Emily Summers

AS OCR Biology Revision Pack

UNIT f211 Cells, exchange, transport

Module 1 Cells

Cell Structure

1. State the resolution and magnification that can be achieved by a light microscope, a transmission electron microscope and a scanning electron microscope.

Light Microscope TEM SEMMaximum Resolution

0.2 micrometres 0.0001 micrometres 0.005 micrometres

Maximum Magnification

X 1500 Over x 1,000,000 Under x 1, 000, 000

2. Explain the difference between magnification and resolution

Magnification How much bigger the image is than the specimen.

Magnification = Length of Image / Length of specimen

Resolution How well a microscope distinguishes between two points that are close together.

3. Explain the need for staining samples for use in light and electron microscopy

In Light microscopes and TEM’s the beam of lights/electrons pass through the object, and there is an image produced as some parts of the specimen absorb more light/electrons than others, but sometimes the specimen is transparent so it will look white because light/electrons pass through so the object is stained

Light Microscope Electron MicroscopeDye- usually methylene blue/eosin Specimen dipped in metal like lead, the

metal ions scatter electrons to contrast.

4. Calculate the linear magnification of an image

Magnification = Length of Image / Length of specimen

5. Outline the functions of the structures.

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Picture Description Function

Large and contains chromatin. Enclosed by a

nuclear envelope double membrane.

Nuclear pores go through the envelope. Nucleolus

inside.

Nucleus contains the cell’s genetic material.

Chromatin contains DNA and proteins which

regulate cell activities. Instructions for making

proteins.

Flattened membranous sacs called cisternae, rough is studded with

ribosomes, smooth is not.

RER transports proteins and SER is involved in

lipid synthesis.

Stack of flat, membrane bound stacks. [Pitta

bread!]

Golgi body receives proteins from ER and

modifies them. Packages proteins into

vesicles to transport them exocytosis

Sausage shaped. Double membrane separated by fluid filled space. Inner membrane is folded to form cristae and the middle part of the mitochondria is called the matrix.

Site of aerobic respiration, ATP is

produced.

In plant cells. Double membrane. Membranous

sacs called thylakoids, plural=granum. Plural=grana.

Site of photosynthesis, carbohydrate molecules

made.

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Spherical sacs surrounded by a single

membrane, with no clear internal structure.

Contains enzymes.

Enzymes break down cells. E.g. white blood cell lyosomes break

down invading microorganisms and

lyosome in the sperm’s head breaks down the

material surrounding the egg.

TINY.Bound to ER to make

RER and also in cytoplasm. Consist of two

subunits.

Site of protein synthesis, they are like an

assembly line where mRNA from the nucleus is used to make proteins

from amino acids.Eukaryotic- 80SProkaryotic- 70S

Small tubes of microtubules. A pair can

be found next to the nucleus in animal cells.

Also in some protocytists.

Involved in cell division to make spindles which move chromosomes in

nuclear division.

Membrane bound sac found in plants filled with

cell sap.

Keep the plant supported, rigid and

turgid. Also like a garbage disposal for

plants.

Network of protein fibres Support, movement. E.g. Chromosome

movement in mitosis.

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Thick layer, in plants. Made of cellulose in eukaryotic cells and murein in prokaryotic

cells.

Gives the cell strength and rigidity

Thin, flexible layer around all eukaryotic cells. Made

of phospholipids and proteins.

It separates the cell contents from external environment and even controls movement of

substances in and out of the membrane with

receptor cells.

Enclosed jelly like substance within the cell

membrane.

In eukaryotic cells it contains organelles, in

prokaryotic cells it contains enzymes

needed for metabolic reactions.

Circular and loose. Unprotected, unlike in

eukaryotic cells.

Genetic instructions

PlasmidSmall circle of DNA Exchange DNA easily

and quickly between eukaryotic cells. Used in

genetic engineering.

A thick polysaccharide layer outside of the cell

wall

Useful for sticking cells together, and as a food

reserve. Protects against phagocytosis

and chemicals.

Rigid tail that rotates. “The motor is embedded in the cell membrane and is driven by a H+ gradient

across the membrane. Clockwise rotation drives the cell forwards, while

Propels the cell

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anticlockwise rotation causes a chaotic spin. This is the only known example of a rotating

motor in nature”

A tightly-folded area of the cell membrane

Contains membrane bound proteins needed

for respiration

6. Explain the importance of the cytoskeleton in providing mechanical strength to cells, aiding transport within cells and enabling cell movement.

