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a level biology > head start (pg 4 - 30) ALSO DO HEART REVISION > Flashcards

head start (pg 4 - 30) ALSO DO HEART REVISION Flashcards

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1
Q

what is metabolism?

A

in a living cell thousands of biochemical reactions take place every single second. The sum of these reactions is called metabolisms

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2
Q

what is a metabolic pathway?

A

a single chain of biochemical reactions that take place in a living cell

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3
Q

what would happen to the biochemical reactions inside a body if they didn’t have enzymes?

A

these reactions would take place very slowly at normal body temperature

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4
Q

what are enzymes?

A

enzymes are biological catalysts that increase the rate of reactions

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5
Q

how do enzymes act as catalysts?

A

a reaction that needs a high activation energy can’t start at a low temperature of 37 degrees (i.e. body temperature). Enzymes reduce the activation energy

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6
Q

what type of proteins are enzymes?

A

globular proteins (because they’re roughly spherical)

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7
Q

what does the order of amino acids in an enzyme determine?

A

its structure, and so how it works

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8
Q

give 2 examples of what enzymes help with

A

enzymes can be involved in breaking down molecules or building molecules. For example:
- Digestive enzymes are important in the digestive system, where they help to break down food into smaller molecules, e.g. carbohydrases break down carbohydrates
- enzymes involved in DNA replication help to build molecules, e.g. DNA polymerase

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9
Q

what is the substance that’s acted upon by an enzyme called?

A

the substrate

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10
Q

what is the active site?

A

the region on the surface of the enzyme molecule where a substrate molecule can attach itself. It’s where the catalysed reaction takes place

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11
Q

how quickly do enzymes break down substrate molecules?

A

almost as soon as the enzyme-substrate complex has formed, the products of the reaction are released and the enzyme is ready to accept another substrate molecule

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12
Q

describe the effect of temperature on enzyme activity

A

as temperature increases, enzyme reactions become faster, because the molecules have more energy. However, at high temperatures the atoms of the enzyme molecule vibrate more rapidly and break the weak bonds that hold the tertiary structure together. The shape of the active site changes and the substrate can no longer fit in. The enzyme is said to be denatured

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13
Q

describe the effect of pH on enzyme activity

A

acids and alkalis can denature enzymes. Hydrogen ions (H+) in acids and hydroxyl ions (OH-) in alkalis disrupt the weak bonds and change the shape of the active site

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14
Q

what is the name of the OH- alkali ion?

A

hydroxyl

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15
Q

what is the function of the cytoplasm of the cell?

A

it contains enzymes that speed up biochemical reactions

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16
Q

what does the vacuole contain?

A

cell sap, a weak solution of sugar and salts

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17
Q

how big are prokaryotic cells compared to eukaryotic cells?

A

prokaryotes like bacteria are roughly a tenth the size of eukaryotic cells

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18
Q

name 3 organelles that prokaryotic cells don’t have

A
  1. nucleus
  2. mitochondria
  3. chloroplasts
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19
Q

what is the largest magnification a light microscope can reach? what does this allow you to see?

A

1500x (this allows you to see individual animal and plant cells along with the organelles inside them)

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20
Q

which parts of a cell can you see with a light microscope?

A
  1. If the cells have been stained you can see the dark-coloured nucleus surrounded by lighter-coloured cytoplasm
  2. tiny mitochondria and the black line of the cell membrane are also visible
  3. in plant cells, the cell wall, chloroplasts and the vacuole can be seen
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21
Q

give a different name for a light microscope

A

an optical microscope

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22
Q

what is the part of the microscope you look down called?

A

the eyepiece

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23
Q

what are the names of the two different knobs on the microscope, and how are they different?

A

the larger knob (the coarse adjustment knob) is to bring the specimen roughly into focus. The smaller knob (usually lower down than the coarse adjustment knob) is called the fine adjustment knob and is to finely tune the focus.

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24
Q

what is the name of the area where you put your microscope slide?

A

the stage

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25
Q

what do the high and low power objective lenses do?

A

they magnify the specimen

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26
Q

how many times can electron microscopes magnify objects?

