2.1 Cell structure and function

Cells are the building blocks of life, housing complex structures that keep them functioning. From the nucleus storing genetic material to mitochondria powering cellular processes, each organelle plays a crucial role in maintaining cell health and function.

Understanding cell structure is key to grasping how our bodies work at a microscopic level. By exploring the differences between prokaryotic and eukaryotic cells, we gain insight into the diversity of life forms and their unique adaptations for survival.

Organelles and their functions

Nucleus and endoplasmic reticulum

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• The nucleus contains the cell's genetic material (DNA) and is the site of DNA replication and transcription
  • Surrounded by a double membrane called the nuclear envelope, which contains nuclear pores that regulate the passage of molecules between the nucleus and cytoplasm
• The endoplasmic reticulum (ER) is a network of membrane-bound channels and sacs that play a role in protein and lipid synthesis, as well as calcium storage
  • The rough ER is studded with ribosomes and is involved in protein synthesis
  • The smooth ER lacks ribosomes and is involved in lipid synthesis (phospholipids and steroids) and detoxification of harmful substances (alcohol and drugs)

Golgi apparatus, mitochondria, and other organelles

• The Golgi apparatus is a stack of flattened membrane sacs that modify, package, and sort proteins and lipids for transport to various destinations within the cell or for secretion outside the cell
  • Proteins and lipids are modified by the addition of carbohydrates (glycosylation) or other chemical groups
  • Packages molecules into vesicles for transport to lysosomes, the plasma membrane, or secretion outside the cell
• Mitochondria are the powerhouses of the cell, generating ATP through the process of cellular respiration
  • Have a double membrane structure, with the inner membrane being highly folded to form cristae, which increase the surface area for energy production
  • The matrix contains enzymes involved in the Krebs cycle, while the inner membrane contains the electron transport chain and ATP synthase
• Lysosomes are membrane-bound organelles containing digestive enzymes that break down and recycle worn-out organelles, macromolecules, and foreign particles that enter the cell
  • Contain hydrolytic enzymes (acid hydrolases) that function optimally in the acidic environment of the lysosome
• Peroxisomes are small, membrane-bound organelles that contain enzymes involved in the breakdown of fatty acids and the detoxification of various molecules, including hydrogen peroxide
  • Contain oxidative enzymes, such as catalase, which converts hydrogen peroxide (H2O2) into water and oxygen
• Ribosomes are the sites of protein synthesis and are composed of ribosomal RNA (rRNA) and proteins
  • Can be found freely in the cytoplasm or attached to the rough ER
  • Consist of a large and a small subunit that come together to translate mRNA into polypeptides

Prokaryotic vs Eukaryotic cells

Structural differences

• Prokaryotic cells, such as bacteria and archaea, lack membrane-bound organelles and a true nucleus, while eukaryotic cells, such as those found in plants and animals, have a true nucleus and various membrane-bound organelles
  • In prokaryotic cells, the genetic material (DNA) is located in the cytoplasm in a region called the nucleoid
• Prokaryotic cells are generally smaller than eukaryotic cells
  • Prokaryotic cells have a typical size range of 1-5 µm, while eukaryotic cells are typically 10-100 µm in size
• The cell wall composition differs between prokaryotic and eukaryotic cells
  • Prokaryotic cell walls are composed of peptidoglycan, while eukaryotic cell walls, when present, are made of cellulose (plants) or chitin (fungi)
  • Animal cells lack cell walls

Genetic material and specialized structures

• Prokaryotic cells have a single circular chromosome, while eukaryotic cells have multiple linear chromosomes contained within the nucleus
• Prokaryotic cells lack membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, and mitochondria
  • However, prokaryotic cells may have specialized structures such as plasmids (small, circular DNA molecules), pili (surface appendages for adhesion and DNA transfer), and fimbriae (short, thin surface appendages for adhesion)
• Eukaryotic cells have a more complex cytoskeleton composed of microfilaments, intermediate filaments, and microtubules
  • Prokaryotic cells have a simpler cytoskeleton primarily composed of homologs of actin (MreB) and tubulin (FtsZ)

