Biology Unit 2- First Half

The Variety of cells
1. Eukaryotic cells
  1. These are defined as cells that have a nucleus and other membrane bound organelles.
    1. Nucleus- Controls the activities of the cell and contains all the DNA
      1. Cell surface membrane- control the netry and exit of substances into and out of the cells
        The SER- Transports and produces lipids.
        The RER- Transports the ribosomes made at the ribosomes
        The mitochondria- Carries out aerobic respiration and produces ATP which releases energy for respiration
        Golgi body- transports and chemically modifies substances such as glycoproteins
        Lysosomes- Gets rid of unwantedstructures inside the cell
    2. Plants contains 3 additional structure;
      1. A cell wall:This is made of cellulose( beta glucose) and helps support the cell and stops it bursting from osmosis
        It is permeable and so some things can go through it.
        The space between each cell is called the intercellular space.
        The plasmodesmata is the gap between each cell, it allows cytoplasmic connection between plant cells.
        The biological glue, also known as the middle lamella stick the cell walls together.
      2. Chloroplasts: These are structures found inside the cell. They contain different pigments such as chlorophyll to absorb light of different intensities. The chloroplasts carry out photosynthesis.
        Structure:
        Contains a chloroplast enevelope i.e a double membrane
        Contains lipid droplets
        Contains starch grains
        Contains a solution called stroma
        Contains the thylakoid membrane, which what they collectively called the granum (bunch of membranes) and the lamella (linking membrane)
        Contains circular DNA
        Also contains ribosomes.
      3. Vacuole: Help support the plant and keep the cell turgid
        Contains cell sap
        This is a solution of weak salts and sugars etc.
2. Cell differentiations
  1. This is when cells become specialised to carry out a specific function
    1. A group of similar cells that carry out a specific function are called tissue
      1. A group of DIFFERENT tissue that work together to carry out a specific function is called an organ
        Organs are organised into organ systems e.g. the exchange system and the digestive system
    2. We need to know two:
      1. The epithelial cells
        Are adapted for absorption
        These are found in the small intestine
        They have many mitochondria so that they can carry out respiration and so produce ATP which releases energy for active transport.
        They have microvilli on their surface which increases the surface area of a higher rate of diffusion
      2. The palisade cells
        Are adapted for photosynthesis
        Contains large amounts of chloroplasts
        These contain different pigments to absorb light of different wavelengths.
        The chloroplasts can move within the cell
        The cells are elongated, i.e. they are tall and thin so you can fit more of them at the top of the cell.
        There are thin gaps between them so that CO2 can diffuse to every cell inside the leaf
        Cell wall is thin for a short diffusion pathway
Carbohydrates
1. Starch- made of alpha glucose
  1. This is where the OH is at the bottom
  2. This is a major energy store in plants
    1. There are two types of chain:
      1. Straight: Amylose
      2. Branced: Amylopectin
      3. There are hydrogen bonds within the molecules that holds the structure in its specific shape.
        It's good for storage because:
        It's readily broken down by enzymes
        Amylase
        It's insoluble, so doesnt affect water potential
        It's coiled and compact, so you can fit more within a given volume
2. Glycogen: made of alpha glucose
  1. A major energy store in animals
    1. It consists of highly branched chains
      1. This is to allow rapid enzymic hydrolysis from many ends when needed.
        To glucose
3. Cellulose: made of beta glucose
  1. With the OH at the top
  2. Has more of a structural function
    1. Each glucose molecule is rotated 180* compared to the one next to it
      1. This is called alternate bonding and allows the chain to be produced straight rather than coiled.
        Many of these paralled, straight chains will be bundled together into what's called microfibrils, linked by hydrogen bonds
        These bundles are bundled together into bigger bundles into fibres, and are also linked by hydrogen bonds.
        These fibres are strong, flexible and permeable,
        Cellulose is strong because: There are MANY, LONG, STRAIGHT, PARALLEL, CHAINS, crosslinked by hydrogen bonds into fibres that hold the chain together
  3. Hard to digest, because the enzyme cellulase is rare.
Size and Surface Area
1. Large organisms;
  1. Have a small surface area: volume ratio
  2. Larger organisms have more cells, so requires more oxygen and have a higher rate of respiration.
    1. Therefore they need specialised exchange systems and a transport system in order to transport the oxygen and glucose around the body fast.
