# data[].attributes.attachment_link.title
BBC Bitesize - GCSE Combined Science - Cell structure - AQA - Revision 12

# data[].attributes.attachment_link.description
How are cells structured? Learn about the size and function of plant and animal cells for GCSE Combined Science, AQA.

# data[].attributes.attachment_link.title
Biotechnology: Genetic Modification, Cloning, Stem Cells, and Beyond

# data[].attributes.attachment_link.readable_text
Main Difference - Tracheids vs Vessels

Tracheids and vessels are two types of conducting elements found in the xylem of plants. Both tracheids and vessels are involved in providing the mechanical support to the plant as well. Both these conducting elements consist of tubular structures. The main difference between tracheids and vessels is that tracheids are narrow and less efficient in water conduction whereas vessels are wide and highly efficient in water conduction. Tracheids are the major conducting elements in ferns and gymnosperms. In contrast, vessels are the major conducting elements in angiosperms. The other two cellular components of the xylem are xylem fibers and xylem parenchyma.

Key Areas Covered

1. What are Tracheids
- Definition, Characteristics, Function
2. What are Vessels
- Definition, Characteristics, Function
3. What are the Similarities Between Tracheids and Vessels
- Outline of Common Features
4. What is the Difference Between Tracheids and Vessels
- Comparison of Key Differences
Key Terms: Angiosperms, Ferns, Gymnosperms, Lignin, Perforation Plate, Pits, Tracheids, Vessels, Water Conduction, Xylem
What are Tracheids

Tracheids are one of the two conducting elements in the xylem of angiosperms. Tracheids are also present in ferns and gymnosperms as their major conducting element. Angiosperms have both vessels and tracheids; hence tracheids are only secondary conduction elements in angiosperms.

Tracheids consist of a single cell with pointed ends. The diameter of tracheids is around 30 μm. During secondary cell wall thickening, the tracheids are highly lignified, forming a polygonal cross section. Only the regions of the pits are not lignified. After lignification, tracheids become dead cells. Their protoplast becomes empty with the maturation. In addition to water conduction, tracheids are capable of providing mechanical strength to plants. They provide mechanical strength to softwood plants. Due to the high surface to volume ratio, tracheids can hold water against the gravity. A tracheid of oak is shown in figure 1.

What are Vessels

Vessels are the other type of conducting elements found only in angiosperms. Vessels do not contain a protoplasm at maturity, and secondary cell wall thickening by lignification produces a dead, tubular cell for the conduction of water. These lignified vessels are also involved in providing mechanical strength to plants. Wood is mainly made up of vessels.

Vessels are shorter cells with a wider lumen than tracheids. Due to the increased diameter, vessels conduct water more efficiently than tracheids. Moreover, vessels are arranged end to end, forming a tube. The ends of the vessels are made up of perforation plates. Some perforation plates consist of a single opening. Some may consist of several elongated openings. Another type of perforation plates may consist of several round openings or net-like openings. The presence of a perforation plate increases the efficiency of water conduction. The vessel elements of oak are shown in figure 2.

Similarities Between Tracheids and Vessels

Tracheids and vessels are two components of the xylem.

Both tracheids and vessels are tubular cells

Both tracheids and vessels are dead at maturity since they possess secondary lignification.

Both tracheids and vessels are found in both primary and secondary xylem.

Both tracheids and vessels are involved in water conduction along the stem as well as providing mechanical support to the plant.

Difference Between Tracheids and Vessels

Definition

Tracheids are tubular cells in the xylem of vascular plants, involving in the water conduction from roots to the leaves.

Vessels: Vessels are elongated dead cells found in the xylem of flowering plants, consisting of perforated cell walls through which the water flows.

Found in

Tracheids: Tracheids are present in all vascular plants.

Vessels: Vessels are only present in angiosperms.

Origination

Tracheids: Tracheids are originated from a single cell.

Vessels: Vessels are originated from a longitudinal file of cells. Therefore, they produce continuous tubes.

Diameter of the Lumen

Tracheids: The tracheids contain a narrow lumen.

Vessels: Vessels contain a wide lumen.

