Knowt
Scaling Up
I. Definitions and Concepts
1. Active Transport
The movement of particles from an area of low concentration to an area of high concentration, against the concentration gradient.
This requires energy.
2. Adult Stem Cell
A stem cell found in the bone marrow that can form many types of cells.
3. Alveoli
Small air sacs in the lungs that serve as the gaseous exchange surface.
They provide a large surface area for efficient exchange.
4. Aorta
The main artery that takes oxygenated blood away from the heart to the body.
5. Artery
A blood vessel that carries blood away from the heart under high pressure.
6. Atrium (pl. atria)
The upper chamber of the heart that receives the blood from the veins.
7. Biconcave
Describes the shape of red blood cells which increases the surface area for gaseous exchange.
8. Blood
A tissue that contains red blood cells, white blood cells, plasma and platelets.
9. Bone Marrow
A human tissue that contains stem cells which can develop into red blood cells.
10. Cell Cycle
A series of events that take place in a cell in preparation for cell division.
12. Cell Differentiation
The production of specialised cells allows organisms to become more efficient.
13. Companion Cells
The active cells of the phloem.
They provide energy for the phloem to transport substances.
14. Concentration Gradient
The difference in concentration between two areas.
15. Cytokinesis
The third stage of the cell cycle in which two identical diploid daughter cells are formed.
16. Diffusion
The net spreads out of particles from a high concentration to a lower concentration (down their concentration gradient).
Energy is not required.
17. Double Circulatory System
A circulatory system found in mammals in which the blood passes through the heart twice in a full body circuit
18. Embryo
An organism in its early stages of development.
19. Embryonic Stem Cell
A type of stem cell found in very early embryos that can differentiate into any cell type.
20. Guard Cells
Cells that control the opening and closing of the stomata.
21. Heart
The organ that pumps blood around the body.
22. Hypertonic
The net movement of water out of the cell via osmosis.
23. Hypotonic
The net movement of water into the cell via osmosis.
24. Interphase
The first stage of the cell cycle in which cells grow, new proteins are synthesised and chromosomes are replicated.
25. Lignin
A material that lines the xylem vessels and provides strength and support.
26. Meristematic Cell
A type of cell that can differentiate into any plant cell type.
27. Meristem Tissue
A plant tissue that contains many undifferentiated cells.
28. Mitosis
A type of cell division that produces two identical diploid daughter cells (i.e. contain a full set of chromosomes) from one parent cell.
It is the second stage of the cell cycle and is important for growth, development and the replacement of damaged cells.
29. Multicellular Organism
An organism that has more than one cell.
30. Muscle Cell
A specialised animal cell that contracts or relaxes causing muscle movement.
31. Nerve Cell
A specialised animal cell that transmits electrical impulses.
32. Osmosis
The net movement of water molecules from a region of high concentration to a region of low concentration through a partially permeable membrane
33. Phloem
A plant tissue that transports sugars from the source to the sink.
34. Plasma
A pale yellow liquid found in the blood that carries water, enzymes, salts, nutrients, proteins, urea and hormones.
35. Plasmolysis
The net movement of water out of a plant cell, causing the cell membrane to move away from the cell wall. This results in cell death.
36. Potometer
A piece of capillary tube that is used to investigate the rate of transpiration. Water loss from the surface of the leaf is measured by the distance that the air bubble travels over a certain period of time.
37. Pulmonary Artery
The main artery that carries deoxygenated blood away from the heart to the lungs
38. Pulmonary Vein
The main vein that carries oxygenated blood back to the heart from the lungs.
39. Red Blood Cells
Cells in the blood that carry oxygen and remove carbon dioxide
40. Root Hair Cells
Specialised cells that provide a large surface area for the uptake of water and minerals from the soil.
41. Sieve Tubes
Plant cells that have no nuclei and are connected via the cytoplasms.
42. Sperm Cell
A specialised animal cell that carries the male DNA to the egg for reproduction.
43. Stem Cell
Cells that are unspecialised and capable of differentiating into a range of different cell types.
44. Stomata
Small pores in the epidermis of the leaves that facilitate gaseous exchange.
45. Translocation
The process of transporting sucrose around the plant.
46. Transpiration
The loss of water from the surface of the leaves by evaporation from the open stomata.
47. Turgid
When the vacuole of a plant cell becomes swollen and enlarged with water
48. Undifferentiated Cell
A cell that is not specialised for its function.
49. Valves
Structures found at each end of both ventricles that prevent the backflow of blood (ensuring blood flows in only one direction).
50. Vein
A blood vessel that carries deoxygenated blood to the heart at low pressure.
51. Vena Cava
The main vein that carries deoxygenated blood back to the heart from the body.
52. Ventricles
The lower chambers of the heart that receive blood from the atria and pump it to the arteries.
The heart has two ventricles.
53. Villi
Small projections from the small intestine that increase the surface area for food absorption.
54. Water Potential
A measure for the tendency of water to move from one area to another area.
It is represented by the sign Ψ (Psi).
55. Xylem
A specialised plant tissue that transports water and dissolved minerals from the roots to the leaves of the plant.
