Topic 3: Exchange of substances

Question 1

What is tissue fluid?

How is it kept constant and why?

Answer 1

The environment around the cells of multicellular organisms.

Most cells are too far from exchange surfaces or the external environment for diffusion to supply and remove materials to keep its composition constant. Mass transport systems are used for this instead.

Question 2

What do organisms need to exchange with their environment? Give examples

Answer 2

• Respiratory gases (e.g O2/CO2)
• Nutrients (e.g glucose, fatty acids, amino acids)
• Excretory products (e.g urea, faeces, CO2)
• Heat

Question 3

What is the difference between passive and active exchange?

Answer 3

Active exchange requires metabolic energy, passive exchange does not

Question 4

How do very small organisms exchange substances with their environment? Why?

Answer 4

Simple diffusion of substances across the outer surface is sufficient to meet their needs because they have a very large surface area to volume ratio.

Single-celled organisms have a surface only covered by a cell-surface membrane or cell wall so there is a short diffusion pathway.

Question 5

Why do large organisms not exchange substances with their environment via diffusion?

Answer 5

As organisms get larger, their volume increases at a faster rate than surface area, decreasing surface area to volume ratio.

Metabolic rate is proportional to volume but diffusion is proportional to surface area, so the rate of diffusion would not be sufficient enough to support their needs.

It would also take too long to reach the centre of the organism as most cells aren’t in contact with the environment.

Question 6

How have organisms evolved to exchange substances?

Answer 6

• A flattened shape so no cell is far from the surface for simple diffusion
• Specialised exchange surfaces with large areas to increase surface area to volume ratio

Question 7

What is the equation for the surface area of a sphere?
What is the equation for the volume of a sphere?

Answer 7

SA = 4 x pi x r^2

V = 4/3 x pi x r^3

Question 8

Give some features of exchange surfaces and state how they are an advantage

Answer 8

• Large SA:V - increases rate of exchange
• Very thin - short diffusion pathway to increase rate of exchange
• Selectively permeable - allows selected materials to cross
• Movement of environmental medium - maintains concentration gradient to increase rate of exchange
• Transport system - maintains internal medium movement to maintain concentration gradient to increase rate of exchange

Question 9

Why are exchange surfaces usually located inside the organism?

Answer 9

They are very thin, so are easily damaged and dehydrated. They would dessicate

Question 10

How have insects evolved to reduce water loss?

Answer 10

• Small SA:V to minimise the area over which water is lost
• Waterproof cuticle over their rigid chitin exoskeleton
• Spiracles can be closed to reduce water loss. Conflicts with the need for oxygen so usually occurs at rest
• Spiracle hairs trap water being evaporated, maintaining a humid environment in the spiracle, reducing water loss. Also minimises bulk air movement through the gap, reduces the area of the spiracle and allows for filtering of the air

Question 11

Describe the structure of the insect gas exchange system

Answer 11

Tracheal system:
- Spiracles - tiny pores on the body surface that can be open/closed by a valve
- Tracheae - internal network of tubes attached to spiracles. Supported by rings of chitin ( a nitrogen-containing polysaccharide) to prevent them from collapsing at low pressures
- Tracheoles - smaller, dead-end tubes that branch from tracheae into body tissues - atmospheric air is brought directly to respiring tissues, providing a short diffusion pathway to any body cell

Question 12

Give three ways that respiratory gases move in/out the insect tracheal system

Answer 12

• By concentration gradients
• Using mass transport (abdominal pumping)
• Using the water-filled ends of tracheoles

Question 13

Describe how the tracheal system in insects uses concentration gradients to exchange gases

Answer 13

• Oxygen is used in respiring cells, creating a concentration gradient in the system
• Oxygen diffuses from the atmosphere, through the tracheae and tracheoles into the cells
• The inverse happens with carbon dioxide

Question 14

Describe how the tracheal system in insects uses abdominal pumping to exchange gases

Answer 14

• Creates mass transport
• Muscles pull segments of the skeletal plates together
• This squeezes air into sacs deeper in the tracheal system
• Reduces pressure in the system to lower than atmospheric pressure
• Spiracles open and air moves in

Question 15

Describe how the tracheal system in insects uses the water-filled ends of tracheoles to exchange gases

Answer 15

• In periods of activity, muscles respire partially anaerobically, producing soluble lactate
• Lowers the water potential of cells
• Water moves from the tracheole into the cell by osmosis
• Air is drawn into the space in the tracheole vacated by water
• Increases the rate of diffusion as there is a smaller diffusion pathway and diffusion occurs more quickly in the gas phase than liquid.

