Cell Structure and Function: Prokaryotic, Eukaryotic, Animal, and Plant Cells

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Cell Structure and Function

Introduction to Cells

Cells are the basic structural and functional units of life. All living organisms are composed of cells, and all physiological, biochemical, genetic, and metabolic functions occur within cells. Cells can be classified into two main types: prokaryotic and eukaryotic cells.

Cell: The smallest unit of life, capable of performing all life processes.
Structural unit: Cells provide the physical structure for organisms.
Functional unit: Cells carry out essential functions such as metabolism, growth, and reproduction.

Classification of Cells

Prokaryotic vs. Eukaryotic Cells

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

Prokaryotic cells: Cells without a true nucleus or membrane-bound organelles. Found in domains Bacteria and Archaea.
Eukaryotic cells: Cells with a true nucleus and membrane-bound organelles. Found in domain Eukarya (includes plants, animals, fungi, and protists).

Key Features of Prokaryotic and Eukaryotic Cells

	Cell membrane	Cytoplasm	DNA	Nucleus	Cell size	Complexity
Prokaryotic cell	✓	✓	✓	No	Smaller	Simple
Eukaryotic cell	✓	✓	✓	Yes	Larger	Complex

Common Features of All Cells

All cells are enclosed by a plasma membrane.
All cells contain genetic code in the form of DNA.
All internal structures are floating (suspended) in cytosol (the fluid portion of the cytoplasm).

Prokaryotic Cell Structure

Main Components

Prokaryotic cells have a simple structure but contain all the essential components for life.

Plasma membrane: Boundary that separates the cell from its environment.
Chromosome: Single, circular DNA molecule containing genetic information.
Ribosomes: Structures that build proteins.
Cytoplasm: Jellylike fluid inside the cell.
Cytosol: The fluid portion of the cytoplasm.
Nucleoid: Region where the chromosome is located (not membrane-bound).
Cell wall: Protective layer outside the plasma membrane (in most prokaryotes).
Capsule: Sticky outer coat used to glue prokaryote to surfaces or each other.
Fimbriae: Structures for attachment to surfaces or other cells.
Flagella: Structures for movement (swimming).

Example: Escherichia coli is a common prokaryotic cell found in the human gut.

Eukaryotic Cell Structure

Main Components

Eukaryotic cells are more complex and contain many membrane-bound organelles.

Nucleus: Contains the cell's DNA and controls cellular activities.
Plasma membrane: Encloses the cell and regulates movement of substances.
Cytoplasm: Includes cytosol and organelles.
Ribosomes: Build proteins; found free in cytosol or attached to endoplasmic reticulum.
Mitochondria: Site of cellular respiration and energy production.
Endoplasmic reticulum (ER): Synthesizes proteins and lipids.
Golgi apparatus: Modifies, sorts, and packages proteins and lipids.
Lysosomes: Contain digestive enzymes (mainly in animal cells).
Centrosome: Organizes microtubules (mainly in animal cells).
Chloroplasts: Site of photosynthesis (in plant cells).
Cell wall: Rigid outer layer (in plant cells).
Central vacuole: Large storage organelle (in plant cells).

Comparison of Animal and Plant Cells

Key Differences and Similarities

Animal and plant cells share many structures but also have unique features.

Animal Cell	Common Structures	Plant Cell
Centrosome Lysosome	Nucleus Plasma membrane Cytoplasm Cytosol Ribosomes Chromosome (DNA) Flagella	Central vacuole Chloroplast Cell wall

Example: Plant cells have chloroplasts for photosynthesis, while animal cells have lysosomes for digestion.

Functions of Cell Structures

Summary of Major Organelles and Their Functions

Nucleus: Stores genetic material and coordinates cell activities.
Plasma membrane: Controls entry and exit of substances.
Cytoplasm: Site of metabolic reactions.
Ribosomes: Synthesize proteins.
Mitochondria: Produce ATP (energy).
Chloroplasts: Convert solar energy to chemical energy (photosynthesis).
Cell wall: Provides structural support (plants).
Central vacuole: Stores water and nutrients (plants).
Lysosome: Digests cellular waste (animals).

Genetic Material in Cells

DNA as the Genetic Code

Both prokaryotic and eukaryotic cells contain genetic information in the form of DNA.

DNA: Deoxyribonucleic acid, the molecule that carries genetic instructions for life.
In prokaryotes, DNA is found in the nucleoid region; in eukaryotes, it is enclosed within the nucleus.

Equation:

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
Cell size	Smaller	Larger
Complexity	Simple	Complex
Domains	Bacteria, Archaea	Eukarya

Conclusion

Understanding the differences between prokaryotic and eukaryotic cells, as well as the unique features of animal and plant cells, is fundamental in biology. These distinctions help explain the diversity of life and the specialized functions of cells in various organisms.