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The Movement of Substances Across Cell Membranes

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Student: Zoe Wood Tutor: Len Taylor The Movement of Substances Across Cell Membranes ONCW - Human Physiology, Level 3 Date of Submission: 18th January 2006 The Movement of Substances Across Cell Membranes All of the living organisms on Earth are divided in pieces called cells. These are small compartments that hold all the biological equipment necessary to keep an organism alive and successful on Earth. The human body is made up of about ten trillion cells. Within each cell there is a network of organells, that all have unique functions. These organelles allow the cell to function properly. They are surrounded by membranes, which separate various activities within the cell. The nucleus is present in most living cells and controlls the cells activities. It is like the brain of the cell and contains all the cells DNA. It is the largest organelle surrounded by a nuclear membrane, which has several openings, called pores, for the RNA and proteins to pass through. Mitochondria provide the energy a cell needs to move, divide, produce secretary products and contract. They are membrane-bound organelles, and like the nucleus have a double membrane. The outer membrane is fairly smooth. But the inner membrane forms folds called cristae. The cristae increase the inner membrane's surface area. ...read more.


Passive transport occurs in living and non-living systems and occurs with the use of natural kinetic energy of molecules or ions. There are three types of passive transport, diffusion, facilitated diffusion and a special kind of selective diffusion called osmosis. Diffusion is the net movement of ions or molecules from a region of their higher concentration to a region of their lower concentration. This is a process in which molecules randomly move about, from an area where there is more of that molecule to where there is fewer. It is a fairly slow process as there is no energy to push the molecules through the membrane, and it only works if the molecules are small enough to actually get through the tiny pores of the membranes surface. The molecules move about freely, eventually hitting one of the tiny gaps in the membrane, any molecules that don't find the gap, just bounce back off until they do, so it is a very random process. A man named Adolf Eugene Flick did a lot of experiments, which revealed important information about diffusion. He's 'law' showed that the rate of diffusion doesn't just happen on the difference of concentration, but also on the surface of the membrane and the thickness of the membrane. ...read more.


Another active process is Exocytosis. This is a process in which useful materials are secreted out of the cell from a vesicle. The vesicle, often produced by the ER or the Golgi body, fuses with the cell membrane and release its contents out. This process is used in the small intestine by discharging tiny droplets of fat into the lacteals. It also happens in the pancreas, by secreting pancreatic enzymes into the lumen where their apical surfaces meet and leads eventually to the pancreatic duct draining into the small intestine. Another form is Endocytosis, which is the opposite of Exocytosis. The membrane surrounds the particles and takes it in, sealing it off to form a vesicle, trapping the material inside. The useful material is absorbed from the vesicle into the cell and the vesicle, again, fuses with the cell membrane secreting the remaining products. This is known as Phagocytotis, meaning 'cell eating'. Another form of Endocytosis is Pinocytosis, meaning 'cell drinking'. It is similar to Phagocytosis, but takes in droplets of moisture in liquid form, instead of molecular particles. This occurs continuously in almost all cells in the human body, allowing cells to pick up all the critical components that the cell needs. Various substances can pass through the cell membrane, dependent on their solubility, size and contents, different processes will occur to help the substances get across, and enable cells to function properly and keep all organisms alive on Earth. ...read more.

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