Cell Membrane Components & Transport

• Cell membranes determine organization and function based on their components.
• The cell membrane plays a role in cell recognition and adhesion.
• Simple diffusion, osmosis, and facilitated diffusion are types of passive transport across the cell membrane.
• Active transporters vary in type and energy source.

Membrane Components: Lipids, Proteins, and Carbohydrates

• The physical organization and function of biological membranes relies on its constituents.
• Lipids, proteins, and carbohydrates are key components of the cell membrane.
• The phospholipid bilayer forms a "lipid lake" in which proteins float.
• The fluid mosaic model describes the cell membrane as composed of many discrete components that can move freely

Lipids

• Phospholipids and cholesterol are examples of lipids in cell membranes.
• Phospholipids are the major lipid component of biological membranes
• The nonpolar, hydrophobic fatty acid "tails" interact inside the bilayer.
• Charged, polar, hydrophilic "head" portions interact with polar water.
• Phospholipids vary in fatty acid chain length, degree of unsaturation, and the presence of polar groups.
• Saturated fatty acids in phospholipids allow close packing.
• Unsaturated fatty acids in phospholipids introduce kinks for less dense, more fluid packing.
• Cholesterol, a steroid, is abundant in animal cell membranes.
• The hydroxyl group of cholesterol interacts with polar phospholipid ends.
• Nonpolar rings and hydrocarbon chains of cholesterol interact with the membrane's interior.
• Cholesterol maintains membrane integrity and modulates membrane fluidity.
• A phospholipid molecule can travel from one end of the cell to the other.
• It is unlikely for a phospholipid to spontaneously flip to the other side of the bilayer.
• Cholesterol and long-chain saturated fatty acids reduce membrane fluidity
• Movement is limited when cholesterol and long-chain saturated fatty acids are present.
• Shorter-chain fatty acids, unsaturated fatty acids, and less cholesterol increase membrane fluidity.
• Lower temperatures slow molecular movement and decrease fluidity.
• Replacing saturated, long-chain fatty acids with unsaturated, shorter-chain fatty acids helps with fluidity at lower temperatures.

Proteins

• Integral, peripheral, and anchored are types of membrane proteins.
• Membrane proteins make up about ¼ of the eukaryotic genome.
• Integral membrane proteins are partially embedded in the bilayer.
• Hydrophilic domains of integral proteins interact with water inside or outside the cell.
• Hydrophobic domains of integral proteins interact with fatty acids in the phospholipid bilayer.
• A transmembrane protein extends through the entire bilayer.
• Transmembrane proteins have different domains and functions on the inner and outer membrane sides.
• Peripheral membrane proteins lack exposed hydrophobic groups and aren't embedded in the bilayer.
• Peripheral proteins interact with exposed integral protein parts or phospholipid polar heads.
• Anchored membrane proteins attach covalently to fatty acids or other lipid groups.
• Hydrophobic lipid groups insert into the phospholipid bilayer allowing association with the membrane.

Carbohydrates

• Glycoproteins and glycolipids are examples of carbohydrates found in the cell membrane
• Carbohydrates are located on the outer cell surface and serve as recognition sites for other cells and membranes.
• Carbohydrates may covalently bond to lipids (forming a glycolipid) or to proteins (forming a glycoprotein).
• Proteoglycans are more heavily glycosylated proteins with longer carbohydrates compared to glycoproteins.

Selective Permeability of Biological Membranes

• Biological membranes allow only some substances to pass through, hence selective permeability.
• Specific proteins present in the membrane determines which substances enter or leave.
• Passive and active transport describe two fundamental processes for substances crossing membranes.
• Passive transport doesn't require chemical bond energy input
• Active transport requires energy from chemical bonds.

Passive Transport

• With passive transport, energy is derived from a substance's concentration difference across the membrane (concentration gradient).
• In a solution, there is a tendency for all components to be evenly distributed
• Solution at Equilibrium does not mean molecules stop moving
• It means their overall distribution does not change
• Diffusion is the process of random movement toward equilibrium.
• Motion of each particle is random, but net movement is directional until equilibrium.
• Net movement in diffusion goes from high to low concentration regions.
• A substance's rate of diffusion depends on five factors: size/mass, temperature, density, the concentration gradient and the area across which a substance diffuses

Simple Diffusion

• Is the method by which small molecules pass across phospholipid bilayer
• Hydrophobic molecules (soluble in lipids) use this method

Electrically Charged or Polar Molecules

• Do not readily pass through the membrane due to not being very soluble in the hydrophobic interior of the bilayer
• Form hydrogen bonds with water and ions in aqueous environments of cytoplasm or cell exterior

Aquaporins

• Water can cross membranes through protein channels called aquaporins

Osmosis

• Is the movement of water molecules through a membrane towards a higher solute concentration
• Passive and does not require metabolic energy

Tonicity

• There are three terms to compare two soluitions based on solute concentration
• Isotonic solutions have equal solute concentrations
• A hypotonic solution has a lower solute concentration compared to the other solution
• A hypertonic solution has a higher solute concentration compared to the other solution

Facilitated Diffusion

• Channel or carrier proteins aid in passive transport (no energy input required)
• Integral membrane proteins form hydrophilic channels (tunnels) allowing certain substances to pass
• Gated channels only open when a stimulus changes their 3D shape
• Chemcial signals act as an example of a stimulus (ligand)
• Carrier protiens bind substances and speeed their diffusion accros the phospholipid bilayer
• Polar molecules and amino acids eg. glucose employ this method

Glucose Transporters

• The process of faciliting glucose transport is undertaken by a glucose transporter protein
• A specific 3D site will bind glucose on the protein
• Which causes it to change shape and release glucose to the other side of membrane
• Diffusion rate also increases as the concentration gradient increases
• The rate of increase slows
• The facilitated diffusion system saturates at a certain point

Active Transport

• Active transport is neccessary because a cell must sometimes move substances against their concentration gradients
• It requires energy, often in the form of ATP
• It is directional, depending on need of the cell
• There are there kinds of membrane proteins that undertake active transport
• Uniporters transport one substance in one direction
• Symporters transport two different substances in the same direction
• Antiporters transport two different substances in opposite directions
• There are two basic types of active transport in terms of energy source: primary and secondary active transport
• Primary active transport involves direct hydrolysis of ATP
• Secondary active transport does not draw energy from ATP directly
• it draws it's energy from an ion concentration gradient
• a key instance of primary active transport is the sodium-potassium pump
• Secondary active transport employs the energy “regained” by letting ions move across the membrane with their concentration gradients

Sodium-Potassium Pump

• Found in all animal cells.
• Is an intergal membrane glycoprotein.
• Draws energy from ATP to facilitate transport.
• Is an antiporter.
• Three Na+ are transported out of the cell.
• Two K+ are transported into the cell.

Endocytosis

• Macromolecules and particles enter the cell by endocytosis
• The cell membrane surrounds a part of the exterior environment and buds off as an internal vesicle
• three types: phagocytosis (“cellular eating”), pinocytosis (“cellular drinking”) and receptor-mediated endocytosis
• all three types, the cell membrane invaginates, forming a small pocket around materials from the environment
• the pocket deepens to form a vesicle
• the vesicle separates from the membrane and migrates to the cell’s interior

Exocytosis

• = the process by which materials packaged in vesicles are secreated from a cell