Cells and Their Environment

Sections 1 & 2

• Passive transport• Movement that does not require energy from the cell

• Examples: Diffusion, Osmosis, and Crossing the cell membrane

• Active transport• Movement across the cell membrane against its concentration gradient

• Movement against a concentration gradient

• Movement in vesicles

• Membrane receptor proteins

Passive Transport

• Movement across the cell membrane that does not require energy from the cell

• Substances move from areas of high concentration to areas of low concentration

• This is known as the concentration gradient

Equilibrium

• When the substances are equally divided among all areas then they have reached equilibrium

• Concentration of a substance is equal throughout a space

Movement of Substances

• Diffusion describes the movement of a substance from an area of high concentration to an area of lower concentration

• This is caused by the random motion of particles in a substance

Diffusion Across the cell Membrane

• Many molecules and ions enter or exit the cell by diffusing across the membrane

• Concentration inside the cell verses concentration outside the cell

• Cell membrane is selectively permeable

Osmosis

• The process in which water diffuses through the selectively permeable cell membrane.

• Type of passive transport (high to low)

• The direction of water movement depends on relative concentrations of water molecules

Directions of Water Movement

• Hypertonic - water moves out of the cell (shrinks)

• Hypotonic - water moves into the cell (bursts)• Isotonic - no net movement of water (same concentration inside/outside of the cell)

Crossing the Cell Membrane

• Channels - transport proteins provide polar passageways through which ions and polar molecules can move across the cell membrane

• Each channel only allows a specific substance to pass through

Diffusion Through Ion Channels

• Ion channels - a transport protein within which an ion can pass through (Na+, Cl-, K+, Ca2+)

• Ion can pass through the cell membrane without contacting the nonpolar interior of the lipid bilayer

Ion Transport (Electrical Charge)

• Movement across the membrane is also influenced by the particle’s positive or negative charge

• Opposite charges attract, like charges repel

• Typically, inside of cell is more negatively charged

Facilitated Diffusion

• Facilitated Diffusion is a type of passive transport that moves substances down their concentration gradient without using the cells’ energy

• Carrier proteins are used to move amino acids & sugars

Active Transport

• The transport of substances across the membrane against its concentration gradient is called active transport

• Amino acids, sugars, and other substances must be transported from outside the cell into cytoplasm

• Active Transport requires cell energy (ATP)

Sodium-Potassium Pump

• Na+K+ Pump transports 3 Na+ ions out of a cell and 2 K+ into the cell.

• Na+ is more concentrated outside the cell and K+ is more concentrated inside the cell - against concentration gradient

Purposes of Na/K Pump

• Prevents Na from accumulating inside cell. Too much Na would cause water to enter into cell by osmosis, causing cell to burst

• Na/K pump helps maintain ion concentration levels, which are used to transport glucose and other substances across the cell membrane.

Movement In Vesicles

• Movement into cell by a vesicle- endocytosis

• Vesicles form pouch, may fuse w/ a lysosome to move into cell.

• Movement out of the cell by vesicle -exocytosis

• Cells use exocytosis to export proteins (examples are nerve cells and glands)

Receptor Proteins

• Cells must respond to important information sent by hormones. Cells can receive messages b/c of receptor proteins in cell membrane bind to signal molecules.

• The binding of a signal molecule by its complementary receptor proteins causes a change in the receiving cell.

• The change can occur in 3 ways.– Changing permeability of cell (allows ions to cross)

– Triggering formation of second messenger inside cell

– Activating enzymes inside the cell (speed up reactions)

A Brief Review

• Passive Transport:• Movement w/o energy• Diffusion - movement from high to low conc.

• Osmosis- movement of water through c.m.

• Ion channels - helps ions across c.m.

• Facilitated Diff.- substance moves down concentration gradient

• Active Transport:• Movement w/ energy

• Na/K Pump - uses ATP to move Na/K

• Endo/Exocytosis• Signal molecules become bound to receptor proteins