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A-level biology > AS - Unit 1 - Cells > Flashcards

AS - Unit 1 - Cells Flashcards

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1
Q

What is the maximum magnification of a light microscope?

A

X1500

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2
Q

What is the definition of magnification?

A

The degree to which the size of an image is larger than the object itself.

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3
Q

What is the definition of resolution?

A

The degree to which it is possible to distinguish between two objects that are very close together. The higher the resolution, the greater the detail you can see

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4
Q

What is the highest resolution of the light microscope?

A

200 nm

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5
Q

What kind of organism can you see under a light microscope?

A

A living one

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6
Q

What is an artefact?

A

When a microscope specimen is distorted during the preparation process and therefore isn’t an accurate representation

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7
Q

How do you prepare a specimen for a light microscope?

A
  1. Staining - coloured stains are applied to allowed the specimen to be seen.
  2. Sectioning - specimens are embedded in wax, thin sections are then cut without distorting the structure of the specimen
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8
Q

What is the order of the units of measurement?

A 
Metre (m) 
Decimetre (dm) 
Centimetre (cm)
Millimetre (nm)
Micrometre (um)
Nanometre (nm)
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9
Q

What is the highest resolution for the human eye?

A

100 um

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10
Q

What is the highest resolution for the electron microscope?

A

0.20nm

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11
Q

How do you calculate magnification and size of a specimen?

A

Actual size X magnification

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12
Q

What are the two types of electron microscope?

A

Transmission electron microscope (TEM)

Scanning electron microscope (SEM)

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13
Q

How do TEM’s work??

A

The electron beam passes through a very thin prepared sample

Electrons pass through the denser parts of the sample less easily giving some contrast in the final image

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14
Q

How does an SEM work??

A

The electron beam is directed onto a sample

The electrons don’t pass through the specimen they are ‘bounced off’ the sample

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15
Q

Which electron microscope produces a 3D image??

A

A scanning electron microscope (SEM)

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16
Q

What is the max magnification of a TEM microscope??

A

X500 000

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17
Q

What is the max magnification of an SEM?

A

X100 000

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18
Q

Give 3 advantages of the electron microscope?

A

The resolution is 0.1nm (2000X better than a light microscope)
Can be used to produce detailed images of the organelles inside cells
SEM produces 3D images that can reveal the detail of contours and cellular or tissue arrangements

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19
Q

Give 3 limitations of the electron microscope

A

Samples have to be dead
Expensive
Preparing samples for and using the electron microscope requires a high degree of skill and training

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20
Q

Give the definition of staining (referring to microscopy)

A

Refers to any process that helps to reveal or distinguish difference features

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21
Q

What do electron microscope images look like?

A

Black and white

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22
Q

What is division of labour?

A

Each organelle has a specific role to play in the function of the cell, all the different organelles work together in a cell, each contributing it’s part to the survival of the cell

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23
Q

What is the cytoskeleton?

A

A network of fibres made of protein that keep the cell’s shape stable by providing an internal framework

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24
Q

Some fibres are called actin filaments, what role do they play in the cell?

A

Actin filaments move against each other and move some organelles around inside cells

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25
Q

What are microtubules?

A

They are cylinders inside the cell that are about 25nm in diameter. They are made of a protein called tubulin. They are used to move a microorganism through a liquid or to waft a liquid past the cell. They move chromosomes during mitosis, vesicles from the er to the Golgi. They use ATP to drive these movements

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26
Q

What is a eukaryote?

A

A cell with a nuclei

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27
Q

What are flagella or cilia?

A

Hair-like extensions that stick out from the surface of cells. Each one is made up of a cylinder that contains 9 microtubules arranged in a circle.

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28
Q

What are prokaryotes?

A

Cells without a nuclei

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29
Q

What are vesicles?

A

Membrane-bound sacs found in cells. They are used to carry many different substances around the cell

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30
Q

What does the vacuole do in a plant cell??

A

Maintains cell stability

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31
Q

What is a vacuole filled with and what is its purpose?

A

Water and solutes, to maintain cell stability and push the cytoplasm against the cell wall making the cell turgid. This helps to support the plant

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32
Q

What are plant cell walls made from??

A

Cellulose

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33
Q

What is the structure of the nucleus?

A

Largest organelle

Surrounded by a nuclear envelope

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34
Q

What is the function of the nucleus?

