Organisms exchange substances with their environment Flashcards
Do smaller or larger organisms have a larger surface area to volume ratio? Why?
Smaller organisms
As size increases, volume increases disproportionately compared to surface area
Why is transport across a small organisms body surface more sufficient?
- distances are short (less than 0.5mm)
- surface area to volume ratio is relatively large
Why can’t large active organisms rely upon their body surface for exchange?
- distances are too great
- surface area relative to volume is insufficient
- nutrients and waste removal for cells is larger
What two systems are needed for a large active organism?
- a specialised exchange surface
- an transport system to deliver materials to and from the exchange surface
Features of a specialised exchange surface:
- a large surface area
- thin barrier to reduce diffusion distance
- large concentration gradient
-fresh supply of molecules on one side
-removal of molecules on the other side
What is the tracheal system?
Series of tubes that deliver oxygen directly to the cells. this process is diffusion (simple)
What are the function of the spiracles?
The way air enters the tracheal system
Spiracles can be opened and closed - this helps to reduce water loss
How can the delivery of oxygen be maximised in the tracheal systemin an insect?
- cells using oxygen generates an oxygen concentration gradient
- pumping of the abdomen ventilates the tracheal system, maintains gradient
- production of metabolites during respiration reduces water potential of cells
How does water leave the tracheoles
Via osmosis
how is oxygen transported directly to tissues undergoing respiration in insects?
from spiracles, trachea or tracheoles, supply it with gases to the body. Gases move in and out through diffusion, mass transport of muscle contraction and water volume change in the tracheoles
Structure of fish gills:
- two rows of filaments
- filaments covered with folds called lamella (increases SA)
- extensive network of blood capillaries (conc. gradient)
Where does gas exchange in a fish take place and why?
Lamella
Distance between water and the blood cells is 5um (short diffusion path)
What is counter current flow?
Blood flows in an opposite direction to water, maintains a favourable concentration gradient, across the entire exchange surface
Why would parallel flow not be efficient in a gill of a fish
Concentration gradient will decrease as blood flows along the lamella preventing any further uptake of oxygen by diffusion
how is ventilation maintained in a fish?
mouth opens and lowers floor of buccal cavity. water flows in. fish closes its mouth, the buccal cavity raises, increases pressure. water is forced over gill filaments by the pressure difference. The operculum acts as a valve, lets water out and in.
how have plants adapted to increase gas exchange in their leaves
- small holes called stomata (in mesophyll) which allow gases to enter and exit the leaves. Large no. of these means cells are close to the stomata, reducing the diffusion distance.
- air spaces allow gases to come in contact with photosynthesising mesophyll cells.
how alveoli have adapted to maximize gas exchange:
- alveoli are one cell thick. These are surrounded by capillaries. This reduces the diffusion pathway for gases.
- constant blood supply by capillaries, creates a steep concentration gradient.
- large number of alveoli (~300 million)
how does the trachea, bronchi and bronchioles play its role in keeping the airway clear
- air enters through the nose, trachea, bronchi and bronchioles
- airway has rings of cartilage, in trachea allows passage of food down the oesophagus
- gas exchange takes place in the walls of alveoli
trachea and bronchi structure
- bronchi are narrower.
- layers make up a thick wall. wall is composed of cartilage, as incomplete C rings
- inside surface of the cartilage is glandular tissue, elastic fibres, smooth muscle and blood vessels
structure of bronchioles
- are narrower than the bronchi.
- larger bronchioles contain cartilage.
- wall is made out of smooth muscle and elastic fibres
- alveoli clusters at the ends
what is pulmonary fibrosis
scarring of the lung, tissue in lungs thicken difficulty to absorb o2 into the bloodstream (loose elasticity)
how dos pulmonary fibrosis effect gas exchange
- fall in lung volume. loss of elasticity, leads to decreased ventilation, leads to a lower tidal volume, due to lack of expansion
- increase diffusion distance
what is asthma?
occurs due to an obstruction in the airways, inflammation and bronchoconstriction, caused by an allergic reaction, so they narrow producing mucus
effect of asthma on gas exchange
- bronchoconstriction disrupt breathing, restriction of ventilation, deprives body of o2
- impaired exchange of o2 between air sacs in and blood in the lungs.
