EUkaryotic Cell Structure
Cells:
- Prokaryotes are single celled organisms with a simple structure and undivided internal area called the cytoplasm.
- Eukaryotic cells have a more complicated structure, with membrane bound organelles.
- Reactions within cells often take place within the cytoplasm.
- The cytoplasm can be divided into many different membrane bound compartments called organelles.
- The separation of different parts of the cell with different functions by using membranes is called compartmentalisation, providing distinct conditions for different processes.
- Membranes are selectively permeable, so are effective barriers in controlling what goes in and out of cells.
- Different organelles have distinct structures and functions.
Organelles in Animal Cells:
- Nucleus: contains DNA which directs the synthesis of proteins required by the cell and in this way controls all the metabolic processes of the cell. The DNA in the nucleus associates with histone proteins which form a complex called chromatin, which condenses into chromosomes which can be seen during cell division.
- Nucleolus: Area within the nucleus which is responsible for producing ribosomes, and is made of proteins and RNA. rRNA is combines with proteins to form ribosomes for proteins synthesis.
- Mitochondrion: an organelle which is the site of cellular respiration, where ATP is produced. They have a double membrane, of which the inner is highly folded (cristae) which increase surface area for respiration. The fluid interior is called the matrix. Mitochondria contains a small amount of DNA called mtDNA.
- Vesicles: Membranous sacs which store and transport substances within the cell. They are a single membrane with a fluid inside.
- Lysosomes: These are specialised versions of vesicles which contain hydrolytic enzymes for breaking down waste material and pathogens in the cytoplasm. In some cells they are important for apoptosis - programmed cell death.
- The Cytoskeleton: present throughout the cytoplasm, a network of fibres necessary for the shape and stability of a cell, also controlling cell movement, and movement within the cell.
- Microfilaments: contractile proteins made from actin, responsible for movement of organelles within the cytoplasm and for cell cleavage during cell division. Their length changes with the addition/removal of monomers, which have to change shape to be added at the minus end, but do not need to change shape to be added at the plus end meaning they are added faster at the plus end, so increase in length faster in one direction. Treadmilling is where monomers are added at one end of the microfilament, but removed at the other end due to the availability of monomers.
- Microtubules: globular tubulin proteins which polymerise to form tubes which create a scaffold like structure, determining the shape of the cell.
- Intermediate Fibres: give mechanical strength to cells.
- Centrioles: not present in flowering plants and fungi, these are composed of microtubules. Two associated centrioles form the centrosome which is involves in the assembly of spindle fibres in division. They also play a role in positioning flagella and cilia.
- Flagella: whip-like structures protruding from a cell which enable cell motility, but can be used as sensory organs.
- Cilia: hair-like structures which can beat rhythmically. They can be stationary too. Each cilium contains two central microtubules, surrounded by 9 other pairs of microtubules.
- Rough Endoplasmic Reticulum: a network of flattened sacs called cisternae which ribosome bind to and is responsible for the synthesis and transport of proteins.
- Smooth Endoplasmic Reticulum: a network of cisternae which are responsible for lipid and carbohydrate synthesis/storage.
- Ribosomes: Made from rRNA molecules, they are the site of protein synthesis and are not surrounded by a membrane.
- Golgi Apparatus: Made of cisternae which play a role in post-translational modification of protein and packaging of proteins into vesicles.
Protein Synthesis and Secretion:
- Proteins are synthesised at ribosomes at the RER.
- Pass into the cisternae and are packaged into transport vesicles.
- Vesicles are moved to golgi apparatus by cytoskeleton.
- Proteins are structually modified before leaving the golgi apparatus in a secretory vesicle.
- Secretory vesicle leaves the cell in exocytosis.