Biology | Cell Structure, Reproduction and Development

Welcome to the Cellular World: Structure, Replication, and Life!

Hello! This chapter is the foundation of all Biology. We are going to explore the incredible world inside the cell—the basic unit of life—and learn how a single cell divides, specialises, and builds an entire organism. Don't worry if some concepts look complex; we will break them down into easy, bite-sized pieces!

Think of a cell as a highly efficient, tiny city. Each part has a specific job, and everything must work perfectly together for the city (the organism) to survive. Let's start with the fundamental differences between cell types.

Section 1: Prokaryotes vs. Eukaryotes

All life falls into one of two major categories based on cell structure: Prokaryotic or Eukaryotic.

1.1 Prokaryotic Cells (The Simple Builders)

Prokaryotes (like bacteria) are the simplest type of cell. They are ancient and very efficient, but they lack internal complexity.

• They have no nucleus. Their genetic material (DNA) floats freely in the cytoplasm, usually as a single circular chromosome.
• They lack membrane-bound organelles (like mitochondria or Golgi).
• They often have smaller rings of DNA called plasmids, which can be shared between bacteria.
• They may possess a flagellum (tail) for movement or pili (hairs) for attachment/sharing plasmids.
• Their cell wall is made of peptidoglycan.

Analogy: A prokaryote is like a basic tent. Everything is open-plan and functional, but simple.

1.2 Eukaryotic Cells (The Complex Architects)

Eukaryotes are found in plants, animals, fungi, and protists. They are much larger and far more complex than prokaryotes.

• They possess a true nucleus, which holds the genetic material.
• They contain numerous membrane-bound organelles, allowing specialised tasks to occur in separate compartments.
• Their DNA is linear and wrapped around proteins called histones, forming chromosomes.

Key Takeaway: The fundamental difference is the presence of a nucleus and membrane-bound organelles in Eukaryotes, which Prokaryotes lack.

Section 2: The Eukaryotic Cell – The Organelles

These notes focus on the detailed ultrastructure of the eukaryotic cell. Remember, each organelle is surrounded by its own membrane, allowing it to maintain a unique internal environment.

2.1 The Control Center and Factories

The Nucleus

• Function: Contains the genetic material (DNA), controls the cell's activities by directing protein synthesis.
• It is surrounded by a nuclear envelope (a double membrane) containing nuclear pores, which allow molecules (like mRNA) to move in and out.
• Inside is the nucleolus, responsible for making ribosomes.

Ribosomes

• Function: The site of protein synthesis (translation).
• Found either free in the cytoplasm or attached to the rough endoplasmic reticulum (RER).

Endoplasmic Reticulum (ER)

• The ER is a network of membranes forming sacs (cisternae).
  • Rough ER (RER): Studded with ribosomes. Its job is to synthesise, fold, and process proteins that are destined to be secreted outside the cell or embedded in membranes.
  • Smooth ER (SER): Lacks ribosomes. Its job is mainly for lipid synthesis (e.g., steroids) and detoxification (especially in liver cells).

2.2 Processing and Energy

Golgi Apparatus (or Golgi Body)

• Function: Modifies, sorts, and packages proteins and lipids received from the ER into vesicles (sacs) for transport to their final destination (the cell surface or other organelles).
• Analogy: This is the cell’s post office!

Mitochondria

• Function: The site of aerobic respiration, generating large quantities of ATP (adenosine triphosphate – the cell’s energy currency).
• Structure: Double membrane. The inner membrane is highly folded into structures called cristae, increasing the surface area for respiration enzymes. The inner space is called the matrix.

Lysosomes

• Function: Sacs containing powerful hydrolytic enzymes. They break down waste materials, worn-out organelles (autophagy), or even the entire cell (autolysis).
• Analogy: The cell’s recycling and clean-up crew.

2.3 Structure and Movement

The Cytoskeleton

• A network of protein filaments throughout the cytoplasm.
• Function: Provides mechanical strength, maintains cell shape, anchors organelles, and enables cell movement (e.g., using flagella or cilia) and movement of materials within the cell.

Section 3: Cell Reproduction – The Cell Cycle and Mitosis

Cells don't live forever; they must divide to allow for growth, repair of damaged tissues, and replacement of old cells. This process is called the Cell Cycle.

3.1 Interphase (The Preparation Stage)

This is the longest part of the cycle, where the cell prepares for division. The chromosomes are long, thin threads (chromatin) and not visible under a light microscope.

Interphase is broken into three crucial sub-phases:

• G1 (Gap 1): Cell grows, new organelles and proteins are made.
• S (Synthesis): The cell replicates its entire DNA so that each new daughter cell receives a full set. After S phase, each chromosome consists of two identical sister chromatids joined at the centromere.
• G2 (Gap 2): Cell keeps growing and checks the duplicated DNA for errors before entering mitosis.

