Organisms exchange substances

How does an organisms size relate to their SA:V ratio?

The larger the organism, the lower the SA:V ratio

How does an organism’s SA:V ratio relate to their metabolic rate?

The lower the SA:V ratio, the lower the metabolic rate

Why do smaller organisms not require specialised exchange or transport systems?

They have a large SA:V ratio which allows efficient exchange directly across the body surface

Why do smaller mammals have a high metabolic rate?

Due to their high SA:V ratio, they lose heat rapidly. Therefore, a high metabolic rate provides the energy needed to provide more heat and maintain a stable body temperature

Give some adaptations that large organisms have to increase SA:V ratio (7)

• Large, folded body parts

• Villi and microvilli

• Alveoli and bronchioles

• Spiracles and tracheoles

• Gill filaments and lamellae

• Thin wide leaves

• Capillary networks

Why do multicellular organisms require specialised gas exchange surfaces?

Their small SA:V ratio means the distance that needs to be crossed to larger and substances cannot easily enter the cells

Name 3 features of an efficient gas exchange surface

1. Large SA:V ratio

2. Short diffusion distance

3. Maintained a concentration gradient

Explain the process of inspiration and the changes that occur during it

• External intercostal muscles contract, pulling ribs upwards + outwards

• Internal intercostal muscles relax

• Diaphragm contracts and flattens

• Lung volume increases

• Air pressure in lungs initially drops

• Air moves into the lungs due to gradient between atmosphere and lungs

Explain the process of expiration and the changes that occur during it

• Internal intercostal muscles contract, pulling ribs downwards and inwards

• External intercostal muscles relax

• Diaphragm relaxes and domes upwards

• Lung volume decreases

• Air pressure in lungs initially greater than atmosphere

• Air moves out of lungs from high pressure in lungs to lower pressure in atmosphere

Describe alveoli and their function in gas exchange

• Tiny air sacs

• 300 mil in each lung so very large SA for gas exchange

• Walls are one cell thick and covered with a network of capillaries so minimises diffusion distance

• Network of capillaries also allows a constant blood supply which maintains a conc gradient to remove exchanged gases

What is the nasal cavity and what’s its structure?

The internal chamber of the nose that air passes through.

It is lined with ciliated epithelial cells and goblet cells

It also contains bones called turbinates that increase SA

What’s the function of the nasal cavity?

• Moistens air entering the lungs which protects the delicate alveoli

• Goblet cels secrete mucus which traps dust and bacteria

Describe the trachea and its function

• Wide tube supported by C-shaped cartilage to keep the air passage open during pressure changes

• Lined with ciliated epithelial cells which move mucus towards the throat to be swallowed, preventing lung infections

• Carries air to the bronchi

Describe the bronchi and their function

• Two tubes that branch off from trachea, one leading to each lung

• Supported by smaller and less complete rings of cartilage to keep airways open

• Lined with epithelial cells which move mucus up towards throat to be swallowed

• Lined with goblet cells which secrete mucus

• Elastic fibres found in the walls which allow them to stretch and recoil during ventilation

• Smooth muscle found in the walls which can contract to narrow the airway and control airflow

• Allow passage of air into the bronchioles

Describe the bronchioles and their function

• Very small airways that branch from the bronchi

• No cartilage

• Smooth muscle present in walls so they can contract and relax to control airflow

• Elastic fibres present in walls so they can recoil during exhalation

• Epithelium cells become simpler and thinner to provide a short diffusion pathway

• Allow passage of air into the alveoli

Describe the pathway taken by air as it enters the mammalian gaseous exchange system

Nasal cavity → trachea → bronchi → bronchioles → alveoli

What is tidal volume?

Volume of air that enters and leaves the lungs at normal resting breath (0.5dm³)

What is vital capacity?

Max volume of air we can inhale and exhale in one breath

What is residual volume?

The volume of air left in the lungs after the strongest exhalation

What is total lung capacity?

