Cell Structure and Function

Early Cell Observation and Microscopy

Historical Context: Early scientists made detailed sketches of their cellular observations.
Light Microscopes (LM):
- Magnify cells (living and preserved) significantly, typically up to $1,000$ times (e.g., LM $1,000 \times$ ).
- Different types utilize various techniques to enhance contrast and selectively highlight specific cellular components.
Electron Microscopes (EM):
- Provide much greater magnification and reveal intricate cellular details not visible with light microscopy.
- Scanning Electron Microscope (SEM): Provides detailed surface views (e.g., SEM $2,000 \times$ ).
- Transmission Electron Microscope (TEM): Provides internal structural details (e.g., TEM $2,800 \times$ for general views, $15,000 \times$ for cell type comparisons, $130,000 \times$ for Golgi, $0.1$ µm for mitochondria/Golgi, $0.25$ µm for nuclear details).

Cell Size and Surface Area-to-Volume Ratio

Scale of Biological Structures: Cells themselves are largely microscopic, but biological structures span a vast range.
- Human height: $10$ m
- Length of some nerve/muscle cells: $1$ m to $100$ mm ( $10$ cm)
- Chicken egg: $10$ mm ( $1$ cm)
- Frog egg: $1$ mm
- Unaided eye viewing range: from $10$ m down to approximately $100$ µm
- Light microscope viewing range: from approximately $1$ mm down to $100$ nm
- Electron microscope viewing range: from approximately $100$ µm down to $0.1$ nm
- Representative Sizes:
  - Most plant and animal cells: $10-100$ µm
  - Mitochondrion: $1$ µm
  - Most bacteria: $1$ µm
  - Mycoplasmas (smallest bacteria): typically $1$ µm
  - Viruses: $10-100$ nm
  - Ribosome: $10$ nm
  - Proteins: $1-10$ nm
  - Small molecules / Lipids: $1$ nm
  - Atoms: $0.1$ nm
Importance of Surface Area-to-Volume Ratio:
- A smaller cell possesses a significantly greater ratio of surface area to volume compared to a larger cell of the same shape.
- Example: One large cube with sides of $30$ µm has a surface area of $5,400$ µm $^2$ . In contrast, twenty-seven smaller cubes, each with sides of $10$ µm (collectively occupying the same total volume), have a total surface area of $16,200$ µm $^2$ .
- This higher ratio in smaller cells is crucial for efficient exchange of substances, such as nutrient uptake and waste removal, which occur across the cell's surface.

Types of Cells: Prokaryotic vs. Eukaryotic

Cells are broadly categorized into two fundamental types:
- Prokaryotic Cells:
  - Lack a true, membrane-bound nucleus; their genetic material (bacterial chromosome) is located in a nucleoid region.
  - Do not contain membrane-bound organelles.
  - Components: Fimbriae (for attachment), Nucleoid, Ribosomes (for protein synthesis), Plasma membrane, Bacterial chromosome, Cell wall (for structural support), Glycocalyx (outer coating), Flagella (for motility).
  - Typically measure around $0.5$ µm.
  - Example: A typical rod-shaped bacterium like Corynebacterium diphtheriae (colorized TEM shown).
- Eukaryotic Cells:
  - Characterized by the presence of a true, membrane-bound nucleus that houses the genetic material.
  - Contain numerous membrane-bound organelles, which are specialized subcellular structures performing distinct functions.
  - Generally larger and more structurally complex than prokaryotic cells.
  - Animal Cell Organelles (examples): NUCLEUS (Nuclear envelope, Nucleolus, Chromatin), ENDOPLASMIC RETICULUM (Rough ER, Smooth ER), CYTOSKELETON (Microfilaments, Intermediate filaments, Microtubules), Flagellum, Centrosome, Plasma membrane, Microvilli, Ribosomes, Golgi apparatus, Peroxisome, Lysosome, Mitochondrion.
  - Plant Cell Organelles (including those unique to plants): NUCLEUS (Nuclear envelope, Nucleolus, Chromatin), Rough ER, Smooth ER, Golgi apparatus, Central vacuole, Microtubules (part of cytoskeleton), Mitochondrion, Peroxisome, Plasma membrane, Cell wall, Wall of adjacent cell, Chloroplast, Plasmodesmata.

The Nucleus: Control Center

Function: Serves as the cellular control center, containing the cell's DNA and directing all cellular activities.
Structure:
- Nuclear Envelope: A double membrane consisting of an outer and an inner membrane, forming the boundary of the nucleus. It is continuous with the rough endoplasmic reticulum.
- Nuclear Pores: Channels through the nuclear envelope that regulate the transport of macromolecules (e.g., proteins, RNA) between the nucleus and the cytoplasm. These are complex structures often associated with a pore complex (TEM close-up: $0.25$ µm).
- Chromatin: The complex material composed of DNA and associated proteins (histones) that condenses to form chromosomes.
- Nucleolus: A dense, non-membranous structure within the nucleus, primarily responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits.
- Nuclear Lamina: A network of protein filaments lining the inner nuclear membrane, providing structural support to the nucleus (TEM shown).

The Endomembrane System

The endomembrane system is a complex network of internal membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.

Endoplasmic Reticulum (ER)

An extensive network of interconnected membranous tubules and sacs (cisternae) that accounts for a substantial portion of the total membrane in a eukaryotic cell. The ER lumen is the internal compartment.
Smooth Endoplasmic Reticulum (Smooth ER):
- Lacks ribosomes on its surface.
- Primary Functions:
  - Lipid Synthesis: Synthesizes various lipids, including oils, phospholipids, and steroids.
  - Detoxification: Processes and detoxifies drugs and poisons, particularly abundant in liver cells.
  - Calcium Storage: Stores and releases calcium ions ( $\text{Ca}^{2+}$ ), crucial for many cellular processes, especially muscle contraction.
Rough Endoplasmic Reticulum (Rough ER):
- Has ribosomes studded on its outer surface, giving it a