Chapter 4: Cells and Organelles – Structure and Function

Cells and Organelles

This chapter provides an overview of the structure and function of cells and their organelles, highlighting the differences between cell types and the specialized roles of cellular components. Understanding these structures is fundamental to cell biology and underpins many cellular processes.

Cell Size Limitations

• Surface Area to Volume Ratio: As cells increase in size, their volume grows faster than their surface area, limiting the rate of material exchange with the environment.

• Diffusion Rate: Efficient diffusion of molecules is critical for cell survival; larger cells face challenges in maintaining adequate concentrations of substances.

• Example: Microvilli increase surface area in intestinal cells to enhance absorption.

Equation: $\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

Comparison of Cell Types

The following table summarizes key differences among Bacteria, Archaea, and Eukaryotes:

Property	Bacteria	Archaea	Eukaryotes
Typical size	Small (1–5 μm)	Small (1–5 μm)	Large (10–100 μm)
Nucleus and organelles	Absent	Absent	Present
Cytoskeleton	Actin and tubulin-like proteins	Actin and tubulin-like proteins	Actin and tubulin proteins
Cell wall	Peptidoglycan	Varies (proteinaceous or pseudopeptidoglycan)	Cellulose (plants), chitin (fungi), variable (others)
Mode of cell division	Binary fission	Binary fission	Mitosis or meiosis

Additional info: Table also includes DNA structure, ribosome size, and membrane lipid differences.

Generalized Animal and Plant Cells

• Animal Cells: Lack cell walls, contain lysosomes, centrioles, and small vacuoles.

• Plant Cells: Have cell walls, large central vacuole, chloroplasts, and plasmodesmata.

• Example: Chloroplasts in plant cells enable photosynthesis, absent in animal cells.

Plasma Membrane

• Structure: Composed of a phospholipid bilayer with embedded proteins, sterols, glycolipids, and glycoproteins.

• Function: Defines cell boundaries, regulates transport, and facilitates communication.

• Dynamic Nature: Membrane is not homogeneous or static; proteins and lipids can move laterally.

Nucleus

• Nuclear Envelope: Double membrane with nuclear pores for molecular transport.

• Nucleolus: Site of rRNA transcription and ribosome assembly.

• Internal Organization:

  • Nuclear Lamina: Intermediate filaments (lamins) supporting nuclear structure.

  • Chromosome Territories: Distinct regions for individual chromosomes.

  • Speckles: Sites of pre-mRNA splicing factor storage.

  • Cajal Bodies: Assembly sites for small nuclear ribonucleoproteins (snRNPs).

  • PML Bodies: Involved in transcriptional regulation and DNA repair.

Nuclear Pores

• Each mammalian cell contains ~3000–4000 nuclear pores.

• Nuclear pore complex (NPC) is composed of ~30 different nucleoporins, arranged with octagonal symmetry.

• NPCs regulate import/export of macromolecules via nuclear localization and export signals, mediated by karyopherins.

Mitochondria and Chloroplasts

• Mitochondria: Site of aerobic respiration; inner membrane forms cristae to increase surface area for ATP production.

• Chloroplasts: Site of photosynthesis in plants and algae; contains thylakoid membranes for light-dependent reactions.

• Endosymbiosis Theory: Both organelles likely originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Plastid Types (Plants)

Plastid Type	Main Function
Chloroplast	Photosynthesis, pigment synthesis and storage
Chromoplast	Pigment synthesis and storage (carotenoids)
Amyloplast	Starch storage
Leucoplast	Synthesis of fatty acids and amino acids

Endoplasmic Reticulum (ER)

• Structure: Network of cisternae (flattened sacs) and lumen (internal space).

• Rough ER: Studded with ribosomes; synthesizes membrane and secreted proteins.

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

Golgi Complex and Protein Secretion

• Consists of cisternae and lumen, similar to ER.

• Functions as a packaging and processing station for proteins and lipids.

• Forms secretory vesicles for transport to the plasma membrane or other destinations.

Lysosomes

• Found in animal cells; contain hydrolytic enzymes (hydrolases) for digestion and breakdown of macromolecules.

• Involved in autophagy (self-digestion) and phagocytosis (engulfing external particles).

• Carbohydrate lining protects the lysosomal membrane from degradation.

Peroxisomes

• Present in plants, animals, fungi, protozoa, and algae.

• Carry out oxidation reactions, producing hydrogen peroxide ($H_2O_2$) as a by-product.

• Detoxify harmful compounds and break down long-chain fatty acids.

• Specialized forms (glyoxysomes) convert fats to carbohydrates in plants.

Vacuoles

• Animal and Yeast Cells: Temporary storage and transport; phagosomes for engulfing particles.

• Plant Cells: Large central vacuole for storage, digestion, and maintaining turgor pressure.

Naked "Organelles"

• Ribosomes: Sites of protein synthesis; composed of large (60S) and small (40S) subunits in eukaryotes; found free in cytosol or bound to ER.

• Cytoplasm: Includes cytosol (fluid matrix) and cytoskeleton.

• Cytoskeleton: Network of microtubules, actin filaments (microfilaments), and intermediate filaments providing structure, transport, and motility.

Outside the Cell

• Extracellular Matrix (ECM): Complex network of proteins and polysaccharides outside animal cells; provides structural support and mediates cell signaling.

• Cell Wall: Rigid structure outside the plasma membrane in plants, fungi, and some protists; composed mainly of cellulose (plants) or chitin (fungi).

Viruses, Viroids, and Prions

• Viruses: Acellular, obligate intracellular parasites that invade cells, disrupt function, and may kill the host cell.

• Viroids: Small, circular RNA molecules causing plant diseases.

• Prions: Infectious proteins responsible for neurological diseases (e.g., scrapie, kuru, mad cow disease).