Chapter 1 Plasma membranes

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1 1 of 5 TEXTBOOK ANSWERS Chapter 1 Plasma membranes Recap The plasma membrane: keeps internal contents of the cell confined to one area keeps out foreign molecules that damage or destroy the cell s components and molecules controls what enters and exits a cell according to needs of the cell enables communication with the environment and with other cells. 2 The plasma membrane is a flexible structure that can change shape easily. It is able to grow and expand as the cell contents increase. It can repair itself quickly if damaged. The membrane is made up of phospholipid molecules. 3 The plasma membrane is described as a fluid mosaic model because it is composed of two major components: the phospholipid layer (fluid) and embedded protein (mosaic) molecules. The phospholipid bilayer is capable of sideways movement and is highly mobile and flexible allowing the cell to change shape easily. The various proteins embedded in the bilayer form the mosaics of the title. 4 Cholesterol interferes with the strong inflexible bonds that exist between the lipid tails, therefore making the membrane more flexible. Cholesterol also increases the fluidity between the phospholipid molecules in lower temperatures; however, it works to decrease fluidity at higher temperatures as it limits the movement of the phospholipid tails. So cholesterol maintains membrane fluidity within specific limits. Recap Receptor proteins, transport proteins, recognition proteins and adhesion proteins 2 Receptor molecules have receptor sites on their surface that detect molecules, such as hormones. Each receptor is specific for a single molecule or a small number of molecules with a related structure to which the receptor binds. Recognition proteins act as markers that identify the cell as a normal body cell belonging to the individual. The immune system recognises self cells and leaves them be, and can identify non-self molecules and mount an immune response against them. Adhesion proteins link cells together to maintain both the 3D structure and normal functioning of tissues. The cytosol facing end of these proteins is usually connected to parts of the internal cytoskeleton. Transport proteins allow the movement of specific substances across a membrane, usually by forming a channel through it.

2 2 of 5 Recap Extracellular refers to the area outside the plasma membrane and intracellular is the area inside the plasma membrane. 2 The membrane s permeability properties ensure that essential molecules, such as glucose, amino acids and lipids enter the cell readily, necessary cell contents remain in the cell and waste compounds leave the cell. Its selective permeability keeps the concentration of substances inside the cell fairly constant, thus allowing the cell to maintain a constant internal environment that maximises its functioning capacity. 3 Water molecules are highly polar; that is, they consist of charged particles. Water is the solvent in which most things will dissolve. But these things must also be polar. When trying to mix oil and water, this does not occur as water is polar and the lipid in oil is not. Therefore, the oil will not dissolve. However, the non-polar lipid may dissolve in another non-polar substance. Hydrophilic substances are therefore polar substances and hydrophobic substances are non-polar. So like dissolves like. 4 Two types of substances that are able to diffuse through the lipid bilayers are small uncharged polar molecules (such as water, ethanol and glycerol) and small hydrophobic molecules (such as oxygen, carbon dioxide and nitrogen). Experiment 1.1 Discussion 1 The evidence is that only some colour comes out of the cells, not all the cell contents. 2 It was necessary to ensure that the ruptured chromoplasts containing the red colour (betacyanin) was removed completely. Otherwise, it would be difficult to know if colour that was recorded after the procedure was a result of the procedure, or of excess colour that had not been washed away originally. 3 High temperatures would act in denaturing the protein elements in the membrane that would then allow the pigment to leak out of the cells (and the organelles inside the cell). Recap Transport Protein Structure Type of Movement Energy required Channel protein Carrier protein Carrier transport protein Narrow passages through which small ions diffuse Bind to specific molecules and change shape as they bind and release the molecule on the other side of the membrane ATP powered one way valves that only allow the movement of substances into the cell against their gradient Facilitated diffusion Facilitated diffusion Active transport No No Yes 2 The plasma membrane s permeability is like a sieve in that the size of the molecule entering or leaving the cell determines whether it may pass through or not. It is different in that the size of the holes do not alter they are always open. The channels in the plasma membrane may open or close depending on the needs of the cell.

3 3 of 5 3 The type of organelles present in each cell will vary according to that cell s role within the organism. Different organelles require different substances and the different proteins ensure different substances enter and leave the cell as required. Transport proteins in lysosomes allow amino acids to cross into the cytosol; in plants, membrane proteins facilitate the transport of water around the large vacuole. Recap Both endocytosis and exocytosis use the plasma membrane to move substances in and out of the cell, and both processes are active. Exocytosis moves substances made in the cell out of the cell, whereas endocytosis draws substances into the cell from the extracellular environment. 2 Endocytosis is the bulk transport of substances into the cell. Phagocytosis is when solids are engulfed and enter the cell; pinocytosis is the process that takes on droplets of liquid. Recap The nucleus contains the code that allows the cell to produce the necessary proteins. This code is found in the nucleic acid, DNA, which is housed in the nucleus. 2 Ribosomes consist of two subunits that are composed of RNA and various proteins. The RNA in ribosomes is synthesised in the nucleolus of eukaryotic cells. Ribosomes are the site of protein synthesis producing polypeptide chains from amino acids. The order of the amino acids is determined by the code on the DNA. 3 Both the endoplasmic reticulum (ER) and the Golgi apparatus consist of interconnecting membranes. The ER is a transportation system more like a highway that allows substances to move along it from one part of the cell to the other. The Golgi apparatus is able to pinch off into small vesicles that contains the final product of the protein synthesis process. These vesicles move to the plasma membrane where that merge with it and discharge their contents to the outside of the cell. 4 Rough ER is studded with ribosomes and transports the proteins to other parts of the cell as well as to other cells. The smooth RE does not have ribosomes attached and its main role is to transport proteins, synthesise lipids and assist in the manufacture of plasma membranes. Chapter review questions Remembering 1 a Phospholipid b Phospholipid and cholesterol c Phospholipid 2 Plasma membrane is described as a lipid bilayer as it is composed of a double layer of phospholipids. 3 Protein Receptor proteins Recognition proteins Adhesion proteins Transport proteins Function Have receptor sites on their surface that detect molecules such as hormones. Act as markers that identify the cell as a normal body cell belonging to the individual. Link cells together to maintain the three dimensional structure and normal functioning of tissues. Allow movement of specific substances across a membrane typically by forming a channel through it.

