Eukaryotic cells are intricately structured, encompassing a multitude of components each fulfilling specific roles. With a complexity surpassing that of prokaryotic cells, understanding these structures provides clarity to the workings of organisms like fungi, plants, animals, and protists.
Plasma Membrane
- Definition: The plasma membrane is a selective barrier that surrounds the cell.
- Function: It regulates the entry and exit of substances, ensuring essential molecules enter the cell whilst waste products are efficiently expelled.
- Composition:
- Phospholipid Bilayer: Forms the basic structural framework, ensuring semi-permeability. The hydrophilic (water-attracting) heads face outward, while hydrophobic (water-repelling) tails face inward.
- Proteins: Scattered throughout the membrane and play roles in signal reception, molecule transport, and cell recognition.
- Carbohydrates: Typically found on the exterior surface, they contribute to cell recognition and signalling.
Cytoplasm
- Definition: The cytoplasm, predominantly water, serves as the cellular medium where numerous metabolic reactions occur.
- Organelles: Within the cytoplasm lie numerous organelles, each optimally positioned to perform its specific role.
- Cytosol: The fluid portion, distinct from the organelles, containing dissolved nutrients, ions, and waste products.
Nucleus
The nucleus stands as a sentinel, guarding the genetic code and regulating cellular activities.
- Nuclear Envelope: A double-membraned structure punctuated with nuclear pores, which regulate substance exchange between the nucleus and cytoplasm.
- Nucleolus: Located within the nucleus, it is pivotal in ribosome synthesis. It amalgamates ribosomal proteins and rRNA into ribosomal subunits.
- Chromatin and Chromosomes: These structures of DNA and proteins carry the genetic code. When the cell isn't dividing, the DNA exists as chromatin, a loose coil. During division, it condenses into distinguishable chromosomes.
Image courtesy of LadyofHats (Mariana Ruiz)
Mitochondria
- Function: These double-membraned organelles are ATP factories, transmuting energy from nutrients into usable cellular currency.
- Inner Membrane: Highly folded into structures called cristae, maximising surface area and consequently enhancing ATP production.
- Matrix: The internal compartment where essential enzymes execute the final steps of cellular respiration.
Image courtesy of CNX OpenStax
Endoplasmic Reticulum (ER)
A vast membranous network, the ER serves as a production and processing hub.
- Rough ER (RER):
- Function: Engaged in protein synthesis due to its ribosome-studded surface.
- Ribosomes: Synthesise proteins that are either excreted from the cell, incorporated into the cell’s plasma membrane, or sent to an organelle called the lysosome.
- Smooth ER (SER):
- Function: Specialised in lipid synthesis and detoxification processes.
- Role in Calcium Storage: Muscle cells use the SER for calcium storage, vital for muscle contractions.
Image courtesy of OpenStax
Golgi Apparatus
- Function: Often considered the cell’s 'post office', it modifies, sorts, and packages proteins and lipids.
- Cisternae: Distinctive flattened membranous sacs receive molecules from the ER for modification and dispatching.
- Directionality: Has a cis face (receiving side) and a trans face (shipping side) to ensure the unidirectional flow of materials.
Image courtesy of Kelvinsong
Vesicles/Vacuoles
- Vesicles: These tiny membranous sacs transport substances between cellular components or to the exterior. Some, like the peroxisomes, neutralise toxins.
- Vacuoles: Larger than vesicles, they're storage sacs. In plants, the central vacuole dominates the cell's volume, maintaining turgor pressure.
Lysosomes
- Function: These cellular 'recycling centres' digest unneeded materials.
- Acidic Environment: Maintains a pH of 5, optimal for its hydrolytic enzymes.
- Autophagy: Lysosomes digest and recycle worn-out organelles, reusing the organic monomers.
Image courtesy of lumoreno
Cytoskeleton
The cell's skeletal framework ensures its structural integrity, motility, and more.
- Microfilaments (Actin Filaments):
- Composition: Built from actin protein.
- Functions: Provide structural support, determine cell shape, enable cell movement (as in muscle contraction) and cell division.
- Microtubules:
- Composition: Comprised of tubulin proteins.
- Functions: They shape and support the cell and act as tracks for intracellular movement. Centrioles, involved in cell division in animal cells, are made of microtubules.
