2- movement of substances in and out of cells and nutrition in plants and humans Flashcards
(174 cards)
1
Q
diffusion
A
the movement of molecules from a region of its higher concentration to a region of its lower concentration
2
Q
molecules move down a concentration gradient as a result of
A
their random movement
3
Q
diffusion helps living organisms-
A
obtain many of their requirements, get rid of waste products, carry out gas exchange for respiration
4
Q
diffusion in small intestine
A
digested food products from limen of small intestine to blood/lymph in villi found covering small intestine walls
5
Q
diffusion in a leaf- oxygen
A
oxygen moving from air space between mesophyll cells to mitochondria in all cells
6
Q
diffusion in leaf- carbon dioxide
A
carbon dioxide moving from air spaces between mesophyll cells to chloroplasts in mesophyll cells
7
Q
diffusion in leaf- water vapour
A
water vapour moves from stomatal pores to air outside stomata
8
Q
diffusion in lungs- oxygen
A
oxygen moves from alveolar air space to blood in capillaries around alveoli
9
Q
diffusion in lungs- carbon dioxide
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carbon dioxide moves from blood in capillaries around alveoli to alveolar air space
10
Q
difference in concentration gradient affect on diffusion
A
the greater the difference in concentration between two regions the faster the overall rate of diffusion because the higher concentration the more random collisions against the membrane will occur
11
Q
temperature affect on diffusion
A
the higher the temperature the more kinetic energy the particles of that substance will have so they move and spread faster compared to when at a lower temp and they have less kinetic energy
12
Q
surface area affect on diffusion
A
a membrane with a greater surface area will have a greater rate of diffusion
13
Q
rate of diffusion is influenced by
A
temperature, surface area, concentration gradient, diffusion distance
14
Q
how can you investigate the temperature affect on diffusion
A
by using beetroot as it contains a dark purple-red pigment and heating above 45°c can damage the membrane meaning that the pigment can leak out so the speed at which this pigment leaks out of the cell tells us about the rate of diffusion
15
Q
apparatus needed to investigate the effect of temperature on diffusion
A
beetroot, knife, cutting board, ruler, test tube, water bath, stopwatch
16
Q
method to investigate the effect of temperature on diffusion
A
1) using a knife cut 2 equally sized cubes of beetroot, they have to be the same dimensions so they all have equal surface area/volume as these factors could affect the rate at which the pigment leaks out
2) rinse the beetroot to remove any pigments that is released during cutting
3) put 5cm3 of water into 2 test tubes labled A and B
4) keep test tube A at room temperature and then transfer test tube B a hot water bath at 90 degrees c
5) leave the test tubes fir 2 minutes then add a piece of beetroot into each tube
6) after 10 minutes observe the colour of the liquid in both test tubes
17
Q
results and analysis of investigating temperature affect on diffusion
A
you should notice that the higher temperature more of the pigment has leaked out because the cell membrane of the beetroot cells has become damaged so more pigment can leak out. at a higher temperature particles have more kinetic energy so this results in faster movement of particles compared to when they have less energy
18
Q
limitations of investigating temperature affect on diffusion
A
the beetroot pieces may not be identical in size and shape meaning one test tube could have slightly more beetroot tissue than the other - the solution to this is cut the beetroot as accurately as possible using a knife and ruler and repeat each investigation several times to find a mean
some parts of the beetroot tissue could have more pigment in their cells than others- solution is conduct several repeats using different parts of the beetroot and find a mean
our results would be more reliable if we tested a range of temperatures rather than just two- solution is to set up 5 test tubes in water baths at different temperatures ( etc 10-50 going up in 10s)
observing colour can be very subjective which means it is difficult to compare the differences in diffusion between test tubes- solution use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid
19
Q
corms investigation for investigating temperature affect on diffusion
A
c- we are changing the temperature of the environment
o- the beetroot cubes will all be taken from the same beetroot or beetroot of the same age
r- we will repeat the investigation several times to ensure our results are reliable
m- we will observe the colour change of the liquid after 10 minutes
s- we will control the volume of water used, the dimensions of the beetroot cubes and each cube must be blotted before it is weighed each time
20
Q
water can move in and out of cells by
A
osmosis
21
Q
all cells are surrounded by
A
a partially permeable cell membrane
22
Q
osmosis
A
the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
23
Q
osmosis is water moving down a
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concentration gradient
24
Q
the cell is partially permeable which means it allows
A
small molecules (like water) through but not the larger molecules (like solute molecules)
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animal cells lose and gain water as a result of
osmosis
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if an animal cell is placed in a solution with a lower water potential than the cell
it will lose water by osmosis and become crenated (shrivelled up)
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if an animal cell is placed into distilled water with a higher water potential than the cell
