AS Biology, Unit 1 (Module 1) notes (OCR)


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This presentation features key notes and diagrams from the unit 1, module 1 of AS biology. These notes have been mad heavily using OCR text books, however other sources have been used.

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AS Biology, Unit 1 (Module 1) notes (OCR)

  1. 1. AS BIOLOGY REVISION NOTES Unit 1 : Module 1
  2. 2. Microscopy 1.1.1-3 • • I A Magnification = image size ÷ actual size Resolution is the ability to see two distinct points separately. M Light microscope Transmission electron microscope (TEM) Scanning electron microscope (SEM) • • • • • • • • • • • • • • • • • • • Light beam Glass lens focus 1000-1500X magnification Resolution of 200nm Living specimens can be used Can be fixed and stained. Coloured stains used cheap • • • Electron beam Electromagnets 500,000X magnification Resolution of 2nm Dead specimens are used Must be fixed and stained. stained with heavy metals like uranium and lead Expensive Produces 2D images as electron beams pass through the specimen. • • Electron beam Electromagnets 200,000X magnification Resolution of 2-10nm Dead specimens are used Stained with metal particles or salts. Expensive. Produces a 3D image as the electron beam bounces off the specimen.
  3. 3. Cells and cell contents 1.1.4 • • • • • • • • • • • • • • Internal structures of a cell are called organelles. These make up the cell‟s ultrastructure. Within cells there is a network of protein fibres which make up the cell‟s cytoskeleton. Actin filaments can move against each other allowing movement and allows the movement or organelles within the cell. They are like fibres found in muscle cells. Microtubules are cylinders around 25nm in diameter. Made of a protein called tubulin. These can be used to move a microorganism through a liquid or waft a liquid past a cell. There are proteins known as microtubule motors which move organelles along these fibres. They use ATP to drive these movements. Flagella (correctly named Undulipodia in eukaryotes) and cilia are structurally the same. They are hair-like extensions on the surface of cells. Made up of nine microtubules arranged in a circle with two microtubules in the middle. Undulipodia are longer than cilia. Undulipodia attached to sperm cells can move the whole cell. In ciliated epithelial tissue the cilia move substances such as mucus across the surface of the cells. Undulipodia usually occur in ones or twos on a cell. Cilia occur in large numbers on a cell. Some bacteria have flagella, internal structure very different to that of eukaryotic Undulipodia. They are true motors. They are made of a spiral of protein – called flagellin - attached by a hook to a protein base. Using ATP the disc rotates and spins the flagellum. Vesicles are membrane bound sacs found in cells and used to carry different substances. Vacuoles in plant cells maintains stability. Filled with water and solutes. Makes the cell turgid to help support the plant. Plant cell walls are made up of cellulose – a carbohydrate polymer made up of glucose subunits. Held rigid by the pressure of the fluid inside the plant – turgor pressure – and so helps to support the whole plant.
  4. 4. Cell ultrastructure
  5. 5. Internal cell structures 1.1.5 • • Most organelles in a cell are membrane-bound. Which means they have their own membrane to stop them mixing with the rest of the cell contents. Membrane bound organelle‟s structure and function: Structure Function Nucleus: largest organelle. When stained shows darker patches know as chromatin. Surrounded by nuclear envelope – made of two membranes with fluid between. Holes in the nuclear envelope called nuclear pores. Contains a nucleolus. Stores most of the cell‟s genetic material and the instructions for making proteins. Chromatin consists of DNA and proteins. Chromatin condenses into chromosomes. The nucleolus makes RNA and ribosomes. Endoplasmic reticulum (ER): series of flattened membrane-bound sacs called cisternae. Rough ER is studded with ribosomes. Rough ER transports proteins made on the attached ribosomes. Some proteins may be secreted from the cell. Smooth ER is involved in making lipids the cell needs. Golgi apparatus: stack of membrane-bound sacs (which look very much like pitta bread) Responsible for receiving proteins and modifying them. Receives proteins from the ER and may add sugar molecules to them. It packages the modified proteins into vesicles to be transported. Some may go to the surface of the cell so they can be secreted. Mitochondrion: singular is spherical or sausage shaped. Has two membranes separated by a fluid filled space. Inner membrane highly folded to form cisternae. This central part is the matrix. Site where ATP is produced during respiration. Sometimes called universal carrier energy. Drives most of the cellular processes. Chloroplasts: found in only plant cells. Two membranes separated by a fluid filled space. Inner membrane is continuous, with a elaborate network of flattened sacs called thylakoids. A stack of thylakoids is called a granum. Chlorophyll molecules are present on the thylakoids and in the intergranal membranes. Site of photosynthesis in plants. Light energy used to drive reactions, in which carbohydrate molecules are made from carbon dioxide and water. Lysosomes: spherical sac surrounded by a single membrane. Contain powerful digestive enzymes which break down materials.
