8.2 Applications and skills

8.2.2 Perspectives on mitochondria and chloroplasts

  • That structure correlates to function is a common theme in biology. The relationship can be explained by evolution and natural selection.
  • Both mitochondria and chloroplasts have DNA. Genetic mutations that allow mitochondria to produce ATP more efficiently, and chloroplasts that harvest light more efficiently, would increase an organism’s chance of survival and reproductive fitness.

Figure 8.2.2a – TEM micrograph of a mitochondrionFigure 8.2.2a – TEM micrograph of a mitochondrion

Figure 8.2.2b – TEM micrograph showing two large chloroplastsFigure 8.2.2b – TEM micrograph showing two large chloroplasts

Skill: Annotate diagrams to show structural adaptations

Use the information from the tables below to annotate diagrams: 

Figure 8.2.2c – Structure of mitochondriaFigure 8.2.2c – Structure of mitochondria
Mitochondria are the site of aerobic respiration.

Outer membrane

Function: Separate contents from rest of cell

Structure: Phospholipid bilayer

Inner mitochondrial membrane

Function: Oxidative phosphorylation

Structure: Electron transport proteins, and ATP synthase are embedded in the membrane

Intermembrane space

Function: Maintain high proton concentration for oxidative phosphorylation and chemiosmosis

Structure: Small volume

Cristae

Function: Oxidative phosphorylation

Structure: Projections of the inner mitochondrial membrane that increase surface area

Matrix

Function: Krebs cycle, Link reaction

Structure: High concentration of enzymes

 

Figure 8.2.2d – Structure of a chloroplastFigure 8.2.2d – Structure of a chloroplast

Thylakoids

(Grana)

Function: Light absorption

Structure: Arranged in stacks (grana) to increase surface area

Thylakoid membrane

Function: Phoactivation, photolysis

Structure: Contains photosystems, large proteins with pigment molecules (i.e. chlorophyll) concentrated in the reaction centre

Function: Photophosphorylation

Structure: ATP synthase embedded in the membrane

Thylakoid lumen

Function: Photophosphorylation and chemiosmosis

Structure: Small volume allows proton gradient to be maintained

Stroma

Function: Reactions of the Calvin cycle

Structure: High concentration of enzymes

Imaging active mitochondria with electron tomography

  • Electron tomography is an imaging technique that creates three-dimensional images of internal structures.
  • A beam of electrons is passed through an object at different angles on different planes to create a number of two-dimensional pictures that are pieced together using visualisation software.

Figure 8.2.2e – Principle of electron tomographyFigure 8.2.2e – Principle of electron tomography

  • Electron tomography (ET) is an improvement on traditional electron microscopy (TEM) because images can be taken while structures are active. In the case of mitochondria, this technique has shed light on many features of that were previously unknown.

Figure 8.2.2f – Comparison of TEM (a, b) and ET image (c, d) images of mitochondria from chick cerebellum.  (Frey, 2002)Figure 8.2.2f – Comparison of TEM (a, b) and ET image (c, d) images of mitochondria from chick cerebellum.  (Frey, 2002)

  • Through electron tomography, researchers have found that the inner membrane of mitochondria is dynamic and flexible. Cristae can change in diameter, as well as form junctions with other parts of the inner membrane.
  • The changes are not random – they seem to be controlled by specific regulatory mechanisms that could allow for more efficient production of ATP.

Figure 8.2.2gFigure 8.2.2g
Electron tomography is an improvement on traditional TEM.

Nature of Science

  • Developments in scientific research follow improvements in technique: Electron tomography was first used in the life sciences, but is now used to produce images of crystal structures in the physical sciences.

TOK

Chemiosmotic theory faced years of opposition before being accepted. Why would falsification of a current theory not result in acceptance of a new theory or paradigm shift?

Course links

References

Frey, T.G., C.W. Renkin, and G.A. Perkins. 'Insight into mitochondrial structure and function from electron tomography' in Biochemica et Biophysica Acta. 1555(2002), p 196-203.