Keep cells organelles in position with support Strengthen the cell to maintain it’s shape Transport material within the cell Help the cell to move, e.g. cilia and flagella by protein filaments.

7. Compare and contrast, with the aid of diagrams and electron micrographs, the structure of prokaryotic cells and eukaryotic cells.

Prokaryotic Cells Eukaryotic CellsProkaryotic cells are smaller (0.2-2.0 m) Eukaryotic cells are bigger 10-100 mDon’t have a nucleus, DNA floats free in cytoplasm and is circular

DNA is protected in nucleus and is linear

Less organelles and no membranous ones Many organelles, plant & animal70S Ribosomes 80S Ribosomes

8. Compare and contrast, with the aid of diagrams and electron micrographs, the structure and ultrastructure of plant cells and animal cells.

Cell Membranes

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9. Describe, with the aid of diagrams, the fluid mosaic model of membrane structure

10. Describe the roles of the components of the cell membrane; phospholipids, cholesterol, glycolipids, proteins and glycoproteins

Phospholipid molecules form a bilayer that is fluid as they always move, with hydrophilic heads and hydrophobic tails the protein molecules are scattered and can move. Some of these proteins have a carbohydrate chain attached to them, and these are called glycoproteins. Some lipids have a carbohydrate chain attached to them which are called glycolipids. Cholesterol is present in the membrane to provide mechanical stability.

11. Outline the effect of changing temperature on membrane structure and permeability

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Below 0 degrees the phospholipids have little kinetic energy so can’t move a lot, the membrane’s rigid and phospholipids are closely packed. Channel proteins and carrier proteins in the membrane denature to increase permeability of the membrane and ice crystals can form to pierce the membrane and make it highly permeable when it eventually thaws.

0-45 degrees phospholipids can move and aren’t packed tightly- partial permeability. P’lipids move more as they have more kinetic energy + membrane permeability.

+ 45 degrees the bilayer melts and membrane is more permeable. Water in cell expands and puts pressure on membrane, channel proteins and carrier proteins denature and increases membrane permeability.

Emily Summers

12. Explain the term cell signalling.

Cell Signalling How cells communicate with each other,

One cell releases a messenger molecule (e.g. a hormone) The molecule travels to another cell (E.g. in blood) The messenger molecule is detected by the cell as it binds to a receptor on It’s

cell membrane

13. Explain the role of membrane-bound receptors as sites where hormones and drugs can bind.

Membrane bound proteins can act as receptors for messenger molecules

Receptor proteins have specific shapes so messenger molecule shapes are complementary on binding.

Different cells have different receptor types and respond to different messenger molecules

A cell that responds to a messenger molecule is a target cell

Drugs either trigger a response in the cell or block the receptor to stop it working

14. Explain what is meant by passive transport (diffusion and facilitated diffusion including the role of membrane proteins), active transport, endocytosis and exocytosis.

Diffusion the movement of particles from an area of higher concentration to an area of lower concentration

Facilitated diffusion uses carrier and channel proteins

Active transport Moves substances against a concentration gradient using ATP

Endocytosis Cells take substances in, with part of a cells cell membrane surrounding it, the membrane pinches off to make a vesicle inside the cell containing the substance.

Exocytosis Cells secrete substances. Vesicles with these substances pinch off from golgi body sacs and move towards the cell membrane. The vesicles fuse with the cell membrane and release their contents outside of the cell.

15. Explain what is meant by osmosis, in terms of water potential.

Osmosis is the diffusion of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential

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16. Recognise and explain the effects that solutions of different water potentials can have upon plant and animal cells.

Cell Division, Diversity and Cellular Organisation

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Hypertonic- Solution with lower water potential than cell, net movement of water molecules is out so the cell will shrink/cerenate.

Hypotonic- Solution with higher water potential than cell. Net movement of water is into the cell so it will burst/haemolyse.

Isotonic- Same. No net movement, water in and out is equal.

