Cell Metabolism

Metabolism is the sum of all chemical reactions in a cell. It is critical to survival in unicellular and multicellular organisms alike. Metabolic reactions fall into two classes: catabolic reactions break down organic substances and release energy, while anabolic reactions synthesise new products and require energy.

METABOLIC PATHWAYS

A metabolic pathway is a series of chemical reactions that forms the basis of a biological process. The first enzyme converts a substrate into a product, and that product becomes the substrate for the next reaction, continuing until the end product is made. Each step is catalysed by a specific enzyme whose active site is complementary to its substrate.

A METABOLIC PATHWAY — each step requires its own enzyme, and each product becomes the substrate of the next step.

The key characteristics of a metabolic pathway are:

1. Many small, regulated steps - large unregulated steps would quickly produce unfavourable conditions, such as excessive temperatures or highly acidic environments. Each step produces an intermediate compound.
2. Each step is controlled by a specific enzyme - this allows the pathway to be tightly regulated according to the cell's needs; if the required enzyme is absent, the reaction does not take place (or proceeds extremely slowly).
3. Each step releases a small amount of energy - some of which is lost as heat.

Organising chemistry this way has four advantages:

1. Control - a different enzyme at each step means the cell can start or stop the pathway at any point by switching one enzyme on or off.
2. Flexibility - other substrates can enter the pathway at different points, letting the cell use other resources.
3. Useful intermediates - many intermediates are essential molecules the cell needs anyway, produced without a separate pathway.
4. Gradual energy release - energy is released in small portions, some captured by energy coupling processes such as ATP synthesis, rather than in one destructive burst.

INTERNAL MEMBRANES AND LOCATION

Folded internal membranes provide surface area for these reactions. The cristae of mitochondria hold the enzymes of respiration; the thylakoid (grana) stacks of chloroplasts hold the machinery of photosynthesis — more membrane means more bound enzymes and faster reactions. The internal fluids matter equally: the matrix (mitochondrial DNA, ribosomes, soluble enzymes, ions) and the stroma (chloroplast DNA, ribosomes, starch) hold all the necessary components for their pathways in one place.

FACTORS AFFECTING METABOLIC RATE

• Temperature - more heat means more kinetic energy and faster reactions; but beyond the tolerance range, proteins and lipids change structure and reactions fail.
• pH - enzymes hold their three-dimensional shape only within a defined pH range; outside it they denature.
• Nutrient availability - pathways need reactants; scarcity slows metabolism, abundance stimulates it.
• Enzyme availability - without the required enzyme, the step cannot proceed.
• Chemical inhibitors - can block enzymes and halt a whole pathway.
• Location - a reaction only proceeds where all components meet: enzymes, substrates and cofactors together.

CHEMICALS INTERFERING WITH CELL METABOLISM

Certain chemicals interfere with cell metabolism. They do so by binding to enzymes and changing their function, binding to cell membranes and altering their properties, modifying protein synthesis, or binding to DNA and affecting cell division. Some are naturally occurring; others are synthetic.

An inhibitor that disables one enzyme stops everything downstream of it — this is how metabolic poisons act.

• Carbon monoxide - binds to haemoglobin, blocking its reaction with oxygen, and also disables an enzyme in the respiratory pathway.
• Cyanide - disables the enzyme cytochrome oxidase, halting cellular respiration.
• Herbicides and insecticides - act in the same way on enzymes in their target organisms.
• DDT and CFCs - linked to cancer: DDT is a probable carcinogen, while CFCs act indirectly by depleting the ozone layer and increasing UV exposure.
• Illicit drugs - amphetamines and marijuana interfere with neural pathways.
• Medications - can have serious side effects; thalidomide, prescribed for morning sickness in pregnant women, caused severe birth defects.

Humans also use chemicals to great benefit — and the harms often surface only after years of use:

Weighing a chemical's benefit against its harm — with evidence that may arrive decades apart — is a difficult, real-world judgement.