Muscles and Respiration

Sketetal Muscles, Tendons and Ligaments

  • Skeletal muscle is the muscle you move with e.g. biceps and triceps.
  • Tendons hold skeletal muscles to bones.
  • Ligaments attach bones to other bones and hold them together.
  • Skeletal muscles relax and contract at a joint to move bones.
  • Muscles that work together to move a bone are ANTAGONISTIC PAIRS.
  • Once example is your elbow- when your biceps contract, your triceps relax and vice versa.
  • A muscle that bends a joint when it contracts is called a flexor.
  • A muscle that straightens a joint when it contracts is called an extensor.
1 of 14

Structure of Skeletal muscle


2 of 14

Structure of skeletal muscle continued

  • It is made up of large bundles of long cells called muscle fibres.
  • The cell membrane of muscle fibre cells is the sarcolemma.
  • Bits of the sarcolemma fold inwards and stick into the sarcoplasm- these are called transverse tubules which help spread electrical impulses so they reach all parts of the muscle fibre.
  • The sarcoplasmic reticulum runs through the sarcoplasm- it stores and releases calcium ions needed for muscle contraction.
  • Muscle fibres have lots of mitochondria to provide ATP.
  • They are multinucleate (contain many nuclei).
  • Muscle fibres have lots of long, cylindrical organelles called myofibrils- made up of lots of protiens and are highly specialised for contraction.
3 of 14


  • Contain bundles of thick and thin myofilaments that move past each other to make muscles contract.
  • Thick myofilaments are made of myosin, thin ones are made of actin.
  • They have dark and light bands: Dark ones contain think myosin filaments and some overlapping thin actin filaments (A bands)
  • Light bands contain thin actin filaments only- these are I bands.
  • A myofibral is made up of many short units called sarcomeres- the ends of these are marked with a Z line.
  • The middle of each sarcomere is marked with an M line, and it is also the middle of the myosin filaments.
  • Around the M line is the H zone- this only contains myosin filaments.
4 of 14

Myofibrils continued


5 of 14

Sliding filament theory

  • Myosin and actine filaments slide over each other to make sarcomeres contract- myofilaments don't contract themselves.
  • Similtaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract.
  • Sarcomeres return to their original length as the muscle relaxes.
6 of 14

Muscle contraction

Binding sites in resting muscles are blocked by tropomyosin which is held by troponin until an action potential reaches the muscle and contraction starts:

  • The muscle depolaries the sarcolemma and the depolarisation spreads to the sarcoplasmic reticulum- this causes the Ca 2+ (Calcium) ions to be released into the sarcoplasm.
  • This pulls the tropomyosin out of the binding site on the actin filament, and the exposure of the site allows the myosin head to bind.
  • The calcium ions also activate the enzyme ATPase ti break down ATP in ADP + P to provide energy- this energy moves the myosin head which pulls the actin filament along.
  • The ATP energy also breaks the bond between the myosin head and actin filament so the myosin head detaches. The myosin head then reattaches to a different binding site and the cycle repeats many times (as long as calcium ions are present and bound to troponin).
  • When the muscle stops being stimulated the calcium ions leave and the troponin molecules are moved back by active transport back into the sarcoplasmic reticulum (ATP is used here).
  • The troponin returns to it's original shape which pulls the tropomyosin back into place, blocking the binding sites; the actin filaments also slide back into their relaxed position, lengthening the sarcomere.
7 of 14

Slow and fast twitch muscle fibres

Slow Twitch:                                                                Fast Twitch:

Contract slowly                                                            Contract quickly

Muslces used for posture                                            Muscles used for fast movement

Good for endurance activities                                     Good for short bursts of speed/power

Can work for a long time without tiring out                 Get tired very quickly

Energy released slowly- lots of mitochondria             Energy relased quickly- few mitochondria

Aerobic respiration                                                     Anaerobic respiration

Reddish colour- rich in myoglobin                              Whitish colour- little myoglobin 

Lots of blood vessels                                                  Few blood vessels

8 of 14

Aerobic respiration- Glycolysis

Glycolysis splits one molecule of glucose into two smaller molecules called PYRUVATE and it happens in the cytoplasm of cells- it doesn't need oxygen so is an anaerobic process:


  • Glucose is phosphorylated by adding 2 phosphates from 2 ATP molecules- this creates 2 molecules of triose phosphate and 2 of ADP.