Emily Summers

Explain the meaning of the term homologous pair of chromosomes

Humans have 46 chromosomes in total 23 pairs. One chromosome in each pair comes from the mother, and then the other comes from the father. Same size, same genes although they can have different versions of the genes (alleles).

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1. A bud forms at the cell surface

2. The cell undergoes interphase3. The cell undergoes mitosis4. Nuclear division is complete

budding cell’s nucleus has an identical copy of parent cell dna

5. The bud separates off from the parent cell with a genetically identical yeast cell

Meiosis:

1. Gametes are found in all sexually reproducing organisms

2. Male & Female join at fertilisation forming a zygote dividing into a new organism

3. (Sperm and Egg)4. (Pollen grains and ovules)5. Normal body cells of plants

and animals have diploid (2n) number of chromosomes, each cell contains two of each chromosome from each parent

Emily Summers

Define the term stem cell

Stem cells are cells that are not specialized and can differentiate into specialized cells with mitosis and the correct stimulation.

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.

Bones are living organs containing nerves and blood vessels, and the main bones have marrow in the middle, adult stem cells divide and differentiate to replace worn out erythrocytes and neutrophils to fight infection.

In plant cells stem cells are in the cambium. In the root and stem the stem cells of the vascular cambium divide to differentiate into the xylem and phloem, the vascular cambium then forms a ring inside the root and shoots. These cells divide and grow from the ring differentiating and moving away from the cambium.

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), neutrophils, epithelial cells, sperm cells, palisade cells, root hair cells and guard cells.

Neutrophills protect the body against illness, they are flexible so they can engulf pathogens and they have lots of lysosomes with digestive enzymes that can break

down the pathogens.

Erythrocytes carry oxygen in the blood and they have a biconcave disc shape to give a large surface area to volume ratio for gaseous exchange, they don’t have a nucleus so they have more room for haemoglobin.

Epithelial cells cover organ surfaces and cilia can beat to move particles, and other like microvilli can fold in the cell membrane to increase surface area to volume ratio

Sperm cells have a flagellum that enables them to swim to the egg and they have lots of mitochondria to provide energy to swim, the acrosome contains digestive

enzymes so the sperm can penetrate the egg surface.

Explain the meaning of the terms tissue, organ and organ system.

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Meiosis:

1. Gametes are found in all sexually reproducing organisms

2. Male & Female join at fertilisation forming a zygote dividing into a new organism

3. (Sperm and Egg)4. (Pollen grains and ovules)5. Normal body cells of plants

and animals have diploid (2n) number of chromosomes, each cell contains two of each chromosome from each parent

Emily Summers

A tissue is a group of similar cells that are specialized to work together to carry out a particular function.

E.g. Ciliated epithelium, xylem tissue, squamous epithelium tissue, phloem tissue

Organs are groups of different tissues that work together to form a function.

E.g. Lungs squamous epithelium, ciliated epithelium, elastic connective tissue and vascular tissue.

Organ systems are different organs working together for a different function, e.g. the respiratory system is made of all of the organs, tissues and cells involved in breathing like the lungs, trachea, larynx, nose, the diaphragm and mouth.

Discuss the importance of cooperation between cells, tissues, organs and organ systems.

Mulitcellular organisms work efficiently as they have different cells that are specialized for various functions

It is beneficial because every different cell can carry out a specialized function in a more efficient way than unspecialized cells could.

Each cell depends on the other cells for the functions it cannot carry out

So cells, tissues and organs in multicellular organisms cooperate to keep the organism alive and working well.

E.g. Muscle cells can move well but to do so they need oxygen, so they need erythrocytes to carry oxygen to them from lungs.

Module 2 Exchange and Transport

Exchange Surfaces & Breathing

Explain, in terms of surface area:volume ratio, why multicellular organisms need specialised exchange surfaces and single-celled organisms do not.

Smaller organisms have a higher surface area to volume ratio, so single celled organisms can diffuse substances directly in or out of the cell across the cell surface membrane.

However, diffusion in multicellular organisms is too slow because:

Some cells are deep in the body, so large diffusion distance from external environment

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Large animals have a smaller surface area to volume ratio so it is difficult to exchange enough substances to supply a large volume of animal through a small outer surface

A lot of multicellular organisms are active so many cells are respiring and so they need a constant rapid supply glucose and oxygen.

