cell structure Flashcards
nucleus structure
- nuclear envelope is double membrane that surrounds the nucleus. Its outer membrane is continuous with the endoplasic reticulum of the cell and often has ribosomes on its surface. Controls the entry and exit of materials in and out of the nucleus and contains reactions taking place within it
- nuclear pores allow the passage of large molecules eg mRNA, out of the nucleus
- nucleoplasm- granular, jelly-like material that makes up bulk of nucleus
-chromosomes consist of protein bound, linear DNA - nucleolus- small spherical region within the nucleoplasm. Manufactures ribosomal RNA and assembles ribosomes.
functions of the nucleus
- act as control centre of the cell through production of mRNA and tRNA and hence protein synthesis
- retain the genetic material of the cell in the form of DNA and chromosomes
- manufacture ribosomal RNA and ribosomes
mitrocondria
- around organelle is double membrane that controls entry and exit of material. The inner of the two membranes is folded to form extensions known as cristae
-cristae are extensions of the inner membrane, which in some species extend across the whole width of the mitrocondrion. Provide a large surface area for the attachment of enzymes and other proteins involved in respiration - matrix makes up remainder of the mitrocondia. Contains proteins, lipids, ribosomes and DNA that allows mitrocondria to control the production of some of their own proteins. Many enzymes involved in resperation are found in the matrix
function of mitrocondria
- site of aerobic stage of resperation (Krebs cycle and oxidative phosphorylation pathway). Responsible for production of energy- carrier molecule, ATP, from respiratory substrates such as glucose
chloroplasts
-chloroplast envelope- double plasma membrane that surrounds organelle. Highly selective to what it allows to enter and leave the chloroplast
- grana are stacks of disk-like structures called thylakoids. Within thylakoids is chlorophyll. Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana. Grana is where light absorption takes place
- stroma: fluid- filled matrix were second stage of photosynthesis takes place. Within stroma there are a number of structures
how are chloroplasts adapted to function:
- granal membranes provide a large surface area for the attachment of chlorophyll electron carriers and enzymes that carry out the first stage of photosynthesis.
- fluid of stroma possesses all enzymes needed for second stage of photosynthesis
- contain both DNA and ribosomes can can quickly manufacture proteins needed for photosynthesis
resolution
Minimum distance two points can be separated and still distinguished as two separate points
magnification
how many times larger an image is when compared to the actual size of the object
magnification equation
Image
Actual x Magnification
cell fractionation
-process when cells are broken up and separated into major organelles
what features does the solution have that the cells are placed in
ice cold- reduce enzyme activity that may break down organelles
pH buffered- so pH doesn’t fluctuate as any change in pH may alter structure of organelles and enzymes.
isotonic- prevent organelles shrinking or bursting as a result of the osmotic entry of water
steps of cell fractionation
- cells undergo homogenisation- breaks up cells to release organelles
resultant fluid then FILTERED to remove any large pieces of debris - span in centrifuge. Large centripetal force draws organelles to bottom of the pellet.
Heaviest organelles eg nuclei forced to bottom of tube where they form pellet/ sediment first - the supernatant then poured into fresh tube and process is repeated, increasing the speed of the centrifuge each time as the required organelles become lighter
TEM
- electron gun that produces a beam of electrons that is focused onto specimen by electromagnet. Beam passes through a thin section of the specimen. Parts of specimen absorb electrons and appear dark, other parts allow electrons to pass through so appear bright. An image is produced and can be photographed to give photomicrograph.
Resolving power of TEM is 0.1 nm, cant always be achieved as: - difficulties preparing specimen limit the resolution that can be achieved
- higher energy electron beam that is required may destroy specimen
Main limitations: - whole system must be in vacuum so living specimens cannot be observed
- complex staining process
- black and white image
-specimen must be extremely thin - image may contain artefacts. These are things that result from the way the specimen was prepared
SEM (scanning electron microscope)
-same limitations as TEM, but doesnt need to be extremely thin as electrons dont penetrate.