Did you know? If DNA replication (S phase) goes wrong, the cell should trigger ‘apoptosis’ (programmed cell death) to prevent damaged DNA from being passed on.

3.2 Mitosis (Nuclear Division)

Mitosis is the process of nuclear division, resulting in two genetically identical diploid (2n) daughter nuclei. The goal is to separate the sister chromatids equally.

Memory Aid: PMAT

P - Prophase

• The chromatin condenses (coils up) into visible, short, thick chromosomes.
• The nuclear envelope breaks down.
• Spindle fibres (made of microtubules) begin to form, extending from the poles.

M - Metaphase

• Chromosomes line up along the cell's equator (the middle line), known as the metaphase plate.
• Spindle fibres attach to the centromeres of the chromosomes.

A - Anaphase

• The centromeres split.
• The sister chromatids separate and are pulled apart to opposite poles by the shortening spindle fibres. (This requires ATP, provided by nearby mitochondria).
• Key Point: Once separated, each chromatid is now considered an individual chromosome.

T - Telophase

• The chromosomes arrive at the poles and begin to decondense (uncoil).
• New nuclear envelopes form around the two sets of chromosomes.
• The cell now has two identical nuclei.

3.3 Cytokinesis

This is the final step: the physical division of the cytoplasm, which happens concurrently with Telophase. This results in two separate, genetically identical daughter cells.

3.4 Importance and Regulation

• Importance of Mitosis: Growth, repair/replacement of damaged tissue, and asexual reproduction.
• Regulation: The cell cycle is controlled by checkpoints (e.g., G1, G2, M). These ensure the cell is ready to proceed. If regulatory genes (like tumour suppressor genes) mutate, uncontrolled division occurs, leading to tumour formation or cancer.

Section 4: Differentiation and Development

How do genetically identical cells created through mitosis become so different? This is achieved through differentiation and controlled gene expression.

4.1 Cell Specialisation and Gene Expression

All body cells contain the same full set of DNA. Specialisation (or differentiation) is the process where cells acquire the necessary structural and functional modifications to perform a specific job (e.g., becoming a muscle cell or a nerve cell).

• Differentiation is controlled by selective gene expression. Specific genes are permanently switched on, producing the necessary proteins (like haemoglobin in red blood cells), while other genes remain switched off.
• Once a cell is fully specialised, it is usually not possible for it to revert to a less specialised state.

4.2 Stem Cells and Potency

Specialised cells are derived from unspecialised cells called stem cells. These are unique because they can self-renew (divide repeatedly) and differentiate into specialised cell types.

Levels of Potency

Potency refers to the number of different cell types a stem cell can produce.

1. Totipotent cells: Can differentiate into ANY cell type, including cells that form the placenta and embryo. (Example: The very first cells of the zygote, immediately after fertilisation.)
2. Pluripotent cells: Can differentiate into ALL cell types that make up the body, but NOT the placenta/support tissues. (Example: Embryonic stem cells.)
3. Multipotent cells: Can only differentiate into a limited number of cell types (usually within a specific tissue or organ). (Example: Adult stem cells, like those in bone marrow making various blood cells.)

Memory Trick: Think T, P, M in decreasing order of flexibility. Total > Plenty > Multiple (limited).

4.3 Gametes and Fertilisation (Start of Development)

Development starts with fertilisation—the fusion of two highly specialised gametes (sex cells).

• Gametes (sperm and egg) are haploid (n), meaning they contain half the number of chromosomes.
• When they fuse, they form a zygote, which is diploid (2n).
• The zygote immediately begins rapid cell division (mitosis) and differentiation to form an embryo. Since the initial cells can form everything, they are totipotent.

4.4 Ethical Considerations of Stem Cells

Stem cell research is vital for future medical treatments (e.g., repairing spinal injuries, treating Parkinson's). However, the source of the cells raises ethical debates.

• Embryonic Stem Cells (Pluripotent): Harvested from early embryos (often from IVF surplus). They offer the greatest potential because they can become any cell type.
  • Ethical Issue: Harvesting destroys the embryo, leading to moral concerns about the destruction of potential human life.
• Adult Stem Cells (Multipotent): Found in adult tissues (e.g., bone marrow). They are more limited in potential but can still be used for treatments.
  • Ethical Advantage: Less ethical concern, as the source is an adult donor, not an embryo.
  • Practical Disadvantage: Difficult to isolate, multiply, and less flexible than embryonic stem cells.

Keep going! Mastering these fundamental concepts about structure and division will make the rest of your studies in Genetics and Physiology much easier.