Cell Structure and Function2.1: General Biology I Study Notes

Cell Structure

Introduction to Cell Theory

The study of cells is fundamental to understanding all living organisms. The cell theory describes the properties and processes of cells, which are the basic units of life.

• Cell Theory: All living things are composed of cells, and all cells arise from pre-existing cells.

• Pattern: Cells are the structural and functional units of life.

• Process: Cells carry out all necessary life functions, including metabolism, growth, and reproduction.

• Implications: Understanding cell theory helps explain the unity and diversity of life.

Example: The discovery of cells in nearly every location on Earth, from deep-sea vents to arid deserts, demonstrates their adaptability and universality.

The Emergence of Cells

Cells are the oldest known forms of life, with evidence suggesting they appeared about 3.5 billion years ago. All current cells can be traced back to these earliest cells.

• First cells: Likely prokaryotic (bacteria-like) in structure.

• Evolution: Over time, cells diversified and evolved into more complex forms.

Example: Modern bacteria are descendants of ancient prokaryotic cells.

Cell Size and Diversity

How Big is a Cell?

Cells vary greatly in size and shape, depending on their function and type.

• Bacterial cells: Typically 1-10 micrometers in diameter.

• Animal cells: Usually 10-100 micrometers in diameter.

• Plant cells: Can be even larger, up to several hundred micrometers.

Example: The human egg cell is one of the largest cells in the body.

Cell Theory: Pattern and Process

Pattern

Cells are surrounded by membranes and contain genetic material (DNA). They may exist as single-celled organisms or as part of multicellular organisms.

• Membranes: Define the boundaries of the cell and regulate the movement of substances.

• Genetic material: DNA is the hereditary material in all cells.

• Cellular organization: Cells may be prokaryotic or eukaryotic.

Process

Cells carry out essential life processes, including metabolism, growth, and reproduction.

• Metabolism: Chemical reactions that provide energy and build cellular components.

• Growth: Increase in cell size and number.

• Reproduction: Cells divide to produce new cells.

Cell Types

Prokaryotic Cells

Prokaryotic cells are simple, single-celled organisms without a nucleus. They include bacteria and archaea.

• No nucleus: DNA is located in a region called the nucleoid.

• Cell wall: Provides structure and protection.

• Ribosomes: Sites of protein synthesis.

• Simple internal structure: Few organelles.

Example: Escherichia coli is a common prokaryotic bacterium found in the human gut.

Eukaryotic Cells

Eukaryotic cells are more complex and contain a nucleus and various organelles. They make up plants, animals, fungi, and protists.

• Nucleus: Contains the cell's genetic material.

• Organelles: Specialized structures that perform specific functions (e.g., mitochondria, endoplasmic reticulum).

• Compartmentalization: Organelles allow for specialized environments within the cell.

Example: Animal and plant cells are both eukaryotic, but plant cells have additional structures like chloroplasts and cell walls.

Cell Specialization

Specialized Cell Types

Both prokaryotic and eukaryotic cells can be specialized for particular functions.

• Prokaryotic specialization: Some bacteria form spores or have flagella for movement.

• Eukaryotic specialization: Muscle cells contract, nerve cells transmit signals, and plant cells perform photosynthesis.

Structure and Function of Cell Components

Prokaryotic Cell Components

• Cell membrane: Selectively permeable barrier.

• Cell wall: Provides rigidity.

• Ribosomes: Protein synthesis.

• Nucleoid: Region containing DNA.

Eukaryotic Cell Components

• Nucleus: Stores genetic information and coordinates cell activities.

• Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

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

• Mitochondria: Site of cellular respiration and energy production.

• Chloroplasts (plants): Site of photosynthesis.

• Vacuoles (plants): Storage and structural support.

Flow of Genetic Information in Eukaryotic Cells

Genetic information flows from DNA to RNA to protein, a process known as the central dogma of molecular biology.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into protein by ribosomes.

Equation:

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

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	Few (ribosomes only)	Many (ER, Golgi, mitochondria, etc.)
Size	Small (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Summary

• Cells are the basic units of life, as described by cell theory.

• Prokaryotic cells are simple and lack a nucleus, while eukaryotic cells are complex and contain organelles.

• Cell specialization allows for diverse functions in multicellular organisms.

• Understanding cell structure and function is essential for studying biology.