-SEM directs a beam of electrons onto the surface of the specimen from above rather than penetrating from below. Beam is then passed back on forth across a portion of the specimin in a regular pattern. Electrons are scattered by the specimen. Can built up a 3D image of the scattered electrons
-has lower resolving power than TEM but still more than light
structure of endoplasmic reticulum
-elaborate 3D system of sheet-like membranes spreading throughout cytoplasm. Continuous with outer nuclear membrane. Membranes contain a closed network of tubules and flattened sacs called cristae
Rough ER- ribosomes present on outer surface
smooth ER- lacks ribosomes on surface and more tubular in appearance
function of RER
- provide large surface area for synthesis of proteins and glycoproteins
-provide pathway for the transport of materials, especially proteins throughout the cell
functions of SER
- synthesise. store and transport lipids and carbohydrates
golgi aparratus structure
- stack of membranes that make up flattened sacs, or cisternae, with small round hollow structures called vesicles.
golgi function
The proteins and lipids produced by ER are passed through golgi in strict sequence. Golgi modifies these proteins. Also labels them allowing them to be accurately sorted. Once sorted the modified proteins and lipids are transported to the Golgi vesicles which are regularly pinched off from the ends of golgi cisternae. These vesicles may move to the cell surface, where they fuse with the membrane and release their contents to the outside of the cell
main functions;
- add carbohydrates to proteins to form glycoproteins
- produce secretory enzymes, eg those produced from pancreas
- secrete carbohydrates
- transport, modify and store lipids
- form lysosomes
lysosome structure
- formed when vesicles produced by Golgi contain enzymes such as proteases and lipases
- contain lysozymes, enzymes that hydrolyse the walls of certain bacteria.
- glycoprotein coat
- sac surrounded by single membrane
lysosome function
- release enzymes to outside of cell, exocytosis, in order to destroy material around the cell
- completely break down cells after they have died (autolysis)
- hydrolyse material ingested by phagocytic cells, eg wbc and bacteria
ribosomes
- small cytoplasmic granules. May be in cytoplasm or RER
-ribosomes have two subunits- one large one small
80S- found in eukaryotes
70S- found in prokaryotic cells, mitochondria and chloroplasts- slightly smaller
-protein synthesis via translation occurs here
cell wall structure
- only found in plant cells
- consists of microfibrils of cellulose. These have considerable strength so contribute to overall strength of cell wall
- also consist of a number of polysaccharides
- thin layer, called middle lamella, which marks boundary between adjacent cell walls and cements adjacent cells together
ALGAE CELL WALLS- cellulose or glycoproteins or mixture of both
FUNGI CELL WALLS- no cellulose, only CHITIN - nitrogen containing polysaccharide, also glycan and glycoproteins
BACTERIA CELL WALL- MUREIN
functions of cell wall
- provide mechanical strength to prevent cell bursting under pressure caused by osmotic entry of water
- give mechanical strength to plant as a whole
- allow water to pass along it and so contribute to the movement of water through the plant
vacuole structure
- fluid- filled sac bounded by a single membrane- tonoplast.
-contains solution of mineral salts, sugars, amino acids, wastes and sometimes pigments such as anthocyanins - within mature plants normally one large, central vacuole
vacuole function
- sugars and amino acids act as a temporary food store
- pigments may colour petals to attract pollinating insects
- support herbaceous plants, and herbaceous parts of woody plants by making cells turgid
differences in prokaryotic and eukaryotic DNA
Prokaryotic:
- No true nucleus
- DNA not associated with histones
- some DNA in form of plasmids (circular)
- no chloroplasts
- smaller 70 S ribosomes
-murein cell wall
- dont contain membrane bound organelles
- may have outer mucilaginous layer called capsule
role of plasmids
- possess genes that aid survival of bacteria in adverse conditions, eg produce enzymes that break down antibiotics. Can reproduce themselves independently
