Cell Membrane Transport

Phospholipid Bilayer and Selective Permeability

The cell membrane is a critical structure that separates the internal environment of the cell from the external environment. It is composed primarily of a phospholipid bilayer, which provides selective permeability, allowing only certain substances to cross.

• Phospholipid bilayer: Consists of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Selective permeability: Small, nonpolar molecules (e.g., O2, CO2) can cross easily; large polar molecules and ions require transport proteins.

• Membrane proteins: Facilitate the movement of specific molecules across the membrane.

Example: Oxygen and carbon dioxide gases diffuse directly through the lipid bilayer due to their small size and nonpolar nature.

Diffusion and Osmosis

Diffusion and osmosis are passive transport mechanisms that move substances across cell membranes without energy expenditure.

• Diffusion: Movement of particles from an area of high concentration to low concentration, down a concentration gradient.

• Osmosis: Diffusion of water across a selectively permeable membrane toward the side with higher solute concentration.

• Dynamic equilibrium: State where concentrations are equal but particles continue to move randomly.

• Facilitated diffusion: Passive movement of large or charged molecules via transport proteins.

• Active transport: Movement against a concentration gradient, requiring energy (often ATP).

Example: Water entering an animal cell in a hypotonic solution by osmosis, causing the cell to swell.

Types of Solutions and Effects on Cells

Cells respond differently to their environment depending on the solute concentration outside relative to inside.

• Hypertonic: Higher solute concentration outside; water moves out, cell shrinks.

• Hypotonic: Lower solute concentration outside; water moves in, cell swells and may burst.

• Isotonic: Equal solute concentration; no net water movement, cell remains stable.

Example: Cells placed in a hypotonic solution will swell and may lyse.

Cell Walls and Membranes

Cell walls provide structural support in plants, fungi, and many prokaryotes. Animal cells lack cell walls but have flexible membranes.

• Phospholipid bilayer: Creates a flexible, selectively permeable barrier.

• Membrane proteins: Aid in transport and communication.

Cellular Respiration

Overview and Equation

Cellular respiration is the process by which cells convert glucose and oxygen into ATP, the energy currency of the cell.

• Equation:

• Reactants: Glucose and oxygen

• Products: Carbon dioxide, water, ATP

• ATP: Transfers usable energy for cell activities

Example: Muscle cells use ATP for contraction during exercise.

Respiratory System and Gas Exchange

Structure and Function

The respiratory system enables gas exchange, supplying oxygen and removing carbon dioxide.

• Trachea: Connects mouth/nose to lungs

• Lungs: Absorb oxygen

• Alveoli: Tiny air sacs for rapid gas exchange

• Diaphragm: Muscle controlling airflow

• Heart: Circulates respiratory gases

Gas exchange: Oxygen diffuses into blood at alveoli; carbon dioxide diffuses out.

Breathing control: Brainstem senses CO2 and pH, adjusting breathing rate.

Cell Size and Surface Area-to-Volume Ratio

Importance of SA:V Ratio

Cell size affects efficiency of material exchange. Surface area-to-volume ratio (SA:V) decreases as cells grow, limiting transport.

• High SA:V: More efficient exchange

• Low SA:V: Less efficient; adaptations needed

Higher the surface area to volume ratio the quicker diffusion can occur

Rate of diffusion was about the same

Example: Small cells exchange nutrients and wastes more efficiently than large cells.

Homeostasis and Feedback Loops

Homeostasis

Homeostasis maintains stable internal conditions despite external changes through ONLY negative feedback .

• Negative feedback: Reverses a change (e.g., blood glucose regulation)

• Positive feedback: Amplifies a change (e.g., oxytocin during childbirth)

Blood Glucose Regulation

• Insulin: Lowers blood glucose by promoting uptake and storage

• Glucagon: Raises blood glucose by promoting release from storage

• Diabetes: Type 1 (no insulin production), Type 2 (cells unresponsive to insulin)

Example: After eating, insulin helps cells absorb glucose; between meals, glucagon releases glucose.

Temperature and Hydration Regulation

• Hypothalamus: Regulates temperature

• Sweating: Cools body

• Shivering: Produces heat

• ADH: Conserves water in kidneys

Example: During exercise, sweating and increased skin blood flow help release heat.

Connecting Concepts: Ultra-Marathons

During prolonged exercise, cellular respiration increases, requiring more oxygen and producing more carbon dioxide and heat. The respiratory and circulatory systems deliver oxygen and remove wastes, while homeostasis maintains temperature, hydration, and glucose levels.

Quick Self-Test and Checklist

• Explain diffusion, dynamic equilibrium, and osmosis

• Identify hypertonic, hypotonic, and isotonic solutions

• Compare passive, facilitated, and active transport

• Describe phospholipid bilayer and selective permeability

• Compare prokaryotic and eukaryotic cells

• Write cellular respiration equation and explain ATP, ETC, oxygen, fermentation

• Show directions of gas movement in lungs and tissues

• Explain cell size and SA:V ratio

• Explain lead toxicity

• Explain feedback loops, glucose regulation, temperature, and hydration