Transport Across Cell Membranes

Cell Membrane/Plasma Membrane  Separates the cell from the non-living outside environment  Without a cell membrane there could be no cell  Performs three functions: 1.It separates the cell from its outside environment 2.It enables communication and 3.movement of the materials between the cell and its environment

Transport across the Cell Membrane Selective permeability – cell membrane allows some substances to pass through but keeps others out (Border control, Customs, Airport security) So the cell membrane is selectively permeable or semi-permeable Cell membrane can also control if the transport of these molecule is fast or slow

Fluid Mosaic Model  Cell membranes are made up of lipids  Protein molecules float within the lipid molecules of the cell membrane  This arrangement of molecules in constant motion is called the fluid mosaic model

The role of Proteins Two Types: Transport proteins and Receptor proteins Transport proteins: Enable much of the transport or movement of materials across the cell membrane. Sometimes called carrier proteins. Receptor proteins: function as receptor molecules – receive signals from the outside and transmit the signal to the inside of the cell enabling communication between cells

Semi-permeable –materials that can pass and materials that can't

Passive Transport – No Energy required  Molecules are always in motion  They naturally move from high to low concentrations  The difference in concentration between two areas is called a concentration gradient

Solute  A substance that is being dissolved to make a solution In the case of sugar and water it is the sugar because it is being dissolved in the water

Solvent  A liquid that is dissolving another substance to make a solution In the case of sugar and water it is the water because the water is dissolving the sugar

Diffusion Diffusion – The random movement of molecules down the gradient or from high to low concentration Example: Perfume

How Does Diffusion Work? Pores in the cell membrane allow these materials to pass thru Diffusion happens automatically if the membrane is permeable to that molecule and there is a difference in concentration on either side This is called passive transport because no energy is used by the cell to move these molecules

Function of Diffusion in the Cell  This is how cells get food and oxygen

Osmosis Osmosis – movement of water across cell membrane. - The movement of water continues until there is the same amount of water on both sides of the membrane this is called equilibrium.

Hypertonic Solution  Higher concentration of solutes outside the cell  Higher concentration of water inside the cell  Water moves out of cell and cell shrinks

Hypotonic Solution  Higher concentration of solutes inside the cell  Higher concentration of water outside the cell  Water moves into the cell and cell expands

Equilibrium (Isotonic)  Osmosis and Diffusion will continue until there is the same concentration of water and solute molecules on each side of the cell this is called isotonic

Why Does Osmosis Happen in Cells?  Cells are surrounded by water so there must be the same amount of water inside as outside the cell. Many solutes can not pass through the membrane, so Water is the only thing that moves!

Facilitated Diffusion  Carrier protein in the cell membrane is needed to help move a specific molecule across  Facilitated means to help or make easier  Carrier protein acts like a gate and increases the rate of transport of certain substances  Does not require energy because molecules are moving down the concentration gradient

Glucose moves in and out of the cell this way

Active Transport  Movement of a substance against the concentration gradient from low to high concentration  Movement of molecules that can't cross the cell membrane  Energy is needed and work is done for this type of transport  This is one of the most important uses of our cell energy!

Channels and Carrier Proteins

Ion Channels

What if I am too large to cross the Cell Membrane?

Endo- and Exo-cytosis Movement of very large molecules (food, waste) into (endo) and out (exo) of the cell The large molecule is enclosed in a "packet" To move into – part of cell membrane surrounds molecule to form a "packet", then pinches off the cell membrane, entering the cell To move out – a "packet" (the molecule surrounded by a membrane) fuses/merges with the cell membrane – molecule released outside the cell

Exocytosis and Endocytosis Exocytosis Exo = Out leaves the cell Endocytosis Endo = In enters the cell