Cell Structure
OCR AS Biology Unit 1: Cells, Exchange and Transport
Learning objectives
• To understand and discuss the basis of cell theory based upon milestones in cell biology
• To carry out practical work on cell size and magnifications using a light microscope
• To be able to draw a plant cell and an animal cell using a light microscope
• To prepare slides using stains to identify cell structure.
Milestones in Cell Biology
• Read through handout
• Group discussion– Discuss the impact of the microscope on cell
biology– List things you think are the most important
Cell theory• The cell is the fundamental unit of life. All organisms,
whatever their type or size, are composed of cells. The modern theory of cellular organisation states:-– All living things are composed of cells and cell products.– New cells are formed only by the division of pre-existing cells– The cell contains inherited information (genes), which is used as
instructions for growth, functioning and development.– The cell is the functioning unit of life; the metabolic reactions of life
take place within the cells.
Cell Theory - Question
• Before the development of cell theory, it was commonly believe that living organisms could arise by spontaneous generation.
• Explain what this term means and why it has been discredited as a theory.
Plan Diagrams – dicotyledonous leaf
Cell Biology and Microscopy
• When Scientists began to observe cells, they started with simple microscopes
• There are two different types of microscope both use a form of radiation to create an image of the specimen:– Light microscope – uses light– Electron microscope – uses electrons
Using a light microscope
• Magnification– Number of times larger an image is compared
with the real size of the object
• Resolution– The ability to distinguish between two separate
points
Structure of a generalised animal cell as seen with a very high power quality light microscope (diameter ~ 20ųm)
Structure of a generalised plant cell as seen with a very high power quality light microscope (diameter ~ 40ųm)
Light Microscope
• For the microscope in front of you, work out– The magnification of each lens– The field of view for each lens • Using a graticule / stage micrometer
• Collect a prepared slide of Squamous epithelium, draw what you can see.– Your diagram should include title, labels,
magnification and a scale bar.
Examination of plant cells• Strip a pieces of epidermis from the inner lining of one of
the fleshy scales of an onion.
• Mount it in dilute iodine solution• Observe and draw a cell under low power and high power
• Repeat the above procedure using water instead of iodine.• what difference does this make to how much you can see?
Examination plant cells - conclusion
• What does this tell you about the value of staining cells before you look at them under the microscope?
• Make sure all diagrams have a title, label, magnification and scale bar.
Microscopy and magnification
OCR AS Biology
Microscopes
• Light microscope• Long wavelength• Can only distinguish between two objects if they are
0.2µm apart.
• Electron microscope• Shorter wavelengths• Can distinguish between objects 0.1nm apart
ionmagnificat
imageofsizeobjectofsize
Magnification
• How many times bigger the image is compared to original subject.
• If asked to calculate the size of an object
objectofsize
imageofsizeionmagnificat
Magnification
• When calculating magnification make sure the units of length are the same for both object and image
unit symbol Equiv. in m
Kilometre km 103
Metre m 1
Millimetre mm 10-3
micrometre µm 10-6
nanometre nm 10-9
Magnification - example
• Object 100nm in length• How much is it magnified in a photo?• Measure object in photo (10mm long)
nm
mm
objectofsize
imageofsize
100
10
Magnification - example
• The measurements need converting to the same unit (usually the smallest)
• There are 10 000 000nm in 10mm• Magnification is:
• 100 000 times
times
nm
nm
objectofsize
imageofsize
000100
1
000100
100
00000010
The effect of progressive magnification of a portion
of human skin
Structure of plant and animal cells under an electron microscope
OCR AS Biology
The Electron Microscope
• Two main advantages– High resolving power (short wavelength of
electrons)– As electrons negatively are charged the beam can
be focused using electromagnets• As electrons are absorbed by molecules of air,
a near-vacuum has to be created within the chamber of an electron microscope.
