Cell Division, Cell Diversity and Cellular Organisation

Question 1

Cell Cycle: What is the cell cycle?

Answer 1

The cell cycle is the process that all body cells from multicellular organisms use to grow and divide. It starts when a cell has been produced by cell division and ends with the cell dividing to produce two identical cells. The cell cycle consists of a period of cell growth and DNA replication, called interphase, a period of cell division called mitosis.

Question 2

Cell Cycle: What are the stages of the cell cycle?

Answer 2

• Mitosis
• Interphase
  
  • Gap phase 1
  • Synthesis
  • Gap phase 2

Question 3

Cell Cycle: What is the mitosis stage?

Answer 3

The cycle starts and ends here and is when the nucleus divides and chromatids separate, after this is cytokinesis, when the cytoplasm divides or cleaves.
-Mitosis only occupies a small percentage of the cell cycle and the remaining percentage includes the copying and checking of genetic information. as well as processes associated with growth.

Question 4

Cell Cycle: What is interphase?

Answer 4

During interphase, the cell carries out normal functions, but also prepares to divide. The cell’s DNA is unravelled and replicated, to double its genetic content. The organelles are also replicated so it has spare ones and its ATP content is increased.

Question 5

Cell Cycle: What is the G1 phase?

Answer 5

Gap phase 1: Biosynthesis - cell grows, organelles replicate and new organelles and proteins are made

Question 6

Cell Cycle: What is the S phase?

Answer 6

Synthesis: cell replicates its DNA and checks the DNA for errors

Question 7

Cell Cycle: What is the G2 phase?

Answer 7

Gap phase 2: cell keeps growing and proteins needed for cell division are made

Question 8

Cell Cycle: How long does the cell cycle take?

Answer 8

The length of time for a parent cell to divide into two daughter cells, and for each to grow to full size, varies between species and cell type. It is also affected by the availability of nutrients for the cells.
-Some prokaryotic cells can go through the whole process in around 30 minutes, whereas yeast cells (single-celled eukaryotes) take about 4 hours.

Question 9

Where is DNA held?

Answer 9

Chromosomes are in the nucleus of eukaryotic cells. Each chromosome contains one molecule of DNA, which includes specific lengths of DNA called genes. So the chromosomes that hold the instructions are sometimes called the blueprint, for making new cells. The daughter cells produced during the cell cycle mist contain a copy of all of these instructions s they must contain a full set of chromosomes, copied exactly from the chromosomes in the parent cell.

Question 10

How many chromosomes do humans have?

Answer 10

In humans we have 23 different types of chromosomes. There are two copies in each cell so we have 46 chromosomes in the nucleus of each cell.
-Different chromosomes have different genes on them, there are 2 copies of each gene.

Question 11

What is the structure of chromosomes?

Answer 11

The DNA double helix is wrapped around histone proteins, this makes nucleosomes. As this continues wrapping up, it produces chromatin which eventually makes chromosomes.

Question 12

What is a histone protein?

Answer 12

Histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes.

Question 13

What is a nucleosome?

Answer 13

Nucleosomes are the basic unit of DNA packaging in eukaryotes (cells with a nucleus), consisting of a segment of DNA wound around a histone protein core.

Question 14

What is chromatin?

Answer 14

The material of which the chromosomes of eukaryotic organisms are composed, consisting of protein, RNA, and DNA.
-Material staining dark red in the nucleus during interphase of mitosis and meisois. It consists if nucleic acids and proteins. Chromatin condenses into chromosomes when supercoiled during prophase of cell division.

Question 15

What is the definition of a chromosome?

Answer 15

A linear DNA molecule wrapped around histone proteins found in the nucleus.

Question 16

What must happen before a cell can divide?

Answer 16

In eukaryotes, the molecules of DNA that make up each chromosome are wrapped around proteins called histones. The DNA and the histone proteins together are called chromatin. Before a cell can divide to produce two new daughter cells, the DNA of each chromosomes must be replicated. Two replicas are produced. Each is an exact copy of the original, and they remain held together at a point called the centromere. This plays an important role an the process of nuclear division.

Question 17

What is produced when a cell’s DNA is replicated?

Answer 17

At this stage you can’t see the chromosomes under a light microscope. Each chromosome now consists of two replica DNA strands. These replicas are called a pair of sister chromatids. When they are separated from each other, each one will end up in a different new daughter cell.

Question 18

What must happen to chromatin before it can be divided?

