Biology | Cell structure

👋 Welcome to the World of Cells!

Hey future biologists! Get ready to zoom in on the fundamental unit of life: the cell. This chapter, "Cell Structure," is the foundation of all subsequent biology topics, linking directly to the "Unity and diversity" section by showing us how all life, despite its incredible diversity, shares common structural components.
Think of cells as tiny, self-contained cities. By understanding how these cities are built and organized, you unlock the secret to how all organisms function, from the smallest bacteria to the largest whale. Don't worry if the names of the organelles sound complicated—we’ll use analogies to make them stick!

1. The Foundation: Cell Theory

Before diving into structure, we must understand the core concept upon which modern biology is built: the Cell Theory. This theory provides the unifying principles for studying life.

The Three Tenets of Cell Theory

1. All living organisms are composed of one or more cells.
   (Whether you are a single-celled amoeba or a multicellular human, you are made of cells.)
2. The cell is the basic unit of structure and function in living organisms.
   (The cell is the smallest thing that can perform all life processes.)
3. Cells arise only from pre-existing cells.
   (Life does not spontaneously generate; cells reproduce.)

🔥 Key Takeaway: Cell Theory emphasizes the unity in life—no matter how complex an organism is, it all starts with the basic cellular unit.

2. Unity and Diversity: The Two Major Cell Types

Although all cells share common features (like a plasma membrane, cytoplasm, and genetic material), they fall into two major structural categories: Prokaryotes and Eukaryotes.

2.1. Prokaryotes (The Simple Architects)

These are the oldest and structurally simplest cells, typically consisting only of bacteria and archaea. They are defined by their lack of internal compartments.

• No Nucleus: Their genetic material (DNA) is located in a region called the nucleoid.
• Simple Structure: They lack membrane-bound organelles (like mitochondria or ER).
• Common Features: They possess a cell wall (for protection/structure), a plasma membrane, cytoplasm, and ribosomes (for protein synthesis).
• Size: Generally much smaller than eukaryotic cells (often 1–10 µm).

2.2. Eukaryotes (The Complex Cities)

These cells are found in animals, plants, fungi, and protists. Their defining feature is the presence of internal membranes, creating distinct compartments (organelles).

• Possess a Nucleus: The genetic material is housed within a double membrane.
• Contain Organelles: They have specialized, membrane-bound structures that perform specific functions. This process is called compartmentalization.
• Size: Generally much larger (often 10–100 µm).

3. Eukaryotic Cell Structure: Organelles and Compartmentalization

Compartmentalization is crucial for eukaryotes. By separating different metabolic pathways and environments into specific organelles, the cell can operate much more efficiently. Think of it like separating the kitchen, bedroom, and bathroom in a house—each space has a dedicated function and environment.

Here is a breakdown of the essential membrane-bound organelles (SL/HL content):

3.1. The Control Centre: The Nucleus

• Structure: Usually the largest organelle, enclosed by a nuclear envelope (a double membrane) containing nuclear pores.
• Function: Contains the cell’s hereditary material (chromosomes/DNA) and controls all cellular activities by regulating gene expression.
• Analogy: The CEO’s Office or the City Hall.

3.2. Protein and Lipid Factories: Ribosomes and Endoplasmic Reticulum (ER)

Ribosomes

Structure: Not membrane-bound (found in both prokaryotes and eukaryotes!). They are made of RNA and protein, composed of a large and a small subunit.
Function: The site of protein synthesis (translation).
Analogy: The Construction Workers.

Endoplasmic Reticulum (ER)

A network of flattened sacs (cisternae) and tubes that extends throughout the cytoplasm.
1. Rough ER (RER): Studded with ribosomes.
Function: Synthesizes, modifies, and transports proteins destined for secretion or insertion into membranes.
2. Smooth ER (SER): Lacks ribosomes.
Function: Synthesis of lipids (like phospholipids and steroids), detoxification of drugs and poisons, and storage of calcium ions.
Analogy: The Cell's Manufacturing and Transport Highway System.

