CH:6 Review The Cell

The Cell Theory

The cell theory is a fundamental concept in biology that states:

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in living things.

• All cells arise from pre-existing cells.

Why Are Cells Small? – Surface to Volume Ratio

Cells are limited in size due to the relationship between their surface area and volume. As a cell grows, its volume increases faster than its surface area, making it difficult for the cell to efficiently exchange materials with its environment.

• Surface-to-volume ratio: The ratio of a cell's surface area to its volume. A higher ratio allows for more efficient transport of materials in and out of the cell.

• Small cells have a higher surface-to-volume ratio, which is advantageous for nutrient uptake and waste removal.

Microscopy

Microscopes are essential tools for studying cells and their structures. There are two main types:

• Light microscope: Uses visible light to magnify objects up to about 1000x. Suitable for viewing living cells and tissues.

• Electron microscope: Uses beams of electrons for much higher magnification and resolution. Includes transmission electron microscopes (TEM) and scanning electron microscopes (SEM).

Basic Features of All Cells

All cells, whether prokaryotic or eukaryotic, share certain features:

• Plasma membrane: A selectively permeable barrier that encloses the cell.

• Cytosol: The semifluid substance within the cell.

• Chromosomes: Structures that carry genetic information (DNA).

• Ribosomes: Complexes that synthesize proteins.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells

Prokaryotic cells are simpler and smaller than eukaryotic cells. They lack a nucleus and most membrane-bound organelles.

• External appendages:

  • Flagella: Tail-like structures used for movement.

  • Pili: Hair-like structures for attachment and conjugation.

• Cell wall: Provides structure and protection.

• Plasma membrane: Controls entry and exit of substances.

• Nucleoid: Region where DNA is located (not membrane-bound).

• Ribosomes: Sites of protein synthesis.

Bacterial Cell Shapes and Arrangements

Bacteria can have various shapes (e.g., cocci, bacilli, spirilla) and arrangements (e.g., chains, clusters).

Eukaryotic Cells

Eukaryotic cells are more complex, containing a nucleus and numerous membrane-bound organelles. They can be classified as animal or plant cells.

• Animal cells: Lack cell walls and chloroplasts; have centrioles.

• Plant cells: Have cell walls, chloroplasts, and large central vacuoles.

Cell Organelles and Internal Structures

Nucleus

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

• Nuclear envelope: Double membrane that surrounds the nucleus.

• Nucleolus: Site of ribosome synthesis.

Endomembrane System

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

• Endoplasmic reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport.

• Lysosomes: Contain digestive enzymes; break down waste materials and cellular debris.

• Vesicles: Small membrane-bound sacs that transport substances within the cell.

Other Organelles

• Mitochondria: Site of cellular respiration; produces ATP (energy currency of the cell).

• Chloroplasts (in plant cells): Site of photosynthesis; converts solar energy to chemical energy.

• Cytoskeleton: Network of protein filaments that provides structural support, cell shape, and movement.

  • Microtubules: Hollow tubes; maintain cell shape, enable movement of organelles.

  • Centrosomes and centrioles: Organize microtubules; important in cell division.

Extracellular Matrix (ECM)

The ECM is a network of proteins and carbohydrates outside animal cells that provides structural support and mediates cell signaling.

Intercellular Junctions

Cells are connected to each other by specialized structures called intercellular junctions, which facilitate communication and adhesion.

• Tight junctions: Prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together.

• Gap junctions: Allow for the exchange of ions and small molecules between cells.

Integration of Organelles and Levels of Organization

Organelles work together to carry out the functions necessary for cell survival. Cells are organized into tissues, organs, and organ systems in multicellular organisms.

Key Terms and Phrases

The following table summarizes important terms related to cell structure and function:

Example: Surface-to-Volume Ratio Calculation

For a spherical cell of radius r:

• Surface area =

• Volume =

• Surface-to-volume ratio =

This shows that as the radius increases, the surface-to-volume ratio decreases.

Additional info:

• Autophagy is the process by which cells degrade and recycle their own components using lysosomes.

• Phagocytosis is the process by which a cell engulfs large particles or other cells.

• Try-Sach disease is a genetic disorder caused by the absence of a lysosomal enzyme, leading to the accumulation of lipids in the brain.