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A Level Biology OCR A > 6 - Cell Division > Flashcards

6 - Cell Division Flashcards

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1
Q

How many pairs of chromosomes does a normal human have?

A

23

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2
Q

What does a gene do?

A

Code for a characteristic

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3
Q

For what 3 reasons does mitosis occur?

A

Growth, repair and asexual reproduction

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4
Q

What occurs during the G1 phase of the cell cycle?

A
  1. Proteins are synthesised 2. ATP is produced 3. Key organelles (including centrioles) grow larger, then duplicate
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5
Q

What is the G1 checkpoint known as in animals?

A

The restriction point

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6
Q

What is the G1 checkpoint known as in yeast?

A

START

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7
Q

Why is the G1 checkpoint important?

A

The cell must be checked to make sure that it is ready for division, as once the chromosomes are duplicated the process cannot be reversed

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8
Q

What factors are checked at the G1 checkpoint?

A
  1. Nutrients- are the necessary nutrients present in the cell and its environment 2. Size- is the cell large enough for division? 3. Molecular signals- Is the cell receiving positive cues (i.e. growth factors) from its neighbours? 4. Mechanical signals- Is the cell attached to a support/crowded by neighbours 5. DNA integrity- is any DNA damaged?
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9
Q

What happens if the cell is rejected at the G1 checkpoint?

A

It passes into a resting state called G0

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10
Q

What are some reasons for passing into G0?

A

Cell age/senescence, damage, lack of need for them to reproduce

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11
Q

Is G0 temporary or permanent?

A

It can be both

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12
Q

Which cells spend all of their existence in G0?

A

Neurons, although many cells spend much of their life in G0 performing functions

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13
Q

What else, apart from carrying out functions, can occur in G0?

A

Cell death

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14
Q

What happens if a cell passes the G1 checkpoint?

A

It passes into the S phase

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15
Q

What occurs during the S phase?

A

Each chromosome is duplicated in the nucleus

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16
Q

Which stages of the cell cycle make up interphase?

A

G1, S and G2

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17
Q

What occurs during the G2 phase of cell division?

A

The duplicated chromosomes are checked for damage, and repaired/replaced if possible and necessary

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18
Q

What is checked at the G2 checkpoint?

A

Cell size, DNA condition (integrity, i.e. is any damaged?, and replication, i.e. is it fully replicated?)

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19
Q

What happens if a problem is detected at the G2 checkpoint?

A

The cell will pause at the checkpoint to undergo repairs

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20
Q

What happens if any problem is fixed at the G2 checkpoint?

A

The cell re-enters the cell cycle, and proceeds with division as normal

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21
Q

What happens if damage found at the G2 checkpoint is irreparable, and why is this important?

A

The cell may undergo apoptosis, which is important in preventing cancer by stopping errors being passed to daughter cells

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22
Q

What is apoptosis?

A

Programmed cell death

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23
Q

What happens if a cell passes the G2 checkpoint without any problems?

A

It proceeds to mitosis

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24
Q

How many chromatids is a chromosome made up of?

A

Either 1, or two genetically identical ones

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25
Q

What is a chromatid?

A

A strand of DNA

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26
Q

How many chromatids does each chromosome have during G1?

A

1 chromatid

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27
Q

How many chromatids does each chromosome have from S phase to the beginning of mitosis?

A

2

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28
Q

How do chromosomes condense?

A

The DNA (in the form of chromatin fibres) is supercoiled around histone proteins

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29
Q

What are the 4 stages of Mitosis?

A

Prophase, Metaphase, Anaphase, Telophase

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30
Q

What is a chromosome?

A

A strip of tighlly coiled DNA

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31
Q

What 6 things occur during Prophase in mitosis?

A
  1. Chromatin fibres begin to coil and condense to form chromosomes. 2. Nucleolus disappears 3. Nuclear envelope begins to break down, and fully disappears by the end of Prophase 4. Microtubules begin to form spindle fibres linking the poles of the cell 5. Centrioles migrate to opposite poles of the cell 6. Spindle fibres attach to centromeres, begin to move chromosomes to cell centre
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32
Q

What is a centromere?