2. Small organisms:
  1. Have a large surface area; volume ratio
    1. Very small organisms can exchange gases over their entire body surface for this reason
3. Exchange of heat with the environment:
  1. If you have a larger surface area to volume ratio:
    1. Your're likely to have a high rate of heat loss
      1. To compensate, you have a high rate of respiration because it produces heat as a by-product
        So if you live in a hot environment, it's better to be smaller as you have a large SA:VR and so a high rate of heat loss
        Or better to have a feature that has a large surface area for heat loss e.g. large thin ears
  2. Larger organisms have a low rate of heat loss for this reason, (small SA:VR)
    1. And a low rate of respiration to go with it
      1. Thererefore if you live in a cold environment it's better to be bigger as there's a low rate of heat loss
        Or to have features that have a small surface area e.g. small ears.
4. Conserving water: Any cell exposed directly to air will lose water by evaporation
  1. Therefore most of the body should be covered in a waterproof layer e.g. skin.
    1. This means diffusion can't occur across the whole body surface and a specialised gas exchange system and transport system is required.
Gas Exchange
1. In a single celled organism;
  1. They have a very large surface area: volume ratio
    1. Therefore diffusion can occur over the entire body surface
      1. The gas exchange surface is the CELL SURFACE MEMBRANE
        It is one cell thick, so there is a short diffusion pathway
        Also, there is a low conc of oxygen inside the cell as it is used up immediately
2. In insects;
  1. Gas exchange occurs through the tracheal system:
    1. It consists of:
      1. Spiracle: Small openings found at the side of the insect
        Valves: These open and close the valves
        Air sacs: used for pumping air into and out of the tracheal system
        Trachea: Thick tubes that are held open by rings of chitin which provides strength and support
        Tracheoles: these are smaller branched tubes, not lined with chitin
        These chitin end at the muscle, there is fluid present there as well.
  2. This occurs by:
    1. Oxygen enters through the spiracle.
      1. It goes down the tracheae
        Down to tracheoles
        Diffuses in fluid at the tips of the tracheole and then diffuses into the water.
        This means diffusion takes longer
        CO2 diffuses in the same way but in the opposite direction
  3. Ventilation mechanism:
    1. The spiracles close
      1. The air sacs are squeezed
        This pushes air deeper into the tips of the tracheoles
  4. In very active insects, where the wing muscles are working hard, repsiration is mainly anaerobic.
    1. This causes the build up of lactic acid in the muscle cells.
      1. The water potential becomes more negative
        Water will move from the tips of the tracheoles to the muscle cells by osmosis
        This shortens the diffusion pathway as oxygen diffuses directly from the tracheoles into the muscle
  5. Adaptations to prevent water loss:
    1. They have an exoskeleton: which is covered in a waxy waterproof layer to prevent the diffusion of water.
      1. The valves can close the spiracles
3. In Fish
  1. Structure:
    1. 4 pairs of gills, in the pharynx
      1. These are covered by a flap called the operculum
        Each gill is made up of many finger like projections called filaments
        On each gill filament are many lamella, these are further projections
        These allow a large surface area
        Each gill is attatched to abone called the gill arch
        This contains blood vessels
        Deoxygentaed blood is supplied to each gill to be removed
        Oxygentaed blod is removed from each gill to be taken to the cells
  2. The gas exchange surface is the lamella wall
    1. It consists if a single layer of squamous cells to allow a short diffusion pathway
      1. The capillary wall is also one cell thick
  3. Counter current flow
    1. This where blood and water flow in OPPOSITE directions.
      1. This is in order to maintain a concentration gradient throughout the whole length of the lamella so diffusion can occur across the entire length of the lamella.
        A parallel flow would mean an equilibrium will have been reaxched and no further diffusion can take place.
  4. Ventillation in fish:
    1. Inspiration (Taking water in)
      1. The mouth is open
        The floor of the pharynx is pulled down (muscle contraction)
        The operculum is closed to prevent water escaping
        The volume increases
        Pressure decreases
        Water is brought in from a high pressure to a lower pressure.
    2. Expiration ( Water out)
      1. Mouth closed
        Operculum open
        Floor of the pharynx is pulled up
        Volume decreases
        Pressure increases
        Water forced out through the gills from high pressure to low pressure
4. In plants:
  1. A dicolydenous leaf is one that has broad leaves and a branching network of veins
    1. During the day ypou have both photosynthesis and respiration
      1. At night, it's just respiration
        Gas exchange occurs between the air spaces inside the leaf
        The gas exchange surface is the surface of the mesophyll cells.
