AQA A Level Biology: 3.3 Organisms exchange substances with their environment

Describe the relationship between the size of an organism and its SA:V ratio

Smaller organisms have high SA:V ratios

Larger organisms have low SA:V ratios

What adaptations do some organisms have for exchange (in terms of SA:V ratio)?

Flattened shape so small diffusion distance

Specialised exchange surfaces with surface areas

What is the relationship between SA:V ratio and metabolic rate?

Smaller organisms ---> higher SA:V ratio ---> higher metabolic rate.

In endothermic and exothermic animals, metabolic rate is inversely proportional to size.

Adaptions of single celled organisms to aid gas exchange

1. Large surface area

2. Thin surface

3. Short diffusion pathway

Adaptions of fish for gas exchange

1. Gill fillaments + lamellae provide large surface area for gas exchange

2. Many capillaries maintain diffusion gradient

3. Counter-current flow

Describe counter current flow

1. Water flows in opposite direction to blood

2. Blood always passing water with a higher oxygen concentration

3. Diffusion gradient maintained throughout length of gill

Describe gas exchange in insects

1. Air moves into trachae through spiracles

2. Oxygen travels down concentration gradient towards cells

3. Trachae branch off into tracheoles with thin permeables that go into cells

4. CO2 travels down concentration gradient towards spiracles to be released

5. Rhythmic abdominal movements move air in and out of spiracles

Describe adaptions to aid exchange

1. Animals with high SA:V ratio lose more water

Therefore, adapted kidney to produce less urine

2. Small mammals in cold regions eat a large amount of high energy foods

3. Small animals have thick layers of fur or hibernate when it is very cold

4. Elephants have large flat ears to increase SA for heat loss

5. Hippos spend lots of time in water to lose heat

Insect adaptions to reduce water loss

1. Muscles close spiralces

2. Waterproof waxy cuticle

3. Hairs around spiracles

All to reduce evapouration

What reduces water loss when plants get dehydrated

1. Guard cells lose water

2. Guard cells become flaccid

3. Stomata close

Explain the limitations of the gas exchange method for insects?

Relying mostly on diffusion for gas exchange means the diffusion pathway needs to be short ---> limits size of insects.

Fossilised insects were much larger. Suggest how the composition of the atmosphere then compares to now.

Then, [O2] in atmosphere was higher.

Short diffusion pathway not as essential as now, as larger [O2] gradient so adequate O2 available for insects to be larger.

Describe the adaptations of the leaf for gas exchange

1. Short diffusion path

2. Diffusion takes place in gas phase - faster than in water.

3. Many small stomata - no cell is far from a stoma and also therefore a short diffusion pathway.

4. Many interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come into contact with mesophyll cells.

5. Large SA of mesophyll cells for rapid diffusion.

How do xerophytic plants limit water loss?

1. Waxy, waterproof cuticle

2. Reduced number of stomata

3. Curled leaves with stomata inside protects from wind

4. Hairs on epidermis traps moist air round stomata

5. Stomata in sunnken pits that trap moist air reduces concentration gradient

Describe the structure of the human gas exchange system

Lungs -> trachea -> bronchi -> bronchioles -> alveoli

Adaptions of alveoli for gas exchange

1. Many alveoli = large surface area

2. Small diffusion distance - alveolar epithelium one cell thick

3. Good blood supply

4. Moist

Inspiration

1. External intercostal muscles contract, internal intercostal muscles relax

2. Diaphragm contracts

3. Rib cage moves upwards and out

4. Volume of thoracic cavity increases

5. Lung pressure decreases

6. Air flows down pressure gradient into lungs

Exhilation

1. Internal intercostal muscles contract, external intercostal muscles relax

2. Diaphragm relaxes

3. Ribcage moves downwards and inwards

4. Thoracic cavity decreases

5. Air forced down pressure gradient out of lungs

Why does rapid diffusion take place in humans?

1. RBCs slowed as they pass through pulmonary capillaries ---> more time for diffusion.

2. Distance between alveolar air and RBCs reduced ---> RBCs flattened against the capillary walls.

3. Capillary endothelium and alveolar epithelium walls are thin (one cell thick) ---> short diffusion distance.

4. Alveoli and pulmonary capillaries have a very large total SA.

5. Constant ventilation by breathing movements and constant circulation of blood by the heart ---> ensure a steep [O2] / [CO2] gradient for gas exchange.

Define Tidal Volume

Volume of air in each breath.

Define Ventilation Rate

Number of breaths per minute.