Question 16

Why do fish need gills for gas exchange?

Answer 16

Need a specialised exchange surface as they can’t do gas exchange just by diffusion because:
- Fish have a waterproof, gas-tight outer covering
- They have a small SA:V
- Oxygen is not very concentrated in water and warm water has less oxygen dissolved in it than cold water

Question 17

Describe the location of the gills and the flow of water through them

Answer 17

Located just behind the head and are protected by the operculum as they are very delicate.

Water passes through the mouth, past the gills and out the operculum opening, maintaining a water current across the gills.

Question 18

Describe the structure of the gills in fish

Answer 18

• Made from gill filaments stacked in a pile (increases surface area)
• Gill lamellae sit at 90 degrees to the filaments (increases surface area)
• Thin epithelium (short diffusion pathway)
• Good blood supply (maintains a concentration gradient)
• Water and blood flow in opposite directions = countercurrent flow (maintains concentration gradient across whole gill lamellae)

Question 19

What is the countercurrent exchange principle?

Answer 19

Blood + water flow in opposite directions through the gills.

Maintains a concentration gradient across the whole length of the lamellae as the concentration of oxygen in the water is always slightly higher than the concentration in the blood.

Circulation replaces oxygen-rich blood, ventilation replaces oxygen-poor water.

Oxygen never reaches equilibrium so about 90% of oxygen from the water can be absorbed.

Question 20

What is parallel flow?

Answer 20

Blood and water flow in the same direction through the gills.
There is no net diffusion once the concentrations of oxygen equalise so diffusion only occurs at the first part of the filament.

Question 21

How does gas exchange in plants change with the activity of the plant?

Answer 21

• When more photosynthesis is happening, CO2 from the air is used and O2 produced. Some CO2 comes from respiration but most diffuses in. Some O2 is used in respiration but most diffuses out
• When photosynthesis slows down/stops, O2 diffuses in as it is used in respiration, CO2 produced by respiration diffuses out

Question 22

Describe the adaptations of leaves for gas exchange

Answer 22

• Flat shape = no living cell is far from external air (short diffusion pathway)
• Diffusion occurs in the gas phase (quicker than as a liquid)
• Air spaces in the spongy mesophyll tissue increases the surface area

Question 23

What are stomata and where are they located?

Answer 23

Small pores on leaves surrounded by a pair of guard cells which open/close the stomatal aperture (the opening).

They are small and mostly on the underside of leaves as it is shaded there, reducing water loss by evapotranspiration

Question 24

Describe how stomata help to regulate water levels in plants

Answer 24

They open/close to balance the need to reduce water loss with the need to maximise gas exchange.

When the plant has a lot of water, guard cells become turgid and open, allowing gas exchange.

When the plant is short of water, guard cells become flaccid and close, preventing water loss.

Sensitive to light (close at night to reduce water loss as photosynthesis can’t occur so there is no need for gas exchange)

The opening/closing is possible due to the thickened inner walls + thin outer walls

Question 25

How do guard cells work?

Answer 25

- Actively transport K+ ions inside to control water movement - Concentration of K+ ions inside guard cell reduces water potential - Water moves into guard cell by osmosis - Guard cell expands and becomes turgid - Rigid inner wall resists expansion + it becomes more curved - Stomata open because the inner wall is less flexible than the outer wall - When they lose water, they become flaccid and collapse, closing the stomata

Question 26

What is a xerophyte?

Answer 26

Plants adapted to living in areas with a limited water supply

Question 27

Name some adaptations of xerophytes

Answer 27

- Thick waterproof waxy cuticle - Rolled leaves - Hairy leaves - Sunken stomata in pits on the epidermis - Stomata at the bottom of ridges - Low stomatal density - Small SA:V - Barrel-like stems - Extensive roots

Question 28

Explain these adaptations of xerophytes and give an example: - Thick waterproof waxy cuticle - Rolled leaves

Answer 28

- Increases length of diffusion pathway and makes leaf impermeable to water, e.g holly - Reduces surface area of exposed leaf. Traps still air inside which becomes saturated with water vapour so has a high water potential. No water potential gradient between the inside and outside of leaf so no water loss, e.g marram grass

Question 29

Explain these adaptations of xerophytes and give an example: - Hairy leaves - Sunken stomata in pits on the epidermis

Answer 29

- Especially on the lower epidermis, traps still moist air next to the leaf, reduces the water potential gradient, reducing water loss, e.g heather - Traps moist air next to the leaf, lowering the water potential gradient. Shelters stomata from conditions that increase transpiration, e.g pine trees

Question 30

Explain these adaptations of xerophytes: - Stomata at the bottom of ridges - Low stomatal density

