Topic 3 - Organisms Exchange Substances with their Environment

What is the problem with having a small SA:V ratio?

it makes it more difficult to exchange materials and heat with the environment fast enough to keep conditions inside the organism constant.

For organisms to be more than 1mm^3, they must become multi-cellular and have a way of increasing SA for exchange of materials and heat

What are the 5 ways organisms can increase their SA:V ratio?

- flattened body

- internal gills

- external projections

- internal tubing system

- internal lungs

How does gas exchange occur in single-cellular organisms?

they have a large SA:V ratio so gas exchange occurs by diffusion through their membranes

How does gas exchange occur in insects?

They have evolved a network of internal tubes called tracheae

What are tracheae supported by in insects?

strengthened rings

What do the tracheae divide into?

Smaller dead-end tubes called tracheoles, which extend all through the body tissues of the insect to bring atmospheric air to all respiring tissues

What area the 3 ways respiratory gases move in and out of the tracheal system?

- along a diffusion gradient

- by mass transport

- the ends of the tracheoles are filled with water

How do gases exchange in the tracheal system using a diffusion gradient?

- O2 used up in respiration so a conc gradient is created. O2 can diffuse from the atmosphere a;peg the tracheae and tracheoles to the cells

- the opposite occurs for CO2

- as diffusion is faster in air than in water, gases are exchanged quickly by the method

How do gases exchange in the tracheal system using mass transport?

the contraction of muscles in insects can squeeze in the trachea, enabling mass movements of air in and out of the

How do gases exchange in the tracheal system using the water in the end of the tracheoles?

1) cells around the tracheoles respire by anaerobic respiration, lactate produced

2) water potential in muscle cells reduced, water moves into cells by osmosis from tracheoles

3) water in ends of tracheoles decreases in volume so more air is drawn in

4) this means the final diffusion pathway is by gas rather than liquid, so rate ofdiffusion is increased (however this increases rate of water evaporation)

What is the limitation of the tracheal system in insects?

It relies on diffusion which needs a short diffusion pathway, limiting the size of insect

How do gases enter and leave the tracheae?

Through tiny pores called spiracles which are opened and closed using valves.

Why do fish need a specialised surface for gas exchange?

- fish are aquatic and have a waterproof and air-tight covering

- they have a small SA:V ratio

Describe the structure of gills in fish.

- gills are made up from gill filaments

- at right angels to the filaments, there are gill lamellae

Why are there gill lamellae?

To increase surface area

How does water flow through gills?

Water goes in through the mouth and is forced across the gills and out at the back of the head

water flows in the opposite direction to the blood called counter-current flow, to maintian concentration gradient

Why is counter-current flow better than parallel flow?

It means that 80% of the oxygen in the water can be absorbed, rather than just 50%.

What is the xylem?

vascular tissue that transports water in the stem and leaves of plants.

How are root hair cells specialised?

thin cell wall to reduce diffusion distance

many mitochondria to produce ATP for active transport of dissolved mineral ions

hair extension to increase SA for absorption

What are the 2 things the xylem is composed from?

tracheids and xylem vessel elements

What are tracheids?

long, thin cells with tapered ends

What is the structure of xylem vessel elements?

- shorter and wider than tracheids, and join end-to-end

- as they develop, the ends of the vessels disappear to form a continuous tube called the xylem vessel

- the contents die leaving a hollow lumen to enable the water and dissolved mineral ions to flow continuously

What is transpiration?

The loss of water vapor through the stomata of leaves.

What is the first formed xylem called?

the protoxylem

How is xylem adapted to its function?

there is lignin to form a secondary cell wall. it strengthens the xylem and acts as a waterproofer because it does not dissolve.

How is lignin laid down in xylem vessels' walls?

rings first, then spirals

What is the mature xylem called?

it is called metaxylem where the walls are covered in lignin, except pits

What theory is used to describe how water moves up through the xylem?

cohesion-tension theory

Describe cohesion tension theory.

- water evaporates from stoma

- water molecules form H bonds between each other so tend to stick together, so form a continuous column of water through the xylem vessel

- this means when transpiration occurs, more water molecules are drawn up into the xylem

- this is called transpiration pull

What is the evidence for cohesion tension theory?

Changes in diameter of a tree-during the day where transpiration rate is increased, the diameter is decreased, due to the cohesive forces of the water.

When a xylem vessel if broken, air is drawn in and no water leaks out (which would happen if the water was under pressure), so this is consistent with the water being under tension.

What is phloem?

the tissue that transports organic substances in plants

What are the two parts of the phloem?

sieve tube elements and companion cells

What are the directions of flow in the xylem and phloem?

xylem - unidirectional (up)

phloem - bidirectional

Describe companion cells.

- found at the side of the sieve tube element. Each sieve tube element has one or more companion cells which contain many mitochondria to produce ATP for active transport, and ribosomes to produce amino acids.

