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Kimball's Biology Pages - Cellular Respiration
The electron transport chain and process of chemiosmosis in aerobic metabolism is the quintessential example of the use of membranes to facilitate energy conversion.

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Special points of emphasis

Biological Membranes

Bioenergetics and Cellular Respiration



A prominent theme in bioenergetics is the segregation of ions by membranes for energy purposes, which allows the storage of free energy in diffusion and in membrane potential. The processes of photosynthesis in plants and oxidative metabolism in eukaryotes involve such functioning of membranes. Photosynthesis on the inner membranes of chloroplasts generates a proton gradient for ATP synthesis between the thylakoid lumen and the stroma. A similar process occurs on the inner membranes of mitochondria, where the electron transport system leads to the formation of a proton gradient between the intermembrane space and the matrix. The free energy of this concentration gradient is then used to drive ATP synthesis.


Heat and Temperature


Chemical Thermodynamics and the Equilibrium State

Biological Membranes

The Eukaryotic Cell

Bioenergetics and Cellular Respiration


The Endocrine System

Homeothermic vertebrates (birds and mammals) have two mechanisms for direct thermogenesis at the cellular level.

The first mechanisms begins with the release of two substances in response to chilling, the thyroid hormone thyroxine and the neurotransmitter norepinephrine, which open channels for the facilitated diffusion of Na+ ions into the cell. The Na+ ions are then returned by active transport to the extracellular space. The movement of sodium ions from inside the cell to outside the cell stores free energy (the movement is against both against a concentration gradient and positive electrical potential). The return of the Na+ ions represents a free energy decrease, and also an enthalpy decrease, meaning that the system generates heat as it cycles.

The second mechanism involves the production of heat in brown fat. Brown fat contains large numbers of mitochondria. In brown fat, chemiosmosis can be decoupled from ATP synthesis, so the free energy decrease involved as protons diffuse from the intermembranous space into the matrix evolves heat rather than leads to substrate level phosphorylation.

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