Cell Structure and Function: Study Notes for General Biology

Chapter 6: Cells

Microscopy: Types, Advantages, and Limitations

Microscopes are essential tools for studying cells and their structures. There are three main types, each with unique capabilities:

• Light Microscope: Uses visible light to illuminate specimens. Allows observation of living cells and tissues, but has limited resolution (cannot see most organelles).

• Transmission Electron Microscope (TEM): Passes electrons through thin sections of specimens, providing high-resolution images of internal cell structures. Specimens must be dead and thinly sliced.

• Scanning Electron Microscope (SEM): Scans the surface of specimens with electrons, producing detailed 3D images of cell surfaces. Only surface features are visible; specimens must be coated and are not alive.

Example: TEM is used to study the internal structure of mitochondria, while SEM is used to visualize the surface of a bacterium.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structure and complexity.

Feature	Prokaryotic	Eukaryotic
Size	1–5 μm	10–100 μm
Nucleus	No nucleus; DNA in nucleoid region	True nucleus surrounded by nuclear envelope
Organelles	Few; no membrane-bound organelles	Many; includes mitochondria, Golgi apparatus, ER, etc.
Kingdoms	Bacteria, Archaea	Protista, Fungi, Plantae, Animalia
Evolution	3.5 billion years ago	1.5 billion to 100 million years ago

Basic Cell Structures

Cells have several key structures that are essential for their function:

• Cell Wall: Provides structural support and protection (found in plants, fungi, bacteria).

• Plasma Membrane: Selectively permeable barrier that controls entry and exit of substances.

• Bacterial Chromosome: Circular DNA molecule in prokaryotes.

• Flagella: Tail-like structures used for movement.

Example: The plasma membrane surrounds all cells, while the cell wall is present in plant and bacterial cells.

Compartmentalization in Eukaryotic Cells

Eukaryotic cells contain membrane-bound organelles that compartmentalize cellular functions, increasing efficiency and specialization.

• Advantages: Separation of incompatible reactions, increased surface area for reactions, and regulation of metabolic pathways.

• Organelles: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.

Nuclear Envelope: Structure and Function

The nuclear envelope surrounds the nucleus and regulates the passage of materials between the nucleus and cytoplasm.

• Structure: Double membrane with nuclear pores.

• Function: Protects genetic material, controls exchange of RNA and proteins.

• Nuclear Pore Complex: Allows selective transport of molecules.

Protein Synthesis and the Nucleus

The nucleus controls protein synthesis by storing genetic information and directing the production of messenger RNA (mRNA).

• Chromosomes: Contain DNA, which encodes instructions for protein synthesis.

• Process: DNA is transcribed into mRNA, which exits the nucleus and is translated into protein in the cytoplasm.

Function of the Nucleolus

The nucleolus is a dense region within the nucleus responsible for ribosome synthesis.

• Ribosomal RNA (rRNA): Synthesized and combined with proteins to form ribosomal subunits.

Major Organelles and Their Functions

Plant and animal cells contain specialized organelles with distinct functions:

• Ribosome: Site of protein synthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosome: Contains digestive enzymes to break down waste.

• Peroxisome: Carries out specialized metabolic reactions.

• Mitochondria: Site of cellular respiration and energy production.

• Cytoskeleton: Provides structural support and facilitates movement.

• Chloroplast (plants only): Site of photosynthesis.

• Vacuole (plants only): Stores water, nutrients, and waste.

• Cell Wall (plants only): Provides rigidity and protection.

Protein Secretion Pathway

Proteins destined for secretion follow a specific pathway:

1. Transcription (DNA to mRNA)

2. Translation (mRNA to protein)

3. Transport to Golgi apparatus

4. Packing in secretory vesicles

5. Exocytosis (release from cell)

Surface Area and Volume Calculations

Cell size affects surface area-to-volume ratio, which influences nutrient exchange and efficiency.

• Formulas:

$V = l \times w \times h$ (Volume of a cube)

$A = 2(lw + lh + wh)$ (Surface area of a cube)

Example: 1000 small cells have a greater total surface area than one large cell of the same total volume, facilitating more efficient exchange.

Phospholipids and Membrane Structure

Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Structure: Hydrophilic head, hydrophobic tail.

• Function: Form bilayers that make up cell membranes.

Membrane Fluidity

Membrane fluidity is influenced by several factors:

• Decreasing Temperature: Membranes become less fluid and more rigid.

• Unsaturated Hydrocarbon Chains: Increase fluidity by preventing tight packing.

• Cholesterol: Maintains fluidity by restraining movement at high temperatures and preventing solidification at low temperatures.

Membrane Proteins: Functions

Membrane proteins perform various essential functions:

Function	Description
Transport	Move substances across the membrane (channels, carriers).
Enzymatic Activity	Catalyze chemical reactions at the membrane surface.
Signal Transduction	Transmit signals from outside to inside the cell.
Cell-Cell Recognition	Allow cells to identify and interact with each other.
Intercellular Joining	Connect adjacent cells via junctions.
Attachment to Cytoskeleton and ECM	Anchor the membrane to internal and external structures.

Additional info: These notes expand on the original handwritten and diagram-based content, providing full academic context and definitions for all major cell biology concepts covered in the file.