Cell Structure and Function: Microscopy, Membranes, and Organelles

4.1 Microscopes Reveal the World

Discovery and Early Study of Cells

The invention and refinement of microscopes allowed scientists to observe and study cells, leading to the foundation of cell biology.

• Robert Hooke: First person to observe and describe cells (from cork) using a microscope. He coined the term "cell" but had limited understanding of their function.

• Antonie van Leeuwenhoek: Improved microscope design and observed living cells, noting their movement and diversity.

Types of Microscopes

• Light Microscope: Uses visible light to magnify specimens. Light passes through the specimen, allowing observation of living cells and tissues.

• Scanning Electron Microscope (SEM): Scans the surface of a specimen with a focused beam of electrons, producing detailed 3D images of cell surfaces. Images are in black and white.

• Transmission Electron Microscope (TEM): Passes electrons through thin sections of specimens, revealing internal structures in high detail. Images are also in black and white.

• Magnification: The increase in an object's apparent size.

• Resolution: The ability to distinguish two close objects as separate; higher resolution reveals more detail.

Cell Theory

• All living things are composed of cells.

• All cells arise from pre-existing cells.

Additional info: The cell theory is a fundamental concept in biology, established in the 19th century by scientists such as Schleiden, Schwann, and Virchow.

4.2 The Small Size of Cells Relates to Their Function

Importance of Cell Size

Cell size is limited by the relationship between surface area and volume. Efficient exchange of materials with the environment is crucial for cell survival.

• Plasma Membrane: A phospholipid bilayer with embedded proteins that forms the boundary of all cells. It regulates the movement of substances in and out of the cell.

• Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, creating a semi-permeable barrier.

• Proteins in the membrane serve as channels, receptors, and enzymes.

Surface Area-to-Volume Ratio

The surface area-to-volume ratio determines how efficiently a cell can exchange materials with its environment. Smaller cells have a higher ratio, which is advantageous for nutrient uptake and waste removal.

• As a cell grows, its volume increases faster than its surface area.

• Cells must maintain a high surface area-to-volume ratio to survive.

	Single Large Cell	Many Small Cells
Total Volume	27 units3	27 units3
Total Surface Area	54 units2	162 units2
Surface Area-to-Volume Ratio	2	6

Additional info: Higher surface area-to-volume ratios in small cells facilitate more efficient exchange of materials.

4.3 Prokaryotic Cells Are Structurally Simpler Than Eukaryotic Cells

Basic Features of All Cells

• All cells have a plasma membrane, DNA, ribosomes, and cytoplasm.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in a region called the nucleoid. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus (enclosed by a nuclear envelope) and various membrane-bound organelles. Examples: Plants, animals, fungi, and protists.

• Most bacteria have a cell wall; some have an additional capsule for protection.

4.4 Eukaryotic Cells Are Partitioned into Functional Compartments

Compartmentalization

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

• Nucleus and ribosomes: Control genetic information and protein synthesis.

• Other organelles are involved in the synthesis and breakdown of molecules.

4.5 The Nucleus Contains the Cell's Genetic Instructions

Structure and Function of the Nucleus

• Nucleus: Contains most of the cell's DNA, which stores genetic instructions for protein synthesis.

• Nucleolus: A dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Subunits of ribosomes are assembled in the nucleolus and then transported to the cytoplasm.

Genetic Information Flow:

• DNA in the nucleus is transcribed into RNA.

• RNA is translated into protein in the cytoplasm.

$\text{Nucleus} \rightarrow \text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

4.6 Ribosomes Make Proteins for Use in the Cell and for Export

Role of Ribosomes

• Ribosomes: Molecular machines that translate genetic information from messenger RNA (mRNA) to synthesize proteins.

• Ribosomes can be free in the cytoplasm or bound to the endoplasmic reticulum (ER), depending on the destination of the proteins they produce.

Additional info: Proteins made by free ribosomes typically function within the cytosol, while those made by ER-bound ribosomes are often secreted or incorporated into cellular membranes.