Surface Area to Volume Ratio

examples of things interchanged between an organism and environment:

• Respiratory gases (oxygen and carbon dioxide)

• Nutrients (glucose, fatty acids, amino acids, vitamins, minerals)

• Excretory Products (urea and carbon dioxide)

• Heat

Exchange takes place at the surface of an organism - minerals absorbed are used by the cells mostly make up its volume

Effective Exchange = Large compared to its volume

Smaller organisms have a larger SA:VOL which allows efficient gas exchange as their volume increases at a faster rate than their surface area - diffusion is unable to reach all cells in an organism

The rate at which gases, heat, nutrients and waste products enter or leave a cell or organism by diffusion depends on the surface area. The rate and which the substances are used or produced depends on the cell or organism’s volume.

Exchange in Different Organisms

• Single-celled organisms (e.g. amoeba) are very small - large surface area to volume ratio and a short diffusion pathway so substances can quickly diffuse in and out across the cell-surface membrane

• Multi-cellular organisms - diffusion is too slow due to a long diffusion pathway - large animals have a large surface area to volume ratio - in order to absorb and excrete substances at a fast enough rate to stay alive - develop specialised exchange surfaces

Size, Metabolic Rate and Heat Exchange

• Metabolic rate of a small animal is relatively more greater than a large animal - heat is released by metabolic processes e.g., respiration - the release of heat is related to the mass or volume of the organism

• Rate of heat loss is determined by the surface area

• Small animals have a larger SA:VOL so lose heat faster so they need a higher metabolic rate so they can respire faster to replace to the lost heat

Gas Exchange Surfaces

All living cells must respire to provide ATP (ENERGY) to survive. - Aerobic Respiration - oxygen needs to be taken and carbon dioxide needs to be removed.

Movement of gases across an exchange surface is by diffusion

• Unicellular Organisms - no special gas exchange surfaces - rely on simple diffusion of gases across the outer surface membrane to respire - Large SA:VOL and a short diffusion pathway

• Multicellular organisms need to develop a specialised gas exchange surface

Factors which allow efficient gas exchange :

• A large surface area - relative to the volume of an organism which increases rate of exchange

• Very thin surface - diffusion distance is shorter so materials cross the exchange surface rapidly

• Selectively Permeable - allow selected materials to cross over

• Movement of the environment medium , e.g. to maintain a steep diffusion gradient

Gas Exchange in Fish

• Fish have a waterproof, gas-tight outer layer (not permeable) with a quite small SA:VOL therefore they need a specialised internal gas exchange system - gills

• Gills of a fish are structures situated in the boy of a fish just behind the head - gills are made up of gills filaments with many gill lamellae sticking up at angles to increase surface area of the gills - gill lamellae is where gas exchange occurs

Ventilation Mechanism - ensures water enters through the mouth and forced over the gill with an opening on each side of the body - ensure constant flow of water over the gills

Gills have a very rich blood supply - many capillaries with a single layer of thin endothelium close to the thin-walled lamellae

Many Capillaries increase surface area whilst the thin endothelium ensures a short diffusion pathway between blood and water

• The flow of water over the gill lamellae and the flow of blood within them flow in opposite directions = counter-current flow system which is important for ensuring maximum possible gas exchange is achieved

• This means that water always has more oxygen than the blood = never reaches equilibrium - the diffusion gradient for oxygen is maintained across the entire length of the gill lamellae

• As a result the diffusion of oxygen into the blood is always maintained so the fish always has enough oxygen for respiration

Gas Exchange in Insects

Body of an insect is protected by the exoskeleton - made of the rigid substance chitin which as a result prevents diffusion so they require a specialised gas exchange

Tracheal System = network of air-filled tubes(tracheae) which open through small holes in the exoskeleton(spiracles)

Gases enter and leave the tracheae through the spiracles - tracheae is supported by strengthened rings to prevent collapsing.

Tracheae divide into smaller dead-end tubes called tracheoles which extend throughout the entire body tissue - site of gas exchange - oxygen diffuses directly into the cells from the tracheoles and carbon dioxide diffuses out.

Large number of small tracheoles = large surface area for diffusion

Thin walls and many tracheoles = shorter diffusion pathway

Respiratory Gases move in and out of the tracheal system by :

• Down a diffusion gradient = when respiring oxygen at the end of the tracheoles in reduced and a diffusion gradient forms so oxygen from the atmosphere move to the lower oxygen conc and carbon dioxide is produced during respiration and forms a diffusion gradient in the opposite way

• Ventilation = movement of abdominal muscles = mass movements in air into and out of the trachea which speeds up the gas exchange as it maintains a diffusion gradient - known as abdominal pumping which increases during flight