Describe the features of an efficient exchange surface, with reference to diffusion of oxygen and carbon dioxide across an alveolus.

a large surface area

a thin permeable surface

a moist exchange surface

Describe the features of the mammalian lung that adapt it to efficient gaseous exchange.

On inhalation the air enters the trachea

The trachea divides into two bronchi, and one bronchus goes to each lung

The bronchus divides into bronchioles, which end in small air sacs called alveoli where

gaseous exchange occurs.

The ribcage, intercostals muscles and diaphragm work together to move air in/out

Describe 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

Goblet cells secrete mucus which traps pathogens and dust in the inhaled air, the cilia

on the surface of cells beat rhythmically to waft mucus at the back of the throat where

it’s swallowed and the stomach’s acidity kills any pathogens.

Elastic fibres in the walls of the trachea, bronchi, bronchioles and alveoli aid ventilation.

They stretch and recoil to push air out when exhaling.

Smooth muscle is in the walls of the trachea, bronchi and bronchioles and it can relax to

dilate the lumen, to allow air in/out easily.

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Rings of cartilage are in the trachea walls and bronchi to give support and flexibility to

stop them collapsing on inhalation and pressure dropping.

Part of lung Cartillage Smooth

Muscle

Elastic

Fibres

Goblet Cells Epithelium

Trachea Large-C

shape

Yes Yes Yes Ciliated

Bronchi Small pieces Yes Yes Yes Ciliated

Larger

Bronchiole

None Yes Yes Yes Ciliated

Small

Bronchiole

None Yes Yes No Ciliated

Smallest

Bronchiole

None No Yes No No cilia

Alveoli None No Yes No No cilia

Outline the mechanism of breathing (inspiration and expiration) in mammals, with reference to the function of the rib cage,

intercostal muscles and diaphragm

Inspiration ExpirationIntercostal and diaphragm muscles contract

Intercostal and diaphragm muscles relax

Ribcage moves upwards and outwards and diaphragm flatten increasing volume of thorax

Ribcage moves downwards inwards and diaphragm curved again

As the volume of the thorax increases lung pressure decreases below atmospheric pressure

The thorax volume decreases causing air pressure to increase above atmospheric pressure

Air flow into the lungs Air forced out of lungsActive process needing energy (ATP)

Passive process not requiring energy.

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Explain the meanings of the terms tidal volume and vital capacity.

Tidal Volume Volume of air inhaled/exhaled in a normal breath- normally 0.4 dm3

Vital Capacity The maximum volume of air that can be inhaled/exhaled

Describe how a spirometer can be used to measure vital capacity, tidal volume, breathing rate and oxygen uptake

1. A spirometer has an oxygen filled chamber with a lid that can move2. The person will breathe through a tube connected to O2 chamber3. On inspiration/expiration the lid of the chamber moves up/down4. The movements are recorded by a pen attached to the lid of the chamber,

and writes on a rotating drum to create a spirometer trace5. The soda lime in the tube the person breathes into absorbs CO2

Analyse and interpret data from a spirometer.

Transport in animals

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Explain the meaning of the terms single circulatory system and double circulatory system, with reference to the circulatory systems of fish and mammals.

In a single circulatory system blood passes through the heart once, whereas in a double circulatory system the blood goes through the heart twice for each complete circuit of the body.

Explain the meaning of the terms open circulatory system and closed circulatory system, with reference to the circulatory systems of insects and fish.

Mammals and fish have closed circulatory system, which means the blood is inside blood vessels. The heart pumps blood into arteries which branch into capillaries, and substances like oxygen and glucose diffuse from blood in capillaries to body cells but blood will stay in the blood vessels, veins take blood back to the heart.

Whereas insects have open circulatory systems meaning that blood isn’t contained in blood vessels, it flows free through the body cavity.

Describe, with the aid of diagrams and photographs, the external and internal structure of the mammalian heart.

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Describe the cardiac cycle, with reference to the action of the valves in the heart

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The left ventricle of the heart is thicker than the right ventricle as the right ventricle only pumps to the nearby lungs, but the left ventricle pumps to the whole body so must contract with power.

The ventricular walls are thicker than atrial walls because the atria only pump to the nearby ventricles, but the ventricles push blood out of the heart.

The AV valves link the atria to the ventricles to prevent backflow of blood into the atria as the ventricles contract.