  6. 6. • Non-membrane-bound organelles Structure Function Ribosomes: tiny organelles that consists of two subunits. Can be found in the cytoplasm or on the rough ER. Site of protein synthesis. They act as an assembly line where coded information (mRNA) from the nucleus is used to assemble proteins from amino acids. Centrioles: small tubes of protein fibres - microtubules – which are present only in animal cells. They are found in a pair next to the nucleus. Used in cell division. From fibres known as spindle which move chromosomes during nuclear division.
  7. 7. Organelles at work 1.1.6 • Division of labour • • • • • • • Instructions to make a hormone are in the DNA in the nucleus. The specific instruction to make a hormone is called a gene for that hormone. A gene is on a chromosome. The nucleus copies the instructions in the DNA into a molecule called mRNA. mRNA leaves via the nuclear pores and attaches to a ribosome (rough ER). The ribosome reads the instructions and uses the codes to assemble the hormone (protein). The assembled protein inside the rough ER is pinched off in a vesicle and transported to the golgi apparatus. The golgi apparatus modifies and packages the protein into a vesicle ready to be transported or secreted out of the cell. • Prokaryotes and eukaryotes • • • • • • • • • Prokaryotic cells are much smaller than eukaryotic cells. they only have one membrane and contain no membrane-bound organelles. Surrounded by a cell wall made from murein. Many have a capsule which provides protection from dehydration and white blood cells. Contain ribosomes – smaller than eukaryotic ribosomes. ATP production happens in specially folded regions of the plasma membrane called mesosomes. DNA is found loose in the cytoplasm in the form of a single loop. Some conation smaller loops of DNA called plasmids. General area containing DNA is called the nucleoid. Many have flagella which function like eukaryotic Undulipodia but have a very different internal structure. • Prokaryotes that help • • • • Food industry uses particular bacteria species e.g. cheese and yoghurt. In mammalian intestines bacteria cells help with vitamin K production and help digest some food. Skin is covered with a „normal flora‟ of bacteria. These help prevent harmful microorganisms getting into the body. Sewage treatment and natural recycling rely on bacterial cells digesting and respiring dead and waste material.
  8. 8. Biological membranes 1.1.7 • • • • • • • • The major roles of membranes include: • Separating cell contents from the outside environment. • Separating cells components from cytoplasm. • Cell recognition and signalling. • Holding components of some metabolic pathways in place • Regulating transport of materials into or out of cells. The basic structure of a membrane consists of a number of arranged phospholipids. They have a hydrophilic phosphate head and two hydrophobic fatty acid tails. When phospholipids are completely surrounded by water they form a phospholipid bilayer where the tails are hidden from the water. The bilayer creates a barrier to molecules and separates the cell contents from the surrounding environment. The membrane is partially permeable. However a simple phospholipid bilayer would be incapable of performing all of the functions of biological membranes. Other components are needed in order to make a functioning biological membrane. The specialisation of cell membranes is a part of the process of differentiation: • Plasma membranes of the cells in a growing shoot have receptors that allow them to detect the molecules that regulate growth. • Muscle cell membranes contain a large number of the channels that allow rapid uptake of glucose to provide energy for muscle contraction. • Internal membranes of chloroplasts contain chlorophyll and other molecules needed for photosynthesis. • Membranes of white blood cells contain special proteins that enable the cells to recognise foreign cells and particles.
