May 1, 2020
3.1.3 Substances are exchanged between organisms and their environment by passive or active transport across exchange surfaces. The structure of plasma membranes enables control of the passage of substances across exchange surfaces.
Cells
- The structure of an epithelial cell from the small intestine as seen with an optical microscope.
- The appearance, ultrastructure and function of plasma membrane, including cell-surface membrane; microvilli; nucleus; mitochondria; lysosomes; ribosomes; endoplasmic reticulum; Golgi apparatus.
- Candidates should be able to apply their knowledge of these features in explaining adaptations of other eukaryotic cells.
Cells are the smallest unit of living things 🧫. For GCE AQA Biology, you should be familiar with their organelles - appearance, function, and ultrastructure. 🦠
Nucleus
Appearance
- The nucleus is contained by a double membrane called a nuclear envelope.
- The outer nuclear envelope is linked with the ER and has ribosomes
- This controls the movement of things in and out of the nucleus and holds all the reactions happening in the nucleus in one place
- Holes in the nuclear envelope are called nuclear pores
- Allow large molecules like mRNA to move in and out of the nucleus
- The nucleus is mainly made up of nucleoplasm, a jelly-like substance
- Nucleolus - small region inside the nucleoplasm
- Makes ribosomal RNA and assembles the ribosomes
Function
- Contains the cell’s hereditary material (DNA)
- Controls the cell’s activities - i.e. controls the protein synthesis in the cell
- Make ribosomal RNA and assemble the ribosomes
Plasma membrane/cell-surface membrane
- All membranes around or within cells or cell organelles are known as plasma membranes
- The cell-surface membrane is specifically the external membrane between a cell and its surrounding environment
Appearance
- Made up of a bilayer of amphipathic phospholipids
- The hydrophobic fatty acid tails (non-polar) face each other and are held together by weak hydrophobic interactions
- The hydrophilic phosphate heads (polar) face the aqueous outer environment/cell interior and are attracted to water
- Contain other molecules such as cholesterol, glycolipids, proteins, and glycoproteins
Function
- Cell-surface membrane: control the movement of substances in and out of the cell
- Lipid-soluble substances can pass through the fatty acid tail layer & move freely
- Water-soluble substances cannot
- Different types of proteins embedded in the membrane help to regulate the movement of specific substances
- Plasma membranes (organelles):
- Compartmentalise organelles so that metabolic reactions can proceed with the best possible conditions
- Isolate damaging enzymes in lysosomes
- Control entry and exit of materials within the cell
- Act as an internal transport system e.g. endoplasmic reticulum
Mitochondria
Appearance
- Surrounded by a double membrane (Mitochondrial membrane)
- The inner membrane has many invaginations and folds called cristae
- The inside of the mitochondria is called the matrix
- Contains proteins, lipids, ribosomes, DNA
Function
- Mitochondria are the site of aerobic respiration
- The Krebs cycle and electron transport chain occur in the mitochondria
Lysosomes
Appearance
- Round vesicle-shaped organelle
- Contains digestive enzymes
- Single-membraned
Function
- Digest used-up organelles to recycle the chemicals used to make them
- Break down cells after cell death
- The enzymes can be exocytosed to destroy materials outside of the cell
- Lysosomes in phagocytes are also responsible for hydrolysing ingested white blood cells or bacteria
Ribosomes
Appearance
- Composed of a large subunit and a small subunit
- Large subunit - 3 tRNA binding sites
- Small subunit - 1 mRNA binding site
- Made of both RNA and protein
- Size: 70S in prokaryotes, 80S in eukaryotes
Function
- Ribosomes on ER: Synthesis of proteins for export
- Free ribosomes: Synthesis of proteins for use inside the cell itself
Rough endoplasmic reticulum
Appearance
- Flattened layers of membrane sacs called cisternae, and tubules
- Continuous with the outer nuclear membrane
- Ribosomes dotted on the surface of the ER (hence, rough)
Function
- Synthesise proteins and glycoproteins
- Transport proteins throughout the cell
Smooth endoplasmic reticulum
Appearance
- Flattened layers of membranes called cisternae, and more tubules
- Continuous with the outer nuclear membrane
- No ribosomes dotted on the surface of the ER (hence, smooth)
Function
- Synthesise, transport, and store lipids and carbohydrates
Golgi apparatus
Structure
- Flattened layers of membrane sacs called cisternae
- Associated with vesicles
Function
- Process and export the proteins and lipids made by the ER
- Modify proteins by adding non-protein molecules
- Finalised proteins and lipids are sent to Golgi vesicles
- Vesicles fuse with the cell surface so the contents can be exocytosed
All together in a cell, the organelles are arranged as follows:
Cell structure & function
Although most animal cells contain some number of these organelles, some cells will contain more or less of specific organelles depending on their function. For example:
- Muscle cells have lots of mitochondria, which allows them to release energy via aerobic respiration more quickly in order to contract rapidly during exercise
- Secretory cells in the pancreas have lots of rER, which allows them to produce a lot of protein (hormones, enzymes)
You will also be asked to examine an intestinal epithelial cell in GCE AQA Biology. Special organelles to look out for include:
- Extensive rough ER and Golgi apparatus, as the cell needs to make lot of enzymes (proteins) for digestion
- Lots of mitochondria as the cell needs lots of ATP for the absorption of digested materials via active transport
- Microvilli - small hair-like projections on the surface of the cell - that help the cell to increase its surface area to volume ratio and increase the rate of absorption
That's all for today!
Reference
Toole, G., & Toole, S. (2015). Aqa biology A level. Oxford: Oxford University Press.
Images from
- https://biologica.concord.org/webtest1/Shout3d_cell_6_1_00/v2_top_right_nucleus.htm
- https://www.britannica.com/science/membrane-biology
- https://opentextbc.ca/biology/chapter/3-3-eukaryotic-cells/
- https://www.pinterest.com/pin/355925176794895874/
- https://twitter.com/rollribosome
- https://personalpages.manchester.ac.uk/staff/j.gough/lectures/the_cell/cell_structure/SER1.jpg
- https://www.britannica.com/science/Golgi-apparatus
- https://alevelbiology.co.uk/notes/characteristics-of-eukaryotic-cellular-structures/
- https://st2.depositphotos.com/1067125/9848/v/950/depositphotos_98484204-stock-illustration-intestinal-epithelial-cell-with-microvilli.jpg
- http://getwallpapers.com/collection/cell-biology-wallpaper