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Transcript
Cell Structure and Function All living things are made of ________. Some, like us, contain many millions of cells. Such organisms are referred to as being _________________. Other organisms are composed of only one cell and are said to be ________________. Diagrams of some common unicellular organisms are drawn below. Most cells are so small that they can only be seen using a microscope. All cells contain _______________ surrounded by a flexible membrane called the _________ membrane. Using a very powerful electron microscope, it is possible to see many of the structures inside a cell. These structures are called cell ______________ and they are responsible for carrying out the chemical processes which maintains the cell. These chemical processes contribute to the cell’s metabolism. Many metabolic reactions are catalysed by ____________. A series of such reactions is called a _____________ pathway. The diagrams on handouts 1 and 2 show the organelles that are present in a typical plant and animal cell, and the table on page 2 summarises the functions of the various organelles within these cells. 1 ORGANELLE NUCLEUS MITOCHONDRION ENDOPLASMIC RETICULUM (rough or smooth) RIBOSOMES * FUNCTION contains chromosomes (DNA) which provide the recipe for making cell _____________. site of aerobic ______________ (which in turn produces ATP). involved in the ____________ of proteins for export from the cell. site of ____________ synthesis – found on the surface of RER or free in the cytoplasm. GOLGI APPARATUS involved in the packaging and processing of (or golgi body) ____________ ready for secretion from the cell. SECRETORY a sac derived from the ________ (GOLGI) VESICLE __________ containing the finished ___________ which it then secretes from cell by fusing with the cell ____________. LYSOSOMES a sac which contains powerful digestive ___________ which are used to destroy bacteria. PLASMA CELL controls what enters or leaves a cell. MEMBRANE CHLOROPLAST only found in _________ cells; contain _____________ used to trap light energy needed for _________________. VACUOLE found in many plant cells; contains water with salts and sugars. Contributes to cell turgor. CELL WALL only found in ___________ cells. Made of cellulose it gives cells strength and rigidity. It is freely (or totally) permeable. * = only found in animal cells 2 Organelles Working Together The organelles inside a cell do not work in isolation. Instead they work together in order to synthesise, transport, package and eventually secrete ____________ from the cell. Handout 3 gives some examples of such proteins. The nucleus contains the _____________ information (or recipe) needed to make ______________. Proteins are then synthesised in the _____________. They are then _______________ by either the rough or smooth _________________ ________________. Then, they are packaged and processed ready for secretion in the ________ _____________. Parts of the golgi apparatus nip off to form secretory _____________ which contain the finished _____________ (e.g. an ___________ like amylase or a _____________ like ADH). The secretory vesicle fuses with the cell _____________ and the ____________ is secreted from the cell. The diagram below shows how all of these organelles work together. Note how sacs which detach from the ER join up to form the _________ ____________. Processing and Secretion of a Protein. The sequence in which these organelles become involved in the production of a protein for secretion is: nucleus → ____________ → → _________ _____________ ______________ → vesicles 3 _______________ What can organelles tell us? We can work out what the function of a cell is by simply looking at the organelles (and the number of them) in a cell. For example: 1. __________ cells and sperm cells contain many _______________ compared to other cells e.g. skin. This is because both of these types of cells require a lot of energy. 2. Palisade mesophyll cells in the leaves of plants contain many, many ______________. This is because this is the part of the leaf where most __________________ occurs. 3. Cells is our salivary glands, pancreas, liver and pituitary gland, are abundant in ________ ______________. This is because all of these types of cells are involved in secreting proteins. 4. White blood cells contain a lot of _____________. The powerful digestive enzymes in these organelles help these cells to destroy bacteria or other foreign bodies (e.g. viruses). 4 Cell Structure Related to Function It is important in Biology to remember that the structure (i.e. how a cell is built) of a cell is closely related to it’s function. Here are some examples. CELL TYPE FUNCTION HOW STRUCTURE IS RELATED TO FUNCTION To surround and then destroy bacteria and other foreign bodies (e.g. viruses). Able to change their _________ which then allows them to surround bacteria. Contain many _____________ which then destroy bacteria. Present a __________ _____________ _______ in contact with the soil and so increases the amount of water it is able to absorb. WHITE BLOOD CELL To absorbs __________ and minerals from the soil. ROOT HAIR CELL To fertilise an egg cell. Has a ________ that it uses to swim towards the egg. It’s tail is packed full of _______________ which supply it with loads of energy needed for it’s journey. These cells (found in our windpipe) are used to sweep mucus away from the lungs and into the nose or mouth. Covered in tiny hairs called ________. The cilia beat upwards, wafting the mucus towards the mouth or nose and away from the lungs. To transport oxygen around the body. These cells are bi-concave (like a refresher). This gives them a large ____________ _______. They also contain _______________ which has an affinity (loves) oxygen. SPERM CELL EPITHELIAL CELL RED BLOOD CELL 5 ABSORPTION AND SECRETION There are three main ways in which substances can be absorbed into or secreted from a cell. These are: 1. ______________ 2. ______________ 3. ___________ _______________ DIFFUSION Diffusion is the _________________ of molecules from a region of _______ concentration to a region of _____ concentration. The difference that exists between the two regions is called the concentration _______________. Diffusion is important to cells because it is the process by which molecules either ________ or _______ a cell. Substances which enter a cell by diffusion include __________ and dissolved food like ___________. Substances that leave a cell by diffusion include ____________ ______________. Water can either enter or leave a cell. OXYGEN CARBON DIOXIDE DISSOLVED FOOD (e.g. glucose) OSMOSIS Osmosis is described as a “special case of diffusion”. Osmosis is defined as: It is the plasma cell membrane of a cell that acts as a ______________ permeable membrane. THE MOVEMENT OF WATER MOLECULES FROM A REGION OF HIGH CONCETRATION TO A REGION OF LOW CONCENTRATION THROUGH A SELECTIVELY PERMEABLE MEMBRANE. 