Study sheet: Cell Structure and Function

📋 Course Outline

1. Cell Types and Shared Structures
2. Eukaryotic Organelles and Functions
3. Endosymbiosis and Cell Efficiency
4. Surface Area to Volume
5. Plasma Membrane Structure
6. Membrane Transport Mechanisms
7. Water Potential and Osmosis
8. Osmolarity and Water Balance

📖 1. Cell Types and Shared Structures

🔑 Key Concepts & Definitions

• Prokaryotic cell : a relatively simple cell whose circular DNA lies in a nucleoid region and that lacks membrane-bound organelles
• Eukaryotic cell : a complex cell containing membrane-bound organelles and linear chromosomes enclosed within a nucleus

📝 Essential Points

• All cells contain:
  • genetic material
  • ribosomes
  • cytosol
  • a plasma membrane

📌 Prokaryotic DNA is generally a circular chromosome in the nucleoid and may include plasmids, whereas eukaryotic DNA is packaged into linear chromosomes inside a membrane-bound nucleus.

💡 Memory Hook

Prokaryotes lack membrane-bound organelles, whereas eukaryotes contain them.

📖 2. Eukaryotic Organelles and Functions

🔑 Key Concepts & Definitions

• Ribosome : a structure made of proteins and ribosomal RNA that assembles amino acids into polypeptide chains during translation

★ Must-know

📌 Rough endoplasmic reticulum contains membrane-bound ribosomes and functions in protein synthesis, whereas smooth endoplasmic reticulum lacks ribosomes and functions in lipid synthesis and detoxification.

• The Golgi complex modifies proteins into their final conformation and packages them into vesicles for transport throughout the cell.

• Mitochondria produce energy, contain a smooth outer membrane and folded inner membrane, and carry out Krebs-cycle reactions in their enzyme-containing matrix.

• Chloroplasts contain thylakoids stacked into grana, stroma surrounding the grana, and membranes and enzymes specialized for light-dependent and light-independent photosynthesis.

Further detail

• Lysosomes can:
  • digest macromolecules
  • degrade worn-out cell parts
  • participate in apoptosis
  • destroy bacteria and viruses

• The centrosome organizes spindle fibers in animal-cell division, amyloplasts store photosynthetic glucose as starch in plant cells, and the cytoskeleton helps maintain cell shape and move materials.

💡 Memory Hook

Ribosomes make, Golgi modifies and packages, lysosomes digest.

📖 3. Endosymbiosis and Cell Efficiency

🔑 Key Concepts & Definitions

• Endosymbiosis hypothesis : states that mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by larger prokaryotes that became interdependent

★ Must-know

• Mitochondria and chloroplasts support the endosymbiosis hypothesis because they contain circular DNA and ribosomes similar to those of prokaryotes.

📌 Compartmentalization increases cell efficiency by separating enzymes involved in different metabolic processes and reducing harmful cross-reactions.

Further detail

• Mitochondria and chloroplasts reproduce by binary fission, a process also used by bacteria.

💡 Memory Hook

Engulfed prokaryotes became interdependent → mitochondria and chloroplasts.

📖 4. Surface Area to Volume

★ Must-know

📐 Formula — For a spherical cell, surface area is $4\pi r^2$, volume is $\frac{4}{3}\pi r^3$, and the surface-area-to-volume ratio is $\frac{3}{r}$.

📌 A larger cell has a lower surface-area-to-volume ratio and exchanges nutrients, wastes, chemicals, and energy less efficiently with its environment.

Further detail

• Folding membranes, such as mitochondrial cristae and intestinal villi, increases the surface area available for reactions or nutrient absorption.

💡 Memory Hook

Larger radius → lower surface-area-to-volume ratio → less efficient exchange.

📖 5. Plasma Membrane Structure

🔑 Key Concepts & Definitions

• Fluid mosaic model : describes a plasma membrane whose mobile phospholipids, proteins, glycoproteins, glycolipids, and steroids move within the membrane surface

★ Must-know

• The plasma membrane is a phospholipid bilayer whose hydrophilic phosphate heads face aqueous environments and whose hydrophobic tails face inward away from water.

• Plasma-membrane proteins transport materials, participate in signaling, anchor cells, and catalyze chemical reactions, while glycoproteins and glycolipids function in cell recognition.

Further detail

📌 Steroids in the plasma membrane adjust membrane fluidity in response to environmental conditions and cellular needs.

💡 Memory Hook

A fluid phospholipid sea with mobile proteins, glycoproteins, glycolipids, and steroids.

📖 6. Membrane Transport Mechanisms

🔑 Key Concepts & Definitions

• Passive transport : the movement of molecules down their concentration gradient from higher to lower concentration without energy input

★ Must-know

• Small hydrophobic molecules such as O2, CO2, and N2 cross the phospholipid bilayer easily, whereas large polar molecules and ions require membrane channels or transport proteins.

• Osmosis is the diffusion of water across a membrane, whereas facilitated diffusion is protein-assisted passive transport of polar molecules or ions.

• The Na+/K+ pump uses ATP to move three Na+ ions out of the cell and two K+ ions into the cell against their concentration gradients.

Further detail

📌 Endocytosis brings water and macromolecules into the cell in vesicles, whereas exocytosis fuses vesicles with the plasma membrane to expel molecules.

💡 Memory Hook

Passive transport moves down the gradient without energy; active transport moves against it using energy.

📖 7. Water Potential and Osmosis

🔑 Key Concepts & Definitions

• Water potential : the potential energy of water in a solution and predicts the direction of water movement

★ Must-know

📌 Water moves from higher water potential, usually a hypotonic solution, toward lower water potential, usually a hypertonic solution.

📐 Formula — Total water potential is calculated as $\psi=\psi_s+\psi_p$, where $\psi_s$ is solute potential and $\psi_p$ is pressure potential.

📐 Formula — Solute potential is calculated as $$\psi_s=-iCRT$$$, where i is the ionization constant, C is solute concentration, R is the pressure constant, and T is temperature in Kelvin.

Further detail

• The ionization constant is 1 for glucose, 2 for NaCl, and 3 for CaCl2 because these substances form one, two, and three particles in solution, respectively.

📐 Formula — Temperature in Kelvin is calculated as $K=^{\circ}C+273$.

💡 Memory Hook

Higher water potential → lower water potential.

📖 8. Osmolarity and Water Balance

🔑 Key Concepts & Definitions

• Osmolarity : the total concentration of solutes in a solution

★ Must-know

📌 A freshwater paramecium has a higher internal solute concentration and lower water potential than its environment, so water enters and the contractile vacuole pumps excess water out.

📌 Saltwater-fish cells have a lower solute concentration and higher water potential than seawater, so the fish drinks seawater, retains water, and excretes excess solute through specialized salt-secreting organs.

Further detail

• Unregulated water entry can cause a cell to burst, while excessive dehydration can cause an organism to die.

💡 Memory Hook

Water imbalance → regulatory organelles or organs restore solute concentration.

📊 Synthesis Tables

Transport Mechanisms

Mechanism	Direction	Energy or protein requirement
Passive transport	High to low concentration	No energy; may use a membrane protein
Facilitated diffusion	High to low concentration	No energy; requires a membrane protein
Active transport	Low to high concentration	Requires energy and a transport protein
Endocytosis/exocytosis	Into or out of the cell in vesicles	Requires energy