Answer 18

Chromatin must be coiled up (supercoiled) to form visible chromosomes. Each one is then short and sturdy enough the be moved around more easily. Supercoiled chromosomes 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.

Question 19

How thick is chromatin and chromosomes?

Answer 19

Chromatin threads are about 30nm thick.

-After supercoiling a chromosome is about 500nm thick.

Question 20

Why are chromosomes often shown as sister chromatids?

Answer 20

Single chromosomes would be too small to be seen with a light microscope.When copied chromatin supercoils and condenses, it becomes denser and more easier to stain and so can be seen easier with a microscope.

Question 21

What is the advantage of supercoiling?

Answer 21

• Easier to move

- More compact

Question 22

What happens as the chromosomes are being replicated?

Answer 22

As chromosomes are being replicated, proof-reading enzymes move along the new DNA strands and check that the copying has been done properly. If the genes are not copied precisely, the resulting mutations may mean the cells fail to function, and could become cancerous.

Question 23

What other than a full set of chromosomes do daughter cells need?

Answer 23

Each new cell, like its parent, must carry out a number of metabolic function. In order to survive, it needs its own membranes, cytoplasm, organelles, enzymes and other proteins.

Question 24

Why do all organisms need to produce genetically identical daughter cells?

Answer 24

• Asexual reproduction: single-celled organisms, such as Paramecium, divide to produce two daughter cells that are separate organisms. Some multicellular organisms, such as Hydra, produce offspring from parts of the parent.
• Growth: multicellular organisms grow by producing new extra cells. Each new cell is genetically identical to the parent cells and so can perform the same function.
• Repair: damages cells need to be replaced by new ones that perform the same function and so need to be identical.
• Replacement: red blood cells and skin cells are replaced by new ones.

Question 25

Mitosis: What is mitosis?

Answer 25

Mitosis refers to the process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus, eventually forming diploid cells. It is a continuous process but is divided into four named stages.

Question 26

Mitosis: What is the structure of a chromosome in mitosis?

Answer 26

As mitosis begins, the chromosomes are made of two strands joined in the middle by a centromere. The separate strands are called chromatids. Two strands on the same centrimere are called sister chromatids. There are two strands because each chromosome has already mad an identical copy of itself during interphase.
-When mitosis is over, the chromatids end up as one-strand chromosomes in the new daughter cells.

Question 27

Mitosis: What are the four stages of mitosis?

Answer 27

-Prophase
-Metaphase
-Anaphase
-Telophase
PrepMiddleApartTwo

Question 28

Mitosis: What happens in prophase?

Answer 28

• Each chromosome has already replicated in interphase. In prophase, the chromosomes condense (due to supercoiling) and become shorter and fatter. It can be seen that they consist of a pair of sister chromatids when using a light microscope at this point.
• At this time, the nuclear envelope breaks down and disappears, allowing the chromatids to move around the cell.
• An organelle called the centriole (a tiny bundle of proteins) divides in two, and each daughter centriole moves to opposite ends (poles) of the cell to form the spindle, a structure made of protein fibres.

Question 29

Mitosis: What happens in metaphase?

Answer 29

The chromosomes (each with two chromatids) move to the middle of the cell, the central region of the spindle (the equator). Each becomes attached to a spindle thread by its centromere.

Question 30

Mitosis: What happens in anaphase?

Answer 30

• The replicated sister chromatids that make up the chromosome are separated from each other when the centromere that holds them together splits. At this point, each of the ‘sisters’ effectively becomes an individual chromosome. Each one is identical to the original chromosome in the parent cell from which it was copied.
• The spindle fibres shorten, pulling the sister chromatids further and further away from each other towards the poles. They assume a V-shape because the centromeres, attached to the spindle fibres lead.

Question 31

Mitosis: What happens in telophase?

Answer 31

• As the separated sister chromatids reach the poles, the spindle breaks down and disappears. The chromatids uncoil and become long and thin again, back into chromosomes.
• A nuclear envelope forms around each groups of chromosomes, so there are now two nuclei.

Question 32

Mitosis: What happens in cytokinesis?

Answer 32

Cytokinesis is the division of the cytoplasm. The cell membrane constricts, pinching the cell into the two daughter cells. Each contains a full set of chromosomes identical to that found in the original parent cell.
-Having identical genetic information, in the form of identical chromosomes, means each daughter cell is capable of doing everything the parent cell could do.

Question 33

Mitosis: Why is cytokinesis different in animal and plant cells?