it will gain water by osmosis as it has no cell wall to create turgor pressure
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plant cells lose and gain water as a result of
osmosis
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as plants have a cell wall they are protected from
cell lysis
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if a plant cell is placed in a strong sugar solution (with a water potential lower than the cell)
it will lose water by osmosis and the vacuole gets smaller and the cell membrane shrivels away from the cell wall and it becomes flaccid or plasmolysed
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if a plant cell is put in distilled water (higher water potential than the cell)
it will gain water by osmosis and the vacuole gets bigger, pushing the cell membrane against the cells wall and the cell becomes turgid due to the high turgor pressure caused by the cytoplasm pushing against the cell wall
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water entering a cell by osmosis makes the cell
rigid and firm
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why is it important for the cell to become rigid and firm after water has entered the cell by osmosis
because when a plant cell is firm it provides support and strength for the plant- making the plant stand upright with its leaves held out to catch sunlight
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what happens if a plant doesn't get enough water
the cells can't become rigid and firm (turgid) and the plant will wilt
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how can we investigate osmosis
using cylinders of potato and placing them in distilled water and sucrose solutions of increasing concentration
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apparatus needed to investigate osmosis
potatoes, cork borer, knife, sucrose solution, test tube, balance, paper towels, ruler, test tube rack
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method to investigate osmosis
1) prepare a range of sucrose (sugar) solutions raging from 0 mol/dm3 (distilled water) to 1 mol/dm3
2) set up 6 labelled test tubes with 10cm3 of each of the sucrose solutions
3) using the knife, cork borer and ruler, cut 6 equally sized cylinders of potato
4) blot each one with a paper towel and weigh on the balance
5) put 1 piece into each concentration of sucrose solution
6) after 4 hours remove them, blot with paper towels and reweigh them
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results and analysis of investigating osmosis
the percentage change in mass can be calculated for each piece of potato by using the equation
((final mass-initial mass)/initial mass) x 100
the potato cylinder in the distilled water will of increased its mass the most because there is a greater concentration gradient in this tube between the distilled water (high water potential) and the potato cells (low water potential). this means more water molecules will move into potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells and making them more turgid. the potato cylinder in the strongest sucrose concentration will, have decreased its mass the most as there is a greater concentration gradient in this tube between potato cells (higher water potential) and the sucrose solution (low water potential). this means more water molecules will move out of potato cells by osmosis making them flaccid and decreasing the mass of the cylinder.
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what if there is a potato cylinder that has not increased or decreased in mass
if there's a potato cylinder which hasnt increased or decreased in mass it means there was a no overall net movement of water in or out of the potato cells. this is because the solution that the cylinder was in was the same concentration as the solution found in the cytoplasm of the potato cells so there was no concentration gradient.
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limitations to investigation osmosis
slight differences in potato cylinders may mean that results aren't reliable or comparable- solution is for each sucrose concentration , repeat the investigation with several potato cylinders making a series of repeat experiments means that any anomalous results can be identified and ignored when a mean is calculated
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corms for investigating osmosis
c- we are changing the concentration of sucrose solution
o- the potato cylinders will all be taken from the same potato or potatoes of the same age
r- we will repeat the investigation several times to ensure our results are reliable
m- we will measure the change in mass of potato cylinders after 4 hours
s- we will control the volume of sucrose solution used, the dimensions of the potato cylinders and each cylinder must be blotted before it is weighed each time
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active transport
the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
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energy is needed in active transport because
particles are being moved against a concentration gradient in the opposite direction from which they would naturally move (by diffusion)
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active transport across the cell membrane involves
protein carrier molecules embedded in the cell membrane
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active transport in animals allows molecules
such as glucose to be transported into the blood stream from the lumen off the small intestine (the gut) when the concentration of sugar molecules in the blood is higher
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photosynthesis- endothermic or exothermic?
