What are the two storage proteins? Exploring Key Examples in Animals and Plants

November 17, 2025 •

5 min read

Storage proteins can account for as much as 25% of the dry weight in many plant seeds, acting as crucial amino acid reserves for the developing embryo. Understanding what are the two storage proteins and their specific functions in different organisms, from animal embryos to germinating seedlings, is fundamental to biology and nutritional science. These specialized proteins provide the essential building blocks for growth and development when an organism's other resources are limited.

Quick Summary

Storage proteins serve as vital reservoirs of amino acids and minerals, with prominent examples including casein and ovalbumin in animals and various proteins in plant seeds. Their primary function is to provide essential nutrients for growth, development, and metabolic processes during periods when other food sources are unavailable or insufficient.

Key Points

Animal Examples: The two most prominent storage proteins in animals are casein (milk) and ovalbumin (egg white), both supplying amino acids for development.
Plant Categories: Plant storage proteins are broadly classified into Seed Storage Proteins (SSPs) for germination and Vegetative Storage Proteins (VSPs) for temporary reserves.
Iron Storage: Ferritin is a crucial animal storage protein that specifically stores and regulates the body's iron supply.
Nutrient Source: The primary function of all storage proteins is to provide a reserve of amino acids and minerals, which are mobilized during periods of rapid growth or nutrient scarcity.
Diverse Sources: Storage proteins are found in a wide variety of sources, including mammalian milk, bird eggs, and plant seeds and tubers.
Nutritional Impact: These proteins are a major source of dietary protein for humans and animals, contributing to nutrition, health, and food product functionality.

Understanding the Concept of Storage Proteins

Proteins are large, complex molecules essential for virtually every cellular process in an organism. While many proteins perform a specific enzymatic, structural, or transport function, a distinct class known as storage proteins acts as a biological reservoir of nutrients. These proteins are strategically accumulated in tissues like seeds, milk, or eggs to be utilized later, often during a crucial developmental phase, such as germination in plants or early growth in animals. The question of "what are the two storage proteins?" has different answers depending on the biological context, with key examples found in both the animal and plant kingdoms.

Animal Storage Proteins: Casein, Ovalbumin, and Ferritin

In animals, two of the most well-known storage proteins are casein and ovalbumin, both serving as a primary nutrient source for developing young. These proteins are critical for providing the amino acids and organic nitrogen needed during rapid growth phases.

Casein

Casein is a complex protein found predominantly in mammalian milk, making up approximately 80% of the total protein content. It is particularly rich in essential amino acids, calcium, and phosphorus, which are all vital for the growth of a newborn mammal. Casein forms micelles, which are large aggregates that remain suspended in the milk, and its structure allows for the slow release of amino acids into the bloodstream when consumed. This slow digestion rate makes it a popular nutritional supplement for athletes aiming for sustained protein supply.

Ovalbumin

Ovalbumin is the main storage protein in egg white (albumen) in birds and reptiles, constituting over 50% of its protein content. It is a monomeric phosphoglycoprotein that acts as a ready supply of amino acids for the developing embryo. Ovalbumin has a unique structure and is known for its heat sensitivity, which causes it to denature and coagulate upon cooking. In addition to its storage function, ovalbumin has properties related to egg white's gelation and emulsifying abilities, making it important in the food industry.

Ferritin

While casein and ovalbumin store amino acids, ferritin is another crucial animal storage protein that holds a different type of nutrient: iron. This globular protein complex is found within the cells of most organisms and can store up to 4,500 iron atoms within its spherical protein shell. Ferritin plays a critical role in regulating the body's iron levels, preventing both iron deficiency and toxic iron overload.

Plant Storage Proteins: SSPs and VSPs

In the plant world, the storage proteins can be categorized into two broad classes: Seed Storage Proteins (SSPs) and Vegetative Storage Proteins (VSPs).

Seed Storage Proteins (SSPs)

SSPs are the proteins that accumulate to high concentrations in seeds during the late stages of development. These proteins serve as the primary source of nitrogen and carbon for the germinating seedling until it can perform photosynthesis. Examples vary widely by plant type:

Cereals (e.g., wheat, maize, rice): Prolamins and glutelins. Gliadin and glutenin are the specific prolamins in wheat that form gluten.
Legumes (e.g., peas, beans): Globulins, such as legumins and vicilins.
Other seeds: Albumins and other globulins, depending on the species.

