Transport across cell membranes CELL BIOLOGY! Transport across cell membranes

Cell membrane: A fluid mosaic REMINDER: Why are phospholipids so special? The membrane is about as fluid as salad dressing, and the components drift about phospholipids are constantly moving and bumping into each other. It’s like a crowded party! https://www.youtube.com/watch?v=Qqsf_UJcfBc ACTIVITY: Membrane Structure

A beautiful mosaic… The membrane has various proteins embedded in its fluid framework These proteins have different functions, depending on the type of cell.

Cholesterol This steroid helps to stabilize the membrane at warm temperatures, and also helps keep it fluid at low temperatures

Cell Membrane – A Mosaic

Functions of Membrane Proteins https://www.youtube.com/watch?v=GW0lqf4Fqpg (a) Transport – active vs. passive PASSIVE  if molecules are moving from [high] to [low] ACTIVE  if molecules are moving from [low] to [high] Active transport requires ATP (b) Enzymes for metabolic pathways (c) Receptors for chemical messages. “Signal Transduction” ACTIVITY: Signal Transduction

Functions of membrane proteins (d) Cells need to recognise other cells around them, for example white blood cells move across the lining of blood vessels (e) Cells join to one another by protein junctions. When the space gets too crowded they know to stop dividing (f)Help maintain cell shape by attaching to the cytoskeleton & extracellular matrix https://www.youtube.com/watch?v=U6uHotlXvPo&feature=related

How do substances cross? http://www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html When molecules diffuse down their concentration gradient, they move from an area of [high] to [low] until equilibrium is reached At equilibrium molecules still move but at an equal rate in and out of the cell.

Selectively Permeable Activity: Selective Permeability of Membranes

Passive Transport ACTIVITY: Diffusion A cell does not have to do work to achieve equilibrium Molecules will still move back and forth randomly, but will always stay in equilibrium Two or more substances diffuse independently of each other, down its own concentration gradient.

Simple Diffusion Simply movement of molecules from high to low concentration across the cell membrane. No protein is needed Ex. Water (slowly), I2 (a non- charged molecule), alcohol, oxygen, carbon dioxide Macromolecules (ex. Proteins) are too large and will not diffuse. Polarity is also a factor.

Amazing Real Life Example… O2 travels from a [high] in the alveoli through cell membranes into pulmonary capillaries, where O2 is in [low] in the blood.

What affects the rate of diffusion? Concentration Gradient Diffusion will be faster if the concentration gradient is large Size of Molecules Smaller molecules diffuse faster Charge Ions need proteins to diffuse across the membrane The number of proteins in the membrane will affect diffusion Temperature Diffusion is faster at higher temperatures Chemical nature of molecules

Facilitated Transport Polar or charged particles can move down their concentration gradient only if there is a special pathway for them! Does not require energy Examples Sugars (glucose), amino acids, Ions (Ca2+) Water in kidney cells, red blood cells, and plant cells by the protein aquaporin. FUN FACT! This allows the entry or exit of up to 3 billion water molecules a second! ACTIVITY: Facilitated Diffusion

Osmosis Simple diffusion of water across a phospholipid bilayer Water moves with its concentration gradient Water molecules will move to a [lower] until equilibrium is reached ACTIVITY: Osmosis and Water Balance in Cells

Hyper, hypo, and isotonic solutions Isotonic – [equal] in & out of cell. Water moves in and out equally Hypotonic – lower [solute] outside cell. Water enters the cell. Bursts, or turgid Hypertonic - higher [solute] outside cell. Water exits the cell. Shriveled.

Active Processes Energy is required to move the molecules Three types: Active Transport Endocytosis Exocytosis GET IT?!?! He’s transporting through the cell wall!

Active Transport Transport proteins are needed Solute binds to protein, but cannot pass through ATP  ADP + P provides energy for protein to change shape Ex. Sodium, potassium

Exocytosis Exo = outside Kytos = cell Used to export bulky materials such as proteins or polysaccharides Vesicle fuses with plasma membrane to secrete material outside of cell This requires energy, so it is an active process EX. When the Toronto Maple Leafs lose and you cry, cells in your tear glands use exocytosis to export a salty solution containing proteins EX.2. steroids (estrogen, testosterone), other hormones (adrenalin, thyroxin)

Endocytosis The formation of a transport vesicle by the in-folding of the cell membrane White blood cells do this A LOT! After the vesicle forms, it will either be released into the cytoplasm or digested by enzymes in lysozomes. Also needs ATP, so is an active process There are 2 types Phagocytosis Pinocytosis

Phagocytosis – “Cellular Eating” Phagocytosis takes in large chunks, such as bacteria, viruses, or old cells A lysosome will fuse with this vacuole and hydrolytic enzymes will digest the contents

Pinocytosis – “Cellular Drinking” The cell “gulps” droplets of fluid in tiny vesicles Any solutes dissolved in the droplets will enter the cell ACTIVITY: Exocytosis and Endocytosis

Bioflix Time BIOFLIX: Membrane Transport Fill out the flow chart in your notes!