Cell Size, Plasma Membrane Structure, and Membrane Transport Mechanisms

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Cell Size and Surface Area-to-Volume Ratio

Importance of Cell Size

The size of a cell is a critical factor in its ability to exchange materials with its environment. The surface area-to-volume ratio (SA:V) determines how efficiently a cell can obtain nutrients, remove waste, and exchange gases.

Surface Area: Determines the rate of exchange of materials.
Volume: Determines the requirements for materials and metabolic activity.
As cell size increases, volume increases faster than surface area, making exchange less efficient.
Cells remain small to maintain a high SA:V ratio, which facilitates efficient exchange.

Mathematical Relationships

For a cube: Surface Area: Volume:
For a sphere: Surface Area: Volume:
As radius increases from 1 to 3, SA increases from 1 to 9, but V increases from 1 to 27.

Example Table: Comparison of SA:V Ratios in Animals

Organism	Surface Area (cm2)	Volume (cm3)	SA:V Ratio
Insect	102	101	10
Mouse	103	102	10
Dog	104	103	10
Elephant	105	104	10

Additional info: As organisms get larger, it becomes more difficult for them to exchange materials with their surroundings due to a lower SA:V ratio.

Plasma Membrane Structure and Function

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol. Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

Hydrophilic heads face the aqueous environments inside and outside the cell.
Hydrophobic tails face inward, away from water.

Membrane Proteins

Integral proteins: Span the membrane and are involved in transport and signaling.
Peripheral proteins: Loosely bound to the surface and play roles in cell signaling and structure.
Proteins determine most of the membrane’s specific functions.

Membrane Carbohydrates

Attached to proteins (glycoproteins) or lipids (glycolipids).
Play a key role in cell-cell recognition and immune response.
Basis for blood group differences and tissue development.

Membrane Models

Davson-Danielli Model: Proposed a protein-phospholipid sandwich.
Fluid Mosaic Model: Describes the membrane as a dynamic mosaic of proteins and lipids that move and change.

Membrane Permeability

Semi-Permeable Nature

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

Small non-polar molecules (O2, CO2) and hydrophobic molecules can pass directly.
Large or polar molecules require transport proteins.
Polar uncharged molecules (H2O) pass in small amounts.

Membrane Transport Mechanisms

Passive Transport

Passive transport is the movement of molecules from high to low concentration without energy input.

Diffusion: Movement of molecules down their concentration gradient.
Facilitated Diffusion: Movement of large or charged molecules via transport proteins (channels or carriers).
Osmosis: Diffusion of water across a semi-permeable membrane.

Active Transport

Active transport requires energy (usually ATP) to move molecules against their concentration gradient.

Protein pumps: Move ions such as Na+ and K+ across the membrane.
Proton pumps: Move H+ ions, important in cellular respiration and photosynthesis.
Endocytosis: Cell engulfs large molecules or particles by forming vesicles.
Exocytosis: Cell expels materials using vesicles.

Equations

Diffusion rate is proportional to the concentration gradient.
For Na+/K+ pump: per ATP hydrolyzed

Tonicity and Osmoregulation

Tonicity refers to the ability of a solution to cause a cell to gain or lose water. Osmoregulation is the control of water and solute concentrations within a cell.

Hypotonic: Lower solute concentration outside the cell; water enters the cell.
Hypertonic: Higher solute concentration outside the cell; water leaves the cell.
Isotonic: Equal solute concentration; no net water movement.

Summary Table: Types of Membrane Transport

Type	Energy Required	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose, Ions
Active Transport	Yes (ATP)	Low to High	Na+/K+ Pump
Endocytosis	Yes (ATP)	Into Cell	Phagocytosis
Exocytosis	Yes (ATP)	Out of Cell	Insulin Release

Key Terms and Definitions

Amphipathic: Molecule with both hydrophilic and hydrophobic regions.
Selective Permeability: Ability of the membrane to allow some substances to pass while restricting others.
Facilitated Diffusion: Passive movement of molecules with the help of transport proteins.
Active Transport: Movement of molecules against their concentration gradient using energy.
Tonicity: The effect of a solution on cell water balance.
Osmoregulation: Regulation of water and solute concentrations in cells.

Examples and Applications

Osmosis in Red Blood Cells: Placing cells in hypotonic or hypertonic solutions causes swelling or shrinking.
Na+/K+ Pump: Maintains membrane potential necessary for nerve impulse transmission.
Facilitated Diffusion: Glucose transport into cells via carrier proteins.

Additional info: These notes cover topics from General Biology chapters on cell structure, membrane function, and transport mechanisms, providing foundational knowledge for understanding cellular processes.