exchange substances (gas exchange) - topic 3

Question 1

how do single celled organisms exchange gases?

Answer 1

oxygen is absorbed by diffusion across their body surface , which is covered only by a cell-surface membrane
carbon dioxide from respiration diffuses across body surface

Question 2

describe the structure of the tracheal system in insects

Answer 2

-tiny pores called spiracles are on the body surface. this is where gases enter and leave.
- the trachea lead on from the spiracles, they are an internal network of tubes which are supported by strengthened rings to prevent them from collapsing. The gases travel down the trachea.
- trachea divide into tracheloes, dead end tubes that extend throughout all living tissue of the insect.
-atmospheric air is brought directly to living cells, as there is a short diffusion pathway from a tracheole to any body cell.

Question 3

In which 3 ways do respiratory gases move in and out of the tracheal system

Answer 3

1. Along a diffusion gradient ( respiring cells use up oxygen and so the conc towards the ends of tracheoles falls. this creates a diffusion gradient that causes oxygen to diffuse from atmosphere to cells. Carbon dioxide creates a concentration gradient in the opposite direction. )
2. Mass transport ( contraction of muscles in insects squeezes the trachea enabling mass movements of air in and out )
3. The ends of tracheoles are filled with water (during major activity, muscle cells around tracheoles respire and carry out some of this anaerobically. This produces lactate , which is a soluble and lowers cells’ water potentials. water is drawn in to cells by osmosis . Water in ends of tracheoles decrease in volume and in doing so draws air further into them. this means the final diffusion pathway is in a gas rather than in a liquid phase , and therefore diffusion is more rapid .

Question 4

how do insects control water conservation?

Answer 4

The spiracles may be opened/closed by a valve. when open, water vapour can evaporate from the insect. Insects mostly keep these closed to prevent water loss. tiny hairs around spiracles also help to trap water vapour leaving insect.

Question 5

what are the limitations to the gas exchange system of insects?

Answer 5

relies mostly on diffusion, and for diffusion to be effective, the diffusion pathway needs to be shorts and this is why insects are a small size. As a result the length of the diffusion pathway limits the size that insects can attain.

Question 6

what is the structure of gills (fish)?

Answer 6

• Gills located within the body of fish behind the head.
  -Made up of gill filaments .
• Gill filaments are stacked up in a pile.
  -At right angles to filaments, are gill lamellae , which increase the surface area over the gills .
• Water is taken in through the mouth and forced over the gills through an opening on each side of the body.
• The flow of water over the lamellae and the flow of blood within them are in opposite directions (countercurrent flow)

Question 7

What is the countercurrent principle in fish?

Answer 7

Blood and water flowing over the lamellae flow in opposite directions.
- Blood that is already well loaded with oxygen meets water, which has its maximum concentration of oxygen. therefore diffusion of oxygen from the water to the blood takes place
- Blood with little oxygen in it meets water which has had most, but not all, of its oxygen removed. Again, diffusion of oxygen from water to the blood takes place
- As a result, a diffusion gradient is maintained across the entire width of the gill lamellae. About 80 percent of oxygen available in the water is absorbed into blood.

Question 8

How do plants exchange gases?

Answer 8

carbon dioxide and oxygen taken in and leave via the stomata. Cells respire and use up oxygen and release carbon dioxide. However, cells in the palisade mesophyll carry out photosynthesis and so use carbon dioxide and release oxygen.

Question 10

What is the structure of a leaf?

Answer 10

Waxy, waterproof cuticle prevents evaporation. Stomata are small pores on the under side of the leaf which allow gases to enter/leave. Guard cells control stomatal opening. Spongy mesophyll has many air spaces to allow for rapid diffusion of gases to the palisade mesophyll, which contains the cells that photosynthesise.

Question 11

What is the function of stomata and guard cells?

Answer 11

Every stoma is surrounded by a pair of guard cells. Guard cells open the stomata when the plant has enough water which allows for gases to enter and water loss. When the plant is losing excessive water, the guard cells close the stomata to avoid water loss. Stomata get closed at night as there is no need for gas exchange to occur as photosynthesis cannot occur as there is no sunlight.

Question 12

What are the adaptations that xerophytes have to control water loss?

Answer 12

waxy waterproof cuticle stops evaporation from occuring.
Leaves that curl up in a way that the stomata are not exposed to air- moist air traps around the stomata to reduce rate of evaporation
Hairs around stomata to trap water vapour
Stomata in pits again to trap water vapour to reduce water loss.

Question 13

What is the structure of mammalian lungs?

