Ultrastructure of Cells Flashcards
What is ultrastructure?
Features that can be seen using an electron microscope
What does metabolism involve?
Vital chemical reaction requiring enzymes and specific conditions - synthesis and breakdown of molecules and the reactions take place in the cytoplasm
What is cytoplasm made up of?
Cytosol - water, salts, organic molecules
How is cytoplasm separated from external environment?
By a cell surface membrane and in eukaryotic cells the cytoplasm is divided into different membrane bound compartments known as organelles that have a variety of functions
Function of membranes?
They are selectively permeable and control the movement of substances into and out of the cells and organelles - effective barriers which control which substances enter and exit cells ; THEY ARE FRAGILE
What does the nucleus contain?
Coded genetic information in the form of DNA molecules - which directs the synthesis of all proteins required by the cell and thus controls all metabolic activities in the cell as many of the proteins are the enzymes necessary for metabolism to take place
Describe the membrane of the nucleus
A double membrane called a nuclear envelope which protects it from damage in the cytoplasm - nuclear envelope contains nuclear pores that allow molecules to move into and out of the nucleus (DNA too large to leave via pores therefore transcribed into mRNA which are then exported)
How is the DNA arranged as in the nucleus?
It associates with proteins called histones to form a complex called chromatin - this coils and condenses to form chromosomes which only become visible when cells prepare to divide (spindle fibres - centrioles mitosis)
What is the nucleoli?
It is within the nucleus and is responsible for producing ribosomes - composed of proteins and RNA ; RNA is used to produce ribosomal RNA which combines with proteins to form the necessary ribosomes for protein synthesis
Function of mitochondria
They are the site of the final stages of cellular respiration where the energy stored in bonds of complex organic molecules is made available to use by the production of ATP - number of mitochondria in the cell reflects the amount of energy it uses therefore very active cells usually have a lot of mitochondria
Structure of mitochondria
Double membrane - inner membrane is highly folded to form cristae structures and the fluid interior is called the matrix
Cristae
INNER MEMBRANE of mitochondria which is highly folded - Membrane forming cristae contain enzymes used in aerobic respiration
Mitochondrial DNA?
Mitochondria contain mtDNA (small amount of DNA) which allows them to produce their own enzymes and reproduce themselves
What are vesicles?
Vesicles are membranous sacs (with fluid inside) that have storage and transport roles - THEY CONSIST OF A SINGLE MEMBRANE (transport materials inside the cell)
What are lysosomes?
Specialised forms of vesicles that contain hydrolytic enzymes that are responsible for breaking down waste material in cells including old organelles ; they play an important role in the immune system as they are responsible for breaking down pathogens ingested by phagocytic cells - PLAY AN IMPORTANT ROLE IN APOPTOSIS (programmed cell death by excessive production of lysosomes)
What are hydrolytic enzymes?
They help in catalysing hydrolysis - use water to break chemical bonds (larger molecules into smaller molecules) - break down old organelles and are responsible for breaking down pathogens (phagocytosis) - play a big role in apoptosis
What is phagocytosis?
Process by which a cell uses it’s membrane to engulf a large particle (including pathogens)
Function of cytoskeleton
Network of fibres essential for shape and stability of a cell - organelles are held in place and it is responsible for cell movement/movement of organelles within cells
3 components of cytoskeleton
Microfilaments
Microtubules
Intermediate fibres
What are microfilaments?
Contractile fibres made of the protein actin - responsible for cell movement and cell contraction during cytokinesis (pinching together during mitosis)
What are microtubules?
Globular tubulin proteins polymerise to form tubes that are used to form a scaffold structure that determines the shape of the cell - act as tracks for the movement of organelles around the cell (including vesicles)
What are spindle fibres made of?
They have a role in physical segregation of chromosomes in cell division - they are composed of microtubules
What are intermediate fibres?
They give mechanical strength to cells and help maintain their integrity
What are centrioles?
A component of the cytoskeleton present in most eukaryotic cells with the exception of flowering plants and most fungi - they are composed of microtubules
What is a centrosome?
Two associated centrioles form centrosome - which is involved in the assembly and organisation of spindle fibres during cell division
Relationship between flagella/cilia and centrioles?
Centrioles are thought to play a role in positions flagella and cilia
Flagella and cilia
They are both extensions (protuberances) - flagella are longer than cilia but cilia are usually present in much greater numbers
Function of flagella?
They enable cells motility - used as a sensory organelle detecting chemical changes in the cell’s environment
Types of cilia
Mobile or stationary
Stationary cilia
Present on cell surface and have an important function in sensory organs such as the nose
Mobile cilia
They beat in a rhythmic manner creating a current - causes fluids and objects adjacent to the cell to move ; they are present in the trachea to move mucus away from the lungs (helping keep air passages clean) AND IN FALLOPIAN TUBES TO MOVE EGG CELLS FROM OVARY TO UTERUS
Cross section of each cilium
Two central microtubules - surrounded by 9 pairs of microtubules arranged like a wheel ; known as the 9+2 arrangement
How do the cilia move?