The SL valves link ventricles to the pulmonary artery and aorta to stop backflow of blood to heart after ventricular contraction.

The cords/tendons attach AV valves to ventricles so they aren’t forced up into the atria after ventricular contraction.

Emily Summers

SAN is a pacemaker and sends out a wave of excitation that spreads over atrial walls

Ventricles are electrically insulated by a collagen tissue band, the right and left atria contract

The wave of excitation spreads to the AVN from the SAN After a small delay of 0.1s so the atria have emptied the AVN passes the wave to

the bundle of His The Bundle of His passes the wave to the Purkyne fibres The Purkyne fibres carry the wave of excitation to the apex of the ventricle walls

causing them to contract simultaneously from the bottom up.

Describe the cardiac cycle, with reference to the action of the valves in the heart

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Interpret and explain electrocardiogram (ECG) traces, with reference to normal and abnormal heart activity.

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P= Contraction/Depolarisation of atria

QRS= Depolarisation of ventricles

T= Repolarisation/Relaxation of ventricles

Emily Summers

Too fast heartbeats (i.e. 120 beats a minute) are fine during exercise, at rest however it shows that the heart doesn’t pump blood efficiently.

With the atria contracting but the ventricles not, e.g. some P’s not followed by a QRS this could indicate a problem with the AVN, i.e. no impulse from the atria to ventricles.

Fibrillation is when the atria lose their rhythm and don’t contract properly, resulting in chest pain, fainting or even death.

Describe, with the aid of diagrams and photographs, the structures and functions of arteries, veins and capillaries.

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Arteries need thick and muscular walls that are elastic so they can withstand the high pressure of the blood inside them, carrying blood from the heart to the rest of the body. The endothelium is folded so the artery can expand, allowing it to cope with high pressures. They have narrow lumens to keep blood at high pressure.

Veins take blood back to the heart at low pressure so have a wide lumen with low amounts of elastic or muscle tissue, they have valves to prevent backflow of blood. Blood flow is helped by the muscular pump system (Muscles contracting squeezes blood back to the heart) to aid venous return.

Capillary walls are one cell thick to shorten diffusion pathway. They are near cells in exchange tissues like the alveoli in the lungs for a short diffusion pathway for gaseous exchange (i.e. O2 and CO2) there are lots of capillaries to increase surface area to volume ratio for exchange, networks are called capillary beds.

Emily Summers

Explain the differences between blood, tissue fluid and lymph.

Blood Tissue Fluid Lymph ExplanationErythrocytes Yes No No Erythrocytes are

too large and cannot get through capillary walls to tissue fluid

Leukocytes Yes Few Yes Most white blood cells are in lymphatic system & only go to tissue fluid on infection

Platelets Yes No No In tissue fluid if capillaries are damaged

Proteins Yes Few Antibodies Too big to go through capillary walls

Water Yes Yes Yes Tissue Fluid and Lymph have a higher water potential than blood

Dissolved Solutes

Yes Yes Yes Solutes like salt can move freely

Describe how tissue fluid is formed from plasma.

Tissue fluid surrounds the cells and is made from substances that leave the blood, like oxygen, water, etc. At the start of the capillary bed the pressure inside the capillaries near the arteries is more than the pressure in tissue fluid, this difference forces fluid out of the capillaries and into spaces surrounding cells to form tissue fluid.

When the fluid leaves the pressure is less in the capillaries, so the pressure is lower at the end of the capillary bed nearest veins.

Because of the fluid loss the water potential at the end of the capillaries nearer the veins is lower than the water potential in the tissue fluid, so some water will re enter the capillaries from the tissue fluid near the veins by osmosis down a water potential gradient.

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Describe the role of haemoglobin in carrying oxygen and carbon dioxide.

Hb + 4O2 HbO8

Erythrocytes contain Hb, a large protein with a quaternary structure (Because it has 4 polypeptide chains)

CONFIRMATIONAL CHANGE

Each chain has a haem group containing iron, and is why Hb is red.

Hb has a high affinity for oxygen and each molecule can carry 4 O2 molecules

It is reversible, and oxygen can dissociate from Hb near the body cells to leave Hb.

The pO2 is a measure of O2 concentration, the greater the concentration the higher the partial pressure. So pCO2 is the measure of CO2 concentration in a cell.