endothermic because energy from sun light is transferred to the chloroplasts in green plants
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photosynthesis
the process by which plants manufacture carbohydrates from raw materials using energy from light
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what do plants use glucose as
a source of energy in respiration
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what other things can plants use glucose for
produce starch, synthesise lipids into energy sources, to form cellulose to make cell walls, produce amino acids
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photosynthesis wird equation
carbon dioxide + water -> glucose + oxygen
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photosynthesis chemical equation
6CO2 + 6H2O -> C6H12O6 +6O2
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limiting factor
something present in the environment in such short supply that it restricts life processes
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main factors to limit the rate of photosynthesis
temperature, light intensity, carbon dioxide concentration
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increasing temperature affect on photosynthesis
increasing temperature increases kinetic energy of particles, increasing the likelihood of collisions between reactants and enzymes which results in more product. however at high temperature enzymes that control the processes of photosynthesis can be denatured and this reduces the overall rate of photosynthesis
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denatured
where the active site changes shape and is no longer complementary to its substrate
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more light intensity on photosynthesis
the more light intensity the more photosynthesis
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carbon dioxide affect on photosynthesis
more carbon dioxide the more photosynthesis
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the more chloroplasts a plant has...
the more photosynthesis occurs because then more light can be absorbed
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amount of chorophyll can be affected by
disease, level of nutrients, loss of leaves
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wax cuticle
protective layer on top of the leaf which prevents water from evaporating
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upper epidermis
thin and transparent to allow light to enter palisade mesophyll later underneath
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palisade mesophyll
column shaped cells tightly packed with chloroplasts to absorb more light, maximising photosynthesis
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spongy mesophyll
contains internal air spaces that increases the surface area to volume ratio for the diffusion of gases mainly carbon dioxide
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lower epidermis
contains guard cells and stomata
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guard cells
absorbs and loses water to open and close the stomata to allow carbon dioxide to diffuse in and oxygen to diffuse out
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stomata
where gas exchange takes place. opens during the day and closes during the night. evaporation of waster takes place here. in most plants its found in a much greater concentration on the underside of the leaf to reduce water loss
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vascular bundle
contains xylem and phloem to transport substances to and from the leaf
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xylem
transports water into the leaf for mesophyll cells to use in photosynthesis and for transpiration from stomata
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phloem
transports sucrose and amino acids around the plant
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large surface are on a plant and how its adapted for photosynthesis
increases surface area for diffusion of carbon dioxide and absorption of light for photosynthesis
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plants are thin- adaptation for photosynthesis
allows carbon dioxide to diffuse to palisade mesophyll cells quickly
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chlorophyll on leaves- adaption for photosynthesis
absorbs carbon dioxide to diffuse to palisade mesophyll cells quickly
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network of veins- adaptation for photosynthesis
allows the transport of water to the cells of the leaf and carbohydrates from the leaf for photosynthesis
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stomata- adaptation for photosynthesis
allows carbon dioxide to diffuse into the leaf and oxygen to diffuse out
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epidermis is thin and transparent- adaptation for photosynthesis
allows more light to reach the palisade cells
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thin cuticle made of wax- adaptation for photosynthesis
to protect the leaf without blocking sunlight
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palisade cell layer at top of leaf- adaptation for photosynthesis
maximises the absorption of light as it will hit chloroplasts in the cells directly
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spongy layer- adaptation for photosynthesis
air spaces allow carbon dioxide to diffuse through the leaf, increasing the surface area
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vascular bundles- adaptation for photosynthesis
thick cell walls of the tissue in the bundles help to support the stem and leaf
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magnesium mineral ion function in plant
magnesium is needed to make chlorophyll
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magnesium mineral ion deficiency in plant
causes yellowing between the veins of leaves (chlorosis)
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nitrate mineral ion function in plant
nitrates are a source of nitrogen needed to make amino acids (to build proteins)
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nitrate mineral ion deficiency in plant
causes stunted growth and yellowing leaves
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how to demonstrate the evolution of oxygen
using water plants such as elodea or camboba (types of pondweed)
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as photosynthesis occurs
oxygen gas produced is released
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how can oxygen released be seen when a plant is in water
as bubbles leaving the cut end of the pondweed
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apparatus needed to investigate photosynthesis
beaker, water plant, funnel, boiling tube, splint, Bunsen burner, heat proof mat
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method for investigating photosynthesis
1) take a bundle of shoots of a water plant
2) submerge them in a beaker of water underneath an upturned funnel
3) fill a boiling tube with water and place it over the end of the funnel
4) as oxygen is produced the bubbles of gas will collect in the boiling tube and displace the water
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results and analysis of investigating photosynthesis
you can show that the gas collected is oxygen by relighting a glowing splint
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why can't leaves be tested for glucose presence
as glucose is produced it is quickly used up and converted into other substances and then transported or stored as starch
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starch is stored in the chloroplasts where photosynthesis occurs so
testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesising
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apparatus needed to investigate light and photosynthesis
beakers, leaf tissue, Bunsen burners, tripod, gauze platform, prongs, ethanol, apron, safety goggles, gloves, iodine solution, white tile
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method to investigate light and photosynthesis
1) destarch the plant by placing it in a dark cupboard for 24 hours- this ensures that any starch already present will be used up and not affect the results of the experiment.
