Eukaryotic cells, characterised by their membrane-bound organelles, display remarkable diversity when we observe the cellular architecture of animals, fungi, and plants. These structural differences cater to the unique life processes and survival strategies of each group.
Cell Walls
Cell walls provide support, protection, and structure. Their composition and presence vary among eukaryotic organisms.
- Animal Cells:
- Absence of Cell Walls: Unlike plants and fungi, animal cells do not have cell walls. Instead, they rely on a semi-permeable plasma membrane.
- Implication: This lack provides flexibility, permitting a variety of cell shapes, which aids in functions like phagocytosis.
- Fungal Cells:
- Chitin-based Walls: Fungal cell walls are chiefly constructed from chitin, a strong nitrogen-containing polysaccharide.
- Role: The wall protects fungal cells from environmental factors, plays a part in preventing desiccation, and confers structural rigidity.
Image courtesy of MDPI
- Plant Cells:
- Cellulose Framework: Cell walls of plants predominantly consist of cellulose. This complex sugar provides sturdiness.
- Role: Apart from structural support, plant cell walls play a critical role in maintaining turgor pressure, especially crucial for plants in water retention.
Image courtesy of LadyofHats
Vacuoles
Vacuoles are vesicles involved in a range of functions, from storage to waste management. Their prominence and roles vary in different cells.
- Animal Cells:
- Smaller Vacuoles: Unlike plant cells, animal cells might contain several small vacuoles.
- Function: They play roles in nutrient storage, waste management, and even ion regulation. Some specialised vacuoles, called lysosomes, contain enzymes to break down cellular waste.
- Fungal Cells:
- Vacuole Structure: Similar to plant cells, fungal cells may contain a prominent vacuole.
- Function: Aids in osmoregulation, thereby preventing the cell from bursting due to osmotic pressure. Also serves as a storage repository and waste disposal unit.
- Plant Cells:
- Central Vacuole: A defining feature, the central vacuole can occupy up to 90% of the cell's volume in mature plant cells.
- Function: Apart from storage, it aids in waste management, maintains turgor pressure, and even plays a role in growth by expanding, pushing the cell wall outward.
Image courtesy of Wizeprep
Chloroplasts
The presence or absence of chloroplasts can determine how an organism obtains energy.
- Animal Cells:
- Lack Chloroplasts: Animals rely on food intake, thus, their cells don’t have chloroplasts.
- Fungal Cells:
- Absent but with Exceptions: While most fungal cells lack chloroplasts, there are rare fungi which exhibit photosynthetic capabilities.
- Plant Cells:
- Presence of Chloroplasts: These organelles are the photosynthesis sites, converting light energy into chemical energy stored as glucose.
- Significance: Responsible for the green hue of plants due to chlorophyll, a pigment essential for capturing light.
Image courtesy of Kelvinsong
Centrioles
Centrioles are cylindrical structures important in cell division.
- Animal Cells:
- Presence and Role: Centrioles, found in the centrosome, play a pivotal role in cell division by organising the spindle fibres.
- Structure: Composed of microtubules arranged in a specific pattern.
- Fungal Cells:
- Generally Absent: Many fungi lack centrioles, but some, like the chytrids, possess them.
- Plant Cells:
- Typically Absent: Plant cells usually form spindle fibres without the need for centrioles during mitosis and meiosis.
Image courtesy of CNX OpenStax
Cilia and Flagella
These structures aid movement, and their presence varies across eukaryotes.
- Animal Cells:
- Cilia and Flagella: Depending on the cell type, animal cells may exhibit cilia or flagella. For instance, respiratory tract cells have cilia, while sperm cells have a flagellum.
- Function: Cilia move in coordinated waves, propelling substances, whereas flagella whip back and forth, propelling the cell.
- Fungal Cells:
- Limited Presence: Most fungi lack these structures, but some, like chytrids, exhibit a flagellated stage in their life cycle.