Answer 30

- Sun only shines directly on stomata in brief periods of the day, reducing transpiration - Reduces water loss by evaporation

Question 31

Explain these adaptations of xerophytes: - Small surface area to volume ratio - Barrel-like stems - Extensive roots

Answer 31

- Small and roughly circular cross sections of leaves reduce rate of evaporation - Lets plant hold a lot of water - Very deep roots can reach and absorb more water. Very shallow, widespread roots easily absorb any rainfall from the surface. Some plants can store water in their roots

Question 32

Give some places that xerophytes can be found

Answer 32

- Deserts (low rainfall + high temperatures) - Sand dunes (high winds on coast increase transpiration, rain drains through sand quickly out of reach from roots) - Cold regions (water is frozen so very difficult to absorb) - Salt marshes (the water has a low water potential due to the high salt concentration so is difficult to absorb)

Question 33

Why do aerobic organisms need to exchange gases?

Answer 33

They need a constant supply of oxygen to release energy in the form of ATP during respiration. The carbon dioxide must be removed as its build-up could be harmful for the body

Question 34

What is the relative volume of gases exchanged for mammals and why? How have they adapted to cope with this?

Answer 34

The volume of oxygen absorbed and carbon dioxide removed is large because: - They're relatively large organisms with a large volume of living cells - They maintain a high body temperature so have high metabolic and respiratory rates They have evolved lungs as a specialised gas exchange surface

Question 35

Why are mammalian lungs not inside the body?

Answer 35

- Air alone is not dense enough to support them - The body would otherwise lose too much water and dehydrate

Question 36

Where are the lungs in the human body and how are they ventilated?

Answer 36

Supported and protected by the ribcage which can be moved by intercostal muscles. Ventilated by a tidal stream of air, ensuring the air within them is constantly replenished

Question 37

Name the structures making up the lungs

Answer 37

- Trachea - Bronchi - Bronchioles - Alveoli

Question 38

Describe the structure of the trachea

Answer 38

- Flexible airway supported by C-shaped rings of cartilage to prevent collapse at low pressures - Can collapse slightly to allow food to pass down the oesophagus - Walls are made from muscle lined with ciliated epithelial cells and goblet cells - Goblet cells produce mucus, trapping dirt and microorganisms - Cilia move mucus up to the pallet, where it passes down the oesophagus into the stomach

Question 39

Describe the structure of the bronchi

Answer 39

Two divisions of the trachea, each leading to one lung. Also have goblet cells, ciliated epithelial cells and cartilage, but the amount is reduced as the bronchi get smaller

Question 40

Describe the structure of the bronchioles

Answer 40

A series of branching subdivisions of the bronchi. Their walls are made from smooth muscle lined with epithelial cells. Muscle allows them to constrict so they can control the flow of air in and out the alveoli

Question 41

Describe the structure of the alveoli

Answer 41

Minute air sacs at the ends of bronchioles, diameter 100-300um. Between them is collagen and elastic fibres. Made from a single layer of flattened epithelial cells. Elastic fibres let them stretch to fill with air during inhalation and recoil in exhalation so carbon dioxide-rich air can be expelled. The exchange surface is the alveolar membrane

Question 42

What is ventilation (breathing)?

Answer 42

The alternate increasing and decreasing of pressure in the lungs relative to the atmosphere. Air moves in and out to maintain diffusion across the alveolar epithelium.

Question 43

Describe the process of inspiration

Answer 43

- An active process - External intercostal muscles actively contract - Internal intercostal muscles relax - Ribs and sternum move upwards and outwards - Diaphragm contracts (moves downwards) + flattens - Volume of the thorax increases - Elastic tissue of the lungs is stretched and lungs expand to fill thoracic cavity - Pressure inside lungs decreases, atmospheric pressure is greater than pulmonary pressure so air is forced in - As lungs fill with air, stretch receptors send impulses to the expiratory part of the respiration centre to end breathing in

Question 44

Describe the process of expiration

Answer 44

- During normal breathing it is largely passive (air forced out by recoil of elastic tissue), it is only active + using muscles during forced expiration - External intercostal muscles relax - Internal intercostal muscles contract - Ribs + sternum move downwards and inwards - Diaphragm relaxes (moves up) and returns to its dome shape - Volume of the thorax decreases - Elastic tissue of the lungs recoil so lung size reduces - Pulmonary pressure increases above atmospheric pressure so air is forced out - As air leaves, stretch receptors are no longer stimulated - the inhibition of inspiration stops so it can start again

Question 45

What is pulmonary ventilation?

