Module 2: Foundations in Biology 2.1.5 Transport Across Biological Membranes

What needs to enter and leave cells?

The fluid mosaic model

Differentiated plasma membranes

How are things transported across cell membranes? These are the main ways:

Diffusion

Diffusion This is the movement of molecules or ions from a region where they are at a high concentration to a region of lower concentration. The difference in the concentration is called the concentration gradient. Movement occurs until an equilibrium is reached ie. there will be an equal distribution of ions or molecules. This process is passive – it does not require metabolic energy.

The rate at which diffusion occurs depends on: The concentration gradient: the greater the difference in the 2 concentrations the greater the rate. The size of the ions or molecules: the smaller they are the greater the rate. The distance over which diffusion occurs: the shorter the distance, the greater the rate.

Cell membranes are fully permeable to the respiratory gases O2 and CO2, they are able to diffuse rapidly in solution, depending on the concentration gradients. Cell membranes are only selectively permeable to other molecules. Due to the hydrophobic nature of membranes, uncharged & lipid-soluble molecules diffuse through more easily than ions and polar molecules such as glucose and amino acids. Polar molecules are thought to pass through membranes via channels formed by transport or channel proteins.

Facilitated Diffusion Some polar molecules diffuse across membranes more rapidly by combining with special transport proteins. Transport proteins with specific binding sites for glucose are present in the cell surface membrane. Once binding has occurred, the protein changes shape and moves the glucose to the other side of the membrane. Once the glucose has detached, the protein changes back to its original shape and position in the membrane. This process does not require energy, and continues until an equilibrium is reached.

The Electrochemical Gradient Diffusion is also affected by an electrical gradient, as well as concentration. Ions are attracted to areas of opposite charge, and move away from areas of similar charge. The inside of most cells tend to be negatively charged. What affect does this have?

Carry out Practical Activity 5 Investigating the rate of diffusion through a membrane

                                      Osmosis

Osmosis Osmosis is the diffusion of water molecules from a region where they are at a high concentration to a region of lower concentration, through a partially permeable membrane.

Set up Practicals: Activity 6 Investigating Osmosis in an artificial Cell Activity 2 Investigating different Water Potentials of Plant Cells

Osmotic Potential A ‘weak’ solution has a high concentration of water molecules, we say it has a high water potential. Pure water has the highest water potential of zero. All lower water potentials have negative values. A ‘strong’ or concentrated solution has a low concentration of water molecules, we say it has a low water potential. Water diffuses out of cells with a high water potential into cells with a lower water potential. Water potential is expressed as the Greek letter psi . High water potential Low water potential

Osmosis in Plant Cells Solute Potential (ψs): This is the concentration of dissolved substances in the cell. It is a measure of the reduction in water potential due to the presence of solute molecules. It is always a negative number. Pressure Potential (ψp): As water enters a plant cell it swells and the cell wall begins to stretch. The pressure that the cell wall develops is called the pressure potential. This is usually positive. The water potential of a plant cell can be calculated using this formulae: water potential = solute potential + pressure potential ψ ψs ψp

Osmosis in Plant Cells Turgidity: If you put plant cell in distilled water there will be an influx of water into the cell. The cell will swell and produce a pressure potential. As more water enters the pressure potential will rise until it equals the solute potential and the water potential is zero. In this state the cell is said to be turgid. Plasmolysis: A plant cell in a strong sugar solution will lose water by osmosis. The cell surface membrane will start to shrink away from the cell wall. In this state the cell is said to be plasmolysed.

Osmosis in Plant Cells

Osmosis in Animal Cells If animal cells are placed in distilled water they will swell and burst, as there is no cell wall to limit the expansion of the cell. When this happens to red blood cells it is called haemolysis.

Osmosis in Animal Cells If animal cells are placed in strong sugar solution they will lose water, so they will shrink. When this happens they have a crinkled appearance, and they are called crenated.

Active Transport

Active Transport This occurs when molecules or ions have to move against a concentration gradient. ie. from a low concentration to a high concentration. Active transport can only take place in a living organism that is actively producing energy by respiration. Temperature and oxygen concentration which affect the rate of respiration, will also affect the rate of active transport.

Active Transport Active transport takes place by means of carriers in the cell membrane. The substance to be transported becomes attached to the carrier molecule on one side of the membrane. Then the carrier molecule changes in such a way that the substance is moved through it to the other side of the membrane. The necessary energy comes from the hydrolysis of ATP (adenosine triphosphate). This means water is added.

Processes that involve active transport are:

Endocytosis & Exocytosis

Endocytosis and Exocytosis These processes are involved in the transport of large quantities of materials in and out of cells, instead of individual molecules and ions. During endocytosis substances enter the cell. During exocytosis substances leave the cell. In both cases the necessary energy comes from the hydrolysis of ATP (adenosine triphosphate).

Endocytosis In endocytosis the cell surface membrane forms a vesicle around the material which is being ingested. The vesicle is closed and the cell surface membrane rejoins. The vesicle moves into the cytoplasm.

Endocytosis There are two types of endocytosis: Phagocytosis solid materials enter the cell in a vesicle. lysomes enter the vesicle, emptying their enzymes into it. the enzymes digest the materials and the products are absorbed into the cytoplasm white blood cells, called phagocytes, remove bacteria and cell debris by phagocytosis. Pinocytosis sometimes called ‘cell drinking’ similar to phagocytosis but involves liquids. the vesicles can be very small. a human ovum can take up nutrients from surrounding cells by pinocytosis.

Exocytosis Exocytosis is the reverse of endocytosis. In exocytosis the vesicle moves from the cytoplasm towards the cell membrane. The vesicle fuses with the cell membrane. The contents of the vesicle are released outside the cell. Often the materials involved in exocytosis are useful secretions, such as digestive enzymes and hormones.

Have a go at building your own cell membrane. http://www. wisc-online http://www.youtube.com/watch?v=2UPqLm-uDnI