Molecular and Cell Biology notes
Lecture 1 – Fundamental features of eukaryotic cells
- Plant cells have a cell wall, provides support and constrains the shape and components of the cell.
- Means that we need something to allow communication – plasmodesmata.
- Site of photosynthesis: Chloroplasts.
- Plant cells tend to have large vacuoles – substances can be stored/secreted inside cells.
- Animal cells tend to have much smaller vacuoles.
- Eukaryotic cells are packed full of membranes – cells are very crowded.
- Protein in cells is 20-30% of the volume of the cytosol. Proteins are very abundant in cells.
- Prokaryotes don’t have a nuclear membrane.
Structures shared by animal and plant cells
- Mitochondria, nucleus, Golgi apparatus, rough endoplasmic reticulum, peroxisome.
Animal cells:
- Centriole: Organise spindle fibres during mitosis.
- Microvilli
- Glycosome: A peroxisome involved in glycogen storage and metabolism.
- Lysosome: Small vacuole.
Plant cells:
- Large vacuole
- Chloroplasts
- Cell wall
- Plasmodesmata
Cell size – why are cells so small?
- Metabolism needs fuel from outside and produces waste products.
- Import fuel and export waste products.
- Exchange limited by surface area to volume area.
- For a protein in the cytoplasm: 10ms to traverse E. coli and 10s to traverse an animal cell.
- E. coli is very small, so it takes a shorter time to move across than the mammalian cell.
- Small so things can diffuse easily through the cell. Very small cubes have a larger surface area : volume ratio.
- Small cells are easier to turnover than large cells, so if damage occurs they are easier to replace.
- Even small cells have problems with surface area for transport – in cells specialised for exchanging materials, the
surface area is dramatically increased and mitochondria abundant.
- Mitochondria provide energy to take up substances from the gut.
- Symbiotic bacteria in the gut.
- Transfer of nutrients in plants: Wall ingrowths increase the surface area of the cell membrane.
- Valonia ventricosa – largest-known plant cell.
- A coenocytic structure with multiple nuclei and chloroplasts and a large central vacuole.
- Coenocytic: Contains components of many cells contained in one big cell.
- Average person comprises about 40 trillion bacteria and 30 trillion human cells.
- Erythrocytes (red blood cells) 84% - most abundant human cell.
Why do cells and organelles have compartments?
- Membranes allow compartmentation in cells.
- Different environments (e.g. pH in vacuoles, mitochondria and chloroplasts).
- Assists metabolic regulation by keeping enzymes, substrates and regulators in separate locations.
- Locally high metabolite concentrations.
- Sequestration of toxic substances.
- Destruction/turnover of substances and organelles.
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,- Cells secrete and internalise large numbers of proteins (ER).
- Internal compartments of chloroplasts and mitochondria have an acidic pH to drive ATP synthesis.
Nucleus
- Contents: Genome, DNA replication, transcription, RNA processing, but translation in cytoplasm.
- Regulation: Compartmenting the genome from the cytoplasm allows gene expression to be regulated (e.g. post-
transcriptional processing, such as alternative splicing).
- Transport: Nuclear pore complexes transport RNA, ribosomes, proteins, carbohydrates, signalling proteins and
lipids, small particles diffuse passively.
- Can regulate the genome independently of the rest of the cell.
- Compartmentation of genetic material.
- Compartments – need transport to move substances around across membranes.
- Nuclear pores provide transport.
- ER is continuous with the outer membrane of the nucleus.
Cytoskeleton
- Protein filaments (actin and microtubules) and motor proteins.
- Microtubules: Cylindrical tubes of tubulin. Highly dynamic.
- Allows things to move within cells – movement.
- Protein filaments form a 3-D mesh – rigidity, shape and structure.
- Movement (trackways).
- In muscle cells, actin comprises 10% of total cell protein.
- In non-muscle cells, it is 1-5% of the cellular protein.
- A typical liver cell has 20,000 insulin receptors but 500 million actin molecules.
Cytoskeleton – shapers and movers
- Microfilaments: Actin fibres (3-6nm diameter).
- Movements: Gliding, contraction and cell cleavage. With myosin, responsible for muscle contraction.
