Cell Membrane & Homeostasis Study Notes

The Cell Membrane: Overview

• Function: separates the components of a cell from its environment; surrounds the cell
• Acts as the “gatekeeper” of the cell: regulates the flow of materials into and out of the cell; selectively permeable
• Helps cells maintain homeostasis: provides a stable internal balance within the cell

Structure and Components

• Also called the Phospholipid Bilayer
• All cells have a cell membrane made of phosphates, proteins, and lipids
• Double layer (bilayer) of phospholipids: each phospholipid has a hydrophilic (water-loving) phosphate head and hydrophobic (water-fearing) fatty acid tails
• The membrane has both water-attracting and water-repelling properties, making it amphipathic: HydroPHILIC heads; HydroPHOBIC tails
• Permeability characteristics allow selective transport
• Cell types and presence of walls:
  • Prokaryotes have a cell wall in addition to a cell membrane
  • Eukaryotes:
  • Animal cells have a cell membrane only
  • Plant cells have a cell membrane plus a cell wall

Role in Homeostasis

• The cell membrane maintains homeostasis by balancing several internal conditions: pH, temperature, glucose (sugar) intake, and water balance
• Maintains through both active and passive transport mechanisms
• Emphasizes that homeostasis aims for a stable internal environment

pH and Homeostasis

• pH defines how acidic or basic a solution is; scale ranges from 0 to 14
• Mathematical sense: $0 \le \mathrm{pH} \le 14$
• Classification:
  • Acidic: $0 \le \mathrm{pH} \le 6$
  • Neutral: $\mathrm{pH} = 7$
  • Basic (alkaline): $8 \le \mathrm{pH} \le 14$
• Buffers: substances that correct pH imbalances by resisting changes in pH

Quick pH Examples

• Orange juice: pH ~ $2$ (acidic)
• Gastric juices (stomach): pH ~ $1$ (acidic)
• Tap water: pH ~ $7$ (neutral)
• Sodium hydroxide: pH ~ $10$ (basic)
• Ammonia: pH ~ $14$ (highly basic)
• pH scale reference: Acidic range (0–6), Neutral at 7, Basic range (8–14)

The Phospholipid Bilayer: Details

• Composition: phospholipids arranged in a double layer; embedded proteins and other molecules
• The bilayer forms the fundamental structure of the membrane
• Key consequence: selective permeability (not all substances cross freely)

Passive Transport (no energy required)

• Definition: transport that does not require cellular energy to move molecules from high to low concentration
• Main processes:
  • Diffusion
  • Facilitated diffusion (uses proteins)
  • Osmosis (water movement)
• Direction: from region of high concentration to region of low concentration

Diffusion

• Movement of small particles across the cell membrane until homeostasis is approached
• Driven by concentration gradient: high concentration to low concentration
• Example: diffusion of scent molecules or aerosols in air; perfume sprays from a high concentration inside the spray can to a lower concentration in the surrounding air
• Facilitated diffusion requires carrier and channel proteins to assist particle movement down the gradient (still from high to low)
• Outside of the cell to inside of the cell or vice versa depending on the gradient

Osmosis

• Special case of diffusion specifically for water across a selectively permeable membrane
• Water moves from areas of high water concentration to areas of low water concentration
• Membrane is semipermeable: permeable to water but not necessarily to solute (e.g., sugar)
• Concept of tonicity governs water movement relative to surrounding solution

Osmosis and Tonicity

• Hypertonic solutions: higher solute concentration outside relative to inside
• Hypotonic solutions: lower solute concentration outside relative to inside
• Isotonic solutions: equal solute concentration outside and inside
• The surrounding fluid for body cells is typically isotonic

Osmosis Concentration Concepts

• Hypertonic: outside solution is saltier (higher solute concentration) than inside; water leaves the cell; cells shrink/shrivel
  • Example cue: adding salt to a slug causes it to shrink; reflects water movement out of cells
  • Mathematical cue: outside solute concentration > inside solute concentration
  • Word roots: Hyper = “more” or “above”
• Hypotonic: outside solution has less solute than inside; water enters cell; cells swell
  • Outside solute concentration < inside solute concentration
• Isotonic: outside and inside solute concentrations are equal; no net water movement; cell size remains unchanged
  • Iso = “equal”

Active Transport (energy required)

• Definition: transport that uses energy (ATP) to move molecules against their concentration gradient (low to high)
• Mechanisms involve protein pumps
• Types:
  • Exocytosis: materials EXIT the cell
  • Endocytosis: materials ENTER the cell
  • Pinocytosis: small particles enter the cell via small vesicle formation (cell drinking)
  • Phagocytosis: larger particles ENTER the cell via vesicle formation (cell eating)
• Example: carbon dioxide removal from body cells into surrounding blood vessels to be carried to the lungs for exhale; CO₂ moves from inside cells (lower CO₂ concentration) to blood (higher CO₂ concentration) requiring energy to be pumped across the membrane

Energy Comparison and Analogy

• No energy needed: Diffusion, Osmosis, Facilitated Diffusion
• Energy needed: Active Transport
• Analogy:
  • Passive Transport = downhill movement
  • Active Transport = uphill movement

Summary of Key Points

• The cell membrane is a selective barrier that maintains homeostasis and separates the cell from its environment
• It is composed of a phospholipid bilayer with hydrophilic heads and hydrophobic tails; proteins are embedded within the bilayer
• Prokaryotes have cell walls in addition to membranes; eukaryotes show variation: animal cells have membranes only, plant cells have membranes plus walls
• Homeostasis involves balancing pH, temperature, glucose, and water via passive and active transport
• pH is on a 0–14 scale; buffers help maintain stable pH
• Passive transport moves substances down their concentration gradient without energy input: Diffusion, Facilitated Diffusion, Osmosis
• Osmosis is water movement across a semipermeable membrane, governed by tonicity (hypertonic, hypotonic, isotonic)
• Active transport moves substances against their gradient using energy, via exocytosis, endocytosis, pinocytosis, and phagocytosis
• Conceptual framework: some processes occur without energy (downhill); others require energy (uphill)