68
5 Cells in Motion
by a protein can be properly called a gate – it may open and close in response to a
voltage change or a chemical signal.
Proteins on the surface of the cell may serve as catalysts or attachment sites able
to recognize and bind incoming molecules, which allows communication between
the cell and the extracellular space. Transmembrane proteins transmit the signal to
the interior of the cell by sending their own messenger there when prompted by a
messenger coming from outside.
5.3 Energetics and Electrochemistry
Maintaining the cell’s chemistry in a non-equilibrium state, expressing genes, assembling proteins, and moving them around, all of this costs energy. It comes in
the first instance from sunlight, as plants, algae, and photosynthetic bacteria convert
CO 2 and water 2 to organic matter, which goes further up the food chain as fuel for
oxygen-breathing organisms.
Once gained, energy must be circulated internally to reach the right places at the
right time. Energy can be compared with money: you pay for work requiring effort.
Nature has not developed electronic money that can be transferred between the bank
accounts of the various molecules or cellular structures, nor anything like our own
symbolic paper money. Her money is material, but far more sophisticated than gold
or silver coin. It is not hoarded, but transferred, used, and recreated; an animal may
use about its own weight of molecular currency a day. Cellular coinage is not made
up of sophisticated proteins, but small molecules that can be reversibly transformed
from a high- to a low-energy state and vice versa.
The energy of organic fuel, either photosynthesized or ingested, is invested in the
most expensive circulating coin: a nucleotide going under the abbreviation NADH.
It releases a large amount of energy and acquires a positive charge turning it into
Krebs
cycle
cycle
NAD +
NADH
NA
high H +
h
ATP
synth
A
h
ADP
ATP
A
Na +
drive
P
D +
motors
Fig. 5.5 Energy exchange in an animal cell. High energy states are in red, and lower energy states
are in blue
2 Both are greenhouse gases, but only CO 2 , which is actually food for plants, is called “poison” by
folks scared of global warming.
5 Cells in Motion
by a protein can be properly called a gate – it may open and close in response to a
voltage change or a chemical signal.
Proteins on the surface of the cell may serve as catalysts or attachment sites able
to recognize and bind incoming molecules, which allows communication between
the cell and the extracellular space. Transmembrane proteins transmit the signal to
the interior of the cell by sending their own messenger there when prompted by a
messenger coming from outside.
5.3 Energetics and Electrochemistry
Maintaining the cell’s chemistry in a non-equilibrium state, expressing genes, assembling proteins, and moving them around, all of this costs energy. It comes in
the first instance from sunlight, as plants, algae, and photosynthetic bacteria convert
CO 2 and water 2 to organic matter, which goes further up the food chain as fuel for
oxygen-breathing organisms.
Once gained, energy must be circulated internally to reach the right places at the
right time. Energy can be compared with money: you pay for work requiring effort.
Nature has not developed electronic money that can be transferred between the bank
accounts of the various molecules or cellular structures, nor anything like our own
symbolic paper money. Her money is material, but far more sophisticated than gold
or silver coin. It is not hoarded, but transferred, used, and recreated; an animal may
use about its own weight of molecular currency a day. Cellular coinage is not made
up of sophisticated proteins, but small molecules that can be reversibly transformed
from a high- to a low-energy state and vice versa.
The energy of organic fuel, either photosynthesized or ingested, is invested in the
most expensive circulating coin: a nucleotide going under the abbreviation NADH.
It releases a large amount of energy and acquires a positive charge turning it into
Krebs
cycle
cycle
NAD +
NADH
NA
high H +
h
ATP
synth
A
h
ADP
ATP
A
Na +
drive
P
D +
motors
Fig. 5.5 Energy exchange in an animal cell. High energy states are in red, and lower energy states
are in blue
2 Both are greenhouse gases, but only CO 2 , which is actually food for plants, is called “poison” by
folks scared of global warming.