Cell Organelles and Nucleus structure

• Cell organelles are the topic of the notes.

The Nucleus

• The nucleus is the most important structure, acting as the control center.
• It is typically spherical and centrally located within the cell.
• Chromosomes inside carry the genes necessary for inheriting traits.
• The nucleus comprises the nuclear envelope, nucleolus, chromatin, and nucleoplasm.

Nuclear Envelope

• It has two unit membranes with fluid-filled space.
• The outer membrane can be continuous with the endoplasmic reticulum.
• It separates the nucleus from the cytoplasm.
• Nuclear pores within it regulate the movement of molecules in and out.

Nucleolus

• This is a small, spherical body inside the nucleus.
• It is a non-membranous matrix composed of ribonucleic acid (RNA), enzymes, and histone proteins.
• DNA is concentrated here when making ribosomal RNA (rRNA).
• The site for the synthesis of ribosomal subunits and ribosome protection.
• Subunits travel through nuclear pores to the cytoplasm and are involved in protein synthesis.

Chromatin

• Chromatin consists of DNA strands wrapped around histone proteins.
• This makes the DNA compact enough to fit inside the nucleus.
• The nucleosome is the basic structural unit.
• DNA exists in different phases like nucleosome, solenoid, and chromatin fiber.
• A nucleosome has a DNA segment wrapped around a core of 8 histone proteins.
• Adjacent nucleosomes connect via linker DNA and H1 histone.
• The solenoid is the coiled form of nucleosomes (like a phone cord).
• Chromatin fiber constitutes a series of nucleosomes.
• Chromosomes are visible only when the cell divides.
• In non-dividing cells, DNA appears as chromatin, condensing into chromosomes during cell division.
• DNA contains genes, which carry instructions that control cell metabolism and heredity.

Cytoplasm

• A jelly-like substance present between the nucleus and the cell membrane
• It includes all materials inside the cell, excluding the nucleus.
• It is composed of cytosol and cell organelles.

Cytosol

• The cytosol is the soluble, internal fluid fraction of the cytoplasm.
• Nutrients, ions, proteins, and waste products are dissolved within it.
• The cytosol contains enzymes and cofactors for pathways such as glycolysis and fatty acid synthesis.

Cell Organelles

• They are subunits located in the cytoplasm that perform specific functions.
• Can be classified as membranous or non-membranous.

Membranous Organelles

• These are covered with a plasma membrane and isolated from the cytosol.
• Examples include the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, and mitochondria.

Non-Membranous Organelles

• These lack a membrane and have direct contact with the cytosol.
• Examples include the cytoskeleton, microvilli, centrioles, cilia, and ribosomes.

Endoplasmic Reticulum (ER)

• A network of internal membranes located around the nucleus.
• Contains cisternae, which are flat, sac-like structures attached to the nuclear envelope.
• Exists in two forms: rough ER (rER) and smooth ER (sER).

Rough ER (rER)

• It has ribosomes on its surface.
• rER transports proteins synthesized by ribosomes.
• Ribosomes synthesize proteins, insert them into the ER, and modify/transport them to the Golgi apparatus.

Smooth ER (sER)

• It lacks ribosomes.
• It participates in lipid synthesis, including membrane lipids like steroids and phospholipids.
• Handles detoxification of drugs and poisons, especially in liver cells.
• Regulates calcium levels, particularly in muscle cells.

Mitochondrion

• This is the "powerhouse" of the cell
• It has a double unit membrane structure.
• The inner membrane folds into cristae, increasing the surface area.
• The inner mitochondrial membrane contains the electron transport chain (respiratory chain) for cellular respiration.
• It uses oxygen to release energy via oxidative phosphorylation.
• The mitochondrial matrix contains enzymes for the Krebs cycle and fatty acid oxidation.

Lysosome

• It is a membrane-bound sac with a single membrane.
• It contains digestive enzymes called acid hydrolases.
• Hydrolytic enzymes use water to break chemical bonds.
• The pH within lysosomes is around 4.5, allowing the enzymes to function effectively.
• Acid hydrolases digest/recycle macromolecules (e.g., proteins) inside the cell or those absorbed/engulfed by the cell.
• Lipases, phosphatases, proteases, and nucleases are key enzymes.
• Lysosomes break down large molecules, digest non-functioning organelles, food particles, and debris.
• They also digest engulfed viruses or bacteria.
• In the absence of nutrients, lysosomes digest organelles to produce nutrients for energy.
• Lysosomes participate in programmed cell death (apoptosis).

Vacuoles

• They have a simple structure with a fluid mass surrounded by a membrane.
• Vacuoles act as storage bubbles, particularly in plant cells, where they store food, water, and other nutrients.
• They help maintain water balance and control cell volume.
• In animal cells, vacuoles are small and sequester waste.

Peroxisomes

• These are present in all eukaryotic cells.
• They have a single membrane separating their contents from the cytosol.
• Peroxisomes generate hydrogen peroxide (H₂O₂) for detoxification.
• Liver and kidney cells are rich in peroxisomes due to their role in toxin removal.
• Peroxisomes contain oxidative enzymes like oxidases that oxidize substrates to produce hydrogen peroxide.

Golgi Apparatus

• It is composed of 5-8 cup-like, flattened sacs called cisternae.
• Receives proteins made by the ER.
• Materials are transported from the rER to the Golgi and then to the cell membrane via vesicles.
• Modifies and packages proteins, hormones, or enzymes for exocytosis.
• Renews and modifies the plasma membrane.
• Packages special enzymes within vesicles for use in the cytoplasm.
• Modify, sort, and package molecules from the ER for storage or transport.

Ribosomes

• They synthesize proteins.
• Each ribosome is made of two subunits composed of rRNA and protein.
• Ribosomes are present in both prokaryotic (70S) and eukaryotic (80S) cells.
• In eukaryotic cells, there are free ribosomes floating in the cytosol.
• Ribosomes located on the rER make proteins for export.
• Some antibiotics, like azithromycin, act on bacterial ribosomes.

Cytoskeleton

• There are three types of filaments in the cytoplasm of most eukaryotic cells.
• Microfilaments are composed of actin and are associated with myosin to cause muscle contraction.
• Intermediate filaments provide tensile strength to the cell.
• Microtubules are composed of tubulin protein and form the spindle fibers for chromosome separation during mitosis.

Centrioles

• Consist of two short cylinders of microtubules at right angles.
• They participate in cell division by arranging/organizing microtubules to form the mitotic spindle.
• They are important in the formation of cell structures like cilia and flagella.
• They also have a role in shaping the zygote cytoskeleton.