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Cell Theory and the Structure and Function of Cells

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Cell Theory

Foundations of 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 framework for understanding the structure and function of all living organisms.

  • All organisms are composed of one or more cells.

  • The cell is the smallest unit that exhibits all the characteristics of life.

  • All cells arise from preexisting cells.

Example: Humans, plants, and bacteria are all made up of cells, whether single-celled or multicellular.

Classification of Cells

Major Types of Cells

Organisms are grouped into two major classifications based on their cellular structure: prokaryotic and eukaryotic cells.

  • Prokaryotes: Bacteria; typically unicellular organisms.

  • Eukaryotes: Animals, plants, fungi, and protists; can be unicellular or multicellular.

Cellular Organization

Prokaryotic vs. Eukaryotic Cells

Cells are classified according to their internal organization. The main distinction is the presence or absence of a nucleus and membrane-bound organelles.

Feature

Prokaryotic Cells

Eukaryotic Cells

Plasma Membrane

Present

Present

Cytoplasm

Present

Present

Nucleus

Absent

Present (membrane-bound)

Membrane-bound Organelles

Absent

Present

Cell Wall

Present (most)

May or may not be present

Additional info: Eukaryotic cells are generally larger and more complex than prokaryotic cells.

Structure Reflects Cell Function

Specialization of Eukaryotic Cells

Although all eukaryotic cells have a nucleus, they can differ greatly in structure and function depending on their role in the organism.

  • Muscle cells: Contain many organelles (especially mitochondria) to provide energy for muscle contraction.

  • Nerve cells: Are long and thin to efficiently carry electrical impulses over distances.

Example: The elongated shape of nerve cells allows for rapid communication within the nervous system.

Cell Size and Efficiency

Surface Area to Volume Ratio

Cells are small to maximize their efficiency in exchanging materials with their environment. A higher surface area to volume ratio allows for more effective transport of nutrients and waste.

  • Small cells: Greater surface area relative to volume, facilitating efficient exchange.

  • Large cells: Lower surface area to volume ratio, which can limit efficiency.

Microscopy and the Study of Cells

Types of Microscopes

Cells are too small to be seen with the naked eye and require magnification for study. Different types of microscopes provide varying levels of detail.

  • Light Microscope: Magnifies up to 1,000x; suitable for viewing live cells and basic structures.

  • Transmission Electron Microscope (TEM): Magnifies up to 100,000x; provides detailed images of internal cell structures.

  • Scanning Electron Microscope (SEM): Magnifies up to 100,000x; provides 3D images of cell surfaces.

Plasma (Cell) Membrane

Structure and Function

The plasma membrane surrounds the cell, separating it from its environment. It is selectively permeable, allowing some substances to pass while blocking others.

  • Phospholipid bilayer: Composed of hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

  • Cholesterol: Adds rigidity to the membrane.

  • Proteins: Serve as channels, transporters, and receptors.

  • Carbohydrates: Provide recognition patterns for cells and organisms.

Additional info: The fluid mosaic model describes the dynamic nature of the plasma membrane, where lipids and proteins can move laterally within the layer.

Solubility Principle

The plasma membrane's lipid nature affects the movement of substances:

  • Lipid-soluble (hydrophobic) substances: Pass easily through the membrane (e.g., O2, CO2, fats).

  • Water-soluble (hydrophilic) substances: Do not pass easily (e.g., glucose, ions).

Transport Across the Plasma Membrane

Types of Transport Processes

Cells use various mechanisms to move substances across the plasma membrane, classified as passive or active transport.

  • Passive Transport: Does not require energy; moves substances down their concentration gradient.

  • Active Transport: Requires energy (usually ATP); moves substances against their concentration gradient.

  • Bulk Transport: Moves large molecules via vesicles (endocytosis and exocytosis).

Passive Transport

  • Simple Diffusion: Movement of small, nonpolar molecules directly through the lipid bilayer (e.g., O2, CO2).

  • Facilitated Diffusion: Movement of larger or polar molecules via protein channels or carriers (e.g., glucose, ions).

  • Osmosis: Diffusion of water across a selectively permeable membrane.

Active Transport

  • Carrier Proteins: Use energy to transport molecules against their concentration gradient.

  • Example: Sodium-potassium pump ( out, in per ATP hydrolyzed).

Bulk Transport

  • Endocytosis: Brings substances into the cell via vesicles.

  • Exocytosis: Expels substances from the cell via vesicles.

Nucleus

Control Center of the Cell

The nucleus contains the cell's genetic material (DNA) and directs all cellular activities.

  • Nuclear envelope: Double membrane with pores for molecular exchange.

  • Chromosomes: DNA-protein complexes carrying genetic information.

  • Nucleolus: Site of ribosome synthesis.

Cytoplasm and Organelles

Major Organelles and Their Functions

  • Ribosomes: Sites of protein synthesis; can be free in cytoplasm or attached to endoplasmic reticulum.

  • Endoplasmic Reticulum (ER): Network for synthesis and transport of proteins and lipids.

    • Rough ER: Studded with ribosomes; synthesizes proteins for export.

    • Smooth ER: Lacks ribosomes; synthesizes lipids.

  • Golgi Apparatus: Modifies, packages, and ships proteins and lipids to their final destinations.

  • Mitochondria: "Power plants" of the cell; site of cellular respiration and ATP production.

Mitochondria and Cellular Respiration

Energy Production

Mitochondria generate ATP, the cell's main energy currency, through cellular respiration. This process involves breaking down glucose and other nutrients to release energy.

  • ATP (Adenosine Triphosphate): Stores and provides energy for cellular processes.

  • Cellular Respiration Stages:

    • Glycolysis (in cytoplasm): Breaks down glucose; yields ATP and pyruvate.

    • Krebs Cycle (in mitochondria): Processes pyruvate; releases CO2 and high-energy electrons.

    • Electron Transport Chain (in mitochondria): Uses electrons to produce large amounts of ATP.

Key Equation:

Example: Muscle cells have many mitochondria to meet high energy demands during contraction.

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