Cell Parts, Structure and Function

CELL PARTS, STRUKTURE AND FUNCTION If you observe a cell with a light microscope in a school environment, you can see the cell nucleus and cytoplasm, but you cannot observe the contents of the cell completely and clearly. The researchers used an electron microscope which can magnify the object of observation up to 1,000,000 x so that various organelles in the cytoplasm can be seen, such as the endoplasmic reticulum, ribosomes, lysosomes, mitochondria and Golgi bodies. a. Plasma Membrane In animal cells, the plasma membrane is the outermost layer that separates the cytoplasm from the surrounding environment, whereas in plant cells it is located between the cell wall and the cytoplasmic contents of the cell. With modern research techniques, it can be seen that the plasma membrane (lipoprotein) consists of two layers of lipids and proteins that are distributed irregularly. Lipids in the plasma membrane are mainly phospholipids (lipids which compounds with phosphate), glycolipids (lipids which combine with carbohydrates) and sterols such as cholesterol. Each lipid molecule consists of The head is in the form of phosphate which is hydrophilic (capable of binding water molecules), and the tail is in the form of fat which is hydrophobic (repels water molecules). Protein molecules, both those located on the outer surface and The inner surface is called peripheral protein and is hydrophilic, protein molecules that penetrate from the outer surface to the inner surface called integral proteins. An integral protein part embedded within the lipid layer is hydrophobic. Cell biologists describe that the proteins in the membrane are solid objects in the fatty fluid, like an empty bottle floating on the lake, both of them always move dynamically, which then called the Fluid Mosaic Model. The presence of bound carbohydrate compounds protein molecules will add hydrophilic properties on both sides membrane surface, this causes the plasma membrane to be selective. There are molecules that can pass through the membrane from outside to inside the cell, or can only pass through the membrane from inside to outside or vice versa. The cell membrane, apart from functioning as a boundary between cells, also has several other functions, as follows. 1) As a protector, namely protecting the cell contents from escaping. 2) Regulates the traffic of various substances because the cell membrane is selectively permeable, in this way the cell membrane maintains its shape, size and chemica reactions. 3) As a receptor (receiver of stimuli) from outside, such as hormones, chemicals, mechanical stimulation and electrical stimulation. b. Transport of Substances through Cell Membranes Various organelles found in the cytoplasm have membranes whose structure is the same as the plasma membrane. Even though the plasma membrane is only ± 0.1 ÿm thick, the plasma membrane is a barrier to the movement of molecules and ions of substances. Free movement of ions and molecules is very important for maintaining appropriate pH stability, controlling ion concentrations in cells for enzyme activity, obtaining a supply of food, energy and other raw materials, and removing metabolic waste that can be toxic. The above is done by diffusion, osmosis, active transport, and endocytosis or exocytosis c. Diffusion through Membranes Diffusion is the spreading of solid, liquid or gas molecules from high concentration to low concentration so that the concentration is the same everywhere. For example, put 1 spoonful of table salt in a glass of water. Without stirring, the salt molecules will spread throughout the water in the glass and the water will taste salty if we taste it (diffusion of solid substances in a liquid medium). Small molecules, such as H2O , CO2 , and O2 can easily and quickly pass through membranes. Other molecules that can diffuse are molecules that can dissolve in fat. These molecules can diffuse through phospholipid membranes, for example fatty acids and glycerol. d. Diffusion with Facilities Sometimes a substance particle cannot diffuse because it is blocked by a membrane that is difficult to penetrate. However, if the membrane contains a supporting factor, namely a transport protein, then the high concentration of material causes the opening of channels in the integral proteins, the particles of the substance can diffuse without involving energy Transport of substances as mentioned above is a diffusion process with facilities. e. Osmosis Osmosis is the diffusion of water molecules through a membrane, from a low concentration solution (hypotonic) to a high concentration solution (hypertonic) until finally the solution becomes the same concentration (isotonic). A solution that has a low concentration means it contains more water molecules than a solution that has a high concentration. Water enters the cell if the concentration of the solution inside the cell is higher than the solution outside the cell. If too much water enters the cell, the cell will swell and may even burst. Conversely, if the concentration of the solution outside the cell is higher than the concentration of the solution inside the cell, cell water will come out. If a lot of cell water comes out, the cell will shrink and even experience plasmolysis (the detachment of the plasma membrane from the cell wall) which is found in plants. To understand better, do the following activities. f. Active Transport Diffusion and osmosis are passive transport systems, do not require energy. This process takes place when there is a difference in concentration outside and inside the cell. Active transport is transport that requires energy to remove or enter molecules or ions through the membrane, molecules move from low concentration to high concentration (in the opposit direction). In contrast to diffusion which can go both ways, active transport is a one-way movement and is influenced by electrical charges inside and outside the cell. In detail, active transport is as follows. 1. From low concentration to high concentration. 2. Through the plasma membrane. 3. Requires transport molecules (transport proteins). 4. Requires energy in the form of ATP. f. Endocytosis and Exocytosis Endocytosis is the introduction of substances into cells, while exocytosis secretes substances out of the cell. Both processes are included in transportation active. Endocytosis is carried out by single-celled organisms and white blood cells. Endocytosis that occurs in solid substances is called Phagocytosis, and endocytosis What occurs in solution is called pinocytosis. Examples of phagocytosis, for example cells White blood eats foreign proteins (disease germs) or amoeba eat bacteria. The substances eaten are put into vacuoles food. Exocytosis can be found in the process of secreting substances by cells gland. For example, the secretion of digestive enzymes into the intestines. Secretions (substances that secreted) is usually encased i membrane sac or vacuole The sacs go to the edge of the cell, open and the secretions come out.