cell structure

• magnification

  • how many times bigger the image is when compared to the object
  • $magnification=$$\frac{image\ size}{actial\ size}$
• resolution

  the ability to distinguish between 2 points of an image
• cell fractionation


  this is the process where cells are broken up and the different organelles they contain are separated out

  before this process begins, the tissue is placed in a solution of the same water potential as the tissue. the solution is:

  • cold - to reduce enzyme activity that may break down organelles
  • isotonic - to prevent organelles bursting or shrinking due to osmosis
  • buffered - to keep the pH the same

  there are 2 stages to cell fractionation:

  • homogenation
  • ultracentrifugation
• homogenation (stage 1 cell fractionation)


  cells are broken up by a homogeniser (blender). this releases the organelles from the cell. the resultant fluid, known as the homogenate, is then filtered to remove any complete cells and large pieces of debris
• ultracentrifugation (stage 2 cell fractonation)

  this is the process by which the fragments in the filtered homogenate are separated in a machine called a centrifuge. this spins tubes of homogenate at high speeds in order to create a centrifugal force. for animal cells, the process is as follows:

  • the tube of filtrate is placed in a centrifuge and spun at a low speed
  • the heaviest organelles, the nuclei, are forced to the bottom of the tube, where they form a thin sedement or pellet
  • the fluid at the top of the tube (supernatant) is removed, leaving just the sediment of nuclei
  • the supernatant is transferred to another tube and spun in the centrifuge at a faster speed
  • the next heaviest cells, the mitochondria, are forced to the bottom the tube
  • the process is continued
• transmission election microscope (TEM)


  consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet. the beam passes through a thin section of the specime. parts of the specimen absorbe electrons so appear dark. other parts allow electrons to pass through and so appear bright. an image is produced on the screen and this can be photographed to give a photomicrograph. the resolving power of the TEM is 0.1nm although this cannot always be achieved because:

  • difficulties preparing the specimen limit the resolution that can be achieved
  • a higher energy electron beam is required and this may destroy the specimen
• limitations of TEM

  • the whole system must be in a vaccum and so living specimens cannot be observed
  • a complex staining process is required and even then the image is not in colour
  • the specimen must be extremely thin
  • the image may contain artefacts. these are things that result from the way that the specimen is prepared
• the scanning electron microscope (SEM)


  all limitations of the TEM also apply to the SEM, except the specimen doesn't need to obe extremely thin as electron do not penetrate. basically, similar to a TEM, the SEM directs a beam of electrons onto the surface of the specimen from above. the beam is then passes back and forth across a portion of the specimen depending on the contours of the specimen surface. the resolving power is 20nm
• nucleus (eukaryote)

  this contains the organisms hereditary material and controls the cells activities. it has several parts:

  • nuclear envelope - double memrane surrounding the nucleus. contiunuous with the ER of the cell, often has ribosomes on its surface. controls entry and exit of materials in the nuclues and contains the reactions taking place within it
  • nuclear pores - allow passage of large molecules e.g. mRNA
  • nucleoplasm - makes up the bulk of the nucleus
  • chromosomes consist of protein-bound, linear DNA
  • nucleolus - small region within the nucleoplasm that manufactures rRNA and assembles the ribosomes
• functions of the nucleus (eukaryote)

  • act as the control centre of the cell through the production of mRNA and tRNA and protien synthesis
  • retain the genetic material of the cell in the form of DNA and chromosomes
  • manufacture rRNA and ribosomes
• mitochondrion (eukaryote)

  • double membrane controls entry and exit of material. inner membrane is folded to form extensions calles cristae
  • cristae provide a large surface area for the attachment of enzymes and other proteins involved in respiration
  • matix - contains proteins, lipids, ribosomes and DNA that allows the mitochondria to control the production of some of their own proteins
• mitochondria function

  the site of aerobic respiration to produce ATP, therefore cells have a large number of mitochondria to support metabolic activity.
• chloroplasts 


  carry out photosynthesis

  • chloroplast envelope- double plasma membrane that surrounds the organelle, controls what enters and leaves the chloroplast
  • grana - stacks of thilakoids. within the thylakoids is clorophyll. some thylakoids have tubular extensions that join with thylakoids in adjacent grana. the grana are where the fist stage of photosynthesis takes place
  • stroma - a fluid-filled matrix where the second stage of photosynthesis takes place. within the stroma are other structures including starch grains
• adaptations of chloroplasts

  • granal membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the first stage of photosynthesis
  • the fluid of the stroma possesses all the enzymes needed for the second stage of photosynthesis
  • chloroplasts contain DNA and ribosomes so they can quickly manufacture proteins for photosynthesis
• rough endoplasmic reticulum


  has ribosomes present on the outer surfaces of the memranes. functions:

  • provide a large surface area for the synthesis of proteins and glycoproteins
  • provide a pathway for the transport of materials throughout the cell
• smooth endoplasmic reticulum


  lacks ribosomes on its surface. functions:

  • synthesise, store and transport lipids and carbohydrates
• golgi apparatus


  functions:

