Table of Contents
Chemical energy storage in cells and organisms means converting easily lost, short‑term energy sources (like light or a steep concentration gradient) into more stable chemical forms that can be used later, elsewhere in the cell, or by another organism. In anabolism, this storage is mainly achieved by building energy‑rich molecules from simpler precursors.
In this chapter, we will focus on:
- How energy is “packed” into different storage molecules
- Short‑term vs. long‑term energy storage
- Major storage forms in plants, animals, and microorganisms
- Why multiple storage systems exist and how they complement each other
Concepts of Chemical Energy Storage
Free Energy and “High‑Energy” Bonds
Chemical energy is stored in the arrangement of atoms and electrons in molecules. When a reaction releases free energy ($\Delta G < 0$), some of that energy can be trapped instead of lost as heat.
Some bonds, often (but imprecisely) called “high‑energy bonds”, release a large amount of free energy when broken in a coupled reaction. In biology, this does not mean the bond is unusually strong; it means that the overall reaction in which the bond is hydrolyzed proceeds with a large, negative $\Delta G$.
ATP is the classic example (covered in detail elsewhere), but there are many other molecules whose synthesis represents energy storage.
Short‑Term vs. Long‑Term Storage
Organisms do not store all energy in one form. Different forms serve different time scales and functions:
- Immediate energy carriers
- ATP, GTP, NADH, NADPH, FADH$_2$
- Turnover is rapid; constant regeneration is needed
- Short‑ to medium‑term storage
- Soluble sugars (e.g. glucose, sucrose)
- Glycogen and starch (polymerized glucose)
- Long‑term storage
- Triacylglycerols (fats and oils)
- Certain storage proteins
- Structural polysaccharides to some degree (e.g. plant starch in seeds, tubers)
Anabolic pathways convert energy from catabolic processes (or from light, in photosynthesis) into these storage forms.
Storage in Carbohydrates
Carbohydrates play a central role as an energy store because they are:
- Chemically versatile and easily mobilized
- Compatible with aqueous cytoplasm
- Biochemically well‑integrated with glycolysis and cellular respiration
Soluble Sugars: Transport and Short‑Term Storage
Many organisms use small, soluble sugars as immediately available energy sources and for transport between tissues.
- Glucose
- Major fuel molecule in many organisms
- Rapidly metabolized via glycolysis and cellular respiration
- Concentration strictly regulated because high levels can damage cells and strongly affect osmotic pressure
- Sucrose
- Disaccharide (glucose + fructose)
- Main transport sugar in most plants
- Synthesized in source tissues (e.g. leaves) and transported via phloem to sinks (e.g. roots, fruits, seeds)
- Chemically more stable than free glucose and less reactive, suitable for long‑distance transport
These sugars represent a readily accessible pool: they can be used directly or polymerized into larger storage forms.
Polymer Storage: Glycogen and Starch
To store glucose without dangerously increasing cytosolic osmolarity, organisms polymerize it into large, insoluble macromolecules.
Glycogen in Animals and Many Microorganisms
- Structure
- Highly branched polymer of $\alpha$‑D‑glucose
- Linkages mainly $\alpha(1 \rightarrow 4)$ in chains and $\alpha(1 \rightarrow 6)$ at branch points
- Properties
- Compact, yet rapidly mobilizable due to many chain ends
- Stored mainly in:
- Liver: maintains blood glucose levels
- Skeletal muscle: local energy store for contraction
- Energy storage role
- Reserve fuel for periods of high energy demand or low external nutrient supply
- Mobilized by glycogenolysis to glucose‑1‑phosphate, feeding into glycolysis
Starch in Plants and Some Protists
- Structure
- Mixture of:
- Amylose: mostly linear $\alpha(1 \rightarrow 4)$‑linked glucose
- Amylopectin: branched polymer with $\alpha(1 \rightarrow 4)$ chains and fewer $\alpha(1 \rightarrow 6)$ branches than glycogen
- Forms semi‑crystalline granules in plastids (chloroplasts and amyloplasts)
- Types
- Transient (temporary) starch
- Accumulates in chloroplasts during the day from excess photosynthate
- Degraded at night to maintain metabolism when photosynthesis stops
- Storage starch
- Deposited long term in seeds, fruits, tubers, and roots (e.g. wheat grain, potato tuber)
- Serves as a major energy reserve during germination and early growth
- Energy storage role
- Efficient long‑term carbohydrate reservoir
- Breakdown products (maltose, glucose) can be transported or metabolized
Structural vs. Storage Carbohydrates
Some polysaccharides have structural rather than storage roles (e.g. cellulose in plant cell walls, chitin in arthropod exoskeletons). They also contain chemical energy, but their primary function is mechanical support, and they are often less accessible to the organism’s own enzymes.
