Cell Basics and Types

Overview

This lecture provides a comprehensive introduction to the cell, the fundamental structural and functional unit of all living organisms. It covers the historical discovery of cells, their diverse types, detailed structure, key organelles, and the processes by which cells divide and reproduce. Understanding cells is essential to grasping the complexity of life, from single-celled organisms to complex multicellular beings.

Discovery and Importance of Cells

Historical Background:
In 1665, Robert Hooke made a groundbreaking observation while examining a thin slice of cork under a self-designed microscope. He noticed that the cork was composed of many tiny, box-like compartments resembling a honeycomb. Hooke coined the term "cells" for these compartments, derived from the Latin word cellula, meaning "little room." This discovery marked the first time anyone had observed that living things are made up of discrete units.
Significance:
Although Hooke observed dead plant cells (cork), his discovery laid the foundation for cell biology. Later, Antonie van Leeuwenhoek (1674) observed living cells in pond water, and Robert Brown (1831) discovered the nucleus within cells. These milestones contributed to the formulation of the Cell Theory by Schleiden and Schwann in the late 1830s, which states:
- All living organisms are composed of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells (added by Virchow in 1855).
Unicellular vs. Multicellular Organisms:
Some organisms, like Amoeba, Paramecium, and bacteria, are unicellular, meaning a single cell performs all life functions. In contrast, multicellular organisms (plants, animals, fungi) consist of many cells that specialize and work together, forming tissues and organs.

Observing Cells

Microscopic Observation:
Using a compound microscope, one can observe cells from various sources such as onion peel (plant cells) and human cheek cells (animal cells). These observations reveal that:
- Cells vary in shape and size depending on their function.
- Despite differences, cells share common structural features.
- Staining techniques (e.g., iodine, safranin, methylene blue) help visualize cell components like the nucleus and cytoplasm.
Cell Diversity:
Different cell types exist even within a single organism, such as nerve cells, muscle cells, blood cells, and reproductive cells, each adapted to perform specific functions.

Cell Structure and Main Components

Every cell, regardless of type, has three fundamental components:

Plasma Membrane (Cell Membrane):
- A flexible, selectively permeable barrier made primarily of lipids and proteins.
- Controls the entry and exit of substances, maintaining the internal environment of the cell.
- Allows passive transport (diffusion and osmosis) and active transport (energy-dependent movement).
- Plays a role in processes like endocytosis (engulfing food particles) and exocytosis (expelling waste).
Nucleus:
- The control center of the cell, enclosed by a double-layered nuclear membrane with pores.
- Contains chromosomes, which are made of DNA and proteins. DNA carries genetic information essential for inheritance and cell function.
- Directs cellular activities by regulating gene expression and protein synthesis.
- In prokaryotes, the nucleus is absent; instead, they have a nucleoid region with loosely organized genetic material.
Cytoplasm:
- The jelly-like fluid inside the cell membrane, excluding the nucleus.
- Contains various organelles suspended in cytosol, each performing specialized functions.
- Site of many metabolic reactions essential for cell survival.

Types of Cells: Prokaryotic vs. Eukaryotic

| Feature | Prokaryotic Cells | Eukaryotic Cells | |-----------------------|------------------------------------------|-------------------------------------------| | Size | Generally small (1–10 µm) | Larger (5–100 µm) | | Nucleus | No true nucleus; nucleoid region present | True nucleus enclosed by nuclear membrane | | Organelles | Lack membrane-bound organelles | Contain membrane-bound organelles | | Chromosomes | Usually a single circular chromosome | Multiple linear chromosomes | | Examples | Bacteria, Archaea | Plants, animals, fungi, protists |

Prokaryotes are simpler and more primitive, lacking compartmentalization.
Eukaryotes have complex internal structures allowing compartmentalization of functions.

Major Cell Organelles and Their Functions

1. Endoplasmic Reticulum (ER)

The endoplasmic reticulum (ER) is an extensive network of membrane-bound tubules and flattened sacs that spread throughout the cytoplasm of eukaryotic cells. It plays a crucial role in the synthesis, folding, modification, and transport of proteins and lipids.

Structure:
The ER is continuous with the outer membrane of the nuclear envelope, forming an interconnected system. It exists in two forms:
- Rough ER (RER): Characterized by the presence of ribosomes attached to its cytoplasmic surface, giving it a "rough" appearance under the microscope.
- Smooth ER (SER): Lacks ribosomes, appearing smooth.
Functions:
- Rough ER: The ribosomes on the RER are sites of protein synthesis. Newly synthesized proteins enter the lumen of the RER where they undergo folding and modifications such as glycosylation (addition of sugar groups). These proteins are then packaged into vesicles and transported to the Golgi apparatus for further processing or secretion.
- Smooth ER: Involved in lipid and steroid hormone synthesis, metabolism of carbohydrates, and detoxification of drugs and poisons (especially in liver cells). The SER also stores calcium ions important for muscle contraction and other cellular processes.
Significance:
The ER acts as a manufacturing and packaging system within the cell. It also provides a structural framework that supports the cytoplasm and facilitates intracellular transport.

2. Golgi Apparatus

The Golgi apparatus, also known as the Golgi complex or Golgi body, is a stack of flattened, membrane-bound sacs called cisternae. It is often located near the nucleus and works closely with the ER.

