GCSE AQA Biology Combined Science - Cell Biology Notes

CELL BIOLOGY
STRUCTURE OF A CELL
 Role of the nucleus: contains genetic material in the form of DNA and controls the cell's
activities
 Role of the ribosomes: responsible for protein synthesis
 Role of the cytoplasm: where chemical reactions take place
 Role of the mitochondria: the site of aerobic respiration and so release energy for the cell to
use
 Role of the cell membrane: controls what goes in and out of the cell
 Role of the chloroplasts: absorbs light energy from photosynthesis, makes the plant green
 Role of the vacuole: contains cell sap, helps maintain the structure and shape of the cell
 Role of the cell wall: made of cellulose, maintains the shape of the cell



EUKARYOTES AND PROKARYOTES
 In a bacterial cell DNA is stored as plasmids
 Bacteria use a flagellum for movement
 The key difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a
nucleus



MICROSCOPY
 Magnification = image size / object size
 Image size = object size x magnification
 Object size = image size / magnification
 Magnification: how many times larger the image is than the object
 Advantages of a light microscope:
o cheaper
o easier to use
 Features of a light microscope
o Illuminated from underneath
o Maximum magnification = x2000
o Resolving power = 200nm
o Used to view tissues, cells and large sub-cellular structures
 Advantages of an electron microscope
o better resolution

o can be used to see sub-cellular structures
 Features of an electron microscope
o Maximum magnification = x2,000,000
o Resolving power = 10nm

, CELL DIFFERENTIATION
 Stem cells must undergo differentiation to form specialised cells
 This involves some of their genes being switched on or off to produce different proteins
which allows the cell to acquire different sub-cellular structures
 Cell differentiation in animals:
o Almost all cells differentiate at an early stage and then lose this ability
o Most specialised cells can make more of themselves through mitosis
o Red blood cells cannot divide and are replaced by adult stem cells
o Cell division mostly only happens to repair or replace damaged cells
 Cell differentiation in plants:
o Many cells retain the ability to differentiate
o Plant cells only differentiate when they reach their final position



CELL SPECIALISATION
 ANIMALS:
o Sperm cells: specialised to carry the male’s DNA to the egg
 Streamlined head and long tail to aid swimming
 Many mitochondria to supply energy for the cell to move
 Digestive enzymes in the top of the head to break down the outer layers of
the egg cell
o Nerve cells: specialised to carry electrical signals quickly around the body
 Axon is long which allows impulses to be carried along long distances
 Lots of dendrites so branched connections can form with other nerve cells
 Nerve endings have many mitochondria to supply energy to make
neurotransmitters
o Muscle cells: specialised to contract quickly to cause movement
 Proteins slide over each causing the muscle to contract
 Lots of mitochondria to provide energy from respiration for contraction
 Store glycogen that is used in respiration by mitochondria
 PLANTS:
o Root hair cells: specialised to take up water by osmosis and mineral ions by active
transport from the soil
 Large surface area so more water can move in
 Large permanent vacuole to allow water to move in quicker
 Lots of mitochondria to provide energy from respiration for the active
transport of mineral ions
o Xylem cells: specialised to transport water and mineral ions from the roots to the
shoots
 Lignin is formed causing the cell to die, the cells become hollow and join
end-to-end to form a tube so substances can move through
 Lignin is deposited in spirals which helps the cell to withstand pressure
o Phloem cells: specialised to carry the products of photosynthesis to all parts of the
plant