AQA A Level Biology - Gas Exchange

what SA:V ratio does a small organism have?

large

what SA:V ratio does a large organism have?

small

why do single celled organisms not need a circulatory system?

• substances can diffuse directly into/out of the cell across the membrane

• there is a high enough rate due to the short distance

why do large organisms require a circulatory system?

• diffusion across outer membranes is too slow

• because some cells are deep within the body so there is a large diffusion distance

• there is a low SA:V ratio so it is difficult to exchange enough substances to supply a large volume through a small SA

• therefore, need it to move substances from exchange surfaces to other parts of the body

how else may large organisms allow efficient exchange of substances around the whole body?

have a body shape that increases surface area without increasing volume

how does metabolic rate relate to SA:V ratio?

• affects rate at which organisms exchange substances with their environment

• slow metabolic rate - small SA:V ratio

• high metabolic rate - large SA:V ratio

describe the structure of the leaf

what are the adaptations of a normal leaf for gas exchange?

• air spaces in spongy mesophyll layer, irregular shaped cells - increase surface area for gas exchange

• guard cells - open to allow CO2 in and O2 out, maintaining a steep conc. gradient

• very thin, large, flat leaves - short diffusion distance, large surface area

what are the adaptations of xerophytes for efficient gas exchange whilst reducing water loss? (don’t explain)

• small/needle shaped leaves

• rolled up leaves

• thick waxy cuticle

• sunken stomata

• hairs on leaves

• green stem

• reduced SA of above ground parts

• thicker epidermis

• compact spongy mesophyll

• stomata close during day

explain the adaptations: small needle shaped leaves, reduced SA of above ground parts, compact spongy mesophyll

• needle leaves - lower surface area so less water loss

• reduced SA - lower surface area of parts exposed to sun, so less SA for water loss by evaporation/transpiration

• compact SM - reduced surface area for gas exchange of water vapour so less water loss

explain the adaptations: thick waxy cuticle, thick epidermis, green stem

• thick cuticle - waterproof so reduces water loss from upper surface of leaf

• thick epidermis - increased diffusion distance so les water can get through to upper surface of leaf, so less water loss from upper surface of leaf

• green stem- can photosynthesize, less need for leaves, less SA exposed to sun, less water loss

explain the adaptations: rolled leaves, sunken stomata, hairs on leaves, closed stomata in day

• rolled leaves: trap water vapour inside of leaves, creating a humid environment, less steep concentration gradient, less water loss and stomata can stay open

• sunken stomata: pits trap a layer of very humid air, so less steep concentration gradient so less diffusion and water loss

• hairs on leaves: trap air that becomes saturated with water vapour, less steep water potential gradient, less water loss

• stomata close: prevents water loss at hottest part of day

why can insects not have diffusion of gases through the outer surfaces?

• have a tough exoskeleton made of chitin

• covered with a cuticle

• completely impermeable so no gas exchange

describe the structure of the insect gas exchange system

• have no lungs

• have small openings in cuticle that can open+ close called spiracles

• these open into tracheae, highly branched gas filled tubes

• branch into smaller open ended tubes called tracheoles, which lead to muscles/organs

what do tracheoles contain?

• fluid containing dissolved oxygen (haemolymph)

• when muscles are at rest, fluid is drawn into tracheoles (stores additional O2)

• when contracting the fluid is drawn into tissues (draws O2 into tissues)

what controls the opening and closing of the spiracles?

increasing carbon dioxide concentration

describe the adaptations of the insect gas exchange system

• high numbers of tracheae and tracheoles

• tracheoles penetrate insect tissues

• fluid movement

• abdominal pumping

• ends of tracheoles are moist and permeable

describe the adaptation of high numbers of tracheae and tracheoles

increases surface area for gas exchange

describe the adaptation of tracheoles penetrating insect tissues

• reduces diffusion distance for gas exchange

• delivers gas directly to cells- diffusion in gas phase is faster than when dissolved

describe the adaptation of fluid movement in tracheoles

• reduces diffusion distance in times of high stress - exercise

• increases surface area

• draws O2 into tissues when need + stores excess when it isn’t

describe the adaptation of abdominal pumping

• moves air in and out of spiracles

• removes CO2 and draws in O2

• maintains a steep concentration gradient

describe the adaptation of the ends of tracheoles being thin, moist and permeable

• short diffusion distance

• allows gases to dissolve facilitating diffusion

how do fish obtain oxygen?

