What Do Animals Cells Have That Plants Don't

Animal cells and plant cells, the fundamental building blocks of life in their respective kingdoms, share many similarities but also possess distinct differences that enable them to perform their specialized functions. While both cell types are eukaryotic—meaning they have a nucleus and other complex organelles—animal cells boast unique structures that are absent in plant cells, allowing them to carry out functions essential for animal life.

Key Distinctions: What Animal Cells Have That Plants Don't

The features that distinguish animal cells from plant cells are primarily related to structural support, cell division, and specific organelle composition. These differences reflect the diverse lifestyles and functional requirements of animals compared to plants.

1. Centrioles

What They Are: Centrioles are cylindrical structures composed of clusters of microtubules arranged in a specific pattern. Typically found in pairs, they are a crucial part of the centrosome, an organelle involved in cell division.

Function in Animal Cells:

• Cell Division: Centrioles play a critical role in organizing the mitotic spindle during cell division. The mitotic spindle is responsible for separating chromosomes accurately into the daughter cells. During prophase, the centrosomes move to opposite poles of the cell, and microtubules extend from them to form the spindle fibers.
• Formation of Cilia and Flagella: Centrioles are involved in the formation of cilia and flagella, which are cellular appendages used for movement. Cilia are short, hair-like structures that can move substances across the cell surface, while flagella are longer, whip-like structures that propel the cell through its environment.
• Organization of Microtubules: Centrioles help organize microtubules within the cell, which are part of the cytoskeleton. The cytoskeleton provides structural support and facilitates intracellular transport.

Why Plants Don't Need Them: Plant cells do not have centrioles because they have evolved alternative mechanisms for organizing the mitotic spindle during cell division. Instead of centrioles, plant cells use structures called microtubule organizing centers (MTOCs), which perform a similar function in spindle formation. Plant cells also have rigid cell walls, which provide structural support, reducing the need for cilia or flagella for movement in most plant cells.

2. Lysosomes

What They Are: Lysosomes are membrane-bound organelles containing a variety of enzymes capable of breaking down different types of biomolecules. They are often referred to as the "recycling centers" of the cell.

Function in Animal Cells:

• Intracellular Digestion: Lysosomes break down cellular waste, damaged organelles, and ingested materials through a process called phagocytosis. The enzymes within lysosomes, such as proteases, lipases, and nucleases, digest proteins, lipids, nucleic acids, and carbohydrates, respectively.
• Autophagy: Lysosomes are involved in autophagy, a process by which the cell degrades and recycles its own components. This process is essential for maintaining cellular health and responding to stress.
• Cellular Defense: In immune cells, lysosomes play a role in destroying pathogens and presenting antigens to other immune cells.

Why Plants Don't Need Them (As Much): Plant cells also perform intracellular digestion but rely more on vacuoles for this purpose. Plant vacuoles can store nutrients, water, and waste products and contain enzymes that can degrade cellular components. While plant cells do have structures similar to lysosomes, they are less prominent and functionally diverse compared to those in animal cells. The large central vacuole in plant cells takes on many of the digestive functions that lysosomes perform in animal cells.

3. Glycogen

What It Is: Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals and fungi. It is similar in structure to starch, which is the primary energy storage molecule in plants.

Function in Animal Cells:

• Energy Storage: Glycogen is primarily stored in the liver and muscle cells. When blood glucose levels drop, glycogen is broken down into glucose through a process called glycogenolysis, providing a rapid source of energy.
• Regulation of Blood Glucose: The liver plays a crucial role in maintaining blood glucose homeostasis by storing and releasing glucose as needed.
• Muscle Energy: In muscle cells, glycogen provides a readily available source of glucose for muscle contraction during physical activity.

Why Plants Don't Need It: Plants use starch as their primary energy storage molecule. Starch is stored in specialized organelles called amyloplasts, which are found in plant cells, particularly in storage tissues such as roots, stems, and seeds. When energy is needed, starch is broken down into glucose through a process called hydrolysis. The difference in energy storage molecules reflects the different energy needs and metabolic pathways in animals and plants.

4. Cell Junctions

What They Are: Cell junctions are specialized structures that connect cells to each other and to the extracellular matrix. They are crucial for tissue organization, cell communication, and maintaining the integrity of multicellular organisms.

Types of Cell Junctions in Animal Cells:

• Tight Junctions: These junctions form a tight seal between cells, preventing the passage of molecules and ions through the space between cells. They are commonly found in epithelial cells, such as those lining the digestive tract.
• Adherens Junctions: These junctions provide strong adhesion between cells and are connected to the actin cytoskeleton. They play a role in maintaining tissue structure and transmitting mechanical forces.
• Desmosomes: These junctions provide robust mechanical strength and resistance to shear stress. They are commonly found in tissues that experience mechanical stress, such as skin and heart muscle.
• Gap Junctions: These junctions form channels between cells, allowing the passage of ions, small molecules, and electrical signals. They are essential for cell communication and coordinating cellular activities.

Why Plants Don't Need Them (In the Same Way): Plant cells are connected by structures called plasmodesmata, which are channels that pass through the cell walls of adjacent cells, allowing for communication and transport of nutrients and signaling molecules. While plasmodesmata serve a similar function to gap junctions, they are structurally different and allow for the exchange of larger molecules. Plant cells also rely on the cell wall for structural support and cell adhesion, reducing the need for complex cell junctions found in animal cells.

