As we navigate the microscopic world of cellular biology, we encounter the complex and intriguing eukaryotic cells, hosting a variety of specialised structures called organelles. In this section, we will deepen our understanding of organelles such as the nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, cytoskeleton, and those unique to plant cells.
The Nucleus
The nucleus serves as the control centre of eukaryotic cells, owing to its role in storing and protecting the cell's genetic material, or DNA. This DNA is meticulously organised into structures known as chromosomes, which condense during cell division. The nucleus is enveloped by a double-membrane structure, the nuclear envelope, perforated with nuclear pores for the controlled exchange of substances between the nucleus and the cytoplasm. Within the nucleus, a dense region known as the nucleolus is dedicated to ribosome synthesis, central to protein production.
Mitochondria
Renowned as the cell's powerhouse, the mitochondria are integral to the cell's energy supply. These double-membraned organelles host a process called cellular respiration, wherein organic molecules are broken down to release energy, stored in molecules of ATP (adenosine triphosphate). The inner mitochondrial membrane forms numerous infoldings, termed cristae, designed to maximise the surface area for enhanced ATP production. Intriguingly, mitochondria have their own DNA, supporting the theory that they were once free-living prokaryotic organisms.
Endoplasmic Reticulum (ER)
The endoplasmic reticulum (ER), a labyrinthine network of tubules and vesicles, participates in protein and lipid synthesis. Based on the presence or absence of ribosomes, ER is categorised into:
- Rough ER: Dotted with ribosomes, the rough ER is actively involved in protein synthesis and quality control. Newly synthesised proteins enter the ER lumen, where they undergo folding and modifications.
- Smooth ER: Devoid of ribosomes, the smooth ER partakes in lipid metabolism, calcium ion storage, and detoxification of harmful metabolic byproducts. Its role extends to the synthesis of steroid hormones in certain cell types.
Golgi Apparatus
Resembling a stack of pancakes, the Golgi apparatus is a series of flattened, membranous sacs called cisternae. It functions as the cell's post office, receiving proteins and lipids from the ER, modifying them if needed, and then sorting and packaging them into vesicles that are dispatched to their designated locations within or outside the cell.
Lysosomes
Dubbed the cell's recycling centre, lysosomes are spherical organelles loaded with hydrolytic enzymes. These enzymes enable lysosomes to digest unwanted or damaged cellular components, foreign particles, and pathogens, reducing them to basic molecules that can be reused by the cell. This process of self-digestion, known as autophagy, is essential for cellular homeostasis.
Peroxisomes
Peroxisomes, small, membrane-bound organelles, harbour enzymes that perform various metabolic reactions, such as fatty acid breakdown and detoxification of harmful substances. One key reaction generates hydrogen peroxide, a potentially damaging molecule, which is swiftly decomposed into water and oxygen by the enzyme catalase within the peroxisome itself.
Cytoskeleton
The cytoskeleton, a dynamic network of protein fibres, lends structure and shape to the cell, enables cellular movement, and anchors organelles in place. The cytoskeleton is comprised of:
- Microtubules: Hollow tubes that provide tracks for vesicle transport and form the spindle fibres during cell division.
- Actin filaments (microfilaments): Thin fibres are involved in muscle contraction, cell division, and cell movement.
- Intermediate filaments: Rope-like fibres provide mechanical strength to the cell and help maintain cellular integrity.
Organelles in Plant Cells
Plant cells boast additional organelles not found in animal cells:
- Cell Wall: Composed primarily of cellulose, the cell wall gives the plant cell rigidity and strength, protects against mechanical stress, and prevents excessive water uptake.
- Chloroplasts: Housing chlorophyll, these double-membraned organelles are the site of photosynthesis, where light energy is converted into chemical energy.
- Central Vacuole: A large, water-filled sac that provides turgidity to the cell, supports the cell's structure, and stores various substances such as nutrients, pigments, and waste products.
FAQ
The Golgi apparatus relies on molecular 'tags' attached to the proteins during their modification. These tags, essentially short amino acid sequences or attached carbohydrate or phosphate groups, serve as molecular addresses. They signal the vesicles to fuse with the correct part of the cell membrane, ensuring the proteins are transported to their correct destinations.
The cytoskeleton is a dynamic network of protein fibres that provide structural support and maintain the shape of the cell. It anchors organelles in their positions, assists in intracellular transport by providing 'tracks' along which vesicles move, and enables cell movement and division. Specific cytoskeletal elements, such as microtubules and actin filaments, also participate in specialised processes like chromosome segregation during mitosis and muscle contraction.
The central vacuole in plant cells plays multiple roles. Primarily, it maintains cell turgor pressure, providing structural support and helping the plant maintain its shape. It also serves as a storage depot for various substances, such as nutrients, pigments, and waste products. In some cases, the vacuole stores toxic substances to deter herbivory. Therefore, the central vacuole is a multi-functional organelle vital to plant cell function and survival.
Lysosomes maintain an acidic environment inside their membrane-bound structure, which is optimal for the activity of the hydrolytic enzymes they contain. The rest of the cell maintains a neutral pH where these enzymes are inactive. Additionally, the lysosomal membrane is composed of special lipids that resist enzymes. In the event of accidental release, the cell can neutralise these enzymes to prevent self-digestion.
Mitochondria and chloroplasts are thought to originate from free-living bacteria that were engulfed by an ancestral eukaryotic cell, a process known as endosymbiosis. Over time, most of their genes were transferred to the host's nuclear genome, yet they retained a small portion of their original DNA. This genetic autonomy supports their self-replication and allows them to synthesise some of their own proteins, crucial for their unique functions of ATP production and photosynthesis, respectively.
Practice Questions
The rough endoplasmic reticulum (RER) plays a significant role in protein synthesis and quality control. It's termed 'rough' due to the ribosomes attached to its surface where protein synthesis occurs. The newly synthesised proteins enter the RER lumen, where they are correctly folded and undergo necessary modifications. The smooth endoplasmic reticulum (SER), lacking ribosomes, is involved in various metabolic processes. It is instrumental in lipid metabolism and also participates in the synthesis of steroid hormones in certain cell types. Moreover, the SER serves as a reservoir for calcium ions and is actively involved in the detoxification of harmful metabolic byproducts.
Mitochondria, the powerhouse of the cell, are double-membraned organelles integral to the cell's energy supply. The outer membrane is smooth, allowing the passage of molecules, whereas the inner membrane is highly folded into structures called cristae. This convoluted structure significantly increases the surface area of the inner mitochondrial membrane, enhancing the capacity for ATP production during cellular respiration. The matrix, enclosed by the inner membrane, contains enzymes for the Krebs cycle and the mitochondrial DNA, supporting the autonomous replication and protein synthesis capabilities of mitochondria. Hence, the structure of mitochondria is highly adapted for its role in energy production.