Answer 33

• In plants, only meristem cells can carry out mitosis and cytokinesis. Plant cells lack centrioles to produce the tubulin spindles, these just form in the cytoplasm instead.
• In animal cells, cytokinesis starts sat the edge of as cell where the cell membrane constricts. This is not what happens in plant cells (due to the cell wall).

Question 34

Mitosis: How does cytokinesis occur in plant cells?

Answer 34

Because plants have a cell wall, cytokinesis is different in plant cells to animal cells. It begins in the centre of a cell wall with a cell plate. This is a double membrane which secretes materials needed to make two cell walls (one for each daughter cells). The cell walls begin to form in the centre and move outwards until they meet the perimeter.

Question 35

Mitosis: How does cytokinesis occur in yeast?

Answer 35

Cytokinesis in yeast involves budding of new cells of the original.

Question 36

Mitosis: How does division occur in plants?

Answer 36

Only meristem cells can divide by mitosis. These are found in the root and shoot tips, forming a ring of tissue in the centre of the tips. This causes the plant shoots and roots to grow in girth, affecting the whole plant.

Question 37

What are other types of cell cycles?

Answer 37

• Gametes (meiosis)
• Bacteria (binary fission)
• Stem cells

Question 38

Sexual Reproduction: What is sexual reproduction?

Answer 38

The production of a new individual formed by the fusing of gametes from two different parent organisms. The offspring have unique combinations of alleles inherited from both parents.

Question 39

Sexual Reproduction: What are gametes?

Answer 39

Gametes are the sperm cells in males and egg cells in females. In sexual reproduction, two gametes join together at fertilisation to form a zygote, which divides and develops into a new organism.
-Gametes have 23 chromosomes, half the amount of normal chromosomes.

Question 40

Sexual Reproduction: What is a homologous pair?

Answer 40

Chromosomes that have the same gene at the same loci. Members of a homologous pair of chromosomes pair up during meiosis. Diploid organisms, produced by sexual reproduction, have homologous pairs of chromosomes - one member of each pair from the male parent and the other member from the female parent.

Question 41

Sexual Reproduction: What is fertilisation?

Answer 41

The combining of the two gametes and their genetic material.
-At fertilisation, a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. Half these chromosomes are from the father and half are from the mother. The diploid cell produced by fertilisation is called a zygote.

Question 42

Sexual Reproduction: What is a zygote?

Answer 42

Fertilised cells

Question 43

Sexual Reproduction: What is an embryo?

Answer 43

Zygote divided by mitosis

Question 44

Sexual Reproduction: What is a foetus?

Answer 44

Developing zygote

Question 45

Sexual Reproduction: What is meiosis?

Answer 45

Meiosis is a type of cell division that happens in the reproductive organs to produce gametes. Cells that divide by meiosis are diploid to start with but the gametes that are formed from meiosis are haploid - the chromosome number halves.
-Cells form by meiosis are all genetically different because each new cell ends up with a different combination of chromosomes.

Question 46

Sexual Reproduction: What are the steps in the process of meiosis?

Answer 46

1. After fertilisation has occurred, the DNA replicates and coils up to form (92) chromosomes. The chromosomes arrange themselves into homologous pairs.
2. The chromosome pairs then swap bits with each other, by crossing over.
3. In the first division these homologous pairs split up (2x46). Any one chromosome from each pair can go into either cell, as long as each cell gets one number 1, one number 2 etc.
4. In the second division, each chromosome splits in half (4x23). Any half can go into any cell.
5. Four new genetically different cells are produced. They’re genetically different from each other because the chromosomes swap bits during meiosis and each gamete gets a combination of half of them, at random.

Question 47

Sexual Reproduction: How does meiosis create genetic variation?

Answer 47

• By crossing over; chromosomes from each parent get so close to each other near the equator they could overlap and swap genes.
• The chromosomes line up in pairs randomly.

Question 48

Sexual Reproduction: How does meiosis differ from mitosis?

Answer 48

• Meiosis produces 4 cells containing half the number of chromosomes.
• Meiosis produces cells that are genetically different from each other and from the parent cell.
  
  • These features, together with the fusion of gametes from different individuals, means that the offspring of sexually reproduced individuals are always different from each other, (apart from identical twins).

Question 49

Asexual Reproduction: What is asexual reproduction?

Answer 49

Asexual reproduction is the production of genetically identical new organisms by a single parent organism.

