Cell Structure and Function: Foundations of General Biology

Cell Theory and the Nature of Cells

Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells, the basic unit of life. It provides the foundation for understanding the structure and function of all living organisms.

• Definition: The cell theory states that all living things are composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.

• Three Parts of Cell Theory:

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

  2. The cell is the basic unit of structure and organization in organisms.

  3. Cells arise only from pre-existing cells.

• Applications: Artificial cells containing biological components are used in medical research, such as encapsulated cells for diabetes mellitus, stem cell therapies, and encapsulation of drugs or microorganisms.

Types of Cells: Prokaryotic and Eukaryotic

Classification of Cells

Cells are classified into two main types based on their structural characteristics: prokaryotic and eukaryotic.

• Prokaryotic Cells: Do not contain membrane-bound organelles. Typically smaller (1–10 μm). Example: Bacteria.

• Eukaryotic Cells: Contain membrane-bound organelles, including a nucleus. Typically larger (10–100 μm). Example: Animal and plant cells.

• Kingdoms: Prokaryotes include Bacteria and Archaea; Eukaryotes include Protista, Fungi, Plantae, and Animalia.

• Single-celled organisms: Many prokaryotes and some eukaryotes (e.g., protists).

• Multi-celled organisms: Most eukaryotes (e.g., plants, animals, fungi).

Cell Size and Surface Area

Cells are generally small because a high surface area-to-volume ratio is essential for efficient exchange of materials with the environment.

• Reason for Small Size: Larger organisms have more cells, not larger cells.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the rate of exchange.

• Example: Microvilli in intestinal cells increase surface area for absorption.

Cell Structure and Components

Basic Structures in All Cells

Despite their diversity, all cells share certain fundamental structures.

• Plasma Membrane: Encloses the cell, controlling the movement of substances in and out.

• Cytoplasm: The jelly-like substance within the cell, containing organelles and cytosol.

• Genetic Material: DNA, which carries hereditary information.

• Ribosomes: Sites of protein synthesis.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic and eukaryotic cells differ in several key ways.

• Prokaryotic Cells:

  • No nucleus; DNA is in the nucleoid region.

  • No membrane-bound organelles.

  • Cell wall present (in most).

  • Smaller and simpler structure.

• Eukaryotic Cells:

  • DNA enclosed within a nucleus.

  • Contains membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Larger and more complex.

Feature	Prokaryotic	Eukaryotic
Nucleus	No	Yes
Organelles	No	Yes
Size	1–10 μm	10–100 μm
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

The Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining the internal environment and mediating communication with the external environment.

• Main Components: Phospholipids, proteins, cholesterol, carbohydrates.

• Functions:

  1. Physical isolation of the cell interior.

  2. Regulation of exchange with the environment.

  3. Sensitivity to the environment (receptors).

  4. Structural support.

  5. Identification (cell recognition).

• Fluid Mosaic Model: Describes the membrane as a fluid structure with a mosaic of proteins embedded in or attached to a bilayer of phospholipids.

• Semi-permeable: Allows some substances to cross more easily than others.

Phospholipid Bilayer

The plasma membrane consists of a double layer of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Hydrophilic heads: Attracted to water.

• Hydrophobic tails: Repelled by water.

• Function: Creates a stable barrier between two aqueous compartments.

Membrane Fluidity

Membrane fluidity is influenced by lipid composition and temperature.

• Unsaturated fatty acids: Increase fluidity due to kinks in the tails.

• Saturated fatty acids: Decrease fluidity, making the membrane more rigid.

• Cholesterol: Acts as a fluidity buffer, stabilizing the membrane at different temperatures.

Membrane Proteins

Types of Membrane Proteins

Proteins embedded in the plasma membrane perform a variety of functions.

• Integral Proteins: Span the membrane; involved in transport and signaling.

• Peripheral Proteins: Attached to the membrane surface; involved in signaling and maintaining cell shape.

• Transmembrane Proteins: A type of integral protein that extends across the bilayer.

Functions of Membrane Proteins

• Transport of molecules across the membrane.

• Enzymatic activity.

• Signal transduction (receptors).

• Cell-cell recognition.

• Intercellular joining.

• Attachment to the cytoskeleton and extracellular matrix.

Cell Walls and Extracellular Structures

Plant Cell Walls

Plant cells have a rigid cell wall outside the plasma membrane, providing structural support and protection.

• Main Component: Cellulose (in plants).

• Function: Maintains cell shape, prevents excessive water uptake.

• Other Organisms: Fungi (chitin), bacteria (peptidoglycan).

Extracellular Matrix (ECM) in Animal Cells

The ECM is a network of proteins and carbohydrates outside animal cells, providing structural and biochemical support.

• Main Components: Collagen, proteoglycans, fibronectin.

• Functions: Anchors cells, transmits signals, regulates cell behavior.

Cell Junctions

Cells in multicellular organisms are connected by specialized junctions.

• Tight Junctions: Prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together.

• Gap Junctions: Allow communication between animal cells.

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

Organelles

Definition and Function

Organelles are specialized structures within eukaryotic cells, each with a specific function, usually surrounded by a lipid bilayer.

• Function: Compartmentalize cellular processes, increase efficiency of reactions, and allow for specialization.

• Examples: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, chloroplasts (in plants).

Key Vocabulary

• Surface area

• Vesicle

• Dynamic instability

• Glycolipid

• Glycoprotein

• Microvilli

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No	Yes
Membrane-bound Organelles	No	Yes
Cell Size	1–10 μm	10–100 μm
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Key Equations

• Surface Area of a Cube: $SA = 6a^2$

• Volume of a Cube: $V = a^3$

• Surface Area to Volume Ratio: $\text{SA:V ratio} = \frac{6}{a}$

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Tables and equations were inferred and formatted for study purposes.