Cell Structure & Function: General Biology Study Notes

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Cell Structure & Function

Introduction

Cells are the fundamental units of life, and their structure is closely tied to their function. Understanding the differences between cell types and their organelles is essential for studying biological processes such as metabolism, growth, and reproduction.

Prokaryotic vs. Eukaryotic Cells

Key Differences

Prokaryotic cells are simpler, lacking a nucleus and most membrane-bound organelles. They are found in Bacteria and Archaea.
Eukaryotic cells have a true nucleus and various membrane-bound organelles. They are found in Protists, Fungi, Plants, and Animals.

Structural Comparison

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Organelles	Few (e.g., ribosomes)	Many (e.g., mitochondria, ER, Golgi)
Size	Smaller (0.1–5 μm)	Larger (10–100 μm)
Cell Wall	Usually present (peptidoglycan)	Present in plants/fungi (cellulose/chitin), absent in animals
DNA	Circular, in nucleoid	Linear, in nucleus

Examples

Bacteria (e.g., Corynebacterium diphtheriae) are prokaryotic.
Animal cells (e.g., human epithelial cells) are eukaryotic.

Plant vs. Animal Cell Structure

Key Differences

Plant cells have a cell wall, chloroplasts, and a large central vacuole.
Animal cells lack a cell wall and chloroplasts but have centrioles and lysosomes.

Structural Comparison

Feature	Plant Cell	Animal Cell
Cell Wall	Present (cellulose)	Absent
Chloroplasts	Present	Absent
Central Vacuole	Large, central	Small or absent
Lysosomes	Rare	Common
Centrioles	Absent	Present

Microscopy and Cell Size

Scale of Biological Structures

Microscopes are essential tools for biologists to observe cells and their components, which range in size from atoms to entire cells.

Human cells are typically 10–100 μm in diameter.
Bacteria are about 1–10 μm.
Organelles such as nuclei and mitochondria are 1–10 μm.
Ribosomes are about 20–30 nm.

Major Cell Organelles and Their Functions

Nucleus

The nucleus is the information center of the cell, containing most of the cell's DNA and directing cellular activities.

Nuclear envelope: Double membrane with pores for molecular transport.
Chromatin: DNA and associated proteins.
Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

Ribosomes

Ribosomes are the sites of protein synthesis, composed of rRNA and proteins. They can be free in the cytosol or bound to the endoplasmic reticulum.

Free ribosomes: Synthesize proteins for use within the cell.
Bound ribosomes: Synthesize proteins for secretion or for use in membranes.

Endomembrane System

The endomembrane system regulates protein processing and performs metabolic functions.

Endoplasmic Reticulum (ER): Network of membranes; rough ER has ribosomes, smooth ER does not.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
Lysosomes: Digestive organelles containing hydrolytic enzymes.
Vacuoles: Storage and waste disposal; large central vacuole in plants.

Endomembrane System Table

Organelle	Structure	Function
Nucleus	Double membrane, nuclear pores	Houses DNA, controls gene expression
Ribosome	Two subunits (rRNA & protein)	Protein synthesis
ER (Rough)	Membrane network with ribosomes	Protein synthesis and processing
ER (Smooth)	Membrane network without ribosomes	Lipid synthesis, detoxification
Golgi Apparatus	Stacked membranous sacs	Modification and sorting of macromolecules
Lysosome	Membranous sac of enzymes	Digestion and recycling
Vacuole	Large membrane-bound vesicle	Storage, waste disposal, water balance

Protein Processing and Trafficking

Pathway of Protein Synthesis

Proteins are synthesized by ribosomes and processed through the endomembrane system before reaching their final destination.

Transcription of DNA to mRNA in the nucleus.
Translation of mRNA to protein by ribosomes.
Proteins enter the ER for folding and modification.
Transported to the Golgi apparatus for further modification and sorting.
Packaged into vesicles for delivery to the cell membrane or other locations.

Energy Conversion Organelles

Mitochondria

Mitochondria are the sites of cellular respiration, converting glucose and oxygen into ATP, the cell's energy currency.

Structure: Double membrane, inner membrane folded into cristae.
Function: ATP synthesis via oxidative phosphorylation.

Equation for Cellular Respiration:

Chloroplasts

Chloroplasts are found in plant cells and are the sites of photosynthesis, converting light energy into chemical energy.

Structure: Double membrane, thylakoid stacks (grana), stroma.
Function: Photosynthesis.

Equation for Photosynthesis:

Endosymbiotic Theory

Mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Both have double membranes and their own circular DNA.
Replicate independently within the cell.

Cytoskeleton

Components and Functions

The cytoskeleton is a network of protein filaments that provides structural support, enables cell movement, and organizes cellular components.

Microtubules: Thickest; made of tubulin; involved in cell shape, transport, and division.
Intermediate filaments: Mid-sized; provide mechanical strength.
Actin filaments (microfilaments): Thinnest; involved in cell movement and shape changes.

Cytoskeleton Table

Type	Structure	Function
Microtubules	Hollow tubes of tubulin	Cell shape, transport, mitosis
Intermediate Filaments	Fibrous proteins coiled into cables	Mechanical support
Actin Filaments	Two intertwined strands of actin	Cell movement, muscle contraction

Cell Membrane Projections

Specialized Structures

Microvilli: Increase surface area for absorption (e.g., in intestinal cells).
Cilia and Flagella: Enable movement of cells or substances across cell surfaces.

Application: Predicting Cell Function

Structure-Function Relationship

The function of a cell can often be inferred from its structure and organelle content. For example, cells with many mitochondria are likely involved in energy-intensive processes, while cells with abundant rough ER are specialized for protein secretion.

Example: Muscle cells have high mitochondrial density for ATP production.
Example: Pancreatic cells have extensive rough ER and Golgi for enzyme secretion.

Summary Table: Major Organelles and Functions

Organelle	Main Function
Nucleus	Genetic information storage and processing
Ribosome	Protein synthesis
Endoplasmic Reticulum	Protein and lipid synthesis
Golgi Apparatus	Modification and sorting of macromolecules
Lysosome	Digestion and recycling
Mitochondrion	ATP production
Chloroplast	Photosynthesis
Vacuole	Storage and waste disposal
Cytoskeleton	Structural support and movement

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