Question 50

Asexual Reproduction: What is a clone?

Answer 50

Clones are genetically identical cells or organisms derived from one parent.

Question 51

Asexual Reproduction: Why does bacteria not divide by mitosis?

Answer 51

• They lack a nucleus
• Mitosis refers to cell division involving chromosomes. (In bacteria, the DNA is a free strand, and is not wrapped around histone proteins).
• They also have plasmids of DNA

Question 52

Asexual Reproduction: What is binary fission?

Answer 52

The DNA replicates and the cell divides in two, each having the same DNA as the parent cell. It does not involve mitosis.
-Bacteria are prokaryotes. They have a single, naked (not associated with histone proteins) strand of DNA that is in the cytoplasm, not in a nucleus. They may also have small plasmids of DNA. These may have genes for antibiotic resistance. Because bacteria can swap plasmids, they are used in genetic engineering.

Question 53

Asexual Reproduction: What is vegetative propagation?

Answer 53

Many plants undergo asexual reproduction using specialised parts of the plant that are derived from adult plant cells. Examples include potato tubers and strawberry plant runners. These specialised parts can produce many new individual organisms that are genetically identical to the original parent. They are also called clones. This form of asexual reproduction is known as vegetative propagation.

Question 54

Asexual Reproduction: What is budding?

Answer 54

Yeast can reproduce asexually by a process called budding. As budding involves mitosis, the offspring produced are genetically identical to the parent cell.

1. The parent yeast cell swells on one side, forming a bud at the surface of the cell.
2. The cell undergoes interphase - the DNA and organelles are replicated ready for the cell to divide.
3. The cell begins to undergo mitosis - the replicated DNA, cytoplasm and organelles move into the bud.
4. Nuclear division is complete - the budding cell contains the nucleus that has an identical copy of the parent cell’s DNA.
5. Finally, cytokinesis occurs and the bud pinches off from the parent cell, producing a new, genetically identical yeast cell.

Question 55

Asexual Reproduction: What is artificial cloning?

Answer 55

Artificial cloning has been practiced by farmers and growers for many years. For example, cuttings taken from some plants, can be made to grow into adult plants. They are all genetically identical to the parent plant from which the cutting was taken, so they are clones of the parent. They are also all genetically identical to each other.

Question 56

Asexual Reproduction: What are the steps in cloning animals?

Answer 56

1. Tissue biopsy is taken from donor cow.
2. Donor cells are grown in tissue culture.
3. Donor cell nucleus is transferred to recipient egg.
4. Clones embryo is transferred to surrogate mother.
5. Pregnancy is monitored b ultrasound, until a cloned cow is born.

Question 57

Asexual Reproduction: Who is Dolly the sheep?

Answer 57

The artificial cloning of animals became big news in 1997. Scientists placed a nucleus from an adult sheep’s udder cell into a sheep egg cell that had had its nucleus removed. The resulting cell was placed into the uterus of another female sheep (a surrogate). At the end of the pregnancy, a lamb was born, and was named Dolly. Dolly the sheep was a clone of another animal. This technique continues to raise many ethical issues.

Question 58

Stem Cells: What are stem cells?

Answer 58

Mitosis produces genetically identical daughter cells that carry a full set of genetic information. They are potentially capable of becoming any one of the cell types found in the fully grown organism, they are unspecialised. Such cells are known as stem cells. They can be described as omnipotent or totipotent, meaning they are capable of developing into any type of cell or forming any type of tissue.

Question 59

Stem Cells: What are stem cells in humans?

Answer 59

In humans, stem cells are found in early embryos and inn a few places in adults.

• In the first few days of an embryo’s life, any of its cells can develop into any type of human cell - they’re all stem cells.
• In adults, stem cells are found in a few places (e.g. bone marrow), but they’re not as flexible - they can only develop into a limited range of cells. They are used to replace damaged cells.

Question 60

Differentiation: What is differentiation?

Answer 60

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

Question 61

Differentiation: Why do eukaryotes have many different organelles in a cell?

Answer 61

Within an eukaryotic cell there are different organelles each performing a particular function. There organelles all contribute to the survival of the cell. From the mitochondria releasing energy for the cell’s needs to the ribosomes assembling cell proteins, each organelle contributes to the processes needed to sustain life.

Question 62

Differentiation: Why can a single cell only grow so large?