Answer 45

The volume of air moved into the lungs in one minute (dm^3/min)

Question 46

What is ventilation rate?

Answer 46

The number of breaths that occur in one minute (min^-1). Usually around 15-20 for a healthy adult

Question 47

What is tidal volume?

Answer 47

The volume of air taken in at each breath during rest (dm^3). Usually around 0.5dm^3

Question 48

Give the equation for pulmonary ventilation

Answer 48

pulmonary ventilation = tidal volume x ventilation rate

Question 49

Give some characteristics of the lungs that aid gas exchange

Answer 49

- Red blood cells slowed as they pass through pulmonary capillaries = more time for diffusion - Red blood cells compressed against capillary walls = short diffusion pathway - Alveolus epithelium + capillary endothelium are 1 cell thick = short diffusion pathway - Alveoli + close network of pulmonary capillaries have a very large surface area - Ventilation constantly replenishes air + heart action constantly circulates blood around alveoli = maintains concentration gradient

Question 50

What is correlation?

Answer 50

When a change in one of two variables is reflected by a change in the other. It is not enough to prove a causal relationship

Question 51

Give some risk factors for lung disease

Answer 51

- Smoking - Air pollution - Genetics - Infections - Occupation

Question 52

Name some lung diseases

Answer 52

- Pulmonary fibrosis - Asthma - Emphysema

Question 53

Describe pulmonary fibrosis

Answer 53

Scar tissue forms on the lungs, lungs stiffen so the elasticity is reduced. Symptoms - dry cough, chest pain, shortness of breath, weakness, fatigue Causes - idiopathic (exact cause unknown) but has a correlation with pollutants

Question 54

Describe asthma

Answer 54

Inflammation of the bronchi and bronchioles. More mucus produced so increased resistance to air flow in/out the lungs = air in alveoli not replaced as efficiently Symptoms - tight chest, difficulty breathing, coughing Causes - genetics, allergens

Question 55

Describe emphysema

Answer 55

Elastin protein in the lungs isn't replaced, causing irreversible damage and decreased elasticity so lungs are unable to force air out alveoli (inability to recoil - lower tidal volume). Walls of alveoli broken down = lower surface area and longer diffusion pathway Symptoms - severe shortness of breath, chronic cough, blueish skin Causes - smoking

Question 56

What is the human digestive system?

Answer 56

A long muscular tube and its associated glands, which produce enzymes that hydrolyse macromolecules into simple molecules ready for absorption. It breaks down food physically and chemically and absorbs the soluble products.

Question 57

What are the major parts of the human digestive system?

Answer 57

- Oesophagus - Stomach - Ileum - Large Intestine - Rectum - Salivary glands - Pancreas

Question 58

Describe the structure and function of the oesophagus

Answer 58

Made from a thick muscular wall, carries food from the mouth to the stomach

Question 59

Describe the structure and function of the stomach

Answer 59

A muscular sac with an inner layer that produces enzymes. Stores and digests food, especially proteins as its glands produce proteases. Other glands in the wall produce mucus, preventing self-digestion from enzymes and hydrochloric acid

Question 60

Describe the structure and function of the ileum

Answer 60

A long muscular tube where food is further hydrolysed by enzymes produced in its walls and glands elsewhere. Its inner walls are folded into villi, making a large surface area that is further increased by microvilli on epithelial cells. This is how it is adapted to absorb the products of digestion into the bloodstream

Question 61

Describe the function of the large intestine

Answer 61

Absorbs water, most of which comes from the secretions of many digestive glands. Food becomes drier and thicker and forms faeces

Question 62

What is the rectum?

Answer 62

The final section of the intestines where faeces are stored before periodically being removed via the anus in egestion

Question 63

What are the salivary glands?

Answer 63

They are near the mouth, and pass secretions containing amylase into the mouth, which catalyses the hydrolysis of starch into maltose

Question 64

What is the pancreas?

Answer 64

A large gland below the stomach. Produces pancreatic juice containing proteases, lipases and more amylase.

Question 65

What are the two stages of digestion?

Answer 65

Physical breakdown Chemical digestion

Question 66

Describe the physical breakdown stage of digestion

Answer 66

Large pieces of food are broken into smaller pieces by structures like the teeth, making it possible to ingest food and gives a large surface area for chemical digestion. Food is churned by muscles in the stomach wall to physically break it up.