Pits

Tracheids: Tracheids consist of less number of large pits.

Vessels: Vessels contains a large number of small pits.

Perforated/Imperforated Cells

Tracheids: Tracheids are imperforated cells.

Vessels: Vessels are perforated cells.

Efficiency of Conduction of Water

Tracheids: Tracheids are inefficient in water conduction since they are imperforated cells.

Vessels: Vessels are efficient in water conduction.

Cell Wall Thickness

Tracheids: Tracheids contain thin cell walls.

Vessels: Vessels contain highly thickened cell walls.

Cross Section

Tracheids: Tracheids contain polygonal cross sections.

Vessels: Vessels contain circular cross sections.

Average Length

Tracheids: Tracheids are shorter cells (about 1 mm long).

Vessels: Vessels are longer cells (about 10 cm long).

Ends

Tracheids: Tracheids contain tapering end walls.

Vessels: Vessels contain diagonal or transverse end walls.

End to End Connection

Tracheids: Tracheids are laterally connected.

Vessels: Vessels are connected by end to end.

Surface to Volume Ratio

Tracheids: Tracheids consist of a high surface to volume ratio.

Vessels: Vessels consist of a low surface to volume ratio.

Prevention of Air Embolism

Tracheids: Tracheids prevent air embolism due to its high adhesion force in the narrow tube.

Vessels: Vessels do not prevent the air embolism.

Conclusion

Tracheids and vessels are the two water conducting elements found in the xylem. Tracheids are the major conducting element in ferns and gymnosperms. Vessels are only present in angiosperms. The diameter of tracheids is lesser than that of vessels. Furthermore, vessels consist of perforation plates at the ends of the cells. Therefore, the efficiency of water conduction is high in vessels than in tracheids. Both tracheids and vessels are involved in providing mechanical strength to the plants. The main difference between tracheids and vessels is their diameter and the efficiency in water conduction.

Reference:

1. "Tracheid." Encyclopædia Britannica, Encyclopædia Britannica, inc., Available here. Accessed 24 Aug. 2017.
2. Nandkeolyar, Anya. "Xylem Vessels." Prezi.com, 18 Dec. 2013, Available here. Accessed 24 Aug. 2017.

Image Courtesy:

1. "Tracheid of oak (from Marshall Ward)" By Harry Marshall Ward (1854-1906) - Marshall Ward H. The Oak: A Popular Introduction to Forest-Botany. - London: Kegan Paul, Trench, Trübner and Co. Ltd., 1892. - Figure 16, (Public Domain) via Commons Wikimedia
2. "Hardwood Pores" By McKDandy at English Wikipedia (CC BY 2.5) via Commons Wikimedia

# data[].attributes.attachment_link.title
Difference Between Tracheids and Vessels | Definition, Characteristics, Function

# data[].attributes.attachment_link.description
Tracheids and vessels are two types of conducting elements found in the xylem of plants. Both tracheids and vessels are involved in providing the mechanical support to the plant as well. Both these conducting elements consist of tubular structures.

# data[].attributes.body
1/2

# data[].attributes.attachment_link.readable_text
Genetics Basics - Lesson 3: Modes of Inheritance

3.1 Modes of Inheritance

Inheritance patterns describe how a disease is transmitted in families.

These patterns help to predict the recurrence risk for relatives.

In general, inheritance patterns for single gene disorders are classified based on whether they are autosomal or X-linked and whether they have a dominant or recessive pattern of inheritance. These disorders are called Mendelian disorders, after the geneticist Gregor Mendel.

3.2 Autosomal Dominant Inheritance

In autosomal dominant inheritance, only one copy of a disease allele is necessary for an individual to be susceptible to expressing the phenotype.

With each pregnancy, there is a one in two (50%) chance the offspring will inherit the disease allele.

Unless a new mutation has occurred, all affected individuals will have at least one parent who carries the disease allele.

Autosomal dominant inheritance is often called vertical inheritance because of the transmission from parent to offspring.

Across a population, the proportion of affected males should be equal to the proportion of affected females.

Male-to-male transmission can be observed.

