Topic 2.4 MEMBRANES

2.4.1 Draw and Label a Membrane cholesterol

Phospholipid Bilayer – Phosphate head (polar) – Fatty acid tails (nonpolar) – Fluid Mosaic Model – Cholesterol determines fluidity Less cholesterol = more fluid and more permeable More cholesterol = more stable and less permeable

Functions of the Membrane: – Keep the cell contents separate from the outside – Control what substances enter and leave the cell – Proteins assist with bringing substances into the cell

2.4.2 hydrophobic and hydrophilic portions of phospholipids maintain structure Environment inside and outside the cell is mostly water – Polar (hydrophilic) phosphate heads are attracted to water – Nonpolar (hydrophobic) fatty acid tails are repelled by water

Types of Membrane Proteins Integral Proteins – Spans the membrane Peripheral Proteins – Mostly found on the outside of the membrane Some proteins have carbohydrates attached and are called glycoproteins

Cell Membrane Rap K2w K2w

Summary of and Main component of membranes is a phospholipid bilayer Membranes contain integral and peripheral proteins Some proteins are glycoproteins Membranes contain cholesterol which determines fluidity and permeability

2.4.3 List the Functions of Membrane Proteins 1.Hormone binding sites – hormones transported by the blood only act on cells with specific protein receptors 2.Immobilised enzymes (membrane-bound) – ex. electron transport chain on cristae of mitochondria 3.Cell adhesion – integral proteins can stick out and bind to proteins in adjacent cells 4.Cell to Cell communication – via direct contact or signals by hormones or neurotransmitters (signal transduction pathway)

5.Channels for passive transport – membrane is selectively permeable (semipermeable) pass through easily: small nonpolar molecules, gases, water require channels: polar molecules – outside is hydrophobic (near fatty acid tails) – inside of channel is hydrophilic (polar molecules can pass through hydrophobic amino acids hydrophilic amino acids

6.Pumps for active transport – ex. Na + /K + pump in nerve cells – requires ATP (adenosine triphosphate) to transport Na ions back outside the axon and K ions back in –

2.4.4 Define Diffusion and Osmosis Diffusion is the movement of gas or liquid particles from an area of higher concentration to an area of lower concentration Osmosis is the diffusion of water molecules across a partially permeable membrane from a region of lower solute concentration to an area of higher solute concentration Diffusion and Osmosis are both types of passive transport (require no Energy and move particles down their concentration gradient)

2.4.5 Explain Passive Transport Simple Diffusion – Does not require E – Substances move down the concentration gradient (from higher [ ] to lower [ ]) – Mostly small, non-polar molecules Facilitated Diffusion – Does not require E – Substances move down the concentration gradient (from higher [ ] to lower [ ]) – Uses membrane proteins: Channel protein- have a hydrophilic pore through which ions (charged particles ) can diffuse through Transport proteins- has specific binding sites (ex. glucose)

2.4.6 Explain the role of protein pumps and ATP in active transport Active transport: – requires E in the form of ATP (adenosine triphosphate) – moves substances against the [ ] gradient (from low [ ] to higher [ ]) – proteins involved are called pumps or transport proteins – transport proteins are often called carrier proteins or membrane pumps – ex. sodium-potassium pump in nerve cells 3 Na + ions are actively pumped out of the nerve cell using ATP 2 K+ ions are pumped back in

2.4.7 Explain how vesicles are used to transport materials between the ER, Golgi and plasma membrane Intracellular transport often involves the use of vesicles Since the structure of the plasma membrane is essentially the same as the membranes of the ER, Golgi and nuclear envelop, it is possible to exchange membrane sections RER produces proteins intended for export from the cell (secretory proteins) Golgi apparatus prepares newly synthesized proteins for export (exocytosis) These proteins are then wrapped in membrane from the Golgi which eventually joins the cell membrane to be exported by exocytosis

The Endomembrane System Many of the different membranes within the cell are part of the endomembrane system. These membranes are related by: – direct physical continuity – by the transfer of membrane segments through the movement of vesicles.

The endomembrane system includes: nuclear envelope endoplasmic reticulum Golgi and vesicles lysosomes, peroxisomes, vacuoles It does not include mitochondria or chloroplasts. NE RER vesicle

Steps involved in exporting synthesized proteins: 1.The nucleus contains the chromosomes with genes that code for proteins. mRNA is made through transcription and carries the code to the ribosomes on the RER. 2.The ribosomes make proteins (polypeptide chains) through the process of translation 3.The protein is then surrounded by ER membrane (becomes enclosed in a vesicle) and moves to the Golgi apparatus for processing 4.The protein then is wrapped in Golgi membrane and transported in this secretory vesicle to the cell membrane to be exported

2.4.8 Describe how membrane fluidity allows it to change shape, break and reform during endocytosis and exocytosis Endocytosis and exocytosis are types of active transport- require E (ATP) and move substances against the [ ] gradient Endocytosis- taking in large or highly polar molecules ex. white blood cells engulfing bacteria – 2 types: pinocytosis (liquids-cell drinking) phagocytosis (solids-cell eating) Exocytosis- removal of large substance from the cell

Overview of processes by which materials enter and exit a cell ATP required concentration gradient Diffusionnodown (with gradient) to Facilitated Diffusionnodown Osmosisnodown Active Transport with Carrier Proteins yesagainst is possible Endocytosis & Exocytosisyesagainst is possible (with gradient) to