Chapter 6: A Tour of the Cell – General Biology Study Notes

Chapter 6: A Tour of the Cell

Eukaryotic vs. Prokaryotic Cells

Cells are classified into two major types: eukaryotic and prokaryotic. This distinction is fundamental in biology and is based on the presence or absence of a nucleus and membrane-bound organelles.

• Eukaryotic Cells: Have a true nucleus (membrane-bound) where DNA is stored, and possess membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Prokaryotic Cells: Lack a nucleus; DNA is located in a region called the nucleoid. They do not have membrane-bound organelles.

• Examples: Eukaryotes include plants, animals, fungi, and protists. Prokaryotes include bacteria and archaea.

Feature	Eukaryotic Cell	Prokaryotic Cell
Nucleus	Present	Absent (DNA in nucleoid)
Membrane-bound Organelles	Present	Absent
Size	Generally larger (10–100 μm)	Generally smaller (0.1–5 μm)

Cell Size and Surface Area to Volume Ratio

Cell size is limited by physical and metabolic constraints. The surface area to volume ratio is a key factor in determining cell efficiency.

• Too Small: Not enough space for DNA and metabolic machinery.

• Too Large: Surface area to volume ratio becomes too small, limiting the rate of material exchange.

• Typical Sizes: Bacteria can be as small as 0.1 μm; eukaryotic cells can be as large as 100 μm.

Formula:

• Surface Area of a Cube: $SA = 6a^2$

• Volume of a Cube: $V = a^3$

• Surface Area to Volume Ratio: $\frac{SA}{V}$

Example: A large cube (30 μm per side) has a surface area of 5400 μm2, but dividing it into 27 smaller cubes increases the total surface area to 16,200 μm2.

Nucleus

The nucleus is the control center of eukaryotic cells, containing most of the cell's genetic material.

• Double Membrane: The nuclear envelope surrounds the nucleus, separating it from the cytoplasm.

• Chromatin: DNA is stored in the form of chromatin (DNA + proteins).

• Nuclear Pores: Allow passage of proteins and RNA between nucleus and cytoplasm.

• Nucleolus: Site of ribosome (and rRNA) synthesis.

Example: The nucleolus is visible as a dense region within the nucleus and is essential for assembling ribosomal subunits.

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis, translating genetic code from mRNA into polypeptide chains.

• Free Ribosomes: Float in the cytosol; proteins made here usually function within the cell.

• Rough ER Ribosomes: Attached to the rough endoplasmic reticulum; proteins made here are often exported or inserted into membranes.

• Function: Assemble polypeptide chains using the code provided by mRNA and tRNA.

Formula:

• Peptide Synthesis: $\text{mRNA} + \text{tRNA} \rightarrow \text{Polypeptide}$

Example: Ribosomes read mRNA codons and match them with tRNA carrying specific amino acids, forming a growing polypeptide chain.

*Additional info: Further topics such as endomembrane system, energy transfer organelles, cytoskeleton, and cellular junctions are present in the full notes but not shown in the provided images. These would include explanations of the endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, cytoskeletal elements, and cell junctions for a complete study guide.*