Movement of Materials

The transport of water and other types of molecules across membranes is the key to many processes in living organisms. Without the input of oxygen, water, and nutrients, cells would not be able to maintain themselves. Likewise, the elimination of waste products is crucial to the continued activities of the cell. The cell membrane regulates the transport of materials through its chemical and structural composition

Passive Transport Diffusion Facilitated diffusion Osmosis DO NOT REQUIRE ATP ENERGY.

Active Transport Active transport Endocytosis – Phagocytosis – Pinocytosis – Receptor mediated endocytosis Exocytosis THESE ALL REQUIRE ATP ENERGY

Diffusion One of the basic ways substances can get in and out of cells is by simple diffusion. Diffusion is the movement of molecules from a region of high concentration to a region of low concentration by means of random molecular motion. The difference between the high concentration and low concentration is called the concentration gradient.

In cells, small non-polar molecules can easily diffuse through the cell membrane. Oxygen and Carbon dioxide gases diffuse between the cytoplasm and the extracellular fluid by following the concentration gradient created and maintained by cellular respiration.

Factors that affect the rate of diffusion are: 1. Temperature. Temperature measures the average kinetic energy of the molecules. As the kinetic energy increases, the temperature increases. At higher temperatures the molecules move faster and diffuse faster. 2. Size of the solvent molecules. Generally, smaller molecules diffuse faster than larger molecules.

3. “Steepness” of the concentration gradient. The rate of diffusion is proportional to the gradient.

Osmosis Water, though polar, is small enough that it too can move across the cell membrane. Osmosis is the special name given to the movement of water molecules, across a semipermeable membrane, along a concentration gradient.

Water is the usual solvent in cells. The solutes ( ions, minerals, glucose, amino acids) are dissolved in water. Most solutes cannot freely cross the cell membrane. This creates concentration gradients which will lead to the movement of water molecules. Osmotic pressure is created by the difference in water concentration.

HYPERTONIC solutions have more SOLUTE molecules than the cytoplasm.

HYPERTONIC solution: water molecules are drawn OUT of the cell. Effect on Animal cells: Cells shrink: Crenation

Effect on Plant Cells: – Plasmolysis

In a HYPOTONIC solution, there are less solutes than in the cell cytoplasm. Water moves INTO the cell

Effect of Hypotonic solution on Animal cells: The cells will swell and may burst. (Lysis)

Effect of Hypotonic solution on Plant cells Cell will fill with water, but the cell wall will stop it from bursting. Cells are said to be turgid

In an ISOTONIC solution, the concentrations of solute are equal on both sides of the semipermeable membrane. There is no NET movement or water molecules across the membrane.

Facilitated Diffusion Movement of specific molecules down a concentration gradient, passing through the membrane via a specific carrier protein. Each carrier protein has its own shape and only allows one molecule (or one group of closely related molecules) to pass through. Common molecules entering/leaving cells this way include glucose and amino-acids. It is passive and requires no energy from the cell.

Most cells are exposed to extracellular glucose concentrations that are higher than those inside the cell, so facilitated diffusion results in the net inward transport of glucose. Once glucose is taken up by these cells it is rapidly metabolized, so intracellular glucose concentrations remain low and glucose continues to be transported into the cell from the extracellular fluids.

The glucose carriers are “reversible” so glucose can be transported in the opposite direction. This occurs in liver cells when glucose is synthesized from glycogen and released into the circulation.

Ions Channel proteins (porins)form open pores in the membrane, allowing small molecules of the appropriate size and charge to pass freely through the lipid bilayer. Ion channels allow the passage of ions across plasma membranes. They are present in all cells but are especially important in nerve and muscle cells. Their regulated opening and closing is responsible for the transmission of electric signals.

Open and Shut Potassium channels

Three properties of ion channels are central to their function. 1. Transport through channels is extremely rapid. More than a million ions per second flow through open 2. Ion channels are highly selective because narrow pores in the channel restrict passage to ions of the appropriate size and charge. There are specific channel proteins for the passage of Na +, K +, Ca 2+, and Cl - across the membrane. 3. Most ion channels are not permanently open. Instead, the opening of ion channels is regulated by “gates” that transiently open in response to specific stimuli.

The plasma membranes of many cells also contain water channel proteins (aquaporins), through which water molecules are able to cross the membrane much more rapidly than they can diffuse through the cell membrane.

Active Transport Requires ATP energy Transports Substances AGAINST the concentration gradient Proteins act as PUMPS Sodium- Potassium pump is typical example of symport transport (two substances being transported at the same time, in opposite directions)

Endocytosis Transport of materials into cell by means of vesicles Requires energy to change the shape of cell membrane 3 types of endocytosis – Phagocytosis: for “bulky” substances – Pinocytosis: cell “drinking” – Receptor mediated pinocytosis: very specific

Phagocytosis Amoeba, White blood cells of immune system

Pinocytosis

Receptor mediated pinocytosis

Exocytosis Vesicles fuse with cell membrane and release contents to the extra cellular environment Used to excrete metabolic wastes, and export substances, such as hormones. Can be regulated by the cell Vesicles often created by the Golgi Apparatus