Cells, Exchange and Transport (ALL ANSWERS POSSIBLE)
Cells, Exchange and Transport
Cells
(a)state
the resolution and magnification that can be achieved by a light
microscope, a transmission electron microscope and a scanning electron
microscope;
Resolution
|
Magnification
|
|
Light microscope
|
200nm
|
x1,500
|
Transmission Electron Microscope
|
0.1nm
|
x500,000
|
Scanning Electron Microscope
|
0.1nm
|
X100,000
|
(b)explain the difference between magnification and resolution;
Magnification is the degree to which the size of an image is larger than the image itself.
Resolution is the degree to which it is possible to distinguish between two objects that are very close together.
(c)explain the need for staining samples for use in light microscopy and electron microscopy;
A lot of biological material inside a cell isn’t
coloured, so it might be difficult to distinguish between different
features. Coloured stains are used to stain specimens for use with the
light microscope. Chemicals which bind to other chemicals on, or in, the
specimen, which allows the specimen be to seen. Some chemicals bind to
specific structures, such as Acetic orcein staining DNA red.
Electron micrographs start off black and white, with the colour being added by a specialised computer program afterwards.
(d)calculate the linear magnification of an image;
Image size =Actual size x Magnification
(e)describe
and interpret drawings and photographs of eukaryotic cells as seen
under an electron microscope and be able to recognise the following
structures:
Nucleus,
Larges organelle.
Nucleolus,
Dense, spherical structure inside nucleus
Nuclear envelope,
Surrounds the nucleus
Rough and smooth endoplasmic reticulum (ER),
Continuous with the nuclear envelope. RER is studded with ribosomes, SER is not.
Golgi apparatus,
Stack of membrane-bound flattened sacs
Ribosomes,
Tiny. Some are in the cytoplasm and some are bound to the RER
Mitochondria,
Spherical or sausage shaped. Double membrane.
Lysosomes,
Spherical sacs. Single membrane.
Chloroplasts,
Only in plant cells. Two membranes. Contain Thylakoids.
Plasma (cell surface) membrane,
Phospholipid bilayer
Centrioles,
Small tubes of protein fibres. Pair of them next to Nucleus in Animal cells.
Flagella and cilia;
(f)outline the functions of the structures listed in (e); Nucleus,
Houses all of the cell’s genetic material in the form of DNA, which contains the instructions for protein synthesis.
Nucleolus,
Makes ribosomes and RNA which pass into the cytoplasm and are used in protein synthesis
Nuclear envelope,
A double membrane with nuclear pores.
Rough endoplasmic reticulum,
Transports proteins made by the attached ribosomes.
Smooth endoplasmic reticulum (ER),
Involved in the making of lipids.
Golgi apparatus,
Modifies proteins received from the Rough ER and then packages them into vesicles so they can be transported.
Ribosomes,
Site of protein synthesis.
Mitochondria,
Where ATP is made.
Lysosomes,
Contain digestive enzymes that are used to break down material
Chloroplasts,
Site of photosynthesis in plant cells.
Plasma (cell surface) membrane,
Controls the entry and exit of substances into and out of the cell.
Centrioles,
Form the spindle which moves chromosomes during cell division.
Flagella and cilia;
Move by ATP. E.g. wave mucus along trachea or make sperm swim.
(f)outline
the interrelationship between the organelles involved in the production
and secretion of proteins (no detail of protein synthesis is required);
1.The gene containing the instructions for the production of the hormones is copied onto a piece of mRNA
2.mRNA leaves the nucleus through the nuclear pore.
3.mRNA attaches to a ribosome
4.Ribosome reads the instruction to assemble the protein
5.Molecules are ‘pinched off’ in vesicles and travel towards the Golgi Apparatus
6.Vesicle fuses with Golgi Apparatus
7.Golgi apparatus processes and packages the molecules, ready for release
8.The molecules are ‘pinched off’ in vesicles from the Golgi Apparatus and move towards the cell surface membrane
9.Vesicles fuse with the cell surface membrane
10.Cell surface membrane opens to release molecules outside
(g)explain
the importance of the cytoskeleton in providing mechanical strength to
cells, aiding transport within cells and enabling cell movement;
The cytoplasm contains a network of two kinds of
proteins fibres which keep the cell’s shape stable by providing an
internal framework. The types of protein fibres are:
•Microfilaments (small solid strands made of actin, 7nm diameter)
•Microtubules (protein cylinders made of tubulin molecules, 25nm diameter)
Their functions include:
•Supporting organelles
•Strenghtening the cell and maintaining cell shape
•Transporting materials within in the cell (e.g. the spindle during mitosis)
•Cell movement (cilia and flagella)
Microtubules do not move, but they provide an anchor
for protein to move along. E.g. kinesin attaches one end to an organelle
and the other end to a microtubule. Using ATP it ‘swivels’, pushing the
organelle along. The head then reattaches itself to the microtubule and
the process is repeated.
Flagella and cilia are each made from a cylinder
containing 9 microtubules. Flagella move with the aid of the protein,
Dynein. When a molecule of dyneine ‘swivels’ it pulls one microtubule
past the next, causing the cilium to bend.