  9. 9. The fluid mosaic model 1.1.8 • • • • • • • • Fluid mosaic model shows the components found in a biological membrane. Its main features are a phospholipid bilayer providing simple structure, various protein molecules and some extrinsic proteins partially embedded on either side of the layer. Some phospholipids have carbohydrate chains attached to them. These are called glycolipids Some proteins also have carbohydrate chains attached to them. These are called glycoproteins, and they are used in cell signalling. Cholesterol provides mechanical strength giving the membrane stability. Channel proteins allow the transport of certain substances through the membrane because some substances are too large or to polar to be able top pass though the phospholipid bilayer. Carrier proteins actively transport some substances through the membrane. Also present may be enzymes, coenzymes and receptor sites. Receptor sites allow hormones to bind with the cell so that a cellular response can be carried out. Receptor sites are also important in allowing drugs to bind with the cell and so affect the metabolism. Increasing temperature gives molecules more kinetic energy, increasing movement of phospholipids makes membranes „leaky‟ allowing substances that would not normally do so to enter or leave the cell.
  10. 10. Communication and cell signalling 1.1.9 • In order to detect cell signals, cells must have sensors on the plasma membrane which is capable of • • • • • • • • • • • receiving these signals. These are called receptors. They are usually glycoproteins. Multicellular organisms use hormones to communicate between cells. A cell with a specific receptor for a certain hormone is called a target cell. The receptor on the cell is complementary to the hormone and the hormone binds perfectly with this specific receptor on the target cell. For example insulin. Insulin is released and there are many cells with the complementary receptor for insulin. When insulin binds with the receptor on the target cell it triggers an internal response. This leads to more glucose channels (carrier proteins) being present ion the membrane allowing the cell to take up more glucose from the blood. Medicinal drugs have been developed to have complementary shapes to particular receptor sites. Such drugs block receptors. Beta-blockers are used to prevent heart rate increasing for those whom it may be dangerous. Some drugs mimic natural neurotransmitters that some individuals cannot produce – drugs used to treat schizophrenia work in this way. Viruses enter cells by binding with receptors on the cell surface membrane that normally bind to the host‟s signalling molecules. Some poisons bind with receptors. The toxin extracted from the bacterium clostridium botulinum binds with receptors on muscle fibres causing paralysis. Used in small quantities in surgery under the name BOTOX. It paralysis small muscle to prevent wrinkling of the skin.
  11. 11. Crossing membranes 1.1.10 Passive processes • Simple/passive diffusion: • In a gases or liquids molecules constantly move around due to kinetic energy because they are not held together like they are in a solid. • Molecules move down the concentration gradient. From a high concentration to a low concentration. • Molecules distribute evenly, (this does not mean the movement of the molecules themselves stop), this state of no overall net movement of molecules is called an equilibrium. • The rate of diffusion can be affected by a number of factors: • Temperature: increase in temp. causes an increase in kinetic energy so ROD (rate of diffusion) increases. • Concentration gradient: having more molecules on one side of the membrane increases the concentration gradient and so the ROD increases. • Stirring/moving: stirring or movement of air currents in a gas increases the movement of the molecules and so ROD increases. • Surface area: diffusion across membranes occurs more rapidly if it has greater surface area to diffuse across. Red blood cells are biconcave. Epithelial tissues in the small intestine have small folds called microvilli. Alveoli in the lungs increases surface area. • Distance/thickness: the greater the distance of diffusion the slower the ROD. Thick membranes slow down diffusion. • Size of molecule: smaller molecules or ions diffuse more quickly than larger ones. • Lipid based molecules: • The membrane is made from a phospholipid bilayer. Fat soluble molecules can simply pass through the bilayer down the concentration gradient. Steroid hormones are lipid bases and so diffuse through membranes. • Very small molecules and ions: • Carbon dioxide and oxygen are very small molecules and simply pass through the bilayer. Some water molecules will also pass through the bilayer even though they are polar (charged).