A

Houses nearly all the cell’s genetic material

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35
Q

What is the structure of endoplasmic reticulum?

A

Series of flattened, membrane-bound sacs
They are attached to the outer nuclear membrane
Rough ER is studded with ribosomes
Smooth ER does not have ribosomes

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36
Q

What is the function of the endoplasmic reticulum

A

Rough ER - transports proteins that were made on the attached ribosomes
Smooth ER - involved in making lipids that the cell needs

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37
Q

What is the structure of the Golgi apparatus?

A

Stack of membrane-bound, flattened sacs

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38
Q

What is the function of the Golgi apparatus?

A

Receives proteins from the ER and modifies them into vesicles which can then be transported to the surface of the cell so it can be secreted

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39
Q

What is the structure of the mitochondria?

A

Spherical or sausage-shaped

Two membranes separated by a fluid-filled space

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40
Q

What is the function of the mitochondria?

A

The site where ATP is produced during respiration

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41
Q

What is the structure of the chloroplasts?

A

Found only in plant cells and the cells of some protoctists

Two membranes separated by a fluid filled space

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42
Q

What is the function of chloroplasts?

A

The site of photosynthesis in plant cells

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43
Q

What is the structure of lysosomes?

A

Spherical sacs surrounded by a single membrane

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44
Q

What is the function of lysosomes?

A

Break down materials e.g. White blood cell lysosomes help to break down invading microorganisms

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45
Q

What is the structure of the ribosomes?

A

Two subunits

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46
Q

What is the function of ribosomes?

A

Site of protein synthesis in the cell, they act as an assembly line where coded information from the nucleus is used to assemble proteins from amino acids

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47
Q

What is the structure of the centrioles??

A

Small tubes of protein fibres

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48
Q

What is the function of the centrioles?

A

Take part in cell division, form fibres known as the spindle which move chromosomes during nuclear division

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49
Q

What does the term organelle mean?

A

Refers to the particular structure of a cell that has a specialised function. Some organelles are membrane-bound, others are not. All perform a particular role in the life processes of the cell

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50
Q

Where are the instructions the make hormones??

A

In the DNA in the nucleus

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51
Q

Give the 9 steps of protein synthesis

A
  1. mRNA copy of the instructions for insulin is made in the nucleus
  2. mRNA leaves through the nuclear pore
  3. mRNA attaches to a ribosome on the rough ER, ribosome reads the instructions to assemble the protein
  4. Insulin molecules are ‘pinched off’ in vesicles and travel towards the Golgi
  5. Vesicle fuses with Golgi apparatus
  6. Golgi processes and packages insulin molecules ready for release
  7. Packaged insulin molecules are ‘pinched off’ in vesicles from Golgi and move towards cell surface membrane
  8. Vesicle fuses with cell surface membrane
  9. Cell surface membrane opens to release insulin molecules outside
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52
Q

Give 7 features of prokaryotes

A

Only have one membrane
Surrounded by a cell wall
Contain ribosomes that are smaller than eukaryotic ribosomes
DNA in the form of a single loop
DNA is not surrounded by a membrane
ATP production takes place in specialised infolded regions of the cell surface membrane called mesosomes
Some prokaryotic cells have flagella

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53
Q

What is the phospholipid bi-layer?

A

The basic structural component of plasma membranes. It consists of two layers of phospholipid molecules. Proteins are embedded in this layer

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54
Q

Give 5 roles of membranes

A

Separating cell components from the outside environment
Separating cell components from cytoplasm
Cell recognition and signalling
Holding the components of some metabolic pathways in place
Regulating the transport of materials into or out of cells

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55
Q

Explain the basic structure of a phospholipid

A

Phosphate head is hydrophilic (water loving)

Two fatty acid tails are hydrophobic (water hating)

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56
Q

How can a bilayer form?

A

If phospholipid molecules are completely surrounded by water

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57
Q

If phospholipid molecules are mixed with water, how do they arrange themselves?

A

They form a layer at the water surface, the phosphate heads stick into the water and the fatty acid tails stick up out of the water

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58
Q

How does a bilayer effectively create a membrane?

A

The hydrophobic layer formed by the phospholipid tails creates a barrier to many molecules and separates the cell contents from the outside world.

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59
Q

Where do most metabolic reactions take place?

A

In water-based environments

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60
Q

Through which microscopes can you see membranes?

