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: Light (compound) microscope: uses visible light, two or more lenses, and a specimen - usually stained to make structures more visible 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 = 1/10,000th mm

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

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

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

1 2 3 12 4 11 5 6 10 9 7 8 Nucleus Cell membrane Nucleolus Chromatin Cell wall 11 Nuclear envelope 5 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 membrane-bound nucleus and organelles

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

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 Prepare another smear slide in same way but without stain Examine both slides under microscope using correct method - at 100X and 400X Coverslip Glass slide Water droplet with methylene blue Cheek cell smear

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

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 Prepare another slide in same way but without iodine Examine both slides under microscope using correct method - at 100X and 400X Coverslip Glass slide Water droplet Onion layer Mounted needle

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

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