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Gas exchange, Cell recognition, Variation, Digestion and absorption,…
Gas exchange
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Tissue fluid
Partial Pressure:
- The partial pressure of a gas is a measure of concentration of that gas in a mixture of gases (e.g air) or in a liquid (e.g water).
- It is based on how much pressure that gas contributes to the overall pressure exerted by the mixture of gases.
- These substances must diffuse between the organism and the surroundings.
- The rate at which a substance can diffuse is given by Fick's law:
- Rate of diffusion ∝ (surface area * concentration difference) / diffusion distance.
- Tissue fluid is formed by blood plasma (carrying dissolved substances) leaking from the capillaries.
- It surrounds the cells in the tissue and provides them with oxygen and nutrients.
- The movement is by mass flow (the movement of fluids down a pressure gradient).
- There is a net outflow of plasma from the arterial end of capillaries due to higher blood pressure which creates high hydrostatic pressure.
- Some tissue fluid is eventually reabsorbed. Waste products from cell metabolism (e.g CO2 and urea) returned to the capillaries with the tissue fluid.
- Not all fluid passes back into the capillaries. The excess is drained into vessels of the lymphatic
system - this fluid is now known as lymph.
- Lymph is a colourless/pale yellow fluid like tissue fluid but containing more lipids. Lymph passes through the lymphatic system and drains back into the blood via the vena cava.
1) At the arteriole end of the capillary bed, the hydrostatic (liquid) pressure inside the capillaries is greater than in the tissue
fluid.
2) This difference in hydrostatic pressure forces fluid containing small molecules (nutrients and oxygen) out of the blood through tiny gaps between the cells in the capillary walls. This forms tissue fluid.
3) Red blood cells, platelets and plasma proteins remain in the blood as they are too large to be pushed out through the capillary walls.
4) Exchange then occurs between tissue fluid and cells by diffusion, facilitated diffusion and active transport. Oxygen and nutrients enter the cells and carbon dioxide and other wastes (e.g urea) leave the cell and enter the tissue fluid.
5) As fluid leaves the capillaries at the arteriole end it reduces the hydrostatic pressure. This means the blood pressure at the venous end of the capillaries is much lower.
6) As water leaves the capillary but the plasma proteins can’t leave this lowers the water potential of the blood. Therefore, the water potential in the capillaries is lower than in the tissue fluid at the venule so the water moves by osmosis into the capillaries carrying carbon dioxide and other waste substances.
7) Any excess tissue fluid that is not reabsorbed is returned is collected into the lymphatic system which returns it to the circulatory system.
- Lymph contains lymphocytes (a type of white blood cell) which are part of the immune system and help to filter out foreign material from the lymph
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Effects of lung disease
- Ventilation rate (the volume of air
entering the lungs in a minute) and tidal volume (the volume of air in each breath) can tell you about how the lungs are functioning.
- There is always a certain volume of
air that remains in the lungs to make sure they never fully deflate (residual volume).
- Breathing rate is the number of breaths per minute
- You can measure lung function using a spirometer and work out breathing rate (number of breaths per minute), tidal volume and ventilation rate from a spirometer trace:
- Tidal volume: The volume of air in each breath.
- Ventilation rate: The number of breaths per minute. Usually 15 for a healthy person resting.
- Forced vital capacity (FVC): The maximum volume of air it is possible to breathe forcefully out of the lungs after a really deep breath in.
- Forced expiratory volume (FEV): The maximum volume of air that can be breathed out in one second.
- Lung disease can affect both ventilation and gas exchange. All lung diseases reduce the rate of gas exchange in alveoli.
- Less oxygen diffuses into the bloodstream, the body cells receive less oxygen which reduces
the rate of aerobic respiration is reduced.
- This means less energy is released,
so lung disease patients offer suffer with tiredness or weakness in muscles.
Types of lung disease:
- Airway disease --> affects the body's ability to move air in and out of the lungs --> Includes asthma, COPD, bronchitis, emphysema.
- Lung tissue disease --> damaged tissue from scarring or injury --> Includes pulmonary fibrosis, sarcoidosis.
- Lung circulation disease --> affects the circulation of blood to and from the lungs. --> Includes pulmonary hypertension, pulmonary edema.
