topic 2 Flashcards
Describe the structure of the cell membrane (6 marks)
The cell membrane is composed of a phospholipid bilayer. The hydrophilic phosphate head orientates towards the aqueous environment (cytoplasm and tissue fluid) and the hydrophobic fatty acid tails orientate away from the aqueous environment. The phospholipid bilayer has proteins embedded in it. These can be carrier proteins, channel proteins. There are also glycoproteins and glycolipids attached to the surface. These are involved in the recognition of other cells. The cell membrane also has cholesterol, which contributes to the fluidity of the membrane.
osmosis
diffusion of water molecules across a partically permeable membrane down conc gradient, they have a net movement.
facilitated diffusion
aided by channel and carrier proteins
large molecules or charged molecules will move across the membrane through channel or carrier proteins down the conc gradient and does not require energy
active transport
theis moves molecules and ions across the membrane and it requires energy and moves against the conc gradient.
Describe the structure of DNA (6 marks)
DNA is a double stranded molecule arranged into a double helix. The strands are antiparallel and are composed of many mononucleotides to form two polynucleotide strands. Each mono nucleotide is composed of a phosphate group, a pentose sugar called deoxyribose and one of four nitrogenous bases (Adenine, Thymine, Guanine and Cytosine). Each mononucleotide is joined together by a phosphodiester bond between the phosphate group of one mononucleotide and the deoxyribose of another. The two strands are held together as a result of hydrogen bonding between the bases. This is known as complementary base pairing. Adenine will only bond with Thymine and Guanine will only bond with Cytosine.
Describe the structure of RNA (4 marks)
RNA is a single stranded polynucleotide made of many RNA mononucleotides joined together by phosphodiester bonds. Each RNA mononucleotide is composed of a phosphate, a pentose sugar called ribose and one of four nitrogenous bases. The nitrogenous bases are; Adenine, Uracil, Guanine and Cytosine.
Outline the process of DNA replication (5 marks)
The DNA double helix unwinds and the hydrogen bonds between the bases break. The strands separate. An enzyme called DNA helicase is involved in this process. Free DNA mononucleotides enter the nucleus through the nuclear pores and align themselves opposite their complementary base. Hydrogen bonds form between the bases and phosphodiester bonds form between adjacent mononucleotides (the phosphate group on one mononucleotide and the deoxyribose sugar on another. The enzyme DNA polymerase is involved in the formation of the phosphodiester bonds. The two strands rewind into a double helix. DNA replication is semi-conservative.
Compare and contrast the structure of DNA with RNA (5 marks)
DNA and RNA are both classified as nucleic acids and are both polynucleotides however DNA is double stranded and RNA is single stranded. DNA and RNA are both composed of mononucleotides joined by phosphodiester bonds containing a phosphate, a pentose sugar and one of four nitrogenous bases; however DNA contains the sugar deoxyribose and RNA contains the sugar ribose. DNA also contains the bases Adenine, Thymine, Guanine and Cytosine however RNA contains the base Uracil in place of Thymine.
Describe the process of mRNA production in the nucleus of a cell ( 6 marks)
The DNA double helix unwinds at the site of the gene that is to be transcribed and the strands separate. The enzyme DNA helicase is responsible for this. Free RNA mononucleotides align themselves opposite their complementary DNA base (Adenine to Uracil, Guanine to Cytosine) and hydrogen bonds form between the bases. Phosphodiester bonds form between the adjacent RNA mononucleotides between the phosphate of one mononucleotide and the ribose sugar of another. The enzyme RNA polymerase is responsible for this. The mRNA then detaches itself from the DNA strand and undergoes a process known as mRNA splicing, where the non-coding regions known as introns leaving the coding regions, the exons to form a continuous coding sequence. This is known as post transcriptional modification. The mRNA then leaves the nucleus through the nuclear pores.