The total volume of air the lungs can hold after maximum inspiration

Vital capacity + residual capacity

Usually (5-6dm³)

Why can’t fish use their bodies as an exchange surface?

• They have a waterproof, impermeable body surface, so gases can’t dissolve and diffuse across the membrane

• They have a small SA:Volume ratio-their body surface is too small compared to their volume to supply enough oxygen to all cells

What’s the gas exchange organ in fish and their structure?

• Gills

• There are 4 layers of gills on both sides of the head

• Supported by gill arches

• Made up of stacks of gill filaments

• Each gill filament is covered in lamallae which have a capillary network

• Water rushes in and over the gills in the opposite direction to blood flow and then out through a hole in the sides of their head

How are gills efficient for gas exchange?

• Large SA:V ratio created by many gill filaments covered by many gill lamellae

• Short diffusion pathway due to a capillary network in every lamellae and very thin lamellae

• Maintained conc gradient - countercurrent flow mechanism

What is the countercurrent exchange principle?

• Water flows over the gills in the opposite direction to the flow of blood in capillaries

• Ensures that equilibrium isn’t reached

• Ensures that a conc gradient is maintained across the entire length of the lamellae

Why can’t insects use their bodies as an exchange surface?

• They have a waterproof chitin exoskeleton + a lipid layer which prevent gases from diffusing through the surface

• Small SA:V ratio

• No lungs-use a tracheal system

What 3 adaptations do insects have to prevent water loss?

1. Small SA:V ratio where water can evaporate from

2. Waterproof exoskeleton

3. Spiracles open and close to reduce water loss

Name and describe the 3 main features of an insects gas transport system

• Spiracles - round, valve-like openings running along length of abdomen. O2 and CO2 enter and leave via these. Trachea attach to these openings

• Trachea - network of internal tubes extending through the body. Have rings within them to strengthen and keep them open

• Tracheoles - smaller branches dividing off the trachea. Extend throughout all the tissues in the insect and deliver O2 to all respiring cells

Describe the 3 methods of moving gases in the tracheal system

1. By diffusion - when cells respire, they use up O2 and produce CO2, creating a conc gradient from tracheoles to the atmosphere

2. Mass transport - in which an insect contracts and relaxes their abdominal muscles, squeezing the trachea, to move gases on mass

3. When insects are in flight, muscle cells respire anaerobically to produce lactate-lowers water potential of cells so water moves from the tracheoles into cells by osmosis - decreases the volume in the tracheoles so more air from atmosphere is drawn in

How is the tracheal system efficient for gas exchange?

• Large SA - large number of fine tracheoles

• Short diffusion pathway - walls of tracheoles are thin and short distance between spiracles and tracheoles

• Steep diffusion gradient - use of O2 and production of CO2

Where does gas exchange occur in plants?

At stomata

Describe 3 adaptations of a leaf that allow efficient gas exchange

1. Thin and flat to provide short diffusion pathway and large SA:V ratio

2. Many stomata in the underside of the leaf which allow gases to easily enter

3. Air spaces in the mesophyll allow gases to move around the leaf

How do plants limit their water loss while still allowing gases to be exchanges?

Stomata regulated by guard cells which allow them to open and close as needed

What are xerophytic plants?

Plants that are adapted to live in environments with limited water

What structural features do xerophytic plants have to enable efficient gas exchange whilst also limiting water loss? (5)

1. Curled leaves - trap moisture to increase humidity and reduce water potential gradient

2. Hairs - trap moisture to increase humidity

3. Sunken stomata - trap moisture

4. Thicker cuticle - reduces evaporation

5. Longer root network - reach more water at further distances at soil

Define digestion

The hydrolysis of large, insoluble molecules into smaller molecules that can be absorbed across cell membranes

Which enzymes are involved in carbohydrate digestion and where are they found?

• Amylase - found in pancreas and salivary glands

• Membrane-bound disaccharidases - sucrase, lactase and maltase found in small intestine

What are the substrates and products of the carbohydrate digestive enzymes?

