Standard answers Flashcards
ADAPTATIONS OF FISH GILLS
e.g Explain how fish gills are adapted for efficient gas exchange
• 5 points to mention
•Many filaments, covered in many lamellae which increase SA
•Thin epithelium between water and blood, so short diffusion pathway
•Countercurrent flow, allows diffusion along the whole length of the gill by maintaining concentration gradient (blood is always next to water with higher concentration of oxygen
•Large number of capillaries, circulation removes oxygen, maintaining conc gradient
•Pressure changes in ventilation bring in more water to maintain concentration gradient
ADAPTATIONS OF LUNGS
e.g Describe and explain how structure of mammalian breathing systems enables efficient uptake of oxygen
•6 points to mention
•Many alveoli with folded walls to provide a large SA
• Walls of the alveoli are thin, providing short diffusion pathway between alveoli and blood
• Walls of capillaries are thin and have flattened cells, providing a short diffusion pathway
•Many blood capillaries provide large SA
•Cell membrane is permeable to gases
•Intercostal muscles and diaphragm ventilate 🫁, maintains diffusion gradient
STRUCTURE OF ARTERIES
eg. explain 5 ways in which the structure of the aorta/walls of arteries are related to their function
•Elastic tissue to allow stretching when heart beats/recoils when the heart relaxes >smooths out flow of blood, maintaining pressure
•Elastic tissue stretches when ventricles contract/ recoils when ventricles relax
• Muscles that contract, reducing diameter of the lumen > changes flow of blood/pressure
•Thick walls withstand pressure OR stops bursting
•Smooth endothelium reduces friction
AORTA ONLY: aortic valve prevents backflow of blood
STRUCTURE OF CAPILLARIES
eg. Explain how the structure of capillaries relates to it’s function (7 points to mention)
• They have a permeable capillary wall/membrane
•Thin walls only a single cell thick, reducing diffusion distance
• Flattened endothelial cells reduces diffusion distance
• Gaps between cells (fenestrations) that allow large molecules through
•Small diameter, gives a large SA:V ratio/ short diffusion distance
•Narrow lumen, reduces flow rate allowing more time for diffusion to take place
•Red blood cells in contact with the wall, giving short diffusion distance/ more time for diffusion to take place
DESCRIBE THE COHESION-TENSION THEORY IN THE XYLEM
(5 marks)
•Water is lost from leaf in transpiration, diffusing through stomata in the leaves
•The loss of water in evaporation lowers water potential in the mesophyll
• Water moves out of the xylem into mesophyll down water potential gradient, lowering water potential at the top of the xylem
• Water is pulled up the xylem, creating tension at the top of the xylem
• Water molecules are cohesive, they attract to each other by hydrogen bonds
•This creates a continuous column of water
• The water molecules are also adhesive to the walls of the xylem
FORMATION OF TISSUE FLUID
e.g explain how tissue fluid is formed and how it returns to the circulatory system
• At artériole end of capillary, hydrostatic pressure is high (due to contraction of LV)
• As hydrostatic pressure in the blood is higher than osmotic pressure from the tissues, fluid is forced out of capillary through fenestrations
• Proteins and other large molecules remain in the blood
• These proteins lower water potential in the capillaries
• As water potential at the capillaries is lower than at the tissues, water moves back into capillary at the venule end by osmosis
• Lymph system collects any excess tissue fluid which returns to the blood at the subclavian veins
STRUCTURE OF DNA & THE CHROMOSOME e.g describe the structure of DNA and a chromosome (7 points)
•DNA is a polymer of nucleotides
• A nucleotide consists of deoxyribose sugar, a phosphate and an organic base
•The nucleotides are joined together with phosphodiester bonds
•The two strands of DNA are joined together with hydrogen bonds
•Hydrogen bonds form between adenine and thymine and between cytosine and guanine
•DNA is associated with histones this twists to form the chromosome
•When visible in mitosis, the chromosome consists of two chromatids joined at the centromere
TRANSCRIPTION e.g describe how mRNA is produced in a eukaryotic cell (5 marks)
•Hydrogen bonds between the DNA bases are broken
• One of the DNA strands acts as a template
•Free RNA nucleotides align to complementary base pairs. A in the DNA strand pairs with U on the RNA nucleotide, T with A, C with G, and G with C. Uracil is used in place of Thymine in RNA
• RNA nucleotides are joined by RNA polymerase
•This forms pre-mRNA
•Pre-mRNA is then spliced to remove introns, forming mRNA
TRANSLATION
eg. describe how a polypeptide is formed from mRNA (6 marks)
•mRNA attaches to ribosome; the ribosome finds the start codon
• tRNA anticodon binds to the complementary mRNA codon
•tRNA brings a specific amino acid to the ribosome; the ribosome fits around two codons so that the two amino acids can be joined
• Amino acids are joined by peptide bonds
•Amino acids join together with the use of ATP
•tRNA is then released from the ribosome
• the ribosome moves along the mRNA repeating this process to form the polypeptide
MEIOSIS
eg. describe what happens to the chromosomes in meiosis (6 marks)
• chromosomes shorten/thicken/condense
• chromosomes associate in homologous pairs
• the homologous chromosomes twist together with sections crossing over
• chromosomes join to spindle fibres via the centromere at the cell equator
•homologous pairs move to opposite poles
•Pairs of chromatids separate in second division