- There are two types of lung disease which affect ventilation in different ways:
- Restrictive diseases:
- E.g Fibrosis
- make it difficult to fully breathe in (affects elastic tissue).
- Severely reduces FVC as breathing in is difficult but FEV1 is less affected because breathing out is still normal.
- Obstructive diseases
- E.g Asthma
- make it difficult to breathe out as airways are blocked.
- FVC and FEV1 are both much lower than normal.
Gas exchange in humans
Structure:
- In mammals gas exchange takes place in the lungs, these are highly adapted to have a very large surface area in contact with the bloodstream to sustain a high rate of gas exchange required to maintain a high metabolic rate.
Inspiration:
- External intercostal and diaphragm muscles contract 🡨 requires energy
- Rib cage moves upwards and outwards
- Diaphragm flattens
- Increased volume of the thoracic cavity
- Lung pressure decreases to below atmospheric pressure
- Air flows from a higher pressure to a lower pressure (pressure gradient)
- Air flows into the lungs.
Expiration:
- External intercostal and diaphragm muscles relax. (no energy needed).
- Rib cage moves downwards and inwards
- Diagram moves upwards
- Decreased volume of the thoracic cavity
- Air pressure increases to above atmospheric pressure.
- Air is forced down the pressure gradient and out of the lungs.
Forced expiration:
- External intercostal muscles relax
- Internal intercostal muscles contract (Antagonistic)
- Pulls the ribcage further down and in.
Alveoli:
- Surrounded by a network of capillaries.
- The wall of each alveolus is made from a single layer of think, flat cells called ‘alveolar epithelium’.
- The walls contain a protein ‘elastin’ – which helps
them recoil to their normal shape.
Gas exchange:
- Oxygen diffuses from alveoli, across the alveolar epithelium and capillary endothelium and into haemoglobin.
Factors affecting rate of diffusion:
- Thin exchange surface.
- Large surface area
- Steep concentration gradient maintained by
constant blood flow and ventilation
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Cell recognition
The Immune Response
Antigens:
- All cells have specific proteins on their surface membrane that identify it called antigens (like chemical markers).
- An antigen is a protein found on the outside of cells.
- Each antigen has its own unique shape.
- The more closely related two individuals are, the more antigens they have in common.
- Your body recognises the antigens on your cells as your own (self)
- Anything with different antigens (non-self) stimulates an immune response.
- Antigens enable the immune system to identify:
- pathogens (micro-organisms that cause disease)
- toxins (poisonous molecules often released by bacteria – the toxin is the antigen)
- abnormal body cells (e.g cancerous cells or pathogen infected cells)
- cells from other organisms of the same species (e.g in transplanted organs)
Organ transplant:
- Those cells will have some antigens that are different to your own.
- The foreign antigens trigger an immune response where they are rejected.
- Drugs can be taken to suppress a recipients immune system.
Blood transfusion:
- Most important antigens are the ABO blood group antigens
- Anything not recognised by the recipient immune system will trigger a response.
Main stages of immune response:
- Non-specific immune response
- Specific immune response:
- T-cells: TH cells, TC cells
- B-cells: plasma cells
- Antibody production
Phagocytosis (Neutrophils and Macrophages):
- Definition: cellular process of engulfing solid particles using the cell membrane - carried out by phagocytes.
- Part of the non-specific immune response
Phagocytes:
- Made in the bone marrow, travel in capillaries but can squeeze through walls into tissues.
- 2 types: neutrophils and macrophages
- Patrol the body, searching for invaders (non-self antigens)
- Neutrophils: Engulf and digest pathogens (and dead human cells/debris)
- Macrophages: Can punch holes in the bacteria or stick proteins to the outside of the bacteria to make them more appealing for the neutrophils to destroy.
- Endocytosis – infolding of the membrane to create an internal vesicle
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Immunity & Vaccines
- Chromosome: Condensed mass of DNA which appears just before cell division. Each chromosome is one super-coiled DNA molecule containing 1000s of genes.
- Chromatid: One half of a double chromosome (present at the start of cell division)
- Centromere: The point of attachment of two chromatids on a double chromosome.
