Transport Across the Cell Membrane

What is the Function of a Cell Membrane? Acts as a barrier from the external environment Controls substances that enter and exit the cell Selectively permeable: movement across the cell membrane depends on the chemical composition of a substance EX: molecule size, structure, charge, etc.

Transport Across the Cell Membrane Passive Transport Does not require input of energy Active Transport Requires input of energy

Passive Transport Does NOT require the input of energy from the cell Involves movement down a concentration gradient. Difference in concentration (# of molecules) of a given substance between adjoining areas. Number of substances that can pass through using this method is limited due to low energy requirements. Ex: Oxygen, Nitrogen, Carbon Dioxide

Passive Transport Types of Passive Transport Diffusion Osmosis Facilitated Diffusion

Diffusion What is diffusion? What types of molecules can do this? Spontaneous movement of molecules from an area of high concentration to an area of low concentration What types of molecules can do this? Small uncharged molecules Ex: oxygen, carbon dioxide, nitrogen, water How do they do it? By passing through the phospholipid bilayer or through channels formed by transmembrane proteins

Osmosis Movement of WATER across a selectively permeable membrane Water moves across the membrane to maintain equilibrium Concentration of solute (proteins, minerals…) is equal on both sides of the membrane Once equilibrium is obtained, water will continue to move across the membrane but there is no net gain on either side.

Osmosis

Osmosis HYPOtonic solution A solution where the concentration of solute molecules is lower outside the cell than inside the cell There is a higher concentration of water outside of the cell Water moves into the cell and the cell expands (or bursts) Called hemolysis

Osmosis ISOtonic solution A solution where the concentration of solute molecules outside the cell is equal to the concentration of solutes inside the cell Concentration of water is the same on both sides of the membrane

Osmosis HYPERtonic solution A solution where the concentration of solute molecules is higher outside of the cell than inside There is a higher concentration of water inside the cell Water moves out of the cell and the cell shrinks Called crenation

Osmosis

Facilitated Diffusion Some proteins cannot pass through the cell membrane’s hydrophobic middle layer. Carrier proteins can help to pass some molecules through the membrane. Molecules that use this mode of transport include: Large polar molecules Large ions

Simple Vs. Facilitated Diffusion Simple Diffusion Facilitated Diffusion Molecules pass through the phospholipid bilayer or protein channels Molecules pass through the cell membrane via carrier proteins Carrier proteins typically only carry 1 type of molecule

Active Transport Types of Active Transport Bulk Transport Endocytosis Exocytosis Transport through Gates or Channels

Active Transport Used to move substances in or out of a cell AGAINST the concentration gradient Requires ENERGY in order to move substances across the membrane Energy is used to open a channel or a gate in the protein of the membrane to assist in the binding of a substance to the protein Energy can also be used to assist in a shape change that pushes the substance through the membrane (bulk transport)

Bulk Transport Some molecules are too large to cross the membrane through proteins. They must travel in and out of cells through bulk transport. 2 Types Endocytosis Exocytosis

Endocytosis Process by which cells ingest large materials The cell membrane engulfs material from outside of the cell. This portion of the membrane pinches off, forming a membrane bound vesicle inside the cell called a vacuole.

Exocytosis Reverse process of Endocytosis Molecules are carried by vacuoles inside the cell and expelled into the extracellular space (outside of the cell).

videos Passive and Active Transport Endocytosis and Exocytosis