Biology 2.1.1 Cell Structure

how do Transmission Electron Microscopes form an image?

they use electromagnets to transmit a beam of electrons through a specimen

for TEM microscopes why are some spaces on the image darker?

denser regions of the specimen absorb more electrons, thus appear darker while less dense areas allow more electrons to pass through, appearing lighter.

properties of a TEM image?

high resolution; can see internal structures, and within organelles; and are two dimensional

how do Scanning Electron Microscopes form an image?

beam of electrons passed across the surface of a specimen, and rate at which they are bounced back is detected

properties of SEM image?

3D; show the surface of specimens; btw lower maximum resolution than TEM’S

least to most magnification

Light microscope < SEM < TEM

fixing? a specimen

-using formaldehyde

-dehydrating it with a series of ethanol solutions

-impregnating it in paraffin/resin for support then cutting thin slices with a microtome

-paraffin is removed and a stain applied

-specimen mounted using a resin

a way to fix using nitrogen?

-freeze specimen in co2 or liquid nitrogen

-thin slices using a cyrostat

-place specimen on the side and add a stain

how to use an eyepiece graticule to take measurements?

-placed into eyepiece

-as no fixed units must be calibrated for the objective lens that is in use

-done by using a scale engraved on a microscope slide, called a stage micrometer

-by using the 2 scales together the number of micrometres each graticule unit is worth can be worked out

-after this is known the graticule can be used as a ruler in the field of view

differential staining meaning

specimens or sections stained with multiple dyes

stain colour examples

-toluidine-blue

-phloroglucinol-red/pink

-for electron microscopes heavy metal compounds commonly used-black and grey 

light microscopes and resolution

-resolution limited by the wavelength of light

-diffracted as light passes close, so waves spread out

-closer they are to the structure the more the light waves overlap each other as they are diffracted

-points closer together than half the wavelength of visible light cannot be clearly distinguished from each other

electron microscopes and resolution

-resolution is much higher because electrons have a smaller wavelength than visible light

-objects past which the electrons travel can therefore be much closer together before the diffracted beams overlap

cell surface membrane

-made up of phospholipids and proteins (bilayer)

-constantly in motion

-controls the exchange of materials between the internal cell environment and the external environment

-partially permeable

-phospholipid bilayer with a diameter of around 10nm

cell wall

-polysaccharide cellulose in plants and peptidoglycan in most bacterial cells

-narrow cytoplasm threads called plasmodesmata connect the ctyoplasm of neighbouring plant cells

nucleus

-contains chromatin, which makes up chromosomes

-chromosomes are made of sections of linear DNA tightly wound around proteins called histones

-separated from the cytoplasm by a double membrane called the nuclear envelope which has many pores

-allow mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes and signalling molecules to travel in

-nucleolus is the site of ribosome production

mitochondria

-double membrane with the inner membrane folded to form cristae

-the matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP

-small circular pieces of DNA and ribosomes are also found in the matrix, which are needed for replication

chloroplasts

-larger than mitochondria but also has a double membrane

-membrane bound compartments called thylakoids stack up to form chlorophyll containing grana

-grana are joined together by lamellae (thin and flat thylakoid membranes)

-light dependent stage of photosynthesis takes place in the thylakoids, and the light independent stage (the calvin cycle) takes place in the stroma

-also contain small pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

ribosomes

-found in RER in eukaryotic cells or freely in all cells

-each is a complex of ribosomal DNA (rna) and proteins

-80s in eukaryotic cells and 70s in prokaryotic cells, mitochondria, and chloroplasts

endoplasmic reticulum

-ROUGH-

-covered in ribosomes

-formed from continuous folds of membranes continuone continuous with the nuclear envelope

-processes proteins made by the ribosomes

-SMOOTH-

-no ribosomes on surface

-involved in the production, processing and storage of lipids, carbohydrates and steroids

Golgi apparatus

-flattened sacs of membrane similar to the smooth endoplasmic reticulum

-modifies proteins and lipids before packaginig them into golgi vesicles, which then transport the proteins and lipids to their required destination

-proteins that go through the golgi apparatus are usually exported ie hormones such as insulin, put into lysosomes ie: hydrolitic enzymes, or delivered to membrane-bound organelles

