Plasma Membrane Structure and Function: Study Notes

Plasma Membrane Structure and Function

Fluid-Mosaic Model

The plasma membrane is a dynamic and complex structure that surrounds all cells, providing both protection and selective permeability. The Fluid-Mosaic Model describes the membrane as a flexible bilayer of lipids with embedded proteins and carbohydrates.

• Fluid bilayer: Composed of phospholipids arranged in two layers. Each phospholipid has a hydrophilic (polar) head and hydrophobic (nonpolar) tails.

• Mosaic: Refers to the patchwork of proteins, cholesterol, and carbohydrates embedded within the lipid bilayer.

Key Terms:

• Hydrophilic: Water-attracting; describes the phosphate head of phospholipids.

• Hydrophobic: Water-repelling; describes the fatty acid tails of phospholipids.

Membrane Fluidity

Membrane fluidity is essential for proper cell function, affecting the movement of proteins and lipids within the membrane and the ability of the cell to change shape.

Molecule in Plasma Membrane	Double Bonds/Kinks in Tails	How Affect Membrane Fluidity
Phospholipids w/ saturated fatty acids	No double bonds (straight tails)	Decrease fluidity (tails pack tightly)
Phospholipids w/ unsaturated fatty acids	Double bonds (kinked tails)	Increase fluidity (tails pack loosely)
Cholesterol (at moderate temp)	N/A	Reduces fluidity by restraining movement of phospholipids
Cholesterol (at low temp)	N/A	Prevents membrane from solidifying; increases fluidity

• Example: Thermophilic bacteria in hot springs have more phospholipids with unsaturated fatty acids to maintain membrane fluidity at high temperatures.

Plasma Membrane Components

The plasma membrane contains various components, each with specific functions.

Label	Structure	Description
A	Phospholipid	Hydrophilic heads face water; hydrophobic tails are shielded from water
B	Transmembrane protein	Integral protein that spans the membrane; involved in transport and signaling
C	Peripheral protein	Attached to the membrane surface; involved in signaling and structural support
D	Glycoprotein	Protein with attached carbohydrate; functions in cell recognition
E	Glycolipid	Lipid with attached carbohydrate; involved in cell recognition
F	Cholesterol	Steroid found in plasma membrane of animal cells; modulates fluidity

Membrane Proteins: Functions and Visualizations

Membrane proteins perform a variety of functions essential for cell survival and communication.

• Transport proteins: Channels or pumps that assist with passive or active transport of ions and molecules.

• Glycoproteins: Provide cell "signature" or ID tags for recognition.

• Enzymatic proteins: Assist with metabolic pathways.

• Junction proteins: Join adjacent cells via gap or tight junctions.

• Receptor proteins: Bind signaling molecules and initiate cellular responses.

• Attachment proteins: Anchor the membrane to the cytoskeleton and extracellular matrix (ECM).

The Two Faces of the Membrane

The plasma membrane is asymmetric, with different molecules facing the interior and exterior of the cell.

Location	Result	Visualization
Proteins (secretory & membrane) and lipids	Transmembrane proteins are made with carbohydrates facing the exterior	Glycoprotein, Glycolipid
Glycoproteins	Can be further modified and transported in vesicles with carbohydrates facing out	Vesicle fusion and release

Membrane Transport

Permeability of the Membrane

The plasma membrane is selectively permeable, allowing only certain molecules to cross.

Type of Molecule	How Easily Does It Cross the Bilayer?	Examples
Small nonpolar molecules	Can pass through membrane easily	O2, CO2
Small polar molecules	Can slowly pass through membrane	H2O
Large or charged molecules	Cannot pass through membrane	Glucose, ions

• Transport proteins are required for molecules that cannot pass the phospholipid bilayer on their own.

• Example: Aquaporin proteins facilitate rapid water transport across the membrane.

Passive Transport

Passive transport is the movement of substances across the membrane without energy input, driven by concentration gradients.

Type	Visualization	Movement of Ions	Energy?
Simple diffusion	Direct movement through bilayer	High to low concentration	No
Facilitated diffusion	Movement via transport protein	High to low concentration	No

Osmosis and Tonicity

Osmosis is the facilitated diffusion of water across a membrane. Tonicity describes the relative concentration of solutes in solution compared to the cell.

• Isotonic: $[\text{solute}]_{\text{solution}} = [\text{solute}]_{\text{cell}}$ Water has no net movement.

• Hypotonic: $[\text{solute}]_{\text{solution}} < [\text{solute}]_{\text{cell}}$ Water moves into the cell.

• Hypertonic: $[\text{solute}]_{\text{solution}} > [\text{solute}]_{\text{cell}}$ Water moves out of the cell.

Water Balance in Plants & Animals

Cells respond differently to changes in water balance depending on their structure.

	Isotonic	Hypotonic	Hypertonic
Animal Cell	Normal shape	Swells and may burst (lyse)	Shrivels (crenates)
Plant Cell	Flaccid	Turgid (normal)	Plasmolyzed

• Example: Red blood cells in a hypotonic solution will swell and burst, while plant cells become turgid due to their cell wall.