6 THE EFFECT OF OSMOSIS ON CELLS In order to understand how osmosis affects plant or animal cells, we must be familiar with the following terms which describe the concentration of water molecules in a solution surrounding cells compared to the concentration of water molecules inside the cells. TYPE OF SOLUTION So: CONCENTRATION OF WATER MOLECULES has a higher concentration of water molecules than the cells has the same concentration of water molecules than the cells has a lower concentration of water molecules than the cells if cells are placed in a hypotonic solution, water will _________ them by __________ , and the cells will therefore _______ weight. if cells are placed in a hypertonic solution, water will _________ them by __________ , and the cells will therefore _______ weight. if cells are placed in an isotonic solution, the same number of water molecules will enter as leave the cells, therefore their weight will stay the same i.e. they will neither _______ nor ________ weight. In the experiment drawn below, the bag would ______ weight as water would ________ the bag by __________. 7 THE EFFECT OF OSMOSIS ON ANIMAL CELLS The diagram below shows the effect of osmosis on an animal cell when it is placed in a hypotonic, isotonic or hypertonic solution. THE EFFECT OF OSMOSIS ON PLANT CELLS The diagram below shows the effect of osmosis on a plant cell when it is placed in a hypotonic, isotonic or hypertonic solution. 8 Animal cells will _________ when they are placed in a hypotonic solution, but plant cells will not. This is because plant cells have a ______ _______ which prevents this from happening. Summary Effect of solution on cells CELL TYPE HYPOTONIC ISOTONIC HYPERTONIC ANIMAL burst no change shrink PLANT become TURGID no change become PLASMOLYSED (or FLACCID) N.B. Neither diffusion or osmosis require ___________ (ATP) therefore they are not dependent on the process of ____________ _________________. In other words they are not active but ____________ processes. ACTIVE TRANSPORT Apart from diffusion and osmosis, another way in which molecules can enter or leave a cell is by __________ transport. During active transport, molecules move from a region of ______ concentration to a region of _______ concentration. Molecules therefore move against a __________________ ________________. Since molecules are moving against a concentration gradient, active transport requires ____________ (ATP). Since active transport requires energy (ATP), this means that it is dependent on the process of __________ _______________ which of course produces energy (ATP). 9 Active transport differs from diffusion in two ways: 1. During diffusion, molecules move from a high concentration to a low concentration; during active transport, it is the opposite (low to high). 2. Diffusion is not an energy-requiring process; active transport is in other words, diffusion is a _____________ process, active transport is an __________ process. The diagram below shows how substances can either be actively transported into or out of a cell across the plasma cell membrane. Note that it is a _____________ molecule present in the membrane structure that is responsible for actively transporting molecules into or out of a cell. 10 Selective uptake of ions by active transport The table below shows that an aquatic plant is able to select and accumulate certain ions via active transport. The data confirms that: the ions are being absorbed by active transport because there is a __________ concentration of potassium (K) outside than inside it is selective because more ______________ than ______________ ions are being absorbed. Conditions required for active transport Since active transport requires ______________ (supplied by ATP), this means that it is dependent on the process of aerobic respiration (which of course produces ATP). If respiration ceases, so will active transport. Factors that affect aerobic respiration, will therefore affect respiration. Such factors include: temperature (affects the ____________ that control respiration) _____________ concentration (as the O2 concentration decreases, so will the rate of respiration – less ATP = less active transport) _____________ availability (since it is the main respiratory substrate) All of the above factors indirectly affect active transport because they directly affect ____________ _______________. The graph at the top of the next page shows how temperature can affect the rate at which nitrate ions are active absorbed. 