Cell Structures and Functions: Study Guide

Cell Structures and Functions

Key Terms and Definitions

This section introduces essential terminology related to cell biology, providing foundational knowledge for understanding cell structure and function.

• Cell: The basic structural and functional unit of all living organisms.

• Cell membrane (plasma membrane): A selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out.

• Cytoplasm: The jelly-like substance within the cell, excluding the nucleus, containing organelles and cytosol.

• Eukaryote: Organisms whose cells contain a nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists).

• Prokaryote: Organisms whose cells lack a nucleus and most membrane-bound organelles (e.g., bacteria, archaea).

• Ribosome: Molecular complexes that synthesize proteins by translating messenger RNA (mRNA).

• Cell wall: A rigid structure outside the cell membrane in plants, fungi, and some prokaryotes, providing support and protection.

• DNA (Deoxyribonucleic acid): The hereditary material in all living organisms, encoding genetic information.

• RNA (Ribonucleic acid): A nucleic acid involved in protein synthesis and gene regulation.

• Organelle: Specialized structures within eukaryotic cells that perform distinct functions (e.g., mitochondria, chloroplasts).

• Nucleus: The membrane-bound organelle in eukaryotic cells that contains genetic material (DNA).

• Nucleolus: A dense region within the nucleus where ribosomal RNA (rRNA) is synthesized.

• Nuclear envelope: A double membrane that surrounds the nucleus, separating it from the cytoplasm.

• Chromatin: The complex of DNA and proteins found in the nucleus, which condenses to form chromosomes during cell division.

• Nuclear pore: Openings in the nuclear envelope that regulate the passage of molecules between the nucleus and cytoplasm.

• Smooth endoplasmic reticulum (SER): An organelle involved in lipid synthesis and detoxification.

• Rough endoplasmic reticulum (RER): An organelle studded with ribosomes, involved in protein synthesis and modification.

• Golgi body (Golgi apparatus): Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Vesicle: Small membrane-bound sacs that transport substances within or between cells.

• Lysosome: Contains digestive enzymes to break down waste materials and cellular debris.

• Peroxisome: Organelles that break down fatty acids and detoxify harmful substances.

• Mitochondrion: The powerhouse of the cell, generating ATP through cellular respiration.

• Cristae: Folds of the inner mitochondrial membrane, increasing surface area for ATP production.

• Matrix: The innermost compartment of the mitochondrion, containing enzymes for the Krebs cycle.

• Chloroplast: Organelle in plant cells where photosynthesis occurs.

• Thylakoid: Flattened sacs within chloroplasts where the light-dependent reactions of photosynthesis take place.

• Stroma: The fluid-filled space surrounding the thylakoids in chloroplasts, site of the Calvin cycle.

• Granum (pl. grana): Stack of thylakoids within a chloroplast.

• Endosymbiotic theory: The hypothesis that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Cellulose: A polysaccharide that is the main component of plant cell walls.

• Plasmodesmata: Channels between plant cell walls that allow for transport and communication.

• Central vacuole: A large, fluid-filled organelle in plant cells that stores water, nutrients, and waste products.

• Cytoskeleton: A network of protein filaments that provides structural support, shape, and movement for the cell.

• Microtubule: Hollow tubes of tubulin protein that help maintain cell shape and facilitate movement of organelles.

• Microfilament: Thin filaments of actin protein involved in cell movement and shape.

• Intermediate filament: Fibrous proteins that provide mechanical support for the cell.

• Actin: A protein that forms microfilaments.

• Cilia: Short, hair-like structures that move fluid or cells over their surface.

• Flagella: Long, whip-like structures used for cell movement.

• Cellular junction: Structures that connect cells to one another.

• Tight junction: Seals neighboring cells together to prevent leakage of extracellular fluid.

• Anchoring junction (desmosome): Fastens cells together into strong sheets.

• Gap junction: Channels that allow molecules and ions to pass directly from one cell to another in animal cells.

• Animal cell: Eukaryotic cell lacking a cell wall and chloroplasts, typically containing centrioles.

• Plant cell: Eukaryotic cell with a cell wall, chloroplasts, and a central vacuole.

Main Concepts of Cell Theory

The cell theory is a fundamental principle in biology describing the properties of cells.

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in living things.

• All cells arise from pre-existing cells.