2) following the destarch partially cover the leaf of a plant with aluminium foil and place in the sunlight for a day
3) remove the covered leaf and test for starch by-
1) dropping the leaf in boiling water to break down the cell walls and kill the tissue
2) transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes as this removes the chlorophyll so colour changes from iodine can be seen more clearly
3) dip the leaf into boiling water
4) spread the leaf out on a white tile and cover it with iodine solution
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results and analysis of investigating light and photosynthesis
in a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf. the area of the leaf that was covered with aluminium foil will remain orange-brown as it did not receive any sunlight and could not photosynthesise while the area exposed to light could. this proves that light is necessary for photosynthesis and the production of starch
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safety when investigating light and photosynthesis
ethanol is extremely flammable so when this stage of the experiment is happening the Bunsen burner should be turned off. to heat the ethanol use a water bath
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corms for investigating light and photosynthesis
c- we are changing whether there is light or no light
o- the leaves will be taken from the same plant or the same species, age and size of the plant
r- we will repeat the investigation several times to ensure the results are reliable
m- we will observe the colour change of the leaf when iodine is applied after one day
s- we will control the temperature of the room
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apparatus to investigate carbon dioxide and photosynthesis
conical flasks, potassium hydroxide solution, clamps, clamp stands, a plant, beakers, Bunsen burner, tripod, gauze platform, prongs, ethanol, apron, safety goggles, gloves, iodine solution, white tile
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method to investigate carbon dioxide and photosynthesis
1) destarch the plant by putting it in a dark cupboard for 24 hours
2) following the destarch enclose one leaf with a conical flask containing potassium hydroxide- the potassium hydroxide will absorb carbon dioxide from the surrounding air
3) enclose another leaf with a conical flask containing no potassium hydroxide
4) place the plant in a bright light for several hours
5) test both leaves for starch using iodine solution
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results for investigating carbon dioxide and photosynthesis
the leaf from the conical flask containing potassium hydroxide will remain orange-brown as it could not photosynthesise due to lack of carbon dioxide. the leaf from the conical flask not containing potassium hydroxide should turn blue-black as it had all necessary requirements for photosynthesis
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corms for investigating carbon dioxide and photosynthesis
c- we are changing whether there is a carbon dioxide or no carbon dioxide
o- the leaves will be taken from the same plant or same species, age, and size of plant
r- we will repeat the investigation several times to ensure our results are reliable
m- we will observe the colour change of the leaf when iodine is applied after 1 day
s- we will control the temperature of the room and the light intensity
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apparatus to investigate chlorophyll and photosynthesis
beakers, leaf tissues, Bunsen burner, tripod, gauze platform, prongs, ethanol, apron, safety goggles, gloves, iodine solution, white tile
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method to investigate chlorophyll and photosynthesis
1) drop the leaf in boiling water
2) transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes
3) dip the leaf in boiling water
4) spread the leaf out on a white tile and cover with iodine solution
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results and analysis of investigating chlorophyll and photosynthesis
the white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contain chlorophyll strain blue-black. the areas that had no chlorophyll remain orange-black as no photosynthesis is occurring here and so no starch is stored
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corms for investigating chlorophyll and photosynthesis
c- we are changing whether there is chlorophyll or not
o- the leaves will be taken from the same plant or same species, age, and size of the plant
r- we will repeat the investigation several times to ensure our results are reliable
m- we will observe the colour change of the leaf when iodine is applied after one day
s- we will control the temperature of the room and the light intensity
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a balanced diet consists of
carbohydrates, proteins, lipids, dietary fibre, vitamins, minerals, water
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carbohydrate function and source
source of energy and can be found in bread, cereal, pasta, rice, potatoes
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protein function and source
growth and repair found in meat, fish, eggs, pulses and nuts
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lipid function and source
insulation and energy storage found in butter, oil and nuts
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dietary fibre function and source
provides bulk for the intestine to push food through it found in vegetables and whole grains
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vitamins function and source
needed in small quantities to maintain health found in fruits and veg
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minerals function and source
needed in small quantities to maintain health found in fruits and veg, meats, dairy products
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water function and source
needed for chemical reactions to take place in cells, found in water, juice, milk, fruits and vegetables
