3B Mass Transport

How are carbohydrates broken down in digestion?

• by amylase and membrane-bound disaccharidases.

• Amylase catalyses the conversion of starch into maltose and is produced in the salivary glands and by the pancreas.

• Membrane-bound disaccharidases are attached to the epithelial cells lining the ileum and break down the disaccharides like maltose into monosaccharides which can be transported across the membrane of the ileum via specific transporter proteins.

How are lipids broken down in digestion?

• lipase catalyses the conversion of lipids into monoglycerides and fatty acids and is made in the pancreas.

• Bile salts are made in the liver and emulsify lipids, causing them to form smaller droplets. This gives them a bigger surface area for lipase to work on.

• When the lipid has been broken down, the monoglyceride fatty acids stick to the salts to form micelles.

How are proteins broken down during digestion?

• by endopeptidases and exopeptidases into amino acids.

What are endopeptidases?

Enzymes that act to hydrolyse the peptide bond within a protein. Eg. Pepsin is released into the stomach and only works in acidic conditions.

What are exopeptidases?

Enzymes that act to hydrolyse peptide bonds at the ends of protein molecules, removing single amino acids from proteins. Dipeptidases are exopeptidases that work specifically on dipeptides and act to separate the two amino acids. These are often located in the cell-surface membrane of the ileum.

How are monosaccharides absorbed?

by co-transporter proteins with sodium ions (glucose and galactose)

Fructose is absorbed via facilitated diffusion.

How are monoglycerides absorbed?

Micelles help to move the monoglycerides and fatty acids towards the epithelium, where they can be absorbed as they are lipid-soluble. Micelles constantly break up and reform and so can release monoglycerides and fatty acids to be absorbed, but are not taken up whole themselves.

How are amino acids absorbed?

Via co-transporter proteins with sodium ions.

Describe a haemoglobin molecule?

• large protein with quaternary structure- four polypeptide chains.

• Each chain has a haem group containing an iron ion.

• Each molecule can carry four oxygen molecules

What does haemoglobin saturation depend on?

Partial pressure of oxygen- the greater the concentration of dissolved oxygen, the higher the partial pressure. Where there is a high partial pressure, oxygen loads onto Hb to form oxyhaemoglobin. When there is a low partial pressure this is reversed. When cells respire, they use oxygen, lowering the pO2

Why are Hb dissociation curves s-shaped?

When haemoglobin combines with the first O2 molecule, its shape alters to make it easier for other molecules to join too.

How does CO2 concentration affect oxygen unloading?

At high partial pressures of CO2, haemoglobin gives up oxygen more easily. This causes a dissociation curve to shift to the right. The reverse is also true: low partial pressures makes it harder for Hb to give up O2 and the dissociation curve shifts to the left.

How is Hb different in different organisms?

Organisms in environments with a low concentration of oxygen have Hb with a higher affinity for oxygen, so the dissociation curve shifts to the left.

Organisms that are very active and have a high oxygen demand have Hb with a lower affinity for oxygen, so the curve is to the right.

What does it mean that humans have a double circulatory system?

There are two loops- blood is transported from the heart to the lungs and back to the heart to be transported around the body.

How are arteries adapted?

• thick, muscular walls

• Elastic tissue to stretch and recoil, maintaining high pressure and smoothing the flow

• The endothelium is folded, allowing stretch

How are arterioles adapted?

Similar to arteries- these are smaller vessels that the arteries branch into.

How are veins adapted?

Wider lumen

Very little elastic or muscle

Valves to prevent backflow

How are capillaries adapted?

Very near exchange tissues for short diffusion pathway

One cell thick

Large number to increase surface area for exchange

What is tissue fluid?

Fluid surrounding cells, made from small molecules leaving the blood plasma.

Explain pressure filtration:

• at the start of the capillary bed, the hydrostatic pressure is greater in the capillaries than the tissue fluid.

• The overall outward force forces fluid out of the capillaries into the spaces around cells, forming tissue.

• Hydrostatic pressure reduces in the capillaries so it is lower at the venule end, and due to the fluid loss and increasing concentration of plasma proteins the water potential is lower at the end of the capillary bed than in the tissue fluid

• Therefore water reenters the capillaries from the tissue fluid at the venule end by osmosis

• Excess tissue fluid is drained by the lymphatic system, which dumps it back into the circulatory system.

How is the left ventricle of the heart adapted?

Thicker, more muscular wall to contract more powerfully.

Describe the valves of the heart.

AV- link atria to ventricles. Stop blood flowing back to atria when ventricles contract.

SL- link ventricles to P-artery and aorta

Describe the cardiac cycle.

• ventricles relax, atria contract. Blood is pushed into the ventricles as the volume in the atria decreases so pressure increases.

• Ventricles contract, atria relax. Pressure becomes higher in the ventricles, forcing AV valves shut to prevent back flow and forcing the SL valves open so blood can move into the arteries.

• Both ventricles and atria relax. Blood returns to the heart and the atria refill. Pressure in the ventricles fall so the AV valves can open and blood can flow passively in.

How does water move up a plant?

Water evaporates from leaves, creating tension that pulls more water into the leaf through cohesion. This means the whole column of water in the xylem moves upwards.

What is transpiration?

Water evaporates from moist cell walls and accumulates between cells. When the stomata open, water moves out down a concentration gradient.

What factors affect transpiration?

Light

Temperature

Humidity

Wind

What is a potometer?

Equipment used to measure rate of transpiration

Describe phloem tissue.

Sieve tube elements- living cells that form the tube. They have no nucleus and limited organelles.

Companion cells- they carry out living functions for sieve cells.

What is translocation?

The movement of solutes such as sucrose. It is energy requiring and happens in the phloem. Enzymes maintain a concentration gradient from the source to the sink by changing the solutes at the sink.

Explain mass flow hypothesis for phloem transport.

• active transport loads solutes into the sieve tubes from the companion cells at the source. Water potential is lowered so water also enters by osmosis, creating a high hydrostatic pressure in the phloem.

• At the sink solutes are removed from the phloem, increasing the water potential so water leaves by osmosis. This lowers the hydrostatic pressure.

• The process creates a pressure gradient from the source to the sink, pushing solutes along to the sink.

Evidence for Mass Flow

Ring of bark removed- a bulge forms with a higher concentration of sugars.

Radioactive tracers can also be used.

Objections to Mass Flow of phloem transport

Sugar travels to many sinks, not just one with the highest wp.

Sieve plates would create a barrier, so a lot of pressure would be needed for a reasonable rate of transportation.