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?

Key Terms

Selectively permeable Diffusion osmosis Passive transport 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

Molecules move from high to low concentration

Until they are spread out evenly

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.

Effects of Osmosis

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

Cell Size