1.2 - Ultrastructure of cells

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

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

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

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

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

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

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

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

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

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

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

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