Cell Structure and Function: Eukaryotic Organelles and Cellular Processes

Cell Structure and Function

Cell Nucleus

The nucleus is the control center of eukaryotic cells, housing most of the cell's DNA and coordinating activities such as growth, metabolism, and reproduction. It controls protein production by making mRNA.

• Structure: Surrounded by a double membrane called the nuclear envelope.

• Nuclear Envelope: Contains nuclear pores for transport of molecules.

• Nuclear Lamina: lines the envelope and maintains the shape of the nucleus by supporting nuclear envelope.

• Nucleolus: Dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

Example: The nucleus regulates gene expression and cell cycle progression.

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis. They translate the mRNA instructions to build proteins. They can be found free in the cytoplasm or attached to the endoplasmic reticulum.

• Structure: Composed of rRNA and proteins; consists of large and small subunits.

• Function: Translate mRNA into polypeptide chains.

Example: Ribosomes on the rough ER synthesize proteins for secretion or membrane insertion.

Endomembrane System

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of membranes involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; synthesizes proteins.

  • Produces glycoproteins by attaching carbohydrates to proteins in the ER lumen.

  • Separates secretory proteins from cytosolic proteins and packages them to transport vesicles.

  • Functions as a membrane factory

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs.

Example: Liver cells have abundant smooth ER for detoxification.

Golgi Apparatus

The Golgi apparatus acts as a warehouse that modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Structure: Stacks of flattened membranous sacs called cisternae.

  • Has a cis face (receiving side) and trans face (shipping side).

• Function: Glycosylation and processing of macromolecules.

• Process:

  • Vesicles move from ER to Golgi

  • Vesicles coalesce to form new cis Golgi cisternae

  • Cisternal maturation: Golgi cisternae move in a cis-to-trans direction

  • Vesicles form and leave Golgi, carrying specific products to other locations or to the plasma membrane for secretion

  • Vesicles transport some proteins backward to less mature Golgi cisternae, where they function.

  • Vesicles also transport certain proteins back to ER, their site of function.

Example: Secretory cells use the Golgi to package hormones for release.

Organelle	Main Function	Location
Nucleus	DNA storage, gene regulation	Center of cell
Ribosome	Protein synthesis	Cytoplasm/ER
Rough ER	Protein synthesis	Near nucleus
Smooth ER	Lipid synthesis, detoxification	Cytoplasm
Golgi Apparatus	Modification, sorting, packaging	Cytoplasm

Other Eukaryotic Organelles

Lysosomes

Lysosomes are membrane-bound organelles containing hydrolytic enzymes for intracellular digestion.

• Function: Breakdown of macromolecules, recycling of cellular components.

  • Phagocytosis: lysosomes digest food particles engulfed by cells.

    • Lysosome containing active hydrolytic enzymes fuse with food vacuole -> enzyme digests food particles -> release nutrients into the cytosol.

  • Autophagy: lysosomes recycle intercellular materials.

    • The vesicle with damaged organelles fuses with a lysosome -> organelles are digest and their components recycled.

• Role in Disease: Defects can lead to lysosomal storage disorders.

• Functions best in an acidic environment

• Formation: produced by the rough ER and processed in the Golgi Apparatus

Example: White blood cells use lysosomes to destroy pathogens.

Vacuoles

Vacuoles are large vesicles found in plant and fungal cells, serving storage and structural functions.

• Central Vacuole: Stores water and maintains turgor pressure in plant cells.

• Food vacuoles: formed by phagocytosis.

• Hydrolytic Vacuole: enzymatic hydrolysis in plants & fungi

• Protective Vacuole: sore compounds that deter herbivores from being poisonous or unplatable

• Pigment Vacuole: contain pigment like red and blue in details to attract pollinators.

• Contractile Vacuole: Expels excess water in protists.

Example: Plant cells use central vacuoles to store nutrients and waste products.

Mitochondria

Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration.

• Structure: Double membrane, inner membrane folded into cristae.

• Function: Site of aerobic respiration.

Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Example: Muscle cells have many mitochondria to meet energy demands.

Peroxisomes

Peroxisomes are small organelles that contain enzymes for oxidation reactions, such as breaking down fatty acids and detoxifying harmful substances.

• Function: Produce hydrogen peroxide ($\text{H}_2\text{O}_2$) and convert it to water.

Example: Liver cells use peroxisomes for detoxification.

Cytoskeleton

The cytoskeleton is a network of fibers extending throughout the cytoplasm.

• Function: It plays a major role in organizing the structures and activities of the cell.

