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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Edexcel AS Biology - Cells

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Cell Revision

• There are two main types of cells, Prokaryotic and Eukaryotic.
• Prokaryotic cells DO NOT have a nucleus, they are simple cells and their nucleic acids are found in the cytoplasm. They DO NOT have mitochondrion. An example of one is bacterium.
• Eukaryotic cells however DO have a nucleus. They are more complex cells, they also have mitochondrion. *An example of a eukaryotic cell is a plant or animal cell.
• There are certain differences between plant and animal cells. They both contain some things which the other one doesn’t.
• Plant cells contain chloroplasts, a cell wall, a vacuole and cellulose whilst animal cells don’t and an animal cell contains glycogen granules.
• Cells tissue organ organ system organism
• Eukaryotic cells contain:
  • Endoplasmic reticulum. Smooth ER is used for transporting Lipids. Rough ER (containing ribosomes) is used for transporting proteins.
  • Ribosomes, found attached to ER or in the cytoplasm, it is where proteins are made.
  • Cell wall. It surrounds PLANT CELLS, it acts as support.
  • Plasma membrane. It’s found in both plant and animal cells, it is made of both proteins and lipids and it regulates movement in and out of cell. It contains a lipid bilayer. It is also known as a fluid mosaic model due to the proteins embedded in it.
  • The nucleus. It’s surrounded by a nuclear membrane containing pores. It contains chromatins and a nucleolus. It contains the genetic material of the cell. The nucleolus makes RNA.
  • A lysosome. It’s a round organelle, it contains digestive enzymes it is used to destroy or digest invading cells.
  • Golgi apparatus. A group of SER containing flattened sacs with vesicles. It packages substances that are produced by the cell it also makes lysosomes.
  • Microtubule. Made from tubulin, acts as a guide to moving organelles in the cell.
  • Mitochondrion. They have a DOUBLE MEMBRANE, the inner one forms cristae. Inside is the matrix which contains enzymes for respiration. It is also where ATP is produced.
  • Chloroplast. Found only in plant cells. It has a double membrane and is the site where photosynthesis takes place.
  • Centriole. Seen in animal cells during cell division, it is involved with the chromosomes during said division.
• Intrinsic proteins completely span the membrane from inside to outside. They are used in active transport or facilitated diffusion.
• Extrinsic proteins only partly span the membrane. They are either located in the outer phospholipid layer or the inner phospholipid layer. They are used in cell recognition.
• Transport across cell membranes. There are four main ways.
  • Diffusion: Is the passive movement of particles. The particles will gradually move from a highly concentrated area to a lowly concentrated area. It can occur across cell membranes, no energy is required for it to occur. It’s affected by the concentration gradient and temperature.
  • Osmosis: Is when WATER molecules ONLY diffuse through a partially permeable membrane. It doesn’t require ATP. Water molecules will move both ways through the membrane but the net movement will be towards the area with a lower concentration of water molecules.
    • Water potential is the ability for water molecules to move, it is represented by this symbol (Ψ), it is measured in Kilopascals KPa. Pure water has the highest water potential with 0; all other water potentials are NEGATIVE.
    • It can be found out using this equation:

      Water potential (Ψ) =solute potential (Ψs) + pressure potential (Ψp)

  • Solutions can be:
    • Isotonic: same water potential (same concentration)
    • Hypertonic: lower water potential (higher concentrated)
    • Hypotonic: Higher water potential (less concentrated)
  • Facilitated Diffusion: It uses carrier proteins and channel proteins. Some larger molecules e.g. sodium ions can’t diffuse through the cell membrane themselves. Instead they use carrier proteins or channel proteins.
  • Active transport. This REQUIRES ATP and occurs against a concentration gradient. Molecules attach to carrier proteins, then molecules of ATP use energy to change their shape, where they then pass through the cell membrane.
    • Endocytosis takes materials INTO the cell. The material is surrounded by a small vesicle and then absorbed into the cell. There are two types, phagocytosis and pinocytosis. Phagocytosis is where SOLIDS are taken into the cell. Pinocytosis is where LIQUIDS are absorbed.
    • Exocytosis removes materials from the cell. The materials to be removed move through the ER towards the Golgi where they are pinched off by vesicles. The vesicles then secrete then outside the cell.

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• Suitable for: OCR A Level Biology and other exam boards.
• Written by: ~NC~.
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