A

The point on a chromosome onto which a spindle fibre attaches

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33
Q

What occurs in Metaphase in mitosis?

A

Chromosomes are moved by the spindle fibres to form a plane in the cell’s centre, called the Metaphase Plate, and are then held in position

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34
Q

What occurs in Anaphase in mitosis?

A

Centromeres holding pairs of chromatids together in chromosomes divide. Sister chromosomes separated and pulled to opposite poles of the cell by shortening spindle fibres

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35
Q

What shape do chromatids tend to be in during Anaphase?

A

A ‘v’ shape because of them being pulled by the spindle fibres

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36
Q

What occurs during telophase during mitosis?

A

Chromatids, which have now reached the poles of the cell, are referred to as chromatids. Each new set of chromosomes assembles at its respective pole, nuclear envelope and nucleolus reform. Chromosomes start to uncoil.

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37
Q

When does cytokinesis start in both animals and plants?

A

During telophase

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38
Q

How does cytokinesis occur in animals?

A

The cytoskeleton pulls the cell surface membrane inwards, causing it to invaginate (pinch in). This causes a cleavage furrow to form, until the cell surface membrane pinches off and two new cells are formed

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39
Q

What is the difference between cytokinesis in plants and animals?

A

In plants, it begins from the middle of the cell, as opposed to the outside as it does in animals

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40
Q

How does cytokinesis occur in plant cells?

A

Vesicles from the Golgi Apparatus line up in the same position as the metaphase plate, then fuse with each other and the cell surface membrane to form a new cell surface membrane in the middle of the cell called the cell plate. The cell plate then has new cellulose deposited along it.

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41
Q

Why can’t a new cell wall be formed along the cell plate during cytokinesis in plants before the separation of the two daughter cells?

A

Because otherwise the new cells would undergo immediate osmotic lysis

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42
Q

What is osmotic lysis?

A

When a cell membrane bursts (and so, essentially, does the cell) due to the movement of water into the cell via osmosis

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43
Q

What do homologous chromosomes have the same of?

A

They have the same gene loci, so they have genes which code for the same thing. They also have the same length

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44
Q

What happens to homologous chromosomes before cell division?

A

They are copied

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45
Q

What is a bivalent?

A

Where pairs of homologous chromosomes attach to each other at points called chaisma (singular) or chaismata (plural)

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46
Q

What occurs during Prophase 1 of meiosis?

A
  1. Chromosomes condense 2. Homologous chromosomes pair up to form bivalents (resulting in crossing over of chromatids) 3. Nuclear envelope disintegrates 4. Centrioles migrate to poles of cell 5. Spindle fibres begin to form
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47
Q

What is independent assortment?

A

Where pairs of homologous chromosomes line up with random alignments along the metaphase plate during metaphase in meiosis

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48
Q

What occurs during Metaphase 1 during meiosis?

A
  1. Homologous pairs assemble along metaphase plate with random orientations (independent assortment) 2. Spindle fibres from each centriole attach to centromeres
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49
Q

What occurs during Anaphase 1 during meiosis?

A
  1. Homologous chromosomes pulled to opposite poles of cell by spindle fibres, sister chromatids stay attached. 2. Sections of DNA entangled during crossing over break off and rejoin, with recombinant chromatids being formed by this exchange of genes
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50
Q

What is a recombinant chromatid?

A

One with a different genetic makeup to either the father or the mother’s chromatid due to sections of DNA entangled during crossing over being exchanged.

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51
Q

What occurs during Telophase 1 during meiosis?

A
  1. Chromsomes assemble at the pole of each cell and uncoil 2. Nuclear envelope reforms 3. Centrioles move back into cytosol 4. Cell surface membrane invaginates and pinches off to form 2 daughter cells, each with the same amount of genetic information as the parent cell
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52
Q

What happens during Prophase 2 during meiosis?

A
  1. Chromosomes condense and become visible again 2. Nuclear envelope disintegrates 3. Centrioles migrate to poles of cell, spindle fibres begin to form
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53
Q

What happens during Metaphase 2 in meiosis?