11 Remember, temperature affect the ______________ that control aerobic respiration. Description: As temperature increases to 30oC, the rate of active transport _____________. Above 30 oC, the rate of active transport ________________. Explanation: As the temperature increases up to 30oC , the enzyme controlling respiration will be becoming more active. More respiration = more ATP for active transport. As the temperature goes above 30 oC, the rate of active transport decreases, because the enzymes are being ________________. The graph below shows how oxygen concentration can affect the rate at which nitrate ions are active absorbed. Remember, oxygen concentration affects rate of aerobic respiration. Description: As oxygen concentration increases to 30%, the rate of active transport _____________. Above 30%, the rate of active transport remains ________________. Explanation: More oxygen = more aerobic respiration more respiration = more ATP/energy. active transport requires ATP/energy 12 Cell membranes Cells are enclosed by a membrane called the _________ cell membrane. In addition, many organelles such as mitochondria, endoplasmic reticulum, golgi apparatus, chloroplasts and the nucleus itself are bound by one or two plasma membranes. The plasma membrane is composed of ___________ and ________________ (fat) molecules. The phospholipids form two layers (called a bi-layer) which can move – making the membrane ___________ (flexible). The protein molecules are dispersed throughout the phospholipids forming a _________ pattern – and they too can move freely within the membrane. For this reason, the membrane is described as ________ - _________. Because membranes are composed of ____________ and __________________, they are easily damaged by _______ temperatures, acids and by fat solvents such as ___________. This can be demonstrated by using plant cells which have a coloured sap (e.g. beetroot). When the membrane of these cells is damaged, the coloured sap leaks out into the surrounding fluid. The protein molecules in the membrane have many functions. These include: ___________ which speed up many reactions e.g. protein synthesis. receptor sites for hormones which then influence the activity of the cell. proteins which actively transport materials across the membrane using ATP as a source of __________. proteins which form _______ (or channels) in the membrane through which molecules can enter or leave the cell by ______________ . 13 The membrane is described as being ______________ permeable, because it will only allow some molecules to pass through and this depends on the _______ of the molecule. Experiment to demonstrate the structure of the plasma membrane. We can do an experiment to demonstrate the structure of the plasma membrane. To do this we would use a plant whose cells contain a coloured sap (in their vacuoles). An example of a plant we could use is _____________, which has red/purple sap. If a piece of beetroot “bleeds” this means that the plasma membranes surrounding the cells themselves and the plasma membranes surrounding their vacuoles have been damaged. This damage is irreversible. The investigation as shown below was set up for one hour: Results 14 TEST TUBE DOES BLEEDING OCCUR? TREATMENT A B C D Explanation A Bleeding __________ occur because water does not affect the structure of the plasma membrane. B Bleeding __________ occur because the acid denatures the _____________ molecules in the membrane which causes it to break down. This then allows the sap to leak out of the cell and the surrounding liquid turns red. C Bleeding __________ occur because the alcohol dissolves the __________________ molecules in the membrane which causes it to break down. This then allows the sap to leak out of the cell and the surrounding liquid turns red. D Bleeding __________ occur because the high _________________ denatures the _____________ molecules in the membrane which causes it to break down. This then allows the sap to leak out of the cell and the surrounding liquid turns red. NOTE Neither water or a salt or sugar solution will damage the plasma membrane - as long as they are at a ______ temperature. 15 The Cell Wall The cell wall is the non-living outer boundary of _________ cells. It is mainly composed of a structural carbohydrate called _____________, which is laid down in ___________ as shown in the diagram below. This is what gives a plant cell it’s shape and keeps it fairly rigid. Unlike the membrane, the cell wall is totally (or freely) _____________ - in other words it does not prevent anything from leaving or entering a cell. It is the cell wall which prevents a plant cell from bursting when it is placed in a ________________ solution. 16 Aerobic Respiration The whole point of aerobic respiration is to release the ____________ energy in the food we eat. ___________ is needed to release this energy. The equation below summarises aerobic respiration. FOOD + (e.g. glucose) OXYGEN → ___________ ____________ + _____________ + ENERGY Most of this energy ends up being ___________ in our cells in a compound called ATP ( ______________ _______________), but some of this energy ends up being lost as ________. ATP ATP is a compound which is made from adenosine and three phosphates (Pi). It is made by joining a molecule of ADP (adenosine ____________) with a single phosphate (Pi) molecule. To join these two molecules together requires energy. This energy comes from our ________ . So, the chemical energy in food, ends up being stored in _____. The production of ATP from ____ and ____ is called PHOSPHORYLATION, which can be summarised by the following equation: ADP + Pi ATP When cells require an _______________ source of energy, the “high energy” bond between the last two _____________ molecules breaks and energy is released for cellular processes that include: muscle ________________ _________ transport 17 mitosis/meiosis (to separate the chromosomes) DNA ________________ protein synthesis phagocytosis (to allow membrane to move and engulf bacteria). Cells need a constant supply of ATP, so it is re-synthesised as quickly as it is broken down. All living things respire - this is often referred to as “chemical” or “tissue” respiration (to distinguish it from the common use of the word “respiration” to mean “breathing”). ATP acts as a link between aerobic respiration and other cell processes which require ATP to drive them e.g. protein synthesis as shown in the diagram below. Food + oxygen CO2 + H 2O Amino acids ADP + Pi Proteins OXIDATION AND REDUCTION Oxidation is the _______ of _____________ atoms, and when this happens, energy is _______________. (Remember “OIL” = oxidation is loss). Reduction is the _______ of _____________ atoms, and when this happens, energy is _______________. (Remember “RIG” = reduction is gain). [Respiration is often referred to as a process involving the “oxidation of foodstuffs”. This is because during respiration, hydrogen atoms are removed from the respiratory substrate (e.g. glucose) and used to produce energy (ATP)]. 