A

more than 500 000 times

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27
Q

what does an electron microscope allow you to see that you can’t see with a light microscope?

A

electron microscopes allow greater detail to be seen than a light microscope. For example, it allows you to see the detailed structures inside organelles such as mitochondria and chloroplasts

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28
Q

what is the image that’s recorded on an electron microscope called?

A

an electron micrograph

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29
Q

when was the detailed ultrastructure of cells revealed and how?

A

the detailed ultrastructure of cells was revealed in the 1950s when the electron microscope was invented

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30
Q

when can you see chromosomes with a light microscope, and why?

A

during cell division the chromosomes carrying the long DNA molecules coil up, becoming shorter and thicker and visible with a light microscope

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31
Q

what have electron micrographs shown us about the nucleus?

A

that there’s a double membrane around it

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32
Q

how many membranes are there around the nucleus?

A

two

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33
Q

which microscope do you need to study mitochondria?

A

mitochondria are about the size of bacteria, so they can be seen with a light microscope, but you need an electron microscope to see any of the detail

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34
Q

describe a mitochondrion’s membrane(s)

A

each mitochondrion has a smooth outer membrane and a folded inner membrane

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35
Q

what is the job of mitochondria? How do they carry out this job?

A

to capture energy in glucose in a form that the cell can use. To do this aerobic respiration takes place inside the mitochondria

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36
Q

what is the word equation for aerobic respiration?

A

glucose + oxygen -> carbon dioxide + water (+ energy)

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37
Q

what happens to the energy released by respiration?

A

the energy released by respiration ends up in molecules of ATP (adenosine triphosphate). ATP is used in the cell to provide the energy for muscle contraction, active transport and building large molecules from small ones, as well as many other processes

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38
Q

describe the cell wall of plant cells

A
  1. the plant cell wall is relatively rigid and provides support for the cell
  2. It mainly consists of bundles of long, straight cellulose molecules
  3. the cellulose molecules lay side by side to form microfibrils
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39
Q

what is a cell surface membrane?

A

the very thin structure around an individual cell

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40
Q

describe the structure of the cell-surface membrane

A
  1. electron micrographs show that the cell-surface membrane consists of a double layer of phospholipid molecules tightly packed together
  2. bigger protein molecules are embedded in the phospholipid molecules. Some proteins go all the way through the membrane and some only go half way
  3. membranes surrounding the organelles inside cells have the same structure
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41
Q

give another name for cell surface membranes

A

plasma membranes

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42
Q

why are cell surface membranes described as being partially permeable?

A

because the membrane only allows certain substances to pass through it

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43
Q

name the 4 methods by which substances pass through membranes

A
  1. diffusion
  2. osmosis
  3. facilitated diffusion
  4. active transport
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44
Q

give 3 molecules that are small enough to be able to diffuse through the very small gaps between the phospholipid molecules in the cell membrane

A

water, oxygen and carbon dioxide molecules

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45
Q

why can water, oxygen and carbon dioxide molecules diffuse into cells?

A

because they are small enough to pass through the very small gaps between the phospholipid molecules

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46
Q

what is the name for the concentration of water molecules during osmosis?

A

the water potential

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47
Q

what is osmosis?

A

the diffusion of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential

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48
Q

how does facilitated diffusion work?

A
  1. Glucose and many other water soluble molecules are too big to diffuse across the membrane by themselves. They must be helped across by carrier proteins
  2. each substance has its own specific carrier protein
  3. for example, a molecule of glucose fits into the outside end of a glucose carrier protein
  4. this causes the protein to change shape, allowing the glucose molecule to diffuse through the cytoplasm of the cell. The cell doesn’t need to provide any energy
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49
Q

how can mineral ions like sodium (Na+) and potassium (K+) cross the cell membrane?

A

mineral ions like sodium and potassium have electrical charges on them, so they also need help to cross the membrane. Specific channel proteins in the membrane allow them to diffuse through.

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50
Q

how does active transport work?

A
  1. when a cell needs to move substances across the membrane from a region of low concentration to a region of higher concentration, it must provide energy
  2. the substance fits into a specific carrier protein, then molecules of ATP provide the energy to change the shape of the protein
  3. as it changes shape the protein actively transports the substance across the membrane
  4. these special carrier proteins are sometimes called “pumps” because they’re moving substances against a concentration gradient
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51
Q

DNA is a double helix (a double-stranded spiral). What are each of the two DNA strands made up of?

