Cycling of Matter in Living Systems 2.2 The Role of Cell Membrane in Transport

The cell membrane is the organelle responsible for transport of gases, nutrients, and wastes into and out of the cell. The substances that enter and leave the cell may be ions or molecules, or in some cases micro-organisms or other cells. These substances in a physical sense are particles, and behave according to The Particle Model of Matter.

The Particle Model of Matter All matter is made up of particles but the particles in different substances may be different in size and composition. The particles of matter are constantly moving or vibrating; particles move least in solids and most in gases. Adding or taking away energy will affect the movement of particles. The particles of matter are attracted to one another or are bonded together. Particles have spaces between them that are smallest in solids, except for ice and greatest in gases. The spaces may be occupied by the particles of other substances.

Diffusion Diffusion is the natural movement of particles from an area of high concentration to an area of low concentration. Since the particles are in constant motion they move until they are equally distributed throughout the room, resulting in a state of equilibrium. Adding energy and increasing molecular movement (i.e. heat and stirring) can increase the rate of diffusion. Diffusion can also occur in cells. animated.gif

The cell membrane is considered to be selectively permeable because it allows certain particles to pass through it, but not all particles. This is called a semi- permeable membrane. Generally the passage of materials through the membrane is determined by:  The size of the molecules  The charge of the molecules  The solubility of the molecules in lipids (must be soluble)

In general, particles move from areas of high concentration to areas of low concentration. Therefore  Carbon Dioxide diffuses out of the cell, an area of high concentration to an area of low concentration.  Oxygen diffuses into the cell (low concentration) from outside of the cell where the concentration of oxygen is high.

Concentration Gradients Concentration Gradient Must involve different concentrations In cells, different concentrations are separated by a membrane Involves molecules or ions of a single type e.g. different concentrations of sodium or potassium salts in water Different molecules or ions move along their concentration gradients independently of each other Drives diffusion and osmosis

Osmosis The cell is an open system, therefore is always interacting and responding to the conditions of the environment. If the cell membrane is not permeable to the solute mixed with water, the water will pass through the semi-permeable cell membrane, leaving the solute molecules outside of the cell. This is the process of osmosis. Osmosis is the movement of water through a membrane in response to its concentration gradient. The membrane is not permeable to the solute. This is another form of passive transport.

Water moves from high [ ] to low [ ] Look at which side has higher water [ ], water will leave this side and move to the other side, until ‘equilibrium’ is reached Side ASide B Water will move from side B to side A until an equilibrium is reached. (High to Low)

To predict the direction in which a net movement of water will occur, the solute concentration must be compared.  Hypertonic ("hyper" means over) - The concentration of the solutes outside is higher than it is inside the cell. Therefore if the cell is put into this solution, water will leave the cell  Isotonic ("iso" means equal) - The concentration of solutes outside the cell is equal to the inside of the cell. Therefore if the cell is put into this solution, there is no net movement of water molecules, just odd molecules moving in and out of the cell.  Hypotonic ("hypo" means under) - The concentration of solutes outside is lower than it is inside the cell. Therefore if the cell is put into this solution, water will enter the cell.

Facilitated Diffusion There are two forms of protein mediated transport for those molecules that are soluble in water but not in lipids, and to cross the cell membrane. – Channel proteins create pores or channels through which small water-soluble particles are able to move. These molecules move in response to the concentration gradient.

Carrier proteins have the ability to attach to larger molecules that are not able to diffuse across the membrane. The carrier protein changes shape and physically moves the molecules across the membrane and into the cell. Once the molecules have been transported, the protein returns to its original shape. This process is a called facilitated diffusion, which is the movement of substances in response to the concentration gradient but also requires the presence of the protein facilitator. Both forms of protein mediated transport are examples of passive transport, because no added energy needed.

Active Transport Active transport requires energy input to transport molecules. Active transport is necessary when protein carriers need to carry molecules across the concentration gradient, from areas of low concentration to areas of high concentration. The energy required is produced in the mitochondrion through the process of cellular respiration and comes from the substances called adenosine triphosphate or ATP. A series of chemical reactions occurs first in the cytoplasm then in the mitochondrion, to break down glucose and produce the ATP.

Endocytosis and Exocytosis When a molecule needs to be taken in by the cell, but are too large to pass across the cell membrane, even with the help of protein carriers, the process of Endocytosis occurs. Endocytosis is the process that occurs when a vesicle (a small temporary sac) forms around the particle and the cell membrane pinches off around it so that the vesicle is inside the cell.

When a molecule needs to be secreted by the cell, but are too large to pass across the cell membrane, a vesicle surrounds the particle, then moves to the plasma membrane and fuses with it. The vesicle then ruptures, releasing its contents into the surrounding area of the cell. This process of secretion is called exocytosis. Both exocytosis and endocytosis require energy from ATP for the rearrangement of the cell membrane.

Homework: Check and Reflect p. 283 # 1-7