The Electron Microscope
• Two types– Transmission Electron Microscope (TEM)– Scanning Electron Microscope (SEM)
• Activity– Read through the handout on the electron
microscope– Answer discussion questions 1 - 4
Comparison of advantages and disadvantages of the light and electron microscopes
Light MicroscopeAdvantages
Electron Microscope Disadvantages
Small and portable very largeOperated in special rooms
Unaffected by magnetic fields Affected by magnetic fields
Preparation of material is quick and simple
Preparation of material is lengthyRequires expertise
Material rarely distorted by preparation
Preparation may distort material
Natural colour of object observed
Images are in black and white
Cheap to purchase and operate
Expensive to purchase and operate
Light MicroscopeDisadvantages
Electron MicroscopeAdvantages
Magnifies objects up to 1500x only
Magnifies objects more than 500 000X
Depth of field is restricted
Possible to investigate a greater field of depth
Comparison of advantages and disadvantages of the light and electron microscopes
Comparison of pathways of the light and electron microscopes
Ultrastructure of an animal cell as seen through an electron microscope
Ultrastructure of an Animal Cell
Ultrastructure of a plant cell as seen through an electron microscope
Ultrastructure of a Plant Cell
Pupil Activity
• Cell structure – Read through the information on each of the
organelles as you colour them in– Follow the guidance on colouring them in given at
the bottom of the page
– This works on the theory that whilst you are colouring in, you have time to consider and think about the structure and function of the organelles
Cell Structure ActivitiesOCR AS BiologyFOUNDATION
Pupil Activity – Cell StructureAnimal Cell
• In pairs label the diagram of the animal cell given.– How many structures can you identify?– Look at the cells alive animation – how many have
you correctly identified?
• Label the paper copy of the diagram of an animal cell
Animal Cell
Animal cell - answers
Pupil Activity – Cell StructurePlant Cell
• In pairs label the diagram of the plant cell given.– How many structures can you identify?– Look at the cells alive animation – how many have
you correctly identified?
• Label the paper copy of a diagram of a plant cell
Plant cell
Plant cell - answers
Activity:Introduction to organelle function
• Using the cards, match up organelle, position and function.
Organelle Position Function
Nucleus Within cytoplasm Contains genetic code which controls the activities of the cell
Cytoplasm Around nucleus Location of chemical reactions – does the work of the cell
Cell surface membrane
Around cytoplasm Controls exchange of substances between cytoplasm and surroundings
Cell wall around cell membrane Gives cells rigidity, stops it bursting if put in water
Cell vacuole Within cytoplasm Affects concentration of cytoplasm. Is a store of inorganic ions.
Tonoplast Around cell vacuole Controls exchange of substances in plant cells between vacuole and cytoplasm
Large granules Within cytoplasm Usually stores food e.g. starch
Organelle Structure and functionOCR AS BiologyFOUNDATION
Nucleus
Structure of nucleus
• Nuclear envelope• Nuclear pores• Nucleoplasm• Chromatin• nuceolus
Function of the nucleus
• Acts as the control centre of the cell through the production of mRNA and protein synthesis
• Retain genetic material of the cell (DNA / chromosomes)
• Start the process of cell division
Structure of Nucleus
• Chromatin– DNA and associated proteins, chromatin
condenses into chromosomes when the cell divides.
• nucleolus– Manufactures ribosomal RNA and assembles the
ribosomes
Structure of Nucleus
• Nuclear envelope– Controls entry and exit of materials– Outer membrane continuous with endoplasmic
reticulum• Nuclear pores– Passage of large molecules (mRNA) out of nucleus
Chloroplasts
Structure and Function of Chloroplasts
• Chloroplast envelope – Entry and exit of substances
• Stroma– Enzymes for the light independent stages of
photosynthesis• Grana (thylakoids/lamellae)– Light dependent stage of photosynthesis
• Starch grains– Temporary stores of carbohydrates
Mitochondria
Structure of Mitochondria
• Double membrane– Inner membrane folded into cristae which provide
a large surface area• Matrix– DNA, enzymes and ribosomes
Function of Mitochondria
• Site of Krebs cycle and oxidative phosphorylation in aerobic respiration
• Production of energy rich ATP molecules from carbohydrates
Endoplasmic reticulum
Endoplasmic reticulum
• Membranes spreading through the cytoplasm of cells, continuous with the nuclear membrane
• Enclose flattened sacs called cisternae• Rough endoplasmic reticulum– Ribosomes present on outer surface of membrane
• Smooth endoplasmic reticulum– No ribosomes, tubular in appearance.