A

small molecules called nucleotides

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52
Q

what does each nucleotide contain?

A

a part called a base

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53
Q

how many different bases does DNA have?

A

4

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54
Q

name DNA’s four bases

A
  1. Adenine
  2. Cytosine
  3. Guanine
  4. Thymine
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55
Q

what keeps the two DNA strands tightly wound together?

A

each base forms hydrogen bonds to a base on the other strand. This keeps the two DNA strands tightly wound together

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56
Q

which DNA bases always pair up with which?

A

Adenine (A) always joins up with thymine (T), and cytosine (C) always joins up with guanine (G)

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57
Q

what are the pairs of DNA bases called?

A

complementary bases

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58
Q

why do complementary bases join together?

A

because they complement each other in shape - this is called complementary base pairing

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59
Q

what does DNA control?

A

the production of proteins (protein synthesis) in a cell

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60
Q

how are proteins made?

A

by reading the code in DNA:
- proteins are made up of chains of amino acids. Each different protein has its own particular number and order of amino acids.
- this gives each protein a different shape, which means each protein can have a different function
- it’s the order of the bases in a gene that decides the order of amino acids in a protein
- each gene contains a different sequence of bases - which is what allows it to code for a unique protein

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61
Q

what is a section of DNA that codes for a particular protein called?

A

a gene

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62
Q

what is RNA needed for?

A

to make proteins

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63
Q

describe how RNA is used to make proteins

A
  1. DNA molecules (and so genes) are found in the nucleus of a cell, but they can’t move out of the nucleus because they’re very large.
  2. Protein synthesis happens in the cytoplasm at organelles called ribosomes
  3. so when a cell needs a particular protein, a copy of the gene that codes for it is made in the nucleus. This copy is smaller than DNA so it can move in to the cytoplasm, where it can be used to make the protein
  4. the copy of the gene is made from a molecule called messenger RNA (mRNA)
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64
Q

how is the RNA copy of the DNA gene made?

A
  1. the DNA in the gene acts as a template
  2. RNA, like DNA, is made up of nucleotides, which each have a base
  3. the bases on RNA nucleotides line up next to their complementary bases on the DNA template
  4. once the bases on the RNA nucleotides have paired up with the bases on the DNA strand, the RNA nucleotides join together to make an mRNA molecule
  5. eventually, a whole copy of the gene is made and the sequence (order) of bases in the mRNA copy is complementary to the sequence of bases in the DNA template
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65
Q

Which base does RNA not have? What does it have instead?

A

in RNA there’s no thymine (T), so the base uracil (U) binds to any adenine (A) in the DNA instead

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66
Q

how many bases code for 1 amino acid? what is this called?

A

three bases in a row code for 1 amino acid - this is called the genetic code

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67
Q

which amino acid does TAT code for?

A

tyrosine

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68
Q

which amino acid does AGT code for?

A

serine

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69
Q

what are mutations?

A

changes to the base sequence of DNA

70
Q

give an example of a mutation, and explain what the consequences could be

A
  • for example, one base can be substituted for another one. This can cause the base triplet to change. E.g. if C is substituted for A, GCT becomes GAT.
  • so mutations can change the amino acids in the protein that the gene codes for
  • a change in the amino acids can cause a different protein to be produced. Sometimes the different protein can be harmful
71
Q

what controls how frequently mutations happen?

A

mutations happen spontaneously (randomly), but how frequently they happen can be increased by mutagenic agents - factors that increase mutations, e.g. UV radiation in sunlight

72
Q

how can mutations be harmful? give examples

A
  1. mutations can cause cancer because cell division is controlled by proteins. If mutations occur in the genes for these proteins, they can alter the proteins so they no longer work. This can lead to uncontrolled cell division, and the development of a tumour (cancer)
  2. Mutations also cause genetic disorders - mutations that result in altered genes and proteins can be inherited (passed on from your parents), e.g. cystic fibrosis
73
Q

how many genes can each human chromosome contain?

