A level Edexcel BIOLOGY B Chapter 4 - Exchange and transport

A Level Biology Topic 4 Exchange and transport

Give 5 ways that villi increase efficiency of absorption.

1. Increase surface area.
2. Thin walls.
3. Have muscles, can mix and move content of ileum along so maintains gradient.
4. Well supplied with blood vessels, can move blood along so also maintains gradient.
5. Covered in microvilli which further increase surface area

Give two methods of absorbing amino acids and monosaccharides into blood.

Facilitated diffusion and cotransport.

Why can't charged ions diffuse across a membrane?

Hydrophobic nature of the fatty acid tail of the phospholipids in membrane repels charge

Name the two types of facilitated diffusion.

Protein channels and carrier proteins.

Describe how protein channels work.

Protein channels are water filled across membrane allowing specific water-soluble ions to pass through. Ions bind with protein causing it to change it's shape in such a way that it closes one side and the ion is released out of membrane

Describe how carrier proteins work.

Carrier proteins are used when specific molecules bind with the protein to change its shape in such a way that the molecule is release to the inside of the membrane. It can then leave via diffusion

Why can only some ions pass through these channels?

Because each proteins' substrate has a specific shapes, which means ions need to have the complimentary parts which fit the shape of the protein it's binding with

Describe how cotransport works.

Sodium ions inside epithelial cells are constantly actively transported out by a pump which creates a gradient. When sodium ions naturally diffuse back into cell, they carry an amino acid or glucose molecule with them. Can then be passed into blood through other methods.

Does cotransport require energy?

Even though glucose/amino acids are moving against their concentration gradient, they do not require energy because it is powered by sodium ion concentration gradient.

What happens after emulsification of triglycerides?

The monoglycerides and fatty acids remain in association with the bile salts that initially helped them to emulsify the lipid droplets.

What is the name for the collection of monoglycerides and fatty acids?

A micelle

What is the size of a micelle?

Tiny, 4-7nm in diameter

How are the molecules in a micelle released individually into the epithelial cell of the ileum?

Due to movement within the lumen of the ileum the micelle contacts the epithelial cell and the monoglycerides and fatty acids can diffuse across the membrane.

What happens to the lipid molecules when inside the epithelial cell?

Molecules are transported to the endoplasmic reticulum where they are recombined into triglycerides. They then move into the Golgi apparatus and associate with cholesterol and lipoproteins

What is the name given to the structure formed from triglycerides, cholesterol and lipoproteins?

Chylomicrons

What are chylomicrons adapted to do?

Transport lipids

How do chylomicrons move out of epithelial cells?

By exocytosis

What is a lacteal?

The lymphatic vessels of the small intestine that chylomicrons absorb into

Structure - Stomach

A muscular sac with an inner layer that produces enzymes.

Role - Stomach

Its role is to store and digest food, especially proteins. It has glands that produce enzymes, which digest protein.

Structure - Ileum

A long muscular tube, consisting of villi and microvilli.

Role - Ileum

Food is further digested by enzymes produced by its walls and glands. Inner walls are folded into villi, which gives them a large surface area. This alongside microvilli adapts the ileum for its purpose of absorbing the products of digestion into the bloodstream.

Role - Large Intestine

The LI absorbs water.

Role - Rectum

The faeces are stored here before periodically being removed via the anus in a process called egestion.

Role - Salivary Glands

These secretions contain the enzyme amylase, which hydrolyses starch into maltose.

Role - Pancreas

Large gland situated below the stomach. Produces pancreatic juice, which contains proteases to hydrolyse proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch.

Chemical Digestion

Hydrolyses large, insoluble molecules to small and soluble ones.

Carbohydrate Digestion

1. Salivary amylase hydrolyses starch to maltose. It contains mineral salts that help to maintain the pH at around neutral (=optimum)
2. Stomach has acidic pH, which denatures the amylase and prevents further hydrolysis of starch.
3. Small intestine mixes with pancreatic juice, which contains proteases and pancreatic amylase, which hydrolyses remaining starch to maltose. Alkaline salts to maintain pH.
4. Ileum's wall produces maltase, which hydrolyses maltose to alpha-glucose.

Sucrase

Hydrolyses the single glycosidic bond in the sucrose molecule, to produce the two monosaccharides glucose and fructose.

Lactase

Hydrolyses the single glycosidic bond in the lactose molecule, to produce glucose and galactose.

