2.1.1 Cell strucutre

What is magnification

How many times bigger the image produced by the microscope is, compared to the real object under the microscope

What is resolution

The ability to distinguish between 2 separate points as distinct objects

What are the different parts of a microscope

• A = Eyepiece lens - contains eyepiece graticule

• B = Turret

• C = Objective lens

• D = Stage - place stage micrometer and calibrate with eyepiece graticule

• E = Condenser

• F = Light Source

• G = Fine Focus

• H = Coarse focus

How do you calibrate an eyepiece graticule

• Focus on the stage micrometer scale.

• Align the eyepiece graticule with the stage micrometer.

• Count the number of graticule divisions that span a known micrometer distance.

• Divide the known micrometer distance by the number of graticule divisions it spans to calculate the actual length per graticule division

What are the two main types of microscopes

• Optical (or light)

• Electron

  • Transmission electron microscopes (TEMs)

  • Scanning electron microscopes (SEMs)

How is total magnification calculated

Eyepiece lens magnification x objective lens magnification

Optical light microscopes fact file (res, mag, what you can see)

• Maximum resolution: ~ 0.2 micrometers

• Maximum magnification: x1500

• Structures you can visualise: eukaryotic cells, nuclei, (sometimes) mitochondria and chloroplast

What are the advantaged and disadvantages of Light microscopes

• Advantages:

  • Inexpensive

  • small and portable

  • simple slide preparation (doesn’t usually distort sample)

  • specimens can be living or dead

  • Produced a coloured image

• Disadvantages:

  • Lower magnification then electron microscopes

  • Lower resolving power then electron microscopes (200nm)

How do light microscopes produce images

Uses visible light and glass lenses to magnify specimens, producing 2D images of cells and tissues with relatively low resolution

How do transmission electron microscopes work

• Use electromagnets to focus a beam of electrons which is then transmitted through the specimen

• Denser parts of specimen absorb more electrons so appear darker

• In black and white as they don’t absorb light

What are the advantages of transmission electron microscopes

• High resolution images

• Allows the internal structures within cells & organelles to be seen

What are the disadvantages of transmission electron microscopes

• Only to be used with very thin specimens/thin sections

• Can’t be used to observe live specimens

• Specimens take time to prepare which increases the chance of artefacts being produced

  • Artefacts - something not part of the organism e.g dust, air particles

• Do not produce a colourful image

How do scanning emission microscopes work

• They scan a beam of electrons across the specimen

  • Beam bounces off the surface of the specimen

    • Electrons are detected

      • Forms an image

• Produce 3D images that show the surface of specimens

What are the advantages of scanning emission microscopes

• Can be used on thick or 3D structures

• Allows the external 3D structure to be observed

What are the disadvantages of scanning emission microscopes

• Give a lower resolution images then TEMs

• Can’t be used to observe live specimens

• Do not produce a coloured image

Electron microscopes fact file

• Maximum resolution: 0.2 nm

• Maximum magnification: ~ x 1,500,000

• Structures you can visualise: small organelles - ribosomes, endoplasmic reticulum, lysosomes

• Needs a vacuum

Why do electron microscopes have higher resolution than light microscopes

Higher res than light microscopes due to electrons having a shorter wavelength than visible light - allowing finer detail to be resolved

What are the light microscope sample preparation methods

• Dry mount

• Wet mount

• Squash slides

• Smear slides

How does a dry mount work

• Solid specimens

• Viewed whole or sectioned

• Specimen placed in the centre of the slide

• Coverslip placed on top

How does a wet mount work

• Use a pipette to add a drop of water to the slide on the specimen

• Cover slip is placed on top from an angle

• Ensure there are no air bubbles

• Can be used for aquatic samples or other living organisms

How do squash slides work

• Used for soft samples e.g. root tips - to look at cell division

• A wet mount is prepared

• The cover slip is gently pressed down upon

How do smear slides work

• Sample is smeared by the edge of a slide to create a thin even coating/ layer

• The coverslip is then placed on top

• Used with blood

How is a sample prepared for staining in light microscopes

• To prepare a sample for staining:

  • Placed on slide

  • Allowed to air dry

  • Heat fixed by passing through a flame

  • Specimen will adhere to slide and will then take up stains

Why is staining used with light microscopes

• Stains are used to increase contrast as different components within a cell take up stains to different degrees

• Increase in contrast allows components/structures to become visible so they can be identified

How are samples prepared in electron microscopes

• Samples must be prepared in a specific way due to the vacuums inside the microscope