  12. 12. • Facilitated diffusion: • Small charged particles such as sodium, or larger molecules such as glucose, need to be carried across the membrane by a protein. • Channel proteins: these form pores in the membrane, which are often shaped to allow only one type of ion through. Many are also gated meaning they can be opened or closed. • Carrier proteins: these are shaped so that specific molecules e.g. glucose, can fit into them at the cell surface membrane. When the molecule fits the protein changes shape so that the molecule can be carried through to the other side of the membrane. Channel protein Simple/passive diffusion Carrier protein Facilitated diffusion
  13. 13. Crossing membranes 1.1.11 Active processes • Active transport: • • • • • • • • The needs of a cell cannot always be met by the process of diffusion. Sometimes a cell may need to move a substance against the concentration gradient and in other cases it may need to move substances into or out of the cell more quickly than diffusion allows. Some carrier proteins act as pumps. These pumps are complementary to a certain molecule that is carries. These protein pumps carry specific molecules one way across the membrane. They use ATP They carry molecules against the concentration gradient. They carry molecules across at a faster rate than diffusion. Molecules can be accumulated. the energy used in pumping molecules (ATP) changes the shape of the carrier protein. This shape change means that the molecule to be transported fits into the carrier protein on one side only. As the molecule is carried through the carrier uses energy from ATP this changes its shape so that the molecule now leave the carrier protein and cannot enter the carrier protein on the other side as it is now a different shape.
  14. 14. • Moving large amounts – bulk transport • Some cells need to move large quantities of material in or out. • When materials are being brought into the cell it is called endocytosis • When materials are being moved/secreted out of the cell it is called exocytosis (ex=exit) • This is possible because membranes can easily fuse, separate and pinch off. These processes also use ATP. • The energy is used to move the membranes around the materials needed to form vesicles, and to then move these vesicles around the cell. • Examples of bulk transport: • Hormones: pancreatic cells make large quantities of insulin. Insulin is packaged into vesicles by the golgi apparatus. These vesicles fuse with the outer membrane to release insulin into the blood. • White blood cells: these engulf invading micro-organisms by forming a vesicle around them. This vesicle them fuses with lysosomes so the enzymes can digest/destroy the micro-organisms. Such cells are known as phagocytes. • In or out? Solid or liquid? • Endo – inwards For example: bulk movement of a liquid out of the cell would be • Exo – outwards exopinocytosis. • Phago – solid material The bulk movement of a solid into the cell would be endophagocytosis. • Pino – liquid vesicle
  15. 15. Water is a special case 1.1.12 • • • • • • • • Water molecules that are free to move around will diffuse form a region of high water potential to a region of low water potential (high concentration on free water molecules to a low concentration of free water molecules.) If there is another substance dissolved in the water this will affect the concentration of free water molecules. • Solute – a substance that can dissolve. • Solvent – a liquid the solute dissolves in. • Solution – the mixture of solute and solvent. Water potential is the measure of the tendency of water molecules to diffuse from one place to another. Osmosis refers to the diffusion of only water across a partially permeable membrane. As with diffusion, osmosis occurs until an equilibrium is reached. Cells in solutions of high water potential: • Placing an animal or a plant cell in pure water (or any substance with a higher water potential than the contents of the cell), which means the water molecules will diffuse into the cell down the water potential gradient by osmosis and the cell will swell. Animal cells will eventually burst open, (haemolysed). In plant cells the cytoplasm and vacuole will push against the cell wall (the cell is turgid). The cell will not burst as the cell wall will eventually stop the cell getting any larger causing osmosis to stop even if there is still a water potential gradient. Cells in solution of low water potential: • Placing an animal or a plant cell in a concentrated salt or sugar solution (with water potential lower than the cell contents), means that the water molecules will diffuse out of the cell down the water potential gradient by osmosis. The cell will shrink. In animal cells the cell contents will shrink and the membrane will wrinkle. In plant cells the cytoplasm and vacuole will shrink as they lose water and the membrane will pull always form the cell wall. This is called plasmolysis. Water is measured in kiloPascals (kPa). Pure water has the highest water potential with a value of 0kPa. Dissolving a solute in water decreases the water potential and so the value reduces to negative figures. The larger the negative value the more solute that has been dissolved.
  16. 16. Pure water. Water moves in by osmosis down a water potential gradient. Plant cell wall prevents the cell from bursting. The membrane pushes against the wall. The cell is turgid. Animal cells bursts open – it is haemolysed.