- the walls separating the companion cell and the sieve tube elements are thin and perforated with holes called plasmodesmata

Describe the sieve tube elements.

- make up a sieve tube; a cylindrical column of cells joined end-to-end

- these end walls are perforated with holes, called a sieve plate. In these cells, the organelles disappear to allow the cytoplasm to move from one sieve tube element to another in a mass of strands of cytoplasm, linking each sieve tube element to the next one.

What is the name for the process by which organic molecules and some mineral ions are transported from one part of a plant to another?

translocation

What is the currently accepted theory for the mechanism of how translocation occurs?

mass flow theory

What are the three stages of mass flow theory?

1) transfer of sucrose into sieve cells from photosynthesising tissue

2) mass flow of sucrose through sieve tube elements

3) transfer of sucrose from the sieve tube elements into storage or other sink cells.

Describe the transfer of sucrose into sieve cells from photosynthesising tissue.

- sucrose manufactured in chloroplasts diffuses down a conc gradient from photosynthesising cells into companion cells

- hydrogen ions actively transported from companion cells into the spaces between cell walls using ATP

- hydrogen ions diffuse down concentration gradient through companion cells into sieve tube elements

- sucrose molecules are transported along with the H+ ions by co-transport through cotransport proteins

Describe mass flow of sucrose through sieve tube elements

- sieve tubes have a lower water potential after sucrose is transported into them

- xylem next to the phloem therefore has a much higher water potential

- water moves into sieve cells by osmosis, causing a high hydrostatic pressure within them

- at the sink, sucrose is either used up for respiration or converted to starch for storage

- these cells have a low sucrose conent so sucrose is actively transported into them from the sieve tubes, lowering their water potential

- water therefore also moves into the sink cells by osmosis

- hydrostatic pressure of the sieve cells in the region is therefore reduced

- as a result of water enterring the sieve tube elements at the source and leaving the sink, there is a high hydrostatic pressure at the source and a low one at the sink

- there is therefore a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes

Describe the transfer of sucrose from the sieve tube elements into storage or other sink cells.

the sucrose is actively transported tby companion cells out of the sieve tubes and int the sink cells.

What are the xylem and phloem called together?

vascular bundle in stems,

vascular tissue in leaves

What adaptions do the leaves have for rapid diffusion?

- many small pores called stomata so no cell is far from a stoma and the diffusion pathway is short

- numberous inter-connecting air spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells

- large surface area of mesophyll cells

Why are stomata closed at night?

Because carbon dioxide is not needed for photosynthesis, so closing the stomata reduces water loss.

What are the layers of a leaf?

upper cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis (where the stomata and guard cells are found)

What are the adaptions of insects to reduce water loss?

- small SA:V ratio

- waterproof coverings over body surfaces (chitin outer skeleton covered with a waterproof cuticle)

- spiracles can be closed

What are xerophytes?

Plants that are adapted to living in areas where water is in short supply

What are the adaptions of xerophytes?

- thick cuticle

- rolling up of leaves

- hairy leaves

- stomata in pits or grooves

- reduced SA:V ratio of the leaves

- shallow and wide root system

- deep root system

How does rolling up of leaves, hairy leaves and stomata in pits or grooves help reduce water loss in xerophytes?

This traps damp air so that the water potential gradient is reduced and less water is lost by evaporation.

How does a shallow and wide root system aid xerophytes?

Means that max amount overnight condensation can be accessed.

Why are the volumes of CO2 and O2 that need to be exchanged in mammals large?

- they are relatively large organisms with a large volume of living cells

- they maintain a high body temperature which is related to them having high metabolic and respiratory rates

Why are mammalian lungs located inside the body?

- air is not dense enough to support and protect these delicate structures

- the body as a whole would otherwise lose a great deal of water and dry out (desiccation)

What are the parts of the human gas exchange system?

lungs, trachea, bronchi, bronchioles, alveoli

Describe the lungs.

pair of lobed structures made up of a series of highly branched tubules called bronchioles which end in air sacs called alveoli

Describe the trachea.

Flexible airway supported by rings of cartilage, which prevents the trachea from collapsing as the air pressure inside falls when breathing in. The trachea walls are made up of muscle, lined with ciliated epithelium and goblet cells.

Describe the bronchi.

Two divisions of the trachea. They produce mucus to trap dirt particles, and have cilia. They are supported by cartilage.

Describe the alveoli.

Minute air sacs with collagen and elastic fibres found in between them. The fibres allow them to stretch and spring back to allow breathing.

What is the difference between inhaled and exhaled air? Why?

- inhaled has more O2 and less CO2 because of diffusion of these gases in/out of the blood stream in the alveoli.

- exhaled has more water vapour because it diffuses from the moist surface in the alveoli

What is the diaphragm?

a sheet of muscle that separates the thorax from the abdomen

Describe inspiration.

