7.3 – Cell Membrane & Transport

Cell Membrane polar heads are hydrophilic – attracted to water nonpolar tails are hydrophobic – not attracted to water this makes the membrane “selective” in what crosses

cell membrane is called the lipid bilayer because it’s made of 2 layers of phospholipids

due to the membrane’s selective permeability, small molecules can move through easily examples: oxygen (O2) & carbon dioxide (CO2)

ions and large molecules DO NOT move through the membrane on their own

Types of Transport across Cell Membranes

Material moves DOWN the concentration gradient through Three Forms of Transport Across the Membrane Material moves AGAINST the concentration gradient through a transport protein, using ATP (energy) from the cell. Material moves DOWN the concentration gradient through the phospholipids. Material moves DOWN the concentration gradient HELPED by a transport protein.

Passive Transport Diffusion does NOT require energy moves from high to low concentration examples: oxygen diffusing into a cell & carbon dioxide diffusing out

diffusion is a PASSIVE process which means no EXTRA energy is used to make the molecules move - they have a natural KINETIC ENERGY

Diffusion of Liquids

Diffusion through a Membrane cell membrane solute moves DOWN concentration gradient (HIGH to LOW) – think sledding downhill!

Facilitated diffusion Passive Transport Facilitated diffusion doesn’t require energy uses transport proteins to move from high to low concentration examples: glucose or amino acids moving from blood into a cell

Facilitated Diffusion molecules randomly move through the pores in channel proteins due to kinetic energy

some carrier proteins do not extend through the membrane bond and drag molecules through the lipid bilayer and release them on the other side

Osmosis diffusion of water across a membrane moves from HIGH water (low solute) to LOW water (high solute) high H2O & low solute low H2O & high solute

because water is polar, osmosis happens too slowly and needs some “help” cells contain water channel proteins called aquaporins that allow water to pass quickly movement of water in or out of a cell creates a force called osmotic pressure

Cell in Isotonic Solution 10% NaCl 90% H2O ENVIRONMENT CELL NO NET MOVEMENT 10% NaCl 90% H2O What is the direction of water movement? equilibrium The cell is at _______________.

Cell in Hypotonic Solution ENVIRONMENT 10% NaCl 90% H2O CELL 20% NaCl 80% H2O What is the direction of water movement?

Cell in Hypertonic Solution 15% NaCl 85% H2O ENVIRONMENT CELL 5% NaCl 95% H2O What is the direction of water movement?

Cells in Solutions

Active Transport requires energy = ATP! moves materials from LOW to HIGH concentration moves AGAINST concentration gradient

examples: pumping sodium ions (Na+) out and potassium ions (K+) in against strong concentration gradients called sodium-potassium pump

Sodium-Potassium Pump 3 Na+ pumped out for every 2 K+ pumped in

Moving the “Big Stuff” endocytosis is the process where really large molecules move into the cell

Pinocytosis most common form of endocytosis (“cell drinking”) takes in dissolved (liquid) molecules as a vesicle

transport across a capillary cell (blue) mature transport vesicle pinocytic vesicles forming transport across a capillary cell (blue)

Receptor-Mediated Endocytosis some proteins have receptors on their surface to recognize and take in hormones & cholesterol

Phagocytosis used to engulf large particles (solids) such as food and bacteria into vesicles called “cell eating” because this is how some organisms (like amoeba) eat their food

capture of bacteria cells (yellow) by a white blood cell (green)

opposite of endocytosis is exocytosis large molecules that are made in the cell are released through the cell membrane to the outside inside cell cell environment

molecules are moved out of the cell by vesicles that fuse with the cell membrane is how many hormones are secreted and how nerve cells communicate with one another

exocytic vesicle immediately after fusion with cell membrane

Homeostasis all these processes help the cell to maintain homeostasis homeostasis is the balance between the organism and its external environment multicellular organisms have specialized cells, each with their own job, to help maintain homeostasis

Receptor Proteins these proteins in the lipid bilayer are used in intercellular communication signal molecule binding to the receptor causes the receptor protein to release a signal to perform some action

diffusion: http://highered. mcgraw-hill   facilitated diffusion: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html sodium-potassium pump: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html endocytosis & exocytosis: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0072464631/291136/endocytosis_exocytosis.swf::endocytosis_exocytosis.swf cell membrane review: http://www.ibiblio.org/virtualcell/textbook/chapter3/cms.htm