AQA A Level Biology

Revision Notes

2.1.2 Structure of Eukaryotic Cells

The Structure of Eukaryotic Cells

Cell surface membrane

Cell components_Plasma membrane, downloadable AS & A Level Biology revision notes

The structure of the cell surface membrane – although the structure looks static the phospholipids and proteins forming the bilayer are constantly in motion

  • All cells are surrounded by a cell surface membrane which controls the exchange of materials between the internal cell environment and the external environment
    • The membrane is described as being ‘partially permeable’
  • The cell membrane is formed from a phospholipid bilayer of phospholipids spanning a diameter of around 10 nm

Cell wall

Cell components_Cell wall, downloadable AS & A Level Biology revision notes

The cell wall is freely permeable to most substances (unlike the plasma membrane)

  • Cell walls are formed outside of the cell membrane and offer structural support to cell
  • Structural support is provided by the polysaccharide cellulose in plants, and peptidoglycan in most bacterial cells
  • Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells

Nucleus

Cell components_Nucleus, downloadable AS & A Level Biology revision notes

The nucleus of a cell contains chromatin (a complex of DNA and histone proteins) which is the genetic material of the cell

  • Present in all eukaryotic cells, the nucleus is relatively large and separated from the cytoplasm by a double membrane (the nuclear envelope) which has many pores
  • Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes (eg. DNA polymerases) and signalling molecules to travel in
  • The nucleus contains chromatin (the material from which chromosomes are made)
    • Chromosomes are made of sections of linear DNA tightly wound around proteins called histones
  • Usually, at least one or more darkly stained regions can be observed – these regions are individually termed ‘nucleolus’ (plural: nucleoli) and are the sites of ribosome production

Mitochondria

Cell components_Mitochondria, downloadable AS & A Level Biology revision notes

A single mitochondrion is shown – the inner membrane has protein complexes vital for the later stages of aerobic respiration embedded within it

  • The site of aerobic respiration within eukaryotic cells, mitochondria are just visible with a light microscope
  • Surrounded by double-membrane with the inner membrane folded to form cristae
  • The matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP
  • Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication)

Chloroplast

Cell components_Chloroplast, downloadable AS & A Level Biology revision notes

Chloroplasts are found in the green parts of a plant – the green colour a result of the photosynthetic pigment chlorophyll

  • Larger than mitochondria, also surrounded by a double-membrane
  • Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana
  • Grana are joined together by lamellae (thin and flat thylakoid membranes)
  • Chloroplasts are the site of photosynthesis:
    • The light-dependent stage takes place in the thylakoids
    • The light-independent stage (Calvin Cycle) takes place in the stroma
  • Also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

Ribosome

Cell components_Ribosome, , downloadable AS & A Level Biology revision notes

Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein

  • Found freely 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
  • 70S ribosomes (composed of 50S and 30S subunits) in prokaryotes, mitochondria and chloroplasts
  • Site of translation (protein synthesis)

Endoplasmic reticulum

Cell components_Endoplasmic reticulum, downloadable AS & A Level Biology revision notes

The RER and ER are visible under the electron microscope – the presence or absence of ribosomes helps to distinguish between them

Rough Endoplasmic Reticulum (RER)

  • Surface covered in ribosomes
  • Formed from continuous folds of membrane continuous with the nuclear envelope
  • Processes proteins made by the ribosomes

Smooth Endoplasmic Reticulum (ER)

  • Does not have ribosomes on the surface, its function is distinct to the RER
  • Involved in the production, processing and storage of lipids, carbohydrates and steroids

Golgi apparatus (golgi complex)

Cell components_Golgi aparatus, downloadable AS & A Level Biology revision notes

The structure of the Golgi apparatus

  • Flattened sacs of membrane similar to the smooth endoplasmic reticulum
  • Modifies proteins and lipids before packaging them into Golgi vesicles
    • The vesicles then transport the proteins and lipids to their required destination
    • Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles

Large permanent vacuole

Cell components_Vacuole, downloadable AS & A Level Biology revision notes

The structure of the vacuole

  • Sac in plant cells surrounded by the tonoplast, selectively permeable membrane
  • Vacuoles in animal cells are not permanent and small

Vesicle

Cell components_Vesicle, downloadable AS & A Level Biology revision notes

The structure of the vesicle

  • Membrane-bound sac for transport and storage

Lysosome

Cell components_Lysosomes, downloadable AS & A Level Biology revision notes

The structure of the lysosome

  • Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down)
  • Break down waste materials such as worn-out organelles, used extensively by cells of the immune system and in apoptosis (programmed cell death)

Centriole

Cell components_Centriole, downloadable AS & A Level Biology revision notes

The structure of the centriole

  • Hollow fibres made of microtubules, two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division
  • Not found in flowering plants and fungi

Microtubules

Cell Components_Microtubule, downloadable AS & A Level Biology revision notes

The structure of the microtubule

  • Makes up the cytoskeleton of the cell about 25 nm in diameter
  • Made of α and β tubulin combined to form dimers, the dimers are then joined into protofilaments
    • Thirteen protofilaments in a cylinder make a microtubule
  • The cytoskeleton is used to provide support and movement of the cell

Microvilli

Cell components_Microvilli, downloadable AS & A Level Biology revision notes

The structure of the microvilli

  • Cell membrane projections that increase the surface area for absorption

Cilia

Cell components_Cilia, downloadable AS & A Level Biology revision notes

The structure of the cilia

  • Hair-like projections made from microtubules
  • Allows the movement of substances over the cell surface

Flagella

Cell components_Flagella, downloadable AS & A Level Biology revision notes

The structure of the flagella

  • Similar in structure to cilia, made of longer microtubules
  • Contract to provide cell movement for example in sperm cells

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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