AS Biology Osmosis, Diffusion, Circulation

Osmosis 

IS THE MOVEMENT OF WATER FROM A LESS CONCENTRATED SOLUTION TO A MORE CONCENTRATED SOLUTION THROUGH A PARTIALLY PERMEABLE MEMBRANE. 

• Osmosis is important to plants
• They gain water by osmosis through their roots
• water moves into plant cells by osmosis
• making them turgid or stiff
• this makes it able to hold the plant upright. 

Diffusion

IS THE NET MOVEMENT OF MOLECULES FROM A REGION OF HIGH CONCENTRATION TO A REGION OF LOWER CONCENTRATION

The efficiency of diffusion is increased if there is; 

• A large surface area- over which exchange can take place
• A concentration gradient- without which nothing will diffuse
• A thin surface area- across which gases diffuse

Unicellular organisms do not have specialed gas exchange surfaces, instead gasses diffuse in through the cell membrane. 

The smaller something is, the smaller the surface are is but, more importantly, the bigger the surface area is compared to its volume. 

Unicellular organisms have a large surface area to volume ratio. 

They are more efficient when it comes to exchanging gases through their membranes, 

-The diffusion path is short

Multicellular organisms are bigger than unicellular organisms. This makes efficient diffusion of gases more difficult. 

However, if they are small , or large but very thin, the outer surface of the body is sufficient as an exchange surface because the surface are to volume ratio is still high. Larger organisms need specialised exchange surfaces,

E.g-  Gills or Lungs

Artery Every cell in the body needs oxygen in order to respire. Oxygenated blood is carried within the arteries this is generally the flow of blood going away from the heart, the exception being the Pulmonary Artery. When the blood leaves the heart it is under very high pressure and arteries need to be string enough to contain the high pressure blood. In order to do this the Arteries have thick muscular walls with elastic fibres to withstand the pressure which recalls as the heart beats.

Structure- The Arteries have a small lumen compared to that of a vein, maintaining a high blood pressure. The walls surrounding the lumen are made up of layers of muscle and elastic fibre.The total diameter of the artery is large compared to a vein.

Arterioles- Arterioles are able to control blood flow by restricting and opening the lumen. The muscle layer is thicker than in the

Arteries: when the muscle contracts in the Arterioles, the lumen constricts and blood flow becomes restricted. 

The elastic layer is thinner than in the Arteries: there is lower blood pressure in Arterioles so does not need to expand as much.

Veins- carry deoxygenated blood, generally back towards the heart with exception to the pulmonary vein. 

Veins have thinner walls that are flexible and can expand holding large volumes of blood. The blood contained within the veins is at a much lower pressure than in the Arteries. Veins that run within or close to skeletal muscles have blood squeezed as the muscles contract. Movement of the blood is assisted by valves which prevent back flow of blood.

Structure -Veins have a very large lumen and thin walls surrounding the lumen. there are valves within the veins preventing back flow.

Capillaries - Arteries and veins are linked by a series of capillaries. 

These are tiny blood vessels with walls one cell thick, to allow exchange of oxygen, carbon dioxide, nutrients and waste between the blood and tissue. The capillary walls have small pores in the wall through which tissue fluid and White blood cells can pass. Substance movement Diffusion of lipid soluble substances passes through the plasma membrane. Diffusion of lipid Insoluble substances passes though pores in capillary wall. Bulk flow of water with dissolved substances also passes though pores in capillary walls. Osmosis of water passes out of the capillary.