18 INTERNAL STRUCTURE OF A MITOCHONDRION The diagram below shows the internal structure of a mitochondrion. This cell organelle is the site of ___________ ________________. 1μm (micrometer or micron) = 1000th of a millimetre. A typical cell contains around 1000 mitochondria, but those cells which have a high __________ requirement, e.g. _________ and ___________ cells have many more. The central matrix contains the enzymes that are involved in the _________ _________; the cristae contains the enzymes that are involved in the _____________ system (also called the hydrogen transfer system). THE CHEMISTRY OF RESPIRATION Respiration can be divided into 3 separate but continuous processes: 1. ________________ 2. The ________ Cycle (sometimes called the citric acid cycle). 3. The ________________ system (sometimes called the _______________ transfer system). 19 GLYCOLYSIS Glycolysis takes place in the ___________________ of a cell. No ____________ is needed for glycolysis to occur. Glycolysis involves the splitting of ___________ (6C) into two molecules of _____________ acid (2 X 3C). This produces ________ molecules of ATP, but since _______ molecules of ATP are needed to split the glucose in the first place, the NET GAIN is 2 molecules of ATP. So, whether oxygen is available or not, the end products of glycolysis are: 2 molecules of _____________ _______ a net gain of _______ molecules of ATP In order for glycolysis to occur in the first place, both ______ and ____________ must be available to the cell. If oxygen is not available to the cell - in other words if conditions are ______________, then the anaerobic phase of respiration occurs. The pyruvic acid remains in the ________________ of the cell and is then respired anaerobically as follows: During anaerobic respiration, the only source of ATP comes from _______________. Note that plant cells include fungus (like yeast) and animal cells include bacteria. THE KREB’S CYCLE If oxygen is available, the pyruvic acid diffuses from the _______________ into the central matrix of the mitochondrion. It is here that the Kreb’s Cycle 20 occurs. Unlike glycolysis, ____________ is needed for this stage. The diagram below outlines the Kreb’s Cycle. Each molecule of pyruvic acid loses a ___________ atom (as carbon dioxide). This produces a 2-carbon compound called __________. (This joins with an enzyme called CoA, forming a compound called Acetyl CoA). This newly formed 2C compound then joins with a 4C compound already present in the central matrix of the ____________________. This produces a ___carbon compound called _________ acid. Each molecule of citric acid then loses a carbon atom (as __________ _____________) to produce a 5carbon compound which in turn loses a carbon atom to produce a _____ carbon compound. It is this 4C compound that joins with the 2C acetyl compound and the cycle begins again. _____ molecules of ATP are produced as a direct result of the Kreb’s cycle. 21 THE CYTOCHROME SYSTEM The cytochrome system is sometimes called the _______________ transfer system - this is because it consists of a series of _____________ carriers (called cytochromes). The cytochrome system occurs in the ___________ of the mitochondria. Oxygen is needed. 34 molecules of ATP are produced from one molecule of ____________ during the cytochrome system. During glycolysis and the Kreb’s cycle, carbon and _________________ atoms are removed from the original ___________ molecule. After hydrogen atoms have been removed, they are picked up by a single hydrogen carrier molecule called ______ , which is reduced to NADH2. NAD is used to transfer the hydrogen atoms from the cytoplasm or the central matrix to the ___________ of the mitochondria. On arriving at the cristae, they are released by NAD and then pass through the cytochrome system. ADP NADH2 CARRIER 1 Pi ADP CARRIER 2 Pi ADP CARRIER 3 Pi CARRIER 4 OXYGEN WATER NAD ATP ATP ATP As the hydrogen atoms pass through the cytochrome system, the most important compound that is produced is _____. ___________ function is to act as the final hydrogen acceptor. When oxygen combines with hydrogen at the end of the cytochrome system, __________ is produced. Apart from glucose (a carbohydrate) other respiratory substrates include fats, fatty acids, glycerol and proteins. Cell proteins (like the body’s muscles e.g. the heart) are only ever used as a respiratory substrate in times of acute ____________________. Handout 4 on page 23 shows a more detailed outline of chemical respiration. 22 23 ENZYME INHIBITORS An enzyme inhibitor is a substance that can either _____________ an enzyme or block it’s active site. This means that the enzyme can no longer join with it’s ______________. The diagram below illustrates how an inhibitor can stop an enzyme’s activity by blocking it’s active site. (DO NOT ATTEMPT TO MEMORISE NAMES!) Because the inhibitor mimics the shape of the enzyme’s substrate, the inhibitor can block the active site by fitting into it. This results in the enzyme becoming inactive i.e. it can no longer function. Inhibitors can therefore a affect a metabolic pathway as shown in the diagram below. compound A enzyme 1 compound B enzyme 2 compound C If an enzyme inhibitor affects enzyme 2, then this would result in compound B _______________ and compound C ________________. Although the inhibitor affects enzyme 2, it does not affect the activity of enzyme 1. 