Cell Structure and Function: Key Organelles and Components in General Biology

Cell Structure and Function

Overview of Cell Structures

Cells are the fundamental units of life, and their internal structures (organelles and components) perform specialized functions necessary for survival and growth. This section summarizes the main cell structures found in eukaryotic and prokaryotic cells, with emphasis on their functions and distinguishing features.

Cytoplasm

• Definition: Semi-fluid substance that suspends all subcellular substances of the cell (excluding the nucleus).

• Cytosol: The aqueous matrix inside the plasma membrane but outside the organelles.

• Function: Site for many metabolic reactions and houses organelles.

Nucleus

• Definition: Membrane-bound organelle containing most of the cell's DNA.

• Function: Controls cell activities by regulating gene expression; contains nucleolus (site of rRNA synthesis).

• Example: Eukaryotic cells (plants, animals, fungi, protists) have a nucleus; prokaryotes do not.

Nuclear Envelope

• Definition: Double membrane surrounding the nucleus, with nuclear pores.

• Function: Regulates passage of molecules between nucleus and cytoplasm.

Ribosomes

• Definition: Complexes of rRNA and protein; found free in cytoplasm or bound to endoplasmic reticulum.

• Function: Site of protein synthesis (translation of mRNA).

• Structure: Composed of large and small subunits.

Chromatin / Chromosomes

• Chromatin: DNA and associated proteins (histones); less condensed form.

• Chromosomes: Condensed chromatin visible during cell division.

• Function: Storage and transmission of genetic information.

Nucleolus

• Definition: Dense region within the nucleus.

• Function: Site of rRNA synthesis and ribosome assembly.

Endomembrane System

The endomembrane system is a group of organelles that work together to modify, package, and transport lipids and proteins.

• Includes: Nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, plasma membrane.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs.

• Function: Extensive membrane network for biosynthesis and transport.

Golgi Apparatus

• Definition: Stacked, flattened membrane sacs.

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

Lysosomes (Mainly Animal Cells)

• Definition: Membrane-bound vesicles containing hydrolytic enzymes.

• Function: Digestion of macromolecules, recycling of cellular components.

• Optimal pH: Acidic (low pH).

Vacuoles

• Definition: Large membrane-bound sacs, prominent in plant cells.

• Function: Storage of water, ions, nutrients; maintenance of cell turgor pressure.

Peroxisomes

• Definition: Small, membrane-bound organelles containing enzymes for oxidation reactions.

• Function: Breakdown of fatty acids, detoxification of hydrogen peroxide ($2H_2O_2 \rightarrow 2H_2O + O_2$).

Mitochondria

• Definition: Double-membrane organelles; site of cellular respiration.

• Function: Generate ATP by extracting energy from sugars, fats, and other fuels.

• Contains: Own DNA (maternal inheritance).

• Equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Chloroplasts (Plants and Algae)

• Definition: Double-membrane organelles; site of photosynthesis.

• Function: Convert solar energy to chemical energy (glucose).

• Contains: Own DNA, ribosomes, thylakoid membranes (site of light reactions).

• Equation: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Cytoskeleton

• Definition: Network of protein filaments throughout the cytoplasm.

• Function: Maintains cell shape, enables movement, organizes cell contents.

• Components:

  • Microfilaments (actin): Smallest diameter; involved in cell movement and shape.

  • Intermediate filaments: Medium diameter; provide structural support.

  • Microtubules (tubulin): Largest diameter; involved in cell division, transport, and structure.

Centrosome / Centrioles

• Definition: Microtubule organizing center; contains centrioles in animal cells.

• Function: Organizes microtubules during cell division.

Cell Wall (Plants, Fungi, Some Protists, Bacteria)

• Definition: Rigid outer layer outside the plasma membrane.

• Function: Provides structural support, protection, and prevents excessive water uptake.

• Composition: Cellulose (plants), chitin (fungi), peptidoglycan (bacteria).

Plasma Membrane

• Definition: Phospholipid bilayer with embedded proteins.

• Function: Selectively permeable barrier; regulates entry and exit of substances.

Plasmodesmata (Plants)

• Definition: Channels that perforate cell walls and connect adjacent plant cells.

• Function: Allow passage of water, ions, and small molecules between cells.

Gap Junctions (Animals)

• Definition: Channels allowing cytosol to pass between adjacent animal cells.

• Function: Enable cell-to-cell communication.

Extracellular Matrix (ECM) (Animals)

• Definition: Network of proteins and carbohydrates outside the plasma membrane.

• Function: Provides structural support, regulates cell behavior.

Summary Table: Major Cell Structures and Their Functions

Structure	Function
Cytoplasm	Suspends organelles; site of metabolic reactions
Nucleus	Stores DNA; controls cell activities
Ribosomes	Protein synthesis
Endoplasmic Reticulum (Rough/Smooth)	Protein and lipid synthesis; detoxification
Golgi Apparatus	Modification and packaging of proteins/lipids
Lysosomes	Digestion and recycling (mainly animal cells)
Vacuoles	Storage and support (mainly plant cells)
Mitochondria	ATP production (cellular respiration)
Chloroplasts	Photosynthesis (plants and algae)
Cytoskeleton	Cell shape, movement, organization
Cell Wall	Structural support and protection
Plasma Membrane	Selective barrier; communication
Plasmodesmata	Intercellular communication (plants)
Gap Junctions	Intercellular communication (animals)
Extracellular Matrix	Support and regulation (animals)

Comparison: Animal vs. Plant Cells

• Animal Cells: No cell wall, no chloroplasts, small or no vacuoles, have lysosomes and centrioles.

• Plant Cells: Have cell wall, chloroplasts, large central vacuole, plasmodesmata.

Additional info:

• Some structures (e.g., cell wall, chloroplasts) are unique to plants and algae, while lysosomes and centrioles are mainly found in animal cells.

• Prokaryotic cells (bacteria and archaea) lack membrane-bound organelles but have cytoplasm, plasma membrane, ribosomes, and usually a cell wall.