3.3. The Post Office: Golgi Apparatus (or Golgi Complex/Body)

• Structure: Stacks of flattened, membrane-bound sacs called cisternae (but usually fewer and wider than ER cisternae).
• Function: Processes, sorts, modifies, and packages proteins and lipids received from the ER into vesicles for secretion or delivery to other organelles.
  Did you know? Proteins pass through the Golgi, entering the cis face and exiting the trans face.
• Analogy: The Post Office/Shipping Department.

3.4. Energy and Waste Management

Mitochondrion (Plural: Mitochondria)

• Structure: Double membrane structure. The outer membrane is smooth, while the inner membrane is highly folded into cristae. The central space is the matrix.
• Function: Site of aerobic cell respiration, producing the cell's main energy currency, ATP.
• Analogy: The Power Plant.

Lysosomes

• Structure: Spherical sacs bound by a single membrane, containing hydrolytic enzymes.
• Function: Digests ingested food, worn-out organelles (autophagy), or entire cells (apoptosis). They are the cell's "cleanup crew."
• Analogy: The Recycling Centre/Garbage Disposal.

3.5. Cell Boundaries (Plasma Membrane)

The plasma membrane is the boundary of the cell. It is made primarily of a phospholipid bilayer and proteins. While its detailed structure (fluid mosaic model) and transport functions are covered in a separate chapter ("Membranes and membrane transport"), remember its essential structural role:

• It separates the internal cell environment (cytoplasm) from the external environment.
• It controls which substances enter and leave the cell (selective permeability).

4. Comparing Plant and Animal Cell Structures

Both plant and animal cells are eukaryotes, meaning they share the nucleus, mitochondria, ER, and Golgi. However, plant cells have three major structures that animal cells lack:

Structure	Plant Cells Only	Animal Cells Only
Cell Wall	Yes (rigid, outside the plasma membrane, made of cellulose)	No
Chloroplasts	Yes (site of photosynthesis)	No
Vacuole	One large, central vacuole (maintains turgor pressure)	Small, temporary vacuoles (if present)
Centrioles/Centrosomes	No (higher plants)	Yes (involved in cell division)

🔥 Key Takeaway: Compartmentalization (using membranes to create specialized compartments) is the defining feature of eukaryotes that allows them to perform complex, simultaneous reactions efficiently.

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5. 🔬 HL EXTENSION: The Origin of Cells and Eukaryotic Complexity

The syllabus requires HL students to understand the origin of cells, focusing on how early life transitioned from simple prokaryotes to complex eukaryotes. The most widely accepted explanation for the origin of two key eukaryotic organelles (mitochondria and chloroplasts) is the Endosymbiotic Theory.

5.1. The Endosymbiotic Theory

This theory explains that mitochondria and chloroplasts originated as independent prokaryotic cells that were engulfed by a larger host cell. Instead of being digested, the smaller cell remained and developed a symbiotic relationship with the host.

Step-by-Step Explanation:

1. An early anaerobic (non-oxygen using) eukaryotic cell engulfed an aerobic (oxygen-using) prokaryote (a bacterium).
2. The aerobic prokaryote was protected and supplied with raw materials by the host cell.
3. In return, the aerobic prokaryote processed food to produce large amounts of ATP (energy) for the host cell. This became the mitochondrion.
4. Later, in a separate event, some of these host cells also engulfed photosynthetic prokaryotes (cyanobacteria).
5. The photosynthetic prokaryote provided the host cell with sugar, becoming the chloroplast.

5.2. Evidence Supporting Endosymbiotic Theory

The unique structural and genetic characteristics of mitochondria and chloroplasts strongly support their prokaryotic ancestry:

• Double Membranes: They both have two membranes. The inner membrane represents the original prokaryotic plasma membrane, and the outer membrane is derived from the host cell's engulfing vesicle.
• Naked DNA: They have their own circular, naked DNA (like prokaryotes), distinct from the nucleus's linear DNA.
• Ribosomes: They possess their own ribosomes, which are 70S ribosomes—the same size found in prokaryotes (eukaryotic cytoplasmic ribosomes are 80S).
• Reproduction: They reproduce independently via a process similar to binary fission (the way prokaryotes reproduce).

🔥 Key Takeaway (HL): The Endosymbiotic Theory highlights how complex eukaryotic cells evolved through cooperation and fusion of simpler life forms, giving them a significant advantage in energy production (mitochondria) and food production (chloroplasts).