Describe the process of protein synthesis that occurs outside of the nucleus ( 6 marks)
The mRNA attaches to a ribosome in the cytoplasm. tRNA, which contains an anticodon and an amino acid binding site carries specific amino acids to the mRNA at the ribosome. Hydrogen bonds form between the codon on the mRNA and the anticodon on the tRNA. The tRNA molecules are held in place two at a time to allow a peptide bond to form between adjacent amino acids. The enzyme peptidyl transferase forms the peptide bond. The tRNA then detaches and goes to pick up another amino acid. This process is known as translation and is terminated when a stop codon is read. The polypeptide detaches itself and is now said to be a protein in its primary structure.
Outline the process of protein synthesis ( 6 marks)
The double helix unwinds and the strand separate. The enzyme DNA helicase is involved in this process. Free DNA mononucleotides enter the nucleus and align themselves opposite their complementary base. Hydrogen bonds are formed and then the enzyme DNA polymerase forms phosphodiester bonds between adjacent mononucleotides. The mRNA then detaches and leaves the nucleus through the nuclear pores. It attaches to a ribosome and translation occurs. tRNA transports specific amino acids to the mRNA strand and hydrogen bonds form between the codons on the mRNA and the anticodons on the tRNA. Peptide bonds form between adjacent amino acids. Translation ends when a stop codon is read and the polypeptide is released.
Compare and contrast DNA replication and transcription (5 marks)
DNA replication and transcription both involve mononucleotides. However DNA mononucleotides are used in replication and RNA mononucleotides are used in transcription. DNA replication uses the enzyme DNA polymerase whereas transcription uses the enzyme RNA polymerase. DNA is semiconservative and produces a double stranded DNA double helix whereas transcription is not semiconservative and produces a single strand of mRNA. During DNA replication, all of the double helix unwinds and both DNA strands are copied whereas during transcription the DNA only unwinds at the site of the gene to be copied (transcribed) and only one of the strands acts as a template.
Describe how the primary structure of a protein determines the tertiary structure of a protein (3 marks)
The primary structure of a protein is the linear sequence of amino acids joined by peptide bonds in the polypeptide chain. The position of the amino acids and the R group on the amino acid will determine the interactions between the amino acids e.g. hydrogen bonds, disulphide bonds, ionic bonds. This will then determine the degree of folding in the tertiary structure.
Compare and contrast fibrous and globular proteins
Both fibrous and globular proteins are composed of chains of amino acids joined by peptide bonds; however, fibrous proteins have a structural role and globular proteins have a metabolic role. Fibrous proteins have a repetitive sequence of amino acids in their polypeptide chain whereas globular proteins have an irregular sequence of amino acids in their polypeptide chain. Globular proteins are soluble as they have a high proportion of polar R groups (or Hydrophilic R groups) on the outside whereas fibrous proteins are insoluble as they have a high proportion of non-polar R (Hydrophobic R groups) on the outside. Both fibrous and globular proteins have hydrogen bonds and disulphide bonds holding their tertiary structure together. Fibrous proteins are in long parallel strands and globular proteins are compact and spherical. Globular proteins have tertiary or quaternary structure whereas fibrous proteins have little or no tertiary structure
Compare and contrast the MOLECULAR structure of fibrous and globular proteins
Both fibrous and globular proteins are composed of chains of amino acids joined by peptide bonds. Fibrous proteins have a repetitive sequence of amino acids in their polypeptide chain whereas globular proteins have an irregular sequence of amino acids in their polypeptide chain. Globular proteins are soluble as they have a high proportion of polar R groups (or Hydrophilic R groups) on the outside whereas fibrous proteins are insoluble as they have a high proportion of non-polar R (Hydrophobic R groups) on the outside. Both fibrous and globular proteins have hydrogen bonds and disulphide bonds holding their tertiary structure together. Fibrous proteins are in long parallel strands and globular proteins are compact and spherical. Globular proteins have tertiary or quaternary structure whereas fibrous proteins have little or no tertiary structure
