Cell biology

• what is the underneath of a leaf called?

  an exchange surface

  • its covered in stomaa which the carbon dioxide diffuses in through
• The process of binary fission

  • the circular DNA and plasmid(s) replicate
  • the cell gets bigger and the plasmid strands move to opposite poles (ends) of the cell
  • the cytoplasm begins to divide and new cell walls begin to form
  • the cytoplasm divides and 2 daughter cells are produced. both cells ahve 1 copy of the circular DNA but have a variable number of copies of the plasmid(s)
• what is active transport ?

  movement of particles against the concentration gradient, it requires energy from respiration
• how easy it is for an organism to exchange substances with its environment depends on the organisms surface area to volume ratio (SA:V)
• the larger an organism is the smaller its surface area is compared to its volume
• in single celled organisms gasses and disolved substances can diffuse directly on to (or out of) the cell across the cell membrane, this is due to their large surface area to volume ratio so enough substances can be exchanged across the membrane to supply the volume of the cell
• multicellular organisms need some sort of exchange surface for efficient diffusion, this allows enough of the necessary substances to pass through
• exchange surfaces adaptations :

  • thin membrane - substances only have a short distance to diffuse
  • large surface area - lots of substance can diffuse at once
  • (in animas) have a lot of blood vessels- to get stuff in and out of the blood
  • gas exchange surfaces (in animals) are often ventilated - air moves in and out
• gas exchange in the lungs:

  • the job of the lungs is to transfer oxygen to the blood and remove waste CO2 from it
  • to do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place
  • they are specialised to maximise the diffusion of O2 and CO2, they have :
  • enormous surface area
  • moist lining for dissolving gases
  • very thin walls
  • good blood supply
• the villi

  • the inside of the small instestine is covered in millions and millions of these tiny little projections called villi
  • they increase surface area in a big way so that digested food is absorbed much more quickly into the blood
  • they have:
  • a thin layer of surface cells (usually only 1 cell thick)
  • a very good blood supply to assist quick absorbtion
• gas exchange in leaves:

  • oxygen and water diffuse out of the stomata
  • CO2 diffuses in through the stomata
  • the size of stomata is cotrolled by guard cells
  • they close the stomata if the plant is loseing water faster than it is being replaced by the roots
  • the flattened shape of the leaf increases the area of this exchange surface so that its more effective
  • the walls of the cells inside the leaf form another exchange surface, the air spaces inside the leaf increase the area of this surface so its more likely for CO2 to get into the cells
• diffusion is the movement of particles from an area of high concentration to low concentration until equilibrium is reached
• equilibrium is when there is no net change in the number of molecules moving in either direction
• concentration gradient is the difference between the concentrations of a substance in two different places
• gills are the gas exchange surface for fish
• gas exchange in fish :

  • water enters the fish through its mouth and passes out through its gills. as this happens oxygen diffuses from the water into the blood in the gills and CO2 diffuses from the blood into the water
  • blood flows through the lamellae in one direction and water flows over in the opposite direction, this maintains a large concentration gradient between the water and the blood
  • the concentration of oxygen in the water is always higher than that in the blood so as much oxygen as possible diffuses from the water into the blood
• what is osmosis?

  The movement of water molecules from an area of high concentration to an area of low concentration across a semi-permeable membrane.
• what is a partially permeable membrane ?

  a membrane with very small holes in it, only tiny molecules e.g. water can pass through them and bigger molecules (e.g. sucrose ) cant
• if the solution outside the cell has a higher solute concentration than inside the cell then water will leave the cell by osmosis until the solutions on both sides of the membrane have the same solute concentration
• Light microscopes:

  • have a lower resolution
  • have a lower magnification
  • are cheaper
• Electron microscope :

  • have a higher resolution
  • have a higher magnification
  • are more expensive
• equation for magnification :

  magnification = image size / real size
• nucleus

  contains genetic material that controls the activities of the cell
• cytoplasm

  gel- like substance where most of the chemical reactions happen, it contains enzymes that control these chemical reactions
• cell membrane

  holds together the cell and controls what goes in and out
• mitochondria

  where most of the reactons for aerobic respiration take place
• ribsomes

  where protiens are made in the cell
• cell wall

  made of cellulose

  supports and strengthens the cell
• permenant vacuole

  contains cell sap ( a weak solutions of sugar and salts)
• chloroplasts

  where photosynthesis occurs, which makes food for the plant

  contain chlorophyll which absorbs the light needed for photosynthesis
• plasmids

  small rings of DNA found in prokaryotic cells
• what is cell differentation?

  the process where a cell changes to become speciallised for its job
• adaptations of a sperm cell :

  • long tail
  • streamlined head (both help it swim to the egg)
  • lots of mitochondria (to provide the energy needed )
  • enzymes in the head (to digest through the egg cell membrane)
• adaptations of a muscle cell

  • ability to contract and relax
  • lots of mitochondria
• adaptations of a nerve cell

  • long (to cover more distance )
  • branched connections at their ends to connect to other nerve cells and form a network throughout the body
• adaptations of a root hair cell

  they are specialised for absorbing wter and minerals

  • large surface area
• what is a chromosome?

  coiled up lengths of DNA molecules

  • each chromosome carries a large number of genes
  • humans have 23 pairs of chromosomes in a cell
• the 2 main stages of the cell cycle :

  • growth and DNA replication
  • mitosis
• growth and DNA replication :

  • its a cell thats not dividing the DNA is spread out in long strings
  • before it divides the cell has to grow and increase the amount of subcellular structures such as mitochondria and ribosomes
  • it the duplicates it DNA- so theres only 1 copy for each new cell, the DNA is copied and forms X- shaped chromosomes, each arm of the chromosome is an exact duplicate of the other
• mitosis

  • the chromosomes line up at the centre of the cell and cell fibres pull the apart, the 2 arms of each chromsome go to opposite ends of the cell
  • mebranes form around each of the sets of chromosomes, these become the nuclei of the 2 new cells- the nucleus has divided
  • lastly the cytoplasm and cell mebrane divide
  • the cell has now produced 2 new daughter cells, the daughter cells contain exactly the same DNA- theyre identical, their DNA is also identical to the parent cell