• Amylase - starch into maltose

• Maltase - maltose into glucose

• Sucrase - sucrose into glucose and fructose

• Lactase - lactose into glucose and galactose

Name the 3 enzymes involved in protein digestion and explain their roles

• Endopeptidases - hydrolyse peptide bonds between amino acids in the middle of a polypeptide chain

• Exopeptidases - hydrolyse peptide bonds between amino acids at the end of a polypeptide chain

• Membrane-bound dipeptidases - hydrolyse dipeptides into amino acids

How are certain molecules absorbed into the ileum despite a negative concentration gradient?

By co-transport

How does the ileum maximise absorption?

• Ileum wall is covered in villi which have thin walls surrounded by a network of capillaries

• Epithelial cells have even smaller microvilli

• These features increases surface area, decrease diffusion distance and maintain a concentration gradient

Which molecules require co-transport to be digested into the ileum?

Amino acids and monosaccharides

Where does protein digestion occur?

Starts in the stomach, continues in the duodenum and are fully digested into the ileum

What are lipids digested by?

Lipase and the action of bile salts

Where are lipids digested?

The small intestine

Where is lipase produced?

In the pancreas

What needs to happen before lipids can be digested?

• They must be emulsified by bile salts

• This breaks them down into tiny droplets called micelles, which increases the surface area for lipase to act on

What are micelles?

Water soluble vesicles formed of fatty acids, glycerol, monoglycerides and bile salts

They deliver fatty acids, glycerol and monoglycerides to the epithelial cells of the ileum for absorption

Give 3 ways micelles help with lipid absorption

• They make the fatty acids more soluble in water

• They carry the fatty acids to the epithelial cells of the ileum

• They help to maintain a higher concentration of fatty acids outside the epithelial cells by continuously delivering them to the cell surface

How are lipids digested?

Lipase hydrolyses the ester bonds between the monoglycerides and fatty acids

Why do fatty acids and monoglycerides not require co-transport?

They are non polar, so can easily diffuse across the phospholipid membrane of epithelial cells

What happens to fatty acids and monoglycerides once they enter the cell?

They are modified back into triglycerides inside of the endoplasmic reticulum and Golgi body

How do modified lipids leave the epithelial cell?

Vesicles containing the triglyceride are released and move towards the cell membrane

They leave the cell by exocytosis

Give 2 features of the cardiac muscle

1. It is myogenic - can contract and relax without nervous or hormonal stimulation

2. Never fatigues as long as it has a supply of oxygen

What are coronary arteries?

• Supply the cardiac muscle with oxygenated blood

• Branch off from the aorta

Relate the structure of the atria to their function

• Thinner muscular walls - do not need to contract as hard as only pumping blood to ventricles

• Elastic walls - stretch when blood enters

Relate the structure of the ventricles to their function

• Thicker muscular walls to enable bigger contractions - creates a higher blood pressure to enable blood to flow longer distances

• Left ventricle has thicker muscular wall to enable larger contractions to create higher blood pressure so blood reaches all the cells in the body

Label the heart

Which side of the heart carries oxygenated blood and which carries deoxygenated blood?

• Right side - deoxygenated blood

• Left side - oxygenated blood

Describe the pathway blood takes in the heart

• Right side: deoxygenated blood from the body enters through the vena cava and leaves through the pulmonary artery to go to the lungs

• Left side: oxygenated blood from the lungs enters through the pulmonary vein and leaves through the aorta to go to the rest of the body

What does the septum do?

• Separates the deoxygenated and oxygenated blood

• Maintains high conc of oxygen in oxygenated blood to maintain conc gradient to enable diffusion at respiring cells

What are the 2 types of valves in the heart?

• Semi-lunar valves - between ventricles and arteries (pulmonary valve and aortic valve)

• Atrioventricular valves - between atria and ventricles (bicuspid/mitral and tricuspid)

Whats the function of valves?

Prevent backflow of blood

What pressure changes cause valves to open/close?