Examples of diseases with autosomal dominant inheritance include myotonic muscular dystrophy and Huntington disease.

3.3 Autosomal Recessive Inheritance

In autosomal recessive inheritance, two copies of a disease allele are required for an individual to be susceptible to expressing the phenotype.

Typically, the parents of an affected individual are not affected but are gene carriers.

With each pregnancy of carrier parents:

There is a one in four (25%) chance the offspring will inherit two copies of the disease allele and will therefore have the phenotype.

There is a one in two (50%) chance the offspring will inherit one copy of the disease allele and will be a carrier.

There is a one in four (25%) chance the offspring will inherit no copies of the disease allele and will not express the phenotype or be a carrier. This individual would not be at risk for passing the disorder on to his/her offspring.

As with autosomal dominant inheritance, the proportion of affected males should be equal to the proportion of affected females in a given population.

Examples of diseases with autosomal recessive inheritance include sickle cell anemia and cystic fibrosis.

Clinical Information on Diseases

3.3.1 Autosomal Recessive Diseases

Autosomal recessive diseases are observed more frequently in consanguineous relationships (relationships in which the couple is related by blood, such as first cousins) because the individuals are descendants of the same ancestors and are, therefore, more likely to carry the same gene mutations.

Many autosomal recessive disorders are seen more frequently in individuals of certain ethnic backgrounds than others because these individuals are descendants of the same ancestors. However, because these common ancestors are generally more distantly related to these individuals, couples of the same ethnic background would generally have fewer genes in common than consanguineous couples.

3.4 X-Linked Dominant Inheritance

As in autosomal dominant inheritance, only one copy of a disease allele on the X chromosome is required for an individual to be susceptible to an X-linked dominant disease.

Both males and females can be affected, although males may be more severely affected because they only carry one copy of genes found on the X chromosome. Some X-linked dominant disorders are lethal in males.

When a female is affected, each pregnancy will have a one in two (50%) chance for the offspring to inherit the disease allele. When a male is affected, all his daughters will be affected, but none of his sons will be affected.

Examples of diseases with X-linked dominant inheritance are hypophosphatemic ricketsm, oral-facial-digital syndrome type I, and Fragile X syndrome.

3.4.1 Oral-Facial-Digital syndrome type I

A disorder characterized by oral, facial, digital, brain, and kidney manifestations.

The clinical features include oral (lobed tongue, cleft palate, hypodontia and other dental abnormalities), facial (hypertelorism, hypoplasia of alae nasi, cleft lip, micrognathia), digital (brachydactyly, syndactyly, clinodactyly, duplicated hallux, polydactyly), brain (atrophy, agenesis of corpus callosum, Dandy-Walker malformation), and kidney (polycystic kidney disease) manifestations. Affected males are usually miscarried.

The incidence of OFDI is approximately 1 in 50,000.

The disorder is X-linked dominant. The gene is called OFD1 and is located at Xp22.3-p22.2.

Approximately 75% of cases are sporadic and males are usually miscarried. Therefore, when the mother is affected, 1/3 of the female offspring will be unaffected, 1/3 of females will be affected, and 1/3 of males will be unaffected.

When the mother of an affected child does not carry the gene mutation, there is a 1% chance female offspring will be affected due to either a second new mutation or germline mosaicism.

3.5 X-Linked Recessive Inheritance

As in autosomal recessive inheritance, two copies of a disease allele on the X chromosome are required for an individual with two X chromosomes (a female) to be affected with an X-linked recessive disease.

Since males are hemizygous for X-linked genes (they have only one X chromosome), any male with one copy of an X-linked recessive disease allele is affected.

Females are usually carriers because they only have one copy of the disease allele. Affected males are related through carrier females.

For a carrier female, with each pregnancy there is a one in two (50%) chance her sons will inherit the disease allele and a one in two (50%) chance her daughters will be carriers.

Affected males transmit the disease allele to all of their daughters, who are then carriers, but to none of their sons.

Women are affected when they have two copies of the disease allele. All of their sons will be affected, and all of their daughters will be unaffected carriers.