Cilia move out of time with each other to create a ‘wave’
Cells, Exchange and Transport
(h)compare
and contrast, with the aid of diagrams and electron micrographs, the
structure of prokaryotic cells and eukaryotic cells;
Prokaryotic cells do not have a nucleus. They are bacteria and are much smaller than Eukaryotic cells. They have:
•One membrane
•No membrane-bound organelles
•Cell wall made of peptidoglycan not cellulose
•Their ribosomes are smaller
•Circular DNA
•DNA is not surrounded by a membrane.
•ATP production takes place in specialised infolded regions of the cell surface membrane
•Some have Flagella
(i)Compare
and contrast, with the aid of diagrams and electron micrographs, the
structure and ultrastructure of plant cells and animal cells.
Plant cells have a cell wall. This is outside the cell surface membrane and it made of cellulose, which forms a sieve-like
network of strands which make the cell wall strong. This is kept rigid
by the pressure of the fluid inside the cell, so supports the cell and
therefore the entire plant.
Plant cells also contain a Vacuole. This maintains
the cell stability by making the cell turgid as it increases the
pressure inside the cell. This in turn helps support the plant.
Cells, Exchange and Transport
Cell Membranes
(a)outline the roles of membranes within cells and at the surface of cells;
•Separate cell contents from the outside environment
•Separate cell components from the outside environment
•Cell recognition and signalling
•Holding the components of some metabolic pathways in place
•Regulating the transport of materials into or out of cells
(b)state that plasma (cell surface) membranes are partially permeable barriers;
(c)describe, with the aid of diagrams, the fluid mosaic model of membrane structure;
A bilayer of phosopholipid molecules forms the main
structure. Various proteins are studded in the bilayer. Some are
partially embedded (extrinsic) whereas some completely span the membrane
(intrinsic)
(d)describe the roles of the components of the cell membrane; Phospholipids,
Have a hydrophobic head and a fatty acid tail. They
form a bilayer separating the cell from the outside. They are fluid so
components can move around freely. They are permeable to small and/or non-polar molecules, but impermeable to large molecules and ions.
Cholesterol,
Gives the membranes mechanical stability by sitting
between fatty acid tails and therefore making the barrier more complete,
preventing molecules like water and ions from passing through the
membrane.
Glycolipids,
Phosopholipid molecules that have a carbohydrate part
attached. They are used for cell signalling, cell surface antigens and
cell adhesion.
Proteins
Channel proteins allow the movement of some
substances, such as the large molecule sugar, into and out of the cell
as they can’t travel directly through the cell surface membrane
Carrier proteins actively move substances across the cell surface membrane.
Glycoproteins;
Phospholipid molecules with a protein attached. Same functions as glycolipids.
(e)outline the effect of changing temperature on membrane structure and permeability;
Increasing the temperature means that the molecules
have more kinetic energy. This increased movement makes the membrane
leaky, so molecules which would not normally do so can move into and out
of the cell.
(f)explain the term cell signalling;
Process that leads to communication and coordination
between cells, e.g. hormones binding to their receptors on the cell
surface membrane
(g)explain the role of membrane-bound receptors as sites where hormones and drugs can bind;
Hormones are used in cell signalling. The Target
Cells have a receptor which is complementary to the hormone, meaning
that it can bind to the receptor cells, triggering the desired internal
response.
Drugs have also been developed which bind to the receptor molecules on cells. Beta-blockers
are used to prevent a muscle from increasing the heart rate to a
dangerous level, and some drugs used to treat schizophrenia mimic a
natural neurotransmitter which some individuals cannot produce.
(h)explain what is meant by
Passive transport (diffusion and facilitated diffusion including the role of membrane proteins),
Passive transport is the transport of a molecule
without using energy. Diffusion is the net movement of molecules from a
region of high concentration of the molecule to an area of lower
concentration of the molecule down a concentration gradient.
Large and charged molecules need to be transported
across the phospholipid bilayer, they can’t just diffuse across. They
travel either through channel proteins, which are shaped to allow only
one molecule through and are often gated, or carrier proteins, whose
shape can fit a specific molecule, and they then change shape to allow
the molecule through to the other side of the membrane.
Active transport
The movement of molecules or ions across membranes, using ATP to drive ‘protein pumps’ within the membrane
Endocytosis
When large quantities of a material are brought into the cell. Uses ATP.
Exocytosis;
When large quantites of a material are moved out of the cell. Uses ATP.
(i)explain what is meant by osmosis, in terms of water potential. (No calculations of water potential will be required);
The movement of water molecules from a region of
higher water potential to a region of lower water potential across a
partially permeable membrane
(i)recognise and explain the effects that solutions of different water potentials can have upon plant and animal cells
Type of cell
|
Pure water (high water potential)
|
Solution with a v
|
Animal
|
Water moves in. Cell bursts-
|
Water moves out. Cell is crenated
|
haemolysed
|
||
Plant
|
Water moves in. Cell is tugid
|
Water moves out. Cell is plasmolysed.
|
Cells, Exchange and Transport
Cell Division, Cell Diversity and Cellular Organisation
(a)state
that mitosis occupies only a small percentage of the cell cycle and that
the remaining percentage includes the copying and checking of genetic
information;
(b)describe,
with the aid of diagrams and photographs, the main stages of mitosis
(behaviour of the chromosomes, nuclear envelope, cell membrane and
centrioles);
•In Interphase (Pre-Mitosis):
oThe DNA replicates.
•In Prophase:
oThe chromosomes supercoil & become visible under a light microscope.
oThe nuclear envelope breaks down.
oThe centriole divides in two and move to opposite ends of the cell to form a spindle.
•In Metaphase:
oThe chromosomes like up along the middle of the cell.
oThey attach to a spindle thread by their centromere.
•In Anaphase:
oThe replicated sister chromatides are separated when the centromere splits.
oThe spindle fibres shorten, pulling the chromatids apart.
•In Telophase:
oAs the separated sister Chromatids reach the poles of the cells.
oA new nuclear envelope forms around each set.
oThe spindle breaks down.
oThe Chromosomes uncoil so they are no longer visible under a light microscope.
•In Cytokinesis (Post-mitosis):
oThe whole cell splits to down two new cells, each one identical to each other and to the parent cell.
(b)explain the meaning of the term homologous pair of chromosomes;
Chromosomes that have the same genes at the same
loci. Members of a homologous pair of chromosomes pair up during
meiosis. Diploid organisms produced by sexual reproduction have
homologous pairs of choromosomes- one member of each pair from the
mother and one from the father.
(c)explain the significance of mitosis for growth, repair and asexual reproduction in plants and animals;
Growth- multicellular organisms produce new extra
cells to grow. Each new cell is genetically identical to the parents
cell, and so can perform the same function
Repair- damaged cells need to be replaced by new ones
that perform the same functions and so need to the genetically
identical to the parent cell, as with growth.
Asexual reproduction- single celled organisms divide
to produce two daughter cells that are separate organisms. Some
multicellular organisms produce offspring from parts of the parent.
(d)outline, with the aid of diagrams and photographs, the process of cell division by budding in yeast;
Yeast cells undergo cytokinesis by producing a small ‘bud’ that nips of the cell, a process called budding.
(f)state that cells produced as a result of meiosis are not genetically identical (details of meiosis are not required);
(g)define the term stem cell;
Undifferentiated cells that are capable of becoming differentiated to a number of possible cell types.
(h) define the term differentiation, with reference
to the production of erythrocytes (red blood cells) and neutrophils
derived from stem cells in bone marrow, and the production of xylem
vessels and phloem sieve tubes from cambium;
The changes occurring in the cells of a multicellular
organism so that each different type of cell becomes specialised to
perform a specific function.
Erythrocytes and neutrophils both originate as
undifferentiated stem cells in bone marrow. The cells destined to become
erythrocytes lose their nucleus, golgi apparatus and rough endoplasmic
reticulum. They are filled with haemoglobin, the shape of the cell
changes to become a biconclave disc so that it is capable of
transporting oxygen from the lungs to tissues.
The cells destined to become neutrophils keep their
nucleus; a huge number of lysosomes are produced, so their cytoplasm
appears granular. The lysosomes contain enzymes so that the neutrophil
can ingest invading microorganisms.
Xylem and phloem both come from meristem cells.
In xylem, the meristem cells elongate and the walls
become elongated and waterproofed by deposits of lignin, which kills the
cell contents. The ends of the cell break down so they become long
tubes with wide lumen. They are suited to transporting water and
minerals up the plant, and also support the plant.
In the phloem, the cells also elongate, but their
ends do not break down completely, but form sieve plates between the
cells. Next to each sieve plate is a companion cell which is very
metabolically active and used in moving the products of photosynthesis
up and down the plant.
(i) describe and explain, with the aid of diagrams
and photographs, how cells of multicellular organisms are specialised
for particular functions, with reference to
erythrocytes (red blood cells),
Biconclave disc shape to maximise surface area No nucleus = more room for haemoglobin
neutrophils,
Flexible shape to engulf foreign particles or pathogens
Many lysosomes contain digestive enzymes to break down the engulfed particles epithelial cells,
Some have cilia to move particles
Some have mnicrovilli to increase surface area sperm cells,
Organelle content
Many mitochondria to generate energy for movement of undulipodium
Specialised lysosome in sperm head which contains an enzyme specialised to break down the egg wall
Shape
Very small, long and thin to help in easing their movement Undulipodium to move
Content
Nucleus contains half to number of chromosomes of an adult cell in order to fulfil its role as a gamete.
palisade cells,
Contain chloroplasts to absorb light
Thin walls so that Carbon dioxide can diffuse in root hair cells
Hair like projections to increase surface area to absorb water and minerals from the soil. guard cells;
Thin outer wall, thick inner wall.
In light they absorb water to become turgid and allow exchange of gases.
(j)explain the meaning of the terms tissue,
A group of similar cells that perform a particular function
organ
A collection of tissues that work together to form a specific overall function or set of functions within a multicellular organ
organ system
A number of organs working together to form a life function
(k)explain,
with the aid of diagrams and photographs, how cells are organised into
tissues, using squamous and ciliated epithelia, xylem and phloem as
examples;
There are four main types of animal tissue:
•Epithelial tissue
oLayers and linings
•Connective tissues
oHold structures together and provide support
•Muscle tissue
oCells specialised to contract and move parts of the body
•Nervous tissue
oCells that convert stimuli to electrical impulses and conduct those impulses.
Within these main types, there are smaller groups of tissues
Squamous epithelial tissue
•Flattened cells that form a thin, smooth, flat surface.
•Line the insides of tubes such as blood vessels
•Also form thin walls
oAlveoli
•Held in place by basement membrane
oMade of collagen and glycoproteins
oSecreted by epithelial calls
Cells, Exchange and Transport
Ciliated epithelial tissue
•Column- shaped
•Exposed surface covered with cilia
•Move in synchronised waves
•Found on surface of tubes (e.g. bronchi, oviduct)
•Waft mucus in lungs, egg in oviduct
Xylem
•Composed of xylem vessel cells and parenchyma cells
•Parenchyma cells fill the gaps between xylem vessels to provide support
Phloem
•Comprises of sieve tubes and companion cells
•Companion cells are highly metabolically active, moving products of photosynthesis up and down the phloem.
(l)discuss the importance of cooperation between cells, tissues, organs and organ systems
Movement: the muscular and skeletal system must work
together for movement to take place, but this can only happen if the
nervous system ‘instructs’ muscles to coordinate their actions. As
muscles and nerves work, they use energy, so they require a supply of
nutrients and oxygen from the circulatory system, which in turn receives
the chemicals from the digestive and ventilation systems.
Cells, Exchange and Transport
Exchange Surfaces and Breathing
(a)explain, in terms of surface area:volume ratio, why multicellular organisms need specialised exchange surfaces and single-celled organisms do not;
Organisms need to absorb certain substances, (e.g.
oxygen, glucose, proteins, fats, water and minerals) from the
surrounding environment and remove waste products (carbon dioxide,
oxygen and other wastes). Single celled organisms have a large surface-area-to-volume ratio so they can exchange the necessary gases, nutrients and wastes.
Multicellular organisms not only need more supplies as they have more cells, but they also have a smaller surface-area-to-volume
ratio, meaning that the outer surface is not large enough to enable
gases and nutrients to enter the body fast enough to keep all of the
cells alive.
Nutrients and gases also have to travel a larger distance to the centre of the organism.
So, larger organisms need a large area to exchange
more substances, so often they combine this with a transport system to
move substances around the body.
(b)describe
the features of an efficient exchange surface, with reference to
diffusion of oxygen and carbon dioxide across an alveolus;
Large surface area to provide more space for molecules to pass through Thin barrier to reduce the diffusion distance
Fresh supply of molecules on one side to maintain the diffusion gradient
Carbon dioxide is brought in the blood to the
lungs. The concentration is higher in the blood than in the alveoli, so
it diffuses across.
Breathing fills the lungs with air, so there is
more oxygen in the alveolus than in the blood Removal of required
molecules on the other side to maintain the steep diffusion gradient
Blood carries oxygen away from the lungs Breathing removes Carbon Dioxide from the lungs
(c)describe the features of the mammalian lung that adapt it to efficient gaseous exchange;
Many, many alveoli meaning that the total surface area is about 70m2.
Alveolus wall is one cell thick
Capillary wall is one cell thick
Both walls consist of squamous cells
Capillaries in close contact with the alveolus wall
Narrow capillaries
Red blood cells are closer to the capillary wall
Closer to air in the alveoli
Reducing the rate at which the red blood cells flow past in the blood
Total barrier is only two flattened cells, or 1μm thick
(d)describe,
with the aid of diagrams and photographs, the distribution of
cartilage, ciliated epithelium, goblet cells, smooth muscle and elastic
fibres in the trachea, bronchi, bronchioles and alveoli of the mammalian
gaseous exchange system;
The trachea and bronchi have a similar structure, but the bronchi are narrower than the trachea
Thick walls made of several layers of tissue Much of the wall consists of cartilage
Regular C-rings in the trachea Less regular in the bronchi
On the inside surface of the cartilage is a layer
of glandular tissue, connective tissue, elastic fibres, smooth muscle
and blood vessels
The inner layer is an epithelium layer than has two
types of cells. Most of the cells are ciliated epithelium, and there
are goblet cells amongst them
Bronchioles
Much narrower than the bronchi
Larger bronchioles have some cartilage, but the smaller ones don’t. The wall is made mostly of smooth muscle and elastic fibres
Alveoli
Wall is one cell thick 100-300μm diameter Good blood supply
Cells, Exchange and Transport
(e) describe the functions of cartilage,
Structure.
Holds the trachea and bronchi open
Prevents collapse when the air pressure is low during inhalation
cilia,
Move in a synchronised pattern to waft mucus up the
airway to the back of the throat. Once there, the mucus is swallowed and
the acidity of the stomach will kill any bacteria
goblet cells,
Secrete mucus.
Traps tiny particles from the air Reduces risk of infection
smooth muscle
Can contract to restrict airway
Prevents harmful substances from reaching the alveoli
elastic fibres
Reverses the effect of the smooth muscle
When the smooth muscle constricts it deforms the
elastic fibres. As the smooth muscle relaxes, the elastic fibres recoil
to their original size and shape, helping to dilate the airway
in the mammalian gaseous exchange system;
(f)outline
the mechanism of breathing (inspiration and expiration) in mammals,
with reference to the function of the rib cage, intercostal muscles and
diaphragm;
Inspiration
1.Diaphragm contracts to becoming flatter, pushing digestive muscles down
2.External intercostal muscles contract to raise ribs
3.Volume of chest cavity increases
4.Pressure in chest cavity drops below atmospheric pressure
5.Air moves into lungs
Expiration
1.Diaphragm relaxes and is pushed up by displaced organs underneath
2.External intercostal muscles relax and ribs fall
3.Volume of chest cavity decreases
4.Pressure in lungs increases and rises about atmospheric pressure
5.Air moves out of lungs
(g)explain the meanings of the terms tidal volume
The volume of air moved in and out of the lungs during breathing when at rest vital capacity
The largest volume of air that can be moved into and out of the lungs in any one breath
(g) describe how a spirometer can be used to measure
A spirometer consists of a chamber filled with
oxygen floating on a tank of water. A person breaths from a mouthpiece
attached to a tube connected to the oxygen tank. Breathing in takes
oxygen from the chamber so it sinks down, and breathing out pushing air
back into the chamber which floats up. The movements of the chamber is
recorded using a datalogger.
vital capacity,
Asking a person to breathe in and out as much as they can tidal volume,
Asking a person to breathe normally breathing rate
Asking a person to breathe normally, and then
dividing the number of breathes by the time in minutes to calculate the
number of breaths per minute
oxygen uptake;
Divide (the amount of oxygen (dm3) times 60) by the time taken in seconds.
(i) analyse and interpret data from a spirometer.
Cells, Exchange and Transport
Transport in Animals
(a)explain the need for transport systems in multicellular animals in terms of size,
Once an animal has several layers of cells any
oxygen or nutrients diffusing in form the outside will be used up by the
other layers of cells and the cells deeper in the body will not get any
oxygen or nutrients.
level of activity
If an animal is very active then it will need a good supply of nutrients and oxygen to supply the energy for movement.
surface area:volume ratio;
To allow animals to grow to a large size, it needs a
range of tissues and structural support to give the body strength.
Their volume increases as the body gets thicker, but the surface area
does not increase as much. So the surface-area-to-volume
ratio of a large animal is relatively small. Larger animals do not have a
large enough surface area to supply all of the oxygen and nutrients
that they need.
(b)explain the meaning of the terms
single circulatory system
A circulation in which the blood flows through the heart once during each circulation of the body e.g. fish
double circulatory system
A circulation in which the blood flows through the heart twice during each complete circulation of the body e.g. mammals
with reference to the circulatory systems of fish and mammals;
(c) explain the meaning of the terms Open circulatory system
The blood is not always in vessels e.g. insects
Closed circulatory system,
The blood is always in vessels e.g. fish
with reference to the circulatory systems of insects and fish;
(d)describe, with the aid of diagrams and photographs, the external and internal structure of the mammalian heart;
External
The largest parts of the heart are the ventricles.
Above the ventricles les the atria which are much smaller. The coronary
arteries lie over the surface of the heart and carry oxygenated blood to
the heart muscle.
At the top of the heart are the veins that carry blood to the heart, and arteries that carry blood away from the heard.
Internal
Divided into four chambers.
The two upper chambers are atria which receive blood
from the major veins (deoxygenated blood from the body flows into right
atrium from the vena cava, and oxygenated blood flows from the lungs
into the left atrium).
The two lower chambers are the ventricles. They are
separated from each other by the septum and from the atria by the
atrioventricular valves which prevent the blood from flowing the wrong
way. These are attached to tendinous cords which prevent the valves from
turning inside out.
(e)explain,
with the aid of diagrams, the differences in the thickness of the walls
of the different chambers of the heart in terms of their functions;
The walls of the atria are very thin. They do not
need to create much pressure are they are only pushing the blood into
the ventricles
The walls of the right ventricle are thicker than
the atria as only need to pump blood to the lungs. The blood vessels of
the lungs are also very thin, so they would burst of the blood was under
too much pressure.
The walls of the left ventricle are two or three
times thicker than the right as the blood is pumped around the entire
body and so needs to be under high pressure.
Cells, Exchange and Transport
(f)describe the cardiac cycle, with reference to the action of the valves in the heart;
1.When both the atria and the ventricles are relaxed, blood flows into the atria from the major veins.
2.The blood flows through the atrioventricular valves into the ventricles
3.The atria contract simultaneously, pushing blood into the ventricles
4.Blood fills the atrioventricular valve, causing them to snap shut and preventing the blood from flowing back into the ventricles
5.When the pressure in the arteries is higher than the pressure in the ventricles, the semilunar valves shut
6.The walls of the ventricles contract, starting from the bottom
7.When
the pressure in the ventricles is higher than the pressure in the
arteries, the semilunar valves is pushed open and blood is pushed out of
the heart. The contraction only lasts for a short time
8.The ventricles relax
9.When the pressure in the ventricles drops to below that of the atria, the atrioventricular valves open again
10.When the pressure in the ventricles drops to below that of the arteries, the semilunar vavles shut again
(g)describe
how heart action is coordinated with reference to the sinoatrial node
(SAN), the atrioventricular node (AVN) and the Purkyne tissue;
The SAN is the pacemaker, situated at the top of the right atrium. The SAN initiates a wave of excitation at regular intervals.
The wave of excitation quickly travels over the walls of both atria. As it passes, it causes the muscle cells to contract.
At the base of the atria is a disc of tissue that
cannot conduct the electrical impulse, so the only route through to the
ventricles is via the AVN, which is at the top of the septum.
The excitation is delayed here to allow for the atria to finish contracting and for the blood to flow into the ventricles.