Lipid Digestion

Lipase, produced by the pancreas, hydrolyses the ester bond found in triglycerides to form fatty acids and monoglycerides. Lipids are firstly split up into tiny droplets (micelles) by bile salts, produced by the liver. This process is called emulsification and increases the surface area of the lipids so lipase action is sped up.

Protein Digestion

Peptidases hydrolyse large, complex molecules called proteins.

Endopeptidases

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

Exopeptidases

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

Dipeptidases

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 is the oesophagus?

A muscular tube that connects the mouth to the stomach.

What is the function of the large intestine?

Absorbs water

what is tissue fluid?

the environment around the cells of multicellular organisms

what affects the amount of material that is exchanged in an organism?

size and metabolic rate- higher metabolic rate= more materials exchanged

what is passive exchange?

exchange that requires no metabolic energy e.g. through diffusion and osmosis

what is active exchange?

exchange that requires metabolic energy e.g. active transport

how are smaller organisms adapted for exchange in terms of surface area to volume ratio?

they have a surface area that is large enough, compared with their volume

how are bigger organisms adapted for exchange in terms of surface area to volume ratio?

they have a smaller surface area when compared with there ratio so have more specialised mechanisms for exchange

name two ways in which organisms have evolved for efficient gas exchange.

- flattened shape so short diffusion pathway
- specialised exchange surfaces with large surface area:volume

why does a large surface area to volume ratio allow for efficient exchange to take place?

increases the rate of exchange

why does a thin organism allow for efficient exchange to take place?

short diffusion pathway

why does a selectively permeable membrane allow for efficient exchange to take place?

only allows selected materials to cross

name 4 things that need to be exchanged between an organism and their environment.

- respiratory gases
- nutrients
- excretory products
- heat

how do you calculate the surface area to volume ratio?

how does gas exchange occur in a single-celled organism?

they are small organisms so have a large surface area to volume ratio and therefore can exchange substances through diffusion across their body surface

what is tracheae?

the internal network of tubes in insects used for gas exchange, they are supported by strengthened rings

what are tracheoles?

smaller divisions of tracheae which extend throughout all the body tissues of the insect. these touch every cell in the insects body to ensure that respiratory gases reach everywhere

name the 3 ways respiratory gases move in and out of the tracheal system.

- along a diffusion gradient
- mass transport
- the ends of the trachea are filled with water

how are respiratory gases moved in and out of an insect using mass transport?

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

how are respiratory gases moved in and out of an insect along a diffusion gradient?

during cell respiration, O2 concentration at the end of tracheoles decreases so O2 from the atmosphere is drawn in by diffusion along a concentration gradient. the CO2 produced goes the opposite way

how are respiratory gases moved in and out of an insect using water that fills the ends of the tracheoles?

during periods of high activity, muscles carry out anaerobic respiration which produces lactate, a soluble liquid that lowers water potential of muscle cells. this causes water to move from tracheoles to the cells by osmosis. this causes water in tracheoles to decrease in volume which draws air into them through a diffusion gradient

what are spiracles and how do they work?

they are tiny pores on the body surface of an insect that open and close using a valve. they can allow water vapour to evaporate from them and tend to stay shut to avoid water loss.

explain how the tracheal system limits the size of insects

it relies on diffusion to bring oxygen to the respiring tissues. if insects were large it would take too long for oxygen to reach all the tissues quickly enough so the insect would die

why is there a conflict in insects between gas exchange and conserving water?

gas exchange requires a thin, permeable surface with a large area whereas conserving water requires thick, waterproof surfaces with a small area

what are the gills of a fish made up of?

gill filaments

how are gill filaments arranged?

stacked up in a pile at right angles to gill lamellae.

what is the purpose of gill lamellae?

increases the surface area of gills on gill filaments

what is countercurrent flow?

the mechanism in fish where water flows over the gills in the opposite direction to the flow of blood in the fish

why is countercurrent flow important?

it ensures that the maximum possible gas exchange is achieved

how does countercurrent flow work?

blood that is already well-loaded with oxygen will meet water that has its maximum concentration of oxygen. this causes a diffusion of oxygen from the water to the blood
blood with little oxygen in it will meet water that has had most of its oxygen removed, this means diffusion of oxygen from water to blood will take place.

how much available oxygen is absorbed using countercurrent flow?

80%

how much available oxygen would be absorbed if the flow of blood and water was parallel?