Answer 62

There is a physical limit to the size that a single cell can reach. This is governed by the need to support structures within the cell, and by the increasing difficulty of getting enough oxygen and nutrients into a cell to support its needs as its size increases.

Question 63

Differentiation: Why do multicellular organisms need specialised cells, whereas single-celled organisms do not?

Answer 63

Single-celled organisms have a large surface area to volume ratio. They can receive oxygen and remove carbon dioxide by diffusion through the membrane.
Multicellular organisms have a smaller surface area to volume ratio, and not all cells are in contact with the external medium. This means they need specialised cells, forming tissues and organs, to carry out particular function. These functions include delivery of oxygen and nutrients, and removal of waste.

Question 64

Differentiation: In what ways do cells differentiate?

Answer 64

• The number of organelles
  
  • In muscle cells, more mitochondria
  • In leaf cells, more chloroplasts
• The shape of a cell
• The cell’s contents

Question 65

Differentiation: How are all blood cells produced?

Answer 65

In the undifferentiated stem cells in the bone marrow.
-Before differentiation, all blood cells begin with the same set of chromosomes and so each is potentially capable of carrying out the same function.

Question 66

Differentiation: How are erythrocytes differentiated from bone marrow?

Answer 66

RBC

• Lose their nucleus (so can’t carry out mitosis, no DNA. Can only get them from bone marrow).
• Lose mitochondria, Golgi apparatus and rough E.R. (allows more space for haemoglobin so more oxygen can be carried)
• Packed full of haemoglobin to carry oxygen.
• Shape changes to form biconcave discs to allow efficient absorption of oxygen as the surface area is increased.

Question 67

Differentiation: How are neutrophils differentiated from bone marrow?

Answer 67

WBC

• Keep their nucleus (allow them to carry out mitosis and replace because their life span is only a couple of days)
• Their cytoplasm looks granular because it contains many lysosomes containing digestive enzymes to break down engulfed microbes.

Question 68

Differentiation: How are sperm cells differentiated?

Answer 68

In organelles content:
-Energy for movement of the undulipodium is generated by many mitochondria present within the cell.
-The sperm head contains a specialised lysosome (acrosome) that released enzymes onto the outside of the egg so that the sperm nucleus can penetrate the egg in order to fertilise it.
In shape:
-Sperm cells are very small, long and thin to help in easing their movement.
-The single long undulipodium (flagellum) helps to propel the cell up the uterine tract towards the egg.
In content:
-The sperm cell nucleus contains half the number of chromosomes of an adult cell in order to fulfil its role as a gamete.

Question 69

Differentiation: How are root hair cells differentiated?

Answer 69

Root hair cells appear on the outside (epidermal layer) of young plant roots. The cells have a hair-like projection from their surface out into the soil. This greatly increases the surface area of root available to absorb water and minerals from the soil.

Question 70

Specialisation: What are tissues?

Answer 70

A collection of cells that are similar to each other and perform a common function.

• They may be found attached to each other, but not always.
• E.g. Xylem and Phloem, epithelial and nervous tissues

Question 71

Specialisation: What are organs?

Answer 71

A collection of tissues working together to perform a particular function is called an organ.
-E.g. leaves and liver

Question 72

Specialisation: What are organ systems?

Answer 72

An organ system is made up of a number of organs working together to perform an overall life function.
-E.g. the reproductive system and the respiratory system.

Question 73

Specialisation: What is a transport system?

Answer 73

These are used to carry substances between different cells.

Question 74

Specialisation: What is a communication system?

Answer 74

These allow communication between cells in different parts of the organism. Both plants and animals have chemical communication systems that use messenger molecules, such as hormones. Animals also have a nervous system for communication, sending electrical signals to different tissues and organs.

Question 75

Specialisation: Why are plant cells differentiated?

Answer 75

Plants need to move water and minerals from the soil, through their roots and stems, up into the leaves. They also need to move the products of photosynthesis from the leaves to other parts of the plants to use for growth or to store for later use. Xylem and phloem come from dividing meristem cells such as cambium. Meristem cells undergo differentiation to form the different kinds of cells in the transport tissues.

Question 76

Specialisation: How are xylem cells formed from meristem cells?

Answer 76

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

Question 77

Specialisation: How are phloem cells formed from meristem cells?

Answer 77

Phloem tissue consists of sieve tubes and companion cells. The meristem tissue produces cells that elongate and line up end to end to form a long tube. There ends do not break down completely but form sieve plates between the cells. The sieve plates allow the movement of materials up or down the tubes. Next to each sieve tube is a companion cell, which are very metabolically active. There activities play an important role in moving the products of photosynthesis up and down the plant in the sieve tubes.