The wave is then passes away from the AVN down
specialised conduction tissue known as Purkyne tissue. At the base of
the septum, the wave of excitation spreads out over the walls of the
ventricles.
As the excitation spreads upwards from the apex,
the muscles contract, pushing blood up to the major arteries at the top
of the heart
(h)interpret and explain electrocardiogram (ECG) traces, with reference to normal and abnormal heart activity;
(i)describe, with the aid of diagrams and photographs, the structures and functions of
arteries,
Carry blood at high pressure, so artery wall must be able to withstand the pressure Relatively small lumen to maintain pressure
Relatively thick wall containing collagen to give
it strength to withstand high pressure The wall contains elastic tissue
that allows the wall to stretch and then recoil when the heart pumps-this
is a pulse. The recoil maintains the high pressure when the heart
relaxes The wall contains smooth muscle that can contract and constrict
the artery.
The endothelium is folded and can unfold when the artery stretches.
veins
Carry blood at low pressure so the walls do not need to be thick Lumen is relatively large to ease the flow of blood
The walls have thinner layers of collagen, smooth
muscle and elastic tissue. They do not need to stretch and recoil and
are not actively constricted to reduce blood flow.
Contain valves to prevent blood flowing in the
wrong direction. As the walls are thin, the vein can be flattened by the
action of the surrounding skeletal muscles. Pressure is applied to the
blood, forcing it to move along in the direction dictated by the valves.
capillaries;
Walls consist of a single layer of flattened
endothelial cells that reduces the diffusion distance for the materials
being exchanged
The lumen is the same diameter as a red blood cell
(about 7μm). This ensures that the red blood cells are squeezed as they
pass along the capillaries. The diffusion distance is shorter, so they
are more likely to give up their oxygen
Cells, Exchange and Transport
(j) explain the differences between blood, tissue fluid and lymph;
Feature
|
Blood
|
Tissue Fluid
|
Lymph
|
Cells
|
Erythrocytes, Leucocytes
|
Some phagocytic white
|
Lymphocytes
|
and platelets
|
blood cells
|
||
Proteins
|
Hormones and plasma
|
Some hormones, proteins
|
Some proteins
|
proteins
|
secreted by body cells
|
||
Fats
|
Some transported as
|
None
|
More than in blood
|
lipoproteins
|
|||
Glucose
|
Less
|
Less
|
|
Amino acids
|
More
|
Less
|
Less
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Oxygen
|
More
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Less
|
Less
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Carbon dioxide
|
Little
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More
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More
|
(k) describe how tissue fluid is formed from plasma;
At the arterial end of a capillary, the blood is
under high pressure due to contractions of the heart (hydrostatic
pressure). It will tend to push the blood fluid out of the capillaries.
It can leave through tiny gaps in the capillary wall. The fluid consists
of plasma with dissolved nutrients and oxygen.
(l) describe the role of haemoglobin in carrying oxygen and carbon dioxide;
Oxygen
Haemoglobin consists of four subunits. Each subunit
consists of a polypeptide and a haem group. The haem group contains one
iron ion, Fe2+. Because the iron ion attracts
oxygen, it is said to have an affinity for it. A molecule of
haemoglobin, and therefore a red blood cell, can hold four molecules of
oxygen.
Haemoglobin can take up oxygen in a way that produces an S-shaped curve. This is called the Oxygen Dissociation Curve.
At a low oxygen tension the haemoglobin does not readily take up
oxygen. This is because it is difficult for the oxygen molecule to reach
the haem group, due to it being in the centre of the blood cell.
When the oxygen tension rises, the diffusion
gradient into the haemoglobin molecule steeply rises. Once one molecule
of oxygen has associated with a haem group, the shape of the haemoglobin
molecule slightly changes, making it easier for the second the third
molecules of associate. The change in the shape is known as the
‘conformational change’.
But, once the haemoglobin molecule contains three
oxygen molecules, it is difficult for the forth to associate with the
last haem group. This means that it is difficult to achieve 100%
saturation, even at high oxygen pressures. A consequence of this is that
the curve levels off again, meaning that the graph is S-shaped. Carbon Dioxide
5% dissolves in the plasma
10% combines with haemoglobin to form carbaminohaemoglobin
85% is transported as hydrogencarbonate ions
As Carbon dioxide diffuses into the blood, some of
it enters the red blood cells and combines with water to form carbonic
acid, catalysed by carbonic anhydrase.
CO2 + H2O → H2CO3
This carbonic acid then dissociates to form Hydrogen ions and Hydrogencarbonate
ions
H2CO3→ H+ + HCO3-
The Hydrogencarbonate ions diffuse out of the red blood cell. The charge in the red blood cell is maintained by the Chloride Shift; the movement for Chloride ions into the cell.
Hydrogen ions could cause the contents of the cell
to become very acidic, so the haemoglobin acts as a buffer. They
oxyhaemoglobin dissociates, and the hydrogen ions are taken up by the
haemoglobin to form haemoglobonic acid.
(m)describe
and explain the significance of the dissociation curves of adult
oxyhaemoglobin at different carbon dioxide levels (the Bohr effect); `
When tissues are respiring more, there will be more
carbon dioxide, and therefore more Hydrogen ions. This means that more
oxygen will be released from oxyhaemoglobin into the tissues. So, when
more carbon dioxide is present, the oxyhaemoglobin dissociation curve
shifts down and to the right.
