Cell Structure and Function: General Biology Study Notes

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Cell Structure and Function

Introduction to Cell Theory

The cell is the fundamental unit of life in all living organisms. Cell theory states that all living things are composed of cells, and that cells are the basic structural and functional units of life. Cells carry out essential processes that sustain life, including metabolism, growth, and reproduction.

Cell Theory: All organisms are made of cells; cells are the basic unit of structure and function in living things.
Levels of Organization: Cells → Tissues → Organs → Organ Systems → Organisms
Metabolism: The sum of all chemical reactions in the cell that provide energy and synthesize new materials.
Example: Muscle cells contract to enable movement, while nerve cells transmit signals.

Types of Cells

Cells are classified into two main types based on their structure and complexity: prokaryotic and eukaryotic cells.

Prokaryotic Cells: Simpler cells without a nucleus or membrane-bound organelles. Examples include Bacteria and Archaea.
Eukaryotic Cells: More complex cells with a nucleus and membrane-bound organelles. Examples include Protists, Fungi, Animals, and Plants.

Basic Features of All Cells

Despite their diversity, all cells share certain fundamental features:

Plasma Membrane: A selective barrier that encloses the cell and regulates the movement of substances in and out.
Cytosol: The semifluid, jellylike substance inside the cell where subcellular components are suspended.
Chromosomes: Structures that carry genetic information in the form of DNA.
Ribosomes: Complexes that synthesize proteins according to genetic instructions.

Viruses, Viroids, and Prions

Non-Cellular Infectious Agents

Some biological entities are not considered living cells but can infect living organisms and cause disease.

Viruses: Composed of nucleic acid (DNA or RNA) enclosed in a protein coat. They require host cells for replication.
Virion: The complete, infectious virus particle.
Viroids: Small, circular RNA molecules lacking a protein coat; infect plants.
Virusoids: Circular, single-stranded RNAs dependent on viruses for replication and encapsidation.
Prions: Infectious proteins that cause neurodegenerative diseases.
Example: The influenza virus infects human respiratory cells, while prions cause diseases like Creutzfeldt-Jakob disease.

Cellular Structures and Their Functions

Plasma Membrane

The plasma membrane is a dynamic structure that separates the cell from its environment and controls the passage of materials.

Structure: Composed of a phospholipid bilayer with embedded proteins.
Function: Maintains homeostasis by regulating transport, communication, and cell recognition.
Amphipathic Nature: Phospholipids have hydrophilic heads and hydrophobic tails, allowing formation of bilayers in aqueous environments.
Example: Transport proteins in the membrane facilitate movement of ions and nutrients.

Cytosol and Cytoplasm

The cytosol is the fluid component of the cytoplasm, where many metabolic reactions occur.

Cytosol: Contains water, ions, proteins, and other molecules.
Cytoplasm: Includes the cytosol and all organelles except the nucleus.

Chromosomes and Ribosomes

Genetic information and protein synthesis are central to cell function.

Chromosomes: DNA molecules that carry genes; prokaryotes have a single circular chromosome, eukaryotes have multiple linear chromosomes.
Ribosomes: Sites of protein synthesis; prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S).
Example: Ribosomes translate mRNA into polypeptides during gene expression.

Microscopy and Practical Work

Bright-Field Microscopy

Microscopy is essential for studying cell structure. Bright-field microscopy uses light to visualize stained tissues and cells.

Components: Eyepiece, objective lens, stage, adjustment knobs.
Magnification Calculation: Total magnification = eyepiece magnification × objective lens magnification.

Eyepiece Lens	Objective Lens	Overall Magnification
10×	4×	40×
10×	10×	100×
10×	40×	400×
10×	100×	1000×

Comparison of Cell Types

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on the presence or absence of a nucleus and other organelles.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	No membrane-bound organelles	Membrane-bound organelles
Chromosomes	Single, circular DNA	Multiple, linear DNA
Ribosomes	70S (smaller)	80S (larger)
Cell Wall	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi)

Test Yourself: Sample Questions

Which structures are found in prokaryotes? Answer: Cell wall containing peptidoglycan, ribosomes, plasma membrane.
How many layers of phospholipids are in the nuclear envelope? Answer: Two (bilayer).
What is the function of ribosomes? Answer: Protein synthesis.
Calculate the magnification power of a cell using a 10× eyepiece and a 40× objective lens. Answer: 400×.

Summary Table: Achievement vs. Inventory Concepts

Achievement Concepts	Inventory Concepts
Communication (N.S.), Muscular system, Skeletal system, Hormonal coordination, Nutrition, Immunity	Cell structure, Cellular transport, Nucleic acids, Protein synthesis, Plant tissues, Photosynthesis, Cellular respiration, Genetics

Key Terms and Definitions

Plasma Membrane: The boundary of the cell, composed of a phospholipid bilayer.
Cytosol: The fluid portion of the cytoplasm.
Chromosome: DNA-containing structure carrying genetic information.
Ribosome: Organelle responsible for protein synthesis.
Prokaryote: Cell lacking a nucleus and membrane-bound organelles.
Eukaryote: Cell with a nucleus and membrane-bound organelles.
Virus: Infectious agent composed of nucleic acid and protein coat.
Viroid: Infectious RNA molecule without a protein coat.
Prion: Infectious protein molecule.

Important Equations

Magnification Calculation:

*Additional info: Academic context and definitions have been expanded for clarity and completeness.*