- air trapped in lungs
what is emphysema
involves gradual damage of lung tissue, destructs alveoli
effect of emphysema on gas exchange
- breaks down elastin. stops alveoli stretching and recoiling, breaks down alveoli
- alveoli fuse, decrease sa:v
- lung tissue dilates and thickens, diffusion path increase, traps air
what is COPD
chronic obstructive pulmonary disease
build up of pollutants in the lining of lungs, trachea, airways inflaming, caused by pollution & tobacco
what is ventilation
flow of air in & out the alveoli. there are two stages; inspiration and expiration. The process occurs with the intercostal muscles and diaphragm.
what happens during inspiration?
- external intercostal muscles contract
- internal muscles relax
- ribs raise upwards
- diaphragm contracts and flattens
- increase the volume inside the thorax, lowering the pressure. pressure difference between lungs and atmosphere creates a gradient, air is forced into the lungs.
what happens during expiration
- internal intercostal muscles contract
- external muscles relax
- lowering the rib cage
- The diaphragm relaxes and raises upwards.
These actions decrease the volume of the thorax, increasing the pressure and forcing air out of the lungs.
what’s a spirometer?
device that measures lung volume. they breathe in and out of the airtight chamber, moving it up and down, leaving a trace on a graph which is interpreted.
what is vital capacity?
what does it vary on?
maximum volume of air that can be inhaled/exhaled in a single breath.
gender, age, size, height.
what is tidal volume
volume of air we breathe in and out at each breath at rest
what is the breathing rate?
number of breaths per minute, calculated from spirometer trace by counting the number of peaks in a minute
what is the residual volume
The volume of air which is always present in the lungs
what is the expiratory reserve volume
additional volume of air that can be exhaled on top of the tidal volume.
what is pulmonary ventilation? and the calculation?
total volume of air that is moved into lungs in one minute. dm3min-1
tidal vol x ventilation rate
what happens during digestion
large insoluble biological molecules are hydrolysed to smaller molecules that can be absorbed/ assimilated across cell membranes
what is the site of absorption of nutrients
lining of the alimentary canal, ileum of the small intestine
what enzymes break down carbs
- Amylases in the saliva and pancreas hydrolyse glycosidic bonds of starch to form maltose,
- maltases in the ileum hydrolyse maltose to alpha glucose
- sucrases and lactases break down the disaccharides sucrose and lactose
what enzymes break down lipids
lipase hydrolyses the ester bond between the monoglycerides and fatty acid. Before being broken down in the ileum, lipids are emulsified into micelles by bile salts released by the liver. Emulsification increases the surface area and speeds up the chemical reaction.
function of bile
- neutralise stomach aicid which ensure correct pH for enzymes released into the ileum
- digestion of fats
where are bile salts produced
in the liver and stored by the gall bladder
digestion of fats via bile
- triglycerides aren’t water soluble
- fats must be emulsified by bile salts
- triglycerides are split up from large globules into micelles
- surface area increase for enzymes to work on
what do membrane-bound dipeptidases do?
hydrolyse dipeptides into individual amino acids
where are proteins digested
in the ileum and stomach
how is rate of digestion maximised in the ileum:
- villi- increase surface area
- microvilli - increase surface area
- good blood supply (capillaries), maintains a conc gradient
- thin walls, minimise diffusion distance
absorption of triglyceride
- non-polar glycerol & fatty acids move to epithelial cells in micelles. diffuse across the epithelial cell membrane
- er & Golgi body, triglyceride are resynthesized & combined with proteins & cholesterol to from vesicles called chylomicrons
absorption of triglyceride
what happens after chylomicrons are formed?
- chylomicrons ,move into lacteals by exocytosis
- carried through the lymphatic system enter the bloodstream at the vena cava & carried to parts of the body
- triglycerides are hydrolysed by enzyme in the endothelium of the capillaries and then diffuse into cells
co-transport in the epithelial cell
- Na+ & glucose molecules are transported into the epithelial cells lining the small intestine by facilitated diffusion
- Na+ & glucose enter the cells via co-transport proteins
- Na+ are actively transported from epithelial cells into the blood
absorption of amino acids form the lumen of the small intestine into the blood capillary
- amino acid & Na+ bind to carrier protein
- Na+ diffuse into the cell, amino acids are carried to
- amino acids diffuse to the other end of the cell
- then transferred to capillaries via facilitated diffusion
Where is respiratory gases move in and out of the tracheal system in insects?