        They have a large surface area as there are many air spaces in contact with plenty of air space
        Conc gradient: Gases are used as soon as they enter either for respiration or photosynthesis
        Diffusion pathway: Short as gases need only go through a cell wall and cell membrane
    2. Structure:
      1. Cuticle: waxy layer prevents water loss
        Palisade cells: Main region for photosynthesis
        Spongy mesophyll: air spaces for diffusion of gases
        Stoma: Pores on the lower surface. Allows the diffusion of gases into and out the leaf
        Gurad cells: Open and close stomata
Mass Transport:
1. Through large distances, an efficient supply of substances is maintained by mass transport
  1. Mass transport is defined as the bulk movement of substances from an area of high pressure to an area of low pressure
2. Blood vessels
  1. Arteries: Take blood Away from the heart
  2. Arterioles: Smaller arteries
  3. Veins: Take blood back to the heart
  4. Venules: Smaller veins
  5. Hepatic: Of the liver
    1. The hepatic portal vein is what links the small interstine to the liver.
      1. Its contents vary depending on what's been absorbed by the small intestine
  6. Renal: Of the kidneys
  7. Pulmonary: Of the lungs
  8. Veins, venules, arteries, arterioles all the same basic structure:
    1. 1. Endothelium: This is the smooth lining next to the blood which is there to reduce friction
      1. 2. Middle layer: Contains elastic tissue and smooth muscle
        3. Outer layer: Consists of a tough fibrous protein called collagen
  9. Capillaries: Allow exchange of material
  10. Arteries:
    1. These have the thickest walls in able to withstand high pressures without bursting.
      1. They have the most elastic tissue in the middle layer.
        This enable the arteries to dilate when the ventricles contract to accomodate increased blood flow and they are able to recoil when the venrticles relax.
        This is important to:
        Smooth out the flow of blood
        Maintain pressure in the arteries
  11. Arterioles
    1. There is relatively less elastic tissue in the middle layer, they have more muscle
      1. This is important to regulate the flow of blood to area that need it more/less.
        E.g. when exercising and you need more oxygen + glucose to the muscles:
        The muscle in the arterioles will relax
        This increases the size of the lumen
        Which increases the flow of blood to that area
        The opposite happens when you want less blood to a certain area i.e. contract, narrow, decreases
  12. Capillaries
    1. This wall consists of the endothelium only
      1. It consists of a single layer of squamous cells to allow a short diffusion pathway
        The lumen is very narrow, so narrow that blood cells are only able to travel in single form across the capillary.
        The rate of blood flow is very very low as the narrow lumen creates frictional resistance, which needs to be overcome
        This allows time for the exchange of substances between the cells and the blood
        The large network of capillaries allows a large surafce area even though the capillary itself is very small.
  13. Veins/venules
    1. These have thin walls as blood flows under low pressure.
    2. Blood flow is maintained by:
      1. Valves to prevent backflow
      2. Skeletal muscles contract and push on the veins which forces blood up
      3. Residual blood pressure from heart
3. The purpose is to transport substances around the body as diffuison is only efficient over short distances. Mass tranport allows rapid movement of substances over long distances
4. Pressure: Decreases as you get further away from the heart.
  1. Lowest: Vena Cava. Highest: Aorta
  2. Velocity: From arteries to to capillaries, velocity decreases (frictional resistance increases) From capillaries to vein it increases again due to less frictional resistance)
    1. Total cross sectional: capillaries have the greatest amount because there's loads of them
5. Exchange at the capillaries
  1. At the arterial end:
    1. Fluid is forced out of the capillaries at the arterial end even though the water potential gradient should mean water enters the capillaries through osmosis.
      1. There is a more negative water potential gradient at the arterial end because the blood plasma contains more proteins than the tissue fluid does.
      2. The reason for fluid still being forced out is because blood hydrostatic pressure is greater than the water potential gradient.
        Blood hydrostatic pressure is initiated by the contraction of the left ventricle.
    2. Filtration
  2. At the venous end:
    1. WATER will move back into the capillaries by osmosis due to there being a water potential gradient.
      1. The reason this happens is because blood hydrostatic pressure is now weaker than the water potential gradient because we're now further from the heart.
        Any excess tissue that remains will be removed by the lymph vessel (part of the lymphatic system) and will return to the blood via the subclavian vein.
    2. Reabsorption
Oxygen Transport
1. Red blood cells transport oxygen.
  1. They pick up oxygen from the lung capillary and release oxygen at the to the respiring cells in the tissue capillary
    1. The oxygen will bind to the haemoglobins inside the red blood cell
      1. Haemoglobin
        These are a group of chemically similar molecules that are found in many different organisms.
        Different organisms will have different types of haemoglobin depending on their environment and lifestyle
        Some are better at binding and some are better at releasing
        Human haemoglobin is a protein that consists of 4 polypeptide chains. It has quaternary structure.
        In each chain, you have the 'haem' group, which contains the Fe2+ group.Each Fe2+ group can bind to one O2 molecule and so each haemoglobin will bind to 4 O2 molecules
        Haemogloin has an affinity for oxygen i.e. an attraction for oxygen. This will change depending on:
        The temperature
        The concentration of Oxygen
        Haemoglobin will have a high affinity for oxygen when there is a high oxygen concentration.
        This means it will bind better or associate to oxygen at high partial pressures (concentrations) of oxygen to become oxyhaemoglobin
        As in the lung capillary
        At low partial pressures of oxygen, haemoglobin will have a lower affinity for O2 and will be better at releasing or dissociating with oxygen.
        As in the tissue capillary
        Which is what we want to happen as O2 is needed at the tissue for respiration
        The Oxygen dissociation curve shows the relationship between the partial pressure of oxygen and the % saturation of haemoglobin
        As shown in the diagram, when loading the O2, there is a high partial pressure of oxygen in the lung capillary. Haemoglobin will have a high affinity for oxygen and bind with it. It becomes fully saturated.
        In the tissue capillary there is a low partial pressure of oxygen and so haemoglobin has a lower affinity for oxygen and it will be releaased.
        A higher respiration rate means more O2 is used up and so there's a lower partial pressure of O2 and so more O2 is released as haemoglobin has an even lower affinity for oxygen
        Note: As blood flows through the arteries etc. the oxyhaemoglobin does not dissociate because the vessel walls are too thick to allow oxygen to escape so the partial pressure of oxygen remains fairly constant.
        The concentration of CO2
        The curve shows that as the concentration of CO2 increases, the curve is shifted to the right.
        It's called the Bohr effect
        As the cells respire, carbon dioxide is produced which dissolves in the blood to produce carbonic aicd (which leads to more acidic pH)
        Also, there is a higher temperature as heat is a by-product of respiration
        The effect of an increase in CO2 is:
        At any given partial pressure of oxygen, more oxygen is released to the respiring cells
        Meaning oxyhaemoglobin dissociates more efficiently or easily.
        An increase in the conc of CO2 lowers haemoglobins affinity for oxygen
        And ensures that cells are provided with sufficient oxygen
        If there's a higher rate of respiration in a tissue, it means there's a higher conc of CO2 and so shifts the curve to the right which then lowers haemoglobins affinity for O2
        Haemoglobin in different organisms
        Different organisms will have different types of haemoglobin and this depends on its environment and level acitivity
        Curves to the right
        This type of haemoglobin has a lower affinity for oxygen than human haemoglobin
        This means it becomes fully saturated at high partial pressure of O2 but more importantly, it dissociates with O2 at relatively higher partial pressures of O2,
        This then means that at any given partial pressure of oxygen, more O2 is released to the cells.
        This is good for organisms with a high rate of respiration e.g small organisms
        They have a large S.A:VR
        High rate of heat loss
        High rate of respiration to compensate
        Haemoglobin has a lower affinity for O2
        More O2 is released and a high rate of respiration is maintained
        Curves to the left
        This type of haemoglobin has a higher affinity for oxygen
        This means that at any given partial pressure of oxygen, more oxygen is taken up by the haemoglobin
        i.e the haemoglobin becomes fully saturated at lower partial pressures of oxygen
        This is important for organisms that live in oxygen depleted areas such as near the riverbed or at high altitudes.
        Fetal haemoglobin usually has this type of haemoglobin
        This is so the maternal haemoglobin can transfer the oxygen to the fetal haemoglobin at lower partial pressures of oxygen
        i.e. There's less competition
The Roots and Stem
1. Function: To absorb water and minerals from the soil
  1. To anchor the plant in the soil
2. Structure
  1. Epidermis; It's the outer layer and has side roots which increases the surface area
  2. Cortex: A series of unspecialised cells. Its major function is to store starch (energy)
  3. Endodermis: A single layer of cells which surrounds the vascular bundle. Its cell wall contains a waxy substance called suberin. It forms the casparian strip which is impermeable to water
  4. Pericycle: It produces side roots. It a layer of cells between the endodermis and vascular bundle
  5. Cambium: Found near the xylem and phloem. It's a mersitematic growth region where the cells can specialise to become either phloem or xylem
  6. Xylem: Is responsible for the transport of water and other minerals to other parts of the plant
    1. Structure of xylem vessel:
      1. Made of long tube like structures
      2. Made of dead cells called vessel elements
        The contents of these have been removed and so have the ends to allow an unrestricted flow of water
        Cell walls are lined with waterproof material called lignin which provides strength and support
        There are pits in the xylem vessel whiich isn't lined with lignin and allows lateral movement of water between vessels
  7. Phloem: found around the xylem, and its job is to transport organic substances to the plant
3. Transport of water
  1. Needed for:
    1. Photosynthesis
    2. To keep cells turgid
  2. 1. Water enters through root hair cell from the soil. It enters by osmosis from a less -ve wp to a more -ve wp.
    1. 2.It also travels through the cortex by osmosis
      1. 3. Then it reaches the endodermis and can go two ways
        A. Apoplast
        This where water moves through the cell wall. However, the casparian strip means this pathway is blocked as it's impermeable to water.
        B. Symplast
        This is where water moves through the cell i.e. through the cytoplasm and plasmodesmata by osmosis
        4. Therefore water must go through the symplast pathway and allows the control of the passage of water because the cell membrane is partially permeable.
        5. Then the water enters the xylem vessel, and movement is only upwards. There are two theories that are used to explain this:
        Cohesion Tension Theory
        Cohesion: Where water molecules stick to each other by hydrogen bonds
        Adhesion: Where water molecules stick to the inside of the xylem vessel
        Creates a column of water molecules
        As water evaporates from the cells to the air space inside of the leaf and diffuses into the outside air (transpires) the column of water molecules are under tension as they are pulled upwards.
        Evidence
        During midday, the circumference of a tree is thinner.
        The reason for this is because at this time, the stomata are open and there's a higher rate of transpiration
        The water column is under greatest tension because of adhesion, which pulls the xylem vessels inwards.
        And leads to reduced circumference
        Root Pressure:
        This is the movement of water from a higher hydrostatic pressure at the bottom to a lower hydrostatic pressure at the top
        High HP is maintained at the bottom as minerals are actively pumped into the xylem from the endodermis.
        Water then follows by osmosis
        Creates a high HP
        Low HP is maintained at the bottom as minerals will diffuse into the leaf.
        Water will then follow by osmosis
        Creates a low HP at the top of the xylem
        6. Water continues to move by osmosis through a water potential gradient
        7. Transpiration will occur
Transpiration
1. This is the evaporation of water from the cell to the inside air and the diffusion of water to the outisde air.
2. It occurs:
  1. Through the stomata
  2. Through the cuticle, but rare
3. Measured by a potometer
  1. This measures the rate of water uptake from a cut root
    1. 1. A leafy shoot is cut under water
      1. To stop air entering the xylem vessel
        Also, all joints are to be watertight to prevent leakage
      2. 2. The rate is measured following the movement of an intentionally placed air bubble along a scale at a measured time interval (rate).
        The volume water can be measured by finding the volume of the tube (pi x r^2 x l
  2. However, it doesn't take into account the water used to keep the cell turgid or photosynthesis
4. Factors that affect rate:
  1. Light intensity:
    1. Stomata open in the light to allow CO2 in for photosynthesis
      1. Which increases the rate of transpiration
  2. Temperature
    1. Higher temp means molecules have more kinetic energy and so more evaporation and more diffusion takes place
  3. Air movement;
    1. In windy condidtion, the layer of moist air that builds up around the stomata is blown away, which increases the water potential gradient between air inside leaf and outside air.
      1. Increases rate of transpiration
  4. Humidity
    1. This is the concentration of water vapour in the air.
      1. More humidity reduces the WP gradient
        Means a lower rate of transpiration
5. Prevention
  1. Xerophytes are plants with special adaptations to reduce the rate of transpiration
    1. Thick cuticles:
      1. A waxy cuticle is impermeable to water and so it's even better at stopping water from leaving the cuticle.
    2. Smaller leaves:
      1. A smaller surface area for the diffusion of water
    3. Sunken Stomata, Hairs, Rolled leaves:
      1. Traps a layer of moist air around the stomata and reduces the water potential gradient between the leaf airspace and the outside air.

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