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Biology AS CIE

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  1. 1-cell-structure
    10 主题
  2. 2-biological-molecules
    19 主题
  3. 3-enzymes
    13 主题
  4. 4-cell-membranes-and-transport
    16 主题
  5. 5-the-mitotic-cell-cycle
    8 主题
  6. 6-nucleic-acids-and-protein-synthesis
    9 主题
  7. 7-transport-in-plants
    11 主题
  8. 8-transport-in-mammals
    16 主题
  9. 9-gas-exchange
    6 主题
  10. 10-infectious-diseases
    6 主题
  11. 11-immunity
    10 主题
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1-2-cells-as-the-basic-units-of-living-organisms AS eukaryotic-cell-structures-and-functions

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Exam code:9700

Eukaryotic cell structures & functions

  • Cells can be divided into two broad types; eukaryotic and prokaryotic cells

  • Eukaryotic cells have a more complex ultrastructure than prokaryotic cells

    • The term ultrastructure refers to the internal structure of cells

  • The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles

Cell organelles

Cell surface membrane

  • All cells are surrounded by a cell surface membrane which separates the inside of cells from their surroundings

  • Cell surface membranes controls the exchange of materials between the internal cell environment and the external environment

    • The membrane is described as being partially permeable as it allows the passage of some substances and not others

  • The cell membrane is formed from a phospholipid bilayer and spans a diameter of around 10 nm

Diagram of a cell showing the nucleus, cell membrane, proteins, and cytoplasm with arrows indicating their positions and functions.
Cell surface membranes separate cell contents from the surrounding environment and control the passage of substances into and out of cells

Nucleus

  • Present in all eukaryotic cells, the nucleus is a large organelle that is separated from the cytoplasm by a double membrane

  • The nucleus contains the DNA, which is arranged into chromosomes

    • Chromosomes contain DNA and proteins, which are collectively referred to as chromatin

  • The nuclear membrane is known as the nuclear envelope, and contains many pores

  • Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes and signalling molecules to move in

  • The nucleus contains a region known as the nucleolus, which is the site of ribosome production

A nucleus, containing DNA and a nucleolus, and surrounded by a nuclear membrane
The nucleus of a cell contains the genetic material

Rough and smooth endoplasmic reticulum

  • The endoplasmic reticulum (ER) is made up of a series of membranes that form flattened sacs within the cell cytoplasm

  • The ER is linked with the nuclear envelope

  • There are two distinct types of ER, with different roles within the cell

    • The rough endoplasmic reticulum (RER)

      • Continuous folds of membrane that are linked with the nuclear envelope

      • The surface of the RER is covered in ribosomes

      • The role of the RER is to process proteins that are produced on the ribosomes

    • The smooth endoplasmic reticulum (SER)

      • The SER does not have ribosomes on the surface

      • It is involved in the production of lipids, and of steroid hormones such as oestrogen and testosterone

Rough and smooth endoplasmic reticulum, arranged around the nucleus of a cell
The RER has ribosomes on its outer surface and is continuous with the nuclear envelope, while the SER lacks ribosomes

Examiner Tips and Tricks

Be sure to always use the full name of the rough and smooth endoplasmic reticulum when you first refer these structures in an exam; marks are often not awarded for the abbreviations RER and SER in the absence of the full key terms.

Golgi body

  • The Golgi body is often referred to as the Golgi apparatus or the Golgi complex

  • It consists of a series of flattened sacs of membrane

  • It can be clearly distinguished from the ER, as it is not connected to other membrane-bound compartments, and it has a distinctive ‘wifi symbol’ appearance

  • Its role is to modify proteins and package them into vesicles

Golgi body and Golgi vesicles
The Golgi body processes proteins and packages them into vesicles

Mitochondria

  • Mitochondria (singular mitochondrion) are relatively large organelles surrounded by a double-membrane

    • They are smaller than the nucleus and chloroplasts, but can be seen with a light microscope

  • The inner membrane is folded to form cristae

  • Mitochondria are the site of aerobic respiration within eukaryotic cells

  • The mitochondrial matrix contains enzymes needed for aerobic respiration

  • Small, circular pieces of DNA, known as mitochondrial DNA, and ribosomes are also found in the matrix

    • This allows the production of proteins required for respiration

Cross-section through a mitochondrion
Mitochondria have a highly folded inner membrane; this provides a large surface area for embedded proteins that are involved with aerobic respiration

Ribosomes

  • Ribosomes are found in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells

  • Each ribosome is a complex of ribosomal RNA (rRNA) and proteins

  • 80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells

    • Smaller, 70S ribosomes (composed of 50S and 30S subunits) are found in prokaryotes, mitochondria and chloroplasts

  • Ribosomes are the site of translation during protein synthesis

<img alt=”A ribosome, showing the large and small subunits” data-nimg=”1″ decoding=”async” height=”335″ loading=”lazy” sizes=”(max-width: 320px) 320w, (max-width: 640px) 640w, (max-width: 960px) 960w, (max-width: 1280px) 1280w, 1920w” src=”https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=3840/https://cdn.savemyexams.com/uploads/2025/09/32125_httpscdn-savemyexams-comuploads202010cell-components-ribosome-1.png” srcset=”https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=16/https://cdn.savemyexams.com/uploads/2025/09/32125_httpscdn-savemyexams-comuploads202010cell-components-ribosome-1.png 16w, https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=32/https://cdn.savemyexams.com/uploads/2025/09/32125_httpscdn-savemyexams-comuploads202010cell-components-ribosome-1.png 32w, https://cdn.savemyexams.com/cdn-cgi/im

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