BI0004 - Lecture 9 - Transport 2
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What happens in excocytosis?
Give an example.
In excocytosis, transport vesicles fuse with the plasma membrane to release their contents.
- Transport vesicles bud from the Golgi apparatus and move along microtubules to the plasma membrane.
- Specific proteins cause the vesicle and plasma membrane bilayers to fuse.
e.g. secretion of insulin by pancreatic β-cells, neurotransmitters by neurones.
What happens in endocytosis?
What are the three types?
In endocytosis, vesicles form at the plasma membrane to transport substances into cells.
- Receptor-mediated endocytosis
What is phagocytosis?
- Phagocytosis ('cellular eating') imports large particles for nutrition or defense.
- Involves Pseudopodia
What do food lysosomes do to food vacuoles?
The food vacuole fuses with a lysosome, and its contents are digested by hydrolytic enzymes
What is autophagy?
The process in which lysosome are also used to break down damaged organelles.
What is pinocytosis?
Pinocytosis ('Cellular drinking') non-specifically imports fluids and dissolved solutes.
What is receptor mediated endocytosis?
Receptor-mediated endocytosis imports specific substances bound to transmembrane receptors.
e.g. import of LDL provides the cell with cholesterol for membrane synthesis and for synthesis of other steroids
What are plasmodesmata and what do they do?
How does it work?
Plasmodesmata creates gaps that connect plant cells.
In plants, water, small solutes, and in some cases even RNA and protein, can pass directly from cell to cell via plasmodesmata
- The plasma membrane and cytoplasm of the two cells are continuous and sER runs though the plasmodesmata.
- Plasmodesmata also contains proteins that regulate passage of specific proteins, which coordinate the activity of adjacent cells.
What are gap junctions and what do they do?
Gap junctions create gaps that connect animal cells.
In animal tissues, water, ions and small solutes such as amino acids, sugars and nucleotides, can pass directly from cell to cell via gap junctions.
e.g. flow of ions from cell to cell in the heart acts as a signal that coordinates heartbeat
How can transport within cells be speeded up?
Transport within cells can be speeded up by cytoplasmic or protoplasmic streaming.
What to actin and myosin do?
They interact to cause movement.
- When the myosin head interacts with ATP, myosin attaches to actin and changes shape.
- The movement causes the actin filament to slide.
- Actin-myosin interactions can divide cells and move organelles and cytoplasm.
What does cytoplasmic streaming do?
Cytoplasmic streaming within plant cells speeds up the distribution of materials.
- A layer of cytoplasm cycles around the cell moving over a carpet of parallel actin filaments.
- Myosin motors attached to organelles in the fluid cytosol interact with actin to drive streaming.
How do ameobae move?
Amoeboid movement depends on localised contraction brought about by actin and myosin.
They have an outer cytoplasm gel with actin network and an inner cytoplasm sol with actin subunits.
- Pseudopodia extend by assembly of actin microfilaments that convert the cytoplasm from sol to gel inside the projections, while cell surface proteins attach to the substratum.
- Interaction of microfilaments with myosin near the trailing end causes contraction of that region, loosening the cell-surface attachment and pulling the cell forwards.
- Certain white blood cells also use amoeboid movement.
Where might diffusion be adequate for exchange and transport?
Diffusion may be adequate for exchange and transport where organisms are in intimate contact with their environment. e.g. volvox (unicellular alga), paramecium, rod shaped bacteria.
Diffusion may be adequate even in fairly large multicellular organism bathed in their environment. e.g. kelp forests - multicellular alga.
How does volume ratio work?
Larger organisms generally have a smaller surface area to volume ratio
Since for each mm2 of membrane only a limited amount of a particular substance can cross each second, there is a limit to the size of a cell/organism that can meet its metabolic requirements through transport across the plasma membrane alone.
How is diffusion limited?
Diffusion alone is not adequate for transporting substances over distances of more than a few mm.
The time taken for a substance to diffuse from one place to another is proportional to the square of the distance between them.
If it takes 1 second or a given quantity of glucose to diffuse 100μm it would take 100 seconds for the same quantity to diffuse 1mm and it would take nearly 3 hours for the same quantity to diffuse 1cm
Every cell in the body must be able to exchange materials with its environment (O2 and nutrients in, CO2 and waste out)
Most multicellular organisms over a critical size require a specialised transport system.
What must every cell be able to do?
What are two solutions to this?
Every cell must be able to exchange materials with its environment, across its plasma membrane.
Solution 1: Have a size and body that keeps many or all cells in direct contact with the environment.
Solution 2: Have a circulatory system that minimises the distance that substances must diffused to enter or leave a cell.
How can unicellular organisms exchange materials with its environment?
