

-   less and less water molecules enter the cell.

10.  Most minerals

-   are actively transported into the root.

-   there is a gradient of successfully

-   lower water potentials  from root hair to the xylem vessels

-   result in water uptake by osmosis is enhanced.

11.  Water moves by osmosis in the roots follows three pathways:

-   apoplast

-   symplast

-   vacuole.

12.  Apoplastic pathway:

-   water travels along the cell wall

-   and extracelular spaces.

Apoplast pathway

13.  Symplastic pathway:

-   water moves across the cytoplasm of one cell to the next

-   across the plasma membrane

-   through the plasmodesmata.

Symplast pathway

14.  Vacuole pathway:

-   water moves from vacuole to vacuole of one cell to the next

-   across the plasma membrane

-   through the plasmodesmata.
  

Vacuole pathway

Cardiac cycle





Cardiac Cycle

1.       Heart contracts + relaxes in a rhythmic cycle.

2.     Cardiac cycle consists of alternating periods of :

·        Systole - contraction phase.

·        Diastole - relaxation phase.

3.     Cardiac cycle = one complete sequence of systole + diastole

4.     Resting adult - cardiac cycle repeated 72 times per minute = resting heart rate.

5.     Pulse = causes by elastic recoil of arteries.

6.     Ventricular pressure + aortic pressure + ventricular volume:

• changes (in response to the events that occur)
• at different phases of cardiac cycle.

7.     During diastole:

• pressure↓of heart chambers decreases
• volume↑ increases (as blood is pumped in).

8.     During systole:

• pressure↑of heart chambers increases
• volume↓ decreases (as blood is pumped out).

9.     When aortic pressure > ventricular pressure

à semilunar valves shut + vice versa.

10.  When ventricular pressure > atrial pressure

à AV valves shut + vice versa.

11.   Valves = ensure blood only flows in right direction.

Transport In Animals

1. Human heart : 

• consists of 4 chambers : right atrium, right ventricle, left atrium and left ventricle. 
• pump blood to all parts of the body.
• made up of cardiac muscle. 




Front/venrtral  view of heart

 

2. Superior vena cava
    --> carries deoxygenated blood
    --> from the head + arms
    --> into right atrium.

3. Inferior vena cava
    --> carries deoxygenated blood
    --> from the lower body + legs
    --> into right atrium

4. Right atrium
    --> receives deoxygenated blood
    --> from vena cava
    --> to right ventricle. 

5. Tricuspid (antrioventricular) valves
    --> prevent blood (from right ventricle) flowing backwards
    --> to the right atrium.

6. Right ventricle
    --> pumps blood
    --> to lungs
    --> via pulmonary arteries.

7. Semilunar valves
    --> prevent blood (from pulmonary arteries)
    --> from flowing backwards
    --> into right ventricle.

 




Cross-section of heart

  










8.   Pulmonary artery branches
      --> carries deoxygenated blood
      --> from right ventricle 
      --> to lungs.

9.   Pulmonary vein
      --> carrying oxygenated blood
      --> from lungs
      --> to left atrium.

10. Left atrium
      --> receives oxygenated blood
      --> from lungs 
      --> to left ventricle.

11. Bicuspid (atrioventicular) valves
      -- > prevent blood (from left ventricle) flow backwards to left atrium

12. Left ventricle
      --> pumps oxygenated blood
      --> to body tissue (via aorta).

13. Semilunar/aortic valves
      --> prevent blood (from aorta) flowing backwards (into left ventricle).

14. Aorta
      --> carries oxygenated blood
      --> from left ventricle
      --> to the rest of the body.

15. Septum separates the right and left side of the heart.

Simplified version of heart