2.1 Cell structure and function

Cells are the building blocks of life, with intricate structures that enable them to function. This section explores the components of eukaryotic and prokaryotic cells, highlighting their unique features and roles in cellular processes.

The cytoskeleton, plasma membrane, and organelles work together to maintain cell shape, facilitate movement, and carry out essential functions. Understanding these structures is key to grasping how cells operate and interact within living organisms.

Eukaryotic Cell Organelles and Functions

Membrane-bound Organelles and their Roles

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• Eukaryotic cells contain specialized structures called organelles, each surrounded by a membrane and performing specific functions that contribute to the overall functioning of the cell
• The nucleus is the control center of the cell, housing the genetic material (DNA) and serving as the site of DNA replication and transcription into RNA
• The endoplasmic reticulum (ER) is a network of membranous channels and sacs involved in the synthesis, modification, and transport of proteins and lipids
  • The rough ER is studded with ribosomes and is the site of protein synthesis
  • The smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification
• The Golgi apparatus is a stack of flattened membrane sacs that modifies, packages, and sorts proteins and lipids for transport to various destinations (lysosomes, plasma membrane, or secretion)
• Lysosomes are membrane-bound sacs containing digestive enzymes that break down and recycle damaged organelles, macromolecules, and foreign particles (bacteria, cellular debris)

Energy Production and Cellular Respiration

• Mitochondria are the powerhouses of the cell, generating ATP through the process of cellular respiration
  • The inner membrane of mitochondria is highly folded, forming cristae that increase the surface area for energy production
  • The matrix, the innermost compartment of mitochondria, contains enzymes involved in the citric acid cycle and oxidative phosphorylation
• Peroxisomes are small, membrane-bound organelles involved in the breakdown of fatty acids and the detoxification of harmful substances (hydrogen peroxide, alcohol)
  • Peroxisomes contain enzymes such as catalase and oxidases that neutralize toxic compounds

Prokaryotic vs Eukaryotic Cells

Structural Differences

• Prokaryotic cells (bacteria, archaea) lack membrane-bound organelles and a true nucleus, while eukaryotic cells (plants, animals, fungi) possess these structures
• The genetic material in prokaryotic cells is a single, circular DNA molecule located in the nucleoid region, whereas eukaryotic cells have multiple linear DNA molecules housed within a membrane-bound nucleus
• Prokaryotic cells are generally smaller (1-5 μm) than eukaryotic cells (10-100 μm) and have a higher surface area-to-volume ratio
• Prokaryotic cells have a cell wall composed of peptidoglycan, while eukaryotic cells may have a cell wall made of cellulose (plants) or lack a cell wall entirely (animals)

Functional Differences

• Prokaryotic cells have ribosomes (70S) that are smaller than those found in eukaryotic cells (80S), reflecting differences in protein synthesis
• Eukaryotic cells have a more complex cytoskeleton, which includes microfilaments, intermediate filaments, and microtubules, while prokaryotic cells have a simpler cytoskeleton (FtsZ rings, MreB filaments)
• Prokaryotic cells lack membrane-bound organelles, so cellular processes (DNA replication, transcription, translation) occur in the cytoplasm, while in eukaryotic cells, these processes are compartmentalized within organelles

Structure and Function of Cells

Plasma Membrane and Cell Surface

• The plasma membrane is a selectively permeable barrier that controls the exchange of materials between the cell and its environment
  • The large surface area-to-volume ratio of the plasma membrane facilitates efficient exchange of nutrients, waste products, and signaling molecules
• The fluid mosaic model describes the plasma membrane as a fluid phospholipid bilayer with embedded proteins that can move laterally within the membrane
• Membrane proteins perform various functions, such as transport (channels, carriers), enzymatic activity, cell signaling (receptors), and cell adhesion

Compartmentalization and Organelle Function

• The compartmentalization provided by membrane-bound organelles allows for the spatial separation of different cellular processes, enhancing efficiency and preventing interference between incompatible reactions
• The specific shapes and structures of organelles optimize their functions
  • The cristae in mitochondria increase the surface area for energy production through cellular respiration
  • The flattened sacs of the Golgi apparatus enable efficient modification, sorting, and packaging of proteins and lipids
• The arrangement and composition of the cytoskeleton provide structural support, enable cell movement, and facilitate the transport of organelles and macromolecules within the cell

The Cytoskeleton: Shape, Movement, and Organization

Components of the Cytoskeleton

• The cytoskeleton is a dynamic network of protein filaments that provides structural support, enables cell movement, and organizes the internal components of the cell
• Microfilaments are thin, flexible filaments composed of actin monomers
  • Microfilaments are involved in cell motility (pseudopodia formation, cytoplasmic streaming), muscle contraction, and the maintenance of cell shape
• Intermediate filaments are rope-like fibers that provide mechanical strength and resistance to shear stress
  • Intermediate filaments help maintain cell shape and organize the internal structure of the cell (nuclear lamina, keratin filaments in epithelial cells)
• Microtubules are hollow, cylindrical tubes composed of α- and β-tubulin dimers
  • Microtubules are involved in cell division (mitotic spindle), intracellular transport, and the maintenance of cell shape and polarity (centrioles, cilia, flagella)

Motor Proteins and Cellular Movement

• Motor proteins use ATP to drive the movement of organelles and macromolecules along the cytoskeleton
• Myosin motors are associated with microfilaments and are responsible for muscle contraction and cell movement (cytokinesis, phagocytosis)
• Kinesin and dynein motors are associated with microtubules and transport cargo (vesicles, organelles) within the cell
  • Kinesin moves cargo towards the plus end of microtubules (cell periphery), while dynein moves cargo towards the minus end (centrosome)
• The cytoskeleton plays a crucial role in cell migration, allowing cells to move and change shape in response to external stimuli or during processes such as wound healing and embryonic development
  • Actin polymerization drives the formation of protrusions (lamellipodia, filopodia) at the leading edge of migrating cells
  • Myosin-mediated contraction of actin filaments at the rear of the cell propels the cell forward