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