Lecture 10 Transport In/Out of Cells

Transport Materials are exchanged between the cytoplasm and external cell environment across the plasma membrane by several different processes, some require energy (active transport), some do not (passive transport).

Keeping Things In! The cell membrane functions as a semi- permeable barrier, allowing a very few molecules across. Keeps the majority of organically produced chemicals inside the cell.

The Cell Membrane Proteins are suspended in the inner layer. The more hydrophilic areas of these proteins "stick out" into the cells interior as well as the outside of the cell. These integral proteins are sometimes known as gateway proteins. Integral proteins function in: 1) cellular recognition, 2) are binding sites for substances to be brought into the cell, through channels that will allow materials into the cell via a passive transport mechanism. 3) as gates that open and close to facilitate active transport of large molecules.

Diffusion The net movement of a substance (liquid or gas) from an area of higher concentration to one of lower concentration. Molecules move down a concentration gradient. Does not require energy, happens spontaneously-an example of passive diffusion. Eventually, if no energy is input into the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system. Factors that affect the rate of diffusion: - The greater the difference in concentration the faster the rate of diffusion. E.g when you exercise breathing becomes more rapid. -The greater the surface area of a membrane the faster the rate of diffusion. Microvilli on animal cells. -The greater the distance the slower the rate of diffusion. Cells rely on diffusion for internal transport of molecules.

Osmosis Osmosis is the diffusion of water across a semi-permeable membrane. The presence of a solute decreases the water potential of a substance. Thus there is more water per unit of volume in a glass of fresh-water than there is in an equivalent volume of sea-water. Therefore in a cell, which has so many organelles and other large molecules, the water flow is generally into the cell. Water, carbon dioxide, and oxygen are among the few simple molecules that can cross the cell membrane by osmosis. It is one principle method of movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane. Carbon dioxide is produced by all cells as a result of cellular metabolic processes. Since the source is inside the cell, the concentration gradient is constantly being replenished/re-elevated, so it will move out of the cell to an area of low concentration.

Hypertonic solutions are those in which more solute is present - lower water potential. Hypotonic solutions are those with less solute - higher water potential. Isotonic solutions have equal (iso-) concentrations of substances. Water potentials are thus equal, although there will still be equal amounts of water movement in and out of the cell, the net flow is zero.

Low water potential inside Blood cells compared to Distilled water so water enters cause cells to burst The salt solution has an even lower water Potential than the blood Cells and so water moves out

Selective Permeability of Cell Membranes The respiratory gases carbon dioxide and oxygen diffuse rapidly through membranes. Water molecules are small enough to pass between the hydrophobic phospholipid molecules. However ions and larger polar molecules e.g amino acids, sugars, fatty acids and glycerol are repelled by the hydrophobic region. Require alternative method of transport.

Carrier-assisted Transport Certain proteins integrated into the cell membrane are able to move molecules into the cell through water filled hydrophilic channels. The channels have a specific shape hence they are highly selective about the chemicals they allow to cross. Some of these proteins can move materials across the membrane only when assisted by the concentration gradient, a type of carrier-assisted transport known as facilitated diffusion. Both diffusion and facilitated diffusion are driven by the potential energy differences of a concentration gradient. Glucose enters most cells by facilitated diffusion. Glycolysis maintains the concentration gradient.

Types of Passive Diffusion

Diffusion of Gases and Water Molecules Across The Lipid Bilayer

Facilitated Diffusion

Active Transport Is the movement against the concentration gradient. E.g the sodium-potassium pump in nerve cells. Na + is maintained at low concentrations inside the cell and K + is at higher concentrations. The reverse is the case on the outside of the cell. When a nerve message is propagated, the ions pass across the membrane, thus sending the message. After the message has passed, the ions must be actively transported back to their "starting positions" across the membrane. Up to one-third of the ATP used by a resting animal is used to reset the Na-K pump.

Active Transport

Types of Transport Proteins Uniport proteins transport one solute at a time. Symport proteins transport the solute and a cotransported solute at the same time in the same direction. E.g In the membrane of kidney cells transporting glucose and sodium into the cell. Antiport proteins transport the solute in (or out) and the co-transported solute goes in the opposite direction. One goes in the other goes out or vice- versa.E.g Sodium and Potassium Pump.

Vesicles/vacuoles Vesicles and vacuoles that fuse with the cell membrane may be utilized to release or transport chemicals out of the cell or to allow them to enter a cell. Exocytosis is the term applied when transport is out of the cell. Endocytosis is the case when a molecule causes the cell membrane to bulge inward, forming a vesicle. Phagocytosis is the type of endocytosis where an entire cell is engulfed. Pinocytosis is when the external fluid is engulfed. Receptor-mediated endocytosis occurs when the material to be transported binds to certain specific molecules in the membrane. Examples include the transport of insulin and cholesterol into animal cells.

Endocytosis and Exocytosis

Other Features of The Cell Membrane The outer surface of a membrane is usually rich in glycolipids. These, along with carbohydrates attached to the integral proteins, are thought to function in the recognition of self. Multicellular organisms may have some mechanism to allow recognition of those cells that belong to the organism and those that are foreign. When a cell does not display the chemical markers that say "Made in Mike", an immune system response may be triggered. This is the basis for immunity, allergies, and autoimmune diseases. Organ transplant recipients must have this response suppressed so the new organ will not be attacked by the immune system, which would cause rejection of the new organ. Allergies are in a sense an over reaction by the immune system. Autoimmune diseases, such as rheumatoid arthritis, the immune system begins to attack certain cells and tissues in the body

Assignment 4 Due Thursday 26 th Dec Read pp Q1. Plant cells are able to generate ATP in chloroplasts, why then do they need mitchondria? Q2. The level of CO 2 is lower at higher altitudes, how is it that plants are still able to grow in such conditions?