A detailed summary of all modules of the AQA course specification booklet for biology (combined science, trilogy), with all the important information. I used these revision notes to achieve a 9-9 grade for my combined science GCSE.
Revision Notes
Cell Biology
Prokaryotes and Eukaryotes
Plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm, and
genetic material enclosed in a nucleus.
Bacterial cells (prokaryotic cells) are much smaller in comparison.
They have cytoplasm and a cell membrane surrounded by a cell wall.
The genetic material isn’t enclosed in a nucleus. It is a single DNA loop and there may
be one or more small rings of DNA called plasmids.
Animal and Plant Cells
Animal cells have:
A nucleus – contains genetic material that controls the cell’s activities.
Cytoplasm – gel-like substance where most of the chemical reactions happen. These
reactions are controlled by enzymes.
A cell membrane – holds the cell together and controls what goes in and out.
Mitochondria – where most of the reactions for aerobic respiration take place.
Respiration transfers energy that the cell needs to work.
Ribosomes – where proteins are synthesised.
Plant cells also have:
A cell wall – supports and strengthens the cell and is made out of cellulose.
Chloroplasts – where photosynthesis occurs, and contains a green substance called
chlorophyll, which absorbs light.
A permanent vacuole – contains cell sap made of a weak solution of sugar and salts.
Cell Specialisation
Sperm Cells
Long tail and is streamlined to help it swim.
Lots of mitochondria to provide energy.
Carries enzymes in its head to digest through the egg cell membrane.
Nerve Cells
Long and have branched connections at their ends to connect to other nerve cells
(neurones).
Form a network around the body.
Muscle Cells
Long, so they have space to contract, and lots of mitochondria to provide enough
energy for contraction.
,Root Hair Cells
Gives the plant a large surface area for absorbing water and minerals from the soil.
They have no chloroplasts as they are underground, and no light is reaching them.
Phloem and Xylem Cells
They are long so that they can form tubes.
Xylem cells are hollow at the centre.
Phloem cells have very few subcellular structures, so that stuff can flow through
them.
Cell Differentiation
Differentiation is a process by which a cell changes to become specialised, allowing it
to carry out specific functions.
Most types of animal cells differentiate at a young age, and then lose that ability.
In mature animals, cells that differentiate are mainly used to repair or replace cells.
Most plant cells can differentiate throughout their lives.
Chromosomes
The nucleus of a cell usually contains chromosomes made of DNA molecules.
Each chromosome carries a large number of genes.
In body cells, the chromosomes are normally found in pairs. There are 23 of them in
humans.
Mitosis and the Cell Cycle
During the cell cycle, the genetic material is doubled and then divided into two
genetically identical cells.
Before a cell can divide, it needs to increase the number of subcellular structures
(ribosomes, mitochondria). The DNA replicates to form two copies of each
chromosome.
In mitosis, one set of chromosomes is pulled to each end of the cell and the nucleus
divides.
Finally, the cytoplasm and cell membranes divide to form two genetically identical
cells.
Diffusion
Substances move into/out of cells across the cell membrane by diffusion.
Diffusion is the spreading out of the particles, resulting in a net movement form an
area of high concentration to a lower concentration.
In humans, examples of diffusion are urea from cells into the blood plasma for
excretion via the kidney.
Factors that affect the rate of diffusion are:
, The difference in concentrations (concentration gradient).
The temperature.
The surface area of the membrane.
Specialisations in multicellular organisms, helping with diffusion:
Having a large surface area.
A thin membrane, providing a short diffusion path.
An efficient blood supplies.
Being ventilated (for gaseous exchange).
Examples of exchange surfaces are the alveoli, cilia, gills, root hair cells, leaf.
Osmosis
Water moves across cell membranes by osmosis.
Osmosis is the diffusion of water from a dilute solution to a concentrated solution
through a partially permeable membrane.
Active Transport
Active transport moves substances from a more dilute solution to a more
concentrated solution, against the concentration gradient.
This requires energy from respiration.
Active transport allows mineral ions to be absorbed into plant root hairs from very
dilute solution in the soil. Plants require ions for healthy growth.
Is also allows sugar molecules to be absorbed from lower concentrations in the gut
into the blood which has a higher sugar concentration. Sugar molecules are used for
cell respiration.
Organisation
Principles of Organisation
Cells are the basic building blocks of all living organisms.
A tissue is a group of cells with a similar structure and function.
Organs are tissues performing specific functions.
Organs are organised into organ systems, which work together to form organisms.
The Human Digestive System
The digestive system is an example of an organ system in which several organs work
together to digest and absorb food.
Enzymes
Enzymes are biological catalysts that control and speed up useful chemical reactions
in the body.
Enzymes are large proteins and are made out of chains of amino acids.
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