- Intermediate Filaments:
- Composition: Various proteins like keratins, lamins, etc.
- Functions: Provide tensile strength and reinforce cell shape.
Image courtesy of Laboratoires Servier
FAQ
Intermediate filaments, one of the three main components of the cytoskeleton, are essential for maintaining cell structural integrity. Unlike microfilaments (involved in movement) and microtubules (concerned with cell shape and intracellular movement), intermediate filaments provide tensile strength to cells. They are rope-like assemblies of fibrous proteins that resist pulling forces. For example, in epithelial cells, these filaments prevent the cell from being torn apart under stress. Keratin, found in hair and nails, is a type of intermediate filament. Thus, while all cytoskeletal components provide structural support, intermediate filaments specifically prevent the cell from being stretched excessively.
The nucleus earns the title "control centre" of the cell because it houses the cell's genetic material, DNA. DNA contains the instructions required for building and maintaining the organism. It regulates all cellular activities by controlling protein synthesis. The nucleus decides which genes are expressed, determining which proteins are produced and, consequently, how the cell functions. Moreover, the nucleus oversees the replication of DNA during cell division and safeguards the DNA from potential damage, ensuring genetic information's fidelity is passed to subsequent cell generations.
Vesicles and vacuoles are both membrane-bound sacs, but they differ in size, structure, and primary function. Vesicles are typically small and are involved in transporting substances within the cell. For instance, transport vesicles move proteins from the ER to the Golgi apparatus. Contrastingly, vacuoles are larger and primarily used for storage. Plant cells have a central vacuole that stores water, nutrients, and waste products. This vacuole also maintains turgor pressure, keeping the plant upright. Some animal cells have vacuoles, but they're smaller and often used to store water or ions. In essence, while both are storage and transport tools, their primary roles and prominence vary across cell types.
The Golgi apparatus plays a pivotal role in the post-translational modification, sorting, and dispatching of cellular products. Its structure, composed of cisternae, has distinct cis (receiving) and trans (shipping) faces to maintain directionality. Proteins or lipids coming from the ER enter the cis face, move through the Golgi's layers, undergoing modifications like glycosylation or phosphorylation. As these molecules exit the trans face, they are packed into vesicles. The vesicles have molecular "tags" that ensure they're directed to the appropriate destinations, be it within the cell, the plasma membrane, or outside the cell. This tagging system is integral for the cell to function harmoniously and efficiently.
The fluidity and flexibility of the plasma membrane are primarily attributed to the phospholipid bilayer's inherent structure and the presence of cholesterol molecules. The phospholipids, with their hydrophilic heads and hydrophobic tails, naturally orient themselves into a bilayer. The fatty acid tails of these phospholipids can move laterally, allowing the membrane to remain fluid. Additionally, cholesterol interspersed between the phospholipids plays a crucial role. At high temperatures, cholesterol reduces membrane fluidity by limiting the movement of phospholipids, while at low temperatures, it prevents them from packing too closely, thereby averting rigidity. This balance ensures the membrane's adaptability across various conditions.
Practice Questions
The endoplasmic reticulum (ER) is an extensive membranous network crucial for various cellular functions. There are two types: the Rough ER (RER) and Smooth ER (SER). The RER is studded with ribosomes, facilitating protein synthesis. These proteins may be incorporated into the cell's plasma membrane, secreted, or transported to the lysosome. Conversely, the SER specialises in lipid synthesis and detoxification processes. Additionally, in muscle cells, the SER is vital for calcium storage, which plays a pivotal role in muscle contractions. In essence, the ER is integral for the synthesis, modification, and transport of biologically essential molecules within eukaryotic cells.
Mitochondria, often termed the "powerhouses" of the cell, are double-membraned organelles primarily responsible for ATP production. Their structure is optimally tailored to their function. The inner membrane, highly convoluted, forms cristae, increasing the surface area and thereby optimising ATP synthesis. This membrane houses enzymes vital for the electron transport chain in cellular respiration. Within this inner compartment lies the matrix, enriched with enzymes that facilitate the Krebs cycle, another crucial step in ATP production. By converting the energy from nutrients into a usable form (ATP), mitochondria play a foundational role in powering cellular processes.
