AQA GCSE Biology B2.1 Cells, Tissues and Organs Revision Notes

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B2.1.1 Animal and Plant Cells

  • All living things are made up of cells, most of which are very small-they can only be seen with a microscope.
  • Light microscopes at school are used to maginify things hundreds of times.
  • Electron microscopes can magnify things more than a hundred thousand times.
  • Most cells have:
    • Nucleus-to control the cell's activities.
    • Cytoplasm-where chemical reactions take place.
    • Cell membrane-controls the movement of materials in and out of the cell.
    • Mitochondria-where energy is released during aerobic respiration.
    • Ribosomes-where protein synthesis takes place. All proteins in the cell are made there.
  • Plant and algae cells also have:
    • A cell wall made of cellulose-it strengthens the cell and gives it support.
  • Many but not all plant and algae cells also have:
    • Chloroplasts-found in all green parts of the plant. They are green because they contain chorophyll, which absorbs light energy to make food using photosynthesis. Root cells don't have chloroplasts because they are underground and don't photosynthesise.
    • Permanent vacuole-a space in the cytoplasm filled with cell sap, to keep the cells rigid to support the plant.
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B2.1.2 Bacteria and Yeast part 1

  • Bacteria- single-celled living organisms much smaller than animal and plant cells-only seen with a powerful microscope.
  • When it is cultured on an agar plate, millions of bacteria is grown, enabling you to see a bacterial colony with a naked eye.
  • Plasmids carry extra genetic information.
  • The flagellum is a long protein strand that lashes around and is used to move the bacterium around.
  • The genes aren't in a nucleus, but the strand of DNA is usually circular and found in the cytoplasm.

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  • Some cause disease, but many are harmless-some even useful. They can be used to make food like yoghurt and cheese, or used for sewage treatment and to make medicines.
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B2.1.2 Bacteria and Yeast part 2

  • Yeast is a single-celled organism.
  • They are bigger than bacteria but still very small, their sizes varying between 3-4µm.
  • They reproduce by the method of asexual budding-a new yeast cell growing out from the original cell to form a new seperate yeast organism.
  • They are specialised to be able to survive for a long time even when little oxygen is available. When there is sufficient oxygen available, they use aerobic respiration-using oxygen to break down sugar to provide energy for the cell. During this process they produce water and carbon dioxide as waste products.
  • They can use anaerobic respiration-producing ethanol and carbon dioxide instead.
  • Ethanol is commonly referred to as alcohol-the anaerobic respiration of yeast is sometimes called fermentation.
  • Yeast has been used to make bread and alcoholic drinks for thousands of years-it was used to make bread in Egypt 6000 years ago, and to make wine in Iran over 7000 years ago.
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B2.1.3 Specialised Cells Part 1

  • Cells in an organisms can be specialised in order to carry out certain tasks.
  • When a cell becomes specialised, its structure is adapted to suit the job it does. Therefore, it looks a lot different to a normal plant or animal cell. Some cells are specalised to have only one function in the body-such as sperm, eggs, red blood cells and nerve cells.
  • Fat cells:
    • They store excess fat.
    • They have a small amount of cytoplasm and loads of fat.
    • Few mitochondria; little energy needed.
    • Can expand up to 1000 times the orginal size as it fills with fat.
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B2.1.3 Specialised Cells Part 2

  • Cone cells from the human eye:
    • The are the light-sensitive layer of your eye-retina, making it possible for you to see in colour.
    • Outer segment-has visual pigement changing in coloured light and needs energy to return to its original form.
    • Middle section-many mitochondria, providing the energy for the visual pigment to reform, allowing a person to see continually in colour.
    • Final part-a specialised synapse which connects to the nerve cells in the optic nerve-when coloured light makes the pigment change, impulses are triggered, and they cross the synapse, travelling through the optic nerve to the brain.
    • I couldn't find a diagram anywhere, sorry. There's a diagram in the textbook, though.
  • Root hair cells:
    • These are found close to the tips of growing roots, and help plants to take up water more effectively.
    • They are always close to the xylem tissue, which carries water and mineral ions up to the plant.
    • They increase the surface area for water to move into them.
    • Have a large permanent vacuole speeding up movement of water by osmosis from the soil across the root hair cell.
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B2.1.3 Specialised Cells Part 3

  • Sperm cells
    • These are usually released a long way from the egg that they will fertilise.
    • They have the genetic information from the male parent in sexual reproduction. Depending on the type of animal, they have to move through water or the female reproductive system to reach the egg, and then break into the egg.
    • The have a long tail which wips from side to side and helps the sperm move.
    • The middle section is full of mitochondria, providing the energy for the tail to work.
    • The acrosome stores digestive enzymes for breaking down the outer layers of the egg.
    • A large nucleus has the genetic information in.
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B2.1.4 Diffusion

  • Molecules in gases and liquids move around randomly because of their energy.
  • Diffusion- the spreading out of particles of a gas or of a substance in a solution, resulting in a net (overall) movement  of particles. It is from a high concentration area to a low concentration area, taking place due to random movement.
  • Net movement=particles moving in-particles moving out.
  • Two different particles-Particles A and particles B are not mixed at all, at first. Then, the particles move randomly and begin to mix with each other. As they spread, they collide with each other, causing to them to continue spreading randomly and eventually the particles are completely mixed, completing diffusion.
  • The difference in concentration between the two areas-concentration gradient.
  • The steeper the concentration gradient, the faster the rate of diffusion.
  • Examples:
    • Diffusion of oxygen into body cells from the bloodstream as the cells are respiring and using up oxygen.
    • Diffusion of carbon dioxide into actively photosynthesising plant cells.
    • Diffusion of simple sugars and amino acids from the gut through cell membranes.
      diffusion (
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B2.1.5 Tissues and Organs

  • Large multicellular organisms have to overcome the problems linked to their size and develop ways of exchanging materials-during development, cells differentiate, becoming specialised.
  • Tissues
    • It is a group of cells with similar structure and function working together-muscular tissue can contract to make movement. Glandular tissue has secretory cells producing substances like enzymes and hormones.
    • Epithelial tissue covers the outside of your body as well as internal organs.
    • Plants- Epidermal tissues cover and protect surfaces. Mesophyll tissues contain many chloroplasts and carry out photosynthesis. Xylem and phloem are the transport tissues in plants.
  • Organs
    • They are made up of tissues, all working together.
    • For example, the stomach has muscular tissue to churn food and digestive juices, and glandular tissue, which produces the juices, and epilithelial tissue, which covers the inside and outside of it.
    • Many organs have developed to enable exchange to take place, such as the exchange of gases in the lungs.
    • These organs also have adaptations which make the exchange of materials easier such has by increasing the surface area, which increases the rate of diffusion.
    • Other adaptations increase the concentration gradient across membranes.
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B2.1.6 Organ Systems

  • These are made up of organs to perform a particular function.
  • The digestive system is one example, it has:
    • Glands like the pancreas and salivary glands which produce digestive juices.
    • The stomach and small intestine, where digestion occurs.
    • The liver, producing bile.
    • The small intestine, where soluble food is absorbed. It has a very large surface area, which increases the rate of diffusion from the gut to the blood.
    • The large intestine, where water is absorbed to produce faeces.
  • Plants have organ systems too:
    • They have specialised tissues like mesophyll, xylem and phloem and in the plant, tissues form organs, which carry out different functions. Examples of organs are the leaves and stem and roots.
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