• They open when pressure is higher behind the valve

• They close when pressure is higher in front of the valve

Why are two pumps needed in the heart instead of one?

To maintain blood pressure around the whole body - when blood passes through the narrow capillaries of the lungs, the pressure drops and therefore wouldn’t be flowing strongly enough to continue around the whole body. It is returned to the heart to increases the pressure.

What are the 3 stages of the cardiac cycle?

Diastole

Atrial systole

Ventricular systole

Describe what happens during diastole

• The atria and ventricular muscles are relaxed

• Blood enters the atria via vena cava and pulmonary vein

• Volume and pressure in atria increase

• Atrioventricular valves are open

• Semilunar valves are closed

• Volume of ventricles slowly increase as blood flows in passively

Describe what happens during atrial systole

• Atria muscular walls contract

• Pressure in atria increases

• Volume in atria decreases

• Atrioventricular vales are open

• Semilunar valves are closed

• Blood enters ventricles - volume increases and pressure stays low

Describe what happens during ventricular systole

• After a short delay, ventricle muscular walls contract

• Pressure in ventricles increases

• Volume in ventricles decreases

• Atrioventricular valves close

• Semi lunar valves open

• Blood flows into the arteries

What 3 main layers are the walls of arteries and veins made up of?

• A thin inner lining of epithelial cells

• A middle layer of elastic tissue and smooth muscle

• An outer layer of collagen fibres

Relate the structure of arteries to their function

• Muscle layer - thick so that constriction and dilation can occur to control volume of blood

• Elastic layer - thick to help maintain high blood pressure by stretching and recoiling

• Walls are thick to prevent the vessels bursting due to the high pressure

• No valves

Relate the structure of veins to their function

• Muscle layer - thin as blood is under low pressure so doesn’t need to be constricted

• Elastic layer - thin as pressure is low so don’t need to stretch and recoil

• Walls are thin as pressure is low so low risk of bursting and also means that vessels are easily flattened which helps blood flow

• Have valves to ensure blood doesn’t flow backwards

Relate the structure of arterioles to their function

• Muscle layer - thicker than arteries to help restrict blood flow into the capillaries so they’re not damaged

• Elastic layer - thinner than arteries as pressure is lower

• Walls are thinner than arteries as pressure is lower

• No valves

Describe the structure of haemoglobin

A globular protein consisting of 4 polypeptide chains, each carrying a haem group which contains iron

Describe the role of haemoglobin

Present in red blood cells

Oxygen molecules bind to the haem groups and are carried around the body to where they are needed in respiring tissues

What is partial pressure (PO2)?

A measure of oxygen concentration in the surrounding area

How does partial pressure affect oxygen-haemoglobin binding?

When partial pressure of oxygen is high, haemoglobin has a high affinity for oxygen, so becomes saturated with oxygen (association)

When partial pressure of oxygen is low, haemoglobin has a low affinity for oxygen, so oxygen unloads (dissociation)

How does partial pressure of carbon dioxide affect oxygen-haemoglobin binding?

The Bohr effect - as partial pressure of CO2 increases, conditions becomes acidic causing haemoglobin to change shape. The affinity for oxygen therefore decreases so more of it unloads.

What do oxyhaemoglobin dissociation curves show? Describe their shape and what causes it

Oxyhaemoglobin saturation (%) against PO2.

They are S-shaped as when the first oxygen molecule combines with the first haem group, the shape of the haemoglobin become distorted (cooperative nature)-makes it easier for the next 2 oxygen molecules to bind. The final one has difficulty again as there is a low chance of finding a binding site.

How does carbon dioxide affect the position of an oxyhaemoglobin dissociation curve?

