Revision:OCR A Level Biology - Mammalian Physiology and Behaviour

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CARBOHYDRATE METABOLISM

• Carbohydrates are normally transported as glucose substances
• BUT glucose is not suitable for storing in liver and muscle cells as it is diffusable
  • so it's converted to glycogen

• So as the blood glucose level rises
• Beta cells in the Islet of Langerhans (tissue in the pancreas)
• Secretes insulin
• This stimulates the conversion of glucose in glycogen as insulin enables cells to take-up glucose
• Insulin also has this effect on muscle cells and fat in adipose tissue
  • it increases the rate at which glucose is metabolised so the blood glucose levels are brought back down to fluctuate within the norm

• IF the level is too low
  • then the Alpha cells in the Islets of Langerhans secrete glucagon
• Glucagon converts the stored glycogen into glucose
• It does this by binding to glycoprotein receptors on the plasma membrane of hepatocytes
• This results in a series of events known as glycogenolyisis
• The glucose then passes out of the hepatocytes, raising the blood glucose level as a result

• ADRENAL GLANDS also play a part
• Adrenaline has a similar effect as glucagon
• Under stress glycogen is broken down into glucose
• So the respiratory substrate is available as extra fuel - adrenaline is associated with the fight or flight response, so it prepares the body for the "fight"

• GLUCONEOGENESIS - Conversion of amino acids, lipids, pyruvate and lactate into glucose

• This happens in response to glucagon when all glycogen in liver and muscle cells has been exhausted

• Amino acids are converted into glucose by the process of deamination
  • the nitrogen containing part is converted to urea and excreted by the kidneys; the carbon skeleton is converted to keto acids

• The rest are converted into pyruvate by breaking off 2C fragments from the carbon skeleton, which are converted to molecules of triose phosphate and then to glucose

• The lactate is used in anaerobic respiration in the muscles
• It is then taken in by hepatocytes and converted into pyruvate
  • the pyruvate is then converted into triose phophate then to glucose
  • the lipids (glycerol) is converted into triose phosphates which is then converted into glucose

• FATTY ACIDS are broken down into 2C fragments to give acetyl coA

PROTEIN METABOLISM

• TRANSAMINATION is the conversion of one type of amino acid into another
• However some amino acids can only be obtained from the diet, these are known as the essential amino acids

• Excess amino acids cannot be stored by the body so are converted into urea in the ORNITHINE CYCLE
  • the NH3 is deaminated from amino acid
• ONLY THE NH3 GROUP IS EXCRETED
  • rest is used by body

• 2NH₃ + CO₂ + ATP react with ORNITHINE. Another ATP molecule is used to form CO(NH₂)₂ = UREA which is released by hepatocytes into the blood (in humans) and excreted via the kidney
• Ornithine is reproduced and the cycle repeats

MUSCLE CONTRACTION

• OK so muscle contraction is when the actin filaments slide into the Myosin filaments

• This will cause the sarcomere to decrease in length

• The actin filament has two extra proteins
  • Troponin and Tropomyosin

• In the myosin head, there are enzymes called ATPases which catalyse the reaction of

ATP --> ADP + Pi

• When a nerve impulse arrives at the neuromuscular junction, (like at synapses by the post synaptic membrane is muscle) nerve transmition accross the junction is similar to that at normal synapse.

• The arrival of the action potential causes Calcium ion channels to open

• Inlux of Ca²⁺ - cause vesicles of Acetylcholine (ACh) to fuse with presynaptic membrane - EXOCYTOSIS
• ACh bind to receptors in sarcolemma and sodium channels open
• Na⁺ ions flood through - Action potential is continued across the membrane

• This depolarisation spreads to the T- tubule
• Causes Ca²⁺ channels to open - Ca²⁺ diffuse out of Sarcoplasmic reticulum

• Ca²⁺ binds to tropomyosin and troponin
• This causes the Tropomyosin and troponin to move and expose the myosin head and its binding sites
  • the Actin filament can now bind to myosin head.
• The Myosin head tilts (45 deg.) so the actin filaments move
  • this causes the release of ADP + Pi
• Now ATP binds to myosin head
• Enzymes in myosin head hydrolyse ATP --> ADP + Pi
  • this relseases energy causing the myosin head and actin filament to unbind

• Actin filament moves back and myosin head flips away

Comments

These notes are aimed at people studying Edexcel Biology A Level, but will be suitable for other exam boards too.

Originally written by DBSK<3 on TSR Forums.