Detailed and comprehensive AQA biology A-level, cells module revision notes. High-quality detailed notes with diagrams and images. Notes directly reflective of the specification with exam technique in mind, with key terms and definitions.
All living things are made of cells, and cells are the smallest units that can be alive. There are
thousands of different kinds of cell, but the biggest division is between the cells of the prokaryote
kingdom [the bacteria] and those of the other four kingdoms [animals, plants, fungi and
protoctista – e.g. amoeba], which are all eukaryotic cells.
Prokaryotic cells are smaller and simpler than eukaryotic cells, and do not have a nucleus.
The structures observed in electron micrographs [a digital image taken through an electron
microscope] allowing the visualisation of the fine detail of cells [e.g. individual organelles].
Euakryotic Cells:
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,[1] Cytoplasm [or Cytosol]: This is the solution within the cell membrane. It contains enzymes for
glycolysis [part of respiration] and other metabolic reactions together with sugars, salts, amino
acids, nucleotides and everything else needed for the cell to function.
[2] Nucleus: This is the largest organelle. It is surrounded by a nuclear envelope, which is a double
membrane with nuclear pores [that control the exit of substances such as mRNA from the
nucleus].
The interior is called the nucleoplasm, which is full of chromatin [a linear DNA / protein complex].
During cell division the chromatin becomes condensed into discrete observable chromosomes.
The nucleolus is a dark region of chromatin, involved in making ribosomes.
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, [3] Mitochondrion [pl. mitochondria]: This is where aerobic respiration takes place in all
eukaryotic cells [anaerobic respiration takes place in the cytoplasm].
Mitochondria release energy [in the form of the molecule ATP] from carbohydrates, lipids and
other energy-rich molecules. Cells that use a lot of energy [like muscle cells] have numerous
mitochondria.
Mitochondria are surrounded by a double membrane: the outer membrane is simple and quite
permeable, while the inner membrane is highly folded into cristae, which give it a large surface
area.
The space enclosed by the inner membrane is called the mitochondrial matrix, and contains small
circular strands of DNA. The inner membrane is studded with stalked particles, which are the
enzymes that make ATP.
[4] Ribosomes: These are the smallest and most numerous of the cell organelles, and are the sites
of protein synthesis.
Ribosomes are either found free in the cytoplasm, where they make proteins for the cell's own use,
or they are found attached to the rough endoplasmic reticulum, where they make proteins for
export from the cell.
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