• If there is a low partial pressure of CO2 (eg in alveoli) curve shifts to left due to increased affinity for O2

• If there is a high partial pressure of CO2 (eg in respiring tissues) curve shifts to the right due to decrease affinity for O2

Explain why oxygen binds to haemoglobin in the lungs

• Partial pressure of oxygen is high

• Partial pressure of carbon dioxide is low

• Affinity for oxygen is high

• Oxygen associates with haemoglobin to form oxyhaemoglobin

Explain why oxygen is released from haemoglobin in respiring tissues

• Partial pressure of oxygen is low

• Partial pressure of oxygen is high

• Affinity for oxygen is low

• Oxygen dissociates from haemoglobin

What is tissue fluid?

Fluid containing water, glucose, amino acids, fatty acids, ions and oxygen which bathes the tissues. Supplies these materials to the cells while also removing any waste.

How is tissue fluid formed?

• Capillaries have small gaps in the walls so that liquid and small molecules can be forced out

• As blood enters the capillaries from arterioles, the smaller diameter results in a high hydrostatic pressure which forces tissue fluid out (ultrafiltration)

When tissue fluid is formed, what is forced out and what remains in the capillary?

What’s forced out:

-Water

-Dissolved minerals and salts

-Glucose

-Small proteins and amino acids

-Fatty acids

-Oxygen

What remains:

-Red blood cells

-Platelets

-Large proteins

How is tissue fluid reabsorbed?

• Towards the venue end of the capillary, the hydrostatic pressure is low due to the loss of liquid

• Large molecules remain in capillaries which creates a lowered water potential

• This creates an osmotic gradient so water moved back into the capillary by osmosis

What happens to the remaining tissue fluid that doesn’t get reabsorbed?

• It is drained into the lymphatic system, becoming lymph

• It eventually re-enters the bloodstream near the heart

What is transpiration?

The loss of water vapour from the stomata by evaporation

Name and explain 4 factors that affect transpiration

• Light intensity - more light causes more stomata to open so larger surface area for evaporation

• Temperature - more heat means more kinetic energy so faster moving molecules therefore more evaporation

• Humidity - more water vapour in the air makes water potential outside the leaf higher so reduces water potential gradient therefore less evaporation

• Wind - more wind blows away humid air which maintains the water potential gradient therefore increasing transpiration

How is water transported in plants?

Through xylem vessels

Explain the cohesion-tension theory

• Cohesion - water molecules form hydrogen bonds causing them to stick together ad travel up the xylem as a continuous water column

• Adhesion - water sticks to xylem walls (capillarity)

• Tension - as water is pulled up the xylem, it creates tension which pulls the xylem in to become narrower

• Root pressure - as water moves into roots it increases volume of liquid inside the root therefore pressure inside the root increase which forces water upwards

Explain how water moves up the xylem (5)

1. Water vapour evaporates out of stomata on leaves. This loss in water volume creates a lower pressure

2. When this water is lost by transpiration, more water is pulled up the xylem to replace it

3. Due to the hydrogen bonds between water molecules, they are cohesive. This creates a column of water within the xylem

4. Water molecules also adhere to the walls of the xylem. This helps to pull the water column upwards

5. As this column of water is pulled up the xylem, it creates tension which pulls the xylem in to become narrower

Name the process whereby organic materials are transported around the plant

Translocation

How are organic substances transported around a plant?

Via the phloem

List the 2 key cells inside of phloem vessels

• Sieve tube elements

• Companion cells

Explain the structure of sieve tube elements

• Living cells

• Contain no nucleus

• Contain few organelles

Explain the function of companion cells

Provide ATP required for active transport of organic substances

Is translocation an active or passive process?

Active

According to the mass flow hypothesis, what is the source and what happens here?

• The source is the leaf

• Here, sucrose is made in photosynthesis

How does sucrose in the leaf move into the phloem?

• Sucrose is transported by facilitated diffusion through plasmodesmata into companion cells

• Hydrogen ions are pumped out (with proton pump) of companion cells by active transport into the cell wall space (apoplast), creating a concentration gradient

• H+ then re-enter companion cells through co-transport proteins which carry sucrose as well against their conc gradient

• This results in a high concentration of sucrose in the companion cells

• Sucrose then diffuses from companion cells into sieve tube elements - via plasmodesmata or membrane transport