Cell Theory and Historical Contributions

Contributions of Early Scientists

The development of cell theory was made possible by the work of several scientists:

• Anton van Leeuwenhoek: First to observe living cells (microorganisms) using a microscope.

• Robert Hooke: Coined the term "cell" after observing cork tissue.

• Matthias Schleiden: Proposed that all plants are made of cells.

• Theodor Schwann: Proposed that all animals are made of cells.

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

• Robert Brown: Discovered the cell nucleus.

Cell Theory

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

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

• All cells arise from pre-existing cells.

Cell Structure and Function

Plasma Membrane vs. Cell Wall

• Plasma Membrane: A selectively permeable phospholipid bilayer that surrounds the cytoplasm of all cells, controlling the movement of substances in and out.

• Cell Wall: A rigid outer layer found in plants, fungi, and some prokaryotes, providing structural support and protection. Not present in animal cells.

• Comparison: The plasma membrane is flexible and present in all cells, while the cell wall is rigid and only in certain organisms.

Cytoplasm and Cytosol

• Cytoplasm: The entire contents within the cell membrane, excluding the nucleus in eukaryotes; includes cytosol and organelles.

• Cytosol: The fluid portion of the cytoplasm, where many metabolic reactions occur.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region (e.g., bacteria, archaea).

• Eukaryotic Cells: Have a true nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists).

Phospholipid Characteristics

• Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• This amphipathic nature allows them to form bilayers, creating a semi-permeable membrane.

Fluid-Mosaic Model

• The plasma membrane is a fluid structure with a "mosaic" of proteins embedded in or attached to a double layer of phospholipids.

• Proteins and lipids can move laterally within the layer, contributing to membrane fluidity.

Extracellular Matrix and Cell Junctions

• Extracellular Matrix (ECM): A network of proteins and carbohydrates outside animal cells that provides structural support and mediates cell signaling.

• Cell Junctions: Structures that connect cells to one another (e.g., tight junctions, desmosomes, gap junctions in animals; plasmodesmata in plants).

Nucleus Structure and Function

• Nuclear Envelope: Double membrane that encloses the nucleus, containing nuclear pores for transport.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Ribosomes

• Function: Synthesize proteins by translating mRNA.

• Location: Free in cytosol or bound to rough endoplasmic reticulum (ER).

DNA and Protein Production

• DNA contains genetic instructions for protein synthesis.

• Transcription (DNA to RNA) occurs in the nucleus; translation (RNA to protein) occurs at ribosomes.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes and processes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals, stores calcium ions.

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Lysosomes

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

Vacuoles

• Storage organelles; large central vacuole in plant cells maintains turgor pressure and stores nutrients/waste.

Chloroplasts

• Site of photosynthesis in plant and algal cells; contain chlorophyll pigment.

Mitochondria

• Powerhouse of the cell; site of cellular respiration and ATP production.

Cytoskeleton and Microtubules

• Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support, cell shape, and movement.

Flagella and Cilia

• Both are hair-like structures for movement; flagella are longer and usually singular, cilia are shorter and numerous.

Bioenergetics and Metabolism

Energy Forms and Usage

• Energy exists as kinetic, potential, chemical, thermal, and more.

• Cells primarily use chemical energy stored in molecules like glucose and ATP.

Kinetic vs. Potential Energy

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy due to position or structure.

First and Second Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed (conservation of energy).

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

Entropy

• A measure of disorder or randomness in a system.

One-way Flow of Energy

• Energy flows through ecosystems in one direction: from sunlight to chemical energy (via photosynthesis) to heat.

Calorie vs. calorie

• calorie (lowercase "c"): Amount of energy needed to raise 1 gram of water by 1°C.

• Calorie (uppercase "C"): Also known as a kilocalorie (kcal); equals 1,000 calories.

Metabolism

• All chemical reactions in a cell, including catabolism (breaking down molecules) and anabolism (building molecules).

ATP and Enzymes

ATP Function

• Adenosine triphosphate (ATP) is the main energy currency of the cell.

• Provides energy for cellular processes by hydrolysis of its phosphate bonds.

Phosphorylation and ATP/ADP Cycle

• Phosphorylation: Addition of a phosphate group to a molecule, often activating or deactivating it.

• ATP is converted to ADP (adenosine diphosphate) when a phosphate is removed, releasing energy; ADP can be recharged to ATP.

Enzymes

• Biological catalysts that speed up chemical reactions without being consumed.

• Features: Specificity, efficiency, regulation, and ability to lower activation energy.

Active Site and Allosteric Site

• Active Site: Region on the enzyme where the substrate binds and the reaction occurs.

• Allosteric Site: Site other than the active site where molecules can bind and regulate enzyme activity.

Activation Energy

• The minimum energy required to start a chemical reaction.

• Enzymes lower the activation energy needed for reactions.

Enzyme Activity and Environmental Factors

• Temperature and pH can affect enzyme structure and function; each enzyme has optimal conditions.

Enzyme Inhibition

• Feedback Inhibition: End product of a pathway inhibits an earlier step, regulating metabolic pathways.

• Competitive Inhibition: Inhibitor competes with substrate for the active site.

• Noncompetitive Inhibition: Inhibitor binds to an allosteric site, changing enzyme shape and function.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Key Equations

• First Law of Thermodynamics: Where is the change in internal energy, is heat, and is work.

• ATP Hydrolysis: