Role of haemoglobin in carrying oxygen
- Haemoglobin consists of four sub-units. Each subunit contains a polypeptide chain and a haem group with 1 iron ion (Fe2+). Because Fe2+ attracts oxygen, the haemoglobin has an affinity for oxygen. A molecule of haemoglobin (and so an erythrocyte) can hold 4 molecules of oxygen.
- Haemoglobin takes up oxygen in a way that produces an s-shaped curve called the oxygen dissociation curve.
- At low oxygen tensions, it is difficult for an oxygen molecule to reach the haem group because it is in the centre of the blood cell.
- However, when the oxygen tension rises so does the diffusion gradient. When one molecule of oxygen has associated with a haem group a 'conformational change' in shape occurs so the 2nd and 3rd molecule can associate easier.
- But once 3 oxygen molecules are in the haemoglobin it is difficult for a fourth to associate, even at high oxygen pressures. This makes it difficult for the haemoglobin to achieve 100% saturation, so the curve levels off again, producing an s-shaped curve.
Describe the role of haemoglobin in carrying CO2
- Carbon dioxide is transported from respiring tissues to the lungs by the blood.
- 5% is dissolved in blood plasma.
- 10% is combined with haemoglobin to form carbaminohaemoglobin.
- 85% is transported as hydrogencarbonate ions.
- Carbon dioxide diffuses into the blood, some into erythrocytes. Combined with water it forms carbamino acid, which is catalysed by carbonic anhydrase.
- CO2 + H2O --> H2CO3
- The carbonic acid dissociates to release H+ ions and hydrogencarbonate ions (HCO3-).
- H2CO3 --> HCO3- + H+
- The hydrogencarbonate ions diffuse from the erythrocytes into the plasma. Charge in the cell is maintained by the chloride shift (movement of chloride ions into the erythroyctes from the plasma).
- Haemoglobin acts as a buffer (compound that maintains a constant pH) by taking up acidic H+ ions to form haemoglobinic acid.
Fetal and adult haemoglobin significance to oxygen
- Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.
- This is because it needs to 'pick up' oxygen from an environment that makes adult haemoglobin release oxygen, such as the placenta.
- Fetal haemoglobin absorbs oxygen from the mother's blood fluid which reduces oxygen tension and so causes maternal haemoglobin to release oxygen.
- The oxyhaemoglobin dissociation curve for fetal haemoglobin is to the left of the curve for adult haemoglobin.
How tissue fluid is formed from plasma
- At the arteriolar end of a capillary, hydrostatic pressure due to the contractions of the heart is high.
- This pushes blood fluid through tiny gaps in the permeable capillary walls.
- Erythrocytes, platelets and the majority of white blood cells are too large to leave this way.
- The fluid that does manage to leave is made of plasma with dissolved nutrients and oxygen. We call this tissue fluid.
Describe and explain the bohr effect
- Respiring tissues such as contracting muscles produce more carbon dioxide.
- More H+ ions will be produced in the erythrocytes.
- This means more oxygen will be released from oxyhaemoglobin into the respiring tissues.
- So when more carbon dioxide is present, the oxyhaemoglobin dissociation curve shifts downwards and to the left because haemoglobin is less saturated with oxygen.
- This is called the bohr effect.
Production and secretion of protein
- The gene containing the instructions for the production of the protein is copied onto a piece of mRNA.
- The mRNA leaves the nucleus through a nuclear pore.
- The mRNA attaches to a ribosome.
- The ribosome reads the instruction to assemble the protein.
- The assembled molecules are ‘pinched off’ in vesicles and travel to the golgi apparatus.
- The vesicle fuses with the golgi apparatus.
- The golgi apparatus modifies the molecule and packages it, ready for secretion.
- The molecules are ‘pinched off’ from the golgi apparatus and travel towards the cell surface membrane.
- Exocytosis (vesicle fuses with cell surface membrane which opens to release molecules outside the cell).
Compared ultrastructure of plant & animal cells
- Plant cells have a cellulose cell wall outside the cell surface membrane.
- The cellulose forms a sieve-like network of strands which make the cell wall strong.
- This is kept rigid by the pressure of the fluid inside the cell, so supports the cell and therefore the entire plant.
- Plant cells also contain a vacuole which maintains cell stability through making the cell turgid by increasing the pressure inside the cell.
- This in turn helps support the plant.
Role of membranes
- Separates organelles from the cytoplasm.
- Holds the contents of some metabolic pathways in place.
- Nuclear pores provide exit point for mRNA.
- Mitochondria have folded double membrane (cristae) where aerobic respiration takes place.
- Regulates the transport of materials into and out of the cell.
- Cell recognition and signalling.
Components of phospholipid bilayer
- Phospholipids: Hydrophilic phosphate head and hydrophobic fatty acid tail. Fluid, so components can move around freely. Permeable to small and/or non-polar molecules, impermeable to large molecules and ions.
- Cholesterol: Gives membranes mechanical stability by sitting in the gaps between fatty acid tails and preventing water molecules and ions from passing through the membrane.
- Glycolipids: Carbohydrate part attached to phospholipids. Used in cell signalling, cell surface antigens and cell adhesion.
- Proteins: Channel proteins allow the diffusion of some substances, i.e. the large molecule sugar, in and out of the cell when they can’t travel directly through the membrane. Carrier proteins use ATP powered active transport to move substances across the plasma membrane.
- Glycoproteins: See glycolipids, but attached to proteins.
Membrane-bound receptors, drugs and hormones
- Hormone Target Cells have a receptor which is complementary to it’s specific hormone. This allows the hormone to bind to the receptor site and trigger the desired internal response. Hormones are used in cell signalling.
- Drugs which bind to receptor molecules on cells have also been developed. Beta-blockers are used to prevent a muscle from increasing the heart rate to a dangerous level, and some drugs used to treat schizophrenia mimic a natural neurotransmitter which some individuals cannot produce.
Passive & active transport; endo & exocytosis
- Passive transport: The transport of a molecule without using ATP. Diffusion is the net movement of molecules from a region of high concentration of the molecule to an area of lower concentration of the molecule down a concentration gradient. It is a passive process. But large and charged molecules are unable to use simple diffusion to cross the phospholipid bilayer. They use the proteins in the bilayer to cross, which is called facilitated diffusion. Channel proteins are shaped to allow only one molecule through and are often gated. They transport substances like calcium and sodium ions. Carrier proteins, whose shape can fit a specific molecule, change shape to allow molecules such as glucose or amino acids through to the other side of the membrane.
- Active transport: This is the process used to move larger molecules and ions across the membrane. It uses ATP energy to drive ‘protein pumps’ within the membrane.
- Endocytosis: The process of taking materials into a cell by surrounding them with part of the plasma membrane, which then ‘pinches off’ to form a vesicle inside the cell. This is an active process and requires ATP. These materials often travel in bulk.
- Exocytosis: The process of removing materials from a cell by fusing vesicles containing the material with the plasma membrane. These materials often travel in bulk.
Effects of water potential
Type of cell
Solution of high Ψ
Solution of low Ψ
Water moves in, cell is haemolysed (bursts)
Water moves out, cell is crenated
Water moves in, cell is turgid
Water moves out, cell is plasmolysed
Functions of organelles
Houses genetic material as DNA, which contains instructions for protein synthesis
Makes ribosomes and RNA which pass into the cytoplasm as part of protein synthesis
Double membrane with nuclear pores to allow entry and exit from the nucleus
Rough Endoplasmic Reticulum
Transports proteins made by the attached ribosomes
Smooth Endoplasmic Reticulum
Involved in lipid production
Modifies and packages proteins
Site of protein synthesis
Site of ATP production
Contain digestive enzymes to break down materials
Site of photosynthesis in plants
Controls the entry and exit of substances in and out of a cell
Forms spindle during cell division which moves chromosomes
Undulipodia and Cilia
Move with ATP energy: sperm ‘tails’ for movement and waft mucus along trachea
Structure of organelles
Spherical structure inside nucleus
Surrounds the nucleus
Rough Endoplasmic Reticulum
Studded with ribosomes
Smooth Endoplasmic Reticulum
Like R.E.R. without ribosomes
Stack of membrane bound, flattened sacs
Small, seen on R.E.R. and free in cytoplasm
Roundish, double membrane
Spherical, single membrane
Only in plants, thylakoids
Tubes of protein fibres, pairs in animal cells n
Flagella and Cilia
Hairlike projections on cell surface
- Interphase (pre-mitosis): DNA replicates.
- Prophase: Chromosomes supercoil (shorten and thicken) and become visible under a light microscope. The nuclear envelope breaks down. The centriole divides in two and moves to polar ends of the cell to form the spindle.
- Metaphase: The chromosomes line up along the centre of the cell (spindle equator), attached to the spindle by their centromere.
- Anaphase: The spindle fibres contract, splitting the centromere of the replicated sister chromatids and pulling them apart to polar ends of the cell.
- Telophase: A new nuclear envelope forms around each set of sister chromatids as they reach opposite ends of the cell. The spindle breaks down. The chromosomes uncoil so they are no longer visible under a light microscope.
- Cytokinesis (post-mitosis): The cytoplasm splits to form two new genetically identical cells.