Cell Structure and Function: General Biology Study Notes

The Cell Theory

Definition and Principles

The Cell Theory is a foundational concept in biology that describes the properties and significance of cells in living organisms. It states:

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

• The cell is the basic unit of life in all living things.

• All cells arise from preexisting cells.

These principles guide our understanding of what constitutes life and how organisms are structured.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main categories: Prokaryotic and Eukaryotic. This classification is based on the presence or absence of a nucleus and membrane-bound organelles.

• Prokaryotic Cells: Do not have a nucleus or membrane-bound organelles. Their DNA is located in a region called the nucleoid. Prokaryotic cells are generally smaller and simpler than eukaryotic cells. Examples include Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Eukaryotic cells are typically larger and more complex. Examples include Animal, Plant, Fungi, and Protist cells.

Both cell types share basic structures: plasma membrane, cytoplasm, chromosomes, and ribosomes.

Cell Membrane Structure

Phospholipid Bilayer

The plasma membrane surrounds all cells and regulates the movement of substances in and out. It is primarily composed of a phospholipid bilayer with embedded proteins and carbohydrates.

• Phospholipids: Each molecule has a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails.

• Bilayer Arrangement: Phospholipids arrange themselves so that the hydrophobic tails face inward, away from water, while the hydrophilic heads face outward toward the aqueous environment.

This structure creates a semi-permeable barrier, allowing selective transport of molecules.

Cellular Components

Cytoplasm

The cytoplasm is the jelly-like substance within the cell membrane that contains organelles and is the site of many metabolic reactions. It is mainly water-based and is described as aqueous.

Nucleus

The nucleus is the control center of eukaryotic cells, housing the DNA. It is surrounded by a double membrane called the nuclear envelope. The nucleus is the site of DNA replication and transcription (the first step of the Central Dogma of molecular biology: DNA → RNA → Protein).

Ribosomes

Ribosomes are complexes of RNA and protein that synthesize proteins by translating messenger RNA (mRNA). They can be found free in the cytoplasm or attached to the endoplasmic reticulum.

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. It includes:

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis. Two types: Rough ER (with ribosomes, protein synthesis) and Smooth ER (lipid synthesis, detoxification).

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

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

• Vacuoles: Membrane-bound sacs for storage and transport. Plant cells have a large central vacuole that maintains cell structure via turgor pressure.

Energy-Related Organelles

Mitochondria

Mitochondria are the "powerhouses" of the cell, generating ATP through cellular respiration. They have a double membrane and their own DNA, reflecting their evolutionary origin (Endosymbiont Theory).

• Aerobic Respiration: Uses oxygen to produce ATP.

• Anaerobic Respiration: Produces ATP without oxygen (less efficient).

Chloroplasts

Chloroplasts are found in plant and algal cells. They perform photosynthesis, converting light energy into chemical energy (glucose). Like mitochondria, they have a double membrane and their own DNA.

Cellular Support and Movement

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, maintains cell shape, and enables movement. It consists of:

• Microtubules: Largest fibers, hollow tubes, maintain cell shape, aid in cell division, and serve as tracks for organelle movement.

• Microfilaments: Smallest fibers, made of actin, involved in cell movement and shape changes.

• Intermediate Filaments: Medium-sized fibers, provide structural stability.

Centrosome and Centrioles

The centrosome is the main microtubule organizing center in animal cells. It contains a pair of centrioles that help organize microtubules during cell division.

Flagella and Cilia

Flagella are long, whip-like structures that enable cell movement. Cilia are shorter, hair-like projections that move substances across the cell surface or enable cell locomotion.

Extracellular Matrix

The extracellular matrix is a network outside the cell composed of proteins, carbohydrates, and polysaccharides. It provides structural support and helps cells communicate.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Size	Smaller (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists
DNA Location	Nucleoid region	Nucleus

Key Terms and Concepts

• Plasma Membrane: Selectively permeable boundary of the cell.

• Cytoplasm: Internal fluid containing organelles.

• Organelle: Specialized structure within a cell performing a specific function.

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Turgor Pressure: Pressure exerted by the central vacuole in plant cells, maintaining rigidity.

• Endosymbiont Theory: Theory that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Important Equations

• Cellular Respiration (Aerobic):

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Photosynthesis:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Summary of Cell Functions

• Genetic Code System: DNA stores genetic information.

• Delivery System: Proteins and molecules are synthesized and transported.

• Disposal System: Lysosomes degrade waste.

• Energy System: Mitochondria and chloroplasts produce and store energy.

• Structural System: Cytoskeleton and extracellular matrix provide support and shape.

Additional info: These notes expand on the original material by providing definitions, examples, and context for each cell structure and function, ensuring a comprehensive and self-contained study guide for General Biology students.