- Microtubules: Cylindrical tubes (20-25nm diameter) of tubulin.
1. Determine cell shape.
2. Provide a trackway for movement of cell organelles and vesicles.
3. Form spindle fibres in mitosis; inside flagella and cilia.
- Intermediate filaments: (8-12nm diameter).
1. Anchor and position nucleus.
2. Give cell flexibility.
Cytoskeleton – movers:
- Movers (motor proteins) all powered by ATP: kinesin, dynein, myosin (muscle).
- Molecules and cargo-containing vesicles (and organelles) are moved around the cell by motor proteins. Actin
filaments also form trackways.
- Kinesis travels towards the “plus” end AWAY from the nucleus.
- Dynein travels towards the “minus” end TOWARDS the nucleus.
- Two sets of motor proteins that move things around the cell.
- Carry things along microtubule pathways.
Microtubules pattern plant cell wall synthesis
- Cortical microtubules form template for deposition of cellulose, get bands of cellulose around the cell.
- Tugor-driven plant growth is constrained along the axis of elongation (can only grow up).
- Microtubules form bands around the cell. Causes the cell to elongate in one direction – can’t grow outwards.
Lecture 2 – The secretory pathway in eukaryotic cells
- Cells produce molecules that have to be delivered to other places inside the cell, or exported out of the cell, at
exactly the right moment. These molecules are parcelled within vesicles.
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, Organelles and their functions
- Lycosome: Contains digestive enzymes that break down cell parts or substances entering by vesicles.
Transport vesicle:
- Takes proteins to Golgi apparatus.
- Takes lipids to Golgi apparatus.
- Golgi apparatus: Modifies lipids and proteins from the ER, sorts and packages them into vesicles.
- Smooth endoplasmic reticulum: Synthesises lipids and has various other functions.
- Rough endoplasmic reticulum: Synthesises proteins and packages them in vesicles.
- The ER/Golgi apparatus modifies and sorts proteins for transport throughout and outside the cell.
- In man, 1/3 of genes code for proteins that enter the ER.
- Polypeptides must fold and be modified correctly in the ER before being transported via the Golgi apparatus to
the cell surface or to lysosomes/vesicles.
The endoplasmic reticulum
- The rough ER processes folding and assembly of its own proteins and proteins destined to other locations.
- Protein translocation via translocation pore. N-terminal signal peptide is removed while the nascent polypeptide
is emerging into the ER lumen.
- ER lumen is specialised for folding, assembly, modification, quality control and recycling.
- Proteins undergo processes such as glycosylation and disulphide bond formation to enhance stability before
secretion.
- Glycoproteins have sugar attached.
ER in cells is dynamic:
- It moves around the cell rather than just remaining stationary.
- Plants – requires myosin and actin.
- Can show this by adding latrunculin – actin inhibitor, movement is facilitated by actin trackways.
- Melanocytes: Melanophores used by fish, amphibians, crustaceans, cephalopods and reptiles to change colour.
- Microtubules provide a template for growth.
Golgi apparatus
- The distribution, shipping and manufacturing department for many of the cell’s chemical products (carbs,
proteins and lipids).
- Doesn’t make proteins but makes lipids.
- Carbs of plant cell walls made here.
- Vesicle traffic: A protein complex enables vesicles to dock and fuse with their target membranes precisely.
- There are usually many vesicles moving around a cell.
Some proteins secreted via the Golgi apparatus in vertebrates:
Protein type Example Cellular site of synthesis
Serum proteins Albumin Hepatocyte (liver)
Extracellular matrix Collagen (structural protein) Fibroblasts
Proteins
Peptide hormones Insulin Pancreatic B-islet cells
Digestive enzymes Trypsin, amylase Pancreatic acini
Milk proteins Casein, lactalbumin Mammary gland
Glycoprotein (sugar) Mucus Widespread
Role of ER and Golgi in milk secretion
1. Transport of ions and water into the lumen.
2. Smooth ER: Formation of cytoplasmic lipid vesicles and lipid secretion.
3. Rough ER: Synthesis of milk protein (casein) and vesicle delivery via Golgi. Metabolic synthesis of lactose from
Glc and UDP-Gal occurs in Golgi.
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