A

Electron microscopes

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61
Q

How thick are cell membranes?

A

7-10nm

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62
Q

What is the protein called that allows water molecules through it?

A

Aquaporin

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63
Q

What is the name for a model of the biological membrane?

A

The fluid mosaic model

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64
Q

What are the three main features of the fluid mosaic model?

A

A bilayer of phospholipid molecules
Various protein molecules floating in the bilayer
Some extrinsic proteins partially embedded in the bilayer and some intrinsic proteins spanning the bilayer completely

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65
Q

What is the difference between a glycoprotein and a glycolipid?

A

Glycolipids are carbohydrates coming straight from the bilayer
Glycoproteins are carbohydrates coming from a protein embedded in the bilayer

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66
Q

What is the role of cholesterol in the fluid mosaic model?

A

The provide eukaryotic cells mechanical stability. This is by fitting between the fatty acid tails and helps make the barrier more complete, so water and ions cannot pass easily and directly through the membrane

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67
Q

What do channel proteins do in the fluid mosaic model?

A

They allow the movement of some substances across the membrane. E.g. Glucose

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68
Q

What do carrier proteins do in the fluid mosaic model?

A

Actively move some substances across the membrane. E.g. Magnesium ions in plant cells

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69
Q

What do receptor sites do in the fluid mosaic model?

A

Some allow hormones to bind with the cell so that a response can be carried out. They also let drugs bind which affects cell metabolism

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70
Q

What factor causes cell membranes to become ‘leaky’?

A

Increasing temperature. This gives cells more kinetic energy so they move faster allowing gaps to be made, making it leaky

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71
Q

In order to detect signals what must cells have on their surface?

A

‘Sensors’ capable of receiving signals. These sensors are called receptors, they are often protein molecules or modified protein molecules

72
Q

What is the name of a cell with a receptor for hormone molecules?

A

A target cell

73
Q

How is a chemical message passed on to a cell?

A

The hormone molecule binds to a receptor on a target cell surface membrane because the two have complimentary shapes. This binding causes the target cell to respond in a certain way

74
Q

What kind of respiration do cells need?

A

Aerobic

75
Q

How did the fluid mosaic model get its name?

A

Fluid mosaic refers to the model of the cell membrane structure. The lipid molecules give fluidity and proteins in the membrane give it a mosaic appearance. The molecules can move about

76
Q

What is cell signalling?

A

Cells communicate with one another by signals. Many molecules act as signals - some signal during processes taking place inside the cells others signal from one cell to others. Cytokines are an example of cell signals

77
Q

What is diffusion?

A

The movement of molecules from a region of high concentration to a region of lower concentration down a concentration gradient

78
Q

What is the rate of diffusion affected by?

A 
Temperature
Concentration gradient
Stirring/moving
Surface area
Distance/thickness
Size of molecule
79
Q

How does temperature affect diffusion?

A

Increasing temp gives molecules more kinetic energy, the rate of random movement increases and so the rate of diffusion increases

80
Q

How does concentration gradient affect diffusion?

A

Having more molecules on one side of a membrane increases the concentration gradient. This increases the rate of diffusion

81
Q

How does stirring/moving affect diffusion?

A

Moving a liquid or creating an air current in a gas increases the movement of molecules and thus the rate of diffusion

82
Q

How does surface area affect diffusion

A

Diffusion occurs more rapidly if there is a greater surface area to diffuse across

83
Q

How does distance or thickness affect diffusion

A

Diffusion is slowed down by thick membranes, there is a greater distance for the molecule to travel

84
Q

How does the size of the molecule affect diffusion?

A

Smaller molecules or ions diffuse more quickly than larger ones

85
Q

How do lipid based molecules travel across the cell membrane?

A

They can simply pass through the bilayer, they diffuse down a conc gradient

86
Q

How do very small molecules and ions travel through the cell membrane?

A

Carbon dioxide and oxygen molecules are small enough to pass through the bilayer, some water molecules will pass directly through the membrane even though they are polar

87
Q

How do larger or charged molecules pass through the cell membrane?

A

Small, charge particles such as sodium ions, or larger molecules such as glucose cannot pass through the lipid bilayer. These molecules are moved via protein channels by facilitated diffusion

88
Q

What are channel proteins?

A

They are pores in the membrane which are shaped to only allow one type of ion through, many are also gated meaning they can be open or closed

89
Q

What are carrier proteins?