Function of endoplasmic reticulum
• RER– Provide LSA for synthesis of proteins– Provides a pathway for the transport of materials
(esp. proteins) throughout the cell.• SER– Synthesis, stores and transports lipids and
carbohydrates– Contains lytic enzymes (liver cells)
Ribosomes
Structure and function of ribosomes
• Two types– 80S – eukaryotic cells– 70S – prokaryotic cells
• Make up 25% of dry mass of cell
• Important in protein synthesis
Golgi Apparatus
Structure of golgi apparatus
• Stack of membrane bound, flattened sacks
Functions of Golgi Apparatus
• Modifies and packages proteins– Adds carbohydrates to proteins to form
glycoproteins– Produces secretory enzymes
• Secretes carbohydrates • Transports, modifies and stores lipids• Forms lysosomes
Lysosomes (animal cells only)
Structure of lysosome
• Spherical sac surrounded by a single membrane
• Contains powerful digestive enzymes
Functions of Lysosomes
• Destroy foreign material inside or outside the cell.– Breakdown material ingested by phagocytic cells– Release enzymes outside the cell – Digest worn out organelles (autophagy)– Autolysis break down cells after they have died.
Cilia
Structure and function of cilia
• Structure– Threads that extend from cell surface– Made of nine sets of 3 microtubules
• Function– Move an entire organism– Move material within an organism• E.g. cilia lining respiratory tract move mucus towards
the throat.
Flagella / undulipodia
• Structure– 9 sets of microtubules in a circle– 2 central microtubules
• Function– Movement• Tail of a sperm• Whole of unicellular organism
Centrioles (animal cells only)
Structure and function of centrioles
• Hollow cylinders of microtubules• Microtubules form spindle fibres for nuclear
division• Maybe involved in formation of microtubules
that make up cells cytoskeleton
Plant cell wall
• Structure– Made of cellulose
• Function– Supports the cell• Cell turgor pressure
Homework – in for next lesson
• Similarities between plant and animal cells– Make a list of the structures plant and animal cells
have in common– Make a table of the differences between plant and
animal cells
• Include all structures in plant and animal cells not just the ones observed through a Light microscope
Movement and Stability in cells
Learning Outcomes
• explain the importance of the cytoskeleton in providing mechanical strength to cells, aiding transport within cells and enabling cell movement;
Cytoskeleton
• Cells contain a network of fibres made of protein, providing an internal framework.
• Fibres can move organelles round within a cell.• Microtubules
– Move chromosomes around in cell division– Move vesicles from endoplasmic reticulum to Golgi apparatus
• ATP is used to drive some of these movements
Division of LabourProtein synthesis
Learning Outcomes
• outline the interrelationship between the organelles involved in the production and secretion of proteins
Protein Production
• Instructions in DNA is nucleus• Instructions copied onto mRNA• mRNA moves to ribosome, where protein is
assembled• Protein transported to Golgi apparatus• Protein modified and packaged• Protein moves in a vesicle to the cell surface
membrane• Protein secreted.
Protein Production
Prokaryotic CellsOCR AS BiologyFOUNDATION
Prokaryotic Cells“pro” – before “karyo“– nucleus
• Prokaryotes were probably the first forms of life on earth. Their heredity material (DNA) is not enclosed within a nuclear membrane. There are no membrane bound organelles within a prokaryotic cell.
Prokaryotic Cells
• The absence of a true nucleus only occurs in 2 groups.– Bacteria– Blue green algae (cyanobacteria)
• Five structures, which are always present in a prokaryotic cell, are:– cell wall, plasma membrane, cytoplasm, ribosome,
circular DNA
Structure of a bacterial cell
Activity• Copy and complete the following table
• Include organelles, DNA, RNA and protein synthesis, ribosome, cell division, cell wall, cellular organisation.
prokaryotic Eukaryotic
organisms
Cell size
metabolism
Comparison of prokaryotic and eukaryotic cells
prokaryotic Eukaryotic
organisms Bacteriacyanobacteria
Protista, fungi, plants, animals
Cell size 1 – 10 µm 10 –100 µm
metabolism Anaerobic and aerobic
Aerobic
organelles No membrane bound organelles
Nucleus, mitochondria, chloroplasts, RER, SER
prokaryotic Eukaryotic
DNA Circular DNA in cytoplasm
DNA organised into chromosomes bounded by nuclear envelope
RNA and Protein Synthesis
Synthesised in same compartment
RNA synthesised and processed in nucleusProtein synthesis in cytoplasm
Ribosomes 70S Type 80S Type
Cell Division Binary fission Mitosis or meiosis
Comparison of prokaryotic and eukaryotic cells
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