Cell Membranes and Transport

Phospholipid Bilayer

Keeps hydrophobic tails away from water and allow hydrophilic heads to interact with water

Sterols

Dual solubility properties, packed inside membrane

Cholesterol in animals

keeps membrane fluid

Semi-permeability

Regulates the entry of molecules

Transport Proteins

Form channels that allow selected polar molecules and ions to pass across a membrane (specific proteins recognize specific molecules)

Cell-Cell Recognition Proteins

Identify a cell as part of same individual or as foreign

Receptor Proteins

Recognize and bind molecules to begin a cell signal transduction

Cell Adhesion Proteins

Bind cells together by recognizing and binding receptors or chemical groups on other cells (gap junctions, tight junctions)

Enzymatic Proteins

Specific to particular membranes (Brush border enzymes in the small intestine)

Glycolipids

Lipids + Carbohydrates

Glycoproteins

Proteins + Carbohydrates

Glycocalyx

Surface coat of carbs of the glycoproteins and glycolipids

Fluid Mosaic Model

Model used to demonstrate membrane makeup

Where are glycolipids + glycoproteins found?

In parts of the membrane that face outside the cell

What does Glycocalyx do?

Protects cells against physical/mechanical damage

Integral Proteins

Proteins embedded in the membrane

Peripheral Proteins

Held to the membrane by noncovalent bonds

Passive Transport

Ions or molecules diffuse with their concentration gradient from high → low

Simple Diffusion

Nonpolar/very small molecules can pass through membrane without help

Facilitated Diffusion

Polar/charged molecules need transport proteins to get them through membrane

Channel Proteins

Proteins that span the membrane and form channels that molecules can pass through

Aquaporins

Channels that water passes through (increase rate of diffusion)

Ion Channels

Facilitate the passage of ions, often gated (Na, K, Cl)

Gated Channels

Closed channels that need the specific ion to bind to the protein for the channel to enter

Carrier Proteins

Proteins change shape to bring selective solutes through the membrane

Active Transport

Directly uses ATP to move against concentration gradient (ex: Na/K pump → uses ATP to pump 3 Na out of a cell and 2 K in)

Secondary Active Transport

Indirectly uses ATP; use a favorable concentration gradient (often built up by primary active transporters) to transport

Symport

Both molecules move in the same direction

Antiport

Molecules move in opposite directions

Osmosis

How water diffuses across membrane due to solute concentrations (passive diffusion high → low)

Osmotic Pressure

The amount of pressure applied to a solution required to prevent water from diffusing across a membrane

Tonicity

Concentration of solute in solution relative to the amount of water present across a specific membrane

Hypotonic

Less solutes relative to water

Hypertonic

More solutes relative to water

Isotonic

Equal parts of solutes and water

Exocytosis

Molecules are released from the cell through vesicles that fuse with the membrane when they reach the cell membrane

Endocytosis

Molecules are taken up into the cell through pit like indentations in the membrane that pinch off and form vesicles within the cells

Bulk endocytosis

All substances in the pit are taken up into the vesicle; nonspecific

Receptor

Mediated endocytosis - bind specific molecules and take the target molecules up into the cell