for AS Biology

CELL STRUCTURE

Sizes of cells

What kind of scale is this?

What are the effective limits of light microscopy?

What are the effective limits of electron microscopy? What makes the difference?

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cellscells

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prokaryotic prokaryotic cellscells

Prokaryotic cells•include the BACTERIA and the CYANOBACTERIA

•are much smaller than eukaryotic cells (1 – 10 m, as opposed to 10 – 100 m)

•lack a membrane-bound nucleus and other membrane-bound organelles such as mitochondria, chloroplasts, e.r.

•have small (70s) ribosomes (eukaryotic cells have 80s ribosomes)

•contain DNA in the form of a closed loop (the bacterial chromosome or nucleoid

Structure of bacterial cells

Bacterial flagella

Flagella are long hollow threads made of a protein called flagellin, which rotate by means of a "motor" located just under the cytoplasmic membrane, and so propel the bacterium through a liquid medium.Bacteria may have one, a few, or many flagella in different positions on the cell. Peritrichous flagella of

Salmonella

Lophotrichous flagella of Spirillum

Polar flagella of Vibrio cholerae

Bacterial flagella

When flagella rotate anti-clockwise, their actions combine to propel the bacterium is a more or less straight line. In unfavourable conditions these ‘runs’ are more frequent.

When flagella rotate clockwise, their separate actions cause the bacterium to tumble randomly. In favourable conditions ‘runs’ are shorter and ‘tumbling’ more frequent.

For an animation of this behaviour, go to http://www.bact.wisc.edu/MicrotextBook/BacterialStructure/Flagella.html

PiliPili are hollow, hairlike structures made of protein allow bacteria to attach to other cells. A specialized pilus, the sex pilus, allows the transfer of genetic material from one bacterial cell to another. Pili (sing., pilus) are also called fimbriae (sing., fimbria).

Pili of Escherichia coli

The cytoskeleton is a network of fibrous proteins in the cytoplasm, responsible for maintaining the shape of the cell as well as anchoring organelles, moving the cell and controlling internal movement of structures. Microtubules function in cell division and serve as a "temporary scaffolding" for other organelles. Actin microfilaments are thin threads that function in cell division and cell motility. Intermediate filaments are between the size of the microtubules and the actin filaments.

Recall as much as you can about the structure and functions of the cytoskeleton, then click for a summary.

How much did you recall? Read up again if necessary!

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Actin microfilaments consist of two chains of globular monomers entwined around each other (like two entwined strings of beads).

With another protein called myosin, they are responsible for muscle contraction.

Intermediate filaments consist of fibrous proteins entwined like rope. They are important in maintaining the shape of cells, and in holding neighbouring cells together.

Microtubules are hollow cylinders with walls made of a helix of tubulin dimers. They lengthen or shorten by adding or subtracting dimers. Microtubules form the spindle that moves chromosomes during cell division, and form ‘pathways’ for the movement of vesicles in cells. Special arrangements of microtubules in cilia and flagella can propel cells, or move substances over cells (next slide)

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In cilia and flagella nine fused pairs of microtubules surround a central unfused pair. ‘Arms’ made of a protein called dynein ‘walk’ up adjacent microtubules, thus bending the cilium or flagellum.

This electron micrograph shows a section of a cluster of cilia on the surface of an

epithelial cell (e.g. in the trachea or oviduct).

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Endoplasmic reticulumis a network of interconnected membranes involved in the synthesis, modification and transport of proteins and other cell products.

Rough endoplasmic reticulum (RER) is so-named because of its rough appearance due to the numerous ribosomes that occur along the ER. Rough ER connects to the nuclear envelope through which the messenger RNA (mRNA) that is the blueprint for proteins travels to the ribosomes.

The membranes and cisternae (cavities) of RER are more flattened than those of SER.

Smooth ER lacks the ribosomes characteristic of Rough ER and is thought to be involved in transport and modification of lipids. Smooth ER is usually more tubular than RER.

Cisternae

Rough endoplasmic reticulum

is concerned with the synthesis and

packaging of proteins, especially those which

are to be secreted from the cell. This

secretion occurs via the

Golgi apparatus

Proteins synthesised by ribosomes on the r.e.r. are ‘packaged’ in transport vesicles

The transport vesicles fuse with the cis

portion of the Golgi

apparatus: proteins pass

through the medial Golgi

where they are modified, and are

then released in Golgi vesicles from the trans

Golgi.

Golgi vesicles may contain proteins for

secretion: these secretory vesicles fuse with the cell

surface membrane and release their

contents in the process called

exocytosis.

Other Golgi vesicles may contain proteins for use inside the cell: lysosomes are formed in this way.