50%

why is countercurrent flow is an efficient means of gas exchange?

a steady diffusion gradient is maintained over the whole length of gill lamellae so more oxygen diffuses from water to the blood

what might be the difference between the gills of an active fish and a less active fish?

faster fish will have larger surface area of gills, more gill filaments and/or more gill lamellae

why is one-way flow of water in fish an advantage compared to two-at flow mechanisms in mammals and insects

less energy is required because the flow does not have to be reversed and water is dense so would be hard to move backwards.

how is gas exchange in plants similar to gas exchange in insects?

- no living cell is far from the external air
- diffusion takes place in the gas phase making it more rapid than in water
- need to avoid excessive water loss
- diffuse air through pores in outer covering

how are leaves adapted for rapid diffusion?

- stomata which mean there is a short diffusion pathway to all cells
- many interconnecting air spaces in mesophyll so gases can readily come into contact with mesophyll cells
- large surface area of mesophyll cells for rapid diffusion

what are stomata?

tiny pores that occur mainly on the underside of leaves.

what surrounds a stomata and what does it do?

guard cells open and close the stomatal pore to control the rate of gaseous exchange

why are stomata important?

it prevents excessive water loss by evaporation/ transpiration

how are the gas exchange systems of plants and insects different?

- insects may crease mass air flow, plants never do
- insects have smaller surface area to volume ratio than plants
- insects have specialised structures for air to diffuse across (tracheae) plants do not

how have insects evolved to reduce water loss?

- small surface area to volume ratio
- waterproof coverings
- spiracles

how does a small surface area to volume ratio help reduce water loss in insects?

minimises area over which water is lost

how do waterproof coverings help reduce water loss in insects?

stops water passing out through osmosis

how do spiracles help reduce water loss in insects?

they can close to stop water pouring out of them

why can't plants have a small surface area to volume ratio?

they need to photosynthesise which requires them to have a large leaf surface to catch light

what is a xerophyte?

a plant that is adapted to living in areas where water is in short supply

how do xerophytes reduce water loss?

transpiration

how does a thick cuticle reduce water loss in plants?

less water can escape through it

how do hairy leaves reduce water loss in plants?

it traps moist air next to the leaf surface so the water potential gradient between the outside and the inside of the leaf is reduced therefore less water is lost by evaporation

how do stomata in pits or grooves reduce water loss in plants?

traps moist air next to the leaf and reduces water potential gradient

how does a reduced surface area to volume ratio of the leaves reduce water loss in plants?

plants have small leaves which are rounded so water loss is reduced

how do leaves rolling up reduce water loss in plants?

water evaporating from the leaf is trapped and the region where the leaf is rolled becomes saturated with water vapour. there is no gradient between the inside and outside of a leaf so water loss is reduced

what is the main problem that insects and plants share?

they both need to achieve efficient gas exchange but the factors needed for this conflict with the features needed for reducing water loss

How does air get to the lungs from being breathed in?

Flows down trachea
Trachea splits into 2 bronchi
Each bronchus branches off into many bronchioles
At the end of bronchioles are many alveoli

Describe what happens to the diaphragm and intercostal muscles when you breathe in (inspiration) ?

Diaphragm contracts
Moves down and flattens
External intercostal muscles contract
Pull the ribcage up and out

Why is the movement of the diaphragm and intercostal muscles important in inspiration?

Increases volume of thorax
Decreases pressure
Lung pressure lower than atmospheric pressure
Air flows into lungs down pressure gradient

True or False
Inspiration is an active process (required energy)

True

Describe what happens to the diaphragm and intercostal muscles during non-forced expiration?

Diaphragm relaxes
Moves up and domes
External intercostal muscles relax
Ribcage moves down and in

Why is the movement of the diaphragm and intercostal muscles important in expiration?

Volume of thorax decreases
Pressure increases
Lung pressure greater than atmospheric pressure
Air forced out of lungs down pressure gradient

True or False
Expiration is always a passive process

False
Usually breathing out does not require energy
BUT during forced expiration it does

Why does forced expiration require energy?

Internal intercostal muscles contract
Move ribcage further down and in

Describe the structure of an alveolus

Wall made of single layer of thin, flat alveolar epithelium
Wall contains protein - elastin
Surrounded by network of capillaries

What is the importance of ventilation?

Removes air with low oxygen concentration
Brings in air with high oxygen concentration
Maintains concentration gradient for oxygen to diffuse into blood from alveolus

What is another name of the thorax?

Thoracic cavity