24 DNA AND PROTEIN SYNTHESIS THE STRUCTURE OF DNA DNA = ________________________ ________ Chromosomes are found in the _____________ of a cell. Each chromosome consists of many _________. These genes contain the _____________ information which determines an organism’s characteristics. Genes are made of _______. DNA is an example of a _______________ acid. Nucleic acids like DNA are made from millions of repeating units called _______________. The structure of a single nucleotide is shown below. A SINGLE NUCLEOTIDE OF DNA There are _______ different bases in DNA. Their names are: 1. ______________ (A) 2. _______________ (T) 3. _______________ (G) 4. _______________ (C) 25 It is only the ________ which makes one nucleotide different from another. So, since there are only four different bases, there can only be four different _________________ in DNA . DNA is a double stranded structure. It’s two strands are twisted like a spiral staircase. This structure is called a ___________ _________. The phosphate and _______________ sugar molecules form it’s backbone. The bases pair off and are held together by weak ________________ bonds. Adenine always pairs with ______________ and vice versa, and guanine always pairs with _____________ and vice versa. For this reason they are referred to as __________________ base pairs. The diagram below shows the DNA double helix. DNA REPLICATION A huge coiled and folded strand of DNA makes up one chromosome. Chromosomes are found in the _____________ of cells. Before cells divide, an exact copy of the DNA must be made. This process is called DNA ________________ and it is important because it ensures that each new cell 26 receives the correct amount of ____________ information for it to grow and ___________ properly. DNA replication occurs at the very end of mitosis just before the _______________ divides the parent cell into two genetically identical daughter cells In order for DNA replication to occur, the following four things must be present in the cell. 1. 2. 3. 4. many of the four free _________________ in DNA the original DNA molecule (which acts as a template to copy) the correct ______________ _____ which provides the energy needed for this process. Stages of DNA Replication DNA replication occurs as follows: 1. 2. 3. 4. 5. 6. 7. the DNA double helix __________ from one end only the weak _______________ bond between the complementary bases __________. the two strand of DNA ____________ exposing their bases free ________________ find their complementary nucleotide on the open DNA strands weak hydrogen bond form between the _________ bonds then form between the phosphate of one nucleotide to the _________________ of the next each new DNA molecule coils to form a double __________. HANDOUT 5 on page 28 shows the process of DNA replication. One half (i.e. one of the strands) of each new DNA double helix comes from the original DNA molecule. The other half has been made from free DNA ________________ joining together. Since 50% or half of the original DNA molecule ends up in the new DNA molecule, DNA replication is referred to as being semi -_____________________. 27 28 DNA Content of Cells Not all cells in our body contain the same amount of DNA. Since ____________ (sex cells) eggs and __________ in animals; pollen grains and __________ in plants, contain ________ the number of chromosomes as somatic (non-sex) cells, they contain half the amount of DNA as somatic cells. Mature red blood cells do not have a nucleus therefore they have no chromosomes and in turn will have no DNA. The table below summarises the DNA content of some human cells. CELL TYPE liver sperm UNITS OF DNA 6.0 mature red blood cell muscle gamete mother cell egg (ovum) young red blood cell skin cell (just before cell completely divides) 12.0 If a DNA molecule contains 1000 bases, and 200 of these bases are adenine, then _______ bases will be thymine. This leaves 600 bases which will be divided equally between cytosine and ____________. So, in this particular molecule of DNA 300 bases will cytosine and 300 will be guanine. The Genetic Code The sequence of _______ in our DNA contains a “code” or recipe for making cell _____________ e.g. enzymes, hormones etc. Proteins are made from single units called ________ ________ . There are approximately ____ different amino acids which need to be “coded” for. If only one base coded for 1 amino acid, we could only code for ____ different amino acids (1 per base) leaving _____ amino acids that will not be “coded” for. If two bases “coded” for 1 amino acid, _____ different amino acids could be “coded” for, leaving ____ amino acids that will not be “coded” for. If three bases “code” for 1 amino acid, _____ different amino acids could be “coded” for which is more than enough. 29 So, THREE BASES CODE FOR 1 AMINO ACID - this makes the genetic code a triplet code. Each triplet of _________ , only codes for one specific amino acid and no other. Each amino acid however may have more than one triplet of bases “coding” for it. For example the triplet CAA will only code for the amino acid called __________ and only ___________ , but the triplet CAC also codes for valine (and only valine). The diagram below shows the genetic code. DO NOT ATTEMPT TO LEARN ANY OF THIS CODE . 30 The Role of RNA in Protein Synthesis RNA stands for _________________ _________ and as the name suggests, like DNA it is also a ______________ acid. There are two types of RNA. These are: 1. 2. mRNA tRNA (messenger RNA) (transfer RNA) The Structure of mRNA Like DNA, mRNA is made from many single ________________. However the sugar in mRNA is called __________. In mRNA, the base ____________ does not exist - it is replaced by a base called ____________. mRNA is single-stranded unlike DNA which is doublestranded. The table below summarises the structural differences between DNA and mRNA. DNA mRNA SUGAR BASES STRANDS DNA is only found in the _____________ of a cell, whereas mRNA is found in both the ______________ and the ________________ of a cell. TRANSCRIPTION (of DNA into mRNA) During the process of transcription, the genetic code in our DNA is transferred to mRNA. This needs to happen because this code is going to provide the recipe for making a ____________ and proteins are synthesised in the _____________. This is the only way that the genetic code (in our DNA) can get to the ribosomes. During transcription, a section of the DNA molecule unwinds and unzips. (This section represents one gene) This allows mRNA complementary bases (A-U; TA; C-G and G-C) to bind temporarily (via the formation of weak ______________ bonds) with the exposed bases on this section of one of the DNA strands. 