Cell membrane regulation

Structure and selective permeability

• The cell membrane, also known as the plasma membrane, is a selectively permeable phospholipid bilayer that separates the cell's interior from the external environment
  • Regulates the passage of molecules into and out of the cell, maintaining the cell's homeostasis
• The fluid mosaic model describes the structure of the cell membrane
  • Consists of a fluid phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol
  • Phospholipids have hydrophilic heads and hydrophobic tails, which spontaneously arrange to form the bilayer structure
• The cell membrane's selective permeability allows the passage of small, nonpolar molecules (oxygen and carbon dioxide) and small, uncharged polar molecules (water) through the phospholipid bilayer
  • Larger or charged molecules require specialized transport mechanisms to cross the membrane

Transport mechanisms

• Passive transport mechanisms move molecules down their concentration gradient without requiring energy input from the cell
  • Simple diffusion occurs through the phospholipid bilayer and is driven by the concentration gradient
  • Facilitated diffusion involves the use of carrier proteins or channel proteins to transport molecules across the membrane
• Active transport mechanisms move molecules against their concentration gradient using energy input from ATP or the electrochemical gradient
  • Primary active transport, such as the sodium-potassium pump, directly uses ATP to transport molecules across the membrane
  • Secondary active transport, such as symporters and antiporters, uses the electrochemical gradient of one molecule to transport another molecule against its concentration gradient
• Endocytosis and exocytosis involve the formation of vesicles to transport larger molecules or particles across the cell membrane
  • Endocytosis includes phagocytosis (engulfing solid particles) and pinocytosis (engulfing liquid droplets)
  • Exocytosis involves the fusion of intracellular vesicles with the plasma membrane to release their contents to the extracellular space

Cytoskeleton structure and function

Components and their roles

• The cytoskeleton is a dynamic network of protein filaments that provides structural support, enables cell movement, and plays a role in intracellular transport and cell division
  • The three main components are microfilaments, intermediate filaments, and microtubules
• Microfilaments, also known as actin filaments, are the thinnest of the cytoskeletal elements (diameter of about 7 nm)
  • Composed of globular actin (G-actin) monomers that polymerize to form filamentous actin (F-actin)
  • Involved in cell movement (muscle contraction and amoeboid movement), formation of cellular structures (microvilli and stress fibers)
• Intermediate filaments are slightly thicker than microfilaments (diameter of about 10 nm)
  • Composed of various proteins (keratins, vimentins, and lamins) depending on the cell type
  • Provide mechanical strength and resistance to shear stress, anchor organelles, and maintain cell shape
• Microtubules are the thickest of the cytoskeletal elements (diameter of about 25 nm)
  • Hollow cylinders composed of α-tubulin and β-tubulin dimers that polymerize to form protofilaments
  • Involved in intracellular transport, cell division (forming the mitotic spindle), and the maintenance of cell shape and polarity

Motor proteins and cell division

• Motor proteins use energy from ATP hydrolysis to move along the cytoskeletal filaments, enabling intracellular transport and generating force for cell movement
  • Myosin is associated with microfilaments and is involved in muscle contraction and cytokinesis
  • Kinesin and dynein are associated with microtubules and transport organelles and vesicles within the cell
• The centrosome, located near the nucleus, is the main microtubule-organizing center (MTOC) in animal cells
  • Consists of a pair of centrioles surrounded by pericentriolar material, which nucleates and organizes microtubules
• The cytoskeleton plays a crucial role in cell division, particularly during mitosis
  • The mitotic spindle, composed of microtubules, is responsible for separating the duplicated chromosomes and ensuring their equal distribution to the daughter cells
• The cytoskeleton enables various types of cell movement
  • Amoeboid movement of white blood cells, flagellar movement of sperm cells, and ciliary movement of epithelial cells in the respiratory and reproductive tracts