Chapter 6 Cell Structure Leaving Certificate Biology Higher

Chapter 6: Cell Structure Leaving Certificate Biology Higher Level

The Microscope • A microscope is used to view very small living organisms and cells • “Be familiar with and know how to use a light microscope”

The Microscope • Two types of microscope you need to know for the Leaving Certificate are: 1. Light (compound) microscope: uses visible light, two or more lenses, and a specimen - usually stained to make structures more visible 2. Transmission electron microscope: uses a beams of electrons (e-), a number of electromagnetic lenses (that focus and diverge the beam of e-), a piece of photographic film (like X-ray film), and a specimen

The Light Microscope • Eyepiece: magnifies the image; closest to the observer’s eye • Objective: magnifies the image; closest to the specimen • Turret: holds the objective lenses • Stage: holds the specimen (slide) • Clips: holds the slide in place • Diaphragm: controls the amount of light • Light source/mirror: sends light up through the stage and specimen • Fine/coarse focus wheels: make fine/large adjustments to the clarity of the image

The Electron Microscope • The beam of e- from tungsten filament travels through specimen and onto photographic film • Everything in the microscope is held in a vacuum • The specimen must be coated with a heavy metal and must also be very thin as e- cannot penetrate thick specimens • Electromagnetic lenses focus the high-velocity e- beam as it hits the specimen and diverges the beam as it approaches the photographic film - thereby magnifying the image amount of divergence dictates what the magnification will be • If the divergence of the beam is too big the magnification will be such that we would lose good resolution – thereby producing a blurry image • The maximum resolution of the electron microscope is: 0. 1 nanometre (nm) = 0. 0001 mm = 1/10, 000 th mm

Using a Light Microscope 1. Ensure low-power lens (4 X) is in position before placing specimen on stage 2. Separate stage and objective as much as possible using coarse wheel before placing specimen on stage 3. Adjust mirror or turn on light underneath stage 4. Ensure diaphragm is fully open to allow light pass through 5. Place specimen slide on stage so that specimen is directly above hole in stage 6. Bring 4 X close to specimen - it is easier and safer to do initial focusing low-power lens than higher-power lenses 7. Looking through eyepiece bring image into focus by turning the coarse and fine focus wheels towards you slowly

Using a Light Microscope (cont. ) 8. Adjust diaphragm if necessary during focusing as image may be too bright to view specimen 9. Once in focus, move slide around gently on stage to see different fields of view 10. Change the objective to the 10 X lens 11. Draw a sketch of the field of view 12. Change the objective to the 40 X lens carefully as this lens may hit the slide and cause damage 13. Refocus using fine focus wheel 14. Draw a sketch of the field of view 15. Once finished move the 4 X lens back to the main position - only then remove slide

Animal Cell Structure

Animal Cell Structure and Function • Cell Membrane: contains the contents of the cell and controls what enters and leaves the cell • Cytoplasm: liquid of cell – consists of cytosol and cell organelles and is where chemical reactions occur • Nucleus: contains the DNA in the form of either chromosomes or chromatin and controls all of the activities of the cell • Nucleolus: where ribosomes are made • Nuclear pores: allow movement of chemical substances in and out of nucleus

Animal Cell Structure and Function • Nuclear envelope: double phospholipid membrane keeping the DNA separate from the cytoplasm • Chromatin: form DNA takes when the cell is not dividing - elongated chromosomes • DNA: deoxyribonucleic acid - organised into chromosomes; where genes are located • Mitochondrion: site of respiration - supplies energy for metabolism • Ribosome: site of protein synthesis

Plant Cell Structure

Plant Cell Structure and Function • Cell wall: made of cellulose; gives shape and support to plant cell; permeable to water, salts and minerals; prevents cell from bursting; protects cell from bacteria and viruses • Chloroplast: contains green pigment chlorophyll; responsible for photosynthesis • Vacuole: large, fluid-filled (sap) sac that takes up the majority the plant cell and is separated from the cytoplasm by a membrane called the tonoplast. Water, food (glucose, amino acids, etc), minerals, ions and wastes are stored here