A

between a couple of hundred and a few thousand genes

74
Q

what is the technical name for a pair of chromosomes? How many are there in humans?

A

homologous pairs - humans have 23 homologous pairs of chromosomes, e.g. two number 1s, two number 2s, two number 3s, etc.

75
Q

what is the same and different about each chromosome in a homologous pair?

A

both chromosomes in a pair are the same size and carry the same genes (which is why they’re called homologous pairs)> But they usually have different alleles (versions of the genes)

76
Q

in an X shaped chromosome, is each half one of the chromosomes in the homologous pair?

A

no - an X-shaped chromosome is actually one chromosome attached to an identical copy of itself (only right after DNA replication)

77
Q

when are chromosomes X shaped?

A

they’re only X-shaped just after the DNA has been replicated (e.g. in cell division)

78
Q

in an X shaped chromosome, what is each side of the X referred to as?

A

a chromatid

79
Q

what is the name for the bit in the middle of an X shaped chromosomes where the two chromatids attach?

A

the centromere

80
Q

describe 3 uses of mitosis

A
  1. If you have damaged tissue, the cells around the damaged area divide by mitosis to replace the damaged cells
  2. Cells also divide by mitosis to produce new tissue for growth
  3. Asexual reproduction also involves mitosis
81
Q

how does asexual reproduction work?

A
  1. in asexual reproduction, a single organism produces offspring by dividing into two organisms or by splitting off a piece of itself
  2. the cells divide by mitosis (like most cells)
82
Q

give 2 examples of things that reproduce asexually

A
  1. bacteria
  2. many plants
83
Q

what are the 4 stages of mitosis in order?

A

prophase, metaphase, anaphase and telophase

84
Q

when does interphase happen?

A

before mitosis starts, there’s a period called interphase

85
Q

what happens during interphase?

A

before the cell starts to divide, every DNA molecule (each chromosome) must replicate so that each new cell has a full copy of DNA. The new molecule remains attached to the original one at the centromere

86
Q

what happens during prophase?

A

mitosis can now begin. Each DNA molecule becomes supercoiled and compact. Each chromosome can now be seen with a light microscope and appears as two chromatids lying side by side, joined by the centromere (i.e. X shaped)

87
Q

what happens during metaphase?

A

the nuclear membrane breaks down and the chromosomes line up along the equator (middle) of the cell (not nucleus)

88
Q

what happens during anaphase?

A

the centromeres split and the chromatids separate and are dragged to opposite ends of the cell

89
Q

what happens during telophase?

A

a nuclear membrane forms around each set of chromatids (exact copies of the original chromosomes) and the cytoplasm divides

90
Q

are gametes haploid or diploid?

A

haploid (they only have half the genetic information)

91
Q

is a zygote haploid or diploid?

A

diploid

92
Q

are normal body cells haploid or diploid?

A

diploid - they have two of each chromosome

93
Q

what happens during and after fertilisation?

A

the nuclei of the gametes join together to form a zygote. The zygote then grows by simple cell division (mitosis) to form the embryo

94
Q

what are the only cells in the human body that divide by meiosis?

A

special cells in the testes and ovaries

95
Q

what are the steps to meiosis?

A
  1. the DNA replicates, so each of the 46 chromosomes become two chromatids joined by a centromere
  2. the 46 chromosomes sort themselves into the 23 homologous pairs, then the pairs separate. One of each pair goes to one side of the cell and one goes to the other
  3. the cytoplasm now divides. Each of the new cells contains 23 chromosomes (consisting of two chromatids joined by a centromere)
  4. in both of these new cells the chromatids separate and the cytoplasm divides to form two cells
  5. at the end of meiosis, four haploid cells have been produced from every original diploid cell
96
Q

does a smaller object have a smaller or larger surface area to volume ratio?

A

larger

97
Q

does a larger object have a smaller or larger surface area to volume ratio?

A

smaller

98
Q

what is surface area important for?

A

exchange

99
Q

what do cells and organisms need to exchange with their environment?

A

materials and heat

100
Q

why do larger organisms have more of a problem with getting enough oxygen, nutrients and waste products?