• obtain dissolved oxygen in water

• water passes over gills, gas exchange takes palce

why is it difficult for fish to obtain enough oxygen?

• lower concentration of oxygen in water than in air

• water is more dense than air so more energy required to move it

describe the structure of the gills

• made up of lots of filaments

• filaments are made up of a lot of slices called lamellae

describe the adaptations of the fish gas exchange system

• lots of filaments and lamellae increase surface area, also contain lots of capillaries for short diffusion distance and conc gradient

• very thin lamellae - water flows across and between each lamellae reducing diffusion distance

• countercurrent exchange

describe the principle of counter current exchange

• water and blood flow in opposite directions through the lamellae

• so equilibrium concentration of O2 is never reached

• so concentration gradient is maintained for full length of the lamellae

• blood is always meeting water with a slightly higher concentration of O2

describe the structure of the human gas exchange system/lungs

• trachea (windpipe) splits into 2 bronchi

• each of these split into many bronchioles, which lead to alveoli

• 2 layers of muscle between ribs - internal and external intercostal muscles

what does ventilation consist of?

• inspiration

• expiration

describe the process of inspiration

• ACTIVE process

• external intercostal muscles contract

• internal intercostal muscles relax

• rib cage moves up and out

• diaphragm contracts, moves down and flattens

• volume of thorax increases and pressure decreases

• air is drawn into lungs

describe the process of expiration

• PASSIVE process (can become forced, particularly during exercise)

• external intercostal muscles relax

• (internal intercostal muscles only contract during forced)

• rib cage moves down and in

• diaphragm relaxes, moves up, curves upwards

• volume of thorax decreases, pressure increases, air expelled from lungs

describe the adaptations of the alveoli

• moist - gases can dissolve, facilitates diffusion

• one cell thick, permeable, walls - short diffusion distance

• millions of alveoli - large surface area for diffusion

• dense capillary network - short diffusion distance, maintenance of concentration gradient

• constant ventilation - maintains steep concentration gradient

• alveoli walls are elastic - stretch on inhalation, recoil on exhalation - aids exhalation by forcing air out

• blood arriving at alveoli - high in CO2, low in O2, blood that leaves is low in CO2 and high in O2 - steep concentration gradient

what can be used to measure lung function?

• spirometer (device you breath into)

• produces a spirometer trace

what does a spirometer trace show you?

• tidal volume

• breathing rate

• residual volume

• vital capacity

what is tidal volume?

volume of air in each breath

what is breathing rate?

number of breaths per minute

what is residual volume?

volume of air always in the lungs even when fully compressed (to prevent collapse)

what is vital capacity?

• maximum volume of air that can be breathed out during a single breath

• tidal volume + expiratory reserve volume

what is the ventilation rate?

• tidal volume x breathing rate

• measured in litres/second or similar

describe the disease tuberculosis

• caused by alveoli destroying bacteria that live in nodules in lungs - can remain dormant

• destroy alveoli and form scar tissue

• reduces SA of lungs, less gas exchange can occur, increases diffusion distance

describe the condition asthma

• lungs/airways are inflamed, goblet cells produce a lot of mucus, fluid enters airways, smooth muscle contracts constricting airways

• reduces air flow in and out of lungs, reduces concentration gradient in alveoli

describe the condition emphysema

• caused by long term smoking

• elastic tissue is broken down and elastic tissue is trapped

• alveoli burst, bronchioles collapse and trap air in alveoli, alveoli cannot recoil

• difficult to expel air, less SA for gas exchange, lower concentration gradient