Pairs of parallel microtubules slide over each other causing the cilia to move in a beating motion
What organelles are linked in protein synthesis and secretion?
Ribosomes, ER, Golgi apparatus ; cytoskeleton coordinates protein synthesis
What is the Endoplasmic Reticulum?
Network of membranes enclosing flattened sacs called cisternae
Function of Smooth ER?
Responsible for lipid and carbohydrate synthesis (as well as steroid production/storage
Function of rough ER?
Ribosomes bound to the surface and responsible,e for the synthesis and transport of proteins
What do secretory cells (release hormones/enzymes) have?
They have more rough ER than cells that do not release proteins
Structure of ribosomes
Can be free floatin pg in cytoplasm or attached to ER (forming rough ER) - they are NOT SURROUNDED BY MEMBRANE ; constructed of RNA molecules made in the nucleoli of the cell and ribosomes are the site of protein synthesis
Mitochondria and chloroplasts contain ribosomes as do prokaryotic cells
Structure of Golgi Apparatus
Similar in structure to the Smooth ER and is a compact structure formed of cisternae that does not contain ribosomes - MODIFIES AND PACKAGES PROTEINS into vesicles :
1) Secretory vesicles - if proteins are destined to leave the cell
2) Lysosomes - stay in the cell
Protein synthesis stages
1) Proteins are synthesised on ribosomes bound to the ER
2) Pass into its cisternae and packaged into transport vesicles
3) Vesicles containing the proteins move towards the Golgi Apparatus via the transport function of the cyroskeleton
4) Vesicles fuse to the cis face of the Golgi apparatus and the proteins enter ; they are structurally modified before leaving the Golgi apparatus in vesicles from the trans face
5) Secretory vesicles carry proteins that are to be released from the cell ; they fuse with the cell surface membrane and release their contents by excocytosis ; some may form lysosomes which contain enzymes for use in cell
Cell wall features
Freely permeable - substances can pass into and out of the cell through the cell wall
Cell contents press against cell wall making it rigid
Defence mechanism - protects against invading pathogens
Vacuoles in plant cells vs animal cells
Plant - large and permanent
Animal - small and transient
Role of vacuole in plant cells
Maintain turgid pressure (contents push against cell) - membrane of vacuole is called tonoplast (selectively permeable allowing some molecules through)
Membrane in chloroplasts
Double membrane structure like mitochondria
What is fluid enclose in chloroplast called?
Stroma
Describe thylakoids
Internal network of membranes in chloroplast which form flattened sacs called thylakoids - many thylakoids = granum
Grana (several granum) are joined by lamellae
Importance of grana in chloroplasts
Chlorophyll pigments where light dependent reactions occur during photosynthesis
Like mitochondria… (endosymbiosis theory)
Chloroplasts also contain circular DNA (like prokaryotes), both have double membrane and ribosomes allowing them to make their own proteins (endosymbiosis theory - once they were prokaryotic microbes)
Mitochondria and chloroplasts are same size and both divide by binary fission
What is purpose of internal membranes in chloroplasts?
Internal membranes provide large SA for photosynthesis
2 evolutionary domains of unicellular organisms
Archae
Bacteria
Prokaryotic organisms 3.5 billion years ago
Adapted to living in extremes of ph and temperature - extremophiles and can be found in hydrothermal vents (archae)
Describe prokaryotic organisms
Unicellular
DNA not within nucleus
None of their organelles are membrane bound
DNA in prokaryotes
Same as in eukaryotes but packaged differently - prokaryotes only have one chromosome supercoiled to make compact and genes on chromosome are grouped into operons ; genes switched on or off at same time
Ribosomes in prokaryotes
SMALLER - 70S (composed of 30 and 50 subunits)
80S (eukaryotic ribosome) involved in MORE complex protein synthesis
Prokaryotic cell wall
Made of peptidoglycan (murein) - polymer of sugars and amino acids
Flagella in prokaryotes
Thinner than in eukaryotes and no 9+2 arrangement - energy to rotate filament is supplied from process of chemiosmosis (not from ATP like in eukaryotic cells)
Structure of flagella
Attached to bacterium by basal body and rotated by a molecular motor which causes hook to rotate creating a whip like propelling movement
Describe endosymbiosis theory
Mitochondria and chloroplasts were formerly free living bacteria (prokaryotes) and these were taken inside another cell (ingested) as an organism that lives within a cell leaving to evolution of eukaryotic cells
Evidence for endosymbiosis
Mitochondria (have their own double membrane)
Reproduction occurs via binary fission
Own DNA for self replication
Similar size to prokaryotes
They have own ribosomes which are 70S - same as prokaryotic ribosomes
They have extra chromosomal DNA like plasmids
Fungi cell wall
Chitin
