Homeostasis ll

1.   The pancreas

- functions as exocrine + endocrine gland.

- has 2 groups of cell in the pancreas.

- α & β-cells in the islet of Langerhans play (major role in blood glucose concdntration).

- α-cells - secrete glucagons

- β-cells - secrete insulin into blood capillaries.


α & β-cells in the islet of Langerhans



2.   Blood glucose level rises↑à due to carbohydrate breakdown.

3.   Insulin + glucagons have opposite effects.

4.   Insulin:

-  secreted by β-cells (when glucose↑ level increases).

-  target organs = liver + muscle tissues.

-  convert glucose à to glycogen.

5.   Glucagon:

-  secreted by α-cells (when the glucose↓ level decreases)

-  Target organ = liver cells.

-  convert glycogen à glucose.


Pancreas response to different level of glucose



6.   Ingestion of glucose à increase insulin level ↑.

7.   2 types of diabetes: Type I & Type II.

Blood glucose & insulin level in normal & diabetic person.

8.   Formation of tissues fluid is a physical process.

9.   At arteriol end:  

-  smaller blood plasma content is forced out

-  when hydrostatic pressure (HP) > osmotic pressure (OP).

10. At venous end:

-  tissue fluid re-enters capillary

-  when osmotic pressure (OP) > hydrostatic pressure (HP).

Homeostasis

1.   Homeostasis

- maintenance of internal environment within cells (tissue fluid + blood plasma).

- has 3 functional components: receptor, control centre + effector.

- 2 types of mechanism:

a.  Negative feedback.

b. Positive feedback.

- 2 types of animals (that respond to the fluctuations in temperature): endotherms + ectotherms.




Ectotherms



Endotherms











 

 

  








 

2.   To maintain a stable body temperature à organism needs to balance heat gain with heat loss.

3.   Heat gained/lost thru:

-  conduction,

-  convection

-  radiation,

- evaporation (heat lost only).

4.   Ectotherms - exhibit behavioural thermoregulation.

5.   Endotherms - show physiological + behavioural thermoregulation

6.   Internal body temperature à sends impulses to hypothalamus (via blood vessels + afferent nerves respectively).

7.   In thermoregulation:

-   receptors (thermoreceptors) = nerves beneath the skin,

-   control centre = hypothalamus

-   effectors = sweat glands + blood capillaries (beneath skin).



Endotherms show physiological and behavioural thermoregulation

 

Ectotherms exhibit behavioural thermoregulation


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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