Forced Expiratory Volume

The maximum volume of air that can be breathed out in 1 second

Forced Vital Capacity (FVC)

The maximum volume of air it is possible to breathe forcefully out of the lungs after a very deep breath in

Equation for calculating the Pulmonary Ventilation Rate

Pulmonary ventilation rate = tidal volume x breathing rate.

Examples of lung diseases

Pulmonary Tuberculosis

Fibrosis

Asthma

Emphysema

Describe the formation of Pulmonary Tuberculosis

1. Tuberculosis bacteria infects lungs

2. Immune system cells build a wall around bacteria forming small hard lumps - tubercles

3. Infected tissue in tubercles dies and gaseous exchange surface damaged

4. Tidal volume decreased

5. TB also causes fibrosis reduces TV as well

6. As TV decreases, Ventitation rate increases

Causing persistant cough, chest pains, shortness of breath, fatigue

What is fibrosis?

The formation of scar tissue in the lungs as a result of an infection or exposure to substances like asbestos or dust

Explain the affects of Fibrosis

1. Scar tissue forms from infection or exposure to asbestos/ dust

2. Scar tissue is thicker and less elastic than normal lung tissue

3. Therefore lungs less able to expand, therefore can't hold as much air, therefore TV and FVC decreased

4. Reduction in gaseous exchange rate - diffusion slower across thicker scarred membrane

5. Symptoms = shortness of breath, dry cough, chest pain, fatigue

Asthma

A respiratory condition where the airways become inflamed and irritated, usually from an allergic reaction to substances such as pollen and dust

Describe an asthma attack

Smooth muscle lining bronchioles contracts + large amount or mucus produced

This causes constriction of the airways, meaning reduced air flow.

Therefore FEV1 reduced.

Symptoms = Wheezing, tight chest and shortness of breath

Emphysema

A lung disease caused by smoking or long-term exposure to air pollution, causing foreign particles to become trapped in the alveoli

Explain emphysema

1. Foreign particles become trapped in alveoli

2. This causes inflammation, attracting phagocytes

3. Phagocytes produce enzyme that breaks down elastin ( protein found in walls of alveoli)

4. Loss of elastin means alveoli can't recoil air as well + destruction of alveoli walls, reducing surface area for gas exchange

Therefore less AEROBIC respiration

Symptoms = Shortness of breath + wheezing

How to measure volumes of air involved in gas exchange

Three way taps, manometers, simple respirometers

Define Digestion

The process in which large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell-surface membranes and assimilated.

Outline the structure of the alimentary canal

- Salivary glands.

- Oesophagus.

- Stomach.

- Ileum.

- Large intestine.

- Rectum.

- Anus.

- Pancreas.

Describe the salivary glands

Situated near mouth they pass the amylase enzyme in their secretions.

Describe the ileum

- Long muscular tube ---> food further digested in ileum by enzymes produced by its walls and by glands that pour secretions into it.

- Has an adapted structure for absorption.

Monomers of sucrose

Glucose + Fructose

Outline the basic process of digestion

1. Physical Digestion:

- Food (if large) broken down into smaller pieces by chewing - then ingested - and then by churning muscles in stomach wall.

- Provides a large SA for chemical digestion.

2. Chemical Digestion:

- All digestive enzymes function by hydrolysis ---> several different enzymes all work together to hydrolyse a molecule.

Explain how carbohydrates are digested in mammals?

1. Saliva from salivary glands mixed with food in mouth during chewing.

2. Salivary amylase starts hydrolysing glycosidic bonds in starch ---> maltose

3. Food then enters stomach. Amylase denatured, preventing further starch hydrolysis.

4. Food passed into small intestine (churning) ---> mixes with pancreatic juices.

5. Pancreatic amylase continues starch hydrolysis to maltose. Alkaline salts produced by pancreas/intestinal wall keep pH neutral ---> amylase can still function.

6. Intestine wall muscles push food along ileum. Epithelial lining produces the membrane-bound disaccharidases maltase (part of epithelial cell membrane), sucrase and lactase.

How are lipids digested?

1. Lipids hydrolysed by lipase enzymes, produced in the pancreas, that hydrolyse the ester bond found in triglycerides ---> fatty acids and monoglycerides.

2. Lipids (fats + oils) firstly split up into tiny droplets called micelles by bile salts (produced by liver) ---> emulsification ---> increases SA of the lipids so speeds up the action of lipases.

How are proteins digested?

1. Endopeptidases - hydrolyse peptide bonds between amino acids in the central region of a protein molecule ---> forms a series of shorter peptide molecules.

2. Exopeptidases - hydrolyse peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases ---> forms single amino acids and dipeptides.

