Transport through Membranes:

Transport: There are various ways in which membranes control what enters and leaves the cell, the transport may be either Passive (no energy is needed) or Active (requires energy). Passive methods include: Diffusion and Osmosis Active methods include: Active transport and Cytosis

1. Diffusion this is the movement of substances from an area of high concentration to an area of lower concentration. Liquids and gases diffuse because the molecules have random kinetic energy. The process is passive so it does not need ATP from respiration so poisons that inhibit respiration do not effect it

There are several factors that effect the rate of diffusion: –Surface area of the membrane: the larger the surface area the faster the diffusion as there is more surface for molecules to pass through eg. Villi increase the surface area in the small intestine –Difference in concentration: the greater the concentration gradient the faster the diffusion, substances diffuse down a concentration gradient until they reach equilibrium

–Thickness of the membrane: the thicker the membrane the greater the diffusion distance so the longer it will take. Usually internal surfaces for exchange are just one cell thick eg. Alveoli –Size of the molecule: the smaller the molecules the easier it is for them to slip through the phospholipid molecules. Also molecules that are lipid soluble will get through faster than those that are water soluble. –Temperature: the higher the temperature the higher the kinetic energy of the molecules so the faster they get through ie. Higher temperature = faster diffusion.

There are 2 types of diffusion: simple and facilitated. Simple diffusion is when small molecules like Oxygen and Carbon dioxide pass through the membrane unassisted, no proteins are involved and no energy is needed. Both gases are soluble in lipids and pass through easily, some water can also pass through in this way. Facilitated diffusion is when proteins of a specific shape (called protein carriers) have binding sites which match the shape of substances which they then carry across the membrane. This process is passive and does not need energy it also moves from a high to low concentration.

2. Osmosis the movement of water from a region of high concentration to a region of lower concentration through a semi permeable membrane This is the diffusion of WATER only A concentrated solution has lots of solute and only a little free water molecules A weak solution has less solute and more free water molecules Hypertonic = high solute concentration Hypotonic = low solute concentration Isotonic = equal solute concentration

The tendency of water movement depends on the Water Potential Water Potential is measured in KPa and is a negative scale with pure water having a water potential of 0 (this is the highest water potential). The more negative the value for wayer potential the more concentrated the solution is. If a cell is placed in distilled water, water will enter by osmosis until the cell swells and bursts this does not happen in plant cells because of the strong cell wall.

3. Active transport the movement of a substance from a low concentration to a high concentration ie. Against a concentration gradient by specific protein carriers requiring energy from ATP. Energy needed for this process is produced in the mitochondria during respiration The protein carriers may change shape and allow molecules to enter and leave the cell Because active transport relies on energy it would be inhibited by respiratory poisons

4. Bulk transport this involves moving substances across the membrane in bulk. This is done by small membrane bound sacs called vesicles. Endocytosis is the transport of substances INTO the cell, it needs energy and involves breaking open the membrane and then reforming it. If it brings solid into the cell it is called Phagocytosis, if it brings liquid into the cell it is called Pinocytosis. Exocytosis is the opposite and involves substances going OUT of the cell in bulk.

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