Cells

Eukaryotes = everything else (fungi, plants, animals) (eu = true)

Fluid-mosaic membranes

• found around all cells
• found around organelles and as an internal membrane structure surrounding fluid-filled sacs in eukaryotes
• currently accepted model (based on EM evidence) = Singer Nicholson fluid mosaic model (1972)
• comprises a fluid phospholipid component with a mosaic of proteins embedded in it


• Phospholipids are a bilayer with hydrophobic tails towards each other
• glycolipids (lipids with a carbohydrate attached - carbohydrate projects outwards - i.e. always found in outer layer) also present in cell surface membranes
• cholesterol also present
• proteins are of two types:
• intrinsic - span the whole membrane
• extrinsic - found in one half only

Function of membranes

• forms a selective barrier: controls movement of substances in and out

Prokaryotes

Structure of Bacterial Cell and its inclusions (illustrated by E. coli)

no internal membranes (e.g. nuclear membrane)

no organelles

do contain: DNA, ribosomes (smaller than in eukaryotes [70S rather than 80S]), cytoplasm

size range:

length 1 to 10 µm

diameter 1 µm

cell wall

• made of murein (a peptidoglycan - polysaccharide chains linked by short peptides)
• some bacteria have a capsule or slime layer - basically a peptidoglycan goo - outside the cell wall - provides protection from phagocytosis

cell surface (plasma) membrane

• may be infolded (invaginated) to perform functions similar to those of organelles in eukaryotes
• e.g. photosynthetic pigments and enzymes found on invaginations called thylakoids
• aerobic respiration enzymes

• long chains of globular protein flagellin
• rotates (like a propeller)
• provides motility
• may be single, few or all over
• some have pili (similar but shorter - 1 nm rather than 5 µm)

• single, circular strand of ds DNA

• small circular strands of DNA
• contain genes which confer antibiotic resistance
• transferable between bacteria

• type of inclusion
• storage structure for glycogen

• type of inclusion
• storage structure for lipids



• identify the above structures in an electron micrograph

Eukaryotes

illustrated by leaf palisade cell and liver cell

• visible under light microscope
• appears as dense spherical
• appears as surrounded by single membrane
• under EM - seen to have 2 membranes = nuclear envelope
• outer membrane continuous with ER
• nuclear pores present - point of exit of mRNA
• nucleus contains nucleoplasm - jelly like - contains chromosomes
• chromosomes - DNA associated with protein histones
• normally appears as amorphous chromatin
• chromosomes only visible during cell division
• function of nucleus - regulates proteins synthesis thus controls cell activity

• circular
• granular
• site of rRNA manufacture
• site of ribosome assembly

• continuous with the outer membrane of the nuclear envelope
• made up of network of flat sacs = cisternae
• Rough ER bears ribosomes
• Rough ER transports proteins
• Smooth ER synthesises lipids
• Particularly prominent in lipid producing cells and in cells that produce steroid hormones
• liver cells convert glucose to glycogen up to a maximum amount of glycogen. If the maximum is exceed the excess glucose is converted to lipid

• similar in structure to ER
• vesicles pinch off the RER and fuse with Golgi
• vesicles pinch off Golgi and fuse with plasma membrane
• function - to modify proteins synthesised by ribosomes by adding carbohydrates to them to form glycoproteins
• also transport lipids around
• also form lysosomes

• membrane-bound vesicles containing digestive enzymes
• destroy (and allow recycling of) old organelles

• 2-5 µm in length
• 1 µm in diameter
• only in plants
• disk shaped
• contain thylakoids aka lamellae - flattened membrane-bound sacs
• on the thylakoid membrane are the pigments (e.g. chlorophyll) of photosynthesis
• several thylakoids stacked together = granum
• stroma contains the enzymes of light-independent reactions
• surrounded by a double membrane
• have their own circular DNA

• 7 µm in length
• 1 µm in diameter
• double membrane
• inner membrane highly infolded to form cristae
• liquid component = matrix
• matrix contain the enzymes of TCA or Krebs Cycle
• cristae bear stalked or elementary particles involved in the synthesis of ATP
• contains own circular DNA

ribosomes

• small structures made up of protein and rRNA
• site of protein synthesis
• found in association with RER - makes proteins destined to be exported from cell
• found free in cytoplasm - make proteins used within the cell e.g. enzymes

microtubules

• long thin, straight tubes made of protein = tubulin
• make up the cytoskeleton
• are constantly being extended and shortened by addition or removal of tubulin
• give the cell shape
• involved in transport of granules and vesicles
• make up the spindle so move chromosomes during cell division

centrioles

• animal cells (and fungi)
• made of microtubules
• found in pairs - lie at right angle to each other
• involved in formation of spindle during cell division
• when the centrioles have microtubules radiating outwards from them the structure is known as a centrosome
• plants have no visible centrioles but do produce a centrosome-like structure (i.e. a structure with radiating microtubules)

 

• found in plants (fungi and bacteria have different types of cell wall)
• rigid - give support and protection
• resist expansion of cell when water enters so prevents bursting
• made of cellulose microfibrils embedded in a matrix of other polysaccharides including pectin and hemicelluloses
• contain pores called plasmodesmata
• cytoplasm is continuous between adjacent cells via the plasmodesmata - gives movement of materials between cells
• cell surface (plasma) membrane



• identify the above structures in light and electron micrographs

Magnification

• how much larger than actual the image appears
• best light microscopes have maximum mag of 3000x
• can increase this indefinitely but resolution is lost (maximum resolution = 200nm)
• because of wavelength of light (uses 550nm)

Resolution

• how close two objects can be together and still be distinguished
• beam of electrons has wavelength of 0.006nm (6pm)
• maximum resolution is 0.2 nm
• maximum magnification is 100,000x

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