Cell 1 membrane Nucleus 2 3 Nucleolus Chromatin 4 Nuclear 5 pore 6 Nuclear envelope 9 Ribosomes Cytoplasm 8 7 Mitochondrion

Cell membrane 1 12 Cell wall 2 Nucleus 3 Nucleolus 4 Chromatin Nuclear 5 envelope 11 Central vacuole 6 Nuclear pore 10 Ribosome 9 Cytoplasm 7 Mitochondrion Chloroplast 8

Prokaryotic and Eukaryotic Cells • Two major kinds of cells - prokaryotic and eukaryotic - can be distinguished based on their structural organisation • Prokaryotic cells have a tough outer cell wall and have no membrane-bound nucleus nor membrane-bound organelles • Eukaryotic cells have a membranebound nucleus and organelles

Mandatory Experiment: To prepare and examine one animal cell and one plant cell, unstained and stained using the light microscope at 100 X and 400 X

To prepare and examine an animal cell • Apparatus/Chemicals: – Cotton buds; microscope slides; coverslips; methylene blue; needle; tissue; water; light microscope • Method: – Scrape the inside of your mouth with cotton bud once and immediately smear on the centre of clean glass slide - do not let smear dry – Immediately add a few drops of methylene blue - leave 5 min – Blot excess stain with tissue and add a drop of water

To prepare and examine an animal cell • Method (continued): – Place coverslip over smear by lowering slowly from a 45º angle using the needle Coverslip Glass slide Water droplet with methylene blue Cheek cell smear – Prepare another smear slide in same way but without stain – Examine both slides under microscope using correct method - at 100 X and 400 X

To prepare and examine an animal cell • Results: – Images are visible stained and unstained cheek cells at both 100 X and 400 X – Based on your experience of viewing cells at 100 X and 400 X, at what magnification do you think this image was obtained? ? X • Conclusion: – Cells can be examined in detail using the light microscope – Cell numbers, structure, and organelles can be better examined by staining the cells

To prepare and examine a plant cell • Apparatus/Chemicals: – Microscope slides; coverslips; iodine; needle; blades; tissue; water; light microscope • Method: – Using the blade carefully incise out shallow and small squares from an internal leaf of an onion – Separate a single-celled layer of tissue from one of the squares – Place this epidermal strip onto the glass slide – Add few drops of iodine - leave 5 min – Blot excess iodine with tissue and add drop of water

To prepare and examine a plant cell • Method (continued): – Place coverslip over epidermal strip by lowering slowly from a 45º angle using the needle Mounted needle Coverslip Glass slide Water droplet Onion layer – Prepare another slide in same way but without iodine – Examine both slides under microscope using correct method - at 100 X and 400 X

To prepare and examine a plant cell • Results: – Images are visible of stained and unstained onion cells at both 100 X and 400 X – Based on your experience of viewing cells at 100 X and 400 X, at what magnification do you think this image ? X was obtained? ? X • Conclusion: – Cells can be examined in detail using the light microscope – Cell numbers, structure, and organelles can be better examined by staining the cells

To prepare and examine an animal cell • Safety: – Care must be taken with the slides and coverslips. They are broken very easily - broken glass should be reported immediately to the teacher and/or laboratory technician. – Use a clean cotton wool buds (and not your finger) to obtain a smear of cells. – Extreme care must be taken when using the blade to obtain the epidermal strip of onion cells - a wooden chopping board should be used to minimise the risk of the onion slipping whilst cutting it – Wear latex or nitrile gloves when handling the methylene blue and iodine solutions - they easily stain the skin and their effects on humans are unknown so care must be taken (especially with substances that stain nuclei of living cells as they are potential carcinogens). – Lab coats must be worn to protect clothing