31 Once a single complementary strand of mRNA has been synthesised, the weak hydrogen bonds between the bases _________. This then allows the mRNA strand to peels off. It then leaves the _____________ through a pore in the ______________ membrane and makes its way to a ______________ . Ribosomes are found free in the cytoplasm or attached to the rough ________________ ________________. A diagram showing the transcription of DNA into mRNA is shown below. The mRNA now holds the genetic code which provides the recipe to make a ______________ e.g. amylase, ADH etc. Three bases in a row (one after the other) on the mRNA strand “code” for one specific _________ ________. A triplet of three bases on an mRNA strand is called a _________. If there are 549 nucleotides (or bases) on a single strand of mRNA, it will provide the recipe to synthesise a protein molecule that consists ____ amino acids (i.e. 549 ÷ 3 = ____ ). 32 The diagram below shows the 64 mRNA codons and the amino acids that they “code” for. Amino acids need to be carried to the ______________ where they will be joined together to form a protein. tRNA molecules act as the amino acid carriers. The Structure of Transfer RNA (tRNA) tRNA is not a strand – it is a molecule on its own. One such tRNA molecule is drawn below. The exposed triplet of bases on a tRNA molecule is called an _______________ of which there are _____ different ones. Each anticodon “codes” for one specific amino acid. It is this _____________ 33 that will determine the amino acid that the tRNA molecule will carry to the ribosome. The above tRNA molecule will carry the amino acid called ___________. Functions of tRNA to transfer amino acids to the _______________ to allow codon/ _____________ links to form to bring amino acids side by side (to allow _____________ bonds to form between them TRANSLATION Translation refers to the process when mRNA is translated into a protein. Translation therefore takes place at the _______________, because these are the organelles that are responsible for protein synthesis. It is the sequence of the _________ in mRNA that will determine the sequence of the _______ _______ in the protein that will be synthesised. It is this sequence of amino acids that makes one protein different from another. Once the protein has been synthesised, it is then _________________ by the rough endoplasmic reticulum to the _________ ___________ where it is packaged and processed ready for secretion. HANDOUT 6 which follows shows the process of translation. MUTATIONS If a gene mutates, this will alter the sequence of the _______ in the DNA that the gene is made from. This in turn, will alter the sequence of the amino acids of the _____________ that this particular gene holds the recipe to make. As a result a different protein will be synthesised. Depending on the circumstances this may give the organism and advantage or a disadvantage -see unit 2. 34 Proteins Proteins are organic* compounds composed of atoms of carbon, ______________ , oxygen and ____________. They are large, complex, three-dimensional molecules. There are thousands of different proteins. Each protein molecule is composed of a large number of building blocks called _______ _______ which are linked together by __________ bonds. Proteins have many important roles to play in both plants and animals. These roles include: all ___________ are proteins and they control metabolic pathways e.g. respiration and photosynthesis many hormones are proteins ______________ which protect our body from disease and infection, are proteins the plasma membrane consists of ________________ and protein molecules – see page 13. muscle tissue is composed mainly of protein our red blood cells contain ________________ which is the protein that transports oxygen around the body Proteins can be classified as FIBROUS or GLOBULAR. A) FIBROUS PROTEINS These proteins are ______________ in water. They are structural proteins that can resist being pulled or stretched and so they are quite strong. Fibrous proteins are arranged in long parallel strands as shown in the diagram below. B) GLOBULAR PROTEINS These proteins are not completely soluble in water. They are usually involved in some type of metabolic reaction. Globular proteins are roughly spherical in shape as shown in the diagram below. Examples of each type of protein are given in the table below. 35 *organic compounds contain the element carbon FIBROUS PROTEINS collagen (found in bone, tendons and ligaments) actin and myosin (found in muscles) keratin (found in hair, nails and skin) elastin (another protein found in ligaments) GLOBULAR PROTEINS enzymes (e.g. amylase which breaks down starch to maltose) hormones (e.g. ADH which controls the level of water in the blood) antibodies (e.g. immunoglobulin which defends the body against infection) plasma membrane proteins (e.g. like the ones in the cell membrane that are involved in active transport). Remember, it is the sequence (order) of amino acids in a protein that makes one protein molecule different from another (not only in function but in shape), and it is the sequence of ______ in DNA that will determine what the sequence of amino acids will be. The shape of a protein molecule is very important to its function, especially enzymes that are specific to their ___________ and antibodies that are produced in response to specific _____________. 36 CELLULAR DEFENCE IN PLANTS AND ANIMALS VIRUSES Viruses are extremely small, and are capable of causing disease. Antibiotics don’t affect them, and we therefore have to rely on our own body’s defence systems to deal with them. Viruses also attack other animals, plants and even bacteria. STRUCTURE OF A VIRUS All viruses contain either DNA or ________ that is surrounded by a ____________ coat like the one shown in the diagram below. VIRAL REPLICATION As the name suggests, viruses are able to make __________ copies of themselves. In doing so, they interfere with the host cells biochemistry, and use compounds supplied by the host cell to replicate. In doing so, they destroy the host cell. In order for viruses to replicate, four things must be supplied by the host cell for replication to be possible. The four things are: ______ which supplies the energy needed for replication free __________________ (to make copies of it’s DNA or RNA) _______ _________ to make the protein coat that surrounds the virus ___________ involved in replication and protein synthesis 37 STAGES OF VIRAL REPLICATION Using ATP, free nucleotides, amino acids and enzymes supplied by the host cell, viruses replicate. The diagram below shows the stages of viral replication. ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ 38 ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ ____________________________________________________ CELLULAR DEFENCE MECHANISMS IN ANIMALS ___________ blood cells help us to defend our body against disease and infection. There are two types of white blood cells: 1. _________________ 2. _________________ PHAGOCYTES These types of white blood cells work by ______________ foreign particles e.g. bacteria. This is a process called ___________________ which is shown in the diagram below. Once ingested, the bacteria are destroyed by the 39 powerful digestive enzymes contained in special cell organelles inside the phagocytes called ______________. Phagocytic white blood cells like the one drawn above, is a good example of a cells whose structure is related to it’s function: 1. phagocytes are able to change their _________, and this allows them to surround and engulf bacteria. 2. phagocytes contain many _____________. The powerful digestive ___________ inside these organelles destroy the bacteria and render them harmless. LYMPHOCYTES These types of white blood cells produce globular proteins called ________________ in response to foreign proteins called ______________. (Viruses and bacteria have a distinctive protein coat which is individual to them). Antibodies are said to be _____________ because different antibodies are produced in response to different antigens. Only one specific type of ____________ can attack a specific type of ____________, so we have thousands of different _______________ , each one designed to produce a ___________ antibody. 40 REJECTION OF TRANSPLANTED TISSUES OR ORGANS When a tissue or an organ (e.g. a kidney) is transplanted from one person to another there is the possibility that it may be _______________ (unless the donor is an identical twin). This is because the transplant patient’s immune system regards the transplanted tissue or organ as a collection of foreign _____________, and attempts to destroy it - this is known as tissue rejection. Phagocytes and lymphocytes treat the transplanted tissue or organ like any other antigen, but rejection can be minimised in two ways: 1. By matching a donor who is ________________ as similar as possible to the recipient. This is why family members make good donors. 2. Using special drugs called ______________________ drugs which unfortunately weaken the body’s immune system, and these drugs must be taken for the rest of their life. An advantage of taking these drugs is that the transplanted tissue or organ is not “attacked” by our own phagocytes and lymphocytes, but a disadvantage is that the transplant patient is open to attack from other pathogens (disease-causing microorganisms). CELLULAR DEFENCE MECHANISMS IN PLANTS Plants too can be attacked by pathogens, but they don’t have an immune system like us and so don’t produce phagocytes or lymphocytes (and therefore do not produce _______________ ). Many plants defend themselves against infection and disease by producing chemicals that are either _______________ , or are enzyme ______________ . For example: NICOTINE TANNINS RESINS Resins are sticky chemicals. They help to prevent the spread of infections by ________________ holes in the bark of trees for example. 41 PHOTOSYNTHESIS During photosynthesis, the energy change is from ________ to ______________. Photosynthesis can be summarised by the following equation. light energy CARBON DIOXIDE + WATER FOOD chlorophyll + OXYGEN (e.g. glucose) So, photosynthesis is the process by which plants with chlorophyll manufacture carbohydrates (e.g. glucose) from carbon dioxide and __________ using _______ energy from the Sun. Oxygen is released as a by-product. The Fate of Light Energy Three things can happen to the light that strikes a leaf. It can either be absorbed, ______________ or transmitted, as shown in the diagram below. B _______________ (12%) A ______________ (5%) C ______________ (83%) Visible light is made from 7 different colours. Each colour is determined by it’s wavelength. In Biology, we refer to these colours of light as the _________________ spectrum (R O Y G B I V). Red 700 Orange Yellow Green 600 Blue Indigo 500 wavelength 42 Violet 400 Absorption of Light Light energy is absorbed by “chlorophyll” which is found in the ________________. Not all plant cells have chloroplasts e.g. the cells in the roots of plants. This is because these cells never see the light of day! The diagram below shows the internal structure of a chloroplast. So, chloroplasts are the organelles found in plants cells where the process of ___________________ occurs. The “chlorophyll” whose function is to absorb light energy is found in the __________ (pleural = grana) of the chloroplasts. The synthesis of glucose takes place in the __________ of the chloroplasts, during the Calvin Cycle (see page ) The Role of Chlorophyll in Photosynthesis Apart from chlorophyll (a and b), plants also contain other ________absorbing pigments. These are referred to as ____________ pigments. They can be extracted from leaves and then separated using a simple technique called ________________. 43 Chromatography Leaves are firstly ground up using a mortar and pestle, and _______ which helps to rupture the cells. __________ (or propanone) is the solvent that is used to extract these pigments from the leaves. The resulting green liquid is then spotted repeatedly onto chromatography paper. When a _________________ green spot has been produced, the paper is dipped in another solvent and left for a few minutes. Chromatography works on the basis of _______________. Some of the lightabsorbing pigments are more soluble than others. The ______ soluble the pigment, the further up the paper it travels. The pigments always appear in the same order as shown in the diagram below. MOST SOLUBLE CAROTENE ( ___________ ) XANTHOPHYLL ( ____________ ) CHLOROPHYLL a ( _____________ ) CHLOROPHYLL b ( _____________ ) LEAST SOLUBLE Function of Having Different Pigments The advantage to the plant of having more than one light-absorbing pigment is that it EXTENDS THE ___________ OF WAVELENGTHS OF LIGHT THAT THE PLANT CAN ___________ FOR PHOTOSYNTHESIS. This additional ________ energy absorbed by the accessory pigments is passed on from chlorophyll b, xanthophyll and _____________ to chlorophyll a molecules which carry out photosynthesis. 