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calcium function and sources
needed for strong teeth and bones and involved in the clotting of blood. deficiency can lead to osteoporosis later in life. found in milk, cheese and eggs
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vitamin D function and source
helps the body to absorb calcium and so require for strong bones and teeth. found in oily fish, dairy products, also made naturally by the body in sunlight
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vitamin c function and source
form essential parts of collagen protein which makes up skin, hair, gums and bones. deficiency causes scurvy. found in citrus fruits, strawberries and green vegetables
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vitamin a function and source
needed to make the pigment in the retina for vision. found in meat, liver, dairy, leafy green veg, eggs
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iron function and source
needed to make haemoglobin, the pigment in red blood cells that transports oxygen. can be found in red meat, liver, leafy green veg
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age- dietary needs
the amount of energy that young people need increases towards adulthood as this energy is needed for growth. children need a higher proportion of protein in their diet than adults as this is required for growth. energy needs in adults decrease as they grow
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activity levels- dietary needs
the more active someone is the more energy required for movement as muscles are contracting more and respiring faster
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pregnancy- dietary needs
during pregnancy energy requirements increase as energy is needed to support the growth of the developing foetus, as well as the larger mass that the mother needs to carry around. extra calcium and iron are also needed in the diet to help build bones, teeth and blood of the foetus
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breast feeding- dietary needs
energy requirements increase and extra calcium still needed to make high quality breast milk
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gender/sex- dietary needs
men average energy requirements tend to exceed that of females due to them having a larger proportion of muscle compared to fat
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the digestive system is an example of
an organ system in which several organs work together to digest and absorb food
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digestion is a process in which
relatively large, insoluble molecules in food are broken down into smaller, soluble molecules that can be absorbed into the bloodstream and delivered to cells in the body
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small soluble molecules are used to either
provide cells with energy or with materials with which they can build other molecules to grow, repair and function
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the human digestive system is made up of
the organs that form the alimentary canal and accessory organs
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the alimentary canal is
the channel or passage through which food flows through the body, starting at the mouth and ending at the anus
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digestion occurs within
the alimentary canal
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accessory organs to digestion produce
substances that are needed for digestion to occur (such and enzymes and bile) but food does not pass directly through these organs
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mouth/salivary glands
the mouth is where mechanical digestion takes place- teeth chew food to break it down to increase its surface area to volume ratio. amylase enzymes in saliva start digesting starch into maltose. the food is shaped into a ball by the tongue and lubricated in saliva so it can be swallowed easily
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oesophagus
tube that connects the mouth to the stomach where the food bolus goes after being swallowed. wave-like contractions will take place to push the food bolus down without relying on gravity
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stomach
food is mechanically digested by churning actions while protease enzymes start to chemically digest proteins. hydrochloric acid is present to kill bacteria in foods and provide the optimum ph for protease enzymes to work
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small intestine (duodenum)
there first section of the small intestine is called the duodenum and it is where the food coming out of the stomach finishes being digested by enzymes produced here and also recreated from the pancreas- ph of the small intestine is slightly alkaline so around ph 8-9
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small intestine (ileum)
the second section of the small intestine is called the ileum. and where the absorption of digested food molecules take place. the ileum is long and lined with villi to increase the surface area over which absorption can take place
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large intestine
water is absorbed from remaining material in the colon to produce faeces. faeces is stored in the rectum and removed through the anus
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pancreas
produces all three types of digestive enzyme- amylase, protease and lipase. secretes enzymes in an alkaline fluid into the duodenum for digestion to raise the ph of fluid coming out of the stomach
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liver
produces bile to emulsify fats (breaks large droplets into smaller droplets)- an example of mechanical digestion. amino acids not used to make proteins break down here (deamination) which produces urea
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gall bladder
stores bile to release into duodenum as required
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ingestion