  17. 17. Concentrated sugar or salt solution. Very low water potential, Water moves out of the cells by osmosis down a water potential gradient. Plant cell membrane pulls away from the cell wall as the water leaves. The cell is plasmolysed. Animal cell shrinks and appears wrinkled. The cell is crenated.
  18. 18. New cells 1.1.13 • Chromosomes have the instructions: • Chromosomes are in the nucleus of eukaryotic cells. Each chromosome contains one molecule of DNA – which includes specific lengths of DNA called genes. Daughter cells are produced during the cell cycle. They must contain an exact copy of chromosomes from the parent cell. • Human – 46 chromosomes • Onion – 12 chromosomes • Chimpanzees – 48 chromosomes • Dogs – 78 chromosomes • Copying and separating: • Molecules of DNA are wrapped around proteins called histones. The DNA and the histones together are chromatin. Chromosomes (chromatin super-coiled into chromatids that pair up) must be replicated for the cell to divide. Chromosomes are held together at the centromere. • Checks and balances • Proof reading enzymes move along the new DNA strands as chromosomes are replicated to ensure there has been no „mistakes‟ resulting in mutation meaning the cell may fail to function. G1 – biosynthesis (proteins are made and organelles are replicated) S – indicates “synthesis” where each chromosome is duplicated so that each has two chromatids . The cell „checks itself‟ after this stage to ensure it has two copies of each chromosome, if not, the cell cycle stops G2 – this is the growth stage of the cell – the enlargement of the developing cell. M – mitosis (prophase, metaphase, anaphase, telophase. Followed by cytokinesis.)
  19. 19. Tow nuclei from one 1.1.14 • The importance of making new cells: • Asexual reproduction – single-celled organisms divide to produce two new daughter cells such as paramecium. Some multi-cellular organisms such as hydra produce offspring from parts of the parent. • Growth • Repair • Replacement – red blood cells and skin cells are replaced with new ones. • Four stages of mitosis: • Prophase – replicated chromosomes supercoil. • At this stage the nuclear envelope and the nucleolus breaks down and disappears and. The centriole divides into two and begins to move to opposite poles of the cell to from the protein spindles. • Metaphase – chromosomes line up in the equator of the cell. • Anaphase – chromosomes pulled apart to opposite poles of the cell. • • • The chromosomes line up in the equator of the cell and each centromere attaches to one of the spindle fibres. The sister chromatids are separated from each other when the centromere that holds them together splits. The spindle fibres shorten pulling the chromatids apart to opposite poles of the cell. Telophase – two new nuclei are formed. • As the chromatids reach opposite poles of the cell a new nuclear envelope forms around each set of chromosomes and they begin to uncoil. • Two new cells can be made: • Cytokinesis is the splitting of the cell to form two new cells completely identical to the parent cell. Cytokenesis starts form the outside and pinches the cell membrane. • A time and a place: • Mitosis only occurs in meristem cells in plants. • Plants cells do not have centrioles. The tubulin protein threads are made in the cytoplasm. • In plant cells cytokinesis starts with the formation of a cell plate on the equator. New cell membrane and cell wall material is laid down along this cell plate.
  20. 20. Cell cycles and life cycles are not the same thing 1.1.15 • • • • • • Some new cells produced by mitosis are new and separate organisms, e.g. amoeba daughter cells. Genetically identical cells are called clones. Bacteria are prokaryotes. They divide by a process known as binary fission. They have a single naked strand of DNA in the cytoplasm (nucleoid), and a capsule for protection to stop dehydration and white blood cell attacks. They may also have small plasmids of DNA. They may have genes for antibiotic resistance. Bacteria can swap plasmids and so are used to genetic engineering. Many plants undergo asexual reproduction using specialised parts of the plant that derive from the adult plant cells, e.g. potato tubers and strawberry plant runners. These specialised parts can produce a clone of the original parent. This is known as vegetative propagation. Artificial cloning of animals became big news in 1997. a nucleus from an adult sheep‟s udder cell was placed into a sheep egg cell (with the nucleus removed) the egg was then placed into the uterus of a surrogate sheep. The lamb was born and was a clone of another animal. Stem cells: • Stem cells retain the ability to divide and differentiate into a range of specialised cell types. There are two broad types of stem cells: embryonic stem cells and adult stem cells found in bone marrow which partake in the repair and replacement of blood, skin and intestine lining cells. • Embryonic stem cells are taken from the embryo. In the very early embryo (morula stage) they are totipotent. Research suggests that they could be used to treat conditions such as Parkinson's disease, spinal cord injuries and to grow new organs and tissues. Stem cells can be described as: • Totipotent – can make all cell types – zygotes and embryo cells. • Pluripotent – capable of producing all cells derived from a particular germ layer. • Multipotent – can make a restricted range of related cell types e.g. haemopoietic stem cells can make red blood cells, white blood cells and platelets. • Unipotent – able to make only one cell type e.g. muscle stem cells.
