Cells and Their Environment Chapter 4

4.1 Passive Transport Part 1: Diffusion Remember: Living things need to maintain homeostasis (responds to external conditions to maintain a stable internal condition) One way to do this is controlling the movement of things in and out of a cell

Passive Transport Requires no energy Diffusion is the movement of a substance from an area of high concentration to an area of low concentration by random movement A difference in concentration of a substance across a space is a “concentration gradient”

Equilibrium is where the concentration is equal Since the interior of a cell is different from its external environment substances move back and forth through the cell membrane The membranes lipid interior repels ions and polar molecules. WHY!!

The diffusion of small or nonpolar particles across the cell membrane is the simplest form of passive transport

Part 2: Osmosis The diffusion of water through a selectively permeable membrane Water moves down the concentration gradient When something is dissolved in water its molecules are attracted to the water molecules This causes a difference between “free water” and “non-free water” This causes the concentration gradient

Hypertonic Solution Fluid outside has higher concentration of dissolved substances than fluid inside the cell Causes water to diffuse out of cell Cell shrinks

Hypotonic Solution Fluid inside has higher concentration of dissolved substances than fluid outside cell Causes water to diffuse into cell Cell swells

Isotonic Solution Fluid inside has same concentration of dissolved substances as fluid outside cell Causes no net water movement Water moves in and out at equal rates Equilibrium – no change in size

How does this affect you?

Cells have ways of dealing with swelling or shrinking Cell walls keep cells from expanding too much Some unicellular eukaryotes have contractile vacuoles to collect excess water and force it out Animal cells remove dissolved particles to stop osmosis

Part 3: Crossing the Cell Membrane Transport Proteins called channels help polar substances cross cell membrane Each only allows 1 kind of substance through

Diffusion through ion channels Many ions are important for cell functions Important in heartbeat and nerve cells Ion channels are transport proteins with a polar pore so ions can pass through membrane Ions pass without touching nonpolar lipid tails

No use of energy so Passive Transport Ion channel gates allow some to open and close in response with stimuli Stretching of cell membrane Change in electrical charge Binding of specific molecules to channel No use of energy so Passive Transport

Electrical Charge and Ion Transport Positive or negative charge affects movement of charged particle across membrane Inside of cells are negatively charged compared to outside This causes positively charged particles to want to move in and the negative ones to want to move out (trying to reach equilibrium)

Facilitated Diffusion Carrier Proteins (a kind of transport protein) bind to a specific substance and carry it across membrane and release it Still moving down concentration gradient Carrier protein attaches to substance on one side Shape of protein changes bringing substance through Substance protected from lipids and is released and protein goes back to original shape Passive Transport

4.2 Active Transport Part 1: Movement Against a Concentration Gradient When concentration is higher inside than outside, cells must move things against gradient from low to high Called Active Transport Requires energy, usually ATP

Sometimes uses carrier proteins Since moving against gradient, called membrane pumps

Sodium-Potassium Pump 3 sodium ions, Na+, out and two potassium ions, K+, in Active transport of both Sodium higher outside and potassium higher inside Energy supplied by ATP

How Does ATP Work? ATP is Adenosine TriPhosphate 3 phosphate groups attached to the sugar Ribose and the base, Adenine

Phosphates are negative so they repel each other and are unstable A little energy in causes one phosphate to break off and more energy is given off This causes ATP to become ADP, Adenosine DiPhosphate Cell uses energy given off from this reaction

Sodium-Potassium Pump 3 Na+ inside bind to pump and a phosphate group leaves ATP and binds to pump Pump changes shape and 3 Na+ are released outside 2 K+ bind to pump and are transported across 2 K+ and P from ATP is released inside cell

Allows K+ to be brought in Pump prevents Na+ from building up in cell since it constantly flows in due to diffusion This build up would cause water to flow in due to osmosis, causing the cell to swell or burst Allows K+ to be brought in Many cells use Na+ pump to bring in things they need

Part 2: Movement in Vesicles Some things are too large to use carrier proteins Vesicles used instead

Endocytosis when moved in Cell membrane forms a pouch around a substance, closes up and pinches off forming vesicle Vesicle may fuse with lysosomes or other organelles

Exocytosis when moved out Vesicles inside fuse with membrane and release contents Used for proteins modified by Golgi apparatus Nerve cells and gland cells release proteins