Cell Structure, Function, and Membrane Transport: General Biology Study Guide

Unit 2: Cell Structure and Function

Topic 2.1: Subcellular Components and Organelles

This topic covers the structure and function of major cell organelles, their roles in cellular processes, and differences between cell types.

• Ribosomes: Ribosomes are molecular machines responsible for protein synthesis. They are found in both prokaryotic and eukaryotic cells. Prokaryotic ribosomes (70S) are smaller than eukaryotic ribosomes (80S).

• Endoplasmic Reticulum (ER): There are two types:

  • Rough ER: Studded with ribosomes; involved in protein synthesis and modification.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus/Complex: A stack of flattened membranes that modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Made of a double membrane; the inner membrane is highly folded into cristae. They are the site of cellular respiration and ATP production.

• Lysosomes: Membrane-bound vesicles containing hydrolytic enzymes for intracellular digestion.

• Vacuole: Functions in storage, waste disposal, and maintaining turgor pressure in plant cells.

• Chloroplast: Double-membraned organelle found in plant and algal cells; site of photosynthesis. Contains structures such as thylakoids and stroma.

Example: Plant cells contain both chloroplasts and large central vacuoles, while animal cells contain lysosomes and smaller vacuoles.

Topic 2.2: Organelle Contributions to Cell Function

Subcellular components work together to accomplish essential cellular functions.

• Endoplasmic Reticulum: Synthesizes proteins (rough ER) and lipids (smooth ER), and transports them within the cell.

• Mitochondria: Generate ATP through cellular respiration.

• Lysosomes: Break down macromolecules and recycle cellular components.

• Vacuoles: In animal cells, vacuoles store substances and help with waste removal; in plant cells, the central vacuole maintains cell rigidity and stores nutrients.

• Chloroplasts: Capture light energy and convert it to chemical energy via photosynthesis.

Example: The mitochondria and chloroplasts both have their own DNA and are involved in energy conversion processes.

Topic 2.3: Surface Area-to-Volume Ratio and Material Exchange

The surface area-to-volume ratio affects the efficiency of material exchange between cells and their environment.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the rate of exchange.

• Equation: $\text{Surface Area-to-Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

• Cells maintain a high surface area-to-volume ratio to facilitate efficient exchange of materials.

• Plasma membrane adaptations (e.g., microvilli) increase surface area.

Example: Small cells or cells with folded membranes (like intestinal epithelial cells) have higher ratios for efficient nutrient absorption.

Topic 2.4: Cell Membrane Structure and Function

The cell membrane maintains the internal environment and regulates transport.

• Phospholipids: Form a bilayer with hydrophilic heads and hydrophobic tails, creating a semi-permeable barrier.

• Embedded Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Fluid Mosaic Model: Describes the dynamic arrangement of lipids and proteins in the membrane.

Example: Membrane proteins facilitate the transport of ions and molecules across the membrane.

Topic 2.5: Biological Membranes and Selective Permeability

Membrane structure determines which substances can pass through.

• Selective Permeability: Only certain molecules (e.g., small nonpolar molecules) can diffuse freely; others require transport proteins.

• Role in Cell Structure: Maintains cell integrity and allows compartmentalization.

• Transport of Polar Molecules and Ions: Requires channels or carriers due to hydrophobic core of membrane.

Example: Water moves through aquaporin channels; ions move through ion channels.

Topic 2.6: Mechanisms of Transport Across Membranes

Cells use passive and active transport to move substances across membranes.

• Passive Transport: Movement of molecules down their concentration gradient without energy input (e.g., diffusion, osmosis).

• Active Transport: Movement against the concentration gradient, requiring energy (ATP).

• Endocytosis: Uptake of large molecules by engulfing them in vesicles.

• Exocytosis: Release of substances from the cell via vesicle fusion with the membrane.

Example: Sodium-potassium pumps use ATP to maintain ion gradients across the membrane.

Topic 2.7: Molecular Structure and Membrane Passage

The ability of molecules to cross membranes depends on their size, polarity, and transport mechanisms.

• Facilitated Diffusion: Passive movement of molecules via transport proteins.

• Types of Molecules: Glucose, ions, and amino acids use facilitated diffusion.

• Active Transport Requirements: Requires energy (ATP) and specific carrier proteins.

• Sodium/Potassium Pumps: Transport Na+ out and K+ into the cell, maintaining electrochemical gradients.

Example: The Na+/K+ pump is essential for nerve impulse transmission.

Topic 2.8: Osmoregulation and Water Potential

Osmoregulation maintains water and solute balance in cells and organisms.

• Water Potential Equation: $\text{Water Potential} = \text{Pressure Potential} + \text{Solute Potential}$

• Solute Potential Equation: $\text{Solute Potential} = -iCRT$

• Osmoregulation is vital for homeostasis and survival.

Example: Plant cells use vacuoles to regulate water potential and maintain turgor pressure.

Topic 2.9: Ion and Molecule Movement Across Membranes

Cells use various processes to move ions and molecules across membranes.

• Channels and Carriers: Facilitate movement of specific ions and molecules.

• Electrochemical Gradients: Drive movement of charged particles.

Example: Calcium channels regulate muscle contraction by controlling Ca2+ flow.

Topic 2.10: Eukaryotic Cell Membrane-Bound Structures

Eukaryotic cells have specialized organelles that compartmentalize functions.

• Membrane-Bound Organelles: Include nucleus, mitochondria, ER, Golgi apparatus, lysosomes, and chloroplasts.

• Compartmentalization: Allows for specialized environments and efficient metabolic processes.

Example: Lysosomes isolate digestive enzymes from the rest of the cell.

Topic 2.11: Compartmentalization and Endosymbiosis

Cells differ in their degree of compartmentalization and organelle origin.

• Prokaryotic vs. Eukaryotic Cells: Prokaryotes lack membrane-bound organelles; eukaryotes possess them.

• Endosymbiotic Theory: Mitochondria and chloroplasts originated from free-living bacteria engulfed by ancestral eukaryotic cells.

• Functional Relationships: Endosymbiotic organelles retain some features of their ancestors, such as circular DNA and double membranes.

Example: Mitochondria perform aerobic respiration, similar to their bacterial ancestors.

Summary Table: Major Cell Organelles and Functions

Organelle	Main Function	Found In
Ribosome	Protein synthesis	All cells
Endoplasmic Reticulum	Protein and lipid synthesis	Eukaryotic cells
Golgi Apparatus	Modification and packaging of proteins/lipids	Eukaryotic cells
Mitochondria	ATP production	Most eukaryotic cells
Lysosome	Intracellular digestion	Animal cells
Vacuole	Storage, turgor pressure	Plant and some protist cells
Chloroplast	Photosynthesis	Plant and algal cells

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard General Biology curriculum.