- mass transport
- along a diffusion gradient
- Via the end of the tracheoles filled with water
- Gas enters and leaves through spiracles
- Spiracles closed to minimise H2O loss
Mass transport in gas exchange of insects?
Contraction of muscles, squeeze the trachea, enabling movement of air in and out
how is a diffusion gradient in insects created?
- oxygen used up, concentration towards the end of tracheoles fall
- Concentration gradient is created
- Oxygen diffuses from the atmosphere, trachea, tracheoles, cells
- Carbon dioxide is produced by cells
- Diffusion gradient is in the opposite direction
- Carbon dioxide diffuses from cells to tracheoles, trachea, atmosphere 
Tracheoles filled up with water in insects during gas exchange
- lactic acid decreases water potential of muscle cells
- Water moves into the cells, via osmosis
- Water in the tracheole ends decreases in volume, so more can be drawn further into them
How rapid diffusion occurs in gas exchange in plants:
- stomata, short diffusion pathway
- air spaces - gas comes in contact with mesophyll cells
-Large surface area
Limiting water loss in insects:
- small surface area to volume ratio
- Waterproof coverings
- Spiracles closing
Modifications to reduce water loss, in plants:
- thick cuticle - waterproof barrier, less water can escape
- Rolling of leaves- no water potential gradient between inside and outside of a leaf so no water loss
- Hairy leaves - traps moist air. This decreases wp gradient between inside and outside of leaf, less water is lost by evaporation
- Reduce sa:v of leaves
- stomata
why does the vol of O2 that has to be absorbed & the vol of CO2 that has to be removed are large in mammals
- have a large volume of cells
- maintains high body temp is related to them having a high metabolic and respiratory rate
why are lungs (site of gas exchange) located inside the body?
- air isn’t dense enough to protect them
- body would lose water & dry out
what is the ileum?
long muscular tube, inner walls (villi) contain micro villi on them which increase surface area. absorbs products of digestion
what is hydrolysis
splitting up of molecules by adding water to the chemical bonds
types of digestion
- mechanical (physical)
- chemical
process of digestion for starch
- saliva enters from salivary glands & is mixed with food
- saliva contains amylase. hydrolyses to maltose
- food enters the stomach, acid denatures amylase and prevents further hydrolysis into starch
- food passes into small intestine, mixes with pancreatic juice
- pancreatic juice contains amylase, this hydrolyses
- muscle in intestine push food along the ileum
- epithelial lining produces maltase. this hydrolyses maltose into alpha glucose
Villi’s properties increase the efficiency of absorption in the following ways:
- increase the surface area
- thin walled, reducing the diffusion distance
- contain muscle so are able to move
- supplied with blood vessels
- The epithelial cells lining the villi possess microvilli These increase the surface area.
Absorption of triglycerides
First stage
- Monoglycerides and fatty acids are mixed with bile salts, these form micelles
- Micelles breakdown when they come in contact with epithelial cells, lining the villi of the ileum
- releases monoglycerides and fatty acids these are nonpolar so they diffuse into the epithelial cells
Absorption of triglycerides
Second stage in the epithelial cell
- Monoglycerides and fatty acids enter, and are transported to the endoplasmic reticulum to form triglycerides
- In the Golgi triglycerides are associated with cholesterol, and Lipo proteins to form chylomicrons
- these, move out of the epithelial cell by exocytosis
Absorption of triglycerides
Third stage in the lymphatic system
- chylomicrons enter lymphatic capillaries called lacteals (centre of each villus)
- Chylomicrons pass lymphatic vessel where they are hydrolysed
- they diffuse out of cells
What is haemoglobin?
A globular protein in a red blood cells, which transport oxygen
What makes haemoglobin soluble?
Hydrophilic, side chains, facing outwards, and hydrophobic side chains, facing inwards. Therefore, it’s good for transporting oxygen in the blood.
What is a haemoglobin made up of?
Four Polypeptide chains, each bound to one haem group.
How much oxygen can each haem group bind to?
One oxygen molecule. This causes the formation of oxyhaemoglobin.
How is haemoglobin efficient at transporting oxygen?