A

“1. Individual chromosomes line up along metaphase plate, again with independent assortment 2. Spindle fibres attach to centromeres “

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54
Q

What happens during Anaphase 2 in meiosis?

A

Chromatids of individual chromosomes pulled to poles of cell by shortening spindle fibres after division of centromeres

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55
Q

What happens during Telophase 2 in meiosis?

A
  1. Chromosomes assemble at poles, then uncoil and form chromatin again 2. Nuclear envelope reforms 3. Each cell’s cell surface membrane invaginates and pinches off to form two new daughter cells, each with half the genetic information of the original parent cell
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56
Q

Why is mitosis used in repair?

A

Old and damaged cells can be replaced by new, identical ones

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57
Q

Why is mitosis used in growth?

A

Allows accelerated growth to increase the size of the organism

58
Q

What is a diploid number of chromosomes?

A

The full number of chromosomes in a normal cell (46 in humans)

59
Q

What is a haploid number of chromosomes?

A

Half the normal amount of chromosomes (23 in humans)

60
Q

What is a gametophyte?

A

An organism which produces gametes

61
Q

What is a sporophyte?

A

An organism which produces spores

62
Q

What is a spore?

A

A cell which is adapted for survival and dispersed in unfavourable conditions

63
Q

How does meiosis ensure variation within a species? (methods)

A
  1. Crossing over between homologous chromosomes 2. Fertilisation (1/2 of genetic information from both mother’s and father’s gametes) 3. Independent assortment and segregation of both homologous and non-identical sister chromatids
64
Q

What is a specialised cell?

A

A cell which has adaptations which help it carry out a specific function

65
Q

What is a tissue?

A

A group of the same specialised cells, which work together to carry out a function

66
Q

What is another name for an erythrocyte?

A

A red blood cell

67
Q

What shape do erythrocytes have and why?

A

A flattened biconcave shape to increase their surface area and SA:V ratio

68
Q

What are the 3 adaptations which erythrocytes have in order to help them transport oxygen round the body?

A
  1. Flattened biconcave shape 2. Flexible, to fit through narrow capillaries 3. In mammals, they lack nuclei or other organelles, to leave more space for haemoglobin
69
Q

What is the function of a sperm cell?

A

To deliver genetic information to the female gamete, the ovum

70
Q

What adaptations does a sperm cell have?

A
  1. A flagellum containing microtubules and many mitochondria, giving them the ability and energy to swim. 2. An acrosome on its head containing digestive enzymes, which are released to digest the protective layers around the ovum and allow the sperm to penetrate 3. Protein fibres to strengthen the tail
71
Q

What type of nucleus does a sperm cell have?

A

A haploid nucleus

72
Q

What adaptations does a palisade mesophyll have?

A
  1. Contain many chloroplasts to absorb lots of light for photosynthesis. 2. Cells rectangular, so can be closely packed to form continuous layer 3. Thin cell walls, increasing rate of CO2 diffusion 4. Chloroplasts can move in cytoplasm to absorb more light 5. Large vacuole to maintain turgor pressure
73
Q

What is a neutrophil a type of?

A

Type of white blood cell

74
Q

Why do neutrophils have a multi-lobed nucleus?

A

To make it easier for them to squeeze through small gaps to get to the site of infections

75
Q

What adaptations does a neutrophil have?

A

Multi-lobed nucleus, granular cytoplasm, many lysosomes in cytoplasm

76
Q

What is the function of a palisade mesophyll cell?

A

Take in light via chlorophyll for photosynthesis

77
Q

What is the function of a neutrophil?

A

Plays an important role in the immune system

78
Q

What is the function of a root hair cell?

A

Uptakes water and minerals from the soil

79
Q

Where are root hair cells found?

A

Surface of the roots near the growing tips

80
Q

What adaptations does a root hair cell have?

A

Long, thin extension to maximise SA:V ratio, vacuole containing cell sap (solution of ions and sugars) to lower water potential and maximise concentration gradient to take up water by osmosis

81
Q

What is the function of a pair of guard cells?

