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What are cells?

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Biology Essay 2 Cell membranes are almost made entirely of protein and lipid. The lipid of membranes is phospholipids. Phospholipids have a very similar structure to triglyceride, except that one of the fatty acid groups is replaced by a phosphate group. This molecule has a top part that is composed of a glycerol group to which an ionised phosphate group is attached. This part is known to have hydrophilic properties (water-loving). Below the head there are two long fatty acids, composed of hydrocarbon chains. These fatty acids are known to have hydrophobic properties (Water-hating). When a small batch of phospholipids comes into contact with water, they will float with the hydrocarbon tails exposed above the water forming a monolayer. If more phospholipids are added the molecules arrange themselves in a bilayer with the hydrocarbon tails facing each other. It is called a lipid bilayer (See figure 1 below) Figure 1 Proteins are spread across the membrane, and are either only on one surface or going from one surface to the other. ...read more.


They move in between the phospholipids. In facilitated diffusion, substances that might not diffuse across the membrane at all are aided to do so by the action of a particular molecule in the membrane. This is a large molecule that forms itself into a pore of a size large enough to allow a particular substance to diffuse through it. Particles in the membrane enable substances to diffuse across the membrane. Osmosis is the diffusion of water molecules from high concentration to low concentration across a partially permeable membrane. Water always diffuses from regions of lower solute concentration to regions of higher solute concentration. When the external solute concentration is the same as that of the cell, there is no net movement of water. Two systems which are the cell and environment, with the same solute concentration are termed isotonic. Osmosis is also a passive process. Water is crucial to all living organisms and an example of osmosis is found in digestion, where water is absorbed into body cells. Figure 2{Water Moves from AREAS of LOW Solute Concentration to AREAS of HIGH Solute Concentration. ...read more.


Pinocytosis is sometimes known as 'cell drinking.' Solutes or Fluids outside the Cell Membrane can be brought into the Cytoplasm by Pinocytosis. Phagocytosis is like pinocytosis, except the cell takes in a food particle or other cells instead of a drop of liquid.It is also known as 'cell eating; Figure 3 Water is of great importance to all living organisms. Water makes up 80% of the cell contents. It is the environment for the chemical reactions of life. It is used in different ways. Water can be used as a habitat to living organisms. It retains heat for a longer period of time allowing animals such as fish to survive and due to cohesion (when the water molecules stick together because of hydrogen bonds) animals can live on the surface of water. Blood which is mainly composed of water can carry heat from warmer parts of the boy to cooler parts. Many different substances dissolve in water because of its polarity. Solvent properties of water allow many substances to be carried and dissolved in water in the blood of animals and the sap of plants. Evaporation of water from plant leaves (transpiration) and the human skin (sweat) has useful cooling effects. ...read more.

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