44 Absorption Spectrum and Action Spectrum The wavelengths of light absorbed by a pigment can be displayed as a graph called the ______________ ___________. Because “chlorophyll” absorbs mainly _____ and _____ wavelengths of light and ________ green wavelengths of light, “chlorophyll” appears green to our eyes. The absorption spectrum for chlorophyll a is shown in the graph below. If an absorption spectrum graph peaks at the green wavelength of light, this is not due to absorption by chlorophyll a or b (which reflects green), but will in fact be due to light being absorbed by either ____________ or _______________. If a plant lacks magnesium, it will not be able to absorb as much light as a plant that is not deficient in this element. This is because magnesium is needed by plants to make _______________ . The less chlorophyll a plants make, the less light they are able to absorb. Is absorbed light actually used by the plant for photosynthesis? This can be checked by growing plants in different _________ of light using different coloured light bulbs and noting how well they grow. If the results of such an experiment are plotted, a graph called an _________ _________ is obtained. The action spectrum for a plant is drawn on the following page. 45 ACTION SPECTURM GRAPH (measures the rate of photosynthesis at each wavelength of light) The rate of photosynthesis can be measured as: volume of ____ absorbed, or volume of ____ released, or increase in dry mass } all measured over a period of time If an absorption spectrum graph and an action spectrum graph follow a similar pattern, then we could conclude that the light being absorbed by the pigment is actually being used for _________________. If the graphs do not follow a similar pattern, then it is likely that other pigments are involved in absorbing different wavelengths of light. 46 THE CHEMISTRY OF PHOTOSYNTHESIS Photosynthesis can be divided into two stages. The products of the first stage however, are required for the second stage to occur. These two stages are: Stage 1 the ________-dependent stage (photolysis and photophosphorylation) Stage 2 the __________ Cycle or the ___________ fixation stage Stage 1 This stage (which requires light energy) takes place in the __________ of the chloroplast. The light energy which is absorbed by ______________ can either be used to: 1. split __________ molecules into ______________ atoms and oxygen gas which is then released by the plant as a by-product. This process of using light energy to split water molecules is called PHOTOLYSIS. The hydrogen atoms are picked up by a “hydrogen carrier” molecule called NADP which is reduced to NADPH2. NADP carries the hydrogen atoms to the _________ of the chloroplast where they are needed for the Calvin Cycle. 2. make ATP. Light energy is used to join _____ and _____ together to make ATP. This process is called PHOTOPHOSPHORYLATION. ATP is also needed for the Calvin Cycle to occur. The two processes of the light-dependent stage ( ________________ and _______________________ ) are outlined in the diagram on the next page. 47 THE LIGHT-DEPENDENT STAGE (occurs in the ___________ of a chloroplast) light energy NADPH2 NADP hydrogen atoms CHLOROPHYLL energy energy WATER (H20) ADP + Pi ATP oxygen gas (released as a by-product) When light energy splits a molecule of water into ______________ atoms and __________ gas, the hydrogen atoms are picked up and carried by NADP (as NADPH2) from the ___________ to the stroma of the chloroplast. The source of the oxygen released during photosynthesis is therefore ____________. BOTH NADPH2 AND ATP ARE NEEDED FOR STAGE 2. WITHOUT THEM, THE CALVIN CYCLE CANNOT OCCUR. 48 Stage 2 The second stage of photosynthesis (i.e. the Calvin Cycle), does not require light energy. In simple terms, during this stage, carbon dioxide is ______________ by the hydrogen atoms which have come from __________ to make carbohydrates (CHO’s) e.g. ____________. The Calvin Cycle is a series of metabolic pathways, each controlled by a different ____________ . The Calvin Cycle takes place in the ___________ of the chloroplast. 49 Carbon dioxide is picked up by the carbon dioxide acceptor molecule called _____________ __________________ (RuBP). When they combine, glycerate phosphate (GP) is produced. GP is then reduced using hydrogen atoms supplied by _________. This requires energy which comes from ______. The end product is glucose. The glucose is broken down by the plant during respiration to produce ATP. The plant however, might join some of these glucose molecules together to form either: 1. 2. __________ for storage or _____________ which is the structural carbohydrate that plant _______ ________ are composed of. Limiting Factors Three factors can limit the rate at which a plant can photosynthesise. The three limiting factors are: 1. _________________ (which affect enzyme activity) 2. light _______________ 3. ___________ ______________ concentration Below shows a typical limiting factors graph that often appears in higher exam. 50 IMPORATANT EXAM INFORMATION You will often be asked to name the compounds in both RESPIRATION and the CALVIN CYCLE. Not only do you need to know their names, you need to know how many CARBON ATOMS these compounds contain. These tables should help you to do this!! RESPIRATION Name of compound Number of carbon atoms (in a single molecule) GLUCOSE PYRUVIC ACID ACETYL (Co A) CITRIC ACID CARBON DIOXIDE (CO2) PHOTOSYNTHESIS Name of compound Number of carbon atoms (in a single molecule) GLYCERATE PHOSPHATE (GP) RIBULOSE BIPHOSPHATE (RuBP) GLUCOSE CARBON DIOXIDE (CO2) 51 Handout 1 52 Handout 2 53