Common Components of All Cells

Despite diversity, all cells share certain structures:

• Plasma membrane

• Cytoplasm

• Genetic material (DNA)

• Ribosomes

Prokaryotic Cell Structure and Function

Prokaryotic cells are simpler than eukaryotic cells and lack membrane-bound organelles.

• Cell wall: Provides shape and protection.

• Plasma membrane: Controls entry and exit of substances.

• Cytoplasm: Contains enzymes and ribosomes.

• Nucleoid: Region containing circular DNA.

• Ribosomes: Protein synthesis.

• Flagella and pili: Movement and attachment.

Cytoplasm

The cytoplasm is the site of many metabolic reactions and contains organelles in eukaryotic cells.

• Composed of cytosol (fluid) and suspended organelles.

• Facilitates movement of materials within the cell.

Cell Size

Cells are small due to the need for a high surface area-to-volume ratio, which facilitates efficient exchange of materials.

• As cell size increases, volume grows faster than surface area.

• Efficient diffusion and transport require small cell size.

Nucleus Structure and Function

The nucleus is the control center of eukaryotic cells.

• Nuclear envelope: Double membrane with pores for molecular exchange.

• Nucleolus: Site of ribosomal RNA synthesis.

• Chromatin: DNA-protein complex; condenses into chromosomes during cell division.

Endomembrane System

The endomembrane system is a network of membranes involved in synthesis, modification, and transport of cellular materials.

• Smooth ER: Lipid synthesis, detoxification.

• Rough ER: Protein synthesis (with ribosomes), protein folding.

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids.

• Vesicles: Transport materials between organelles.

• Lysosomes: Digestive enzymes for breakdown of macromolecules.

Peroxisomes

Peroxisomes are organelles that detoxify harmful substances and break down fatty acids.

• Contain enzymes that produce and degrade hydrogen peroxide ($2H_2O_2 \rightarrow 2H_2O + O_2$).

Mitochondria Structure and Function

Mitochondria are the sites of cellular respiration and energy (ATP) production.

• Outer membrane: Encloses the organelle.

• Inner membrane: Folded into cristae to increase surface area.

• Cristae: Location of electron transport chain.

• Matrix: Contains enzymes for the Krebs cycle.

Chloroplast Structure and Function

Chloroplasts are the sites of photosynthesis in plant cells.

• Outer and inner membranes: Enclose the organelle.

• Thylakoids: Membranous sacs containing chlorophyll.

• Grana: Stacks of thylakoids.

• Stroma: Fluid surrounding the grana; site of the Calvin cycle.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts.

• Both organelles contain their own DNA and ribosomes.

• They replicate independently of the cell.

• Double membranes suggest engulfment by ancestral eukaryotes.

• Similarities to prokaryotic cells support this theory.

Plant Cell Wall and Central Vacuole

Plant cells have unique structures for support and storage.

• Cell wall: Made of cellulose; provides rigidity and protection.

• Central vacuole: Stores water, nutrients, and waste; maintains turgor pressure.

Cytoskeleton Components

The cytoskeleton maintains cell shape and enables movement.

• Microtubules: Hollow tubes; move organelles and chromosomes.

• Microfilaments: Thin actin filaments; involved in cell movement and shape.

• Intermediate filaments: Provide mechanical support.

Cilia and Flagella

Cilia and flagella are structures for cell movement.

• Cilia: Short, numerous; move substances over cell surface.

• Flagella: Longer, usually one or few; propel cells (e.g., sperm).

• Both have a "9+2" arrangement of microtubules.

Cell Membrane Structure

The cell membrane is a fluid mosaic of lipids and proteins.

• Composed of a phospholipid bilayer with embedded proteins.

• Regulates transport, communication, and cell recognition.

Cell Junctions

Cell junctions connect cells and facilitate communication.

• Tight junctions: Seal cells together (animal cells).

• Anchoring junctions (desmosomes): Attach cells to each other (animal cells).

• Gap junctions: Allow passage of ions and molecules (animal cells).

• Plasmodesmata: Channels between plant cells for communication.

Comparison of Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells differ in complexity and structure.

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Size	Small (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Comparison of Animal and Plant Cells

Animal and plant cells share many features but also have key differences.

Feature	Animal Cell	Plant Cell
Cell wall	Absent	Present (cellulose)
Chloroplasts	Absent	Present
Central vacuole	Small or absent	Large, central
Shape	Round/irregular	Rectangular/rigid

Additional info: This guide expands on the provided objectives with academic context, definitions, and comparative tables for clarity and exam preparation.