IB Biology HL - All Extended Response Answers

Unit 1: Cell Biology 1.Outline the cell theory. [6] •The cell theory states that all living things are made of cells. •This is proven by microscopic evidence. •It also states that the cell is the smallest possible living organism to exist on its own. •The exceptions to this are mitochondria and chloroplasts that can exist on their own for up to 30 days. •All cells arise from preexisting cells •The falsification of the theory of vitalism proves this. 2.Outline the exceptions to the cell theory. [6] •The acetabularia challenges the idea that the cell is the smallest and simplest form of life. •It can grow up to 10cm and is complex, containing a rhizoid. •Aseptate hyphae challenges the idea that the cell is simple, containing a single nucleus and cell membranes. •It is multinucleate and long, without cellular membranes. •The striated muscle cells challenge the idea that the cell is small, with a single nucleus. •It is too long, and is multinucleate. 3.Explain the importance of surface area:volume ratio in cells and how this allowed for adaptation. [8] •Large surface area is required as several metabolic reactions take place on the plasma membrane. •It is also important for the exchange of substances between the cell and the outside. •The volume of the cell is important for the production and use of materials. The volume determines how fast substances are used up/produced. It also determines the need for materials. •Large surface area:volume ratio is beneficial to the cell as it ensures that the exchange of substances is taking place at a sufficient rate to sustain the need for materials in the cell. •When cells grow, surface area:volume ratio decreases. •Hence, cells divide to maintain ratio. •Cell size is limited by surface area:volume ratio. •High surface area:volume ratio has advantages. •It provides a shorter distance for diffusion. •It also makes a concentration gradient much easier to produce. 4.Explain the features of the Paramecium that enable it to live. [7] •Metabolism takes place in the cytoplasm. Enzymes are present to catalyze these reactions. •Contractile vacuoles expel water from the cell to maintain homeostatic conditions. •Cilia beat back and forth to move the cell. •Food is stored in the cell in vacuoles. This can be broken down by respiration in the cytoplasm to provide energy for growth. •The nucleus divides by asexual reproduction. •The cell membrane controls what substances enter and leave the cell. For example, oxygen enters the cell through the membrane. The cell membrane regulates excretion. 5.Explain the features of the Chlamydomonas that enable it to live. [7] •Metabolism takes place in the cytoplasm. There are enzymes to catalyze these reactions. •Reproduction takes place by asexual and sexual reproduction. •Contractile vacuoles expel water to maintain homeostatic conditions and to regulate excretion. •Beating of the two flagella regulates movement of the cell. •The cell wall is completely permeable to substances like water. The cell membrane inside the cell wall controls the entry and exit of substances in the cell. This maintains homeostatic conditions and regulates excretion. •It has a light-sensitive eye-spot that senses the light and allows the cell to respond to this by the movement of the flagella to move the cell toward the light. •Photosynthesis occurs inside the chloroplasts for growth. In the dark, carbon compounds for growth are also absorbed through the cell membrane. 6.Explain the features of E. coli that enable it to live. [7] 7.Outline the process of differentiation in cells. [4] •Differentiation in cells leads to specialized cells. •An example is the red blood cell in humans that has no nucleus to maximize surface area for transport of substances in blood. •Differentiated cells contain all genes. •However, only some genes are expressed/switched on. •Groups of differentiated cells are known as tissues. •Hormones and other chemicals determine how a cell develops. 8.Describe emergent properties. [3] •Emergent properties are properties that arise as a result of interactions between parts of an organism. •They are an adaptation in eukaryotic organisms. •For example, the stomach and small intestine work together to carry out digestion. 9.Explain how stem cells can be used to cure diseases. [6] •Stem cells are undifferentiated cells, and can be used to differentiate into any type of tissue and replace any damaged tissues. •They are of different types: totipotent, pluripotent, multipoint, and unipotent. •They can be used to cure leukemia. •Chemotherapy kills the cancerous white blood cells but also kills the healthy WBCs. •Stem cells can be used to replace these lost WBCs. •Stargardt’s disease can also be cured by using stem cells. •Stargardt’s disease results from degeneration of cells in the retina. •Stem cells are differentiated into these retina cells and are used to replace them. 10.Outline the ethical implications behind stem cell research. [3] •Stem cells are of three main types: embryonic, umbilical cord, and bone marrow. •However, only embryonic stem cells are totipotent. •The implications of this include deciding whether an embryo is living or non-living. •Creating embryos by IVF is also controversial. •Using stem cells has been thought of as tampering with nature. 11.Define ultrastructure. [1] •Ultrastructure is all the structures of a biological specimen that are at least 0.1nm in their smallest dimension. •These structures can only be seen using an electron microscope. 12.Outline the differences between the structures of prokaryotic and eukaryotic cells. [6] 13.Explain why eukaryotic cells are compartmentalized. [4] •Compartmentalization means that the pH or extreme conditions of some parts of the cell do not disturb other parts •Substances can be moved around in vesicles •Concentrations of some substances can be increased •Harmful substances, such as the enzymes in lysosomes, could kill other parts of the cell if they were not stored in vesicles. 14.Explain how the structure and properties of phospholipids help to maintain the structure of cell membranes. [8] •The phospholipid bilayer consists of two layers of phospholipids. •Each phospholipid has a phosphorylated alcohol head and two fatty acid tails. ProkaryotesEukaryotesNaked DNA: DNA is not associated with proteinsDNA bound in nucleus using histonesCell walls present; made of peptidoglycanCell walls sometimes present; made of cellulose or chitinNo membrane bound organellesMembrane bound organelles like chloroplasts and mitochondria are presentNo compartmentalizationCompartmentalizationNo vesiclesVesicles presentNo cell membraneCell membrane present - made of phospholipid bilayer •The fatty acid tails are hydrophobic and face inward, facing each other. They are away from water. •The alcohol heads are hydrophilic and face outwards, so that they are in contact with water. •One of the fatty acid tails is saturated, while the other is unsaturated, and contains double bonds. •The alcohol head contains glycerol. 15.Describe the structure of cell membranes. [8] •Cell membranes consist of phospholipid bilayers. •Each bilayer consists of phosphorylated alcohol heads and fatty acid tails. •The alcohol head is hydrophilic and polar and faces outwards so that it is in contact with water. •The fatty acid tails are hydrophobic and non-polar so face inwards and attract each other, so that they are not in contact with water. •One fatty acid is saturated and the other is unsaturated. •Cell membranes also have proteins: integral and peripheral. •They contain cholesterol which maintains the fluidity and integrity of the membrane at extreme temperatures. •The alcohol head consists of a glycerol molecule. 16.Describe the various functions of membrane proteins. [8] •Membrane proteins are used for transport of substances across the cell membrane. For example, sodium-potassium pump. •They are used as anchorage for other substances such as cytoskeletons •They are used as receptors for substances such as neurotransmitters or hormones •They are used for cell recognition of other cells. •They are used for intercellular joining •They are used for enzymatic activity. 17.Describe the structure and function of cholesterol in the cell membrane. [4] •Cholesterol is a steroid which is a lipid. •Cholesterol has hydrophobic and hydrophilic parts. •The hydrophobic parts of the cholesterol molecule are situated on the inside of the bilayer, attacked to the fatty acid tails. •The hydrophilic parts are on the outside of the bilayer, attached to the phosphorylated alcohol heads. •Cholesterol maintains the integrity of the bilayer by limiting movements •It prevents crystallization of the bilayer at low temperatures so it maintains fluidity of the bilayer. •It allows the bilayer to change its shape; this allows for vesicle formation during endocytosis. •It allows animal cells to exist without cell walls.