Movement Across The Cell Membrane

Cell Membranes Control Movement Across the Membrane

Cell membranes control what passes through the membrane.  Their job is to keep the organelles inside the cell and protect them from the outside world.  Mechanisms can require energy for movement, or not require energy. Active process requires energy in the form of ATP.  Passive process does not require ATP to be directly spent moving molecules.

Passive Processes – No Energy Spent

Diffusion – moving from high concentration levels to lower; moves down its concentration gradient. Random movement of molecules until equilibrium is reached.

Facilitated diffusion utilizes transport proteins to allow passage through the membrane.  Substances also move down its concentration gradient.

Osmosis – the movement of water across a selectively permeable membrane.  Water moves from an area of greater water concentration to an area of lesser water concentration (lesser osmotic pressure to area of higher osmotic pressure).  Osmotic pressure can raise a column of water. Reduces concentration of other substances by having water molecules move to “water down solution,” reducing the osmotic pressure.

Filtration- using hydrostatic pressure to force molecules through membranes.  

Active Process – Requires Energy For Transport

Active Transport Mechanisms use ATP or energy for transport.  They move molecules from low concentrations to high concentrations, against concentration gradient.  Active transport utilizes carrier molecules in cell membranes. As much as 40% of a cell’s energy may be spent on active transport processes.

Endocytosis – the cell engulfs materials and brings molecules into the cell by altering the cell membrane.  The cell membrane “extends fingers to wrap around molecules.” Endocytosis is used when objects are too large to travel through membranes.  Objects coming into the cell are wrapped inside of vesicle.

Three forms of Endocytosis:

1. Pinocytosis – cell drinking.  Bringing smaller and fewer molecules into cell.

2. Phagocytosis – brings in large and very large molecules.  Think of a while blood cell attacking and engulfing bacteria.

3. Receptor mediated endocytosis – specific receptor binding onto the cell triggers endocytosis.  Cell very specifically brings inside exactly what it wants.

Exocytosis – reverse of endocytosis; “spitting molecules” outside of cell.