- readily associates (loads) with oxygen at the gas exchange surface
- Readily dissociates (unloads) from oxygen at those tissues, requiring it for respiration
What is partial pressure of oxygen? (pO2)
The measure of oxygen concentration
What is affinity?
(Liking) of haemoglobin for oxygen depends on the pO2
- Haemoglobin loads oxygen at high pO2
- Haemoglobin unloads oxygen low pO2
What is saturation?
Determines the number of haem groups bound to O2
Relationship between partial pressure and affinity?
If you have a high partial pressure, haemoglobin will have a high affinity
What is an oxygen dissociation curve?
A graph showing the amount of oxygen combining with haemoglobin at different partial pressure is
What is a double circulatory system?
Blood passes through the heart twice in every circulation of the body, mammals have this
What are the two circuits in a double circulartory system?
- pulmonary circuits (to do with the lungs)
- Systemic circuit
Advantage of a double circulation
- oxygenated and deoxygenated blood do not mix - this maintains a higher concentration in the blood
- Allows different blood pressures in a systematic and pulmonary circuits - protects the structure of the lungs, while allowing efficient delivery of oxygen and nutrients to all parts of the body
Features of the internal structure of the heart:
- Left and right ventricles are separated by the septum
- Valves prevent backflow of blood
- Opening of the valves is controlled by the relative pressure in the chambers of the heart
What are the walls of a heart made of?
Cardiac muscle called myocardium
Features of myocardium:
- it is myogenic - able to generate its own electrical activity
- Never fatigues (constant) that is a very sensitive to a lack of oxygen
Trace the pathway of blood from the aorta
Aorta, vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta
Why is the left ventricle have a thicker wall to the right?
Pumps oxygenated blood to the body, so more muscle means you exert a higher force generating a high-pressure
What is a cardiac cycle?
The sequence of events in one heartbeat
Features of a cardiac cycle
- consist of alternate contraction (systole) and relaxation (diastole)
- On average, each cycle/heartbeat lasts around 0.8 seconds
Pressure changes in the cardiac cycle:
- Muscle contraction in the chamber, reduces the volume of a chamber and increases pressure
- Blood flows from the area of high pressure to an area of low pressure
- Valves will open if there is a high pressure behind them and close, if there is a high pressure in front of them
What happens during atrial systole?
- Most blood moves into the ventricles by passive flow
- Both atria contract, atrioventricular valves, open, blood flows into ventricles
- Pressure in atrium is higher than pressure in ventricle
- Bicuspid and tricuspid valves open
What happens during ventricular systole?
- Atria, relax (atrial, diastole)
- Ventricles contract and pressure shuts atrioventricular valves, open semilunar valves
- Blood flows into arteries
- Bicuspid and tricuspid valves closed
What happens during ventricular diastole?
- ventricles, relax
- High pressure in arteries, then ventricles, shut the semi lunar valves
- Blood flows into the atria
- Bicuspid and tricuspid valves open
Explain why oxygen uptake is a measure of metabolic rate in organisms
Oxygen is used in respiration, which provides energy/ATP
Name for blood vessels that carry blood to the heart muscle
Coronary arteries
Describe the pathway taken by an oxygen molecule from an alveolus to the blood
- across alveolar epithelium
- Endothelium of the capillary
What factors effect the cardiac output:
- the stroke volume - volume of blood pumped by the ventricle in each heartbeat
- the heart rate - the number of times the heart beats per minute
Cardiac output equation
Stroke volume x heart rate
Control of the cardiac cycle
1. Sino atriol node
impulse starts in the sino atriol (SA) node located in the right atrium. Aka pacemaker
Control of the cardiac cycle
1. Atrioventricular node
The impulse travels through the atria walls. This causes both atria to contract. The cardiac impulse then reaches the atrioventricular node. The av node helps delay the impulse to allow the atria to finish their contraction before the ventricles contract
Control of the cardiac cycle
3. Bundle of HIS
From the av node the impulse spread down the bundle of HIS- located in the septum
Control of the cardiac cycle
4. Purkinje fibres
impulse spreads around the ventricle walls through a network of purkinje fibres. This causes both ventricles to contract
what do most arteries carry
oxygenated blood away from the heart
what do most veins carry
deoxygenated blood to the heart
what do arteries become?