Examples of diseases with X-linked recessive inheritance include Duchenne muscular dystrophy, hemophilia A and hypohidrotic or anhidrotic ectodermal dysplasia.

(A) Hair and (B) teeth of a male with ectodermal dysplasia.
(Reprinted from Emery's Elements of Medical Genetics, 10th ed., Mueller R.F. and Young I.D., ©1998, with permission from Elsevier Science.)

3.5.1 Hypohidrotic or anhidrotic ectodermal dysplasia

©This condition is characterized by three cardinal features that become apparent in childhood: hypotrichosis (sparse or brittle hair), hypohidrosis (reduced ability to sweat and problems with heat regulation), and hypodontia (missing teeth, conical teeth). Other clinical features include facial dysmorphism (including saddle nose), occasional mental retardation, increased susceptibility to infection, and raspy voice.

Hypohidrotic ectodermal dysplasia occurs in approximately 1 in 17,000 newborns.

There are three clinically similar forms with different genetic causes:

X-linked recessive (most common), caused by mutations in the ED1 gene at Xq12-q13.1. The molecular abnormality is in the protein ectodysplasin, which is important in the development of hair, teeth, and sweat glands.

Autosomal dominant and autosomal recessive forms caused by mutations in the EDAR at 2q11-q13 and EDARADD at 1q42.2-q43 genes.

Females who are carriers for the X-linked form may have partial expression, such as missing teeth and inability to sweat in parts of body.

The diagnosis is often made on the basis of clinical features. However, sequence analysis of the ED1 and EDAR coding regions is available on clinical basis; EDARADD testing available on research basis.

3.6 Complex Inheritance

Complex disorders (once known as multifactorial disorders) are caused by the interactions of variations in multiple genes and environmental factors.

The genes involved may make a person susceptible to the disorder, and the environmental factors may trigger this susceptibility.

The liability to exhibit the phenotype of the complex disorder is determined by both genetic and environmental factors. Only individuals with enough genetic liability (multiple genes) who are in the presence of certain environmental factors will exhibit the phenotype. The threshold is the point at which these factors combine sufficiently for the individuals to exhibit the phenotype.

Complex disorders are often common disorders in the population and include heart disease, diabetes, asthma, and many birth defects, such as cleft lip +/- cleft palate.

(A) Severe and (B) mild forms of cleft lip +/- palate.
(Reprinted from Emery's Elements of Medical Genetics, 10th ed., Mueller R.F. and Young I.D., ©1998, with permission from Elsevier Science.)

3.6.1 Cleft Lip +/- Cleft Palate

Cleft lip with or without cleft palate (CL/P) occurs when the frontal maxillary process fails to fuse at 35 days gestation. Cleft lip can be unilateral or bilateral. Isolated cleft palate is a separate entity with different risks and causes.

The clinical features differ depending on the cause of the CL/P, with some cases being syndromic. In isolated cases, bilateral cleft lip is more severe than unilateral cleft lip.

CL/P is relatively common, with an incidence of 1-2 per 1,000 births. There is a male to female ratio of 3:2. The rates differ in different ethnic backgrounds, with CL/P occurring 1/1000 Caucasians, 1.7/1000 Japanese, 0.4/1000 American Blacks.

CL/P is very heterogenous. Approximately 75-80% cases are sporadic. Syndromic cases may be monogenic, due to a chromosome abnormality (trisomy 13), or environmental factors (maternal rubella). Other cases are familial.

All inheritance patterns have been described and depend on the specific cause. However, the majority of cases are of complex inheritance, resulting from multiple genes and environmental factors.

Recently, a gene for van der Woude syndrome (VWS), which includes lip pits and CL/P, was identified; the gene is the interferon regulatory factor 6 (IRF6). VWS is the most common clefting syndrome. Evaluation of the IRF6 gene in non-syndromic cases of CL/P suggests it may be involved in its etiology as well.