• External intercostal muscles contract, pulling the ribcage up and out

• Diaphragm contracts, pulling it from a domed to a flattened shape

• Combined effect is:

Volume of thorax and lungs increases

Pressure is reduced so atmospheric pressure is greater than pulmonary pressure

Air enters and goes down the pressure gradient

Describe expiration.

external intercostal muscles relax

ribcage moves down and in

diaphragm curves

Combined effect:

volume of thorax and lungs decreases

INCREASE IN PRESSURE so air moves out down a pressure gradient as the atmospheric pressure is lower than the pulmonary pressure

aided by alveolar elastic recoil

What is the pulmonary ventilation rate?

total volume of air moved into the lungs during 1 minute;

tidal volume x breathing rate

What is tidal volume?

Volume of air inhaled in each breath

What is breathing rate?

breaths per minute

How can you investigate the decrease in mass of a plant due to transpiration?

1) select 4 leaves of similar SA from the same plant ( same species and age)

2) cover different surfaces of each leaf in petroleum jelly (1 control)

3) record initial mass

4) after 24hrs record mass again and calculate percentage decrease

What can you conclude from an investigation on the decrease in mass of a plant due to transpiration?

- coating lower surface causes similar water loss to coating both sides of the

- coating upper surface causes similar water loss to coating no surface at all

During forced expiration, which muscles are described as being antagonistic and why?

internal and external intercostal muscles, because the external ones relax while the internal ones contract

What are the parts of the digestive system?

aesophagus, stomach, ileum, large intestine, rectum, salivary glands, pancreas, gall bladder

Describe the oesophagus.

carries food from the mouth to the stomach

Describe the stomach.

- Muscular sac with an inner, enzyme-producing layer ---> stores and digests food (especially proteins) and has glands that produce enzymes which digest proteins.

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.

What is the role of the large intestine?

it absorbs water

What is the role of the rectum?

stores faeces before periodically being reomved via the anus in a process called egestion

What do the salivary glands do?

secrete amylase to hydrolyse starch into maltose

Describe the pancreas.

gland situated below the stomach and secretes pancreatic juice, which contains protease, lipase and amylase

What does the gall bladder do?

It stores bile before it's released into the small intestine

Describe carbohydrate digestion.

-begins in the oral cavity with salivary amylase, which breaks down starch into maltose

-In the ileum, pancreatic amylase breaks down any remaining starch to maltose

- the epithelial lining of the ileum produces maltase, a membrane-bound disaccharidase. The maltase hydrolyses the maltose from starch breakdown into alpha-glucose

What does sucrose do?

hydrolyses the single glycosidic bond in the sucrose molecules, producing glucose an fructose

What does lactase do?

hydrolyses the single glycosidic bond in the lactose molecule, producing glucose and galactose

What is the role of saliva?

1. provides lubrication

2. has salivary amylase

3. has mineral salts that help maintain the pH at around neutral, the optimum for amylase

What are the two parts of digestion?

physical breakdown and chemical digestion

Describe physical breakdown.

breaking fown of large food by teeth and churning by muscles in the stomach. This increases SA for enzyme action.

Describe chemical digestion.

breaking down of large insoluble molecules into small soluble ones by hydrolysis by enzymes.

Describe lipid digestion.

Occurs in small intestine: emulsification by bile salts --> lipase converts triglycerides to fatty acids and glycerol; esterase converts cholesterol esters to fatty acids and cholesterol

What are the three enzymes used in protein digestion?

endopeptidase, exopeptidase, dipeptidase

What does endopeptidase do?

hydrolyse peptide bonds between amino acids in the central region of a protein molecule, forming a series of peptide moelcules

What does exopeptidase do?

hydrolyses peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidase. In this way they progressively release dipeptides and single amino acids.

What does dipeptidase do?

hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane- bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.

What are the two factors that the presence of a specialised transport medium depends on?

- SA:V

- how active the organism is

What type of circulatory system do mammals have?

a closed double circulatory system

Which blood vessels are associated with the liver?

renal artery and vein

Which organs does blodd flow directly between? What type of blood?

deoxygenated blood flows from the stomach and intestines to the liver

Why is blood returned to the heart before flowing to other tissues?

There is a drop in pressure when it flows through the lungs, which would mean the circulation would be too slow otherwise.

Why do mammals need a double circulatory system?

Double circulatory systems have blood at a higher pressure, allowing it to flow faster and move substances quickly around the body, which is needed because mammals have high body temperatures and hence a high metabolic rate.

What are the two chambers on each side of the heart?

the atrium and the ventricle

Describe the atrium.

thin-walled and elastic and stretches as it collects blood

Describe the ventricle.

thick muscular wall to pump blood around the body or to the lungs

Which ventricle is thicker and why?

the left side is thicker because it has to pump the blood all over the body.

What are the two valves in the heart?

left atrioventricular (bicuspid) valve

right atrioventricular (tricuspid) valve

They are between the atrium and ventricle on each side.

Describe the aorta.

connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs