Cell Structure and Function

Structures Found in All Cells

All living cells share certain fundamental structures that are essential for life.

• Plasma Membrane: A selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out.

• Cytoplasm: The semi-fluid substance inside the cell where metabolic reactions occur.

• Ribosomes: Complexes of RNA and protein that synthesize proteins.

• Genetic Material: DNA (or RNA in some viruses) that contains instructions for cell function.

Prokaryotic vs. Eukaryotic Cells

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

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. Examples: Bacteria, Archaea.

• Eukaryotic Cells: Have a true nucleus and various membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

Organelles and Their Functions

Functions of Organelles

Organelles are specialized structures within eukaryotic cells that perform distinct functions.

• Nucleus: Contains genetic material; controls cell activities.

• Nucleolus: Site of ribosome synthesis within the nucleus.

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Site of photosynthesis in plant cells.

• Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

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

• Lysosomes: Contain digestive enzymes; break down waste.

• Vacuoles: Store substances; large central vacuole in plants maintains turgor pressure.

Chloroplasts & Mitochondria: Evolutionary Importance

Both organelles are thought to have originated from free-living prokaryotes through endosymbiosis.

• Chloroplasts: Convert solar energy to chemical energy via photosynthesis.

• Mitochondria: Convert chemical energy from food into ATP via cellular respiration.

• Endosymbiotic Theory: Suggests these organelles were once independent prokaryotes engulfed by ancestral eukaryotic cells.

Structures Common to Animal & Plant Cells

• Nucleus

• Mitochondria

• Endoplasmic Reticulum

• Golgi Apparatus

• Plasma Membrane

• Cytoskeleton

Structures Found Only in Plant or Animal Cells

• Plant Cells Only: Chloroplasts, cell wall, large central vacuole.

• Animal Cells Only: Lysosomes, centrioles.

Intercellular Junctions in Animals & Plants

Cells communicate and adhere to each other via specialized junctions.

• Animal Cells: Tight junctions, desmosomes, gap junctions.

• Plant Cells: Plasmodesmata (channels through cell walls for transport and communication).

Cell Membrane Structure and Function

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which depicts a dynamic structure composed of various molecules.

• Phospholipid Bilayer: Forms the basic structure; amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Proteins: Embedded or attached; serve as channels, receptors, enzymes.

• Carbohydrates: Attached to proteins/lipids; involved in cell recognition.

Tripartite Nature of Membrane

• Phospholipids: Provide fluidity and barrier properties.

• Proteins: Facilitate transport, signaling, and structural support.

• Carbohydrates: Mediate cell-cell recognition.

Amphipathic Nature of Membrane Components

Phospholipids and some proteins are amphipathic, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Hydrophilic Regions: Typically carry a charge (+ or -), interact with water.

• Hydrophobic Regions: Nonpolar, avoid water.

Selectively/Differentially Permeable Membrane

The plasma membrane allows certain substances to pass while restricting others.

• Small, nonpolar molecules: Pass easily (e.g., O2, CO2).

• Large or charged molecules: Require transport proteins.

Transport Mechanisms

Passive vs. Active Transport

Cells move substances across membranes using passive or active mechanisms.

• Passive Transport: No energy required; substances move down their concentration gradient. Includes diffusion, osmosis, and facilitated diffusion.

• Active Transport: Requires energy (usually ATP); substances move against their concentration gradient.

Diffusion vs. Osmosis

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Water Potential

Water potential () predicts the direction water will move; it combines solute potential and pressure potential.

• Formula:

• = solute potential (osmotic potential)

• = pressure potential

• Water moves from areas of higher to lower water potential.

3 Solution Types & Effects on Cells

Cytolysis and Plasmolysis

• Cytolysis: Bursting of animal cells in hypotonic solutions due to excessive water intake.

• Plasmolysis: Shrinking of plant cell cytoplasm in hypertonic solutions; cell wall remains intact.

What Prevents Cytolysis in Plant Cells?

• Cell Wall: Provides structural support and prevents bursting in hypotonic environments.

Turgid vs. Flaccid Plant Cells

• Turgid: Plant cell is firm due to water uptake; ideal for plant structure.

• Flaccid: Plant cell loses water; becomes limp and may wilt.

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, facilitates movement, and organizes cell contents.

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

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Tubulin polymers; involved in cell division and transport.

Nucleolus/Nucleoli and Nucleus/Nuclei

• Nucleus: Membrane-bound organelle containing DNA.

• Nucleolus: Dense region within the nucleus; site of ribosome assembly.

• Nuclei/Nucleoli: Plural forms; refer to multiple structures in multicellular organisms or multinucleate cells.

Additional info:

• Endosymbiotic theory is supported by the presence of double membranes and unique DNA in mitochondria and chloroplasts.

• Water potential is crucial for understanding plant water uptake and movement.