- Centriole: Bundles of protein fibres, found near the nucleus. Centrioles migrate to opposite poles of the cell during mitosis and form the spindle.
- Cytokinesis: Division of the cytoplasm to produce two new cells.
Active Immunity:
- A type of immunity you get when your immune system makes its own antibodies after being stimulated by an antigen.
- Natural – when you become immune after catching a disease
- Artificial – when you become immune after being given a vaccination
- Requires exposure to antigen
- Takes a while for protection to develop.
- Memory cells are produced.
- Protection is long term because the antibody is produced (after activation of memory cells) in response to complementary antigen being present in the body
Passive Immunity:
- A type of immunity you get from being given antibodies made by a different organism – your immune system doesn’t produce any antibodies of its own…
- Natural – a baby becomes immune due to the antibodies it receives from its mother, through the placenta and breast milk.
- Artificial – when your body become immune after being injected with antibodies from someone else. E.g. blood donations against the tetanus toxin.
- Doesn't require exposure to antigen.
- Protection is immediate.
- Memory cells aren't produced.
- Protection is short-term because the antibodies given are broken down.
Vaccination:
- Vaccination is the introduction of a dead or attenuated foreign organism (antigens) into the body in order to stimulate an immune response by stimulating antibody production.
- Due to the production of memory cells, it is long lasting.
- It means that you do not have to suffer the disease whilst your B-cells are building up their number on your first exposure to the pathogen.
How vaccines work:
- One dose induces a primary immune response.
- Multiple doses can increase the number of antibodies and memory cells in the bloodstream through the secondary response
- Vaccines are long lasting because they produce memory cells which can produce complimentary antibodies to the antigen in the future.
Preparing Vaccines:
- Pathogens prepared for viruses are made harmless by:
- Killing but leaving antigens unaffected e.g. Cholera vaccine
- Weakening (attenuation - heating) but leaving antigens unaffected e.g.oral vaccine against polio
- Purified antigens removed from pathogen e.g. vaccine against hepatitis B
- Using inactivated toxins called toxoids that are harmless but trigger same immune response e.g Tetanus injection
Herd Immunity:
- ‘Herd immunity’ is a concept used for vaccination, in which a population can be protected from a certain virus if a threshold of vaccination is reached.
- Herd immunity is achieved by vaccinating a large amount of the population so that enough people are immune to the virus it cannot spread far through the population. This protects those who cannot have the vaccine for medical reasons.
Ethical Issues with Vaccines:
- All vaccines are tested on animals first (same as all drugs)
- Humans in clinical trials may put themselves at risk because they believe they may be “immune”.
- Some people refuse vaccines over fears of side effects, they are protected by herd immunity in the same way that people who can’t get the vaccine.
- If there was a new disease, difficult decisions would be made about who would be the first to receive it 🡪 we are living this now!
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HIV and Viruses
Cholesterol (lipid):
- Maintains membrane shape and stability
- Cholesterol fits between the phospholipids
- It binds to the hydrophobic tails of the phospholipids packing them closely together
- This restricts the movement of the phospholipids
making it less fluid/more rigid at higher temperatures.
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Antibodies in Medicine
- Antibodies are proteins which are specific to an antigen as they have a complementary shape.
- Antibodies can bind to antigens to form an antigen-antibody complex.
- An antigen is a protein found on the outside of cells, that triggers an immune response.
Antibodies Work In Three Ways:
- Agglutination.
- Neutralising Toxins
- Preventing Viruses from Entering Host Cells
Agglutination:
- Antibodies cause microbes to clump together
- This makes it easier for phagocytes to engulf more of them at once
Neutralising toxins:
- Antibodies can neutralise toxins
- Some pathogens make us ill by producing toxins
- Some antibodies work by neutralising these toxins
Preventing Viruses from Entering Host Cells:
- Viruses have proteins on their surface which recognise and bind to receptors on the surface of the host cell
- This is how many viruses enter their host cell
- Antibodies can bind to viruses and stop them attaching to their host cells
What are monoclonal antibodies?:
- Monoclonal antibodies are antibodies produced from a single group of genetically identical plasma cells (B cells).
- They are all identical in structure – including the variable region so they only bind with a single antigen that has a complementary shape.
- We can make monoclonal antibodies that can bind to any substance we want!
- Monoclonal antibodies are identical copies of one type of antibody produced in a laboratory.
1) A mouse is injected with a pathogen
2) Single group of genetically identical B-cells (plasma cells) produce antibodies
3) Lymphocytes are removed from the mouse and fused with rapidly dividing mouse tumour cells
4) The new cells are called hybridomas.
5) The hybridomas divide rapidly and release lots of antibodies which are then collected.
- Killer T-cells find and destroy infected cells that have been turned into virus-making factories.
- To do this they need to tell the difference between the infected cells and healthy cells with the help of special molecules called antigens.
- Helper T-cells don’t make toxins or fight invaders
themselves.
- Instead, they are like team coordinators.
- They use chemical messages to give instructions to the other immune system cells.
- These instructions help Killer T-cells
and B-cells make a lot more of themselves so they can fight the infection.
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Variation
Meiosis
Point Mutations:
- Substitution of bases --> a nucleotide is replaced by another, with a different base.
- E.g.:
- Nonsense --> stop codon formed.
- Mis-sense --> a different amino acid is coded for.
- Silent --> substituted base still codes for the same amino acid.
- Addition or deletion of bases --> a nucleotide is gained or lost from the normal DNA sequence.
- Results in a "frame shift".
- If the mutation takes place in the introns or non-coding region there will be no effect.
- Often a non-functional protein will be formed by the mutations on this slide
MITOTIC PHASES:
- Prophase.
- Metaphase.
- Anaphase
- Telophase
Early prophase:
- DNA condenses; chromatin coils tightly to form dense, visible chromosomes.
- DNA has replicated, so that the chromosomes now have their 2 identical chromatids, joined by a centromere in the familiar “X” shape.
Prophase:
- The nuclear envelope disintegrates
and forms vesicles.
- Nucleolus is no longer visible.
- The centrioles move to opposite poles of the cell, and form a spindle of microtubules.
Metaphase:
- Chromosomes attach to the spindle at their centromere.
- Chromosomes are lined up on the equator of the spindle.
- One chromatid from each chromosome is attached, by the microtubules, to each pole
Anaphase:
- The centromeres divide.
- The spindle contracts.
- The separated chromatids move towards their respective poles.
- Poles move further apart
Telophase:
- The daughter chromosomes are now at the poles of the cell.
- The spindle disappears.
- nuclear envelope reforms.
- Chromosomes unravel to form chromatin
Homologous chromosomes:
- In your body cells, you have 46 chromosomes. 23 came originally from your mother and 23 from your father.
- These form matching pairs, one maternal and one paternal chromosome in a pair, with the same genes, but maybe different alleles.
- These are called homologous chromosomes.
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Genetic diversity
What is genetic diversity?:
- The number of different alleles of genes in a species or population.
- Allele frequency is how often a particular allele occurs within a population.
Why is genetic diversity important?:
- If a population has a low genetic diversity, it might not be able to adapt to change in an environment, for example a disease or climate change.
How is genetic diversity in a population increased?:
- Mutations causing new alleles;
- Migration from other populations – this is called
gene flow.
Genetic drift:
- The process whereby an allele becomes more common in a population due to chance.
- Two ways:
- Genetic bottleneck
- Founder effect
Genetic bottlenecks:
- Ecological events may reduce population sizes dramatically e.g. earthquakes, floods, fires.
- Disasters that are unselective.
- Small surviving populations are unlikely to be representative of the original population.
- By chance alleles may be over-represented among survivors, some may be eliminated completely.
The Founder Effect:
- Occasionally a small group of individuals may migrate away or become isolated from a population
- The ‘founding’ population is only made up of a small number of individuals. Inbreeding may be a problem if individuals are closely related.
- It may have a non-representing sample of alleles from the parent population
- The colonizing population may evolve quite differently from the original population, especially if the environment is different
- Certain alleles may go missing all together as a consequence, resulting in a loss of genetic diversity
Similarities between the founder effect & bottlenecking:
- Is followed by genetic drift which results
in changes in allele frequencies
- Genetic diversity is lost
- involve a small number of individuals breeding with each other, both may involve inbreeding among close relatives
- may result in a new population which carries alleles that are unlikely to be a true representation of the original group
Differences between the founder effect & bottlenecking:
- On a bottleneck, individuals are killed, reducing the choice of mates, in the founder effect individuals are ecologically separated
- Locus --> position of a gene on a chromosome
Natural selection
Natural selection:
- In any generation there is variation
- Or variation arises due to mutations
- Selection pressures exist – competition between organisms – eg change in environment, diseases.
- Most “fit” survive to breed – there is “differential
reproductive success”
- And pass their advantageous alleles to their offspring
Adaptation and selection
- Animals need to be adapted to their environment to help them survive the abiotic conditions there, get the food and mates to survive and reproduce.
- Adaptations can generally be categorised into:
- Anatomical / Structural adaptations --> Eg shape or colour of the organism
- Physiological adaptations --> Related to processes inside an organism’s body. Eg antifreeze in cells
- Behavioural adaptations --> The way that an organism acts can increase its chance of survival Eg migration or basking; courtship rituals
- There are overlaps. For example the Flamboyant Cuttlefish.
- It has acids in its skin that make it deadly (physiological). It advertises how inedible it is with this hypnotic display (behavioural)
Digestion and absorption
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Absorption
Comparison of the ways molecules move into and out of cells:
- Name --> type of transport --> direction of movement --> conditions --> examples.
- Diffusion --> passive --> towards lower concentration --> Concentration gradient --> Water, gases (O2and CO2), and steroid hormones.
- Facilitated Diffusion --> passive --> towards lower
concentration --> Concentration gradient, plus channel or carrier proteins --> Water, glucose, and amino acids.
- The ileum = final section of small intestine where nutrients are absorbed into the bloodstream.
- Enzymes are secreted by glands in its walls.
- Inner walls are folded into villi which gives them a large surface area.
- The small intestine has particular features which allow it to maximise the movement of substances through cells into the blood:
- Internal walls are folded into projections called villi (~1mm in length) which give a large surface area
- Villi have thin epithelium (one cell thick) to help keep the diffusion pathway short
- Villi have lots of capillaries to help maintain the concentration gradient by constantly transporting absorbed nutrients away.
- Villi contain muscles and can move which helps them to mix the contents of the ileum so that villi always have new material next to them to absorb nutrients. This also helps to maintain the
concentration gradient.
Absorption of Monosaccharides:
- Monosaccharides – glucose is absorbed by active transport with sodium ions via a co-transporter protein.
1) Sodium ions are actively transported out of the epithelial cells in the ileum, into the blood, by the sodium-potassium pump.
2) This creates a concentration gradient
3) Higher concentration of sodium ions in the lumen of the ileum than inside the cell
4) This causes sodium ions to diffuse from the lumen of the ileum into the epithelial cell, down their concentration gradient
5) Via the sodium glucose co-transporter proteins
6) The co-transporter carries glucose into the cell with the sodium
7) Concentration of glucose inside the cell increases
8) Glucose diffuses out of the cell, into the blood, down its concentration gradient through protein channels by facilitated diffusion.
Absorption of Amino acids:
- Amino acids – Sodium ions are actively transported out of the epithelial cells into the ileum itself.
- They can diffuse back into the cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino
acids with them.
Absorption of Triglycerides:
- Monoglycerides and fatty acids are in association with bile salts, as micelles.
- At the microvillus membrane, the micelle breaks down, releasing the Monoglycerides and fatty acids.
1) Micelles hit epithelial cells and breakdown allowing monoglycerides and fatty acids to diffuse across phospholipid membrane because they are lipid-soluble and non-polar.
2) Monoglycerides and fatty acids are transported to the Endoplasmic Reticulum where they recombine to form triglycerides again.
3) Inside the golgi they bind with cholesterol and proteins make packages of lipoproteins called chylomicrons.
4) Chylomicrons travel in a vesicle to the cell membrane and leave the epithelial cell through exocytosis.
5) Chylomicrons enter lymphatic capillaries called lacteals which transport them away from the small intestine to adipose, cardiac, and skeletal muscle tissue, where the triglycerides can be hydrolysed and fatty acids used by the tissues.
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