II. Supplying the Cell
1. Transporting Substances
a. Diffusion
Diffusion is the spreading out of the particles resulting in a net movement from an area of higher concentration to an area of lower concentration.
It is a passive process as no energy is required.
The molecules have to be small in order to be able to move across, for example oxygen, glucose, amino acids and water, but larger molecules such as starch and proteins cannot.
Example in Living Organisms
a. Single-Celled Organisms
Can use diffusion to transport molecules into their body from the air- this is because they have a relatively large surface area to volume ratio.
Due to their low metabolic demands, diffusion across the surface of the organism is sufficient enough to meet its needs.
b. Multicellular Organisms
The surface area to volume ratio is small so they cannot rely on diffusion alone.
Instead, surfaces and organ systems have a number of adaptations that allows molecules to be transported in and out of cells.
Examples include alveoli in the lungs, villi in the small intestines and root hair cells in plants.
Many Factors Affect the Rate of Diffusion
Factor | Effect |
Concentration gradient (difference in concentrations) | The greater the difference in concentration, the faster the rate of diffusion. This is because more particles are randomly moving down the gradient than are moving against it. |
Temperature | The greater the temperature, the greater the movement of particles, resulting in more collisions and therefore a faster rate of diffusion. |
Surface area of the membrane | The greater the surface area, the more space for particles to move through, resulting in a faster rate of diffusion. |
b. Osmosis
Osmosis is the movement of water from a less concentrated solution to a more concentrated one through a partially permeable membrane.
A dilute solution of sugar has a high concentration of water (and therefore a high water potential). A concentrated solution of sugar has a low concentration of water (and therefore a low water potential). Water moves from a dilute solution to a concentrated solution because it moves from an area of high water potential to low water potential- down the concentration gradient.
It is passive, as it does not use energy.
If the concentration of sugar in an external solution is the same as the internal, there will be no movement and the solution is said to be isotonic to the cell
If the concentration of sugar in external solution is higher than the internal, water moves out, and the solution is said to be hypertonic to the cell
If the concentration of sugar in external solution is lower than the internal, water moves in, and the solution is said to be hypotonic to the cell
Examples in living organisms
a. Osmosis in animals:
If the external solution is more dilute (higher water potential), it will move into animal cells causing them to burst.
If the external solution is more concentrated (lower water potential), excess water will leave the cell causing it to become shrivelled.
b. Osmosis in plants:
If the external solution is more dilute, water will move into the cell and into the vacuole, causing it to swell, resulting in pressure called turgor (essential in keeping the leaves and stems of plants rigid).
If the external solution is less dilute, water will move out of the cell and they will become soft. Eventually the cell membrane will move away from the cell wall (called plasmolysis) and it will die.
c. Active Transport
Active transport is the movement of particles from an area of lower concentration to an area of higher concentration, i.e. against the concentration gradient.
This requires energy from respiration as it is working against the gradient, which is why it is called active.
Examples in living organisms
a. In root hair cells:
They take up water and mineral ions (for healthy growth) from the soil
Mineral ions are usually in higher concentrations in the cells, meaning diffusion cannot take place
This requires energy from respiration to work
b. In the gut:
Substances such as glucose and amino acids from your food have to move from your gut into your bloodstream
Sometimes there can be a lower concentration of sugar molecules in the gut than the blood, meaning diffusion cannot take place
Active transport is required to move the sugar to the blood against its concentration gradient
2. Mitosis
Mitosis is a type of cell division where one cell divides to form two identical daughter cells.
The cell cycle is a series of steps that the cell has to undergo in order to do this.
a. Stage 1 (Interphase)
In this stage the cell grows, organelles (such as ribosome and mitochondria) grow and increase in number, the synthesis of proteins occurs, all 46 chromosomes are replicated (forming the characteristic ‘X’ shape) and energy stores are increased
b. Stage 2 (Mitosis)
The chromosomes line up at the equator of the cell and spindle fibres pull each chromosome of the ‘X’ to either side of the cell.
c. Stage 3 (Cytokinesis)
Two identical daughter cells form when the cytoplasm and cell membranes divide, each containing the same 46 chromosomes as the original cell.
💡 Cell division by mitosis in multicellular organisms is important in their growth and development, and when replacing damaged cells. Mitosis is also a vital part of asexual reproduction, as this type of reproduction only involves one organism, so to produce offspring it simply replicates its own cells.
3. Differentiation and Specialisation
a. Specialised Cells
Cells specialise by undergoing differentiation
A process that involves the cell gaining new sub-cellular structures in order for it to be suited to its role.
Cells can either differentiate once early on or have the ability to differentiate their whole life (these are called stem cells).
In animals, most cells only differentiate once, but in plants many cells retain the ability.
b. Examples of Specialised Cells in Animals
Sperm cells
Specialised to carry the male’s DNA to the egg cell (ovum) for successful reproduction
Streamlined head and long tail to aid swimming
Many mitochondria (where respiration happens) which supply the energy to allow the cell to move
The acrosome (top of the head) has digestive enzymes which break down the outer layers of membrane of the egg cell
Nerve cells
Specialised to transmit electrical signals quickly from one place in the body to another
The axon is long, enabling the impulses to be carried along long distances
Having lots of extensions from the cell body (called dendrites) means branched connections can form with other nerve cells
The nerve endings have many mitochondria which supply the energy to make special transmitter chemicals called neurotransmitters. These allow the impulse to be passed from one cell to another.
Muscle cells
Specialised to contract quickly to move bones (striated muscle) or simply to squeeze (smooth muscle, e.g found in blood vessels so blood pressure can be varied), therefore causing movement
Special proteins (myosin and actin) slide over each other, causing the muscle to contract
Lots of mitochondria to provide energy from respiration for contraction
They can store a chemical called glycogen that is used in respiration by mitochondria
c. Examples of Specialised Cells in Plants
Root hair cells
Specialised to take up water by osmosis and mineral ions by active transport from the soil as they are found in the tips of roots
Have a large surface area due to root hairs, meaning more water can move in
The large permanent vacuole affects the speed of movement of water from the soil to the cell
Mitochondria to provide energy from respiration for the active transport of mineral ions into the root hair cell
Xylem cells
Specialised to transport water and mineral ions up the plant from the roots to the shoots
Upon formation, a chemical called lignin is deposited which causes the cells to die. They become hollow and are joined end-to-end to form a continuous tube so water and mineral ions can move through
Lignin is deposited in spirals which helps the cells withstand the pressure from the movement of water
Phloem cells
Specialised to carry the products of photosynthesis (food) to all parts of the plants
Cell walls of each cell form structures called sieve plates when they break down, allowing the movement of substances from cell to cell
Despite losing many sub-cellular structures, the energy these cells need to be alive is supplied by the mitochondria of the companion cells.
4. Stem Cells
a. Characteristics of Stem Cells
A stem cell is an undifferentiated cell which can undergo division to produce many more similar cells
Some of these will differentiate to have different functions, such as the specialised cells mentioned above
They are important in development, growth and repair
b. Types of Stem Cells
Embryonic stem cells
Form when an egg and sperm cell fuse to form a zygote
They can differentiate into any type of cell in the body
Scientists can clone these cells (though culturing them) and direct them to differentiate into almost any cell in the body
These could potentially be used to replace insulin-producing cells in those suffering from diabetes, new neural cells for diseases such as Alzheimer’s, or nerve cells for those paralysed with spinal cord injuries
Adult stem cells
If found in bone marrow they can form many types of cells (not any type, like embryonic stem cells can) including blood cells
Meristems in plants
Found in root and shoot tips
They can differentiate into any type of plant, and have this ability throughout the life of the plant
They can be used to make clones of the plant
This may be necessary if the parent plant has certain desirable features (such as disease resistance), for research or to save a rare plant from extinction
III. The Challenges of Size
1. Exchange Systems
As mentioned before, multicellular organisms have a small surface area to volume ratio compared to the amount of substances they need to exchange.
a. Surface Area to Volume Ratio
The size of the surface area of the organism compared to its volume
Calculated by finding the volume (length x width x height) and the surface area (length x width), and writing the ratio in the smallest whole numbers
If this is large, the organism is less likely to require specialised exchange surfaces and a transport system because the rate of diffusion is sufficient in supplying and removing the necessary gases
E.g 15 (surface area): 5 (volume) is written as 3:1
Multicellular organisms have had to adapt to increase this ratio as much as possible.
Adaptation | Why? | Example |
Having a large surface area | The greater the surface area, the more particles can move through, resulting in a faster rate of diffusion | Lungs: the small, spherical alveoli (sites of gaseous exchange) in the lungs create a very large surface area (approximately 75m2 in humans). Small intestine: the cells of the small intestine have millions of villi, which are projections which increase the surface area. This means digested food can be absorbed into the blood faster Fish gills: these contain lamellae to increase the surface area. Leaves: the flattened shape increases the surface area. The air spaces inside the leaf increase the surface area, so more carbon dioxide can enter cells. |
Having a thin membrane | Provides a short diffusion pathway, allowing the process to occur faster | Lungs: alveoli and capillary walls are extremely thin. Small intestine: villi have a single layer of surface cell. |
Having an efficient blood supply OR having good ventilation (in animals | Creates a steep concentration gradient, so diffusion occurs faster | Lungs: the lungs constantly supply oxygen to make the blood from alveoli capillaries oxygenated, by exchanging it for carbon dioxide that can be breathed out. This is a constant process meaning the concentration gradient is always steep. Fish: water flows in one direction and blood flows in the other - this means that a steep concentration gradient is maintained as the concentration of oxygen is always much higher in the water - so it will diffuse across. |
2. Human Circulatory System
The heart is an organ in the circulatory system. The circulatory system carries oxygen and nutrients to every cell in the body and removes the waste products.
The heart pumps blood around the body in a double circulatory system. This means there are two circuits. Mammals require this double system because the metabolic rate is higher and so need a faster system.
System 1
Deoxygenated blood flows into the right atrium and then into the right ventricle which pumps it to the lungs to undergo gaseous exchange
System 2
Oxygenated blood flows into the left atrium and then into the left ventricle which pumps oxygenated blood around the body
a. Structure of the heart
Muscular walls to provide a strong heartbeat
The muscular wall of the left ventricle is thicker because blood needs to be pumped all around the body rather than just to the lung like the right ventricle.
4 chambers that separate the oxygenated blood from the deoxygenated blood
2 atria above and 2 ventricles below
Valves to make sure blood does not flow backwards
Coronary arteries cover the heart to provide its own oxygenated blood supply
Process
Blood flows into the right atrium through the vena cava, and left atrium through the pulmonary vein.
The atria contract forcing the blood into the ventricles.
The ventricles then contract, pushing the blood in the right ventricle into the pulmonary artery to be taken to the lungs, and blood in the left ventricle to the aorta to be taken around the body.
As this happens, valves close to make sure the blood does not flow backwards.
b. Structure of blood vessels
Arteries
Carry blood AWAY from the heart
Layers of muscle in the walls make them strong
Elastic fibres allow them to stretch
This helps the vessels withstand the high pressure created by the pumping of the heart
Veins
Carry blood TOWARDS the heart
The lumen (the actual tube in which blood flows through) is wide to allow the low pressure blood to flow through
They have valves to ensure the blood flows in the right direction
Capillaries
Allow the blood to flow very close to cells to enable substances to move between them
One cell thick walls create a short diffusion pathway
Permeable walls so substances can move across them
c. Structure of blood
Red blood cells
Contain haemoglobin
A red protein that combines with oxygen to allow for transport
No nucleus
To create more space for haemoglobin
Biconcave shape
To maximise surface area for oxygen to be absorbed
Flexible
So they can fit through very narrow blood vessels
Plasma
Plasma is the liquid which carries all of the components of blood, such as blood cells, platelets, amino acids, urea etc.
Plasma is mainly made up of water and many substances that need to be transported around the body, e.g. carbon dioxide, urea, are water-soluble
d. Transpiration and Water Uptake
Transpiration
The loss of water vapour from the leaves and stems of the plant. It is a consequence of gaseous exchange, as the stomata are open so that this can occur.
Water also evaporates at the open stomata
As water molecules are attracted to each other, when some molecules leave the plant the rest are pulled up through the xylem
This results in more water being taken up from the soil resulting in a continuous transpiration stream through the plant
Xylem
Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration.
Xylem is adapted in many ways:
A chemical called lignin is deposited which causes the cells to die.
These cells then become hollow and join end-to-end to form a continuous tube for water and mineral ions to travel through from the roots
Water molecules are attracted to each other by hydrogen bonding
Creating a continuous column of water up the plant
The water evaporates from the leaves of the plant, creating the transpiration stream.
Lignin strengthens the plant to help it withstand the pressure of the water movement
Lignin contains bordered pits, which are holes to allow specific areas for water and therefore minerals to enter the plant
Root hair cell
Water is taken up by plants through root hair cells
These are specialised cells with a very large surface area to absorb water via osmosis.
If the rate of transpiration increases then the rate of water uptake will also increase as the plant attempts to replenish the loss.
Guard cells
Open and close stomata
They are kidney shaped, with thin outer walls and thick inner walls
When lots of water is available to the plant, the cells fill and change shape, opening stomata (they are also light sensitive)
This allows gases to be exchanged and more water to leave the plant via evaporation
More stomata are found on the bottom of the leaf, allowing gases to be exchanged whilst minimising water loss by evaporation as the lower surface is shaded and cooler.
Factors affecting water uptake
Increase in light intensity: This leads to an increased rate of photosynthesis, so more stomata open to allow gaseous exchange to occur. This means more water can evaporate, leading to an increased rate of transpiration and so uptake.
Increase in temperature: The molecules move faster, resulting in evaporation happening at a faster rate and therefore the rate of transpiration increases. The rate of photosynthesis increases, meaning more stomata are open for gaseous exchange, so more water evaporates and the rate of transpiration increases. Therefore, water uptake also increases
Increased air movement (wind): If more air is moving away from the leaf due to it being blown away, then the concentration of water vapour surrounding the leaf will be lower. This will mean there will be a steeper concentration gradient resulting in diffusion happening faster. This will increase the rate of transpiration and also water uptake.
Increase in humidity: If the relative humidity is high, then there will be a reduced concentration gradient between the concentrations of water vapour inside and outside the leaf, resulting in a slower rate of diffusion. This will decrease the rate of transpiration and water uptake.
Potometer
Can be used to investigate how these factors affect water uptake.
It is set up underwater to remove air bubbles in the xylem so that there is a continuous stream of water and the system is made airtight, apart from a singular bubble of air.
The distance this air bubble in the capillary tube moves over time is measured.
If it moves faster then it means that there is a greater rate of water uptake and therefore rate of transpiration.
An environmental condition, such as light intensity, is changed each time the experiment is run in order to see how it affects the plant.
e. Translocation
The movement of food substances made in the leaves up or down the phloem, for use immediately or storage
Phloem adaptations
Found in the roots, stems and leaves
Elongated cells with holes in the cell walls (the end walls are called sieve plates)
Many organelles from the cells are removed so cell sap can move through.
However, there are many mitochondria in companion cells which provide the energy the cells require
Food substances can be moved in both directions (translocation), from the leaves where they are made for use, or from storage (underground) to parts of the plant that need it.
Scaling Up
I. Definitions and Concepts
1. Active Transport
The movement of particles from an area of low concentration to an area of high concentration, against the concentration gradient.
This requires energy.
2. Adult Stem Cell
A stem cell found in the bone marrow that can form many types of cells.
3. Alveoli
Small air sacs in the lungs that serve as the gaseous exchange surface.
They provide a large surface area for efficient exchange.
4. Aorta
The main artery that takes oxygenated blood away from the heart to the body.
5. Artery
A blood vessel that carries blood away from the heart under high pressure.
6. Atrium (pl. atria)
The upper chamber of the heart that receives the blood from the veins.
7. Biconcave
Describes the shape of red blood cells which increases the surface area for gaseous exchange.
8. Blood
A tissue that contains red blood cells, white blood cells, plasma and platelets.
9. Bone Marrow
A human tissue that contains stem cells which can develop into red blood cells.
10. Cell Cycle
A series of events that take place in a cell in preparation for cell division.
12. Cell Differentiation
The production of specialised cells allows organisms to become more efficient.
13. Companion Cells
The active cells of the phloem.
They provide energy for the phloem to transport substances.
14. Concentration Gradient
The difference in concentration between two areas.
15. Cytokinesis
The third stage of the cell cycle in which two identical diploid daughter cells are formed.
16. Diffusion
The net spreads out of particles from a high concentration to a lower concentration (down their concentration gradient).
Energy is not required.
17. Double Circulatory System
A circulatory system found in mammals in which the blood passes through the heart twice in a full body circuit
18. Embryo
An organism in its early stages of development.
19. Embryonic Stem Cell
A type of stem cell found in very early embryos that can differentiate into any cell type.
20. Guard Cells
Cells that control the opening and closing of the stomata.
21. Heart
The organ that pumps blood around the body.
22. Hypertonic
The net movement of water out of the cell via osmosis.
23. Hypotonic
The net movement of water into the cell via osmosis.
24. Interphase
The first stage of the cell cycle in which cells grow, new proteins are synthesised and chromosomes are replicated.
25. Lignin
A material that lines the xylem vessels and provides strength and support.
26. Meristematic Cell
A type of cell that can differentiate into any plant cell type.
27. Meristem Tissue
A plant tissue that contains many undifferentiated cells.
28. Mitosis
A type of cell division that produces two identical diploid daughter cells (i.e. contain a full set of chromosomes) from one parent cell.
It is the second stage of the cell cycle and is important for growth, development and the replacement of damaged cells.
29. Multicellular Organism
An organism that has more than one cell.
30. Muscle Cell
A specialised animal cell that contracts or relaxes causing muscle movement.
31. Nerve Cell
A specialised animal cell that transmits electrical impulses.
32. Osmosis
The net movement of water molecules from a region of high concentration to a region of low concentration through a partially permeable membrane
33. Phloem
A plant tissue that transports sugars from the source to the sink.
34. Plasma
A pale yellow liquid found in the blood that carries water, enzymes, salts, nutrients, proteins, urea and hormones.
35. Plasmolysis
The net movement of water out of a plant cell, causing the cell membrane to move away from the cell wall. This results in cell death.
36. Potometer
A piece of capillary tube that is used to investigate the rate of transpiration. Water loss from the surface of the leaf is measured by the distance that the air bubble travels over a certain period of time.
37. Pulmonary Artery
The main artery that carries deoxygenated blood away from the heart to the lungs
38. Pulmonary Vein
The main vein that carries oxygenated blood back to the heart from the lungs.
39. Red Blood Cells
Cells in the blood that carry oxygen and remove carbon dioxide
40. Root Hair Cells
Specialised cells that provide a large surface area for the uptake of water and minerals from the soil.
41. Sieve Tubes
Plant cells that have no nuclei and are connected via the cytoplasms.
42. Sperm Cell
A specialised animal cell that carries the male DNA to the egg for reproduction.
43. Stem Cell
Cells that are unspecialised and capable of differentiating into a range of different cell types.
44. Stomata
Small pores in the epidermis of the leaves that facilitate gaseous exchange.
45. Translocation
The process of transporting sucrose around the plant.
46. Transpiration
The loss of water from the surface of the leaves by evaporation from the open stomata.
47. Turgid
When the vacuole of a plant cell becomes swollen and enlarged with water
48. Undifferentiated Cell
A cell that is not specialised for its function.
49. Valves
Structures found at each end of both ventricles that prevent the backflow of blood (ensuring blood flows in only one direction).
50. Vein
A blood vessel that carries deoxygenated blood to the heart at low pressure.
51. Vena Cava
The main vein that carries deoxygenated blood back to the heart from the body.
52. Ventricles
The lower chambers of the heart that receive blood from the atria and pump it to the arteries.
The heart has two ventricles.
53. Villi
Small projections from the small intestine that increase the surface area for food absorption.
54. Water Potential
A measure for the tendency of water to move from one area to another area.
It is represented by the sign Ψ (Psi).
55. Xylem
A specialised plant tissue that transports water and dissolved minerals from the roots to the leaves of the plant.
