Cell Membranes and Transport: Structure, Function, and Osmosis

Membranes

Introduction to Cell Membranes

Cell membranes are essential structures that define the boundaries of cells and regulate the movement of substances in and out. They are dynamic and complex, playing a critical role in maintaining cellular homeostasis.

• Cell boundary: Separates the cell from its external environment.

• Regulation: Controls passage of molecules into and out of the cell.

• Production: Membranes are produced by the endoplasmic reticulum.

Plasma Membrane Structure

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which forms a selectively permeable barrier.

• Phospholipid molecule: Consists of a polar (hydrophilic) "head" and non-polar (hydrophobic) "tails".

• Bilayer arrangement: Polar heads face outward toward water, non-polar tails face inward, away from water.

• Selective permeability: Restricts access to certain molecules.

Fluid-Mosaic Model

Fluid Nature of Membranes

The fluid-mosaic model describes the plasma membrane as a dynamic structure with proteins and lipids moving laterally within the layer.

• Fluid: Phospholipid bilayer has the consistency of salad oil due to unsaturated fats.

• Mosaic: Proteins are embedded in and scattered throughout the lipid bilayer.

Role of Cholesterol

Cholesterol is an important component of animal cell membranes, affecting their properties.

• Reduces permeability: Non-polar cholesterol molecules decrease membrane permeability to small molecules.

• Stabilizes membrane: Prevents extremes in fluidity.

• Keeps membrane fluid: Maintains proper membrane consistency.

Membrane Proteins

Transmembrane Proteins

Transmembrane proteins span the entire membrane and are involved in various cellular functions.

• Integral proteins: Embedded within the bilayer, often spanning from one side to the other.

Glycoproteins

Glycoproteins are proteins with carbohydrate chains attached, projecting externally from the membrane.

• Cell recognition: Organize cells during development and defend against foreign materials.

• Medical relevance: Can cause problems with organ transplants due to immune recognition.

Transport Proteins

Transport proteins facilitate the movement of molecules and ions across the membrane.

• Tunnels and bridges: Allow specific molecules or ions to move across the membrane.

• Types: Channel proteins and carrier proteins.

Receptor Proteins

Receptor proteins transmit information into the cell by binding to specific molecules.

• Signal transduction: Bind hormones or neurotransmitters to initiate cellular responses.

Enzymatic Proteins

Enzymatic proteins embedded in the membrane catalyze chemical reactions.

• Speed reactions: Build and break down macromolecules.

• Energy extraction: Involved in processes in mitochondria and chloroplasts.

Crossing the Plasma Membrane

Semi-Permeable Nature

The plasma membrane is semi-permeable, allowing selective passage of substances.

• Small, uncharged molecules: Such as H2O, can pass through easily.

• Ions and large molecules: Such as K+, Na+, Ca2+, and glucose, require transport proteins.

Diffusion and Molecular Motion

Diffusion

Diffusion is the net movement of substances from regions of higher concentration to regions of lower concentration due to random molecular motion.

• Uniform distribution: Particles spread out evenly over time.

• Importance: Cells use diffusion to move substances across membranes and through cytoplasm.

Why Are Most Cells Microscopic?

Cell size is limited by metabolic requirements and the efficiency of diffusion.

• Bacteria: 1–10 μm in diameter.

• Eukaryotic cells: 10–100 μm in diameter.

• Diffusion distance: Too large a cell cannot diffuse substances fast enough to maintain metabolism.

Solutions and Osmosis

Definitions

• Solution (aqueous): Homogeneous, liquid mixture of two or more substances.

• Solvent: The dissolving agent (usually water).

• Solute: The substance dissolved in the solvent.

Osmosis

Osmosis is the diffusion of water across a semi-permeable membrane from a region of higher water concentration to a region of lower water concentration.

• Water movement: Water can pass through the membrane; solute molecules may be too large.

• Equilibrium: Water moves until concentrations are equal on both sides.

Tonicity and Effects on Cells

Tonicity

Tonicity refers to the strength of a solution in relation to osmosis and its effect on cell volume.

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell.

Osmosis in Animal Cells

Animal cells respond to changes in tonicity by gaining or losing water, which can affect cell shape and function.

• Isotonic solution: Cell maintains normal shape.

• Hypotonic solution: Cell swells; may burst (hemolysis).

• Hypertonic solution: Cell shrinks (crenation).

Osmosis in Plant Cells

Plant cells have a rigid cell wall that affects their response to osmotic changes.

• Hypotonic solution: Cell becomes turgid due to water uptake (turgor pressure).

• Hypertonic solution: Cell loses water; membrane pulls away from cell wall (plasmolysis).

Summary Table: Effects of Tonicity on Animal and Plant Cells

Solution Type	Animal Cell Response	Plant Cell Response
Isotonic	No net water movement; cell remains normal	No net water movement; cell remains flaccid
Hypotonic	Water enters; cell swells and may burst (hemolysis)	Water enters; cell becomes turgid (turgor pressure)
Hypertonic	Water leaves; cell shrinks (crenation)	Water leaves; cell undergoes plasmolysis

Key Equations

Diffusion Rate

The rate of diffusion is influenced by concentration gradient, temperature, and membrane permeability.

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

• J: Diffusion flux

• D: Diffusion coefficient

• \frac{dC}{dx}: Concentration gradient

Osmosis

• Osmotic Pressure Equation:

$\Pi = iMRT$

• \Pi: Osmotic pressure

• i: van 't Hoff factor (number of particles per molecule)

• M: Molarity

• R: Gas constant

• T: Temperature (Kelvin)

Examples and Applications

• Organ transplant rejection: Glycoproteins on cell surfaces can trigger immune responses.

• Red blood cell hemolysis: Placing cells in hypotonic solutions causes them to burst.

• Plant turgor pressure: Essential for maintaining plant structure and growth.

Additional info: Academic context and equations have been added to expand upon the original notes and provide a self-contained study guide.