Transport Across The Cell Membrane

Selectively Permeable Cell Membranes act like gates allowing only certain substances in or out of the cell.

Selectively Permeable The molecules are constantly in motion – because of their kinetic energy – and either move through the membrane or “bounce” off depending upon their size and charge.

Selectively Permeable Size and Charge of a molecule determines if the cell membrane will let it through

Two Types of Cell Transport PASSIVE VS ACTIVE

PASSIVE TRANSPORT Always moves substances down a concentration gradient. It requires no energy expenditure on the part of the cell to move these substances across the membrane.

3 Kinds of Passive Transport: Diffusion Osmosis Facilitated Diffusion

DIFFUSION Diffusion is the movement of a solute from an area of higher concentration to an area of lower concentration. When the solute is in equal concentration throughout, it is said to be in equilibrium. The molecules will continue to move, but there will be no net movement in one direction or another

OSMOSIS Water moves by osmosis through a semipermeable membrane. Water will always move from an area of high water concentration to an area of low water concentration. Water’s tendency to move is called water potential (it has the potential to move). It is said to move from an area of higher water potential to an area of lower water potential.

OSMOSIS Since it is the amount of solute that determines which way water moves, we could have 3 different sets of circumstances… a low amount of solute a high amount of solute an equal amount of solute on either side of the membrane. These 3 types of solutions have names

HYPOTONIC SOLUTIONS Hypotonic solutions have less solute (more water) than the solution they are being compared to. Water always moves from a hypotonic solution into the solution containing the greater amount of solute.

HYPERTONIC SOLUTIONS Solutions that contain a greater amount of solute (less water) than the solution they are being compared to are called hypertonic. Water always moves from a hypotonic solution into a hypertonic solution

ISOTONIC SOLUTIONS When two solutions separated by a membrane contain equal amounts of solute, they are said to be isotonic to each other. Water will move back and forth between the solutions, but there will be no net movement in one direction or the other.

OSMOSIS Keep in mind that to use the terms hypotonic, hypertonic, or isotonic, you must be comparing two solutions. These two solutions are on opposite sides of the plasma membrane.

Now let’s see what happens to a living cell placed in one of these three types of solutions. We will use a red blood cell (RBC) as an example of an animal cell.

Now let’s see what happens to a plant cell in our 3 types of solutions Now let’s see what happens to a plant cell in our 3 types of solutions. Why would a plant cell behave differently than an animal cell? Do you know?

PLANT CELLS IN HYPOTONIC SOLUTIONS Plant cells placed in a hypotonic solution will fill with water. They have a central vacuole that can take up to 90% of the space inside the cell. This is where the water goes. The plant will swell and the plasma membrane will push against the dead cell wall. The plant cell will not burst because of the cell wall. The rigid condition of the plant cell is known as turgor. The cell is said to be turgid

PLANT CELL IN HYPERTONIC SOLUTION Plant cells placed in a hypertonic solution will lose water from their central vacuole. The shrinking of the central vacuole will cause the plasma membrane to pull away from the dead cell wall (which is immovable). This is known as plasmolysis. The cell will plasmolyze

PLASMOLYSIS IN ELODEA Plasmolysis in Elodea. Note the shrinking of the cytoplasm and how the plasma membrane has pulled away from the cell wall.

EFFECTS OF PLASMOLYSIS IN PLANTS

PLANT CELL IN AN ISOTONIC SOLUTION In an isotonic solution, water will move in and out of the plant cell. The central vacuole is not completely filled, so the cell will be flaccid or limp

Summarizing what we have learned

FACILITATED DIFFUSION ASSISTANCE BY MEMBRANE PROTEINS: CHANNEL OR CARRIER MOLECULES ARE MOVED DOWN THE CONCENTRATION GRADIENT FROM HIGH TO LOW CONCENTRATION

FACILIATED DIFFUSION

CARRIER PROTEIN Alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. Can move solute in either direction. Specific for certain solutes.

Active Transport Requires use of a carrier protein (like facilitated diffusion) However, Active Transport moves solutes against the concentration gradient from low to high concentration Requires cell to expend energy in the form of ATP Symports carry 2 different molecules in the same direction, while antiports carry two different molecules in opposite directions

SODIUM-POTASSIUM PUMP The sodium-potassium (NaK) pump is the classic example of active transport. It uses an antiport carrier protein to move Na (which is INa) outside and bring K (which is Outside OK) in. The NaK pump is used in nerve conduction and muscle contraction

ION PUMPS Ion pumps are another example of Active Transport H+ ions (protons) are pumped to one side of a membrane by active transport creating a proton gradient. This gives the protons potential energy This energy can be used to do work including a type of cellular transport called co-transport

CO-TRANSPORT

BULK TRANSPORT Transport Of Large Molecules Into or Out of the Cell Involves the use of vesicles Endocytosis – into cell Exocytosis – out of the cell Cell must use ENERGY (ATP)

EXOCYTOSIS Movement of large substances out of the cell Requires ATP

ENDOCYTOSIS Endocytosis is the process of bringing large molecules into the cell. There are 3 types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

PHAGOCYTOSIS – “Cell Eating” Phagocytes are cells that “eat” other cells. White blood cells are phagocytes that patrol the body looking for invaders, such as bacteria, to ingest. Once inside the cell, the invader is broken down into its component molecules.

PINOCYTOSIS – “Cell Drinking” Pinocytosis moves liquids and smaller particles into the cell

RECEPTOR - MEDIATED ENDOCYTOSIS