the taking in of substance
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mechanical digestion
the breakdown of food into smaller pieces without chemical change to the food molecules
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chemical digestion
the breakdown of large, in soluble molecules into small, soluble molecules
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absorption
the movement of small food molecules and ions through the wall of the intestine into the blood
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assimilation
the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells
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egestion
the passing out of food that has not been digested out absorbed through the anus
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how does peristalsis work
firstly muscles in the walls of the oesophagus create waves of contractions which force the bolus along. once the bolus has reached the stomach, it is churned into a less solid form, called chyme, which continues on to the small intestine
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peristalsis is controlled by
circular and longitudinal muscles. circular muscles contract to reduce the diameter of the lumen of the oesophagus or small intestine. longitudinal muscles contract to reduce the length of that section the oesophagus or the small intestine
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mucus is produced to
continually lubricate the food mass and reduce friction
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dietary fibre provides
the roughage required for the muscles to push against during peristalsis
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digestion mainly takes place
chemically where bonds holding the large molecules together are broken to make smaller and smaller molecules
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chemical digestion is controlled by
enzymes which are produced in different areas of the digestive system
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enzymes are
biological catalysts- they sped up chemical reactions without themselves being used up or changed in the reaction
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types of digestive enzymes
carbohydrases, proteases and lipases
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carbohydrases are
enzymes that break down carbohydrates to simple sugars such as glucose
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amylase is a
carbohydrase which is made in the salivary glands, the pancreas, and the small intestine
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amylase breaks down
starch to maltose
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maltase breaks down
maltose to glucose
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proteases are
a group of enzymes that break down proteins into amino acids
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pepsin is an
enzyme made in the stomach which breaks down proteins into smaller polypeptide chains
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proteases made in
the pancreas and small intestine break the peptides into amino acids
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lipases are
enzymes that break down lipids to glycerol and fatty acids
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lipase enzymes are produced
in the pancreas and secreted into the small intestine
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bile is
an alkaline substance produced by cells in the liver
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bile is stored
in the gallbladder before being released into the small intestine
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bile main roles
neutralising the hydrochloric acid in the stomach- the alkaline properties of bile allow for this to occur. the neutralisation is essential as enzymes in the small intestine have a higher (more alkaline) optimum ph than those in the stomach
breaking apart large drops of fat into smaller ones- and so increasing surface are. this is known as emulsification
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the more alkaline conditions and larger surface area
allows lipase to chemically break down the fat (lipids) molecules into glycerol and fatty acids at a faster rate
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absorption
the movement of small digested food molecules from the digested system in to the blood (glucose and amino acids) and lymph (fatty acids and glycerol)
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absorption of small molecules occur through
diffusion and sometimes active transport
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water is absorbed primarily
in the small intestine but also in the colon by osmosis
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after absorption
assimilation and digestion occurs
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how is the small intestine adapted for absorption
as it is very long and has highly folded surface with millions of villi. these adaptions massively increase the surface area of the small intestine, allowing absorption to take place faster and more efficiently
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adaptions of the villi which allow for the rapid absorption of substances
a large surface area- microvilli on the surface of the villus further increase the surface available for absorption
a short diffusion distance- the wall of a villus is only one cell thick
a steep concentration gradient- the villi are well supplied with a network of blood capillaries that transport glucose and amino acids away from the small intestine in the blood. a lacteal runs through the centre of the villus too transport fatty acids and glycerol away from the small intestine in the lymph. enzymes produced in the walls of the villi assist with chemical digestion. the movement of villi helps to move food along and mix with enzymes present
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