A

These are shaped so that a specific molecule (e.g. Glucose) can fit into them at the membrane surface. When the specific molecule fits, the protein changes shape to allow the molecule through to the other side of the membrane.

90
Q

What substances are moved through a membrane by simple diffusion?

A

Gases like oxygen and carbon dioxide

Lipid based molecules like steroid hormones

91
Q

What substances are moved through a membrane by facilitated diffusion using channel proteins?

A

Ions like sodium ions and calcium ions

92
Q

What substances are moved through a membrane by facilitated diffusion using carrier proteins?

A

Larger molecules like glucose and amino acids

93
Q

How do protein pumps (using ATP) differ from protein carriers (using facilitated diffusion)?

A

They carry specific molecules one way across the membrane
Use ATP to carry molecules across
They carry molecules in the opposite direction to the conc gradient
They carry molecules at a much faster rate than by diffusion

94
Q

What is active transport?

A

Refers to the movement of molecules or ions across membranes, which uses ATP to drive protein ‘pumps’ within a membrane

95
Q

Why can carrier proteins only travel molecules one way across the membrane?

A

The energy used by active transport changes the shape of the carrier protein. This means the specific molecule fits into the protein on one side only.

96
Q

What processes are used to transport large quantities of material in or out of a cell?

A

Endocytosis and Exocytosis

97
Q

What does endocytosis do?

A

Brings a large amount of material into the cell

98
Q

What does Exocytosis do?

A

Moves large amounts of materials out of the cell

99
Q

How is endo and Exocytosis able to happen?

A

Membranes can easily fuse, separate and be ‘pinched off’ to form vesicles. This required ATP!

100
Q

Give some examples of bulk transport

A

Hormones - the release of insulin into the blood
Plant cells - materials required to build cell wall
White blood cells engulf microorganisms by forming a vesicles around them

101
Q 
What do the different names mean when used to explain bulk transport? 
Endo
Exo 
Phago
Pino
A

Endo - inwards
Exo - outwards
Phago - solid material
Pino - liquid material

So… The bulk movement of liquid material out of the cell would be described as ‘exopinocytosos’

102
Q

Name three passive processes

A

Diffusion
Facilitated diffusion
Osmosis

103
Q

Name two active processes

A

Active transport

Endo and Exocytosis

104
Q

How does diffusion transport molecules?

A

Down a concentration gradient lipid soluble or very small molecules through lipid bilayer

105
Q

How does facilitated diffusion transport materials?

A

Down a conc gradient charged or hydrophilic molecules or ions via channel or carrier proteins

106
Q

How does osmosis transport materials

A

Down a water potential gradient through bilayer or protein pores

107
Q

How does active transport, transport materials?

A

Against a conc gradient via carrier proteins that use energy from ATP in order to change shape

108
Q

How do endo and exocytosis transport molecules?

A

Bulk transport of materials via vesicles that can fuse with or break from the cell surface membrane

109
Q

What is a solute?

A

A solid that dissolves in a liquid

110
Q

What is a solvent

A

A liquid that dissolves solids

111
Q

What is a solution

A

A liquid containing dissolved solids

112
Q

What is water potential?

A

A measure of the tendency of water molecules to diffuse from one place to another

113
Q

Which way does water always move?

A

From a region of high water potential to a region of low water potential

114
Q

What happens to the water potential when solutes are dissolved in the water?

A

As solutes are dissolved, water molecules cluster around them forming a solution. This lowers the concentration of ‘free’ water molecules, and lowers the water potential. The more solute dissolved, the lower the water potential of the solution

115
Q

What is osmosis

A

The net movement of water molecules across a partially permeable membrane

116
Q

Where is the water potential lower:
Pure water
Cells

A

In cells due to the sugars salts and other substances dissolved in the cytoplasm

117
Q

What happens in plant cells when they are placed in a solution with a high water potential?

A

Water molecules move down the water potential gradient INTO the cells by osmosis. The cell will swell causing the cytoplasm and vacuole to push the membrane against the cell wall. The cell is now turgid

118
Q

What happens to animal cells when they are placed in a solution with a high water potential?

A

Water will move down the water potential gradient INTO the cells by osmosis, the cells will swell and the membrane will eventually burst open. The cell is now haemolysed

119
Q

What happens to animal cells when they are placed in a solution with a low water potential?

