Cell Structure All subtopics

To increase contrast between cell structures / to make specific structures visible

To prevent air bubbles forming between slide and coverslip; air bubbles obscure the specimen / prevent clear focusing

Clean continuous single lines / no shading; correct proportions of cells; ruled label lines touching structures; only visible structures drawn (any 2)

Plan diagram shows distribution of tissues only / no individual cells; cell drawing shows individual cells with cell walls and visible contents

Converting the image size to micrometres $$25\text{ mm}=25 000\text{ µm}$$ Finding the magnification $$M=\frac{\text{image size}}{\text{actual size}}=\frac{25 000}{5}$$ $$M=\times 5000$$

Converting the image size to micrometres $$12\text{ mm}=12 000\text{ µm}$$ Finding the actual size $$A=\frac{\text{image size}}{M}=\frac{12 000}{3000}$$ $$A=4\text{ µm}$$

Eyepiece graticule has arbitrary divisions / no fixed real size; stage micrometer has divisions of known length; calibration converts eyepiece units into real units (µm)

Finding the real length of the matched divisions $$20\times 10=200\text{ µm}$$ Finding the length of one eyepiece division $$\frac{200}{80}=2.5\text{ µm}$$

Higher objective magnifies the specimen more / changes the field of view; one eyepiece division now represents a smaller real distance

The smallest distance between two points that can be distinguished / seen as separate

Magnification is how many times larger the image is than the actual specimen; resolution is the smallest distance between two points that can be seen as separate

Electrons have a much shorter wavelength than visible light; this gives higher resolution / allows smaller structures to be distinguished as separate points; useful magnification is therefore much greater (e.g. ×500 000 vs ×1500); ultrastructure such as ribosomes and membranes can be seen; limitation – specimens must be dead / in a vacuum / cannot view living cells; specimens require complex preparation / staining with heavy metals / risk of artefacts; images are black and white, not coloured (any 6)

mitochondrion / mitochondria

synthesises proteins / proteins for secretion; large surface area provided by stacked cisternae; ribosomes attached to the membrane translate mRNA; vesicles bud off to transport proteins to the Golgi body

receives vesicles containing protein from the RER; modifies the protein (e.g. adds carbohydrate to form glycoprotein); sorts and packages protein into secretory vesicles; vesicles move to and fuse with the cell surface membrane releasing protein by exocytosis

double membrane / envelope; stacks of thylakoids / grana; presence of stroma; (any two)

RER has ribosomes attached to its membrane appearing as dark dots; SER lacks these ribosomes giving a smoother appearance

chloroplasts; cellulose cell wall; large permanent vacuole / tonoplast; plasmodesmata; (any three)

spindle microtubules are organised by other microtubule-organising centres in plants; spindle still forms during mitosis without centrioles

ATP / adenosine triphosphate