Introduction to the Cell

Historical Development of Cell Theory

The discovery and study of cells were made possible by advances in microscopy. Early scientists such as Robert Hooke, Anton van Leeuwenhoek, Matthias Schleiden, Theodor Schwann, and Rudolf Virchow contributed to the development of cell theory, which states that all living things are composed of cells, all cells come from pre-existing cells, and the cell is the basic unit of structure and function in organisms.

• Robert Hooke (1665): First observed cells in cork using a compound microscope.

• Anton van Leeuwenhoek: Improved the microscope and observed living cells in pond water and bacteria.

• Matthias Schleiden & Theodor Schwann: Proposed that all plants and animals are made of cells.

• Rudolf Virchow: Stated that all cells arise from pre-existing cells.

Microscopy and Cell Structure

Types of Microscopes

Microscopes are essential tools for studying cells. The main types include:

• Light Microscope (LM): Uses visible light to observe living cells. Magnification up to ~1000x, resolution ~0.2 μm.

• Electron Microscopes (EM): Use electron beams for much higher resolution.

  • Transmission Electron Microscope (TEM): Reveals internal cell structures by transmitting electrons through the specimen.

  • Scanning Electron Microscope (SEM): Provides detailed images of cell surfaces by scanning the specimen with electrons.

Key Terms:

• Magnification: The increase in an object's image size compared to its actual size.

• Resolution: The clarity of an image; the ability to distinguish two close objects as separate.

Cell Theory

• All living things are composed of cells.

• All cells come from other cells.

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

Cell Size and the Plasma Membrane

Surface Area-to-Volume Ratio

The small size of cells is crucial for efficient exchange of materials across the plasma membrane. As a cell increases in size, its volume grows faster than its surface area, limiting the rate of exchange.

• Plasma Membrane: A phospholipid bilayer with embedded proteins that regulates the movement of substances in and out of the cell.

• Proteins in the membrane act as channels or pumps for transport.

Prokaryotic vs. Eukaryotic Cells

Basic Features of All Cells

• Plasma membrane

• DNA (genetic material)

• Ribosomes (protein synthesis)

• Cytosol (fluid interior)

Prokaryotic Cells

• Domains: Bacteria and Archaea

• No membrane-bound nucleus or organelles

• Generally smaller and simpler

Eukaryotic Cells

• Domain: Eukarya (includes plants, animals, fungi, protists)

• Membrane-bound nucleus and organelles

• Compartmentalization allows specialized functions

Functional Compartments in Eukaryotic Cells

Four Main Functional Groups

1. Genetic Control: Nucleus and ribosomes

2. Manufacture, Distribution, Breakdown: Endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes

3. Energy Processing: Mitochondria (all cells), chloroplasts (plants and algae)

4. Structural Support, Movement, Communication: Cytoskeleton, plasma membrane, cell wall (plants)

The Nucleus and Ribosomes

Nucleus

The nucleus contains the cell’s DNA and directs protein synthesis via messenger RNA (mRNA). The nucleolus within the nucleus assembles ribosome subunits.

Ribosomes

• Composed of ribosomal RNA and proteins

• Synthesize proteins according to genetic instructions

• Can be free in the cytosol or bound to the endoplasmic reticulum

The Endomembrane System

Components and Functions

• Endoplasmic Reticulum (ER): Network of membranes; smooth ER synthesizes lipids and detoxifies, rough ER produces proteins and membranes.

• Golgi Apparatus: Modifies, sorts, and ships products from the ER to other destinations.

• Lysosomes: Contain digestive enzymes to break down macromolecules and recycle cell components.

• Vacuoles: Large vesicles for storage and maintenance; central vacuole in plants stores water and nutrients.

• Peroxisomes: Metabolic compartments that break down fatty acids and detoxify harmful substances (not part of the endomembrane system).

Energy-Converting Organelles

Mitochondria

• Site of cellular respiration, converting chemical energy from food into ATP.

• Structure: Outer membrane, inner membrane, intermembrane space, and matrix (contains DNA, ribosomes, enzymes).

Chloroplasts

• Site of photosynthesis in plants and algae, converting solar energy to chemical energy (sugars).

• Structure: Double membrane, internal thylakoid membranes (site of light reactions).

Endosymbiont Theory

Mitochondria and chloroplasts likely evolved from free-living prokaryotes that were engulfed by ancestral eukaryotic cells, forming a symbiotic relationship.

The Cytoskeleton and Cell Surfaces

Cytoskeleton

• Microfilaments: Actin filaments; support cell shape, involved in movement.

• Intermediate Filaments: Reinforce cell shape, anchor organelles.

• Microtubules: Hollow tubes; shape the cell, guide organelle movement, separate chromosomes during cell division.

Cilia and Flagella

• Locomotor appendages made of microtubules in a "9+2" arrangement.

• Flagella: Long, whip-like; Cilia: Short, numerous, coordinated movement.

Extracellular Matrix (ECM) and Cell Junctions

• ECM: Network of glycoproteins and other molecules outside animal cells; provides support, adhesion, and communication.

• Cell Junctions:

  • Tight Junctions: Seal cells together in a sheet.

  • Anchoring Junctions (Desmosomes): Fasten cells into strong tissues.

  • Gap Junctions: Allow communication between cells.

Plant Cell Walls and Plasmodesmata

• Cell Wall: Rigid structure made of cellulose; provides support and protection.

• Plasmodesmata: Channels between plant cells for transport and communication.

Summary Table: Types of Microscopes and Their Uses

Key Equations

• Surface Area of a Cube: $SA = 6a^2$

• Volume of a Cube: $V = a^3$

• Surface Area-to-Volume Ratio: $\text{Ratio} = \frac{6a^2}{a^3} = \frac{6}{a}$

Additional info: As cell size increases, the surface area-to-volume ratio decreases, limiting the efficiency of material exchange.

Summary of Eukaryotic Cell Structures and Functions