A Tour of the Cell

Concept 4.2: Small Size of Cells and Surface Area-to-Volume Ratio

  • Cells are small to efficiently exchange materials through the plasma membrane.

  • Surface Area-to-Volume Ratio: Critical in determining a cell's ability to interact with its environment.

    • As a cell grows, its volume increases faster than its surface area, leading to a lower surface area-to-volume ratio.

  • Importance:

    • High surface area-to-volume ratio allows for more efficient exchange of materials (nutrients, waste, gases).

    • Essential for maintaining homeostasis within the cell.

  • Microvilli in Intestinal Cells:

    • Microvilli increase the surface area of the epithelial cells lining the intestine, enhancing nutrient absorption.

Example Problem

  • Cell A: Elongated cell measures $125 imes 1 imes 1$ arbitrary units.

    • Surface Area = $2(lw + lh + wh) = 2(125 imes 1 + 125 imes 1 + 1 imes 1) = 2(125 + 125 + 1) = 2(251) = 502$ square units.

    • Volume = $l imes w imes h = 125 imes 1 imes 1 = 125$ cubic units.

    • Surface-to-Volume Ratio = $502 / 125 = 4.016$.

  • Cell B: Cube cell measures $5 imes 5 imes 5$.

    • Surface Area = $6a^2 = 6(5^2) = 6(25) = 150$ square units.

    • Volume = $a^3 = 5^3 = 125$ cubic units.

    • Surface-to-Volume Ratio = $150 / 125 = 1.2$.

  • Comparison:

    • Cell A has a higher surface-to-volume ratio (4.016) compared to Cell B (1.2), indicating that elongated cells are more efficient in material exchange.

Implications

  • Increase in cell size leads to:

    • A drop in the surface area-to-volume ratio.

    • Slower movement of materials in and out of the cell which could hinder its metabolic activities.

Concept 4.3: Eukaryotic vs Prokaryotic Cells

  • Eukaryotic Cells: Contain internal membranes that compartmentalize functions, allowing specialized tasks.

  • Prokaryotic Cells: Lack membrane-bound organelles.

    • Two domains consisting of prokaryotic cells: Bacteria and Archaea.

  • DNA Location:

    • Prokaryotes have their DNA in the nucleoid region, free-floating in the cytoplasm.

    • Eukaryotes have their DNA enclosed in a membrane-bound nucleus.

Prokaryotic Cell Structure

  • Components of a prokaryotic cell:

    • Cell Wall: Provides shape and protection.

    • Plasma Membrane: Regulates what enters and exits the cell.

    • Bacterial Chromosome: Circular DNA found in the nucleoid region.

    • Nucleoid: Area where the bacterial chromosome resides.

    • Ribosomes: Synthesize proteins.

    • Flagella: Facilitate movement.

Concept 4.5 & 4.6: Nucleus, Chromosomes, and Ribosomes

  • Nucleus: Contains genetic material, surrounded by a nuclear envelope.

    • Nuclear Envelope: Double membrane structure made up of two layers.

    • Connectors: Nuclear pores span both layers, allowing selective transport of molecules like mRNA.

  • Nuclear Lamina: Dense fibrillar network providing structural support; involved in nuclear stability.

  • Nuclear Matrix: Framework that organizes the chromosome and aids in regulating gene expression.

  • Chromatin: Composed of DNA and proteins, existing in two forms:

    • Euchromatin: Less condensed, active in transcription.

    • Heterochromatin: Highly condensed, inactive.

  • Chromatin condenses into distinct chromosomes during cell division.

  • Nucleolus: Sub-structure within the nucleus, visible during interphase, where ribosomal RNA synthesis occurs.

  • Ribosomes: Sites of protein synthesis; composed of:

    • Large Subunit: Joins amino acids to form polypeptides.

    • Small Subunit: Binds mRNA and ensures correct codon-anticodon pairing.

  • Types of Ribosomes:

    • Free Ribosomes: Float in cytosol and produce proteins for use within the cell.

    • Bound Ribosomes: Attached to the ER and synthesize proteins for export or for membrane insertion.

Concept 4.7 – 4.12: Endomembrane System

  • Structures of the Endomembrane System include:

    • Endoplasmic Reticulum (ER)

    • Golgi Apparatus

    • Lysosomes

    • Vacuoles

    • Plasma Membrane

    • Nuclear Envelope

  • Endoplasmic Reticulum (ER): Major membranous organelle in eukaryotic cells, consisting of:

    • Rough ER: Studded with ribosomes for protein synthesis. Proteins are folded and modified in the lumen.

    • Functions of Smooth ER:

    • Lipid synthesis (phospholipids, steroids).

    • Carbohydrate metabolism.

    • Detoxification of drugs and poisons.

  • Secretory Proteins: Newly synthesized proteins move from rough ER vesicles to the Golgi apparatus for further processing.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids derived from the ER.

    • Structures include:

    • Cisternae: Flattened membrane-bound sacs.

    • Cis Face: The side of the Golgi facing the ER.

    • Trans Face: The side for export of processed materials.

  • Lysosomes: Membrane-bound organelles containing hydrolytic enzymes for digestion of macromolecules.

    • pH Range: Acidic environment (pH 4.5-5).

    • Phagocytosis: Process where lysosomes digest engulfed materials like bacteria. Professional phagocytes, such as macrophages, carry out this process.

    • Autophagy: Lysosomes recycle cellular components by engulfing damaged organelles, leading to their breakdown and recycling.

    • Tay-Sachs Disease: Caused by accumulation of GM2 gangliosides due to defective lysosomal enzyme Hex-A, leading to neurological dysfunction.

  • Vacuoles: Types include:

    • Food Vacuoles: Formed during phagocytosis, store and digest food particles.

    • Contractile Vacuoles: Maintain osmotic balance in freshwater organisms by expelling excess water.

    • Central Vacuoles: Large vesicles in plant cells; store nutrients, waste products, and help maintain turgor pressure.

Concept 4.13 – 4.15: Mitochondria and Chloroplasts

  • Endosymbiont Theory: Suggests that mitochondria and chloroplasts originated from free-living prokaryotes that entered a symbiotic relationship with a host cell.

  • Support Evidence includes:

    • Similar size and shape to bacteria.

    • Have their own circular DNA resembling bacterial genomes.

    • Double membranes consistent with an engulfing process.

  • Mitochondria: Powerhouse of the cell, responsible for ATP production through oxidative phosphorylation.

    • Structure: Contains an outer membrane, inner membrane, inner membrane space, cristae, and matrix. The cristae maximize surface area for energy production.

  • Chloroplasts: Sites of photosynthesis, contain chlorophyll, and convert solar energy into chemical energy.

    • Structure: Includes outer and inner membranes, thylakoids (stacked in granum), and stroma.

  • Importance of Inner Membranes: Highly folded inner membrane in mitochondria and the thylakoid membranes in chloroplasts increase the surface areas for energy conversion processes.

  • Peroxisomes: Organelles involved in lipid metabolism and the detoxification of harmful byproducts.

  • Compartmentalization: Essential for cellular efficiency; peroxisomes illustrate how specific metabolic processes are segregated within different organelles, preventing potential conflicts and maintaining cellular homeostasis.

Study Tips for Cell Types and Organelles

  • Understanding organelle structure and function aids in determining the primary function of various cell types:

    • Plant Cells: Contain chloroplasts for photosynthesis, large central vacuoles for storage, and cell walls for structure.

    • Animal Cells: Lack chloroplasts but may contain cilia for movement or lysosomes for digestion.

  • The correlation between organelles and cell function is crucial for grasping biological processes in various tissues, such as muscle or epithelial tissues.