Vesicles

membrane bound sac for transport and storage

Lysosomes

-specialist forms of vesicles containing hydrolytic enzymes

-break down waste such as worn out organelles

-used largely by cells of the immune system and in apoptosis/programmed cell death

Centrioles

-hollow microtubule fibres

-two at right angles to each other form a centrosome, which organises the spindle fibres during cell division

-not found in flowering plants and fungi

Microtubules

-in all eukaryotic cells

-makes up cytoskeleton of cell about 25nm in diameter

-made of alpha and beta tubulin to form dimers joined into protofilaments-thirteen of those in a cylinder make a microtubule

-cytoskeleton is used to provide support and movement of the celll

Cytoskeleton

• Within the cytoplasm of cells, there is an extensive network of protein fibres

  • This is known as the cytoskeleton

• The cytoskeleton is made up of two main types of protein fibres: microfilaments and microtubules

  • Microfilaments are solid strands that are mostly made of the protein actin. These fibres can cause some cell movement and the movement of some organelleswithin cells by moving against each other

  • Microtubules are tubular (hollow) strands that are mostly made of the protein tubulin. Organelles and other cell contents are moved along these fibres using ATP to drive this movement

• Intermediate filaments (a third type of fibre) are also found within the cytoskeleton

-key functions include maintaining cell shape, anchoring organelles, intracellular transport, and enabling cell motility and division

• The cytoskeleton is important as it has several different functions, including:

• Strengthening and support:

  • The cytoskeleton provides the cell with mechanical strength, forming a kind of 'scaffolding' that helps to maintain the shape of the cell

  • It also supports the organelles, keeping them in position

• Intracellular (within cell) movement:

  • The cytoskeleton aids transport within cells by forming 'tracks' along which organelles can move

  • Examples of this include the movement of vesicles and the movement of chromosomes to opposite ends of a cell during cell division

• Cellular movement:

  • The cytoskeleton enables cell movement via cilia and flagella

  • These structures are both hair-like extensions that protrude from the cell surface and contain microtubules that are responsible for moving them

Microvilli

-found in specialised animal cells

-cell membrane projections

-used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances

Cilia

-hair like projections made from microtubules

-allows the movement of substances over the cell surface

Flagella

-found in specialised cells

-similar in structure to cilia, made of longer microtubules

-contract to provide cell movement for example in sperm cells

Organelles and the production of proteins

-nucleus, dna is stored, the nucleolus manafactures ribosomes, transcription during which an mRNA copy of DNA is produced

-ribosomes, mRNA leaves the nucleus and attached to a ribosome, translation occurs, during which a chain of amino acids is produced, this chain is known as a polypeptide

-RER, many ribosomes attached to it, after translation the polypeptides are folded and processed to produce proteins

-golgi apparatus, proteins are modified and prepared for secretion

-vesicles, prooteins are transported from the RER to the golgi apparatus, and from the golgi apparatus to the cell surface membrane inside vesicles, vesicles fuse with the cell surface membrane to secrete proteins ie:hormones, from the cell by exocytosis

-cells that produce many proteins will have many of the organelles that are involved with protein production

Extrachromosomal dna

genetic material found within a cell but outside its main chromosomes

-in prokaryotic cells plasmids

-in eukaryotic mitochondrial and chloroplast dna

Linear and circular dna

Linear DNA

• Structure: A long, continuous strand with two distinct ends.

• Location: Found in the nucleus of eukaryotic cells (animals, plants, fungi).

• Replication: More complex due to multiple origins and the "end replication problem" at the tips of the chromosomes.

• Protection: Has telomeres at its ends to protect the genetic information.

• Packaging: Highly coiled and compacted around proteins called histones to fit inside the nucleus. 

Circular DNA

• Structure: A closed loop with no beginning or end.

• Location: Found in the cytoplasm of prokaryotic cells and within organelles like mitochondria and chloroplasts.

• Replication: Simpler replication with a single origin of replication.

• Protection: Does not have telomeres.

• Packaging: Does not require complex packing like linear DNA.

• Example: Plasmids, which are small, circular DNA molecules found in bacteria, are a common example.