Cell Structure, Function, and Membrane Transport: Study Notes for General Biology

Cell Structure and Organelles

Major Cell Organelles and Their Functions

Cells contain specialized structures called organelles that perform distinct functions necessary for cellular life. Understanding these organelles is fundamental to cell biology.

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or attached to the endoplasmic reticulum.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein (rough ER) and lipid (smooth ER) synthesis.

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

• Mitochondria: Powerhouse of the cell; site of aerobic respiration and ATP production.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules and cellular debris.

• Peroxisomes: Involved in lipid metabolism and detoxification of harmful substances.

• Vacuoles: Storage organelles, prominent in plant cells for water and nutrient storage.

• Plasma Membrane: Semi-permeable barrier that controls entry and exit of substances.

Example: Muscle cells have abundant mitochondria to meet high energy demands.

Cell Size and Mathematical Relationships

Surface Area to Volume Ratio

Cell size is limited by the relationship between surface area and volume. As a cell grows, its volume increases faster than its surface area, affecting nutrient uptake and waste removal.

• Mathematical Expression: For a spherical cell: Surface Area = $4\pi r^2$ Volume = $\frac{4}{3}\pi r^3$ Surface Area to Volume Ratio = $\frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r}$

• Smaller cells have a higher surface area to volume ratio, facilitating efficient exchange of materials.

• Larger cells may develop adaptations (e.g., folds, microvilli) to increase surface area.

Example: Intestinal epithelial cells have microvilli to increase surface area for absorption.

Cell Membranes and Their Importance

Plasma Membrane Structure and Function

The plasma membrane is a dynamic structure composed mainly of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It maintains cellular integrity and mediates communication with the environment.

• Separates internal cell environment from the external surroundings.

• Regulates transport of substances in and out of the cell.

• Facilitates cell signaling and recognition.

Example: Receptor proteins in the membrane bind hormones, triggering cellular responses.

Phospholipid Bilayer and Membrane Structure

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a flexible, dynamic structure where proteins float in or on the fluid lipid bilayer.

• Phospholipids: Have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Bilayer arrangement: Hydrophobic tails face inward, hydrophilic heads face outward toward water.

• Proteins: Integral (span the membrane) and peripheral (attached to surface) proteins perform various functions.

Example: Channel proteins facilitate the movement of ions across the membrane.

Membrane Permeability

Selective Permeability

The plasma membrane is selectively permeable, allowing some substances to pass while restricting others.

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

• Large or charged molecules require transport proteins.

• Water moves via specialized channels called aquaporins.

Example: Glucose requires a carrier protein to enter most cells.

Cell Composition and Types

Basic Components of Cells

All cells share certain basic features but can vary in complexity and function.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles (e.g., bacteria).

• Eukaryotic cells: Have a nucleus and organelles (e.g., plant and animal cells).

• Cells are made of water, proteins, lipids, carbohydrates, and nucleic acids.

Example: Red blood cells are specialized for oxygen transport and lack a nucleus.

Cellular Transport Mechanisms

Passive and Active Transport

Cells move substances across membranes using passive and active transport mechanisms.

• Passive Transport: Does not require energy; includes diffusion, osmosis, and facilitated diffusion.

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

Example: Sodium-potassium pump uses ATP to maintain ion gradients in nerve cells.

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across the membrane via transport proteins.

• Requires channel or carrier proteins.

• Moves substances down their concentration gradient.

• Common for glucose, amino acids, and ions.

Example: Aquaporins facilitate water movement across cell membranes.

Endocytosis and Exocytosis

Bulk Transport Mechanisms

Cells use endocytosis and exocytosis to move large particles or volumes of material.

• Endocytosis: Cell engulfs material by forming vesicles from the plasma membrane.

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell.

Example: White blood cells ingest bacteria via endocytosis.

Water Potential and Osmosis

Water Potential ($\Psi$)

Water potential determines the direction of water movement and is influenced by solute concentration and pressure.

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

• Equation: $\Psi = \Psi_s + \Psi_p$ Where $\Psi_s$ is solute potential and $\Psi_p$ is pressure potential.

• Important in plant cells for maintaining turgor pressure.

Example: Water enters plant roots due to lower water potential inside root cells.

Hypotonic, Hypertonic, and Isotonic Solutions

Cells respond differently to external solutions based on their osmotic properties.

• Hypotonic: Lower solute concentration outside; water enters cell, may cause swelling.

• Hypertonic: Higher solute concentration outside; water leaves cell, may cause shrinkage.

• Isotonic: Equal solute concentration; no net water movement.

Example: Animal cells placed in a hypotonic solution may burst (lyse).

Summary Table: Cell Transport Mechanisms

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2 across membrane
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Osmosis	No	Down water potential	Water via aquaporin
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Endocytosis	Yes (ATP)	Into cell	Phagocytosis of bacteria
Exocytosis	Yes (ATP)	Out of cell	Secretion of hormones

Endosymbiotic Theory

Origin of Eukaryotic Organelles

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-livincg prokaryotes engulfed by ancestral eukaryotic cells.

• Both organelles have their own DNA and ribosomes.

• Replicate independently of the cell.

• Double membranes suggest engulfment.

Example: Mitochondria share similarities with aerobic bacteria.

Additional info: Endosymbiotic theory is supported by genetic and biochemical evidence, including similarities in DNA sequences and membrane composition.