Their energy content becomes ecologically relevant as a resource for other organisms (e.g. herbivores, decomposers) that have appropriate enzymes or symbionts.
Storage in Lipids (Fats and Oils)
Lipids represent the densest form of chemical energy storage used by living organisms.
Why Lipids Are Energy‑Dense
- Fatty acids are highly reduced hydrocarbon chains
- Oxidation of fatty acids in cellular respiration yields more ATP per gram than carbohydrates
- Triacylglycerols are hydrophobic and stored anhydrously (without associated water), unlike glycogen or starch, which bind water
- Result: Much higher energy per unit mass and volume
Triacylglycerols as Main Long‑Term Energy Reserve
- Structure
- Glycerol backbone esterified with three fatty acids
- Storage sites
- Animals: adipose (fat) tissue; also within the body cavity or under the skin for insulation and energy
- Plants: oil bodies in seeds and some fruits (e.g. sunflower, rapeseed, olive)
- Biological role
- Long‑term energy reserve for periods of starvation, migration, overwintering, or germination
- In seeds, stored oil supplies energy for early growth before the seedling becomes photosynthetically active
Saturated vs. Unsaturated Fats and Biological Implications
- Saturated fatty acids (no double bonds)
- Pack tightly, solid at room temperature (fats)
- Common in animal storage fat, some plant storage tissues
- Unsaturated fatty acids (one or more double bonds)
- Kinks prevent tight packing, more fluid, often liquid at room temperature (oils)
- Common in plant seeds and in membranes
The degree of saturation influences not only membrane fluidity (discussed elsewhere) but also the physical form of stored energy (fat vs. oil) and its behavior at different temperatures.
Lipid Mobilization
Mobilizing stored fat requires:
- Lipases to hydrolyze triacylglycerols into glycerol and free fatty acids
- Transport of fatty acids to tissues, where they undergo $\beta$‑oxidation and enter cellular respiration
This process is slower than glycogen or starch mobilization but yields significantly more ATP per molecule.
Storage in Proteins and Amino Acids
Proteins are not primarily designed for energy storage, but under certain conditions they become an energy reservoir.
Functional Reserve vs. Dedicated Storage
- Functional proteins (enzymes, structural proteins, transporters) embody a large amount of chemical energy in their amino acid chains
- In starvation or prolonged energy deficit:
- Organisms may break down their own proteins (e.g. muscle protein in animals) to use amino acids as energy substrates
- Storage proteins
- Some organisms synthesize proteins specifically for later mobilization:
- Plant seed storage proteins (e.g. in legumes, cereals)
- Some egg proteins (e.g. yolk proteins) in animals
- Provide both nitrogen and energy to the developing embryo/seedling
Although protein catabolism can supply ATP, it produces nitrogenous waste that must be excreted (ammonia, urea, uric acid), making proteins less convenient as primary energy stores than carbohydrates or fats.
High‑Energy Phosphate and Redox Pools
Beyond “bulk” storage molecules, cells maintain pools of short‑lived but energetically important compounds that buffer and redistribute energy.
Phosphagen Systems (e.g. Creatine Phosphate)
In some animals, especially vertebrates, phosphagen compounds temporarily store high‑energy phosphate bonds:
- Creatine phosphate (phosphocreatine)
- Present in muscle and brain
- Reaction:
$$\text{Creatine~phosphate} + \text{ADP} \rightleftharpoons \text{Creatine} + \text{ATP}$$ - Creatine kinase catalyzes rapid regeneration of ATP during sudden, short‑term energy demands (e.g. sprinting)
Other groups (e.g. many invertebrates) use molecules like arginine phosphate similarly. These systems provide very rapid but short‑lived energy buffering rather than long‑term storage.
