1.2 Ultrastructure of cells Flashcards
Prokaryotic cell structure
- simple cell structure, generally small in size
- cell membrane + cell wall
- no nucleus, only cytoplasm
- divide by binary fission
- only organelles are ribosomes
- DNA is found in a single stranded loop in the nucleod
Cell division in prokaryotes (binary fission)
- Single strand of DNA is replicated
- Two copies move to opposite poles of the cell
- Division of cell and cytoplasm occurs
- Two genetically identical daughter cells are made
Eukaryotic cell structure
- much more internally complex than prokaryotes
- compartmentalised, many compartments made up of single or double membranes (organelles)
- the nucleus (compartment) holds most of the cells chromosomes
Advantages of being compartmentalised
- Enzymes and substrates for a particular process can be much more concentrated than if they were spread throughout the cytoplasm
- Substances that could cause damage to the cell can be kept inside the membrane of an organelle, for example, digestive enzymes of a lysosome could digest and kill a cell, if they were not safely stored inside the lysosome membrane
- Organelles with their contents can be moved around within the cell
- localised conditions - PH and other factors can be kept at optimal levels. the optimal pH level for one process in one part of the cell
Nucleus
the nucleus has a double membrane with pores through it. The nucleus contains the chromosomes. Uncoiled chromosomes are spread through the nucleus called chromatin. There are often dense areas of chromatin around the edge of the nucleus. The nucleus is where DNA is replicated and transcribed to form mRNA. The nucleus controls cell activities.
Rough endoplasmic reticulum
the RER consists of flattened membrane sacs known as cisternae. Attached to the outside of these cisternae are ribosomes. The main function of the RER is to synthesize protein for secretion. It also modifies protein. Synthesised protein passes into the RER’s cisternae and is then carried by vesicles which bud off and are oved to the Golgi apparatus, used outside of the cell.
Golgi Apparatus
This organelle is made of flattened membrane sacs called cisternae, similar to the RER, however, these cisternae are not as long, are often curved, do not have attached ribosomes and have many vesicles nearby. The main function of the Golgi apparatus is to process and modify proteins (by adding carbohydrate or lipid groups) brought in by vesicles from the RER. These modified proteins become glycoproteins or lipoproteins. Most of these proteins (from the lower layer) are then carried in vesicles to the plasma membrane for secretion.
Lysosomes
Lysosomes are spherical with a single membrane. They are formed from Golgi vesicles. They contain high concentrations of proteins. They contain digestive enzymes, which can be used to break down ingested food in vesicles or break down organelles in the cell or even the whole cell. They also secrete outside of the cell and cause autolysis (cell suicide)
Mitochondrion
a double membrane surrounds mitochondria, the inner of these membranes turned in on itself to create structures called cristae. The fluid inside is called the matrix. The shape of mitochondria is variable but is usually spherical or ovoid. They produce ATP for the cell by aerobic respiration. Fat is digested here if it is being used as an energy source in the cell.
Free ribosomes (80S)
These appear as dark granules in the cytoplasm and are not surrounded by a membrane. They are the same size as ribosomes attached to the RER. Their function is to synthesise protein, releasing it to work in the cytoplasm, as enzymes or in other ways. Ribosomes are constructed in a region of the nucleus called the nucleolus. They are made of RNA + protein, split into two sub-units.
Vacuoles and Vesicles
these are organelles that consist of simply a single membrane with fluid inside. Many plant cells have large vacuoles that occupy more than half of the cell volume. Some animals absorb foods from outside and digest them inside vacuoles. Some unicellular organisms use vacuoles to expel excess water. Vesicles are very small vacuoles used to transport materials inside the cell.
Centrioles
In the cytoplasm of cells there are small cylindrical fibres called centrioles (made of microtubules) that have a variety of roles, including moving chromosomes during cell division and forming spindle fibres. Animal cells have structures called centrioles, which consist of two groups of nine triple microtubules. Centrioles form an anchor point for microtubules during cell division and also for microtubules inside cilia and flagella.
Cilia and Flagella
These are whip-like structures projecting from the cell surface. They contain a ring of nine double microtubules plus two central ones. Flagella are larger and usually only one is present. Cilia are smaller and many are present. Cilia and flagella can be used for locomotion. Cilia can be also used to create a current in the fluid next to the cell.
Microtubules
- internal cellular rail tracks to transport vesicles within the cell
- cumulatively form a support structure for the cell
- allow ‘amoeboid’ cellular locomotion
Nucleolus
Where ribosomes are produced
Contains DNA + RNA