Haemoglobin’s affinity for oxygen varies depending on the partial pressure of oxygen, i.e. Oxygen loads onto haemoglobin to form oxyhaemoglobin when there is a high partial pressure, and unloads at lower partial pressures.

Oxygen enters blood capillaries at the alveoli, alveoli have a high partial pressure so oxygen will combine with Hb to form oxyhaemoglobin.

Respiring cells use oxygen and have a lower partial pressure so erythrocytes take oxyhaemoglobin to respiring tissues and the oxygen dissociates.

Then the haemoglobin goes back to the lungs to “pick up” more oxygen

A lot of CO2 diffuses into erythrocytes to form carbonic acid by the enzyme carbonic anhydrase.

10% combines with haemoglobin and is carried to the lungs.

The carbonic acid splits to produce H+ ions and Hydrogencarbonate ions.

The increase in H+ ions causes oxyhaemoglobin to unload oxygen so it can take up H+ ions to stop the cell acidity increasing (BUFFER) to form haemoglobinic acid.

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The Hydrogencarbonate ions diffuse out of the erythrocytes and are transported in blood plasma. When the blood reaches lungs to low pCO2 causes Hydrogencarbonate and H+ ions to recombine into CO2 which then diffuses into the alveoli and is exhaled.

Explain the significance of the different affinities of fetal haemoglobin and adult haemoglobin for oxygen.

The fetus gets oxygen from it’s mother’s blood across the placenta

By the time the mother’s blood reaches the placenta it’s oxygen saturation has decreased as some has been used by the mother’s body.

For the fetus to get enough oxygen to survive it’s haemogobin must have a higher affinity for oxygen than adults.

Transport in plants

Explain the need for transport systems in multicellular plants in terms of size and surface area to volume ratio.

Plants need water, CO2 minerals like nitrates and potassium, and sugars to live and they need to get rid of waste substances. They are multicellular and have a small surface area to volume ratio so need transport systems to move substances to and from cells quickly as diffusion alone is too slow.

Describe, with the aid of diagrams and photographs, the distribution of xylem and phloem tissue in roots, stems and leaves of

dicotyledonous plants.

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100% saturation means that every Hb molecule is carrying four oxygen molecules, 0% means no Hb molecules are carrying any.

Emily Summers

Leaf Cross Section

Root Cross Section

Describe, with the aid of diagrams and photographs, the structure and function of xylem vessels, sieve tube elements and companion cells.

Phloem tissue transports solutes like sucrose around plants, it is only a transport tissue.

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Xylem vessels are long tube structures formed from vessel elements joined end to end. There aren’t end walls so they are not interrupted tubes, and allow water to pass through the middle with ease. The cells are dead and don’t have cytoplasm, the walls are thickened with lignin- a woody substance that supports xylem vessels and stops them collapsing, the lignin quantity increases with age. Water and ions (K+ etc.) move in/out of vessels through pits in walls without lignin.

Emily Summers

Sieve tube elements are living cells that form the tube for transportation of solutes around the plant, they are joined end-end to make sieve tubes. The sieves are end walls with holes in them for solutes to pass through, although they have no nucleus, a thin layer of cytoplasm and few organelles. The cytoplasm of nearby cells is joined through holes in sieve plates.

Companion cells are there for each sieve tube element to carry out metabolic processes for the sieve tube elements that cannot survive on their own as they have no nucleus, etc., and itself- e.g. they provide energy for active transport of solutes

.

Define the term transpiration.

The loss of water from the plant’s surface

Explain why transpiration is a consequence of gaseous exchange.

A plant must open it’s stomata for absorption of carbon dioxide for photosynthesis, which as a consequence allows water to escape because there is a higher water potential inside the leaf than outside. So water moves out of the leaf by osmosis down the water potential gradient.

Describe, with the aid of diagrams, how a potometer is used to estimate transpiration rates.

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Really it measures the water uptake by the plant, but we assume that water uptake is directly related to water loss by leaves.