• Structure: includes microtubules (green) and microfilaments (reddish orange)

• Role: provides mechanical support to the cell, maintaining its shape

  • The cytoskeleton's strength and resilience come from its architecture, stabilized by a balance of opposing forces.

  • It anchors many organelles and cytosolic enzyme molecules

  • Dynamic: capable of being quickly dismantled and reassembled to change the cell's shape.

• Cell motility: movement of cell parts and changes in cell location

  • Motor proteins interact with the cytoskeleton to move whole cells along external fibers and transport vesicles and organelles within the cell.

    • Vesicles, such as those containing neurotransmitters, use motor proteins to "walk" along cytoskeletal tracks to their destinations, like the tips of axons in nerve cells.

• The cytoskeleton is composed of three main types of fibers: microtubules, microfilaments (actin filaments), and intermediate filaments.

• Microtubules:

  • Structure: Hollow tubes made of tubulin dimers (α-tubulin and β-tubulin).

  • Diameter: 25 nm with a 15 nm lumen.

  • Functions: Maintain cell shape, facilitate cell motility, chromosome movements during cell division, and organelle movements.

  • Centrosomes and Centrioles: Microtubules grow from centrosomes, which contain centrioles composed of nine sets of triplet microtubules.

  • Cilia and Flagella: Microtubules form the core of these structures, which are involved in cell movement and fluid movement across cell surfaces. They have a "9+2" arrangement of microtubules.

  • Dynein proteins: Motor proteins that cause bending movements in cilia and flagella by "walking" along adjacent microtubules using ATP.

• Microfilaments (Actin Filaments):

  • Structure: Two intertwined strands of actin.

  • Diameter: 7 nm.

  • Functions: Maintain cell shape, enable changes in cell shape, muscle contraction, cytoplasmic streaming in plant cells, cell motility, and cell division in animal cells.

  • Role in Cell Motility: Actin filaments interact with myosin to cause muscle contraction and amoeboid movement in cells. They also contribute to cytoplasmic streaming in plant cells.

• Intermediate Filaments:

  • Structure: Fibrous proteins coiled into cables.

  • Diameter: 8-12 nm.

  • Functions: Maintain cell shape, anchor the nucleus and other organelles, and form the nuclear lamina.

  • Characteristics: More permanent than microtubules and microfilaments, providing structural stability even after cell death. They are found in some animal cells, including vertebrates, and are made from various proteins, such as keratins.

Cell Membrane Structure

Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, composed of a phospholipid bilayer with embedded proteins.

• Fluid Mosaic Model: Describes the dynamic nature of membrane components.

• Functions: Transport, communication, protection.

Equation:

$\text{Phospholipid bilayer} + \text{proteins} + \text{cholesterol} \rightarrow \text{cell membrane structure}$

Example: Membrane proteins act as channels for ions and molecules.

Cellular Transport

Passive Transport

Passive transport is the movement of substances across the membrane without energy input.

• Diffusion: Movement from high to low concentration.

• Osmosis: Diffusion of water across a membrane.

• Facilitated Diffusion: Uses transport proteins for larger or charged molecules.

Example: Oxygen enters cells by simple diffusion.

Active Transport

Active transport requires energy (ATP) to move substances against their concentration gradient.

• Pumps: Such as the sodium-potassium pump ($\text{Na}^+/\text{K}^+$ ATPase).

• Endocytosis/Exocytosis: Bulk transport of materials into or out of the cell.

Equation:

$\text{ATP} + \text{Pump} \rightarrow \text{Transport against gradient}$

Example: Nerve cells use active transport to maintain ion gradients.

Cell Communication

Signaling Pathways

Cells communicate through chemical signals that bind to receptors, triggering a cascade of events inside the cell.

• Types: Paracrine, autocrine, endocrine, synaptic.

• Second Messengers: Molecules like cAMP amplify the signal.

Example: Hormones like insulin regulate glucose uptake.

Summary Table: Major Eukaryotic Organelles

Organelle	Structure	Main Function
Nucleus	Double membrane, pores, nucleolus	Genetic information storage, regulation
Ribosome	rRNA and proteins, two subunits	Protein synthesis
ER (Rough/Smooth)	Membranous network, ribosomes (rough)	Protein/lipid synthesis, detoxification
Golgi Apparatus	Stacked cisternae	Modification, sorting, packaging
Lysosome	Membrane-bound vesicle	Digestion, recycling
Mitochondria	Double membrane, cristae	ATP production
Peroxisome	Small vesicle, enzymes	Oxidation, detoxification
Vacuole	Large vesicle	Storage, structure (plants)

Additional info: Some details, such as disease examples and specific cell types, were inferred for completeness and academic context.