arterioles, then capillaries where substances are exchanged within cells
where does blood move from the capillaries
into venules into veins and back to the heart
structure of arteries
- tunica externa - contains collagen/elastic fibres
- tunica media - contains smooth muscle, collagen and elastic fibres
- tunica intima - endothelium (single layer of cells), smooth so blood flows easily as there’s less friction
- lumen - hollow centre of tube
arteries structure and function
- transport blood at high pressure so have thick walls
- walls consist of collagen & elastic tissue, give arteries strength and maintains high pressure
- artery recoils, ensure blood is remained at high pressure
- narrow lumen
arterioles structure and function
- lower pressure
- walls thinner with less elastic tissue, more smooth muscle
- smooth muscle allows arterioles to constrict and regulate the flow of blood to different areas of the body
vein structure
- large lumen
- tunica media - very thin containing some smooth muscle and elastic fibres
- valves
structure and function of veins linked
- low pressure, walls are thinner than arteries, large lumen
- contains vales to prevent back flow of blood
capillary structure
- one cell thick
- made of endothelium only
- lumen, just big enough for a red blood cell to squeeze through
structure and function of capillaries
- take blood close to all cells to deliver and remove substances e.g. O2, C6H1206, CO2
- consists of a single layer of cells (endothelium only) to reduce diffusion distance
- vey large numbers to maximise exchange
- small diameter slows the flow of blood to provide time for exchange
3 fluid types
- blood
- tissue fluid
- lymph
What is tissue fluid?
liquid that surrounds all cells. It allows transport between blood & cells (e.g. respiratory gases), via diffusion.
Capillary walls are
Partially permeable
Tissue fluid is the result of the balance between:
1) hydrostatic pressure- pressure of any fluid acting against a container
2) osmotic pressure
Forming tissue fluid - arterial end
- High hydrostatic pressure, forces fluid out of the capillaries at the arterial end. Dissolved gases & nutrients move with it. E.g. NH2, O2. So there is a lower water potential
- large plasma proteins & cells don’t move
Forming tissue fluid - venule end
- Higher osmotic pressure. fluid is pushed out of the capillary as pressure within the capillary is reduced. water potential gradient between capillary and tissue fluid is the same, so water flows into the capillary from the tissue fluid, osmosis
Overall forming tissue fluid process
- there is a net loss of fluid from the capillaries at the arterial end as hydrostatic pressure> osmotic pressure
- there is a net gain at the venous end as osmotic pressure > hydrostatic pressure
Forming tissue fluids lymph
- not all fluid passes back into the capillaries, some is collected to avoid tissue swelling
- net excess drains into the vessels of a lymphatic system and forms lymph
- Lymph passes through the lymphatic system and drains back into the circulatory system
What happens to the lymph once it’s formed
Lymph passes through the lymphatic system and drains back into the circulatory system
atheroma formation
how is it formed?
the effect?
how does it affect the cardiovascular system?
- endothelium is damaged, white blood cells and lipids clump together underlining forming fatty streaks
- over time, connective tissue hardens, builds up and forms a fibrous plaque (atheroma)
- blocks part of the lumen & restricts blood flow & increases blood pressure
aneurysm
what is it?
how is it caused?
what is its affect?
- swelling or the artery
- atheroma weakens arteries when blood travels through artery at high pressure, as it pushes inner layers through outer elastic layer
- this can burst forming a haemorrhage
thrombosis
what is it?
how is it caused?
- atheroma ruptures the endothelium of artery
- platelets & fibrin (protein) accumulates on the damaged wall & forms blood cots, blocks an artery
myocardial infraction
what is it?
how is it caused?
- coronary artery is blocked & heart muscles is cut off from its supply
- didn’t receive any O2
high blood pressure as a risk factor
increased risk of damage to artery walls, increased atheroma formation, increasing blood cot formation, increase myocardial infraction
high blood cholesterol and poor diet as a risk factor
leads to fatty deposits that form atheromas, which increases blood clots & myocardial infraction. diet in high saturated fat is associated with high cholesterol. high salt diet increases risk of high blood pressure
cigarette smoking as a risk factor
CO combines with haemoglobin & reduces O2 to tissues. so myocardial occurs. smoking decreases antioxidants in blood, so theres cell damage in artery walls. leading to atheroma formation
where are the xylem and phloem located?
vascular tissue
what does the phloem do?
transports soluble organic substances (e.g. sucrose) by translocation from sources (leaves) to areas where they are used
what does the xylem do?
transports water & dissolved minerals upwards from the roots and leaves by transpiration and provides structural support
what does phloem sap contain
- water
- soluble assimilates e.g. sucrose, hormones
what is transpiration
the loss of water vapour from leaves by evaporation form the internal surfaces of the leaves and diffusion through the stomata
what is the transpiration pull?
draws water from the roots to the leaves via xylem
what is the cohesion-tension theory?