Question 78

Specialisation: What categories are animal tissues grouped into?

Answer 78

• Epithelial tissue
• Connective tissue
• Muscle tissue
• Nervous tissue

Question 79

Specialisation: What is epithelial tissue?

Answer 79

Layers and linings.

• Squamous epithelial tissue
• Ciliated epithelial tissue

Question 80

Specialisation: What is squamous epithelial tissue?

Answer 80

• Squamous epithelial tissue is made up of cells that are flattened, so that they are very thin. This makes them ideal for lining the insides of tubes such as blood vessels, where fluid can easily pass over them.
• Squamous epithelial tissue also form thin walls, such as the walls of the alveoli in the lungs. It provides a short pathway for the exchange of oxygen and carbon dioxide.
• The squamous cells are held in place by the basement membrane. This is secreted by epithelial cells. It is made of collagen and glycoproteins. The basement membrane attaches epithelial cells to connective tissue.

Question 81

Specialisation: What is ciliated epithelial tissue?

Answer 81

Ciliated epithelial tissue is made up of column-shaped cells. The type of tissue is often found on the inner surface of the tubes, for example in the trachea, bronchi and bronchioles and in the uterus and the oviducts.
-The part of the cell surface that is exposed in the lumen is covered with tiny projections called cilia. Some cells produce mucus. The cilia wave in a synchronised rhythm and move the mucus.

Question 82

Specialisation: Where is ciliated epithelial tissue found?

Answer 82

• In the breathing tract, small particles and microorganism are trapped in the mucus. The cilia waft it up to the back of the throat to be swallowed.
  
  • One of the problems with smoking is that the nicotine in the smoke paralyses cilia, so they can’t sweep to move the mucus. Cilia also becomes damaged by tar in the smoke and are destroyed, so there are fewer of them. The mucus becomes trapped in the lungs and microbes can’t be cleared out of the airways to be killed by the stomach acid.
• The rhythmic waves generated by ciliated epithelium move egg cells from the ovary along the oviduct.

Question 83

Specialisation: What is connective tissue?

Answer 83

Holds structures together and provides support (e.g. cartilage, bone and blood

Question 84

Specialisation: What is muscle tissue?

Answer 84

Cells specialised to contract and move parts of the body. Contains many mitochondria and collagen

Question 85

Specialisation: What is nervous tissue?

Answer 85

Cells that can convert stimuli to electrical impulses and conduct those impulses.

Question 86

Specialisation: Why are leaves organised in the way they are?

Answer 86

Leaves are the major organs of photosynthesis in a plant. Their cells, tissues and overall shape are arranged to help maximise the rate of photosynthesis. The requirements of photosynthesis are: light, water, carbon dioxide and the presence of chlorophyll.

Question 87

Specialisation: How are leaf cells specialised to their function of photosynthesis?

Answer 87

• Cuticle: Waxy coating for waterproofing the prevent transpiration.
• Upper Epidermis: Transparent to allow light through
• Palisade mesophyll cells: where photosynthesis takes place, so contain lots of chlorophyll, so they can absorb a lot of sunlight. walls are thin so carbon dioxide can diffuse in and out. Long and thin to pack more cells in so more photosynthesis can occur.
• Spongy mesophyll cell: Many airspaces to allow circulation of gases easily, increases surface area so carbon dioxide can reach more palisade cells.
• Lower epidermis: has stomata, to allow gases to be exchanged. Stomata have guard cells that can swell to open pore. When not turgid, they close. They close to save water loss.
• A leaf vein system contain xylem and phloem tissues supports the leaf as well as carrying the transport tissues. These tissues transport water into the leaf, and the products of photosynthesis out of the leaf to other parts of the plant.

Question 88

Specialisation: What is the role of the guard cells?

Answer 88

Guard cells are specialised cells that appear in pairs on the lower epidermis. Unlike other lower epidermis cells, they contain chloroplasts and their cell 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.

Question 89

Specialisation: What is an example of system cooperation?

Answer 89

The muscular and skeletal systems must work together in order for movement to take place. But this can happen only if the nervous system ‘instructs’ the muscles to coordinate their actions. As muscles and nerves work they use energy. They require a supply of nutrients and oxygen from the circulatory system, which in turn receives these chemicals from the digestive and ventilation systems.