• Specimens must be fixated using chemicals or freezing, staining with heavy metals and dehydration with solvents

What is differential staining

• The use of multiple stains that bind to different cellular components or cell types, allowing them to be

  distinguished from one another

What is Gram staining

• Type of differential staining

• Uses crystal violet and iodine to help divide bacteria into gram positive (charge) and gram negative groups (charge)

What is acid fast staining

• Differential staining used to identify acid fast organisms (micro organisms with highly impermeable cell walls)

What are the different elements of a eukaryotic cell (12)

• Cell wall

• Cell membrane

• Nucleus

• Mitochondria

• Chloroplast

• Ribosomes

• Rough endoplasmic reticulum (RER)

• Smooth endoplasmic reticulum (SER)

• Golgi apparatus (Golgi complex)

• Vesicles

• Vacuole

• Lysosome

What is the structure and purpose of the cell membrane

• Controls the exchange of materials between the internal cell environment and the external cell environment

• Partially permeable

• Fluid - constantly in motion

  • Due to the 2 layers of fat constantly attracting and repelling each other

• Primarily made up of phospholipids

What is the structure and purpose of the cell wall

• A rigid layer outside the plasma membrane

• Provides structural support to the cell and prevents it from bursting

• Made up of polysaccharides (type of carbohydrate)

  • Cellulose in plants

  • Peptidoglycan in most bacterial cells

What is the structure and purpose of the nucleus

• Membrane-bound organelle

• Contains chromatin (linear DNA bound to histone proteins)

• Separated from the cytoplasm by a double membrane which is full of pores

  • allows MRNA & ribosomes to leave the nucleus and enzymes

• Contains Nucleolus - site of ribosomal RNA synthesis and assembly, some cells have multiple nucleoli

What is the nuclear envelope

• Double membrane surrounding the nucleus

• Containns nuclear pores that control the passage of substances between the nucleus and cytoplasm

What are histone proteins

Condenses DNA into chromosomes

What is the structure and purpose of mitochondria

• Site of aerobic respiration & ATP synthesis

• Surrounded by a double membrane

  • Inner membrane folds in on itself to form a cristae - this increases it surface area

  • Outer membrane is highly permeable to allow for movement of (ATP) molecules

• The cristae forms the matrix, that contain many enzymes needed for respiration and electron carrying

• Smaller, circular mitochondrial DNA (mtDNA) and ribosomes are also found here

  • They need their own to supply themselves to deal with their ‘high work load’

• Can vary in shape and size

What is the structure and purpose of the chloroplast

Site of photosynthesis

• Light dependent phase: Takes place in thylakoids - which stack to form granum (which stack to form lamellae)

• The light independent phase (Calvin cycle): Takes place in the stroma

• Surrounded by a double membrane

• Also contain small, circular DNA and ribosomes

How are lamella built

• Thylakoids (containing chlorophyll) stack to form a granum (grana) - to increase its surface area

• Grana stack to form a lamella (lamella)

What is the structure and purpose of ribosomes

• Found freely in the cytoplasm of all cells or as part of the RER

• Site of translation - protein synthesis

• Each ribosome is a complex of ribosomal RNA (rRNA) and proteins

• Eukaryotes have larger ribosomes due to being larger + making larger proteins

  • Eukaryotic cells contain 80s ribosomes

  • Prokaryotes, mitochondria and chloroplasts contain 70s ribosomes

  • (‘S’ stands for a unit of measurement)

• Found in all cells

What is the structure and purpose of the Rough endoplasmic reticulum (RER)

• Where protein is ‘edited’

• Surface covered in ribosomes

• Formed from continuous folds of membrane that is attached to the nuclear envelope

• Synthesises and transports proteins made by the ribosomes

• Plant & animal cells

What is the structure and purpose of the Smooth endoplasmic reticulum (SER)

• Found in plant & animal cells

• Made up of membrane tubules without ribosomes

• Involved in the processing, synthesis and storage of lipids, carbohydrates and steroids

What is the structure and purpose of the Golgi apparatus (Golgi complex)

• Found in plant and animal cells

• Flattened sacs of membranes - membrane bound organelle

• Responsible for modifying proteins and lipids

  • Pack them into Golgi vesicles

    • Vesicles transport proteins and lipids to their destination

• Proteins that go through the Golgi apparatus are usually exported, put into lysosomes or delivered to membrane-bound organelles

• Proteins enter through a vesicle on the cis face (side) and exists through a vesicle on the trans face