II. Supplying the Cell
1. Transporting Substances
a. Diffusion
Diffusion is the spreading out of the particles resulting in a net movement from an area of higher concentration to an area of lower concentration.
It is a passive process as no energy is required.
The molecules have to be small in order to be able to move across, for example oxygen, glucose, amino acids and water, but larger molecules such as starch and proteins cannot.
Example in Living Organisms
a. Single-Celled Organisms
Can use diffusion to transport molecules into their body from the air- this is because they have a relatively large surface area to volume ratio.
Due to their low metabolic demands, diffusion across the surface of the organism is sufficient enough to meet its needs.
b. Multicellular Organisms
The surface area to volume ratio is small so they cannot rely on diffusion alone.
Instead, surfaces and organ systems have a number of adaptations that allows molecules to be transported in and out of cells.
Examples include alveoli in the lungs, villi in the small intestines and root hair cells in plants.
Many Factors Affect the Rate of Diffusion
Factor | Effect |
Concentration gradient (difference in concentrations) | The greater the difference in concentration, the faster the rate of diffusion. This is because more particles are randomly moving down the gradient than are moving against it. |
Temperature | The greater the temperature, the greater the movement of particles, resulting in more collisions and therefore a faster rate of diffusion. |
Surface area of the membrane | The greater the surface area, the more space for particles to move through, resulting in a faster rate of diffusion. |
b. Osmosis
Osmosis is the movement of water from a less concentrated solution to a more concentrated one through a partially permeable membrane.
A dilute solution of sugar has a high concentration of water (and therefore a high water potential). A concentrated solution of sugar has a low concentration of water (and therefore a low water potential). Water moves from a dilute solution to a concentrated solution because it moves from an area of high water potential to low water potential- down the concentration gradient.
It is passive, as it does not use energy.
If the concentration of sugar in an external solution is the same as the internal, there will be no movement and the solution is said to be isotonic to the cell
If the concentration of sugar in external solution is higher than the internal, water moves out, and the solution is said to be hypertonic to the cell
If the concentration of sugar in external solution is lower than the internal, water moves in, and the solution is said to be hypotonic to the cell
Examples in living organisms
a. Osmosis in animals:
If the external solution is more dilute (higher water potential), it will move into animal cells causing them to burst.
If the external solution is more concentrated (lower water potential), excess water will leave the cell causing it to become shrivelled.
b. Osmosis in plants:
If the external solution is more dilute, water will move into the cell and into the vacuole, causing it to swell, resulting in pressure called turgor (essential in keeping the leaves and stems of plants rigid).
If the external solution is less dilute, water will move out of the cell and they will become soft. Eventually the cell membrane will move away from the cell wall (called plasmolysis) and it will die.
c. Active Transport
Active transport is the movement of particles from an area of lower concentration to an area of higher concentration, i.e. against the concentration gradient.
This requires energy from respiration as it is working against the gradient, which is why it is called active.
Examples in living organisms
a. In root hair cells:
They take up water and mineral ions (for healthy growth) from the soil
Mineral ions are usually in higher concentrations in the cells, meaning diffusion cannot take place
This requires energy from respiration to work
b. In the gut:
Substances such as glucose and amino acids from your food have to move from your gut into your bloodstream
Sometimes there can be a lower concentration of sugar molecules in the gut than the blood, meaning diffusion cannot take place
Active transport is required to move the sugar to the blood against its concentration gradient
2. Mitosis
Mitosis is a type of cell division where one cell divides to form two identical daughter cells.
The cell cycle is a series of steps that the cell has to undergo in order to do this.
a. Stage 1 (Interphase)
In this stage the cell grows, organelles (such as ribosome and mitochondria) grow and increase in number, the synthesis of proteins occurs, all 46 chromosomes are replicated (forming the characteristic ‘X’ shape) and energy stores are increased
b. Stage 2 (Mitosis)
The chromosomes line up at the equator of the cell and spindle fibres pull each chromosome of the ‘X’ to either side of the cell.
c. Stage 3 (Cytokinesis)
Two identical daughter cells form when the cytoplasm and cell membranes divide, each containing the same 46 chromosomes as the original cell.
💡 Cell division by mitosis in multicellular organisms is important in their growth and development, and when replacing damaged cells. Mitosis is also a vital part of asexual reproduction, as this type of reproduction only involves one organism, so to produce offspring it simply replicates its own cells.
3. Differentiation and Specialisation
a. Specialised Cells
Cells specialise by undergoing differentiation
A process that involves the cell gaining new sub-cellular structures in order for it to be suited to its role.
Cells can either differentiate once early on or have the ability to differentiate their whole life (these are called stem cells).
In animals, most cells only differentiate once, but in plants many cells retain the ability.
b. Examples of Specialised Cells in Animals
Sperm cells
Specialised to carry the male’s DNA to the egg cell (ovum) for successful reproduction
Streamlined head and long tail to aid swimming
Many mitochondria (where respiration happens) which supply the energy to allow the cell to move
The acrosome (top of the head) has digestive enzymes which break down the outer layers of membrane of the egg cell
Nerve cells
Specialised to transmit electrical signals quickly from one place in the body to another
The axon is long, enabling the impulses to be carried along long distances
Having lots of extensions from the cell body (called dendrites) means branched connections can form with other nerve cells
The nerve endings have many mitochondria which supply the energy to make special transmitter chemicals called neurotransmitters. These allow the impulse to be passed from one cell to another.
Muscle cells
Specialised to contract quickly to move bones (striated muscle) or simply to squeeze (smooth muscle, e.g found in blood vessels so blood pressure can be varied), therefore causing movement
Special proteins (myosin and actin) slide over each other, causing the muscle to contract
Lots of mitochondria to provide energy from respiration for contraction
They can store a chemical called glycogen that is used in respiration by mitochondria
c. Examples of Specialised Cells in Plants
Root hair cells
Specialised to take up water by osmosis and mineral ions by active transport from the soil as they are found in the tips of roots
Have a large surface area due to root hairs, meaning more water can move in
The large permanent vacuole affects the speed of movement of water from the soil to the cell
Mitochondria to provide energy from respiration for the active transport of mineral ions into the root hair cell
Xylem cells
Specialised to transport water and mineral ions up the plant from the roots to the shoots
Upon formation, a chemical called lignin is deposited which causes the cells to die. They become hollow and are joined end-to-end to form a continuous tube so water and mineral ions can move through
Lignin is deposited in spirals which helps the cells withstand the pressure from the movement of water
Phloem cells
Specialised to carry the products of photosynthesis (food) to all parts of the plants
Cell walls of each cell form structures called sieve plates when they break down, allowing the movement of substances from cell to cell
Despite losing many sub-cellular structures, the energy these cells need to be alive is supplied by the mitochondria of the companion cells.
4. Stem Cells
a. Characteristics of Stem Cells
A stem cell is an undifferentiated cell which can undergo division to produce many more similar cells
Some of these will differentiate to have different functions, such as the specialised cells mentioned above
They are important in development, growth and repair
b. Types of Stem Cells
Embryonic stem cells
Form when an egg and sperm cell fuse to form a zygote
They can differentiate into any type of cell in the body
Scientists can clone these cells (though culturing them) and direct them to differentiate into almost any cell in the body
These could potentially be used to replace insulin-producing cells in those suffering from diabetes, new neural cells for diseases such as Alzheimer’s, or nerve cells for those paralysed with spinal cord injuries
Adult stem cells
If found in bone marrow they can form many types of cells (not any type, like embryonic stem cells can) including blood cells
Meristems in plants
Found in root and shoot tips
They can differentiate into any type of plant, and have this ability throughout the life of the plant
They can be used to make clones of the plant
This may be necessary if the parent plant has certain desirable features (such as disease resistance), for research or to save a rare plant from extinction
III. The Challenges of Size
1. Exchange Systems
As mentioned before, multicellular organisms have a small surface area to volume ratio compared to the amount of substances they need to exchange.
a. Surface Area to Volume Ratio
The size of the surface area of the organism compared to its volume
Calculated by finding the volume (length x width x height) and the surface area (length x width), and writing the ratio in the smallest whole numbers
If this is large, the organism is less likely to require specialised exchange surfaces and a transport system because the rate of diffusion is sufficient in supplying and removing the necessary gases
E.g 15 (surface area): 5 (volume) is written as 3:1
Multicellular organisms have had to adapt to increase this ratio as much as possible.
Adaptation | Why? | Example |
Having a large surface area | The greater the surface area, the more particles can move through, resulting in a faster rate of diffusion | Lungs: the small, spherical alveoli (sites of gaseous exchange) in the lungs create a very large surface area (approximately 75m2 in humans). Small intestine: the cells of the small intestine have millions of villi, which are projections which increase the surface area. This means digested food can be absorbed into the blood faster Fish gills: these contain lamellae to increase the surface area. Leaves: the flattened shape increases the surface area. The air spaces inside the leaf increase the surface area, so more carbon dioxide can enter cells. |
Having a thin membrane | Provides a short diffusion pathway, allowing the process to occur faster | Lungs: alveoli and capillary walls are extremely thin. Small intestine: villi have a single layer of surface cell. |
Having an efficient blood supply OR having good ventilation (in animals | Creates a steep concentration gradient, so diffusion occurs faster | Lungs: the lungs constantly supply oxygen to make the blood from alveoli capillaries oxygenated, by exchanging it for carbon dioxide that can be breathed out. This is a constant process meaning the concentration gradient is always steep. Fish: water flows in one direction and blood flows in the other - this means that a steep concentration gradient is maintained as the concentration of oxygen is always much higher in the water - so it will diffuse across. |
2. Human Circulatory System
The heart is an organ in the circulatory system. The circulatory system carries oxygen and nutrients to every cell in the body and removes the waste products.
The heart pumps blood around the body in a double circulatory system. This means there are two circuits. Mammals require this double system because the metabolic rate is higher and so need a faster system.
System 1
Deoxygenated blood flows into the right atrium and then into the right ventricle which pumps it to the lungs to undergo gaseous exchange
System 2
Oxygenated blood flows into the left atrium and then into the left ventricle which pumps oxygenated blood around the body
a. Structure of the heart
Muscular walls to provide a strong heartbeat
The muscular wall of the left ventricle is thicker because blood needs to be pumped all around the body rather than just to the lung like the right ventricle.
