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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > OCR A Level Biology - Mammalian Physiology and Behaviour

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Contents 1 CARBOHYDRATE METABOLISM 2 PROTEIN METABOLISM 3 MUSCLE CONTRACTION 4 Comments

CARBOHYDRATE METABOLISM

• Carbohydrates are normally transported as glucose substances
• BUT glucose is not suitable for storing in liver and muscle cells
  • so its converted into Glycogen

• So as the Blood glucose level rises
• Beta cells in the Islet of Langerhans (Tissue in Pancreas)
• Secretes Insulin
• This stimulates the conversion of glucose in Glycogen
• Insulin also has this effect on muscle cells and fat in the adipose tissue
  • it increases the rate at which glucose is metabolised so the blood levels return to normal

• IF the level is too low
  • then the Alpha Cells in the Islets of Langerhans secrete Glucagon
• Glucogon converts the stored glycogen into glucose
• It does this by binding to glycoprotein receptors on the plasma membranes of hepatocytes
• This results in a series of events of Glycogenolyisis
• The glucose then passes out of the hepatocytes raising the blood level

• ADRENAL GLANDS also play a part
• Adrenaline has the same effect of Glucogen
• Under Stress glycogen is broken down into Glucose
• So the respiratory substrate is available as extra fuel - adrenaline associated with fight or fly thing? so it prepares body for the "fight"

• GLUCONEOGENESIS - Conversion of Amino acids, lipids, pyruvates and Lactate into glucose

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

• Amino acids are converted into glucose by deamination
  • the nitrogen containing part is converted to urea and excreted by the kidneys

• The rest are molecules are pyruvates which are converted to molecules of Triose Phosphates and then into glucose

• The Lactate is used in anaerobic respiration in the muscles
• It is then taken in by hepatocytes and converted into Pyruvates
  • the Pyruvates are then converted into Triose Phophates then into glucose
  • the lipids (Glycerol) is converted into Triose Phosphates which are then converted into glucose

• FATTY ACIDS NOT CONVERTED

PROTEIN METABOLISM

• TRANSAMINATION is the conversion of one type of amino acid to another
• However some amino acids can only be obtained from diet (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 acids
• ONLY THE NH3 GROUP IS EXCRETED
  • rest is used by body

• NH₃ + CO₂ + ATP react with ORNITHINE. Another ATP molecule is used to form CO(NH₂)₂ = UREA which is released by hepatocytes into the blood 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 the usual thing happens between the presynaptic and post synaptic membrane - which happens to be the muscle fibre

• Due to the arrival of action potential, Calcium channels 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 ans 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.