Membrane Structure and Function

Cells must contain a cell membrane, cytoplasm and genetic material.

The cell membrane is the EDGE, “boundary of life”, while the cytoplasm is the site of all the reactions of life and the genetic material is the information required for life.

SELECTIVELY PERMEABLE means that the CM allows some substances across more easily than others… some it helps and some it inhibits or rejects all together.

Membrane Structure

Development of models as a scientific process (linked to technology)

Models include Overton – proposed lipid base Langmuir (phospholipids) – made membrane Gorter and Grendel (two layers) Davson and Danielli (sandwich image, protein outside)

Singer and Nicolson = Fluid Mosaic Model Required electron microscopy 1972

Membrane Structure

Phospholipids – phosphate and fatty acid tails Hydrophilic phosphates Hydrophobic fatty acid tails

Two layers With tails together

Protein Embedded throughout - integral Some just surface – peripheral Motility

Idea of Fluidity

Cholesterol molecules prevent membrane from becoming ‘crispy’

Proteins ‘bob’ along surface Proteins are anchored by fibers in the

cytoskeleton Phospholipids can even flip flop Merging of membranes ( pg. 128)

Proteins - many purposes within CM

Transport – substance across membrane, specificity to substance, active pumps

Enzymatic activity – sequence reactions Transmit signals – conformational change

when with substrate = message. Junctions – glue cells into tissues Recognition – glycoproteins act as targets or

ID Attach to cytoskeleton – change shape of cell

Other organelles

The cell membrane is continuous with other membranes in the cell

Endoplasmic reticulum starts/ends at CM ER makes CM and it is turned inside out as

the new CM flows out of the ER….

Traffic Across the Membrane

Small particles and ions move in both directions; sugars, amino acids, CO2, NH4 and O2. Also Na+, K+, Ca+2 and Cl-

Hydrophobic molecules ( like CO2) can dissolve in lipid layer

Hydrophilic core of CM impedes movement of ions and polar molecules that are hydophilic.

Proteins built into membrane facilitate the process

Transport Proteins

Span the CM Function as a channel Some also physically move passengers Specificity of fit ( form and function ) allows

for membrane selectivity. (emergent property and homeostasis )

Passive Transport

Molecules have intrinsic kinetic energy They are in constant motion Cytoplasm, extracellular fluid and CM vary in

concentrations and pressure gradients. Concentration refers to the overall

POPULATION of molecules as well as the RATIO in that location compared to another.

Net Movement DYNAMIC EQUILIBRIUM

Passive transport includes

Diffusion Tendency of molecules of any substance to spread out

into the available space. Substances will diffuse DOWN their concentration gradient. (Entropy)

Osmosis The diffusion of water molecules across a selectively

permeable membrane. Hypertonic = solution with higher [conc.] of solutes Hypotonic = solution with lower [conc.] of solutes Isotonic = solutions are equal in solute concentration

Facilitative Diffusion Transport proteins are helping molecules to cross

membrane, but still diffusion (lowering overall free energy) thus doesn’t require energy from cell.

Active Transport

Movement of solutes, UP their concentration gradient, with the help of transport proteins in a CM.

Requires cell to expend energy / do work. Solute becomes more concentrated or

“stock- piled” in a particular area.

Example is Sodium-Potassium Pump. (Na-K)

Electrogenic Pumps

In addition to moving ions, some active transport pump also generate electric current and/ ATP.

An enzyme called ATP synthase is built into the membrane and ADP is phosphorylated as a result of ions (often H+) passing through the membrane.

Examples are found in the Electron Transport Chains of respiration (inner mitochondrial membrane) and photosynthesis (chloroplasts thylakoid membrane)

THE “Answers” for AP Biology

If you don’t know an answer choose either

Hydrogen bond

(water, organic compounds, etc)

OR

Hydrogen ion

(membrane pumps, nerve impulses, etc)