- A unicellular organism's entire surface is in contact with its surroundings.
- Very small animals (rotifers, tardigrades) are small enough that diffusion across their body surface is sufficient.
What is a gastrovascular cavity?
In multicellular organims, a gastrovascular cavity means all cells are bathed in fluid, facilitating exchange of gases and cellular waste.
- e.g. A Hydra is bilayered. Its mouth leads to a central gastrovascular cavity, which extends into its tentacles, so all its cells are in direct contact with the aqueous medium.
- Only the inner layer has direct access to nutrients released by digestion, but these rapidly diffuse to the outer layer.
What is the gastrovascular cavity like in jellies and cnidarians?
In jellies and cnidarians the mouth leads to an elaborate gastrovascular cavity consisting of radial canals leading to and from a circular canal. Ciliated cells lining the cavity circulate fluid within the cavity.
The gastrovascular cavity provides a large area for exchange.
How do Planarians facilitate exchange?
Planarians (e.g. Flat worm) have a gastrovascular facity and a flat body plan.
The flattened body has a high surface area:volume and so optimizes diffusional exchange by minimizing diffusion distances.
Substances enter through pharynx
In a circulatory system, what are three necessary basic components?
What does the circulatory system achieve?
- A circulatory fluid (blood or similar)
- A set of interconnecting vessels
- A muscular pump (heart) that uses metabolic energy to elevate the hydrostatic pressure of the circulatory fluid, which flows through the vessels and back to the heart.
By transporting fluid throughout the body, the circulatory system functionally connects the aqueous environment of the cells to the organs that exchange gases, absorb nutrients, and eliminate waste.
What is haemolymph?
The circulatory fluid in an open circulatory system.
What happens in an open circulatory system?
Where is it found?
In an open circulatory system, the circulatory fluid, haemolymph, also forms the interstital fluid.
Contraction of one or more hearts pumps haemolymph through circulatory vessels into interconnected sinuses, spaces surrounding the organ.
Chemical exchange between haemolymph and body cells takes place here (no need for diffusion across vessel walls).
Relaxation of the heart causes its internal pressure to drop below that in the body cavity and draws the haemolymph back into the vessels through pores which close when the heart contracts.
Body movements that squeeze the sinuses help circulate the haemolymph.
It is found in arthropods and most molluscs. Large crustaceans (lobsters, crabs) additionally have a more extensive system of vessels and an accessory pump.
What happens in a closed circulatory system?
Where is it found?
In a closed circulatory system, the circulatory fluid, blood, is distinct from the interstitial fluid.
The circulatory fluid is confined to the vessels.
One or more hearts pump blood into large vessels that branch into smaller vessels that infiltrate the organs.
Chemical exchange takes place between the blood and the interstitial fluid and between interstitial fluid and body cells.
Found in annelids (including earthworms), cephalopods (including squid, octopus) and all vertebrates.
What are the benefits and limitations of an open circulatory system?
Haemolymph is under relatively low hydrostatic pressure and therefore requires less energy expenditure.
No extensive system of blood vessels means less expensive to build and maintain.
May serve other functions too, e.g. to extend spider legs, and as a hytrostatic skeleton supporting the body in newly moulted aquatic arthropods.
However, low pressure means low flow rate, and without discrete continuous vessels, haemolymph cannot be directed towards tissues with high oxygen demand and CO2 buildup (except in some crustaceans). Most suitable for sedentary species that do not have high oxygen demands.
How do insects overcome the limitations imposed by low pressure haemolymph?
Insects overcome the limitations imposed by low pressure haemolymph by having a tracheal system that delivers oxygen directly to their tissues.
The heart pumps haemolymph through the open circulatory system.
Gas exchange takes place in the branched trachael tubes that infiltrate the body. Pores (Spiracles) open and close to control air flow and water loss.
What are the advantages and disadvantages of closed circulatory systems?
Closed vessels mean a high blood pressure can be generated and maintained.
High pressure systems enable a high flow rate and enable effective delivery of O2 and nutrients to the cells.
More energy is required to build, maintain and operate a closed circulatory system.
Blood can be directed to particular tissues as required, e.g. to the leg muscles during exercise or to the intestines after a meal.
Suitable for meeting the metabolic demands of larger and more active animals.
What specific vessels do closed circulatory systems have?
- Arteries: tough, thick-walled;
- take blood away from the heart under high pressure.
- Elastic recoil helps maintain blood pressure
- Arteries branch into arterioles
- Capillaries: walls are one cell thick.
- Exchange between blood and interstitial fluid takes place in capillary beds.
- Capillaries converge into venules, which converge into veins.
- Veins: Thinner-walled than arteries; return blood to heart under low pressure.
- Valves maintain unidirectional flow.
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