4 chambers that separate the oxygenated blood from the deoxygenated blood
2 atria above and 2 ventricles below
Valves to make sure blood does not flow backwards
Coronary arteries cover the heart to provide its own oxygenated blood supply
Process
Blood flows into the right atrium through the vena cava, and left atrium through the pulmonary vein.
The atria contract forcing the blood into the ventricles.
The ventricles then contract, pushing the blood in the right ventricle into the pulmonary artery to be taken to the lungs, and blood in the left ventricle to the aorta to be taken around the body.
As this happens, valves close to make sure the blood does not flow backwards.
b. Structure of blood vessels
Arteries
Carry blood AWAY from the heart
Layers of muscle in the walls make them strong
Elastic fibres allow them to stretch
This helps the vessels withstand the high pressure created by the pumping of the heart
Veins
Carry blood TOWARDS the heart
The lumen (the actual tube in which blood flows through) is wide to allow the low pressure blood to flow through
They have valves to ensure the blood flows in the right direction
Capillaries
Allow the blood to flow very close to cells to enable substances to move between them
One cell thick walls create a short diffusion pathway
Permeable walls so substances can move across them
c. Structure of blood
Red blood cells
Contain haemoglobin
A red protein that combines with oxygen to allow for transport
No nucleus
To create more space for haemoglobin
Biconcave shape
To maximise surface area for oxygen to be absorbed
Flexible
So they can fit through very narrow blood vessels
Plasma
Plasma is the liquid which carries all of the components of blood, such as blood cells, platelets, amino acids, urea etc.
Plasma is mainly made up of water and many substances that need to be transported around the body, e.g. carbon dioxide, urea, are water-soluble
d. Transpiration and Water Uptake
Transpiration
The loss of water vapour from the leaves and stems of the plant. It is a consequence of gaseous exchange, as the stomata are open so that this can occur.
Water also evaporates at the open stomata
As water molecules are attracted to each other, when some molecules leave the plant the rest are pulled up through the xylem
This results in more water being taken up from the soil resulting in a continuous transpiration stream through the plant
Xylem
Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration.
Xylem is adapted in many ways:
A chemical called lignin is deposited which causes the cells to die.
These cells then become hollow and join end-to-end to form a continuous tube for water and mineral ions to travel through from the roots
Water molecules are attracted to each other by hydrogen bonding
Creating a continuous column of water up the plant
The water evaporates from the leaves of the plant, creating the transpiration stream.
Lignin strengthens the plant to help it withstand the pressure of the water movement
Lignin contains bordered pits, which are holes to allow specific areas for water and therefore minerals to enter the plant
Root hair cell
Water is taken up by plants through root hair cells
These are specialised cells with a very large surface area to absorb water via osmosis.
If the rate of transpiration increases then the rate of water uptake will also increase as the plant attempts to replenish the loss.
Guard cells
Open and close stomata
They are kidney shaped, with thin outer walls and thick inner walls
When lots of water is available to the plant, the cells fill and change shape, opening stomata (they are also light sensitive)
This allows gases to be exchanged and more water to leave the plant via evaporation
More stomata are found on the bottom of the leaf, allowing gases to be exchanged whilst minimising water loss by evaporation as the lower surface is shaded and cooler.
Factors affecting water uptake
Increase in light intensity: This leads to an increased rate of photosynthesis, so more stomata open to allow gaseous exchange to occur. This means more water can evaporate, leading to an increased rate of transpiration and so uptake.
Increase in temperature: The molecules move faster, resulting in evaporation happening at a faster rate and therefore the rate of transpiration increases. The rate of photosynthesis increases, meaning more stomata are open for gaseous exchange, so more water evaporates and the rate of transpiration increases. Therefore, water uptake also increases
Increased air movement (wind): If more air is moving away from the leaf due to it being blown away, then the concentration of water vapour surrounding the leaf will be lower. This will mean there will be a steeper concentration gradient resulting in diffusion happening faster. This will increase the rate of transpiration and also water uptake.
Increase in humidity: If the relative humidity is high, then there will be a reduced concentration gradient between the concentrations of water vapour inside and outside the leaf, resulting in a slower rate of diffusion. This will decrease the rate of transpiration and water uptake.
Potometer
Can be used to investigate how these factors affect water uptake.
It is set up underwater to remove air bubbles in the xylem so that there is a continuous stream of water and the system is made airtight, apart from a singular bubble of air.
The distance this air bubble in the capillary tube moves over time is measured.
If it moves faster then it means that there is a greater rate of water uptake and therefore rate of transpiration.
An environmental condition, such as light intensity, is changed each time the experiment is run in order to see how it affects the plant.
e. Translocation
The movement of food substances made in the leaves up or down the phloem, for use immediately or storage
Phloem adaptations
Found in the roots, stems and leaves
Elongated cells with holes in the cell walls (the end walls are called sieve plates)
Many organelles from the cells are removed so cell sap can move through.
However, there are many mitochondria in companion cells which provide the energy the cells require
Food substances can be moved in both directions (translocation), from the leaves where they are made for use, or from storage (underground) to parts of the plant that need it.