A
  • more chemical reactions happen every second in organisms with a larger volume than in ones with smaller volumes
  • therefore more oxygen, nutrients, waste products and heat need to be exchanged across the membrane of cells of larger organisms
  • with increasing volume this becomes an ever-increasing problem
  • surface area to volume ratio is also higher in larger organisms
101
Q

describe the adaptations of alveoli that increase the diffusion rate

A
  1. the walls of the alveoli consist of a single layer of thin, flattened, epithelial cells. Diffusion happens faster when molecules only have to travel short distances.
  2. Diffusion is faster when there’s a bigger difference in concentrations between two regions. The blood flowing through the rich network of capillaries around the alveoli carries away the oxygen that has diffused through the alveolar walls. This ensures that there’s always a higher concentration of oxygen inside the alveoli than in the blood. The reverse is true for carbon dioxide
  3. The alveolar walls are full permeable to dissolved gases. Oxygen and carbon dioxide can pass easily through the cell membranes of the epithelial cells
102
Q

what does ventilation (breathing) ensure? Why is this necessary?

A

Ventilation (breathing) ensures that air with a high concentration of oxygen is taken into the lungs, and air with a high concentration of carbon dioxide is removed from the lungs. This maintains high concentration gradients between air (inside your alveoli) and blood, increasing the rate of diffusion of oxygen and carbon dioxide

103
Q

describe the mechanical process by which we breathe

A

if the volume of an enclosed space is increased, the pressure inside it will decrease
1. the lungs are suspended in the airtight thorax
2. increasing the volume of the thorax decreases the air pressure in the lungs to below atmospheric pressure. Air flows into the lungs, inflating them until the pressure in the alveoli equals that of the atmosphere
3. decreasing the volume of the thorax increases the pressure in the lungs and air flows out until the pressure in the alveoli drops to atmospheric pressure

104
Q

how do we breathe in?

A
  1. intercostal muscles and diaphragm (a muscular sheet) contract
  2. thorax volume increases
  3. this decreases the pressure so air flows in
105
Q

how do we breathe out?

A
  1. intercostal muscles and diaphragm relax
  2. thorax volume decreases
  3. this increases the pressure, so air flows out
106
Q

give 3 examples of things that can cause disease

A
  1. Pathogens
  2. genetic defects
  3. lifestyle
107
Q

what are pathogens? What type of disease do they cause?

A

pathogens are organisms that can cause disease, e.g. bacteria and viruses. Infectious diseases are caused by pathogens and can be passed from person to person, e.g. TB, malaria

108
Q

what is a diet high in salt a risk factor for?

A

high blood pressure

109
Q

name 6 risk factors for diseases

A

smoking, drinking too much alcohol, high blood pressure, being overweight/obese, an unbalanced diet, over-using sun beds

110
Q

which diseases is smoking a risk factor for?

A
  • mouth, lung and throat cancer
  • emphysema and other lung diseases
  • cardiovascular disease
111
Q

which diseases is drinking too much alcohol a risk factor for?

A
  • mouth, stomach, liver and breast cancer
  • possibly many other cancers
  • cardiovascular disease
112
Q

which diseases is high blood pressure a risk factor for?

A

cardiovascular disease, diabetes

113
Q

which diseases is obesity a risk factor for?

A

various cancers, cardiovascular disease, diabetes

114
Q

which diseases is an unbalanced diet a risk factor for?

A

various cancers, cardiovascular disease, diabetes

115
Q

which disease(s) is over-using sun beds a risk factor for?

A

skin cancer

116
Q

which type of white blood cell detects and engulfs pathogens?

A

a phagocyte

117
Q

which part of the pathogen do the phagocytes detect?

A

the molecules on the surface of the pathogen called antigens

118
Q

how can phagocytes tell the difference between your own cells and foreign pathogens?

A

by the antigens: human cells and pathogens both have antigens on their surface, but phagocytes can tell the difference between self (your own) and foreign antigens

119
Q

what do phagocytes do to pathogens that are carrying foreign antigens?

A

they engulf them and destroy them

120
Q

which white blood cells produce antibodies?