5. Extracellular Matrix (ECM)

What It Is: The extracellular matrix (ECM) is a complex network of proteins and polysaccharides that surrounds cells in animal tissues. It provides structural support, regulates cell behavior, and influences cell-to-cell communication.

Function in Animal Cells:

• Structural Support: The ECM provides a scaffold for cells, giving tissues their shape and integrity.
• Cell Adhesion: The ECM contains proteins that mediate cell adhesion, allowing cells to attach to the matrix and to each other.
• Cell Signaling: The ECM can bind growth factors and other signaling molecules, influencing cell growth, differentiation, and migration.
• Tissue Repair: The ECM plays a critical role in tissue repair and regeneration by providing a template for cell migration and new tissue formation.

Why Plants Don't Need It (In the Same Way): Plant cells have cell walls composed primarily of cellulose, providing structural support and rigidity. The cell wall performs many of the functions that the ECM provides in animal tissues, such as maintaining cell shape, protecting the cell from mechanical stress, and regulating cell growth. While plants do not have an ECM in the same way as animals, they secrete a middle lamella between adjacent cell walls, which helps to hold the cells together.

6. Cilia and Flagella

What They Are: Cilia and flagella are cellular appendages used for movement. Cilia are short, hair-like structures that can move substances across the cell surface, while flagella are longer, whip-like structures that propel the cell through their environment.

Function in Animal Cells:

• Movement: Flagella are used for locomotion in cells such as sperm cells. Cilia are used to move substances across the surface of epithelial cells, such as those lining the respiratory tract.
• Sensory Functions: In some cells, cilia act as sensory organelles, detecting signals from the environment.

Why Plants Don't Need Them (As Much): Most plant cells do not have cilia or flagella. The primary reason is that plants are typically stationary organisms, and their cells do not need to move in the same way as animal cells. However, there are exceptions. For example, the sperm cells of some plants, such as ferns and mosses, have flagella that allow them to swim to the egg for fertilization.

Differences Summarized in a Table

To provide a concise comparison, here's a table summarizing the key differences:

Scientific Explanation of These Differences

The differences between animal and plant cells reflect their distinct evolutionary paths and functional requirements. Animal cells, which are part of heterotrophic organisms, must be able to move, capture food, and respond rapidly to environmental stimuli. Plant cells, on the other hand, are part of autotrophic organisms that can produce their own food through photosynthesis and are generally stationary.

• Centrioles and Cell Division: The absence of centrioles in plant cells is linked to their unique mode of cell division. Plant cells form a cell plate between the two daughter cells, which eventually becomes the new cell wall. This process does not require centrioles for spindle organization.
• Lysosomes and Vacuoles: The difference in the prominence of lysosomes and vacuoles reflects the different strategies for intracellular digestion and storage. Plant cells rely more on vacuoles for these functions because they can store large amounts of water, nutrients, and waste products.
• Glycogen and Starch: The choice of glycogen versus starch for energy storage is related to the energy needs of animals and plants. Glycogen is a more branched molecule than starch, allowing for rapid glucose release, which is essential for the active lifestyle of animals.
• Cell Junctions and Cell Walls: The presence of complex cell junctions in animal cells is necessary for tissue organization and communication in the absence of a rigid cell wall. Plant cells rely on the cell wall for structural support and cell adhesion, reducing the need for these junctions.
• Extracellular Matrix and Cell Walls: The ECM in animal tissues provides a dynamic and adaptable environment for cells, allowing for cell migration and tissue remodeling. Plant cells, with their rigid cell walls, do not require the same level of ECM support.
• Cilia and Flagella: The presence of cilia and flagella in some animal cells reflects their need for movement and sensory functions. Plant cells, which are generally stationary, do not require these structures.

FAQ: Addressing Common Questions

• Do all animal cells have centrioles?

  • No, not all animal cells have centrioles. For example, mature mammalian red blood cells do not have centrioles, as they lack a nucleus and other organelles.

• Can plant cells survive without a cell wall?

  • Plant cells require a cell wall for structural support and protection. Without a cell wall, they would likely burst due to osmotic pressure.

• Are there any animal cells that have structures similar to plant cell walls?

  • No, animal cells do not have structures similar to plant cell walls. Animal cells rely on the extracellular matrix for structural support.

• Why do plant cells have a large central vacuole?

  • The large central vacuole in plant cells serves multiple functions, including storing water, nutrients, and waste products, maintaining cell turgor pressure, and aiding in intracellular digestion.

• Do animal cells have any structures that plant cells completely lack?

  • Yes, animal cells have centrioles, which are completely absent in plant cells (except in some lower plant sperm cells).

Conclusion

The distinctions between animal and plant cells highlight the remarkable diversity and specialization of cell structures in different kingdoms of life. Animal cells possess unique features such as centrioles, prominent lysosomes, glycogen, complex cell junctions, an extracellular matrix, and cilia and flagella in certain cells, which enable them to perform functions essential for animal life. These differences reflect the diverse lifestyles and functional requirements of animals compared to plants, providing a deeper understanding of cellular biology and the evolution of multicellular organisms. By understanding these fundamental differences, we gain insights into the intricate workings of life at the cellular level and the adaptations that allow organisms to thrive in diverse environments.