Cell Structure and Function

Overview of Cell Structure

Cells are the basic unit of life, and all living organisms are composed of one or more cells. The structure and organization of cells are fundamental characteristics that distinguish living things from non-living matter.

• Cell Theory: States that all living things are composed of cells, cells are the basic unit of structure and function in living organisms, and all cells arise from pre-existing cells.

• Hierarchy of Organization: Biological organization proceeds from atoms & molecules, to organelles, cells, tissues, organs, organ systems, and finally the organism.

Two Types of Cells:

• Prokaryotic Cells: Simple cells without a nucleus or membrane-bound organelles. Example: Bacteria.

• Eukaryotic Cells: Complex cells with a nucleus and membrane-bound organelles. Examples: Plant, animal, fungal, and protist cells.

Prokaryotic Cell Structure

Major Components of a Prokaryotic Cell

Prokaryotic cells, such as bacteria, have a simpler structure compared to eukaryotic cells. They lack a nucleus and most organelles but possess specialized structures for survival and reproduction.

• Cell Membrane: Semi-permeable barrier controlling entry and exit of substances.

• Cell Wall: Provides structural support and protection.

• Cytoplasm: Gel-like substance where cellular processes occur.

• Chromosome: Single, circular DNA molecule containing genetic information.

• Plasmid: Small, circular DNA molecules that can carry additional genes.

• Ribosomes: Sites of protein synthesis.

• Pili: Hair-like structures for attachment and genetic exchange.

• Capsule: Outer protective layer aiding in defense and adherence.

• Endospore: Dormant, tough structure for survival in harsh conditions.

• Flagellum: Tail-like structure for movement.

Example: Escherichia coli is a common prokaryotic cell studied in biology.

Eukaryotic Cell Structure

Animal and Plant Cells

Eukaryotic cells are more complex and contain a variety of organelles that perform specialized functions. Animal and plant cells share many organelles but also have unique structures.

• Nucleus: Contains genetic material (DNA) and controls cell activities.

• Endoplasmic Reticulum (ER): Network for protein and lipid synthesis. Rough ER has ribosomes; Smooth ER does not.

• Golgi Complex: Modifies, sorts, and packages proteins and lipids for transport.

• Mitochondria: Site of cellular respiration and ATP production.

• Lysosomes: Contain digestive enzymes to break down waste.

• Vacuole: Storage organelle, larger in plant cells.

• Chloroplasts: (Plant cells only) Site of photosynthesis.

• Cell Wall: (Plant cells only) Provides structure and protection.

• Plasma Membrane: Controls movement of substances in and out of the cell.

Example: Arabidopsis thaliana (plant cell), Homo sapiens (animal cell).

Comparison of Prokaryotic and Eukaryotic Cells

Key Differences and Similarities

Understanding the differences between prokaryotic and eukaryotic cells is essential for studying cell biology.

Subcellular Components and Their Functions

Major Organelles in Eukaryotic Cells

Each organelle within a eukaryotic cell has a specific function that contributes to the cell's overall operation and survival.

Mitochondria and Chloroplasts

Structure and Function

Mitochondria and chloroplasts are specialized organelles responsible for energy conversion in eukaryotic cells.

• Mitochondria: Site of aerobic respiration, producing ATP. Contains an outer membrane and a highly folded inner membrane (cristae) to increase surface area for energy production.

• Chloroplasts: Found in plant cells, responsible for photosynthesis. Contains thylakoid membranes where light-dependent reactions occur, and stroma where the Calvin Cycle (light-independent reactions) takes place.

Equations:

• Cellular Respiration:

• Photosynthesis:

Example: Mitochondria are abundant in muscle cells due to high energy demand; chloroplasts are found in leaf cells for maximum light absorption.

Shared Features and Evolutionary Implications

Common Cell Parts and the Theory of Life

Both prokaryotic and eukaryotic cells share certain features, such as the plasma membrane, cytoplasm, and ribosomes. These similarities suggest a common evolutionary origin.

• Plasma Membrane: Present in all cells, controls movement of substances.

• Cytoplasm: Site of metabolic activity.

• Ribosomes: Universal site of protein synthesis.

Additional info: The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Summary Table: Major Organelles and Their Functions