3. Dipeptidases - hydrolyse peptide bond between the two amino acids of dipeptide ---> membrane-bound - part of epithelial cell-surface membranes.

How is the ileum adapted for absorption of digestion products?

1. Thin-walled ---> short diffusion distance.

2. Contain muscle ---> can move to mix with the ileum contents to help maintain favourable concentration gradients.

3. Many capillaries/rich network on other side of epithelial cells ---> blood can carry away absorbed molecules

4. Many channel/carrier proteins.

5. Microvilli ---> finger-like projections of the cell-surface membrane that further increase the SA for absorption.

Epithelial cells also have many mitochondria to provide ATP for active transport.

How are triglycerides absorbed?

1. Micelle structures formed ---> monoglycerides remain in association with the bile salts that initially emulsified the lipid droplets.

2. Micelles come into contact with villi epithelial cells as they move through ileum. Micelles break down, releasing the monoglycerides and fatty acids ---> non-polar molecules - easily diffuse across the cell-surface membrane into the epithelial cells.

3. Once inside epithelial cells, monoglycerides and fatty acids transported to the ER where they are recombined to form triglycerides.

4. Starting in ER ---> golgi apparatus, triglycerides associate with cholesterol and lipoproteins to form chylomicrons (particles adapted for lipid transport).

5. Chylomicrons move out of the epithelial cells by exocytosis ---> enter lymphatic capillaries called lacteals, found at centre of each villus.

6. Chylomicrons then pass into blood via lymphatic vessels ---> triglycerides in chylomicrons hydrolysed by enzyme in blood capillaries' endothelial cells, from where they diffuse into cells.

What is haemoglobin?

Group of chemically similar molecules adapted for O2 transport, found in a wide variety of organisms.

Describe the structure of haemoglobin

Haemoglobin has a quaternary structure with 4 polypeptide chains linked together, with each chain associated with a prosthetic Fe2+ group.

Each Fe2+ can combine with a single O2 molecule ---> 4xO2 molecules can be carried by each human haemoglobin molecule in humans.

Hb reversible equation

Hb + 4O2 -> HbO8 ( oxyhaemoglobin)

What is the role of Haemoglobin in the transport of oxygen?

1. Readily associate with O2 at gas exchange surface ( high oxygen partial pressures).

2. Readily dissociate from O2 at respiring tissues ( low oxygen partial pressures) .

=> seems contradictory - Hb can change its affinity (chemical attraction) for O2 under different conditions ---> shape changes due to the presence of CO2.

---> in the presence of CO2, the new shape of the Hb molecule binds more loosely to O2 ---> Hb releases its O2.

Bohr effect

Higher CO2 partial pressures cause oxyhaemoglobin to dissociate into Hb + O2, therefore increases rate of oxygen unloading

Is oxygen loaded/unloaded at the gas exchange surface? Explain why.

O2 loaded, because:

High [O2], low [CO2] ---> high affinity for O2.

Is oxygen loaded/unloaded at the respiring tissues? Explain why.

O2 unloaded, because:

Low [O2], high [CO2] ---> low affinity for O2.

Why is the oxygen dissociation curve shaped like an S?

- O2 does not bind evenly to Hb across different partial pressures of O2.

1. Closely packed polypeptide subunits, so hard for O2 to bind ---> low [O2], little binds ---> shallow gradient.

2.Binding of 1st O2 molecule changes the 4 structure of Hb molecule ---> changes shape so that it is easier for other O2 molecules to bind to each subunit.

=> takes a smaller increase in PO2 to bind 2nd O2 molecule than it did to bind 1st O2 => positive cooperativity ---> binding of 1st facilitates binding of 2nd ---> steeper gradient of curve.

3. After 3rd O2 molecule binds ---> more difficulty due to decreased probability of a single O2 molecule finding a free binding site ---> graph flattens off.

NB=> Curve to left = higher affinity for O2.

=> Curve to right = lower affinity for O2.

Describe the Bohr effect

= Increasing [CO2], the more readily the Hb releases its O2 (+ less likely to load O2) due to a reduced Hb affinity for O2.

Curve shifts to right.

Always ensures enough for O2 for respiring tissues:

- Increased respiration rate ---> more CO2 ---> lower pH ---> greater change in shape of Hb ---> O2 more readily unloaded ---> more O2 for respiration.