The recurrence risks for CL/P caused by complex inheritance are higher in families with more than one affected and in cases with a more severe presentation (bilateral vs. unilateral, isolated cleft lip vs. cleft lip and cleft palate. For example, the risk to sibs are shown below:

Presentation

Recurrence Risks to Siblings

Unilateral isolated cleft lip

4.0%

Unilateral cleft lip and palate

4.9%

Bilateral isolated cleft lip

6.7%

Bilateral cleft lip and palate

8.0%

Genetic testing for the nonsyndromic cases is only available under a research basis. With improved technology, CL/P can sometimes be visualized on ultrasound. Clinical and/or research testing is available for some of the clefting syndromes.

# data[].attributes.attachment_link.title
Genetics Basics Lesson 3: Modes of Inheritance

# data[].attributes.attachment_link.description
Genetics Basics - Lesson 3: Modes of Inheritance 3.1 Modes of Inheritance Inheritance patterns describe how a disease is transmitted in families. These patterns help to predict the recurrence risk for relatives. In general, inheritance patterns for single gene disorders are classified based on whether they are autosomal or X-linked and whether they have a dominant or recessive pattern of inheritance.

# data[].attributes.body
Fotosintēzes un šūnu elpošanas salīdzinājums

# data[].attributes.body
goformative.com/join
SLSSQW

# data[].attributes.attachment_link.description
An overview of three types of sexual life cycles.

# data[].attributes.attachment_link.title
Higher Education Support | McGraw Hill Higher Education

# data[].attributes.attachment_link.description
Learn more about McGraw-Hill products and services, get support, request permissions, and more.

# data[].attributes.body
OVOĢENĒZE

# data[].attributes.body
SPERMATOĢENĒZE

# data[].attributes.attachment_link.readable_text
Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells - sperm in males, eggs in females.

During meiosis one cell divides twice to form four daughter cells.

These four daughter cells only have half the number of chromosomes of the parent cell - they are haploid.

Meiosis produces our sex cells or gametes (eggs in females and sperm in males).

Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II):

Meiosis I
1. Interphase:

The DNA in the cell is copied resulting in two identical full sets of chromosomes.

Outside of the nucleus are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division.

During interphase, microtubules extend from these centrosomes.

2. Prophase I:

The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope.

Each chromosome is composed of two sister chromatids containing identical genetic information.

The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on.

The pairs of chromosomes may then exchange bits of DNA in a process called recombination or crossing over.

At the end of Prophase I the membrane around the nucleus in the cell dissolves away, releasing the chromosomes.

The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.

3. Metaphase I:

The chromosome pairs line up next to each other along the centre (equator) of the cell.

The centrioles are now at opposites poles of the cell with the meiotic spindles extending from them.

The meiotic spindle fibres attach to one chromosome of each pair.

4. Anaphase I:

The pair of chromosomes are then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole.

In meiosis I the sister chromatids stay together. This is different to what happens in mitosis and meiosis II.

5. Telophase I and cytokinesis:

The chromosomes complete their move to the opposite poles of the cell.

At each pole of the cell a full set of chromosomes gather together.

A membrane forms around each set of chromosomes to create two new nuclei.

The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.

Meiosis II
6. Prophase II:

Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids).

In each of the two daughter cells the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope.

The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes.

The centrioles duplicate.

The meiotic spindle forms again.

7. Metaphase II:

In each of the two daughter cells the chromosomes (pair of sister chromatids) line up end-to-end along the equator of the cell.

The centrioles are now at opposites poles in each of the daughter cells.

Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.

8. Anaphase II:

The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle.

The separated chromatids are now individual chromosomes.

9. Telophase II and cytokinesis:

The chromosomes complete their move to the opposite poles of the cell.

At each pole of the cell a full set of chromosomes gather together.

A membrane forms around each set of chromosomes to create two new cell nuclei.

This is the last phase of meiosis, however cell division is not complete without another round of cytokinesis.

Once cytokinesis is complete there are four granddaughter cells, each with half a set of chromosomes (haploid):

in males, these four cells are all sperm cells

in females, one of the cells is an egg cell while the other three are polar bodies (small cells that do not develop into eggs).

# data[].attributes.attachment_link.description
Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells - sperm in males, eggs in females. During meiosis one cell divides twice to form four daughter cells.