A

B-cells (they’re sometimes called B-lymphocytes - pronounced: lim-fo-sites)

121
Q

how do antibodies work?

A
  • some white blood cells produce antibodies that bind to antigens.
  • when the antibody binds to the antigen it brings about the death of the pathogen carrying it
122
Q

what are T-cells (or T-lymphocytes) involved in/what do they do?

A

they’re a type of white blood cell that are involved in communication between phagocytes and B-cells:
- when a phagocyte has engulfed a pathogen it signals to the T-cells that it’s found something. The T-cell then activates the B-cells to produce antibodies

123
Q

give three types of white blood cells and briefly explain what they do

A
  1. phagocytes - find & engulf pathogens
  2. B-cells - produce antibodies
  3. T-cells - communicate between phagocytes and B-cells
124
Q

how do vaccines work?

A
  1. If you’re vaccinated against a pathogen you can’t get that disease (you’re immune)
  2. Vaccines contain antigens from a pathogen in a form that can’t harm you, e.g. attached to dead bacteria
  3. your body produces antibodies against the antigens so, if the same pathogen (carrying the same antigens) tries to invade again, the immune system can respond really quickly and you won’t suffer from any symptoms
  4. vaccines don’t stop the pathogen getting into the body, they just get rid of it really quickly when it does
125
Q

why do large animals need a circulatory system?

A

diffusion is only efficient over short distances, so any animal bigger than a simple worm needs a system that will bring glucose and oxygen into close contact with individual cells

126
Q

Give the 3 main types of blood vessels and briefly describe what they do

A
  • arteries carry blood away from the heart
  • veins carry blood to the heart
  • capillaries are where the exchange between the blood and the cells takes place
127
Q

how does our circulatory system get substances such as glucose, oxygen and carbon dioxide into the cells?

A

as the blood flows through the tissues, dissolved substances such as glucose, oxygen and carbon dioxide are exchanged between the blood and the cells

128
Q

what is the main artery in the human body? What does it do?

A

the aorta - it carries oxygenated blood from the heart to the rest of the body

129
Q

give 4 important facts to remember about the heart

A
  1. The heart acts like two separate pumps. The right side sends blood to the lungs and the left side pumps blood around the rest of the body
  2. Blood always flows from a region of higher pressure to a region of lower pressure
  3. Valves in the heart prevent the blood from flowing backwards
  4. no energy is required to make the valves work - it’s the blood pressing on the valves that makes them open and close
130
Q

what does the cardiac cycle mean?

A

the cardiac cycle is the sequence of events that occurs during one heartbeat

131
Q

describe the 4 stages of the cardiac cycle

A
  1. Blood flows into the two atria (the top chambers)
  2. the atria contract to push the blood into the ventricles (the lower chambers)
  3. The ventricles contract, forcing blood into the aorta and pulmonary artery. The ventricles are much more powerful than the atria and, when they contract, the heart valves pop shut automatically to prevent backflow into the atria. The ventricle walls are thicker because they need to push the blood further (e.g. the left ventricle has to push blood all the way round the body)
  4. The blood flows down the arteries and leaves the heart. As soon as the ventricles relax, the valves at the top of the heart pop shut to prevent backflow of blood (back into the ventricles) as the blood in the arteries is now under considerable pressure. The whole cycle then starts again
132
Q

What do most muscles need in order to contract? Why is the heart different?

A

Most muscles require nerve impulses from the central nervous system to make them contract, whereas the heart produces its own electrical impulses

133
Q

how does the heart produce its own electrical impulses to make itself contract?

A
  • a group of specialised cells called the sino-atrial node, in the wall of the right atrium, sends out regular impulses.
  • these spread across the atria, making them contract
134
Q

which artery do the coronary arteries split off from? What are they like in relation to this artery?

A

they come off the aorta - the coronary arteries are quite thin (especially compared to the aorta)

135
Q

what do the coronary arteries do?

A

they provide the heart with oxygen and glucose from the blood

136
Q

What is the name of the smaller vessels that arteries divide into?