Answer 13

-TRACHEA: -airway supported by rings of cartilage to prevent collapsing
- walls made up of muscle
- BRONCHI: - 2 divisions of the trachea. Produce mucus to trap dirt and have cilia to move it up the throat
-BRONCHIOLES: - series of branching subdivisions of the bronchi. Walls made of muscle lined with epithelial cells. This muscle allows them to constrict so that they can control the air flow in/out of the alveoli
-ALVEOLI: - tiny air sacs. Between the alveoli there are some collagen and elastic fibres. The fibres allow the alveoli to stretch as they fill with air when breathing in. They then spring back during expiration in order to expel the carbon dioxide rich air

Question 14

What is the process of inspiration (breathing in)?

Answer 14

-External intercostal muscles contract,while the internal intercostal muscles relax
-The ribs are pulled upwards and outwards, increasing the volume of the thorax
-The diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax
- the increased volume of the thorax results in reduction of pressure in the lungs
-Atmospheric pressure is now greater than pulmonary pressure, and so air is forced into the lungs

Question 15

What is the process of expiration (breathing out)?

Answer 15

-The internal intercostal muscles contract, while the external intercostal muscles relax
-The ribs move downwards and inwards, decreasing the volume of the thorax
-The diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration.The volume of the thorax is further decreased.
-The decreased volume of the thorax increases the pressure in the lungs
-Pulmonary pressure in now greater than that of the atmosphere, and so air is forced out of the lungs

Question 16

Why are the alveoli an efficient gas exchange surface?

Answer 16

-There are millions of alveoli in the lungs, and they are tiny. Therefore, they create a large surface area for diffusion to take place
-The alveolar epithelium is very thin (single layer of cells) , creating a short diffusion pathway for gases to leave/enter
- There is a steep concentration gradient maintained by ventilation and the constant circulation of blood from the heart .

Question 17

Why do we need to ventilate the lungs?

Answer 17

Diffusion alone is not fast enough to maintain adequate transfer of oxygen and carbon dioxide across the lungs. When we ventilate the lungs, the air is pushed across the exchange surface , and air we no longer need it pushed away. Breathing is a form of mass transport, making gas exchange much more efficient

Question 18

Why do we have to digest food molecules?

Answer 18

-Large molecules in food are too big to cross cell membranes, meaning they cannot be absorbed from the gut into the blood.
-Digestion hydrolyses large molecules into smaller molecules that can be absorbed across cell membranes, and easily be absorbed into blood and transported around the body for use of the body cells

Question 19

How are carbohydates digested?

Answer 19

-Amylase catalyses the breakdown of starch, by catalysing the hydrolysis of glycosidic bonds to produce maltose (a disaccharide) . Amylase is produced in the salivary glands and the pancreas.

-Once maltose is produced, a membrane-bound disaccharidase (maltase) that is attached to the cell membranes of epithelial cells lining the ileum (the final part of the small intestine) , help to break down disaccharides into monosaccharides. This again involves the hydrolysis of glycosidic bonds.

Question 20

How are proteins digested?

Answer 20

-Proteins are broken down by a combination of different peptidases. These enzymes catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between them.

Question 21

What is the role of endopeptidases in the breakdown of proteins?

Answer 21

-Endopeptidases act to hydrolyse peptide bonds within proteins. They split the polypeptide chain into smaller parts working from the inside of the protein molecule.

Question 22

What is the role of exopeptidases in the breakdown of proteins?

Answer 22

Exopeptidases act to hydrolyse peptide bond at the ends of protein molecules. They remove singular amino acids from the ends of polypeptide chains.

Question 23

What is the role of dipeptidases in the breakdown of proteins?

Answer 23

Dipeptidases are exopeptidases that work specifically on dipeptides by hydrolysing the peptide bonds between them. Dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine.

Question 24

How are lipids digested?

Answer 24

Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids. Lipases are mainly made in the pancreas into the small intestine where they act.

Question 25

What is the role of bile salts in the breakdown of lipids?

Answer 25

Bile salts are produced in the liver to emulsify lipids - this causes the lipids to form many smaller droplets. This creates a larger surface area that’s available for lipases to work on, rather than a single large droplet.

Question 26

Explain the co-transport of glucose and sodium ions in the ileum

Answer 26

-Sodium ions are actively transported out of the epithelial cell into the blood
-This reduces the sodium ion concentration into the epithelial cell
-Sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell via facilitated diffusion
-The protein the sodium ions diffuse through is a co-transporter protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient.
-Glucose then moves by facilitated diffusion into the epithelial cell to the blood.

Question 27

How are lipids absorbed into the blood?

Answer 27

-fatty acids and monoglycerides leave micelles and enter epithelial cells via diffusion
-these are modified back into triglycerides inside of the endoplasmic reticulum and Golgi body
-Fatty globules combine with proteins to from chylomicrons inside the Golgi apparatus
-Chylomicrons are extruded from the epithelial cell (exocytosis) and enter a lacteal (lymph capillary)
-Lymph in the lacteal transports chylomicrons away from the intestine.