(n)explain the significance of the different affinities of fetal haemoglobin and adult haemoglobin for oxygen
Fetal haemoglobin has a higher affinity for oxygen
than the haemoglobin of its mother. This is because the fetal
haemoglobin must be able to ‘pick up’ oxygen from the haemoglobin from
its mother. This reduces the oxygen tension within the blood fluid, so
the maternal blood release oxygen.
Cells, Exchange and Transport
The oxyhaemoglobin dissociation curve for fetal haemoglobin is to the left of the curve for adult haemoglobin.
Transport in Plants
(a)explain the need for transport systems in multicellular plants in terms of size and surface area:volume ratio;
All living things need to take substances from, and
return wastes to, the environment. Every cell of a multicellular plant
needs a regular supply of water and nutrients. In large plants, the
epithelial cells could gain all they need by simple diffusion, as they
are close to the supply. But there are many cells inside the plant which
are further from the supply, and would not receive enough water or
nutrients to survive. One particular problem is that roots can obtain
water but not sugarsds, and leaves can produce sugars but cannot obtain
enough water from the air.
(b)describe,
with the aid of diagrams and photographs, the distribution of xylem and
phloem tissue in roots, stems and leaves of dicotyledonous plants;
Roots
In roots, the xylem is arranged in an X shape, with the phloem found between the arms of the xylem.
Stem
In the stem, the vascular bundles are found around
the outside of the stem in a ring shape. They xylem is on the inside,
with the phloem on the outside and they are separated by a layer of
cambium, a layer of meristem cells which can divide to produce new xylem
and phloem.
Leaves
They xylem is on top of the phloem in the ‘veins’ of a leaf
(c)describe,
with the aid of diagrams and photographs, the structure and function of
xylem vessels, sieve tube elements and companion cells;
Xylem vessels
Long, thick walls that have been impregnated by
ligin. As the xylem develops, the ligin waterproofs the walls of the
cell. Consequently, the cells die and their end walls and contents break
down. This leaves a long column of collow, dead cells. The ligin
strengthens the walls and prevents the vessel from collapsing- the
vessels stay open even when water is in short supply.
The thickening of the ligin forms patters on the
cell walls. This prevents the vessel from becoming too rigid and allows
the stem or branch to be flexible.
In some places the lignification is not complete.
Pits or Bordered Pits, like pores in the walls, are left which allow
water to leave the vessel to either join another vessel of pass into the
living parts of the cell.
Sieve Tube elements
They are not true cells as they contain very little
cytoplasm and no nucleus. They are lined up end to end to form a tube in
which sugars (usually in the form of sucrose) are transported. At
intervals, there are sieve plates- cross walls which are perforated,
which are at intervals down the tube. Sieve tubes have very thin walls
and are five or six sided.
Companion cells
These are between the sieve tubes. They have a dense
cytoplasm, a large nucleus and many mitochondria to produse ATP for
active processes. They use ATP as a source of energy to load sucrose
into the phloem. There are many plasmodesmata between the companion
cells and they sieve tube, which are gaps in the cell walls allowing
communication and flow of minerals between the cells.
(d)define the term transpiration;
The loss of water vapour from the aerial parts of a plant due to evaporation
(e) explain why transpiration is a consequence of gaseous exchange;
For the exchange of gases to occur, the stomata of plants must be open. This is an easy route by which water can be lost.
To reduce this, plants have many structural and behavioural adaptations
A waxy cuticle waterproofs the leaf preventing water loss through the epidermis
The stomata are often on the underside of leaves, to
reduce evaporation due to direct heating Most stomata close at night-
there is no light, so no photosynthesis can occur, so no need for
gaseous exchange
Deciduous plants lose their leaves in winter when
temperatures are too low for photosynthesis, and the ground may be
frozen, so less water is available, meaning that plants have to conserve
what they’ve got.
Cells, Exchange and Transport
(f)describe the factors that affect transpiration rate;
Number of leaves
More leaves = large surface area which water can be lost from Number, size and position of stomata
If leaves have many, large stomata water vapour is
lost more quickly If the stomata are on the lower surface, water loss is
slower
Presence of cuticle
A waxy cuticle prevents water loss from the leaf surface
Light
In light, the stomata open to allow gaseous exchange for photosynthesis Temperature
Higher temperature will increase the rate of water loss Increase the rate of evaporation
Increase the rate of diffusion as the water
molecules have more kinetic energy Decrease the relative water vapour
potential in the air, causing the rapid diffusion of molecules out of
the leaf
Relative humidity
Higher relative humidity in the air will decrease
the rate of water loss. This is because there will be a smaller water
potential gradient between the air spaces in the leaf and the air
outside.
Air movement or wind
Air moving outside the leaf will carry water vapour away from the leaf. This will maintain a high water potential gradient
Water availability
If there is little water in the soil, plants cannot
replace water lost, so water loss has to be reduce by closing the
stomata, or shedding leaves in winter
(g)describe, with the aid of diagrams, how a potometer is used to estimate transpiration rates;
1.Cut healthy shoot underwater to stop air entering xylem
2.Cut shoot at a slant to increase surface area
3.Ensure apparatus is full of water and that there is only the desired air bubble
4.Insert shoot into apparatus underwater
5.Remove potometer form water and ensure it is airtight around the shoot
6.Dry leaves
7.Keep conditions constant to allow shoot to acclimatise
8.Shut screw clip
9.Keep scale fixed and record position of air bubble
10.Start timing and measure distance moved per unit of time.