Outline the continuous process of oxygen loading and unloading in the body

1. Gas exchange surface, CO2 constantly removed.

2. Slightly higher pH due to low [CO2].

3. Higher in pH changes shape of Hb ---> loads O2 readily.

4. Increase in affinity for O2 ---> O2 not released when being transported in blood to tissues.

5. CO2 produced by respiring cells in tissues.

6. CO2 acidic in solution so pH of blood falls.

7. Lower pH changes Hb shape ---> decreased affinity for O2.

8. Hb releases its O2 into respiring tissues.

Describe cooperative binding

Binding of first oxygen to haemoglobin causes change in shape

Shape change allows more oxygen to bind

Describe the structure of the human heart

Describe the pressure and volume changes during the cardiac cycle that maintain a unidirectional flow of blood

1. Ventricles relax, atria contracts

- Decrease volume of chambers

- Increase pressure inside chambers

- Forces blood into ventricles

2. Ventricles contract, atria relax

- Increase pressure in ventricles compared to atria

- Forces AV valves shut

- Forces SL valves open

- Blood forced into arteries

3. Ventricles relax, atria relax

- Blood returns to heart

- Atria fill again

- Increase pressure in atria

- Ventricles relax, decreases pressure in atria

- AV valves open

- Blood flows passively into ventricles from atria

- Atria contract, cycle begins again

Name the valves present in the heart

Atrioventricular (AV): link atria to ventricles

- prevents blood flowing back intro atria

Semi-Lunar (SL): link ventricles to pulmonary artery

- stops blood flowing back into the heart

Structure of arteries

Thick muscle tissue

Thick elastic tissue

Folded endothelium

Adaptions of arteries

1. Elastic tissue allows stretching which maintains blood pressure as it stretches when ventricles contract - folded endothelium allows stretching

2. Muscle for vasoconstriction

3. Thick wall withstands bursting

4. Smooth endothelium reduces pressure

5. Aortic valve prevents back flow

Structure of arterioles

Tiny branches of arteries that lead to capillaries. These are also under the control of the sympathetic nervous system, and constrict and dialate, to regulate blood flow.

Structure of veins

Thin muscle wall

Less elastic tissue

Wide lumen

Valves

Blood flow helped by contraction of body muscles around them

Function of veins.

Veins carry deoxygenated blood to the heart [except pulmonary veins] where these will be processed through the lungs and given oxygen again.

Adaptions of capillaries

1. Near cells in exchange tissues ( therefore short diffusion path)

2. One cell thick ( shortens diffusion path)

3. Large number increases surface area

Describe the formation of tissue fluid

1. At start of capillary bed, HYDROSTATIC PRESSURE inside capillaries is greater than in tissue fluid

2. Difference in hydrostatic pressure means overall outward pressure forces fluid out of capillaries into space around cells forming tissue fluid

Describe how tissue fluid returns to the circulatory system

1. At the start of the capillary bed, nearest the arteries, the pressure inside the capillaries is greater than the pressure in the tissue fluid.

2. This difference in pressure forces fluid out of the capillaries and into the spaces around the cells, forming tissue fluid.

3. As fluid leaves, the pressure reduces In the capillaries- so the pressure is much lower at the end of the capillary bed that's nearest to the veins.

4. Due to the fluid loss, the water potential at the end of the capillaries nearest the veins is lower than the water potential in the tissue fluid - so some water re-enters the capillaries from the tissue fluid at the vein end by osmosis.

XS tissue fluid drained into lymphatic system

Describe atheroma formation

1. Damage occurs to endothelium ( e.g. by high blood pressure)

2. WBCs and lipids clump together under the lining forming fatty streaks

3. Over time more WBCs, lipids and connective tissue build up and harden to form a fibrous plaque called an atheroma.

4. Plaque partially blocks lumen, restricting blood flow, causing blood pressure to increase.

Cause of CHD

Having lots of atheromas in coronary arteres, restricting blood flow to heart muscle, leading to a myocardial infarction

What is an aneurysm

Balloon-like swelling of the artery

Explain the cause of an aneurysm

1. Atheroma plaques damage and weaken arteries, narrowing arteries and increasing pressure

2. Blood travelling down weakened artery at high pressure pushes inner layers of artery through the outer elastic layer, forming a balloon like swelling

3. Aneurysm may burst, causing haemorrhage

Thrombosis

Formation of a blood clot

Describe the formation of thrombosis

1. Artheroma ruptures endothelium of an artery

2. This damages artery wall leaving a rough surface

3. Platelets and fibrin accumulate at site of damage and form a blood clot ( thrombosis)

4. Blod clot can cause complete blockage of the artery or become dislodged and block a vessel elswhere in body

Describe the formation of a myocardial infarction

1. Coronary arteries become blocked due to thrombosis

2. Heart muscle receives no oxygen

3. No aerobic respiration can occur

4. Death + damage to cardiac muscle

Which factors increase risk of cardiovascular disease?

