What are cells?

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. These proteins include enzymes, receptors and antigens. Proteins can be, amongst other hormone binding sites, enzymes, or pumps for active transport. Some membranes also include the composition of the glycolax. These are glycoproteins and glycolipids whose functions are cell-cell recognition, as receptor sites for chemical signals such as hormone messengers and assisting in the building together of cells to form tissues. Membranes are fluid mosaic because the whole structure that is lipids and proteins is on the move and are called mosaic because the proteins are scattered.

There are various kinds of mechanisms of movement into and out of animal cells; these are diffusion and facilitated diffusion, osmosis, active transport and bulk transport.

Diffusion is a kind of passive transport, which means no extra energy is required. Molecules have energy of motion (kinetic energy) and are constantly colliding with each other. Diffusion always takes place from an area of high substrate concentration to an area of low substrate concentration. Due to this the substrate can be evenly distributed. Non polar substances such as for example glycerol can easily diffuse through. They move in between the phospholipids.

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There are various kinds of mechanisms of movement into and out of animal cells; these are diffusion and facilitated diffusion, osmosis, active transport and bulk transport.

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. Water will cross the Membrane toward the Higher Solute Concentration until the Concentration Gradients of BOTH WATER and SOLUTES EVEN OUT}

Active Transport requires energy. It is the movement of substances across membranes where energy is needed and is the pumping of molecules or ions through a membrane against their concentration gradient. Active transport requires a transporter and energy. The source of this energy is . An example of active transport is in a situation where potassium chloride (K+ and Cl- ions) is available to an animal cell; K+ ions are more likely to be absorbed since they are needed by the cell.

Bulk Transport occurs by movements of vesicles across the membrane. Large molecules of food and other substances are packaged in membrane-bound sacs called vesicles. The fluid mosaic membrane is flexible and makes the activity possible. Energy from metabolism is required, which is ATP and this process is known as cytosis. When substances are being imported into the cells this way, the process is called endocytosis. Pinocytosis and phagocytosis are two types of endocytosis. 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. Overall water has a significantly important role to play for living organisms

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