(h)explain,
in terms of water potential, the movement of water between plant cells,
and between plant cells and their environment. (No calculations
involving water potential will be set);
Between plant cells
Water passes from the cell with the higher (less
negative) water potential to the cell with the lower (more negative)
water potential.
Between plant cells and their environment
Water moves down the water potential gradient. If
the water potential inside the cell is greater than the water potential
outside the cell, water will move out of the cell by osmosis and vice
versa.
Cells, Exchange and Transport
(i)describe,
with the aid of diagrams, the pathway by which water is transported
from the root cortex to the air surrounding the leaves, with reference
to the; casparian strip, apoplast pathway, symplast pathway, xylem,
stomata;
Water enters the root hair cells by osmosis. At the
same time, minerals are actively pumped from the root cortex into the
xylem. The consequence of this is that water moves from the root hair
cell along the symplast pathway to follow the xylem. The symplast
pathway is where water enters the cytoplasm and travels through the
plasodesma (gaps in the cell wall that contain fine strands of
cytoplasm). Water can move through the continuous strand of cytoplasm
from cell to cell. Water can also travel via the apoplast pathway, where
water travels between the cell walls without passing through any plasma
membranes. The Casparian strip blocks the apoplast pathway between the
cortex and the xylem meaning that, to reach to xylem, the water must
join the symplast pathway.
When water reaches the top of the xylem, it enters the leaves, and leaves the leaves through the stomata
(j)explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to
adhesion,
Water molecules in the xylem form hydrogen bonds
with the walls of the xylem vessel. Because the xylem vessels are
narrow, the hydrogen bonds can pull the water up the sides of the vessel
Cohesion and the transpiration stream
Water molecules are attracted to each other by the
forces of cohesion. These forces are strong enough to hold the molecules
together in a long chain. As molecules are lost from the top, the whole
column is pulled up as one chain. This is the transpiration stream
(k)describe,
with the aid of diagrams and photographs, how the leaves of some
xerophytes are adapted to reduce water loss by transpiration;
Smaller leaves- reduced surface area, so less water is lost by transpiration
Densely packed spongy mesophyll- Reduced cell surface area is exposed to the air spaces. Thicker waxy cuticle
Closing the stomata when water availability is low
Hairs on the surface of the leaf- trap a layer of
air close to the surface which can become saturated with moisture and so
will reduce the diffusion of water out of the stomata as the water
vapour potential is low. Stomata in pits- as above
Rolling the leaves so that the lower epidermis is not exposed to the atmosphere- as above
Low water potential inside cells- water potential gradient between the cells and the air space is reduced.
(l)explain translocation as an energy-requiring process transporting assimilates, especially sucrose, between sources (eg leaves) and sinks (eg roots, meristem);
The source is where the sugars come from, and the sink is where they go to.
Sugar is made in the leaves, so they are the source,
and transported to the roots, so they are the sink In early spring, the
leaves need energy to grow, so they sugars are transported from the
roots (now the source) to the leaves (now the sink)
Cells, Exchange and Transport
(m) describe, with the aid of diagrams, the
mechanism of transport in phloem involving: Active loading at the source
and removal at the sink,
1.ATP is used by companion cells to actively transport protons out of the their cytoplasm and into the surrounding tissue
2.This sets up a diffusion gradient and the hydrogen ions diffuse back into the cells
3.This is done through co transporter proteins which enable hydrogen ions to bring sucrose back into the cell with them
4.As the concentration of sucrose molecules builds up, they diffuse into the sieve tube elements through the plasmodesmata
5.The entrance of sucrose into the sieve tube elements reduces the water potential.
6.Water follows by osmosis and increases the hydrostatic pressure in the sieve tube element
7.Water
moves down the sieve tube element from higher hydrostatic pressure at
the source, to lower hydrostatic pressure at the sink.
8.Sucrose moves, via either diffusion or active transport, from the sieve tubes to the surrounding cells.
9.This increases the water potential in the sieve tube element, so water molecules move into the surrounding cells by osmosis
10.This reduces the hydrostatic pressure at the sink
The evidence for and against this mechanism
For
We know that the phloem is used
Supply the plant with radioactively labelled Carbon Dioxide (for photosynthesis), and the labelled CO2 soon appears in the phloem
Ringing a tree to remove the phloem results in
sugars collecting above the ring An aphid feeding on a plant stem can be
used to show that the mouthparts are taking food from the phloem
We know that it needs ATP
Many mitochondria in the companion cells
Translocation can be stopped by using a metabolic poison that inhibits the formation of ATP
The rate of flow of sugars is too fast for it to be done by diffusion alone, so energy must be needed to drive the flow
We know that it uses this mechanism
The pH of the companion cells is higher than that of the surrounding cells (H+ ions actively pumped out)
The concentration of sucrose is higher in the source than in the sink
Against
Not all solutes in the phloem sap move at the same rate
Sucrose is moves to all parts of the plant at the same rate, rather than more quickly to areas with a low concentration
The role of sieve plates is unclear.
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