Cell membranes and transport

Alternative transcript:

such as oxygen, dissolve in the phospholipid and diffuse across the membrane. Water-soluble substances, such as any polar molecules, cannot readily diffuse through the phospholipids and must pass through intrinsic protein molecules, which forms water-filled channels (pores) across the membrane. As a result of this, the cell membrane is selectively permeable to water and some solubles. Diffusion It is the movement of molecules from a region of high concentration to a region of lower concentration i.e. down a concentration gradient. There will be a net movement away from the high concentration area until there is a uniform distribution. The rate of diffusion is affected by: ● ● ● Facilitated diffusion Facilitated diffusion is a special form of diffusion that allows molecules that cannot pass over the membrane naturally to be moved across the membrane. It is a passive process and so happens along the concentration gradient. There are 2 types of proteins that carry out facilitated diffusion: ● ● The thickness of the exchange surfaces' membrane The surface area of the membrane Active transport Active transport is an energy-requiring process in which ions and molecules are moved across the membrane against a concentration gradient. Features of active transport are: ● lons and molecules are moved from a lower to a higher concentration against the concentration gradient The size of the diffusing molecule The nature of the diffusing molecule Temperature ● The process occurs through intrinsic carrier proteins spanning the membrane The rate is limited by the number and availability of carrier proteins ● ● Channel proteins are molecules with pores lined with polar groups. This means that small polar molecules can pass through. This channel opens and closes according to the needs of the cell. Carrier proteins allow diffusion of large polar molecules, such as sugars and amino acids, across the membrane. A molecule attaches to a binding site on the carrier protein and it then changes its shape and releases the molecule on the other side of the membrane. Active uptake of a single molecule or ion occurs as follows: The molecule or ion combines with a specific carrier protein on the outside of the membrane ATP transfers a phosphate group to the carrier protein on the inside of the membrane The carrier protein changes shape and carries the molecule or ion across the membrane to the inside of the cell ● ● The molecule or ion is released into the cytoplasm The phosphate ion is released from the carrier molecule back to the cytoplasm and recombines with ADP to form ATP. The carrier protein then returns Co-transport - type of facilitated diffusion that brings molecules and ions into cells together on the same transport protein. Osmosis A special case of diffusion which involves the movement of water molecules only. Water potential - a measure of the free energy of water molecules and is the tendency for water to move. An addition of a solute to pure water tends to bring water molecules in, lowering the ware potential and giving it a negative value. Solute potential-solute potential measures how easily water molecules move out of a solution. The more solute present, the more tightly the water molecules are held, lowering the tendency of water to move out. Pressure potential - the pressure that the cell wall/membrane experiences is the pressure potential, and as it pushes outwards it is positive. It increases the tendency of water to move out of the cell. Water potential equation => Water potential of cell = pressure potential + solute potential This means that the balance of the solute and pressure potential determine the water potential and whether or not water moves in or out of the cell. Turgor and plasmolysis If the water potential of the external solution is less negative than the solution inside the cell, it is hypotonic to the cell and water flows into the cell. If the water potential of the external solution is more negative than the solution inside the cell, it is hypertonic to the cell and water flows out of the cell. If the cell has the same water potential as its surrounding solution, then they're both isotonic so no net movement of water will occur. Plasmolysis is when a plant cell is placed into a hypertonic solution which causes a net loss of water, this then makes the cytoplasm draw away from the cell wall. This causes the cell to become flaccid. If the external concentration is high enough that the cell has lost just enough water that its membrane begins to be pulled away from the cell wall, the cell is at incipient plasmolysis. Plasmolysed cytoplasm completely pulled away from cell wall A plant cell becomes turgid if it is placed in a hypotonic solution, the cell takes in water until it is prevented by the opposing pressure from the cell wall. As the water enters, the contents expand and push out more on the cell wall increasing the pressure potential. When a cell can take in no more water, it is turgid. When animal cells are put in a Turgid cytoplasm pushed against cell wall cellulose cell wall 4 = 4₂ +4₁ Р nucleus cytoplasm 4 = 0 4₂ = -4₂ incipient plasmolysis cytoplasm begining to pull away from cell wall 4₁ = 0 4 = 4₂ 4₂ = 0 4 > 4, hypotonic solution, because they have no cell wall, they burst. This is called haemolysis. If animal cells are placed in a hypertonic solution, they shrink and become 'crenated'. Bulk transport A cell can transport materials in bulk in, by endocytosis, or out, by exocytosis. Endocytosis occurs when material is engulfed by extensions of the plasma membrane and cytoplasm, surrounding it, making a vesicle. There are two types of endocytosis: Phagocytosis is the uptake of solid materials that are too large to be taken in by diffusion or active transport. Pinocytosis is the uptake of liquid by the same mechanism. Exocytosis is the process by which substances may leave the cell, having been transported through the cytoplasm in a vesicle, which fuses with the cell membrane. Digestive enzymes often secreted it in this way. ● ● phagocytosis plasma membrane lysosomes empty their enzymes into vesicle vesicle bacterium engulfed bacterium bacterium digested exocytosis vesicle containing secretory product e.g. enzyme Golgi body product released When endocytosis or exocytosis occurs, the cell membrane has to change shape and this requires energy. These processes are therefore active, using ATP, generated by the cell's respiration. In both cases, the cell membrane flows with endocytosis decreasing the overall area of the cell membrane and exocytosis increasing it. The property of fluidity of the cell membrane is essential for these processes to occur.