A

To open and close one stoma

82
Q

What happens once a guard cell loses water?

A

It becomes less swollen and changes shape, closing the stoma to prevent further water loss.

83
Q

Why does a guard cell not change shape symmetrically?

A

Because the cell wall is thicker on the inside and thinner on the outside

84
Q

What is the hole between two guard cells called?

A

Stomatal aperture

85
Q

What is a 7 day old embryo called?

A

A blastocyst

86
Q

Where are totipotent stem cells found?

A

A fertilized egg, zygote or the first 8-16 cells made from the fertilized egg

87
Q

What type of cells can totipotent stem cells divide into?

A

Any type of cell

88
Q

Where are pluripotent stem cells found?

A

The blastocyst and early embryos

89
Q

What can totipotent stem cells produce apart from cells for the embryo?

A

Cells for the amnion (amniotic sac) or umbilicus (umbilical cord)

90
Q

What are undifferentiated cells called?

A

Stem cells

91
Q

Why does the replication of stem cells have to be tightly controlled?

A

If they do not divide quickly enough then tissues will be replaced inefficiently, leading to aging, but if there is uncontrolled division then they can form tumours

92
Q

What can be created from pluripotent stem cells?

A

Any type of tissue, but not whole new organisms

93
Q

How can scientists produce pluripotent stem cells?

A

By genetically manipulating cells found in umbilical cords, and some parts of adults (these would be multipotent stem cells before manipulation)

94
Q

Where are the stem cells which create all blood cells located?

A

The bone marrow

95
Q

What can be created from multipotent stem cells?

A

A few cells types, such as erythrocytes

96
Q

What is the name for the type of stem cell which blood cells are created from?

A

Haematopoetic stem cells

97
Q

What are the new cells created from stem cells necessary for?

A

Growth, development and tissue repair

98
Q

What is the name for the ability of a stem cell to differentiate?

A

Potency

99
Q

What is the average lifespan of a human erythrocyte?

A

120 days

100
Q

What 3 steps occur for a haematopoetic stem cell to be converted to an erythrocyte?

A
  1. Nucleus and organelles lost 2. Haemoglobin made 3. Cell shape changed
101
Q

What is the average lifespan of a human neutrophil?

A

6 hours

102
Q

What 3 steps occur for a haematopoetic stem cell to be converted to an neutrophil?

A
  1. More lysosomes created 2. Nucleus becomes multi-lobed 3. Extensive cytoskeleton created
103
Q

When are more neutrophils produced from stem cells?

A

During infection

104
Q

What are the two broad sources of human stem cells?

A

Adult and embryonic stem cells

105
Q

What are some advantages of taking stem cells from the umbilical cord?

A

No invasive surgery needed, plentiful supply of umbilical cords, cells could be stored if owner of umbilicus ever needed them (with no risk of rejection)

106
Q

Where are stem cells found in plants?

A

Meristems, which are found in the cambium between the xylem and phloem, as well as at the tips of roots/shoots or anywhere else growth is occurring in the plant

107
Q

What is the term for meristems found at the tips of roots and shoots?

A

Apical meristems

108
Q

What 4 steps occur for a plant stem cell to be converted into a xylem cell?

A
  1. Loss of cytoplasm and contents 2. Lignin deposited 3. Elongation of cells 4. End walls lost
109
Q

What 2 steps occur for a plant stem cell to be converted to a phloem cell?

A
  1. Some loss of cytoplasm and organelles 2. Sieve plate development
110
Q

What potency of stem cells are plant stem cells all through the plant’s adult life?

A

Pluripotent

111
Q

What diseases could stem cells have the potential to be used in the treatment of?

A
  1. Heart disease 2. Alzheimer’s 3. Type 1 diabetes 4. Parkinson’s 5. Macular degeneration (causes blindness in the elderly and diabetics) 6. Birth defects 7. Spinal injuries
112
Q

What other areas can stem cells be used in? (apart from disease treatment)

A
  1. Burns treatment (growing stem cells on a biodegradable mesh instead of a skin graft) 2. Drug trials (testing drugs on cultures instead of organisms) 3. Developmental biology studies
113
Q

How have stem cells been used in medicine for years already?

