AS Biology Revision Pack UNIT 2

8/6/2019 AS Biology Revision Pack UNIT 2

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Emily Summers

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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 SEM

MaximumResolution

0.2 micrometres 0.0001 micrometres 0.005 micrometres

MaximumMagnification

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 TEMs 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 Microscope

Dye- usually methylene blue/eosin Specimen dipped in metal like lead, themetal 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 containschromatin. Enclosed by anuclear envelope double membrane.

Nuclear pores go throughthe envelope. Nucleolus

inside.

Nucleus contains thecells genetic material.Chromatin contains DNA

and proteins whichregulate cell activities.Instructions for making

proteins.

Flattened membranoussacs called cisternae,rough is studded with

ribosomes, smooth is not.

RER transports proteinsand SER is involved in

lipid synthesis.

Stack of flat, membranebound stacks. [Pitta

bread!]

Golgi body receivesproteins from ER and

modifies them.Packages proteins into

vesicles to transportthem exocytosis

Sausage shaped. Doublemembrane separated byfluid filled space. Innermembrane is folded toform cristae and themiddle part of themitochondria is called thematrix.

Site of aerobicrespiration, ATP is

produced.

In plant cells. Doublemembrane. Membranous

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

Site of photosynthesis,carbohydrate molecules

made.


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Spherical sacssurrounded by a single

membrane, with no clearinternal structure.

Contains enzymes.

Enzymes break downcells. E.g. white bloodcell lyosomes break

down invadingmicroorganisms and

lyosome in the spermshead breaks down thematerial surrounding the

egg.

TINY.Bound to ER to make

RER and also incytoplasm. Consist of two

subunits.

Site of protein synthesis,they are like an

assembly line wheremRNA from the nucleusis used to make proteins

from amino acids.

Eukaryotic- 80SProkaryotic- 70S

Small tubes ofmicrotubules. A pair can

be found next to thenucleus in animal cells.

Also in some protocytists.

Involved in cell divisionto make spindles whichmove chromosomes in

nuclear division.

Membrane bound sac

found in plants filled withcell sap.

Keep the plant

supported, rigid andturgid. Also like a

garbage disposal forplants.

Network of protein fibresSupport, movement.E.g. Chromosome

movement in mitosis.


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Thick layer, in plants.Made of cellulose ineukaryotic cells andmurein in prokaryotic

cells.

Gives the cell strengthand rigidity

Thin, flexible layer aroundall eukaryotic cells. Made

of phospholipids andproteins.

It separates the cellcontents from externalenvironment and evencontrols movement of

substances in and out ofthe membrane with

receptor cells.

Enclosed jelly likesubstance within the cell

membrane.

In eukaryotic cells itcontains organelles, in

prokaryotic cells itcontains enzymesneeded for metabolic

reactions.

Circular and loose.Unprotected, unlike in

eukaryotic cells.

Genetic instructions

PlasmidSmall circle of DNA Exchange DNA easily

and quickly betweeneukaryotic cells. Used in

genetic engineering.

A thick polysaccharidelayer outside of the cell

wall

Useful for sticking cellstogether, and as a food

reserve. Protectsagainst phagocytosis

and chemicals.

Rigid tail that rotates.The motor is embeddedin the cell membrane andis driven by a H+ gradient

across the membrane.Clockwise rotation drivesthe cell forwards, while

Propels the cell


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1. A bud forms at the cell

surface

2. The cell undergoes interphase

3. The cell undergoes mitosis

4. Nuclear division is complete

budding cells nucleus has anidentical 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

6. Gametes have the haploid

number of chromosomes (n)

theres one copy of each

chromosome

7. At fertilisation the haploid

male gamete and female fuse

to make a cell with the diploid

number of chromosomes, half

from sperm half from egg.

Produces cells genetically different-

genetic variation, it creates variation.


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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 dont 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.


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

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.


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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.

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.

In a root the xylem and phloem are

in the centre to give support to the

root as it pushes through the soil.

In stems the xylem and phloem are

near the outside to provide stability

that reduces bending.

In a leaf the xylem and phloem make

up a vein network to support the

thin leaves.

Xylem vessels are long tube

structures formed from vessel

elements joined end to end. There

arent end walls so they are not

interrupted tubes, and allow water


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

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 plants surface

Explain why transpiration is a consequence of gaseous exchange.


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A plant must open its 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.

Really it measures the water upta

by the plant, but we assume that

water uptake is directly related to

water loss by leaves.


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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 water

3. Put the shoot into the apparatus underwater to prevent air entering

4. Remove the photometer from the water and make it air and water tight

5. Dry the leaves, let the shoot acclimatize and shut the tap

6. Keep conditions constant throughout the experiment

7. Record the starting position of the air bubble

8. 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 thestomata open for photosynthesis

Temperature Higher= faster rate as water molecules havehigher kinetic energy so they evaporate from

cells quicker, increasing the water potentialgradient between inside and outside of leafmaking water diffuse out quicker.

Humidity Lower= faster, if the air around the plant is drythe water potential gradient between the leafand air is steeper

Wind Higher= faster, air movement blows the watermolecules from the stomata, steepening thewater 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 Pathway

Goes through living parts of the cells, thecytoplasm. The cytoplasm of nearby cellsconnect through plasmodestmata, which aresmall spaces in cell walls.

Goes through non living parts of the cells, thecell walls, the walls are absorbent and watercan diffuse by osmosis through them and passthrough 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.

y At the source active transport is said to actively load the dissolved solutes into sieve

tubes of the phloem.

y Lowering the water potential inside sieve tubes and water enters them via osmosis.

y Creating a high pressure inside the sieve tubes at the source end of the phloem.

y At the sink the solutes are removed from the phloem to be used

y Increasing water potential inside the sieve tubes so water leaves by osmosis

y Lowering pressure inside the sieve tubes

y Creating a pressure gradient from the source to the sink

y This gradient is responsible for pushing solutes along the sieve tubes to where they are

required in the plant.


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For Against

Removing a ring of bark from a tree takingthe phloem not the xylem from a woodystem a bulge will form above the ring. Onanalysis of the fluid in the bulge, there willbe a higher sugar concentration above the

ring than below- so there must be adownward sugar flow.

Sugar travels to many sinks not one withthe highest water potential, as the modelindicates

Aphids pierce the phloem with theirmouthparts and sap flows into them, thesap flows out quicker nearer the leavesthan further down the stem, so there mustbe a pressure gradient.

Sieve plates would make a barrier to massflow, a lot of pressure would be needed forsolutes to pass at a reasonably quick rate

A metabolic inhibitor stopping ATPproduction in the phloem stopstranslocation, proving it is active transport.

There are experimental mass flow models

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AS Biology Revision Pack UNIT 2