4 4 of 5 4 Hydrophobic refers to molecules that are water fearing or do not dissolve in water; hydrophilic refers to molecules that are water loving or do dissolve in water. 5 Diffusion does not require specific membrane channels very small substances are able to move through the membrane, specifically between the individual phospholipid molecules that make up the fluid part of the membrane. Osmosis is the diffusion of water and this does not require specific channels in the plasma membrane to cross from one side to the other. Understanding 6 a The phospholipid bilayer is described as being fluid; the embedded proteins are described as mosaic as they are studded within the phospholipid bilayer. b The fluidity of the phospholipid bilayer and its ability to undergo exocytosis and endocytosis without leaving a gap in the membrane ensures that the membrane is able to change shape and also maintain an unbroken barrier. The proteins embedded within this phospholipid bilayer carry out a range of functions that the cell requires. The functions are different to the rest of the membrane and therefore offer the appearance of the mosaic. 7 Selectively permeable implies that only some substances are able to pass through the plasma membrane while other substances are not able to do so. Often dependent on size or the concentration gradient on either side of the membrane. 8 Some white blood cells will engulf foreign solid matter. This process is called phagocytosis, therefore cells that carry out this function would be known as phagocytes. 9 The Golgi apparatus is the destination of manufactured proteins after they leave the ribosomes and travel along the endoplasmic reticulum and into the cytosol. These proteins then reach the Golgi apparatus where final preparations are made (such as ensuring the proteins are in their final form) and then packaged in vesicles to exit the cell by exocytosis. The materials produced include hormones and enzymes and other protein products. Applying 10 This would up to individual student responses, however, the cell would need to be a photosynthetic cell with chloroplasts. Therefore, the pathway would include via the stomata due to diffusion into the spongy mesophyll layer. It would then enter the cell and the chloroplast via diffusion and then be used in the production of glucose. It would always move with the concentration gradient and being a tiny molecule it would move between the phospholipid molecules into the organelle that uses it. 11 See Figures 1.11 and A cell that contains more rough ER than smooth ER would be a cell that produces proteins such as hormones and enzymes, so it could be a pancreatic cell that produces both digestive enzymes and hormones for the control of blood glucose levels, such as glucagon and insulin. 13 Substances with similar properties, such as being hydrophilic, will dissolve in other hydrophilic substances. However, substances that are hydrophobic will not dissolve in hydrophilic substances. However, they will dissolve in other hydrophobic substances. Therefore, like dissolves like. This also applies to polar and non-polar substances. 14 a Alcohol, like ethanol, is lipid insoluble, however, it is a small uncharged polar molecule that moves by diffusion through the phospholipid bilayer into the cell. b Caffeine is a larger lipid soluble molecule that easily moves into the cell through the plasma membrane phospholipid bilayer, but because it is larger it takes longer to move into the cell than alcohol.

5 5 of 5 15 a The organelles through which the radioactive amino acids move would be ribosomes, rough ER, smooth ER, Golgi apparatus, vesicles and plasma membrane. b At ribosomes they would enter as individual amino acids. They would leave and move into the ER as part of polypeptide chains, and then leave Golgi in vesicles as finished proteins ready for their specific function. Analysing 16 a The endocytic vesicles b They would contain substances that have been engulfed by the cell and the plasma membrane would pinch off and form vesicles whatever the substance is. c Endocytosis d The vesicles may be larger and would contain solid particles rather than droplets of liquid. 17 Hydrophobic substances do not readily dissolve in water because it is non-polar and water is a polar substance. 18 Each membrane transport protein has a specific function and is structured accordingly. They are all embedded in the phospholipid bilayer and allow substances to pass through them. They differ in what size particle moves through and whether the movement of the particle is movement along the concentration gradient or against it. So channel proteins allow the passive movement of small ions in and out of the cell; carrier proteins bind to specific larger molecules and they move across the membrane passively. Carrier transport proteins are used when substances need to move across the membrane against the concentration gradient. Therefore, they require access to ATP to drive the process. 19 Each organelle possess different transport proteins as different substances are required to move in and out of the organelles. For example, mitochondria would have carrier transport proteins to move glucose into the organelle for cellular respiration; chloroplast would use these to move glucose out of them after being produced in photosynthesis. 20 The concentration of sodium ions would be isotonic to the surrounding environment in culture B. This is because the proteins that monitor the entry of sodium ions would be inactive and the concentration gradient would be eliminated by simple diffusion. Culture A would have a lower sodium ion concentration inside the cell as sodium is only required in small amounts so the channels would actively pump out the excess ions as they entered by diffusion. Evaluating 21 The membrane size would stay relatively constant as exocytosis would contribute to the overall size while endocytosis would deplete the membrane. 22 Substances that move through any of the transport proteins are monitored. The smaller ions moving through the channel proteins enter by diffusion but the channel proteins will close when specific concentration is reached. The carrier proteins need to bind with a specific substance to move it across the membrane. They can only move as much as there are proteins. The carrier transport proteins will move substances against the gradient so a supply of ATP is necessary. In its absence movement will not occur. Creating 23 Responses will vary. 24 Responses will vary.