A

In the form of bone marrow transplants

114
Q

What causes most ethical issues with the use of embryonic stem cells?

A

The fact that extracting stem cells from embryos usually involves destroying the embryo

115
Q

What are some ethical objections to/issues with the use of embryonic stem cells?

A
  1. Religious objections 2. Moral objections i.e. life begins at conception, therefore harvesting embryo is murder 3. Lack of consensus on rights of embryo 4. Lack of consensus on who holds genetic material taken from embryos
116
Q

Why is using embryonic stem cells better and worse than using umbilical stem cells?

A

Embryonic stem cells are pluripotent, so can divides into more things than multipotent umbilical stem cells, but use of umbilical stem cells removes many ethical issues assosciated to embryonic stem cells.

117
Q

What are induced pluripotent stem cells?

A

Adult stem cells genetically modified to act like embryonic (and therefore pluripotent) stem cells

118
Q

What are some disadvantages of using adult stem cells instead of umbilical stem cells?

A

They do not divide as well and are more likely to have acquired mutations

119
Q

What is an organ?

A

A collection of tissues which are adapted to perform a function

120
Q

What is an organ system?

A

A number of organs working together to carry out a major function in the body

121
Q

What are the 4 main categories of tissue in animals?

A
  1. Nervous tissue 2. Epithelial tissue 3. Muscle tissue 4. Connective tissue
122
Q

What is nervous tissue adapted to do?

A

Support the transmission of electrical impulses

123
Q

What is epithelial tissue adapted to do?

A

Cover internal and external body surfaces

124
Q

What is muscle tissue adapted to do?

A

Contract

125
Q

What is connective tissue adapted to do?

A

Hold other tissues together, or act as a transport medium

126
Q

What is the structure of squamous epithelial tissue?

A

A single layer of squamous cells attached to a basement membrane

127
Q

How is squamous epithelial tissue adapted to aid diffusion?

A

Very thin due to only being 1 cell thick

128
Q

What is the structure of the ciliated epithelial tissue?

A

Ciliated epithelial cells (with cilia) mixed with goblet cells, all on a basement membrane

129
Q

What are the 3 types of cartilage?

A
  1. Hyaline (rubbery, but still somewhat tough- found between bones in joints) 2. Elastic- Lots of elastin so very flexible i.e. outer ears 3. Fibrocartilage- Densely packed with collagen so very tough and resistant to shock and compression, found in intervertebral disks
130
Q

What type of tissue is cartilage?

A

Connective

131
Q

What two protein fibres does cartilage contain?

A

Elastin and collagen

132
Q

What is the structure of cartilage?

A

Chrondrocyte cells embedded in an extracellular matrix

133
Q

What are two types of muscle fibres?

A

Skeletal and cardiac

134
Q

What is special about cardiac muscle fibres?

A

They can contract without stimulation from the nerves, instead basing their rhythm on pacemaker cells

135
Q

Why does muscle need to be able to contract?

A

In order to move bones, which in turn move the parts of the body

136
Q

What do skeletal muscles contain?

A

Myofibrils, which contain contractile proteins

137
Q

How are cells connected in muscle fibres?

A

By collagen- when the cell contracts, it pulls on the collagen, causing tension which can cause movement

138
Q

What organelles do muscle cells have more of?

A

Mitochondria and ribosomes (to produce actin and myosin fibres)

139
Q

What are two broad types of tissue in plants?

A
  1. Epidermis tissue, adapted to cover plant surfaces 2. Vascular tissue, adapted for transport of water and nutrients
140
Q

What are 3 specific specialised plant tissues?

A

Xylem, phloem, epidermal

141
Q

What is the structure of the epidermis?

A

A single layer of closely packed cells, usually covered by a waxy cuticle to reduce water loss, with stomata in them to allow gas exchange

A Level Biology OCR A (23 decks)

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