Structure:
The Golgi consists of several cisternae arranged in a stack. It has a distinct polarity with a cis face (receiving side) facing the ER and a trans face (shipping side) where vesicles bud off.
Functions:
- Modification: Proteins and lipids received from the ER are further modified in the Golgi. This includes processes like glycosylation, phosphorylation, and sulfation.
- Sorting and Packaging: The Golgi sorts these molecules and packages them into vesicles targeted to various destinations such as lysosomes, the plasma membrane, or secretion outside the cell.
- Formation of Lysosomes: The Golgi apparatus is responsible for producing lysosomes, which are vesicles containing digestive enzymes.
- Secretion: In secretory cells, the Golgi packages materials for exocytosis, releasing substances like hormones and enzymes outside the cell.
Significance:
The Golgi apparatus acts as the cell’s “post office,” ensuring that molecules are correctly processed, packaged, and delivered to their proper locations.

3. Lysosomes

Lysosomes are membrane-bound organelles filled with hydrolytic enzymes capable of breaking down all types of biological polymers—proteins, nucleic acids, carbohydrates, and lipids.

Structure:
Lysosomes are spherical vesicles surrounded by a single membrane that protects the rest of the cell from the potent digestive enzymes inside.
Functions:
- Intracellular Digestion: Lysosomes digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria.
- Autophagy: They help recycle the cell’s own components by breaking down damaged organelles, allowing the cell to reuse the building blocks.
- Defense: Lysosomes destroy invading pathogens that enter the cell.
- Programmed Cell Death: In some cases, lysosomes release their enzymes into the cytoplasm, leading to controlled cell death (apoptosis). This is why lysosomes are sometimes called “suicide bags.”
Significance:
Lysosomes maintain cellular health by removing waste and recycling materials, preventing accumulation of cellular debris.

4. Mitochondria

Mitochondria are double-membraned organelles often referred to as the “powerhouses” of the cell because they generate most of the cell’s supply of adenosine triphosphate (ATP), the energy currency.

Structure:
Mitochondria have an outer membrane and a highly folded inner membrane. The folds, called cristae, increase the surface area for chemical reactions. The space inside the inner membrane is called the matrix, which contains enzymes, mitochondrial DNA, and ribosomes.
Functions:
- Cellular Respiration: Mitochondria convert energy stored in glucose and other nutrients into ATP through processes like the Krebs cycle and oxidative phosphorylation.
- Regulation of Metabolic Activity: They regulate cellular metabolism and are involved in signaling, cellular differentiation, and apoptosis.
- Genetic Material: Mitochondria contain their own DNA and ribosomes, allowing them to produce some of their own proteins independently of the nucleus.
Significance:
Mitochondria provide the energy required for various cellular activities, making them essential for cell survival and function.

5. Plastids (Plant Cells Only)

Plastids are a group of membrane-bound organelles found only in plant cells and some protists. They are involved in the synthesis and storage of food.

Types and Structure:
- Chloroplasts: Contain the green pigment chlorophyll and are the sites of photosynthesis. They have a double membrane and internal stacks of thylakoids called grana, embedded in a fluid called stroma.
- Chromoplasts: Contain pigments other than chlorophyll, such as carotenoids, giving flowers and fruits their red, yellow, or orange colors.
- Leucoplasts: Colorless plastids involved in the storage of starch (amyloplasts), oils, and proteins.
Functions:
- Chloroplasts: Capture light energy and convert it into chemical energy through photosynthesis, producing glucose and oxygen.
- Chromoplasts: Attract pollinators and seed dispersers through their bright colors.
- Leucoplasts: Store essential nutrients and energy reserves.
Significance:
Plastids are vital for energy production and storage in plants, supporting growth and reproduction.

6. Vacuoles

Vacuoles are membrane-bound sacs within the cytoplasm that serve as storage compartments.

Structure:
Vacuoles vary in size and number depending on the cell type. Plant cells typically have a large central vacuole, while animal cells have smaller, more numerous vacuoles.
Functions:
- Storage: Vacuoles store nutrients, waste products, pigments, and other substances.
- Turgor Pressure: In plant cells, the central vacuole is filled with cell sap, which maintains turgidity and rigidity by exerting pressure against the cell wall. This is crucial for maintaining the plant’s structural integrity.
- Waste Disposal: Vacuoles isolate harmful materials and waste products from the rest of the cell.
- Digestion: In some unicellular organisms, vacuoles function as food vacuoles, digesting engulfed food particles.
Significance:
Vacuoles help maintain cellular homeostasis, support cell structure, and manage storage and waste disposal.

Cell Membrane Transport Mechanisms

Diffusion: Movement of molecules from high to low concentration (e.g., oxygen, carbon dioxide).
Osmosis: Diffusion of water across a selectively permeable membrane toward higher solute concentration.
Tonicity Effects:
- Hypotonic solution: Cell gains water and swells.
- Isotonic solution: No net water movement; cell size remains stable.
- Hypertonic solution: Cell loses water and shrinks.

Cell Division

Mitosis:
- Produces two genetically identical daughter cells.
- Responsible for growth, tissue repair, and asexual reproduction.
- Maintains chromosome number.
Meiosis:
- Produces four genetically diverse daughter cells with half the chromosome number.
- Occurs in reproductive organs to form gametes (sperm and eggs).
- Ensures genetic diversity through recombination and independent assortment.

Summary of Key Concepts

The cell is the smallest unit of life, capable of performing all life processes.
Cells are enclosed by a plasma membrane that regulates their internal environment.
Plant cells have a rigid cell wall that provides structural support.
Organelles within cells perform specialized functions essential for survival.
Prokaryotic and eukaryotic cells differ in complexity and organization.
Cells reproduce through mitosis and meiosis to sustain life and enable reproduction.

Suggested Activities and Further Study

Prepare and observe temporary mounts of onion peel and human cheek cells under a microscope.
Perform osmosis experiments using eggs, potatoes, or raisins to understand water movement.
Research electron microscopes to appreciate how they reveal detailed cell structures.
Compare and contrast plant and animal cells, noting differences in organelles and functions.