1.

Introduction to Cell Biology

Cell biology, also known as cytology, is the branch of biology that studies the structure, function, and
behavior of cells. Cells are the basic building blocks of all living organisms, from the simplest bacteria
to complex multicellular organisms like humans.

The study of cells allows us to understand how life functions at the microscopic level, how organisms
grow, reproduce, and maintain homeostasis, and how diseases affect the body.

Key historical milestones:

 1665: Robert Hooke coined the term “cell” when observing cork under a microscope.

 1839: Matthias Schleiden and Theodor Schwann formulated the Cell Theory.

 1855: Rudolf Virchow added the principle "Omnis cellula e cellula" (“all cells come from pre-
existing cells”).

Cell Theory main points:

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

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

3. All cells arise from pre-existing cells.

2. Types of Cells

Cells can be broadly categorized into two main types:

2.1 Prokaryotic Cells

 Found in Bacteria and Archaea.

 Key features:

o No membrane-bound nucleus; DNA is located in a region called the nucleoid.

o Lack membrane-bound organelles.

o Usually smaller (1–10 μm) and structurally simpler.

o Often have a cell wall, sometimes with a capsule for protection.

o Reproduce asexually by binary fission.

2.2 Eukaryotic Cells

 Found in Protists, Fungi, Plants, and Animals.

 Key features:

o DNA enclosed within a membrane-bound nucleus.

o Contain membrane-bound organelles (mitochondria, endoplasmic reticulum, etc.).

o Larger (10–100 μm) and more complex.

 o Can reproduce sexually (meiosis) or asexually (mitosis).

3. Cell Structures and Organelles

Eukaryotic cells have specialized compartments that perform unique functions.

3.1 Plasma Membrane

 Structure: Phospholipid bilayer with embedded proteins.

 Functions:

o Controls the movement of substances in and out of the cell.

o Facilitates cell signaling and adhesion.

3.2 Nucleus

 Structure: Surrounded by a double membrane (nuclear envelope) with pores.

 Contains chromatin (DNA + proteins) and the nucleolus (site of ribosome synthesis).

 Function: Stores genetic information and controls cell activities.

3.3 Mitochondria

 Known as the “powerhouse” of the cell.

 Double membrane; inner membrane folded into cristae.

 Site of aerobic respiration and ATP production.

3.4 Endoplasmic Reticulum (ER)

 Rough ER: Studded with ribosomes; synthesizes proteins.

 Smooth ER: No ribosomes; synthesizes lipids and detoxifies certain chemicals.

3.5 Golgi Apparatus

 Stack of flattened sacs.

 Modifies, sorts, and packages proteins and lipids for storage or transport.

3.6 Lysosomes

 Contain digestive enzymes to break down waste and macromolecules.

 Important in apoptosis (programmed cell death).

3.7 Peroxisomes

 Contain enzymes that break down fatty acids and detoxify harmful substances.

3.8 Cytoskeleton

 Network of protein filaments (microfilaments, intermediate filaments, microtubules).

 Maintains cell shape, enables movement, and organizes organelles.

3.9 Chloroplasts (in plants and algae)

 Double membrane; contain chlorophyll.

 Site of photosynthesis.

4. Cell Membrane Transport

Cells must exchange materials with their environment. Transport mechanisms include:

4.1 Passive Transport

 Diffusion: Movement of molecules from high to low concentration.

 Osmosis: Diffusion of water through a selectively permeable membrane.

 Facilitated Diffusion: Movement through protein channels/carriers without energy.

4.2 Active Transport

 Movement of molecules against their concentration gradient using energy (ATP).

 Example: Sodium-potassium pump.

4.3 Bulk Transport

 Endocytosis: Cell takes in materials by forming vesicles.

o Phagocytosis (“cell eating”).

o Pinocytosis (“cell drinking”).

 Exocytosis: Vesicles fuse with the plasma membrane to release materials.

5. Energy in the Cell

Energy is vital for all cell processes.

5.1 ATP – Adenosine Triphosphate

 Universal energy currency.

 Produced mainly by cellular respiration in mitochondria.

5.2 Cellular Respiration

 Aerobic respiration: Glucose + O₂ → CO₂ + H₂O + ATP.

 Anaerobic respiration: Occurs without oxygen; produces less ATP.

5.3 Photosynthesis (in plants)

 Occurs in chloroplasts: CO₂ + H₂O + light → glucose + O₂.

6. The Cell Cycle and Division

Cells reproduce to maintain life and enable growth.

6.1 Cell Cycle Stages

1. G₁ phase: Cell growth.

2. S phase: DNA replication.

3. G₂ phase: Preparation for division.

4. M phase: Mitosis (or meiosis in gametes) and cytokinesis.

6.2 Mitosis

 Produces two identical daughter cells.

 Stages:

o Prophase

o Metaphase

o Anaphase

o Telophase

6.3 Meiosis

 Produces four genetically unique gametes.

 Reduces chromosome number by half.

7. Cell Communication

Cells must coordinate their activities.

7.1 Types of Signaling

 Autocrine: Signals affect the same cell that releases them.

 Paracrine: Local signaling between nearby cells.

 Endocrine: Hormones travel through the bloodstream to target cells.

 Synaptic: Neurotransmitters transmit signals between neurons.

7.2 Signal Transduction

 Binding of signaling molecules to receptors triggers a cascade of cellular responses.

8. Specialized Cells and Differentiation

 Cells in multicellular organisms differentiate to perform specific roles.

 Example:

o Neurons transmit electrical impulses.

 o Red blood cells carry oxygen.

o Muscle cells contract.

9. Stem Cells

 Embryonic stem cells: Can become any cell type.

 Adult stem cells: Limited differentiation potential.

 Applications in regenerative medicine and research.

10. Cell Death

 Apoptosis: Programmed cell death; essential for development and homeostasis.

 Necrosis: Unplanned cell death due to injury.

11. Laboratory Techniques in Cell Biology

 Light Microscopy: Observing live or stained cells.

 Electron Microscopy: Detailed ultrastructure.

 Cell Fractionation: Separating organelles.

 Flow Cytometry: Analyzing cell populations.

 Cell Culture: Growing cells in artificial environments.

12. Diseases Related to Cell Biology

 Cancer: Uncontrolled cell division due to mutations.

 Mitochondrial disorders: Defects in energy production.

 Genetic disorders: Due to mutations affecting cellular function.

13. Summary Table of Key Organelles and Functions

Organelle Function

Nucleus Stores DNA, controls cell activities

Mitochondria Produces ATP via respiration

Rough ER Protein synthesis

Smooth ER Lipid synthesis, detoxification

Golgi Apparatus Modifies and packages proteins/lipids

Organelle Function

Lysosomes Digests waste

Peroxisomes Break down fatty acids, detoxify substances

Cytoskeleton Maintains shape, enables movement

Chloroplast Photosynthesis (plants/algae)

14. Conclusion

Cell biology underpins all of modern biology and medicine. Understanding the cell’s structures,
processes, and interactions allows scientists to develop treatments for diseases, improve agriculture,
and explore the origins of life. As technology advances, so too will our understanding of the most
fundamental unit of life.