  21. 21. Binary fission in bacteria cells.
  22. 22. A time and a place: • When and where cells divide: • In plant cells only meristem cells can divide by mitosis. Meristem cells are found in the roots and shoot tips, and in a ring of tissues in the stem. These parts of the plant are responsible for the growth of the whole organism. • Cytokinesis: • Animal cells – cytokinesis starts from the outside, nipping in the cell membrane and cytoplasm along a cleavage furrow. • Plant cells – a cell plate is formed where the spindle equator was and then lays down a cell membrane and a cell wall. • Yeast cells – undergo cytokinesis by producing a small bud that nips off the cell in a process called budding. Animal cell Plant cell Yeast cell
  23. 23. Meiosis • • • • • Sexual reproduction involves the fusing of two cell nuceli. Each cell contributes half of the total genetic information (genome) required by the offspring. Meiosis is the type of cell division that produces such cells, known as gametes which occurs in only sex organs or gonads. When two gametes fuse it is called a zygote. Which can then divide by mitosis to grow into an new individual organism. Normal adult cells are diploid (contain a full set/two sets of chromosomes), their genome consists of pairs of homologous chromosomes, each containing the same genes but not necessarily the same alleles of each gene. During meiosis only one member of each homologous pair goes into each daughter cell, forming a haploid cells. These cells are not genetically identical as they only contain one set of homologous chromosomes. When the two haploid gametes fuse they form a zygote with two sets of chromosomes making it a diploid cells. Female - egg Male – sperm meiosis Fusion Mitosis leading to growth zygote Mitosis leading to growth
  24. 24. Cell specialisation 1.1.16 • • • • • • There is a physical limit to the size that a cell can reach. This is governed by the need to support structures within the cell and by increasing difficulty of getting enough oxygen and nutrients into a cell to support its needs as its size increases. Single-celled organisms have a large surface-area-to-volume ratio, they can receive oxygen and remove carbon dioxide by diffusion through the membrane. Multicellular organisms have a smaller surface-area-to-volume ratio and not all cells are in contact with the external medium, and so need specialised cells forming tissues and organs to carry out particular functions. Differentiation and specialisation: • Differentiation is the term used for cells that become specialised. And cells can differentiate in a number of ways with changes to: • Number of particular organelles. • Shape of the cell. • Contents of the cell Erythrocytes (red blood cells) and neutrophils (a type of white blood cell): • Both are human cells that each began with the same set of chromosomes. All blood cells are undifferentiated stem cells found in bone marrow. • Erythrocytes – lose their nucleus, mitochondria, golgi apparatus and ER. They are then packed with protein haemoglobin. The shape changes to become biconcave discs and are capable of transporting oxygen from lungs to tissues. • Neutrophils – keep their nucleus and produce enormous numbers of lysosomes making the cytoplasm appear to be granular. The role of white blood cells is to ingest invading micro-organisms. Organising the organism: tissues, organs and systems • Tissues – collection of cells similar to each other and perform a common function. They may be found attached to each other. E.g. xylem and phloem in plants, epithelial and nervous tissues in animals. • Organs – collection of tissues working together to perform a particular function. E.g. leaves in plants, liver in animals. • Organ systems – number of organs working together to perform an overall life function. E.g. excretory and reproductive system.