A

Water moves out of the cell by osmosis, the cell contents will shrink and the membrane will wrinkle up, the cell is now crenated

120
Q

What happens to plant cells when they are in a solution with a low water potential?

A

Water will move out of the cell by osmosis, the cytoplasm and vacuole will shrink. The cell surface membrane will pull away from the cell wall. This is called plasmolysis.

121
Q

What is water potential measured in and what is the water potential of pure water?

A

kiloPascals (kPa)
0 kPa
The potential values decrease as more solute is added

122
Q

How many chromosomes do humans have?

A

46

123
Q

In Eukaryotes what is each chromosome wrapped around?

A

Proteins called histones

124
Q

What is chromatin

A

The mix of DNA and histone proteins wrapped together

125
Q

What is the name of the section where two chromosomes are attached?

A

The centromere

126
Q

What is another name for a pair of chromosomes?

A

A pair of sister chromatids

127
Q

Before the DNA can be moved, what do the chromatin strands have to do?

A

Supercoil to form visible chromosomes, each one is sturdy enough to be moved around more easily.

128
Q

Give a disadvantage of supercoiled DNA

A

They can’t perform their normal functions in the cell, so the length of time they spend coiled up needs to be as short as possible

129
Q

What are the stages of the cell cycle?

A

Interphase - DNA replicates
Mitosis - the nucleus divides and chromatids separate
Cytokinesis - the cytoplasm divides or cleaves
Growth phase - each new cell grows to full size

130
Q

Give 4 types/reasons for reproduction of cells

A

Asexual reproduction
Growth
Repair
Replacement

131
Q

What is mitosis?

A

The process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus

132
Q

What are the stages of mitosis?

A

Prophase
Metaphase
Anaphase
Telophase

133
Q

What happens during prophase?

A

Replicated chromosomes supercoil (shorten and thicken)

134
Q

What happens during metaphase?

A

Replicated chromosomes line up down the middle of the cell

135
Q

What happens during anaphase?

A

The replicas of each chromosome are pulled apart from each other towards opposite poles of the cell

136
Q

What happens during telophase?

A

Two new nuclei are formed

137
Q

What happens during interphase?

A

Chromosomes replicate, however they are not yet visible

138
Q

What is the current stage of mitosis when:
Nuclear envelope breaks down and disappears. Centriole divides in two and heads to opposite poles of cell to form the spindle.

A

Prophase

139
Q

What stage of mitosis occurs when:

Chromosomes move to the central region of the spindle and each becomes attached to a spindle thread by its cetromere

A

Metaphase

140
Q

What stage in mitosis occurs when:

Sister chromatids are separated, each is pulled to the centriole on its own side

A

Anaphase

141
Q

What stage in mitosis occurs when:
New nuclear envelope forms around each new chromosome, spindle breaks down and disappears. Chromosomes uncoil so you can no longer see them under the light microscope

A

Telophase

142
Q

What part of the cell cycle is happening when:

Whole cell now splits to form two new cells, each one containing a full set of chromosomes identical to its parent

A

Cytokinesis

143
Q

What is the name of the only plant cells that can divide by mitosis?

A

Meristem cells

144
Q

How does cytokinesis start in animal cells?

A

On the outside of the cell by ‘nipping in’ the cell membrane

145
Q

How does cytokinesis start in plant cells?

A

Starts with the formation of a cell plate where the spindle equator was. New cell membrane and new cell wall material is laid down along this cell plate

146
Q

What is the definition of a clone?

A

Genetically identical cells or organisms derived from one parent

147
Q

How does bacteria divide?

A

Binary fission

148
Q

How does mitosis differ from binary fission?

A

The term mitosis refers only to cell division involving chromosomes.
Bacteria (which uses binary fission) doesn’t have chromosomes it just has one strand of DNA

149
Q

What are stem cells?

A

Cells which are potentially capable of becoming any one if the different cell types found in the fully grown organism it came from.

150
Q

Where are meristem cells located in plants?

A

At the root and shoot tips and in a ring of tissues in the stem or trunk (which allows for an increase in girth).

151
Q

What is a genome?

A

All the genetic material inside an organism (or cell).

152
Q

What are gametes?

A

Cells which contain half the adult number of chromosomes

153
Q

What happens when two gametes (one male and one female) fuse together?

A

They produce a zygote which then divides by mitosis to grow into a new individual organism

154
Q

What are diploid cells?

