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Biology A level - Homeostasis Cheat Sheet (DRAFT) by

This is an OCR Gateway A level biology cheat sheet, Chapter 15 module 5. Specification reference: 5.1.1

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Homeos­tasis

Mainte­nance of internal enviro­nment in constant state despite external changes.
Fluctu­ations within normal range.
Types of dynamic equili­brium:
Negative feedback
Works to reduce change (e.g. body temper­ature control)..
Positive feedback
Works to increase change (e.g. release of oxytocin in childbirth to increase frequency of contra­cti­ons).

Endoth­ermal and ectotherms

Endotherms
Rely on metabolic processes to provide heat.
Ectotherms
Rely on surrou­ndings to provide heat.

Thermo­reg­ulation

Ectoth­erms:
Behavi­oural adapta­tions
Move to shade / Sun, press bodies to heated surfaces.
Physio­logical adapta­tions
Change colour, alter heart rate.
Endoth­erms:
Cooling down
Sweating, vasodi­lation, relaxed erector pili muscles.
Heating up
Vasoco­nst­ric­tion, raising hair/f­eat­hers, shivering.
 

Functions of the liver

Carboh­ydrate metabolism
Control blood glucose, amino acid
Deamin­ation
Transa­min­ation - Conversion of amino acids into another
Deamin­ation - Removal of amine groups from molecules because body cannot store amino acids and proteins (ornithine cycle).
Detoxi­fic­ation
Hydrogen peroxide (+ catalase) --> O2 + H2O
The liver has many functions (around 500 total) not listed here.

Ornithine cycle

Amino acids undergo deamin­ating where they are spilt into amine groups and keto acids.
The amine groups are converted into a miniature and undergo the ornithine cycle as ammonia is toxic.
Urea is still toxic but less so, and is excreted by the kidney in urine.

Structure of the liver

Liver cells = hepato­cytes with adapta­tions such as large nuclei and golgi, lots of mitoch­ondria.
Blood from hepatic artery and portal vein mix in sinusoids to increase overall O2 content of blood.

Lobule structure

Lobules are made up of hepato­cytes and have a high blood supply. The portal vein is the only vein to carry both oxygenated and deoxyg­enated blood.

Cross-­section of lobule

Kupffer cells: macrop­hages for lobule.
Bile ductules: drains into the fall bladder.
  

The kidney

The loop of Henle is found in the medulla of the kidney.

Ultraf­ilt­ration

Happens in the glomerulus.
Blood comes from vein into narrower afferent arteriole which creates high pressures, so blood goes through capillary walls.
Arterioles surrounded by cells called podocytes with arm-like structures called pedicels.
These create slots to ensure molecules such as platelets that got through the epithelial and basement later do not enter the PCT (proximal convoluted tubule).

Selective reabso­rption

In the PCT.
Glucose, amino acids, vitamins and hormones actively cotran­sported from filtrate back into blood.
Adapta­tions:
Microvili
Mitoch­ondria - ATP for active transport.

Osmore­gul­ation

In loop of Henle - descending limb
Top = imperm­eable to water, lower part = permeable to water.
Concen­tration of Na and Cl increases lower down the descending limb.
Water therefore moves out into capill­aries.
Filtrate is very concen­trated.
- ascending limb
Permeable to Na and Cl. Actively pumped out to medulla.
Imperm­eable to water, so more dilute.

Selective reabso­rption in the DCT

Permea­bility varies with ADH.
Na+ can also actively be pumped out.

Osmore­gul­ation in the collecting duct

ADH (primary messenger) binds to receptors which trigger cAMP produc­tion.
cAMP triggered realise of aquapo­rin­-co­nta­ining vesicles.
Facili­tates diffusion of water back into blood i.e. reabso­rption of water.
Water potential of blood detected by hypoth­alamus in brain.
Low water potential leads to release of ADH which affects water permea­bility in collecting duct.

Kidney summary