  25. 25. Sperm cells are specialised in a number of ways: • In organelle content: • Energy for movement of the undulipodium produced by the many mitochondria. • Sperm head contains specialised lysosome (acrosome) which releases enzymes onto the outside of the egg in order for the sperm to penetrate the egg and fertilise it. • In shape: • Very small, long and thin to ease in movement. • Undulipodium helps propel the cell up the uterine tract. • In content: • Nucleus contains only half the number of chromosomes to fulfil its purpose as a gamete. Protein filaments Root hair cells appear on the epidermal layer of young plant roots. They have hair-like projections from their surface into the soil, this increases their surface area of the root and so is able to absorb more water and minerals from the soil.
  26. 26. Organising the organism 1.1.17 • Transport tissues: xylem and phloem • Xylem and phloem come from dividing meristem cells such as cambium. Meristem cells undergo differentiation. • Xylem: consists of xylem vessels with parenchyma cells and fibres. Meristem cells produce small cells which elongate. Lignin reinforces and waterproofs their walls – this kills the contents. The ends of the cells break down so they become long continuous tubes with a wide lumen. Transports water and helps support the plant. • Phloem: consists of sieve tubes and companion cells. Meristem tissue produces cells that elongate and line up end-toend to form a long tube. The ends partly break down forming sieve plates between the cells. Next to each sieve tube is a companion cell which are very metabolically active, which plays a big part n moving products of photosynthesis up and down the plant. Sclerenchyma – fibres strengthen and support Phloem Xylem Cambium – meristematic tissue divides producing phloem to the outside and xylem to the inside
  27. 27. Xylem Phloem
  28. 28. Forming a lining – epithelial tissues in animals. • Epithelial tissue – layers and linings • Connective tissue – holds structures together and provides support e.g. cartilage, bone and blood. • Muscle tissue – specialised to contract and move parts of the body. • Nervous tissue – cells that convert stimuli to electrical impulses and conduct those impulses. • Squamous epithelial tissue – made up of flattened cells that are very thin. The cells together form a thin, smooth, flat surface. Ideal for lining tubes such as blood vessels. Also forms thin walls such as that on alveoli – provides a short diffusion pathway. Squamous tissue are held in place by the basement membrane (secreted by epithelial cells), made of collagen and glycoproteins. The basement membrane attaches epithelial cells to connective tissue. • Ciliated epithelial tissue – made up of column shaped cells. Often found on the inner surface of tubes e.g. trachea, bronchi and bronchioles, uterus and oviducts. The cilia wave in a synchronised rhythm to move such materials as mucus. Harvesting light – co-operation in action: • Leaves are major organs of photosynthesis in plants. Their cells, tissues and overall shape are arranged to help maximise the rate of photosynthesis. Photosynthesis requires: • Light. • Water supply. • Carbon dioxide supply. • Presence of chlorophyll. • As these products build up they need to be moved to parts of the plants where they are needed and oxygen must be secreted. • The leaf is adapted in a number of ways: • Transparent upper epidermis allows light in. • Layer of palisade cells beneath the epidermis, packed with chloroplasts containing chlorophyll. • Loosely packed spongy mesophyll layer with many air spaces to allow for circulation of gases. • Lower epidermis has stomata, which allows gases to be exchanged between the leaf and the surroundings. Each stomata have two guard cells that can swell to open the pore, when the guard cells are not turgid the stoma closes. • Leaf vein system containing xylem and phloem supports the leaf as well as carrying the transport tissues – these transport tissues transport water into the leaf and products of photosynthesis to parts of the plant where it is needed.
  29. 29. • • The role of guard cells: • Guard cells are specialised cells that appear in pairs on the lower epidermis and contain chloroplasts. Their cell wall contains spiral thickenings of cellulose on the inside of the guard cell. When water moves into these cells they become turgid, and only the outer walls stretch. The two guard cells bulge at both ends and the stoma opens. Locomotion – an example of systems co-operation: • The muscular and skeletal systems must work together in order for movement to take place. But this can only happen if the nervous system instructs it to do so. As muscles and nerves work they use energy. They require a supply of nutrients and oxygen from the circulatory system which in turn receives these chemicals from the digestive and ventilation systems. Palisade mesophyll cell Spongy mesophyll cell