Reducing Power: NADPH as a “Storage” of Reducing Equivalents
While NADH and FADH$_2$ are closely tied to ATP generation in catabolism, NADPH functions primarily in anabolism:
- Generated in specific pathways (e.g. light‑dependent reactions of photosynthesis, pentose phosphate pathway)
- Provides reducing power for biosynthetic reactions, including:
- Fatty acid synthesis
- Some steps in amino acid and nucleotide synthesis
- Represents a chemical form of stored reducing power rather than bulk energy storage
NADPH and similar carriers do not serve as long‑term energy stores but act as intermediate “currency” that links energy sources to specific anabolic processes.
Storage Strategies in Different Organisms
Plants
Key characteristics:
- Primary energy input: light (photosynthesis)
- Immediate products: triose phosphates, then sucrose and starch
- Main storage forms:
- Starch in chloroplasts (short term) and storage organs (long term)
- Oils in seeds and some fruits
- Some storage proteins in seeds (energy + nitrogen)
Plants balance daily and seasonal cycles of energy capture and use:
- Day: excess photosynthate stored as transient starch and sucrose
- Night: starch degraded to maintain metabolism
- Growing season vs. dormancy: large reserves stored in seeds, tubers, bulbs, and roots
Animals
Key characteristics:
- Primary energy input: organic food (carbohydrates, fats, proteins)
- Main storage forms:
- Glycogen in liver and muscle (rapidly mobilizable)
- Triacylglycerols in adipose tissue (long‑term, high‑density storage)
- Limited use of structural and functional proteins as emergency reserves
Different species and individuals vary in how much energy they store as fat vs. glycogen, depending on lifestyle (e.g. hibernators, migratory birds, endurance athletes).
Microorganisms
Bacteria, archaea, and unicellular eukaryotes show diverse storage strategies:
- Storage of glycogen‑like or starch‑like polysaccharides
- Polyhydroxyalkanoates (PHAs) (e.g. polyhydroxybutyrate) as intracellular carbon and energy stores in many bacteria
- Lipid droplets or wax esters in some species
- Phosphate storage in polyphosphate granules, which indirectly relate to energy management
These reserves allow microorganisms to survive fluctuating nutrient availability, e.g. feasting on abundant substrates, then enduring long periods of scarcity.
Why Multiple Storage Forms Are Necessary
No single storage molecule fulfills all needs. Multiple storage forms exist because of trade‑offs:
- Speed vs. capacity
- ATP, creatine phosphate: rapid turnover, tiny capacity
- Glycogen, starch: moderate capacity, moderate mobilization speed
- Fats: very high capacity, slower mobilization
- Water association vs. compactness
- Carbohydrate stores bind water, less energy‑dense but quickly accessible
- Fats are anhydrous, compact but slower to mobilize
- Solubility vs. stability
- Soluble sugars useful for transport and immediate needs but affect osmotic pressure
- Insoluble polymers (starch, glycogen) avoid osmotic problems and are more stable
- Additional functions
- Seed storage proteins provide both energy and essential nitrogen and sulfur
- Oils in seeds also influence buoyancy or dispersal
- Structural polysaccharides serve mechanical roles and can later be tapped as energy by other organisms in the ecosystem
As a result, each organism, tissue, and developmental stage uses a specific mix of storage forms tailored to its energy demands, environment, and life cycle.
Summary
- Chemical energy is stored by building energy‑rich molecules from smaller precursors in anabolic pathways.
- Carbohydrates:
- Soluble sugars (glucose, sucrose) serve as transport and quick‑use energy.
- Glycogen (animals, many microbes) and starch (plants) are major polymeric stores.
- Lipids (triacylglycerols) are the most energy‑dense long‑term store, prominent in animal fat and plant seed oils.
- Proteins act mainly as functional molecules but can serve as energy reserves, especially in seeds and eggs or under starvation.
- High‑energy phosphate compounds and redox carriers (e.g. phosphagens, NADPH) buffer and distribute energy but are not bulk stores.
- Different organisms and tissues deploy these storage forms in combinations suited to their metabolic patterns and ecological niches.