Function of Mitochondria

The term thermogenesis refers to heat production in living organisms, mainly mammals. It is classified according to how the heat production is initiated:

• Exercise-associated thermogenesis - due to movement e.g. shivering.
• Non-exercise activity thermogenesis - incl. non-shivering thermogenesis, see below.
• Diet-induced thermogenesis - i.e. heat generated by the body following the digestive processes. See also metabolic rate.

Non-shivering thermogenesis is due to facilitated diffusion of protons into the mitochondrial matrix. This is called "proton leak" or "mitochondrial uncoupling" and only occurs in certain circumstances. When it does happen, this results in the unharnessed potential energy of the proton electrochemical gradient being released as heat.

Non-shivering thermogenesis occurs mainly in brown adipose tissue, also known as "brown fat" (adipose tissue is "fat tissue") because the process is controlled by a protein present in brown fat. The protein is thermogenin, also known as UCP1, and acts as a proton channel that sometimes enables protons (i.e. H⁺ ions) to enter the mitochondrial matrix without contributing to ATP synthesis.

Brown adipose tissue is produced in various amounts by different types of mammals and is at its highest levels in early life and in hibernating animals e.g. bears living in cold environments. Adult humans usually only have small brown fat deposits throughout the body - brown fat is present in humans at birth, but decreases with age.

Mitochondria contain their own genetic material - which is independent of the cell in which they are located.

Mitochondrial DNA (mtDNA) is maternally inherited. At fertilization only nuclear DNA enters from the sperm because although the egg contains mitochondria, sperm cells do not. Sperm are so tiny that mitochondria would hamper their passage toward the egg. (Therefore exercise capacity e.g. for endurance sports tends to be maternally inherited. Maternal ancestral history can also be traced via mtDNA.)

mtDNA accounts for about 1% of the total cellular DNA - recall that the number of mitochondria per cell varies considerably with the type and function of the cell. mtDNA exists in a circular arrangement in the mitochondrial matrix. Mutations of errors in some mitochondrial genes can result in certain diseases - mitochondrial diseases. Mitochondria self-replicate (divide) by fission, as is also true of bacteria cells - which are also known as prokaryotic cells.

Two types of cell death occur in multicellular organisms:

• Apoptosis - the process of programmed cell death (PCD), in which biochemical changes lead to cell changes such as cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation, the ultimate consequence of which is death of the cell.
• Necrosis - traumatic (not pre-planned or "programmed") cell death e.g. due to acute cellular injury.

Apoptosis is advantageous to organisms for many reasons. For example, during development it is necessary for some cells to die in order for normal tissue and organ formation to proceed, e.g. the differentiation of fingers in a human embryo occurs by apoptosis of the cells between the fingers, resulting in separate fingers. Death of abnormal cells such as cells that are cancerous or virally infected is also good for the organism. Unlike necrosis, apoptosis results in cell fragments called apoptotic bodies that phagocytic cells can engulf and quickly remove before the contents of the cell spills over surrounding cells which could cause tissue damage.

Mitochondria help to safeguard cell survival while appropriate and to facilitate apoptosis when necessary. When apoptosis of a cell is stimulated: proapoptotic proteins insert into the mitochondrial membrane, forming pores in the membrane.

1. Proapoptotic proteins insert into the mitochondrial membrane, forming pores in the membrane.
2. A protein called cytochrome leaves the intermembrane space of the mitochondrion via the pores in the mitochondrial membrane.
3. Cytochrome emerges from the mitochondrion into the cytosol (i.e. the intracellular fluid) of the eukaryotic cell.
4. Cytochrome in the cytosol of the cell stimulates a cascade of biochemical changes that lead to apoptotic death of the cell.

Calcium (Ca²⁺) has many important functions in the biochemistry of cells and in physiology generally, e.g. re.

• signal transduction pathways
• neurotransmitter release from neurons
• contraction of muscle cells (See also types of muscle contractions.)
• many enzymes need Ca²⁺ as a cofactor, e.g. in blood-clotting
• fertilization
  ... and extracellular calcium is needed to maintain the potential difference across excitable cell membranes, and for the formation of healthy bones

Storage of Calcium (Ca²⁺):

In the case of mammals, including humans, bone tissue is the main mineral storage site. Release and re-absorption of Ca²⁺ from and into bone is regulated by hormones. At a cellular level, Ca²⁺ is held within two types of organelles, the endoplasmic reticulum and mitochondria. The endoplasmic reticulum (SER & RER) is the main site of cellular storage of calcium. Mitochondria can also transiently store calcium - contributing to the cell's homeostasis of calcium by acting as "cytosolic buffers" for calcium.