A

arterioles

137
Q

starting as the blood is pumped away from the heart, describe which blood vessels the blood moves through in which order until it arrives at the heart again (5 stages)

A
  1. Arteries carry blood away from the heart
  2. they subdivide into smaller vessels called arterioles
  3. arterioles subdivide into microscopic vessels called capillaries
  4. capillaries join up to form veins
  5. veins return blood to the heart
138
Q

Describe the walls of an artery

A

Arteries have a thick wall compared to the diameter of the lumen. There’s an outer layer of fibrous tissue, then a thick layer of elastic tissue and smooth muscle, then a very thin inner layer of folded endothelial tissue

139
Q

how do the arteries maintain the high pressure of the blood that enters them, and why is this important?

A

When the ventricles contract, blood enters the arteries at high pressure. This stretches the folded endothelium and elastic walls. When the ventricles relax, it’s the elastic recoil of the artery wall (when the wall shrinks back to its original size) that keeps the blood pressure up. Important organs, like the kidneys, wouldn’t be able to function if the blood pressure dropped too far between heartbeats.

140
Q

How are arterioles physically different to arteries? What can arterioles do that arteries can’t, and how are they able to do this?

A
  1. arterioles are narrower than arteries and they have a higher proportion of smooth muscle fibres and a lower proportion of elastic tissue
  2. When the circular muscle fibres of an arteriole contract, the diameter of the lumen is reduced, so less blood flows through that vessel. This means that arterioles can control the amount of blood flowing to a particular organ.
141
Q

describe the walls of capillaries

A

capillary walls consist of a single layer of endothelial cells (cells that line the blood vessels). Some capillaries have tiny gaps between the endothelial cells

142
Q

how are capillaries well-suited to their job?

A
  1. the very thin walls and the gaps between the cells allow water and substances like glucose and oxygen to diffuse quickly from the blood into the cells. Waste products, such as carbon dioxide and urea, diffuse from the cells into the blood
  2. Organs contain thousands of capillaries, so altogether there’s a huge surface area for the exchange of substances
  3. blood flows quite slowly through capillaries. This allows more time for diffusion to occur/
143
Q

describe the structure of veins

A

a vein has a large lumen and a relatively thin wall containing some elastic tissue and smooth muscle. Veins also have valves that prevent the blood flowing backwards

144
Q

describe how leg muscles can help blood to return to the heart

A

when the leg muscles contract they bulge and press on the walls of the veins, pushing the blood up the veins. When the muscles relax, the valves close. This action helps the blood to return to the heart

145
Q

what is the main function of blood? What does blood contain that allows it to do this?

A

to transport materials to and from cells - so that the blood can do this, red blood cells are packed with haemoglobin, a protein that contains iron and can carry oxygen

146
Q

what does oxygen form when it combines with haemoglobin?

A

oxyhaemoglobin

147
Q

how much oxygen does one molecule of haemoglobin combine with?

A
  • when there’s a lot of oxygen present, one molecule of haemoglobin can combine with four molecules of oxygen - the haemoglobin is 100% saturated
  • when less oxygen is present, fewer molecules of oxygen combine and the haemoglobin is less than 100% saturated
148
Q

what shape is a graph that plots the “percent saturation of haemoglobin” compared to the “concentration of oxygen”?

A

although it would be reasonable to expect that such a graph would be a straight line (i.e. that the two would be directly proportional). However, when experiments are carried out and the results plotted, the line off best fit is S-shaped

149
Q

what can haemoglobin do that makes it so special?

A
  • haemoglobin has special properties that allow it to become fully saturated with oxygen in the capillaries around the alveoli of the lungs, where there’s a high concentration of oxygen
  • then when it reaches respiring tissue, where there’s less oxygen, it can give up almost all of its oxygen immediately - so the rate of respiration in the tissues isn’t slowed down because of an oxygen shortage
150
Q

how does high carbon dioxide levels affect haemoglobin? What is the name of these effects?

A
  • respiring tissues produce carbon dioxide
  • if there’s a lot of carbon dioxide present, the haemoglobin is less efficient at taking up oxygen (i.e. it needs to be exposed to more oxygen before it becomes fully saturated
  • but, when there’s a lot of carbon dioxide present, the haemoglobin becomes more efficient at releasing oxygen (i.e. it can release more oxygen molecules in areas of fairly high oxygen demand)
  • this is good because it means that rapidly respiring tissues, e.g. contracting leg muscles and brain cells, get more oxygen
  • this effect of changing carbon dioxide concentration on the oxygen-binding properties is known as the Bohr effect
151
Q

what is the Bohr effect?