Question 67

Describe the chemical stage of digestion

Answer 67

Enzymes hydrolyse large, insoluble molecules into smaller soluble ones. Often requires multiple enzymes to hydrolyse a large molecule - usually one hydrolyses it into sections which are then hydrolysed by additional enzymes

Question 68

Give three groups of enzymes, what they hydrolyse and the products

Answer 68

Carbohydrases - hydrolyse carbohydrates, ultimately to monosaccharides Lipases - hydrolyse lipids (fats and oils) into glycerol and fatty acids Proteases - hydrolyse proteins, ultimately to amino acids

Question 69

Describe the steps to carbohydrate digestion

Answer 69

- Saliva, containing amylase, produced by salivary glands is mixed with food in mouth by chewing - Catalyses hydrolysis of glycosidic bonds in starch, producing disaccharide maltose. Saliva also has mineral salts to maintain pH 7 (optimum amylase pH) - Food swallowed, stomach conditions acidic = amylase denatured = no more starch digestion - Food passed to ileum + mixed with pancreatic juice containing pancreatic amylase - Catalyses hydrolysis of starch to maltose. Pancreas + ileum walls produce alkaline salts to maintain pH 7 - Smooth muscle in ileum walls pass food along. Epithelium lining produces maltase, a membrane-bound enzyme that catalyses hydrolysis of maltose to a-glucose - a-glucose absorbed into epithelial cells coupled with a Na+ ion via co-transport protein

Question 70

What is a membrane-bound enzyme?

Answer 70

An enzyme part of the cell-surface membrane of epithelial cells lining the ileum

Question 71

Name four carbohydrases

Answer 71

- Amylase - Maltase - Sucrase - Lactase

Question 72

Describe amylase, including its hydrolysis equation

Answer 72

Not a membrane-bound enzyme, secreted from salivary glands Starch + water -> maltose

Question 73

Describe maltase, including its hydrolysis equation

Answer 73

A membrane-bound enzyme, secreted from the ileum lining Maltose + water -> a-glucose

Question 74

Describe sucrase, including its hydrolysis equation

Answer 74

A membrane-bound enzyme, secreted from ileum lining Sucrose + water -> glucose + fructose

Question 75

Describe lactase, including its hydrolysis equation

Answer 75

A membrane-bound enzyme, secreted from the ileum lining Lactose + water -> glucose + galactose

Question 76

Describe lipid digestion

Answer 76

First emulsified by bile salts, produced in the liver, into tiny droplets called micelles to increase their surface area, This increases the action of lipases - enzymes produced in the pancreas that hydrolyse the ester bond in triglycerides to form fatty acids and monoglycerides (a glycerol molecule attached to a single fatty acid molecule)

Question 77

What are the three enzymes used to digest proteins?

Answer 77

- Endopeptidases - Exopeptidases - Dipeptidases

Question 78

Describe endopeptidases

Answer 78

Produced by the pancreas, the only protease in the stomach. Hydrolyse the peptide bonds between amino acids in the centre of a protein, forming peptide molecules

Question 79

Describe exopeptidases

Answer 79

Produced by the pancreas. Hydrolyse peptide bonds on the terminal amino acids of peptide molecules formed by endopeptidases. Progressively release dipeptides and amino acids

Question 80

Describe dipeptidases

Answer 80

Hydrolyse the peptide bond between two amino acids of a dipeptidase. They are membrane-bound enzymes in the ileum

Question 81

How are amino acids and monosaccharides absorbed?

Answer 81

Absorbed in the ileum by diffusion and co-transport

Question 82

Describe the steps in the absorption of triglycerides

Answer 82

- Bile salts produced in liver split lipids into micelles (emulsification) - increases surface area of lipids so pancreatic lipase can catalyse hydrolysis at greater rate - Pancreatic lipase hydrolyses ester bonds joining 3 fatty acids to glycerol, the free fatty acids + monoglycerides produced packaged into micelles - Movement of material in intestinal lumen = micelles contact epithelial cells of villi in ileum - Micelles break down, releasing non-polar monoglycerides + fatty acids - can diffuse into epithelial cells - Fatty acids + monoglycerides transported to endoplasmic reticulum + recombined = triglycerides - Triglycerides move to golgi apparatus + associate with cholesterol + lipoproteins, forming chylomicrons - Chylomicrons move out epithelial cells by exocytosis, entering lacteals, then pass into bloodstream via lymphatic vessels - Triglycerides in chylomicrons hydrolysed by enzyme in blood capillary endothelial cells, fatty acids diffuse into cells for metabolic processes

Question 83

What is a chylomicron?

Answer 83

A structure adapted for the transport of lipids

Question 84

What is a lacteal?

Answer 84

A lymphatic capillary found at the centre of each villus

Question 85

What is the difference between absorption and assimilation?

Answer 85

Absorption = taking soluble molecules into the body Assimilation = incorporating absorbed molecules into body tissue