- Plant Cells:
- Generally Absent: Plant cells do not typically possess these structures. An exception is seen in certain algae and in the flagellated sperm of some plants.
Image courtesy of Kohidai, L.
FAQ
Certain fungi have developed a symbiotic relationship with photosynthetic organisms like algae or cyanobacteria. In these mutualistic associations, termed lichens, the fungus offers the photosynthetic partner protection and access to nutrients, while the photosynthetic partner provides the fungus with organic compounds produced via photosynthesis. This unique partnership allows the fungal component of the lichen to indirectly benefit from photosynthesis, even though the fungal cells themselves lack chloroplasts. The adaptive advantage of this relationship is evident in the ability of lichens to colonise diverse and sometimes extreme habitats, where either organism alone might struggle to survive.
While both cilia and flagella aid in movement, they differ in size, number, and mode of action. Cilia are shorter and occur in large numbers on the cell surface. They move in coordinated waves, either moving the cell or propelling substances past stationary cells. For instance, cilia in the human respiratory tract move mucus and trapped particles out of the lungs. Flagella are longer and usually fewer in number. They move in a whip-like motion, propelling the entire cell forward. Structurally, both cilia and flagella have a similar arrangement of microtubules, termed the "9+2" arrangement, where nine doublet microtubules surround two central single microtubules.
While centrioles play a significant role in organising spindle fibres in animal cell division, plant cells have developed a different mechanism. Instead of centrioles, plant cells use microtubule organising centres (MTOCs) that do not have the defined cylindrical structure of centrioles. These MTOCs are involved in the assembly of the spindle fibres required for separating chromosomes during mitosis. Additionally, the phragmoplast, a plant-specific structure, forms during cytokinesis and aids in the formation of the cell plate, which eventually becomes the cell wall that separates the two daughter cells.
Flagellated sperm in certain algae are a primitive feature, hinting at their evolutionary lineage. These algae likely evolved in aquatic environments where flagellated sperm could easily swim through water to reach the egg for fertilisation. Even as some algae transitioned to terrestrial habitats, they retained this trait, as their reproductive processes still occur in water or very moist environments. This adaptation ensures successful fertilisation in their specific habitats. Over time, higher plants developed different mechanisms for fertilisation that didn't rely on flagellated sperm, such as the production of pollen in seed plants.
Animal cells differ from plant cells in terms of their energy requirements, metabolic processes, and structural necessities. A central vacuole, like in plant cells, occupies a significant portion of the cell's volume and serves multiple purposes, such as maintaining turgor pressure and storing nutrients. Animal cells, in contrast, utilise multiple smaller vacuoles and lysosomes to handle waste management, ion regulation, and nutrient storage. The absence of a large central vacuole provides animal cells with more space for other organelles and offers flexibility in shape and function, aiding in their diverse roles and specialised processes in multicellular organisms.
Practice Questions
Animal cells do not possess cell walls. They rely solely on a semi-permeable plasma membrane, which offers flexibility and allows for various cell shapes. Fungal cells, on the other hand, have cell walls made primarily of chitin, a nitrogen-containing polysaccharide. This provides rigidity and protection against environmental factors and dehydration. In contrast, plant cells have cell walls that consist predominantly of cellulose. These cellulose-based walls confer structural support, maintain turgor pressure, and play a critical role in water retention, ensuring the plant remains turgid and upright.
Chloroplasts in plant cells are critical organelles responsible for photosynthesis, a process that converts light energy into chemical energy stored as glucose. They contain chlorophyll, a pigment essential for capturing light, giving plants their green colour. This photosynthetic ability allows plants to be autotrophs, producing their own food. Animal cells lack chloroplasts because animals obtain energy by consuming organic substances, making them heterotrophs. They rely on digestion rather than photosynthesis. Most fungal cells also lack chloroplasts because they are saprophytic or parasitic in nature, obtaining nutrients from decaying matter or other living organisms. Their energy acquisition methods make the presence of chloroplasts redundant.