- water in the xylem vessel is pulled (under tension) towards the leaves due to the transpiration pull
- water molecules are dipoles so attract each other. this causes cohesion giving a continuous column of water
- if the column is broken the transpiration stream fails
capillarity
water molecules move up the narrow xylem tubes because of adhesion to the walls
the narrower the tube the higher the water goes
what is the potometer
used to measure the rate of transpiration. it contains a continuous column of water
rate of transpiration
volume of water (vol of a cylinder) / time
what is translocation?
transport of soluble organic substances (assimilates). the solutes are transported in sieve tube elements
what is the mass flow hypothesis?
process by which solutes are thought to be transported
what is the area called where sucrose is produced
source
what is the area where sucrose is used or stored called?
sink
process of the mass flow hypothesis
- sucrose made by source cells moved by active transport into companion cells
- companion cells actively pumping H+ into cell wall spaces
- h+ ions & sucrose enters sieve tube element by cotransport
- water potential inside the sieve tube is lowered & water enters via osmosis from xylem, creating a higher hydrostatic pressure
- at sink, sugars leaves the sieve tube elements by active transport to be used/stored
- water potential inside the sieve tube elements gets higher so water leaves via osmosis. lower pressure inside sieve tubes
result of the mass flow hypothesis
a hydrostatic pressure gradient from source to sink
plant transport from the root hair cells
- root hair cells take up minerals by active transport
- wp is lowered ad. water enters the cells by osmosis, creates root pressure
- water moves towards the xylem creating an increased water pressure
- water leaves the xylem reducing water pressure and drawing water upwards
- water evaporates from mesophyll cells. water loss has reduced water potential of cells
- stomata opens to allow gas exchange, and water vapour diffuses out the cell
ringing experiment
bark and phloem of a tree are removed leaving just the xylem.
Tissues above the missing ring swell with sucrose solution and tissue below dies. showing sucrose is transported in the phloem.
tracer experiments
Plants are grown in an environment that has radioactive CO2 (14CO2). This is in the sugar produced in photosynthesis. movement of sugars is traced using autoradiography. Areas that are exposed appear black. these regions correspond to where the phloem is
aphid stylet experiment
aphids mouth (stylets) penetrate phloem tubes and sup sugary sap. aphids are anaesthesied with CO2, the body is cut off, stylet remains in phloem, phloem sap is collected. aphids enzyme ensures stylet doesn’t get blocked. the solution contains sucrose
puncture experiment
phloem is punctured, sap oozes out .there’s high pressure in the phloem. xylem is punctured, air sucked in, shows low pressure inside. water pulled up xylem, sap pushed down in phloem
describe the advantage of the bohr effect during intense exercise
- increases disassociation of oxygen
for aerobic respiration at the tissues
explain how an arteriole can reduce blood flow into the capillaries
- muscle contracts
- constricts arteriole
give the pathway a red blood cell takes when travelling in the human circulatory stem from the kidney to the lungs
- renal vein
- vena cava to right atrium
- right ventricle to pulmonary artery
describe and explain the effect of increasing CO2 concentration on the dissociation of oxyhaemoglobin
- increases dissociation
- by decreasing pH
binding one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind
- binding of first oxygen changes Quaternary structure of haemoglobin
- creates another binding site
role of the heart in the formation of tissue fluid
- contraction of ventricles produces high blood pressure
- forces water and dissolved substance out of blood capillaries
how does a blockage in the lymphatic system cause lymphodeoma
excess tissue fluid cannot be reabsorbed
how can an ateriole reduce the blood flow into the capillaries
- muscle contracts
- constricts lumen
describe process of crossing over and explain how it increases genetic diversity
- homologous pair of chromosomes form a bivalent
- a chiasmata forms
- produces new allele combinations
- that were not present in original chromosomes