High blood cholesterol + Poor diet

Cigarette smoking

High blood pressure

How does high blood cholesterol + poor diet cause cardiovascular disease?

1.) Cholesterol is one of the main constituents of the fatty deposits that form artheromas

2.) Atheromas lead to increased blood pressure and blood clots

3.) These could block blood supply to coronary arteries, causing myocardial infarction

Diet high in saturated fat associated with high blood cholesterol levels + Diet high in salt increases risk due to high blood pressure

How does cigarette smoking increase cardiovascular disease risk?

1. Nicotine increases high blood pressure

2. Carbon monoxide combines with Hb, reducing amount of oxygen transported in blood, therefore less O2 available for tissues. If cardiac muscles don't have enough O2 = HA

3. Smoking decreases amount of antioxidants in blood, therefore cell damage of coronary arteries more likely = atheroma formation

How does high blood pressure increase risk of cardiovascular disease

Increased risk of damage to artery walls

Increased risk of atheroma formation

Causing increased risk of blood clots

Which could cause myocardial infarction

Define Xylem Vessels

Hollow, thick-walled tubes that transport water in the stem and leaves of plants.

Define transpiration

Evapouration of water from a plant's surface

How does temperature affect transpiration rate?

Warmer molecules have more energy so evapourate from inside leaf faster.

This increases the concentration gradient between inside and outside of leaf, making water diffuse out of leaf faster

Describe how water moves across the cells of a leaf

1. Water lost from mesophyll cells by evaporation from cell walls to leaf air spaces is replaced by water from xylem, via cell walls/cytoplasm.

2. Cells' wp lowered so water moves into them from other cells by osmosis.

3. Wp gradient established that pulls water up from xylem, across leaf mesophyll, into atmosphere.

Outline the cohesion-tension theory

1. Water evaporates from mesophyll cells due to heat from sun leading to transpiration.

2. Water molecules form H-bonds with one another ---> cohesion.

3. Water forms a continuous, unbroken column across mesophyll cells and down xylem.

4. Column of water pulled up xylem as a result of transpiration => transpiration pull.

=> puts xylem under tension ---> negative pressure within xylem.

Evidence for cohesion-tension theory

1. Water flow decreases at night (as would be expected - less photosynthesis).

- Diameter of tree trunk increases - therefore tension has decreased as decrease in negative pressure - xylem not pulled inwards as much.

2. If xylem vessel broken and air enters it ---> tree can no longer draw up water ---> continuous column of water is broken ---> H20 molecules can no longer stick together.

3. Water does not leak out when xylem vessel broken ---> but air drawn in. Suggests water not under pressure but that xylem is under negative pressure.

4. Xylem vessels have no end walls ---> form a series of continuous, unbroken tubes from root to leaves ---> essential to cohesion-tension theory of water flow up the stem.

=> consistent with xylem being under tension.

Define Translocation

Process in which organic molecules (sucrose/amino acids) and some mineral ions are transported from one part of the plant to another.

Define Source

Part of the plant where sugars produced during photosynthesis

Define Sink

Parts of the plant where sugars used directly or stored for future use.

Define Phloem

The vascular tissue in plants which conducts sugars and other metabolic products downwards from the leaves.

Define Mass Flow

Bulk movement of a substance through a given channel/area in a specific area.

What are the phloem made out of

Sieve tube elements

Companion cells ( provide energy needed for active transport of solutes)

Describe the mass flow hypothesis

1. Solutes actively transported from companion cells to sieve tubes at source

2. This lowers water potential inside sieve tubes, so water enters tubes by osmosis from xylem + companion cells

3. Creating high pressure inside sieve tubes at source end of phloem

4. At sink, solutes are removed from phloem to be used up

5. This increases water potential inside sieve tubes, so water leaves the tubes by osmosis

6. Lowering pressure inside sieve tubes

7. This creates a pressure gradient from source end to sink end

8. Gradient pushes solutes along sieve tubes towards the sink

9. At sink, solutes are used or stored

Supporting evidence for mass flow

1. If ring of bark ( including phloem not xylem) removed from woody stem, bulge forms above ring. Bulge has higher concentration of sugars than fluid from below, shows there is downward flow of sugars

2. Radioactive tracer tracks movement fo organic substances in a plant

3. Aphids pierce phloem to access sap, sap flows quicker nearer leaves than further down stem, shows a pressure gradient

4. Metabolic inhibitor stops translocation

Objections (of Mass Flow Hypothesis)

Evidence against Mass Flow Hypothesis

- Sugar travels to many sinks (not just one)

- Sieve plates act as barriers so a lot of pressure is needed