  • TP is oxidised by removing a hydrogen- forms 2 pyruvate molecules.
  • NAD collects the hydrogen ions forming 2 reduced NAD.
  • 4 ATP are produced but 2 were used in the process so it is a net gain of 2 ATP.
9 of 14

Aerobic respiration- Link reaction

The Link reaction happens in the mitochondrial matrix, and converts pyruvate to Acetyl Coenzyme A:

  • Pyruvate is decarboxylated- one carbon is removed in the form of CO2.
  • NAD is reduced- collects the hydrogen removed from pyruvate which changes it into acetate.
  • Acetate is combined with Coenzyme A (CoA) to form Acetyl Coenzyme A (Acetyl CoA).
  • No ATP is produced from this reaction.
  • This reaction occurs twice for every glucose molecule, as 2 pyruvate molecules are produced from each glucose in Glycolysis.
  • So 2 molecules of acetyl CoA go into the next stage, 2 molecules of CO2 are released as waste and 2 reduced NAD also go to the next stage. 
10 of 14

Aerobic respiration- The Krebs Cycle

This reaction also takes place in the mitochondrial matrix and it produces reduced Coenzymes and ATP:

  • Acetyl CoA combines with oxaloacetate to form citrate ( 6 carbon compound); Coenzyme A goes back to the link reaction to be used again.
  • The citrate is then decarboxylised to form a 5 carbon compound and CO2 is released; dehydrogenation also occurs as a hydrogen is removed from the compound and is combined with NAD to form reduced NAD.
  • The 5 carbon compound is then converted into the 4 carbon compound oxaloacetate- decarboxylation and hydrogenation occur which produces 1 molecule of FAD and two of reduced NAD.
  • ATP is produced by the direct transfer of a phosphate group from the 5 carbon compound to NAD- when a phosphate group is directly transfered from one molecule to another it is SUBSTRATE-LEVEL PHOSPHORYLATION.
11 of 14

Aerobic respiration- Oxidative Phosphorylation

  • This reaction produces lots of ATP and is the final stage of aerobic respiration:
  • It is where the energy carried by electrons from reduced Coenzymes (reduced NAD and FAD) is used to make ATP.
  • It involves 2 processes- the electron transport chain and chemiosmosis.

Electron transport chain:

  • Hydrogen atoms are released from the two reduces Coenzymes to leave NAD and FAD oxidised- the hydrogen is then split into protons and electrons.
  • The electrons move along the electron transport chain loosing energy at each carrier.
  • This energy is used to pump protons into the intermembrane space- the concentration of protons is greater than in the mitochondrial matrix which creates an electrochemical gradient.
  • Protons move down the gradient back into the mitochondrial matrix via ATP synthase- this generates the synthesis of ATP and ADP + P.
  • The movement of the H+ ions across the membrane is called chemiosmosis.
  • In the mitochondrial marix at the end of the transport chain, electrons, protons and O2 from the blood combine to form water ( O2 is the final electron acceptor).
12 of 14

Respirometer- measuring respiration

  • Volume of oxygen uptake/ volume of carbon dioxide produced measures rate of respiration- respirometer measures the volume of oxygen taken up.


  • Each test tube has potassium hydroxide (or soda lime) in it which absorbs CO2.
  • Control tube is set up exactly the same as the test tube containing the organism.
  • Syringe is used to set fluid in manometer.
  • Apparatus is then left for a set period of time e.g. 10 minutes.
  • There will be a decrease in volume of air in the test tube as the organism is using it up for respiration.
  • The decrease in air volume will also decrease the air pressure- liquid in manometer moves towards the test tube.
  • Distance moved by liquid is measured and can be calculated into volume of oxygen taken in by the organsim per minute.
  • Any variables that could affect the experiment are controlled e.g. temperature.
13 of 14

Anaerobic respiration

  • It doesn't use oxygen.
  • It doesn't use the link reaction, Krebs cycle or oxidative phosphorylation, only glycolysis.
  • The main type of anaerobic respiration is LACTATE FERMENTATION.
  • It occurs in animals and produces lactate.

Lactate fermentation:

  • Glucose in converted to pyruvate in glycolysis--> Reduced NAD produced in glycolysis transfers hydrogen to pyruvate  to form lactate and NAD.
  • The NAD is then reused in glycolsis and the cycle starts again.

The production of lactate regenerates NAD- glycolysis can continue even with little oxygen around so small amounts of ATP can be produced.

But, lacxtic acid builds up and needs to be broken down. To do this, cells convert the lactic acid back to pyruvate which then re-enters the Krebs cycle. Liver cells can also convert lactic acid back to glucose, either for respiration or for storage.

14 of 14


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Human, animal and plant physiology resources »