Cell Size and Shape

Surface Area-to-Volume Ratio

The surface area-to-volume ratio is a critical factor that influences cell size and function. As a cell grows, its volume increases faster than its surface area, affecting the efficiency of material exchange with the environment.

• Surface Area: For a cube-shaped cell, surface area is calculated as the area of one side multiplied by the number of sides (6 for a cube).

• Volume: The volume of a cube is the product of its height, width, and length.

• Surface Area-to-Volume Ratio: This ratio determines how efficiently substances can diffuse into and out of the cell.

Example Calculation:

• Given a cube-shaped cell with sides of 10 mm:

• Surface Area = (10 mm × 10 mm) × 6 = 600 mm2

• Volume = 10 mm × 10 mm × 10 mm = 1000 mm3

• Surface Area-to-Volume Ratio = 600 / 1000 = 0.6

Formula:

• Surface Area of a cube:

• Volume of a cube:

• Surface Area-to-Volume Ratio:

Importance: A higher surface area-to-volume ratio allows for more efficient exchange of materials, which is why most cells are small.

A Tour of the Cell

Cells are the fundamental units of life, exhibiting a variety of shapes and sizes to perform different functions. Both prokaryotic and eukaryotic cells share basic features, but eukaryotic cells have membrane-bound organelles.

• Prokaryotic Cells: Lack a nucleus and most organelles.

• Eukaryotic Cells: Contain a nucleus and various organelles, each with specialized functions.

The Role of the Nucleus

Structure and Function

The nucleus is the information center of the eukaryotic cell, containing most of the cell's genetic material in the form of DNA.

• Nuclear Envelope: A double membrane that encloses the nucleus, separating it from the cytoplasm.

• Nuclear Pores: Large protein complexes that regulate the entry and exit of molecules between the nucleus and cytoplasm.

• Nuclear Lamina: A network of protein filaments (lamins) that provide structural support to the nuclear envelope.

• Chromatin: DNA and associated proteins (histones) that condense to form chromosomes during cell division.

• Nucleolus: A dense region within the nucleus responsible for ribosomal RNA (rRNA) synthesis and ribosome assembly.

Example: Electron micrographs show the nucleus as a prominent, membrane-bound structure in both animal and plant cells.

Nuclear Pore Complex

• Composed of about 30 different proteins called nucleoporins.

• Functions as a selective gateway for the transport of RNA, proteins, and other macromolecules.

Nucleolus

• Not membrane-bound.

• Site of rRNA gene clusters and ribosome subunit assembly.

• Ribosomal proteins are imported from the cytoplasm and assembled with rRNA in the nucleolus.

Key Function: Synthesis of ribosomal RNA (rRNA) and assembly of ribosomal subunits.

Endomembrane System

The endomembrane system is a group of interconnected organelles that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, transport vesicles, and the plasma membrane.

• Function: Synthesis, modification, and transport of proteins and lipids.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes membrane-bound and secretory proteins.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, calcium storage, and detoxification.

• ER Lumen: The internal space of the ER where protein folding and modification occur.

Golgi Apparatus

• Consists of flattened membranous sacs called cisternae.

• Cis face: Receiving side, located near the ER.

• Trans face: Shipping side, where modified products are sorted and sent to their destinations.

• Functions in modifying, sorting, and packaging proteins and lipids for secretion or delivery to other organelles.

Protein Trafficking (Secretory Pathway)

1. Protein is synthesized by ribosomes on the rough ER.

2. Protein is packaged into vesicles and transported to the Golgi apparatus.

3. Protein is further modified and sorted in the Golgi.

4. Protein is packaged into vesicles for transport to the plasma membrane or other locations.

5. Protein is secreted from the cell or delivered to its final destination.

Table: Comparison of Rough and Smooth Endoplasmic Reticulum

Additional info:

• Some content, such as the details of the cytoskeleton and extracellular structures, is referenced in the outline but not elaborated in the provided slides. For a complete study guide, students should review these topics in their textbook.

• Pulse-chase experiments (mentioned in later slides) are classic methods for studying the movement of proteins through the endomembrane system.