Vegetative Storage Proteins (VSPs)

VSPs accumulate in vegetative tissues like leaves, stems, and tubers during periods of excess nutrient availability. They act as a temporary, mobile reserve of amino acids for use in subsequent growth phases. Compared to SSPs, VSPs are less well-studied but are still vital for plant development and recovery from stress. Examples include patatins in potatoes and sporamins in sweet potatoes.

Functions and Examples of Storage Proteins

Here are some of the key functions and examples of storage proteins:

Nutrient Provision for Embryos: Ovalbumin in eggs and casein in milk ensure that developing animal offspring have a constant supply of amino acids for growth and organ formation.
Germination Support: SSPs in plant seeds provide the amino acid building blocks necessary for the initial stages of seedling growth and development.
Mineral Regulation: Ferritin stores and releases iron in a controlled manner, preventing cellular damage from iron toxicity and ensuring iron availability.
Stress Resilience: VSPs act as a mobile pool of nitrogen, which can be remobilized to support plant growth during periods of nutrient scarcity or environmental stress.
Food Functionality: Storage proteins like gluten in wheat and globulins in legumes influence the texture and properties of many food products.

Comparison of Animal vs. Plant Storage Proteins


Feature	Animal Storage Proteins	Plant Storage Proteins
Primary Function	Provide amino acids, minerals (e.g., iron) to developing offspring or regulate mineral levels in adult organisms.	Provide amino acids and nitrogen for seed germination (SSPs) and act as temporary amino acid reserves (VSPs).
Key Examples	Casein (milk), Ovalbumin (egg white), Ferritin (stores iron)	Globulins (legumes), Prolamins (cereals), Albumins (seeds), Glutelins (cereals), Vegetative Storage Proteins (VSPs)
Location	Milk, egg whites, liver, spleen, bone marrow (ferritin), muscle cells (myoglobin).	Seeds (SSPs), vegetative tissues like leaves, stems, and tubers (VSPs).
Storage Mechanism	Micelle formation (casein), sequestered in egg white (ovalbumin), protein shell (ferritin).	Packed into specialized structures called protein bodies within storage cells.
Breakdown Trigger	Digestion by enzymes after consumption (casein, ovalbumin).	Hydrolyzed by enzymes during seed germination (SSPs) or nutrient stress (VSPs).

The Importance of Understanding Storage Proteins

From an agricultural perspective, understanding storage proteins is vital for improving crop yield and nutritional quality. By genetically modifying crops, scientists can increase the concentration of nutrient-dense storage proteins, helping to address global food insecurity. Furthermore, insights into storage proteins have significant implications for food science, particularly in developing hypoallergenic food products, as certain storage proteins like those in nuts or gluten can cause allergic reactions. Research also explores the functional properties of these proteins for use in food processing.

Conclusion

While there is no single answer to what are the two storage proteins, as different contexts have distinct examples, we can identify key types. In animals, casein (milk) and ovalbumin (egg) are essential for developing young, while ferritin regulates vital iron reserves. In plants, the broad classifications are seed storage proteins (SSPs) and vegetative storage proteins (VSPs), which ensure successful germination and sustained growth. These specialized proteins, with their diverse functions and sources, are fundamentally important across biology, providing the necessary nutrients to sustain life during crucial developmental stages and times of scarcity.

For more detailed scientific information on the topic of storage proteins, a resource like ScienceDirect can provide access to academic research and in-depth analyses.

Frequently Asked Questions

The main difference lies in their primary function and location. Animal storage proteins, like casein and ovalbumin, primarily provide nutrients for developing offspring (milk, eggs), while plant storage proteins (SSPs and VSPs) provide nutrients for seed germination and plant growth.

Casein is slowly digested by enzymes in the mammalian digestive system, releasing a steady supply of amino acids into the bloodstream. This makes it a popular dietary supplement for sustained nutrient release.

During germination, seed storage proteins (SSPs) are rapidly degraded into amino acids. These amino acids are then utilized by the developing seedling as a nutritional source to fuel its initial growth before it can photosynthesize.

Yes, ferritin is a critical storage protein in animals. Its main function is to store iron in a non-toxic form within cells and release it in a controlled manner when needed, regulating the body's iron balance.

Examples include globulins (like legumins and vicilins in legumes), prolamins (like gliadin and glutenin in cereals), and albumins found in many seeds. Patatins in potatoes and sporamins in sweet potatoes are examples of vegetative storage proteins.

When consumed, storage proteins are broken down into amino acids. These amino acids are the fundamental building blocks for synthesizing new proteins, including those required for muscle tissue repair and growth.

In food science, storage proteins are important for both their nutritional value and functional properties. For example, proteins like gluten influence the viscoelastic properties of dough, while others can affect texture, emulsification, and gelation.