Unit 3: The Cell – Anatomy and Physiology

Early Discoveries

Historical Background of Cell Biology

The study of cells began with the invention of the microscope, leading to foundational discoveries in biology.

• Robert Hooke (1665): Used a microscope to examine a thin slice of cork. He observed small, box-like structures and named them "cells" after the rooms in a monastery.

• Anton van Leeuwenhoek: A Dutch scientist who improved the microscope and was the first to observe living microorganisms in pond water and dental scrapings.

Example: Hooke's cork cells were actually the cell walls of dead plant cells, while Leeuwenhoek observed living single-celled organisms (now known as protists and bacteria).

Discoveries Continued

Development of Cell Theory

Further research in the 19th century led to the formulation of the cell theory by several scientists:

• Matthias Schleiden: A botanist who concluded that all plants are made of cells.

• Theodor Schwann: A zoologist who determined that all animals are made of cells.

• Rudolph Virchow: A physician who proposed that all cells arise from pre-existing cells through cell division.

Additional info: Schleiden and Schwann are credited as co-founders of the cell theory.

Cell Theory

Core Principles of Cell Theory

The cell theory is a fundamental concept in biology, describing the properties of cells:

• All living things are composed of one or more cells.

• The cell is the basic unit of structure and function in living organisms.

• All cells arise from pre-existing cells by cell division.

Example: Both unicellular organisms (like bacteria) and multicellular organisms (like plants and animals) follow these principles.

But What About Viruses?

Viruses and the Definition of Life

Viruses challenge the boundaries of the cell theory because they exhibit some, but not all, characteristics of life.

• Viruses are nonliving particles that cannot maintain homeostasis or reproduce independently.

• They consist of an inner core of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid), and sometimes a lipid envelope.

• Viruses require a host cell to replicate, using the host's cellular machinery.

Comparison Table: Living Cells vs. Viruses

Property	Living Cells	Viruses
Cellular Structure	Yes	No
Metabolism	Yes	No
Reproduction	Independent	Only in host
Genetic Material	DNA (and RNA in some)	DNA or RNA

Cell Size

Surface Area to Volume Ratio

Cell size is limited by the relationship between surface area and volume, which affects the efficiency of material exchange.

• Surface Area: The area of the cell's plasma membrane, calculated as length × width (for a rectangular cell).

• Volume: The space inside the cell, calculated as length × width × height.

• As a cell grows, its volume increases faster than its surface area, limiting the rate of exchange with the environment.

Formula:

• Surface Area: $SA = L \times W$

• Volume: $V = L \times W \times H$

Additional info: Cells remain small to maximize their surface area-to-volume ratio, ensuring efficient transport of materials.

The Cell: An Overview

Basic Cell Types and Structures

Cells are classified into two main types based on their internal organization:

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in a nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus and membrane-bound organelles. Examples: Plants, animals, fungi, and protists.

All cells share three basic structures:

• Nucleus (or nucleoid region in prokaryotes)

• Cell membrane

• Cytoplasm

Types of Cells

Prokaryotes vs. Eukaryotes

• Prokaryotes:

  • No membrane-bound nucleus

  • Simple structure

  • Cell wall present

• Eukaryotes:

  • Membrane-bound nucleus

  • Complex structure with organelles

  • Cell wall present in plants and fungi, absent in animals

Cellular Organelles and Their Functions

Nucleus

• Nucleolus: Dense region inside the nucleus; site of ribosome synthesis.

• Nuclear Envelope: Double membrane surrounding the nucleus; contains nuclear pores for material exchange; continuous with the rough endoplasmic reticulum (ER).

Golgi Apparatus (Golgi Body)

• Stack of flattened membranes that modify, sort, and package proteins and lipids for storage or transport out of the cell.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

Lysosomes

• Membrane-bound organelles containing digestive enzymes; break down waste, cellular debris, and foreign invaders.

Mitochondria

• Site of cellular respiration; converts glucose and oxygen into ATP (energy).

• Contains its own DNA; often called the "powerhouse of the cell."

Chloroplasts

• Found in plant cells and some protists; site of photosynthesis.

• Contains chlorophyll and internal membrane structures called thylakoids.

Ribosomes

• Small structures made of protein and rRNA; site of protein synthesis.

• Can be free in the cytoplasm or attached to the rough ER.

Cilia and Flagella

• Hair-like structures that aid in cell movement.

• Cilia: Short and numerous; move substances along the cell surface.

• Flagella: Longer and fewer; propel the cell through its environment.

Centrioles

• Found in animal cells; involved in organizing microtubules during cell division (mitosis and meiosis).

Vacuoles

• Membrane-bound sacs for storage of water, nutrients, and waste.

• Large central vacuole in plant cells helps maintain turgor pressure.

Cytoplasm, Cytoskeleton, and Cell Wall

• Cytoplasm: Jelly-like fluid that fills the cell and surrounds organelles.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments) that provide structural support and aid in movement.

• Cell Wall: Rigid outer layer found in plants, fungi, and some prokaryotes; provides support and protection.

Cell (Plasma) Membrane

• Flexible boundary composed of a phospholipid bilayer with embedded proteins.

• Regulates the movement of substances into and out of the cell.

• Maintains homeostasis by selective permeability.

Additional info: The cell membrane is often described by the fluid mosaic model, which highlights the dynamic arrangement of lipids and proteins.