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The Cell and its Environment
Key Concepts:
How do most small molecules cross the cell membrane?Why is osmosis important to cells?What is the difference between passive transport and active transport?
Diffusion
Main method by which small molecules move across the cell membrane
Diffusion: process by which molecules move from an area of higher concentration to an area of lower concentration. Concentration – amount of a substance in a given volumeWhat causes diffusionCaused by movement of molecules, the more molecules in an
area, the more collisions which cause the molecules to spread out.
Eventually they will be spread evenly throughout an area
Diffusion of Oxygen
Cell membrane is selectively permeable, allows certain materials to pass through but not others
Allows oxygen from water to pass through
More oxygen in water, moves from high concentration to low, so it diffuses into cell
Osmosis Osmosis: the diffusion of water across a
selectively permeable membrane Many cell processes depend on osmosis
Osmosis and Diffusion Water molecules move
from area of high
concentration through a
membrane to an area of
low concentration
Effects of Osmosis
If cell is in solution with same concentration of water, it will retain its normal shape and size.
Effects of Osmosis If cell is in water solution in which
concentration of water outside the cell is less ( lots of solutes like sugar or salt), water will move by osmosis out of the cell and the cell will shrink.
Effects of Osmosis If the cell is in a water solution with
little salt or sugar, the concentration of water outside the cell is higher, water will move by osmosis into the cell and the cell will swell.
Cell Transport Passive transport – movement of
materials through a cell membrane without using cellular energy
Active transport – movement of materials through a cell membrane using cellular energy
Cell must use energy during active transport, but not during passive transport
Transport Proteins Transport proteins pick up molecules
outside the cell and carry them in using energy
Carry molecules out the same way Ex. Sodium (Na), potassium (K), calcium
(Ca)
Transport by Engulfing
Cell membrane surrounds a particle, once engulfed the membrane wraps around the particle and forms a vacuole
Why Cells are Small Size of cell is related to the movement
of materials into and out of cells As cell’s size increases, so does the
distance to the center of cell, materials in the cell move by the streaming of cytoplasm, In a large cell it would take a long time to move substances from the membrane to the center of the cell
If a cell grew too large it couldn’t function very well