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

  

Gaseous Exchange in Mammals

(A) Gaseous exchange in mammals

1. Respiratory tract consists of:  nostril, nasal cavity, pharynx, trachea, bronchi + bronchioles.

2. Gaseous exchange occurs at the ­­­alveoli.
3. Alveolar wall = single layer of squamous epithelium cells.
4. Capillary wall = flattened endothelial cells.
5. Thin layer of alveolar wall + rich network of blood capillaries à enhance gaseous exchange between alveolar sac + blood.

6. Deoxygenated blood - enters the lungs - thrugh pulmonary arteries.
7. Oxygenated blood - leaves the lungs - through pulmonary vein.
8. The rate of O₂ + CO₂ diffusion in the alveolar à depends on partial pressure of the gases in the alveolar air.

   9.   In the lung:

à PO₂ in the alveolar air is higher than blood

à O₂ diffuse rapidly from alveolus

à to blood plasma.

10. In the peripheral tissues:

à PO₂ is lower than blood

à O₂ diffuses out from blood

à to tissues.

11. O₂ +  CO₂ à low solubility in blood plasma
12. CO₂ à mainly transported as hydrogen carbonate ion (HCO₃⁺).
13. In the tissues

à PCO₂ is higher than blood

à  CO₂ diffuses out

à into the capillaries.

(B) Gaseous exchange in mammals

14. Blood carries CO₂ in 3 forms:

·   dissolved gas (8%)

·   HCO₃^- ion in the plasma (73%)

·   carbamino haemoglobin in the red blood cells (19%)

15. A large amount of CO₂

·   combines with H₂O in the red blood cells (RBC)

·   to form carbonic acid (H₂CO₃)

·   then ionizes to form H⁺ +  HCO₃^-

16. HCO₃^- from the red blood cell:

·   diffuse out into the plasma.

·   loss of negatively charge HCO₃^- from the red blood cell

·   balanced by the inward diffusion of -ve chloride ions (Cl^-) = chloride shift.

17. In the lungs.

à PCO₂ is lower than in the blood.

à CO₂ diffuses out

à into alveoli.

18. In the plasma:

1.  dissolved CO₂ diffuses out into the alveolar air.

2.  Carbamino haemoglobin:

à dissociates

à form CO₂ + haemoglobin.

3.  Hydrogen carbonate ions (HCO₃^-):

à diffuse into the red blood cells

à reacts with H⁺

à form carbonic acid (H₂CO₃).

à dissociates

à form H₂O + CO₂

à CO₂ diffuses out into the alveolar air

à exhaled.

(C) Gaseous exchange in mammals

1.  O₂  = transported by the haemoglobin molecule in the red blood cells.

2.  Each haemoglobin molecule à carries 4 mol of O_2.

3.  Each polypeptide chain contains:

·   2 subunits of alpha polypeptides chains

·   2 subunits of beta polypeptides chains.

4.   Haem:

à prosthetic group of haemoglobin.

à as a porphyrin ring with a ferum ion (Fe2+) in the middle.

à each capable of carrying a single O₂ molecule attached to the ferum ion

à porphyrin ring shows strong cooperative O₂ binding to the haemoglobin.

5.   When one mol of O₂ binds to one of the haem group in the haemoglobin

à produces a conformational change in that subunit

à transmitted to the other 3 subunits

à facilitate O₂ binding to the rest of the polypeptide.

à concerted conformational change of the subunits

à increases affinity of haemoglobin for O₂

à resulting in sigmoid shape for the O₂ dissociation curve of the haemoglobin.

     6. The cooperative effect is reversible

à when one subunit of oxyhaemoglobin unloads its O₂,

à other three quickly follow suit

à conformational change

à lowers its affinity for O₂.

     7. Relationship between PO₂ and % saturation of haemoglobin with O₂
         à represented by O₂ dissociation curve = sigmoid.

   8.   The O₂ dissociation curve shows:

à when haemoglobin is exposed to a gradual increase of PO₂,

à it absorbs O₂ rapidly at first

à but more slowly as the PO₂ continues to rise.

     9. The % of O₂ saturation of haemoglobin = 95%

à when blood flows through the lungs (PO₂ = 105 mmHg).

    10. The % of O₂ saturation of haemoglobin = 70%

à when blood flows through a moderately active/resting muscle (PO₂=40 mmHg).

    11. As the blood from the lungs reaches the muscle at rest:

à 25% of the O₂ carried in the heamoglobin is unloaded to the surrounding tissues

à for cellular respiration.

à 70% of the oxygen is still retained by the haemoglobin.

    12. The % of O₂ saturation of haemoglobin is 40%  

à when the blood flows through the active muscle at PO₂ of 20 mmHg.

    13. When the muscle is active:

à haemoglobin readily unloads 55% of the oxygen

à still retains 40%.

    14. During exercise:

à active tissue is in demand for O₂ to produce energy.

à slight drops in PO₂ (between muscle at rest + muscle during exercise.)

à enough to cause a relatively large increase in the amount of O₂ the blood unloads.

    15. The increase of CO₂ conc/decrease in blood pH:

à will induce haemoglobin to unload more O₂.

à affinity of haemoglobin to O₂ is lower.

à O₂ dissociation curve shifts to the right = Bohr effect.

  16. Foetal haemoglobin:

à has a higher affinity for O₂ than the maternal haemoglobin

à therefore, O₂ dissociation curve lies to the left.

   17.  O₂ must easily dissociate from the maternal haemoglobin to the foetal haemoglobin

   à therefore, easily transferred from maternal to foetal blood.

  18. Myoglobin:

   à  higher affinity for O₂ than haemoglobin.

à  Therefore, dissociation curve lies to the left.

Haemoglobin:

à has a lower affinity for O₂ than myoglobin.

19.   In the muscle:

à O₂ dissociates from haemoglobin

à transferred to myoglobin to be stored.