IB DP Biology: HL复习笔记11.2.5 Skills: Anaylising Muscle Contractions in Electron Micrographs

• Many biological structures are too small to be seen by the naked eye
• Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied
• Electron microscopes provide a much higher magnification and resolution so sub-cellular details can be studied
• Using microscopes to calculate the size of specimens requires the use of an eyepiece graticule which should be calibrated to the microscope
• However, it can be very difficult to make out the features of skeletal muscle fibres using an optical microscope
• Only the banding is visible, this is why it is referred to as striated muscle

The dark bands produce a characteristic striped appearance under an optical microscope

• Electron microscopes are often used to see muscle fibres in more detail
• They reveal the structure of myofibrils

The detailed structures of the muscle fibres are visible due to the much stronger magnification of the electron microscope.

In a relaxed sarcomere:

• There will be visible dark lines where the Z-lines are at either end of the sarcomere
• There will also be a darker band in the middle of the sarcomere where the thicker myosin fibres are positioned and in the very centre of that is the M line
• Around the Z-line, lighter bands are seen where the thinner actin fibres are positioned

In a contracted sarcomere:

• The Z-lines and M-lines are still visible with a shorter distance between the two z-lines
• The lighter bands around the z-line will be smaller or not visible
• The darker band will be the same size (although may appear a bit darker).