Golgi apparatus of a plant cell

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Polyribosomes: all of the ribosomes in any one cluster are ‘reading’ a single strand of messenger RNA

Electron micrograph of rough endoplasmic reticulum

Membrane of r.e.r.

Cisterna

Ribosomes on e.r.

surface

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SMOOTH ENDOPLASMIC RETICULUM… is thought to be involved in modifying proteins made by the r.e.r., and possibly other products such as lipids.

Cisterna of smooth e.r.

A peroxisome: reactions that produce toxic peroxides (such as hydrogen peroxide, H2O2) occur here. Peroxisomes contain enzymes that quickly decompose the peroxides to harmless products. They are formed as very small vesicles (microbodies) budding from the e.r.

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Membrane structure is based on the AMPHIPATHIC properties of PHOSPHOLIPIDS:

This end of the molecule is charged and

therefore ‘water-loving’

(HYDROPHILIC)

This end of the molecule is uncharged and therefore ‘water-hating’ (HYDROPHOBIC)

… so in water phospholipid molecules organise themselves with ‘heads’ outwards, facing the water,

and ‘tails’ inwards away from the water ...

This gives rise to the PHOSPHOLIPID BILAYER that is the basis of all biological membranes:

A single phospholipid molecule

Hydrophobic centre

Hydrophilic heads in contact with watery medium

A pure phospholipid bilayer would be very fluid. In real membranes CHOLESTEROL molecules are inserted into the hydrophobic centre as

‘stiffening’: the more cholesterol a membrane contains, the less fluid it is.

INTRINSIC PROTEINS may be embedded in the bilayer, or may span it completely:

Amino acids with hydrophilic side chains here

Amino acids with hydrophobic side chains here

Such protein molecules may be ‘hollow’, forming HYDROPHILIC CHANNELS through which water molecules and water-soluble substances may pass:

Other membrane proteins may act as

TRANSPORT PROTEINS such as

SYMPORTS ...

…whilst GLYCOPROTEINS are often important as RECEPTORS or MARKERS on the outer face of cell surface membranes.

Membrane proteins also include membrane-bound ENZYMES.

GlucoseNa+

SubstrateHormone molecule ‘recognising’ glycoprotein receptor

Same membrane a millisecond later:

This is the FLUID MOSAIC MODEL of membrane structure.

This false-colour electron micrograph shows two opposing nerve cell membranes at a synapse. Each membrane appears as a pair of blue- or mauve-filled ‘tramlines’. The magnification of the micrograph is x436,740. Estimate the thickness of a single membrane in nm.

(NB: before you measure the on-screen thickness of the membranes, use the 1 cm arrow to find how much your monitor is magnifying or diminishing the micrograph, and allow for this in your calculations.)

1 cm

Cytoplasm of cell A

Cytoplasm of cell B

Membranes

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MitochondriaMore …

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This electron micrograph appears on the page at a magnification of x 80 000. The arrow beside it is 1 cm long when the page is seen at its natural size.

What is the actual length of the mitochondrion shown?

What metabolic process takes place in mitochondria?

What is the ENDOSYMBIOTIC THEORY of mitochondrial origins, and what is the evidence for it?

Outline the nature and the functions of the following:

• mitochondrial matrix

• cristae

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Length of mitochondrion in e.m.

• the exact length of the on-screen image will depend on your monitor, but you should have found it to be 90 - 95 mm; suppose you found it to be 95 mm

• image length = 95 mm = 95,000 m

• if image is 80 000 x actual size, then actual length = 95,000 / 80,000

• 95,000/80,000 = 95/80 = 1.1875 = 1.2 m to two significant figures

Cristae

• infoldings of inner membrane, increasing surface area

• site of ELECTRON TRANSPORT CHAIN, by which electrons removed during oxidation of food molecules are passed finally to oxygen, producing water

• most of the useful energy liberated in respiration is made available here

• details will be filled in later, in A2 Unit 4

Mitochondrial matrix

• aqueous solution of enzymes and metabolites

• site of KREBS (TCA) CYCLE, by which small molecules derived from anaerobic respiration in the cytosol are oxidised to CO2; the electrons removed are fed into the electron transport chain on the cristae

Endosymbiotic theory• Proposes that mitochondria (and chloroplasts) originated as aerobic

prokaryotic cells engulfed by the anaerobic ancestors of eukaryotic cells

• instead of being digested the engulfed cells survived inside the larger cells, which used the energy they released; eventually the relationship evolved to the state of mutual dependence

Evidence:

• mitochondria contain their own DNA (termed mDNA), which like bacterial DNA is organised in a single closed loop

• mitochondria contain their own ribosomes, which are the 70s (bacterial) type rather than the larger ribosomes found in eukaryotes

• the double membrane is consistent with an endosymbiotic origin: