Eukaryotic cell ultrastructureEdexcel International A Level Biology: Revision notes
Section 1
Cells and their organisation
All living organisms are made of cells, and all cells share some common features: a cell surface membrane, cytoplasm, genetic material (DNA) and ribosomes for protein synthesis.
In a multicellular organism, cells with the same specialised function form a tissue. Different tissues combine into an organ (for example the liver), and organs work together as an organ system (for example the digestive system). Eukaryotic cells have a nucleus and other membrane-bound organelles, each with a specific function. The structure seen with an electron microscope is called the ultrastructure.
Section 2
The nucleus, nucleolus and ribosomes
The nucleus is surrounded by a nuclear envelope, a double membrane with nuclear pores. It contains DNA (as chromatin) and controls the cell's activities by determining which proteins are made. mRNA passes out through the pores.
The nucleolus is a dark-staining region inside the nucleus that makes ribosomal RNA (rRNA) and assembles ribosomal subunits.
Ribosomes are tiny structures made of rRNA and protein with no membrane. They are the site of protein synthesis (translation). They are found free in the cytoplasm and attached to the rough endoplasmic reticulum.
Ribosomes make the polypeptide; the rER does not. The nucleolus makes ribosomal subunits; it does not make proteins itself.
Section 3
Endoplasmic reticulum, Golgi apparatus and lysosomes
Rough endoplasmic reticulum (rER) is a system of flattened sacs (cisternae) with ribosomes on the outer surface. Polypeptides made on these ribosomes enter the lumen, where they are folded and then transported in vesicles.
Smooth endoplasmic reticulum (sER) has no ribosomes. It makes lipids, such as steroid hormones, and is involved in detoxification.
The Golgi apparatus is a stack of flattened membrane-bound sacs. It modifies proteins (for example adding carbohydrate), packages them into vesicles and forms lysosomes.
Lysosomes are small spherical organelles with a single membrane. They contain hydrolytic enzymes that digest worn-out organelles and material taken in by the cell, such as bacteria.
Section 4
Mitochondria and centrioles
Mitochondria have a double membrane. The inner membrane is folded into cristae, giving a large surface area for the reactions of aerobic respiration, which make ATP. The fluid inside is the matrix. Cells with a high demand for ATP, such as liver cells, have many mitochondria.
Centrioles are found in animal cells in pairs. Each is a hollow cylinder of microtubules. They are involved in forming the spindle during cell division.
Section 5
Recognising organelles in electron micrographs
Use shape and membranes to identify organelles.
- Nucleus: large and round, double membrane with pores; a dark nucleolus inside.
- Mitochondrion: oval or rod shape, double membrane, inner folds (cristae).
- rER: flattened sacs covered in small dark dots (ribosomes).
- sER: tubules with no dots.
- Golgi apparatus: stack of curved flattened sacs with small vesicles at the edges.
- Lysosome: small, round, single membrane, dense contents.
- Ribosomes: tiny dark dots, free or on the rER.
Describe features, then identify: for example 'folded inner membrane, so a mitochondrion'. Remember that a micrograph is a two-dimensional slice, so shapes can look different.
Section 6
Protein transport and extracellular enzymes
The rER and Golgi apparatus work together to make and export proteins such as digestive enzymes.
- The gene is transcribed in the nucleus and the mRNA leaves through a nuclear pore.
- A ribosome on the rER translates the mRNA and the polypeptide enters the lumen of the rER, where it is folded.
- Transport vesicles bud off from the rER and fuse with the Golgi apparatus.
- The Golgi modifies the protein and packages it into secretory vesicles.
- Secretory vesicles move to the cell surface membrane and fuse with it, releasing the protein by exocytosis.
An enzyme released like this is an extracellular enzyme. Each step needs ATP from mitochondria.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Eukaryotic cell ultrastructure
- A student examines an electron micrograph of an acinar cell from the pancreas, which secretes the digestive enzyme amylase. The cell has a large amount of rough endoplasmic reticulum, a prominent Golgi apparatus and many small vesicles close to the cell surface membrane.Explain why this cell having a large amount of rough endoplasmic reticulum is an advantage.2 marks
- Tay-Sachs disease is an inherited disorder in which a lysosomal enzyme that breaks down a lipid is missing. The lipid builds up inside nerve cells, which swell and become damaged.Explain why the lipid builds up inside the nerve cells.2 marks
- Pancreatic cells were given a short pulse of a radioactively labelled amino acid and then transferred to a medium containing only unlabelled amino acids. After 3 minutes most radioactivity was in the rough endoplasmic reticulum. After 20 minutes most was in the Golgi apparatus. After 60 minutes most was in vesicles near the cell surface membrane and in the medium surrounding the cells.Describe the pathway taken by the labelled protein, as shown by these results.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).