Emily Summers

1. Cut a shoot under water to stop air from going into the xylem at a slant to increase surface area to volume ratio for water uptake

2. Check that the apparatus has no air bubbles and is full with water3. Put the shoot into the apparatus underwater to prevent air entering4. Remove the photometer from the water and make it air and water tight5. Dry the leaves, let the shoot acclimatize and shut the tap6. Keep conditions constant throughout the experiment7. Record the starting position of the air bubble8. Start a stopwatch and record the distance moved by the bubble per unit time

Explain, in terms of water potential, the movement of water between plant cells, and between plant cells and their environment.

Light Lighter= faster rate of transpiration as the stomata open for photosynthesis

Temperature Higher= faster rate as water molecules have higher kinetic energy so they evaporate from cells quicker, increasing the water potential gradient between inside and outside of leaf making water diffuse out quicker.

Humidity Lower= faster, if the air around the plant is dry the water potential gradient between the leaf and air is steeper

Wind Higher= faster, air movement blows the water molecules from the stomata, steepening the water potential gradient

Describe, with the aid of diagrams and photographs, how the leaves of some xerophytes are adapted to reduce water loss by transpiration.

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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 and the stomata.

Water travels through the roots via the root cortex into the xylem by two ways

The Symplast Pathway The Apoplast PathwayGoes through living parts of the cells, the cytoplasm. The cytoplasm of nearby cells connect through plasmodestmata, which are small spaces in cell walls.

Goes through non living parts of the cells, the cell walls, the walls are absorbent and water can diffuse by osmosis through them and pass through spaces between them.

When water is in the Apoplast pathway it goes to the endodermis cells in the root, but the path is blocked by the Casparian strip- which is just a waxy strip. The water then must take the Symplast pathway.

This is not a hindrance because the water than has to go through the cell membrane which controls substances entering/leaving.

If the water goes past the barrier it moves into the Xylem.

The main pathway used is the Apoplast pathway as it provides the least resistance.

Explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream.

Cohesion and tension move water up from roots to the leaves against gravity, water evaporates from the leaves at the top of the xylem via transpiration

This creates suction/tension which pulls more water into the leaf

Water molecules are cohesive, meaning they stick together, so if one is pulled into the leaf so are more. The whole column of water in the xylem moves upwards, and it enters the stem through the roots.

Adhesion is the water molecules being attracted to the walls of the xylem vessels, helping water rise up.

Explain translocation as an energy-requiring process transporting assimilates, especially sucrose, between sources (e.g. leaves) and sinks (e.g. roots, meristem).

Translocation is the movement of dissolved substances like sucrose and amino acids when they are needed in a plant- called assimilates. This requires energy and happens in the phloem.

Translocation moves substances from sources (where it is produced- higher concentration) to sinks (where it is used- lower concentration)

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E.g. The source for sucrose is the leaves and the sinks are mainly food storage organs and the meristems (growth areas) in the roots, stems and leaves.

Enzymes maintain the concentration from the source to the sink by changing the dissolved substances at the sink, like by breaking them down or changing them into something else, to make sure there is a lower concentration at the sink than the source to keep a steep concentration gradient.

Describe, with the aid of diagrams, the mechanism of transport in phloem involving active loading at the source and removal at the sink, and the evidence for and against this mechanism.

At the source active transport is said to actively load the dissolved solutes into sieve tubes of the phloem.

Lowering the water potential inside sieve tubes and water enters them via osmosis. Creating a high pressure inside the sieve tubes at the source end of the phloem.

At the sink the solutes are removed from the phloem to be used Increasing water potential inside the sieve tubes so water leaves by osmosis Lowering pressure inside the sieve tubes

Creating a pressure gradient from the source to the sink

This gradient is responsible for pushing solutes along the sieve tubes to where they are required in the plant.

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For AgainstRemoving a ring of bark from a tree taking the phloem not the xylem from a woody stem a bulge will form above the ring. On analysis of the fluid in the bulge, there will be a higher sugar concentration above the ring than below- so there must be a downward sugar flow.

Sugar travels to many sinks not one with the highest water potential, as the model indicates

Aphids pierce the phloem with their mouthparts and sap flows into them, the sap flows out quicker nearer the leaves than further down the stem, so there must be a pressure gradient.

Sieve plates would make a barrier to mass flow, a lot of pressure would be needed for solutes to pass at a reasonably quick rate

A metabolic inhibitor stopping ATP production in the phloem stops translocation, proving it is active transport.There are experimental mass flow models

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