1.2 - Ultrastructure of cells

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Prokaryotic cell structure

first organisms to evolve on Earth

they still have the simplest structure

mostly small in size and found almost everywhere

they have a cell wall outside the cell membrane

protects the cell

maintains its shape

prevents it from bursting

no nucleus is present in a prokaryotic cell

its interior is filled entirely with cytoplasm

there are no compartments

prokaryotes' size is 70 S (Svedberg units)

part of the cytoplasm appears lighter than the rest

this region contains the DNA of the cell

usually one circular DNA molecule

the DNA is not associated with proteins

the lighter area of the cell = nucleoid

contains DNA but isn't nucleus

Cell division in prokaryotes

prokaryotes divide by binary fission

used for asexual reproduction

steps:

the single circular chromosome is replicated

the two copies move to opposite ends of the cell

the cytoplasm divides and the cell divides

each daughter cell has a copy of the chromosome

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Eukaryotic cell structure

more complicate internal structure than prokaryotic cells

eukaryotic cells are compartmentalized

the most important component is the nucleus

it contains the cell's chromosomes

the compartments are known as organelles

each organelle has a distinctive structure and function

advantages in being compartmentalized

enzymes and substrates can be more concentrated than if they were spread throughout the cytoplasm

organelles with their contents can be moved around within the cell

conditions such as pH can be maintained at an ideal level for a particular process

substances that could cause damage to the cell can be kept inside the membrane of an organelle

example: digestive enzymes of a lysosome could digest and kill the cell if they weren't safely stored in the lysosome's membrane

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Nucleus

its membrane is double and has pores

it contains the chromosome

uncoiled chromosomes are spread through the nucleus, they are called chromatin

there are often densely staining areas of chromatic around the edge of the nucleus

the nucleus is where replication and transcription happen,

mRNA is transported through the nuclear pores to the cytoplasm

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Rough endoplasmic reticulum

consists of flattened membrane sacs called cisternae

ribosomes are attached to the outside of the cisternae

the ribosomes arger than in procaryotes and are classified as 80S

the main function of the rER is to synthesize protein for secretion

these proteins

pass into is cisternae

are carried by vesicles

moved to Golgi Apparatus

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Golgi apparatus

consists of flattened membrane sacs called cisternae

like in rER but not as long, often curved, no ribosomes, have many vesicles around

the Golgi apparatus processes proteins borough in by vesicles from the rER

most of the proteins are carried in vesicles to the plasma membrane for secretion

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Lysosome

approximately spherical with a single membrane

formed from Golgi vesicles

contain high concentrations of protein

contain digestive enzymes

used to break down ingested food in vesicles

break down organelles in the cell or the whole cell

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Mitochondrion

a double membrane surrounds it

the inner membranes fold to form structures called cristae

the fluid inside is called the matrix

the shape is variable but is usually spherical or ovoid

ey produce ATP for the cell by aerobic cell respiration

fat is digested here if it's being used as an energy source in the cell

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Free ribosomes

appear as dark granules in the cytoplasm

aren't surrounded by a membrane

they synthesize protein to work in the cytoplasm (as enzymes etc)

constructed in a region of the nucleus called the nucleolus

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Chloroplast

a double membrane surrounds it

inside are stacks of thylakoids

thylakoids are flattened stacks of membrane

the shape is variable but usually spherical or ovoid

they produce glucose and other organic compounds through photosynthesis

starch grains may be present inside if they have been photosynthesizing rapidly

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Vacuoles and vesicles

vacuoles:

consist of a single membrane with fluids inside

many plant cells have large vacuoles (occupy more than half of the cell volume)

some animals absorb food from outside and digest them inside vacuoles

some unicellular organisms use them to expel excess water

vesicles

very small vacuoles used to transport materials inside the cell

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Microtubules and centrioles

microtubules

small cylindrical fibers with a have variety of roles

such as moving chromosomes during cell division

centrioles

consist of two groups of nine triple microtubules

they form an anchor point for microtubules during cell division

and also for microtubules inside cilia and flagella

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Cilia and flagella

whip-like structures projecting from the cell surface

contain a ring of nine double microtubules plus two central ones

flagella re larger and usually only one is present (as in sperm)

cilia are smaller and many are present

cilia and flagella:

can be used for locomotion

also to create a current in the fluid next to the cell

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