  • add carbohydrate to proteins to form glycoproteins
  • produce secretory enzymes
  • secrete carbohydrates
  • transport, modify and store lipids
  • form lysosomes
• lysosomes


  functions

  • hydrolise materials injested by phagocytes
  • release enxymes to the outside of the cell through exocytosis in order to destroy material around the cell
  • digest worn out organelles so that the useful chemicals they are made of can be reused
  • completely break down dead cells through autolysis
• ribosomes


  the site of protein sysnthesis

  have a small and large sub unit containing rRNA and protein
• cell wall

  • consist of a number of polysaccharides, such as cellulose
  • there is a thin layer, called the middle lamella, which marks the boundary between adjacent cell walls and cements adjacent cells together
  • in algae, made of cellulose ot glycoproteins or both
  • in fungi, contain chitin, and glycan and glycoproteins

  functions

  • provide mechanical strength to prevent the cell bursting due to the pressure created by water entering by osmosis
  • to give the whole plant mechanical strength
  • to allow water to pass along it, so contribure to the movement of water through the plant
• vacuoles


  contains a solution of mineral salts, sugars, amino acids, wastes and sometimes pigments

  functions:

  • support herbaceous plants, and herbaceous parts of woody plants, by making cells turgid
  • the sugars and amino acids may act as a temporary food store
  • the pigments may colour petals to attract pollinating insects
• nucleus 


  .

  A) nuclear pore

  B) nucleolus

  C) chromatin

  D) nuclear envelope

  
• mitochondria

  .

  A) inner membrane

  B) outer membrane

  C) cristae

  D) ribosomes

  E) DNA

  F) matrix

  G) intermembrane space

  
• chloroplasts


  .

  A) inner membrane

  B) outer membrane

  C) ribosomes

  D) thylakoid

  E) stroma

  F) starch grain

  G) DNA

  H) lamella

  I) granum

  
• rough endoplasmic reticulum


  .

  A) cisternae

  B) ribosomes

  
• cell specialisation


  first group of cells in embryo are initially identical. as it matures, each cell takes on its own individual characteristics to suit the function that it will perform. every cell contains the genes needed for it to develop into a specialised cell. but only some of these genes are expressed in any one cell. different genes are switched on in each type of specialised cell. the rest are switched off
• tissues


  a collection of similar cells that perform a specific function
• types of tissue

  • epithelial tissues line the surfaces of organs and often have a secretory function. there are many types including ciliated epithelium that lines the trachea
  • xylem is found in plants. it is used to transport water and mineral ions throughout the plant and also gives mechanical support
• organs


  a combination of tissues that are coordinated to perform a function. the stomach is involved in the digestion of food. it is made up of tissues such as:

  • muscle to churn and mix the stomach contents
  • epithelium to protect the stomach wall and produce secretions
  • connective tissue to hold together the other tissues

  in plants, a leaf is made up of the following tissue

  • palisade mesophyll that carry out photosynthesis
  • spongy mesophyll adapted for gas exchange
  • epidermis to protect the leaf and allow gas exchange
• organs 


  organs work together as a single unit known as an organ system. these systems may be grouped together to perform particular functions more efficiently.

  • the digestive system digests and processes food. it is made up of salivary glands, oesophagus, stomach, duodenum, ileum, pancreas and liver
  • the respiratory system is used for breathing and gas exchange. it is made up of the trachea, bronchi and lungs
  • the circulatory system pumps and circulates blood. it is made up of the heart, arteries and veins
• prokaryotic cells


  .

  A) cell wall

  B) cell membrane

  C) pili

  D) flagellum

  E) plasminds

  F) DNA

  G) ribosomes

  H) capsule

  
• viruses


  .

  A) capsid

  B) RNA

  C) enzyme

  D) matrix

  E) lipid envelope

  F) attachment protein

  
• mitosis stages

  • interphase
  • prophase
  • metaphase
  • anaphase
  • telophase
• cell division in prokaryotic cells


  this process is known as binary fission:

  • the circular DNA molecule replicates and both copies attach to the cell membrane
  • the plasmids also replicate
  • the cell membrane begins to grow between the 2 DNA molecules, dividing the cytoplasm into 2
  • a new cell wall forms between the 2 molecules of DNA, dividing the original cell into 2 identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids
• the cell cycle

  • interphase occupies most of the cell cycle and is sometimes known as the resting phase because no division takes place
  • nuclear division when the cell nucleus divides through mitosis or meiosis
  • division of the cytoplasm through mitosis or meiosis
• interphase

  • cell increases in size
  • DNA replication takes place
  • protein synthesis
• prophase

  • chromosomes condense and become visible
  • nuclear envelope disappears
  • nucleolus disappears
  • centrioles move to poles of cell and microtubules begin to form
• metaphase

  • chromosomes line up in equator of cell
  • spindle fibres form and attach to centromere
• anaphase

  • spindle fibres contract
  • centromere splits
  • chromatids pulled towards poles
• telophase

  • nuclear membrane and nucleolus reform
  • spindle fibres disintegrate
  • cells divide (cytokenesis)
  • chromatids uncoil and lengthen
  • centriole replicates