What is the purpose of vesicles

Membrane - bound sacs for transports & storage

What is the structure and purpose of large permanent vacuoles

• Sacs in plant cells surrounded by the tonoplast - a selectively permeable membrane

• Helps maintain tugor pressure

• Stores water, salts, minerals, pigments & proteins within a cell

What is the plasma membrane

• A phospholipid bilayer with embedded proteins that controls the passage of substances into and out of the cell

What are centrioles

• Cyclical structures made of microtubules that organise the spindle fibres during cell division in animal cells

• Not found in flowering plants or fungi

What is the structure and purpose of a lysosomes

• Specialist forms of membrane bound vesicles which contain hydrolytic enzymes (enzymes that break down biological molecules)

• The enzymes break down waste materials such as

  • worn-out organelles

  • Pathogens

  • Indigested material

• Cells of the immune system & cells involved in apoptosis (programmed cell death) use lysosomes a lot

What is the purpose of a microvilli

Cell membrane projections that increase the surface area for absorption

What is the structure and purpose of the cilia

• Short hair-like projections made from micro tubules

• Beat in coordinated waves to move substances across the cell surface or enable cell movement

What are the different elements of prokaryotic cells (10)

• Flagellum - sometimes present

• Capsule - sometimes present

• In folding of cell surface membrane - sometimes present

• Plasmid - sometimes present

• Pili - sometimes present

• Cell wall

• Cell membrane

• Cytoplasm

• Circular DNA

• Ribosomes

What are the structures unique to prokaryotic cells

• Plasmids

• Capsule

• Flagellum

What is the structure and purpose of plasmids

• Small loops of DNA that are separate from the main circular DNA molecule

• Contains DNA that can be passed between prokaryotes

What is the structure and purpose of capsules

Helps protect bacteria from drying out and from attack from by cells from the hosts immune system

What is the structure and purpose of the flagellum

• Long, hair like structure that rotates, enabling movement

• Some have more then 1

• Made of larger micro-tubules

What is the purpose of the infolding of the cell membrane

• May allow for photosynthesis

• May carry out nitrogen fixation

What is the purpose of the pili

• For attachement to other cells or surfaces

• Involved in sexual reproduction

Prokaryotes vs Eukaryotes

• Prokaryotes are 100-1000 x smaller

  • Typically 1-5 micrometers | Eukaryotes 10-100 micrometers

• Cytoplasm lacks membrane-bound organelles

• Smaller ribosomes (70s)

• No nucleus, no histone proteins - free floating, circular DNA

• Cell wall made up of peptidoglycan & as opposed to cellulose

• Both contain: Ribosomes, a plasma membrane, cytoplasm, genetic material

What is the protein production process

1. Genes (instruction to make the proteins) for hormones are transcribed inside the nucleus

2. The nucleus copies the instructions in the DNA onto mRNA.

   • This then leaves the nucleus through a nuclear pore and attaches to a ribosome

3. Proteins synthesised at ribosome

4. Ribosome is already attached to rough endoplasmic reticulum (RER) so protein passes into lumen of RER and is folded

5. The assembled protein in the RER is transported to the Golgi apparatus via a transport vesicle

6. Golgi may modify it so that it is ready for release and packages it into a secretory vesicle

7. Vesicles fuses to the cell surface membrane and release the protein via exocytosis

What is the term for when a vesicle excretes a hormone

Exocytosis

What is the term for when a vesicle takes a hormone

Endocytosis

What is the cytoskeleton

• Maintains cell shape and resists external forces

• A big network of protein fibres

• Made up of mostly fibrous proteins

• Contains: microfilaments, microtubules and intermediate fibres

What are microfilaments

• Solid strands

• Made up mostly of actin

• Can cause some cell movement and the movement of some organelles within cells by moving against each other

• Responsible for cytokinesis, amoeboid movement and muscle contraction

What are microtubules

• Tubular (hollow) strands

• Mostly made up of the protein tubulin

• Organelles and other cell contents are moved along these fibres using ATP to drive it

• Enable movement of cilia and flagella

What are intermediate fibres

Give mechanical strength to cells and help maintain their integrity

What is the cytoskeleton important

• Provides cell strength and support:

  • Provides the cell with mechanical strength, forming a kind of scaffolding that helps to maintain cell shape

  • Support organelles - keeping them in their positions

• Intracellular movement:

  • Aids transport within cells by forming tracks along which organelles can move

  • Examples include: the movement of vesicles & movements of chromosomes to the opposite ends of a cell during cell division

• Cellular movement:

  • Enables cell movement via cilia and flagella

  • Both hair like extensions that contain microtubules that responsible for moving them