Cell Membrane Structure, Diffusion & Osmosis, and Histology Study Guide

Cell Membrane Structure & Function

Phospholipid Bilayer and Fluid Mosaic Model

The plasma membrane is a selectively permeable barrier that separates the cell's internal environment from the external environment. Its structure is best described by the fluid mosaic model, which states that the membrane is a dynamic, fluid structure with proteins embedded within a bilayer of phospholipids.

• Phospholipid bilayer: Composed of two layers of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

• Fluid mosaic model: Lipids move laterally within the layer; proteins are scattered like mosaics.

• Cholesterol: Provides membrane stability and fluidity.

• Glycoproteins: Proteins with attached carbohydrate chains; important for cell recognition.

Membrane proteins serve several functions:

• Transport: Channel and carrier proteins facilitate movement of substances.

• Receptors: Receive and transmit signals.

• Enzymes: Catalyze reactions at the membrane surface.

• Attachment: Anchor the membrane to cytoskeleton or extracellular matrix.

• Recognition: Glycoproteins help identify cells.

Selective permeability means the membrane allows some substances to pass while restricting others, based on size, polarity, and the presence of transport proteins.

Arrangement of Phospholipids

Phospholipids form a bilayer in water because their hydrophilic heads interact with water, while hydrophobic tails avoid water, creating a stable barrier.

Transport Across the Membrane: Diffusion & Osmosis

Diffusion

Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration, driven by the concentration gradient.

• Simple diffusion: No energy required, no protein needed; molecules like O2, CO2, and lipids pass directly through the membrane.

• Facilitated diffusion: No energy required, but needs channel or carrier proteins; used by glucose (GLUT transporters) and ions.

Factors affecting diffusion rate:

• Concentration gradient: Steeper gradient increases rate.

• Temperature: Higher temperature increases rate.

• Molecular size: Smaller molecules diffuse faster.

• Surface area: Larger area increases rate.

• Distance: Shorter distance increases rate.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane, moving from areas of high water concentration to low water concentration. Aquaporins are specialized water channel proteins that speed up osmosis.

• Osmosis is passive: No ATP required.

• Water moves toward higher solute concentration.

Tonicity and Cell Volume

Tonicity describes how a solution affects cell volume, based on the concentration of non-penetrating solutes:

• Isotonic: No net water movement; cell remains normal.

• Hypotonic: Water enters cell; cell swells and may burst (lysis).

• Hypertonic: Water exits cell; cell shrivels (crenation in animal cells, plasmolysis in plant cells).

Clinical application: IV fluids must be isotonic (0.9% saline) to prevent red blood cell damage.

Lab Experiment Conclusions

• Dialysis tubing experiment: Tube with sugar solution gains weight as water moves in by osmosis; membrane is permeable to water, not sugar.

• Agar diffusion experiment: Dye spreads outward, demonstrating diffusion from high to low concentration; rate depends on molecular size.

• Red blood cells: Burst in hypotonic solution (no cell wall); shrivel in hypertonic solution.

• Elodea (plant cells): Become turgid in hypotonic solution; plasmolyzed in hypertonic solution (cell wall prevents bursting).

Histology: Tissue Types

Overview of Epithelial, Muscle, and Nervous Tissues

Histology is the study of tissues. The lab focused on identifying and understanding the structure and function of epithelial, muscle, and nervous tissues.

Tissue Type	Distinguishing Features	Location	Function
Nervous tissue	Neurons with axons, dendrites	Brain, spinal cord	Conduct impulses
Cardiac muscle	Striated, branched, intercalated discs	Heart	Pump blood
Skeletal muscle	Striated, multinucleated, voluntary	Attached to bones	Movement
Smooth muscle	Non-striated, spindle-shaped	Digestive tract	Involuntary movement
Stratified squamous	Multiple layers, flat on top	Skin	Protection
Simple squamous	Single layer, flat cells	Lung alveoli	Diffusion

Each tissue type has unique structural features that relate to its function and location in the body.

Summary Table: Transport Types

Transport Type	Energy?	Protein?	Direction	Example
Simple diffusion	No	No	High → Low	O2, CO2, lipids
Facilitated diffusion	No	Yes (carrier/channel)	High → Low	Glucose via GLUT, ions
Osmosis	No	Aquaporins	High water → Low water	H2O

Key Terms and Concepts

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Plasmolysis: Cytoplasm pulls away from cell wall in hypertonic solution (plant cells).

• Aquaporins: Water channel proteins that speed up osmosis.

• Selective permeability: Membrane allows some substances through, not others.

• Tonicity: Effect of solution on cell volume.

Equations and Formulas

Diffusion rate can be described by Fick's Law:

$\text{Rate} = \frac{(\text{Concentration gradient}) \times (\text{Surface area}) \times (\text{Diffusion coefficient})}{\text{Distance}}$

Osmosis is governed by the movement of water toward higher solute concentration.

Lab and Clinical Applications

• IV fluids must be isotonic to prevent red blood cell lysis or crenation.

• Dialysis tubing experiments demonstrate osmosis and selective permeability.

• Plant and animal cells respond differently to changes in tonicity due to the presence or absence of a cell wall.

Self-Quiz and Practice Questions

• Label the parts of the plasma membrane.

• Compare simple and facilitated diffusion.

• Explain why red blood cells burst in distilled water but plant cells do not.

• Describe the role of aquaporins.

• Identify factors that increase the rate of diffusion.

• Explain the clinical importance of isotonic IV fluids.

Images

Relevant images are included only where they directly reinforce the explanation:

• Nervous tissue overview

• Quick reference for transport types and tissue features

Additional info: Academic context was added to clarify the structure and function of membrane proteins, transport types, and tissue histology, as well as to provide clinical relevance and equations.