• The ends of the tracheoles are filled with water = muscle cells carrying out anaerobic respiration = lactate and lowers WP in cells so water moves into the cells by osmosis - water at the ends of the tracheoles decrease in vol so air is drawn further in - final diffusion pathway is in a gas than liquid so diffusion is more rapid = potential water loss by evaporation

Control Water loss in Insects :

• Small SA:VOL = minimise amount of water loss per area

• Hairs to trap moisture = saturated so it contains a lot of water vapour to lower the water potential gradient

• Waterproof covering = reduce water loss

• Spiracles which open and close = vapour can escape when spiracles are open and close to prevent water loss - open for gas exchange

Gas Exchange in Plants

• Most gas exchange occurs in the leaves - spongy mesophyll layer with large air spaces and thin-walled cells

Thin and Flat shape of the leaf = large surface area and short diffusion pathway

The cells of the spongy mesophyll layer are packed together with air spaces to provide a large surface area for gas exchange and gases to diffuse to the palisade cells

Large surface area of mesophyll cells = rapid diffusion

Many small pores (stomata) - lower epidermis where gases enter and exit by diffusion = short diffusion pathway - surrounded by guard cells to control rate of gas exchange by opening and closing stomata to balance gas exchange with water loss

Control of Water in Xerophytes

• Xerophytes have restricted water supply = adapted to low water environments

• Thick, waxy cuticle = prevents evaporation on the upper surface of a leaf

• Stomata on lower epidermis = protected from direct sunlight = lowers water loss

• Rolling up of the leaves = protects stomata on lower epidermis to trap air and water vapour to stop any water gradient = no water loss

• A thick layer of hairs = traps moisture near the leaf surface so the WP on the inside and outside is reduced so less water is lost by evaporation

• Stomata in pits or grooves = traps air with rolling of leaves and hairs

• A reduced SA:VOL of the leaves = leaves are small and slightly circular meaning rate of water loss is reduced

Gas Exchange in Humans

The lungs are situated in the chest cavity(thorax) surrounded by ribs and above the diaphragm - allow gaseous exchange between air in the lungs and the blood in the capillaries

• Air enters airway through nose/mouth → trachea → splits into 2 bronchi into each side of the lungs

• Mucus membranes line most of the airway containing goblet cells which secrete mucus which are lined by ciliated epithelium

• The bronchi → split into many bronchioles each ending with small air sacs = alveoli(where gas exchange takes place)

The Alveoli

Gas exchange takes place between the alveoli and the blood capillaries

Oxygen diffuses through the epithelium of the alveoli and the endothelium of the blood capillaries into the blood → then combines with haemoglobin in red blood cells then CO2 diffuses from blood → alveoli

• Large surface area

• Short diffusion pathway = alveoli and capillary walls are one cell thick

• A huge network of capillaries =. constant blood flowing around the alveoli and breathing movements constantly ventilate the lungs = large diffusion gradient maintained for oxygen into the blood and carbon dioxide outside of the blood = also increases surface area for exchange of gases

• Each capillary is only 7-10 micrometers = flatten = shortens diffusion pathway adn slows movement of red blood cells for more time for diffusion

• Elastic fibres and collagen between alveoli = stretch when filled with air and recoil during breathing out

• Surfactant to prevent collapsing or sticking of alveoli = always open to increase surface area = reduces surface tension

Process of Ventilation

Inspiration = taking in air into the thorax

Expiration = moving air out of the thorax

Inspiration and Expiration involve the altering of vol of the thorax = air pressure differences between thorax and the atmosphere - pressure is inversely proportional to vol so pressure decreases as volume increases

Ventilation involves the movements of the ribcage, the diaphragm, internal and external intercostal muscles = antagonistic

Inspiration :

• Diaphragm contracts and flattens

• External intercostal muscles contract - ribs up and out

• Vol of thorax increases so pressure decreases

• Atmospheric pressure is higher than the thorax

• Air moves into lungs down the pressure gradient

Expiration :

• Diaphragm relaxes and curves upwards

• External intercostal muscles relax = ribs down and in

• Vol of thorax decreases so pressure increases

• Atmospheric pressure is lower than thorax

• Air is forced out the lungs down the pressure gradient

During rest - expiration is a passive process = no atp required but during forced expiration it pulls the ribcage further down and in which requires energy from ATP

Pulmonary Ventilation

Pulmonary ventilation rate is the total volume of air moved into the lungs during 1 minute

Tidal Volume = the vol of air normally taken in during each breathe at rest

Breathing(ventilation) rate = the number of breaths taken in 1 minute

Pulmonary Ventilation = tidal volume x ventilation rate

Lung Disease and Risk Factors

Asthma = inflamed airways causing constriction

Emphysema = alveoli elastic walls broken down by phagocytosis which reduces surface area

Fibrosis = formation of scar tissue so the lungs are less able to expand

Tuberculosis = bacteria lung disease which damages gas exchange surface

Lung Cancer