biology4alevel600

Plants have 2 transport systems: xylem:  transports water and inorganic ions from the roots to the leaves. phloem:  transports food made in the plant (sucrose and amino acids) from the leaves to the rest of the plant.  Both of these systems are rows of cells that make continuous tubes running the full length of the plant. Plants can be very large, but they have a branching shape which helps to keep the surface area to volume ratio fairly large. Their energy needs are generally small compared with those of animals, so respiration does not take place so quickly. They can therefore rely on  diffusion  to supply their cells with O 2  and to remove CO 2 . Their leaves are very thin and have a large surface area inside them in contact with the air spaces. This means that diffusion is sufficient to supply the mesophyll cells with CO 2  for photosynthesis, and to remove O 2 . Plant transport systems therefore do not transport gase...

7.1    Structure of transport tissues 7.2    Transport mechanisms Flowering  plants do not have compact bodies like those of animals. Leaves and extensive root systems spread out to obtain  the light energy, water, mineral ions and carbon  dioxide that  plants gain from their environment to make organic molecules, such as sugars and amino  acids. Transport systems in plants move  substances from where they  are absorbed or produced to where they  are stored or used. Plants  do not have systems for transporting oxygen  and carbon  dioxide; instead these gases diffuse  through air spaces within stems, roots  and leaves. Learning Outcomes Candidates should  be able to: 7.1    Structure of transport tissues Plants  have two transport tissues: xylem and phloem. a)   draw and label from prepared slides  plan diagrams of transverse sections of stems, roots  and leaves of herb...

The transport system enables the rapid delivery of nutrients and O 2  to as well as the removal of metabolic waste products (including CO 2 ) from all cells of the body for survival. All cells need to take in substances from their environment, and get rid of unwanted substances. In a single-celled organism, this can happen quickly enough by diffusion alone. This is because: no point in the cell is very far from the surface, so it does not take long for gases to diffuse from the cell surface membrane to the centre of the cell, or vice versa; the surface area to volume ratio of the cell is relatively large - that is, it has a large amount of surface area compared to its total volume. In a large organism, diffusion is no longer sufficient. This is because: the centre of the organism may be a long way from the surface, so it would take too long for substances to diffuse all that way; the surface area to volume ratio is much smaller - that Is, It has a small amount of sur...

• The need for, and functioning of, a transport system in multicellular plants Learning Outcomes Candidates should be able to: (a) explain the need for transport systems in multicellular plants and animals in terms of size and surface  area to volume ratios; (b) define the term transpiration (see section 5) and explain that it is an inevitable consequence of gas exchange in plants; (c) [PA] describe how to investigate experimentally the factors that affect transpiration rate; (d) [PA] describe the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous  plants; (e) [PA] describe the structure of xylem vessel elements, phloem sieve tube elements and companion cells  and be able to recognise these using the light microscope; (f) relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their  functions; (g) explain the movement of water between plant cells, and between them and their environment, in te...

1. DNA and RNA are polynucleotides, made up of long chains of nucleotides. 2. A nucleotide contains a pentose sugar, a phosphate group and a nitrogen-containing base. In RNA the sugar is ribose, and in DNA it is deoxyribose. 3. A DNA molecule consists of two polynucleotide chains, linked by hydrogen bonds between bases. There are four bases – adenine always pairs with thymine, and cytosine with guanine. RNA, which comes in several diff erent forms, has only one polynucleotide chain, although this may be twisted back on itself, as in tRNA. In RNA, the base thymine is replaced by uracil. 4. DNA molecules replicate during interphase by semi-conservative replication. Th e hydrogen bonds between the bases break, allowing free nucleotides to fall into position opposite their complementary ones on each strand of the original DNA molecule. Adjacent nucleotides are then linked, through their phosphates and sugars, to form new strands. Two complete new molecules are thus formed from one old one,...

A gene mutation is a change in the sequence of  nucleotides that may result in an altered polypeptide . A mutation is a random, unpredictable change in the DNA in a cell. It may be: • a change in the sequence of bases in one part of a DNA molecule • an addition of extra DNA to a chromosome or a loss of ONA from it • a change in the total number of chromosomes in a cell. Mutations are most likely to occur during DNA replication, for example when a 'wrong' base may slot into position in the new strand being built. Almost all of these mistakes are immediately repaired by enzymes, but some may persist. Single point mutation.  A change in the sequence of bases in DNA may result in a change in the sequence of amino acids in a protein. (Note that this does not always happen, because there is more than one triplet that codes for each amino acid, so a change in a triplet may not change the amino acid that is coded for.) This in turn may result in a change in the 3-D structure of the...

The genetic code specifies the amino acids that are assembled to make  polypeptides . The way that DNA codes for polypeptides is central to our understanding of how cells and organisms function. A polypeptide is coded for by a gene and that a  gene is a sequence of nucleotides that forms part of a DNA  molecule. The sequence of bases in a DNA molecule is a code that determines the sequence in which amino acids are linked together when making a protein molecule. A sequence of DNA nucleotldes that codes for 1 polypeptide, or for 1protein, is known as a gene . The sequence of amino acids in a protein - its primary structure determines its 3-dimensional shape and therefore its properties and functions. For example, the primary structure of an enzyme determines the shape of its active site, and therefore the substrate with which it can bind. A series of 3 bases in a DNA molecule, called a base triplet, codes for 1 amino acid. The DNA strand that is ...