A

the effect of carbon dioxide concentration on the oxygen-binding properties of haemoglobin

152
Q

what makes different organisms of a species similar, but not exactly the same?

A

organisms of the same species are similar because they all have the same genes but they vary because they have different versions of those genes (called alleles). E.g. humans all have a gene for blood type, but they can have A, B or O alleles

153
Q

what is an adaptation?

A

a characteristic that helps an organism to survive and reproduce

154
Q

what is evolution?

A

the gradual change in the characteristics of a population from one generation to the next. The theory of evolution is that all organisms evolved from a common ancestor over millions of years

155
Q

give an example of one mechanism by which evolution occurs (there’s more than one)

A

natural selection

156
Q

explain natural selection (5 stages)

A
  1. organisms from the same population all vary (e.g. different lengths of fur)
  2. organisms compete with each other for food, shelter, water, etc.
  3. those with better adaptations (caused by different alleles) are more likely to find food, shelter, water, etc., survive and reproduce. So they pass on the alleles for their better adaptations. E.g. bears with longer fur will stay warmer and be more likely to survive, and so have kids with longer fur
  4. over time, the number of organisms with the better adaptations (alleles) increases
  5. the whole population of organisms evolves to have the better adaptations (alleles)
157
Q

what is a species?

A

a group of organisms that can reproduce to give fertile offspring

158
Q

how did older classification systems group organisms?

A

based only on how they look, e.g. four limbs, six eyes, etc.

159
Q

give three examples of things that newer classification systems take into account when grouping organisms, other than how they look

A
  1. DNA - how similar and different the base sequence is (e.g. ATTTAC vs. ATTTAT)
  2. Other molecules - e.g. proteins and enzymes
  3. early development - how they grow from an embryo to a baby
160
Q

what does xylem tissue do?

A

transport water and minerals from the roots to the leaves of the plant

161
Q

how does water enter and travel through a plant?

A

water from the soil enters the roots by osmosis. Then it travels through the root to the xylem - this is the tissue that transports water through the plant and up to the leaves.

162
Q

what are the two ways that water can travel through the roots of a plant?

A
  1. the symplast system
  2. the apoplast system
163
Q

how does the symplast system work?

A

some water moves through the root via the cytoplasm of the root cells. The water has to cross the cell membrane, which regulates the passage of the water and dissolved minerals

164
Q

how does the apoplast system work?

A
  • the water moves through the cell walls and the spaces between the cells
  • there are no membranes to regulate the passage
165
Q

describe the cells that make up the tubes (vessels) of xylem tissue

A

these cells are dead, waterproof and hollow. This means water can move through them easily.

166
Q

give three factors that help to pull water up through the xylem tissue

A

cohesion, tension and adhesion

167
Q

describe how cohesion, tension and adhesion help to pull water up through the xylem tissue

A
  1. water evaporates from inside the leaf leaving a higher concentration of solutes
  2. water from the nearest xylem vessel enters by osmosis
  3. water molecules stick together because of weak hydrogen bonds between them - this is called cohesion
  4. as water molecules leave the xylem vessel they pull up further molecules, so the whole column of water is pulled up
  5. Evaporation pulls the water column upwards and gravity pulls it down, so the water column is under tension
  6. the adhesion of water molecules to the sides of the xylem vessels stops the column breaking
168
Q

how are sugars and other organic compounds transported through plants?

A

in phloem tissue - phloem tissue is also arranged in tubes so the solutions of sugar, etc. can move through them easily

169
Q

what does the phloem transport?

A

organic compounds

170
Q

what is translocation?

A

the movement of carbohydrates and other organic compounds in plants

171
Q

where and how does translocation occur?

A

it occurs in the sieve tubes of the phloem tissue. Companion cells next to the sieve tubes are believed to actively transport sugar into the sieve tubes, and then water follows by osmosis

a level biology (3 decks)

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