A

Cells which contain two sets of homologous chromosomes

155
Q

What are haploid cells?

A

Only contain one set of chromosomes. Haploid cells are not genetically identical to each other as each pair of homologous chromosomes separates into haploid cells.

156
Q

Give two ways in which meiosis differs from mitosis

A

Produces cells containing half the chromosomes

Produces cells which are genetically different from each other and from the parent cell

157
Q

What is differentiation in terms of cells?

A

The changes occurring in cells of a multicellular organism so that each different type of cell becomes specialised to perform a specific function

158
Q

What does the role of a specific cell depend on?

A

The number of a particular organelle
The shape of the cell
Some on the contents of the cell

159
Q

What are erythrocytes?

A

Red blood cells

160
Q

What are neutrophils?

A

A type of white blood cell

161
Q

How are the contents of sperm cells specialised to perform its function?

A

Many of mitochondria to create energy of movement for the undulipodium (tail)
Head contains specialised lysosomes (called acrosome) that release enzymes to break down outside of egg so nucleus of sperm can fertilise it

162
Q

How is the shape of sperm specialised to perform its particular function?

A

Very small
Long and thin to help easing their movement
Single long undulipodium (flagellum) helps propel cell towards egg

163
Q

How do the contents of the sperm help it to perform its particular function?

A

Nucleus contains half number of chromosomes of an adult in order to fulfil its role as a gamete

164
Q

How are root hair cells specialised to perform their particular function?

A

Cells have a hair-like projection from their surface out into the soil. Greatly increases surface area of root available to absorb water and minerals from the soil

165
Q

What is the definition of a tissue?

A

Collection of cells that are similar to each other and perform a certain function. They may be found attached to each other, but not always.

166
Q

What is the definition of an organ?

A

Collection of tissues working together to perform a particular function

167
Q

What is an organ system

A

An organ system is made up of a number of organs working together to perform an overall life function. E.g. Excretory system or reproductive system

168
Q

Describe xylem tissue

A

Xylem tissue consists of xylem vessels with parenchyma cells and fibres. Meristem calls produce small cells that elongate. Their walls become reinforced and waterproofed by lignin. This kills cell contents, end of cells break down so they become continuous long tubes with a wide lumen. It is well suited for transporting water and minerals up the plant, also helps support the plant

169
Q

Describe phloem tissue

A

Consists of sieve tubes and companion cells. Meristem tissue produces cells that elongate and line up end to end to form a long tube. Their ends do not break down completely but form sieve plates between cells. Sieve plates allows movement of materials up or down the tubes. Next to each sieve tube is a companion cell, they are metabolically active, their activities play an important role in moving the products of photosynthesis up and down the plant in the sieve tubes

170
Q

What are the four main categories that animal tissues are grouped into?

A

Epithelial tissue -layers and lining
Connective tissues - hold structures together and provide support (e.g. Cartilage, bone and blood)
Muscle tissue - cells specialised to contract and move parts of the body
Nervous tissue - cells that can convert stimuli to electrical impulses and conduct those impulses

171
Q

What are squamous epithelial cells?

A

Cells that are flattened so they are very thin. Together they form a thin, smooth, flat surface. They are ideal for lining insides of blood vessels

172
Q

What is ciliated epithelial tissue?

A

Collection of column shaped cells. Found on inner surface of tubes e.g. Trachea, bronchi and bronchioles
The exposed end of these cells contains cilia. Cilia wave in a synchronised rhythm to move mucus produced by goblet cells

173
Q

What are the requirements for photosynthesis?

A

Light
Water supply
CO2 supply
Presence of chlorophyll

174
Q

Give 5 ways a leaf is adapted to perform its particular function

A

Transparent upper surface to let light in
Palisade layer of long thin tightly packed cells containing lots of chloroplasts which contain chlorophyll
Loosely packed spongy mesophyll layer for gas circulation
Lower epidermis with pores called stomata each containing two guard cells
Leaf vein system containing xylem and phloem tissues supporting and transporting the leaf contents

175
Q

How do guard cells work

A

Contain chloroplasts, cells walls contain spiral thickenings of cellulose. When water is moved into these cells they become turgid and because of the spirals in the walls of the inner edges, only the outer walls stretch. The two guard cells bulge at both ends so a pore opens between them. This pore is known as a stoma (plural stomata)

A-level biology (18 decks)

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