Absorption is the uptake of materials by a cell from the external surroundings.

Secretion is the discharge of materials into the external surroundings.

Molecules being exchanged between cells and their surrounding have to pass through the cell membrane and in plants they also have to pass through the cell wall.

PLANT CELL WALL–

The Plasma Membrane.

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Cellulose fibres

Layer 1

Layer 2

Structure – Made up of bundles of cellulose fibres laid down in layers• The cellulose fibres are ‘glued’ together by calcium

pectate Features

Strong Elastic Hydrophilic – water is attracted to cellulose Non-living Freely permeable – presents no barrier to the movement of molecules. Provides a continuous water conducting channel

Phospholipidbilayer

Channel protein

protein

Functions

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Provides structural support ( strength )

Enzymes – to speed up reaction

Receptors – for cell signalling

Channels – for movement of larger molecules such as glucose

Antigenic markers for cell recognition

Carriers – by active transport

Proteins

The general structure of cell membranes is described by the ‘fluid mosaic’ model where a variety of proteins are closely associated with a lipid bilayer.

This model is called the fluid mosaic model :-• It is fluid due to the rapid movement of the lipids• The term ‘mosaic’ describes the proteins which are

embedded into the lipids in a random fashion

Lipid layer

• The phospholipids have a water soluble hydrophilic ‘head’ and water-insoluble hydrophobic ‘tail’

• The phospholipids form boundary which only allow tiny molecules such as gases and water to pass through into the lipids in a random fashion

1. Diffusion

2. Osmosis

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The cell membrane is said to be selectively permeable allowing the free movement of small molecules, limited movement of larger molecules and prevents the movement of very large molecules

Movement of substances into or out of cells Substances enter or leave a cell in one of 5 ways:-

1. Diffusion

2. Osmosis

3. Active transport

4. Phagocytosis

5. Pinocytosis

Diffusion is the movement of molecules from regions of high concentration to regions of lower concentration

The plasma membrane is selectively permeable

Allows rapid diffusion of gases and water through phospholipid bilayer

Allows slower diffusion of larger molecules such as glucose through membrane proteins

Does not allow diffusion of very large molecules such as starch and proteins

Animal cells in solution

Plant cells in solution

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Time ( mins)

Wat

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Appearance of RBC at time zero

X

I n 0.05% salt Soln Type of solution Appearance -

Y

I n 5% salt SolnType of solution -

Appearance

HYPOTONI C Swollen and Burst

Haemolysed

HYPERTONI CShrivelled

Crenated

The unequal movement of water and solutes results in osmosis

Osmosis is the movement of water molecules from regions of high concentration to regions of low concentration through selectively permeable membranes

A solution is produced when a solute dissolves in a solvent

The greater the solute concentration the lower the solvent concentration

An increase in the solute concentration causes a decease in the solvent concentration

Cells may find themselves in a variety of solutions

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Time ( mins)

Wat

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Appearance PLANT CELL at time zero

X

I n 0.05% salt Soln

Type of solution - Appearance -

Y

I n 5% salt Soln

Type of solution-Appearance -

HYPOTONI C Swollen and Firm

HYPERTONI CCell membrane detached f rom wall

TURGI D

PLASMOLYSED

TYPES OF SOLUTION

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a) Hypertonic ( Higher solute conc. /lower water conc. )

i) Water passes outii) Cell loses weightiii) Cell contents shrink

b) Hypotonic ( lower solute conc. /higher water conc. )

i) Water passes inii) Cell increases in weightiii) Cell swells

c) Isotonic ( same solute conc. /same water conc.)

Equal net movement of water and solutes between cell and solution resulting in no weight change or change in cell contents

COMPULSORY PRACTICAL

Design and carry out an investigation to measure the waterconcentration of cell sap using plant tissue.

You will now use your knowledge of cells in solution to help you calculate the cell sap concentration of potato tissue

Your teacher will explain this practical to you and give you handouts to help you.

The following may be asked in the exam so it is important to understand the design properly and why you have to set things up the way you do.

The following table shows important design procedures

Design feature ReasonPotato cut into thin slices Increase surface area

Results converted to % change in mass

This ensures valid comparison between samples if starting

weights are not identicalCylinders blotted before weighing Removes excess liquid from the

outside of the cell as this may vary between samples

All factors other than sucrose concentration kept equal

To ensure that only one variable is being investigated, experiment

is fair and results are validClass results pooled To improve the reliability of the

results

Key terms Results can always be made more reliable by repeating the

experiment a number of times and calculating the average. The results are only valid if the experimental design ensures only

one variable is altered. Think of the ‘V’ as a big tick which shows the experiment is well designed

Accuracy is always improved by taking more samples which have smaller intervals between them

You must write this up as part of your course and hand in the finished lab report.

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3. ACTIVE TRANSPORT

4. and 5 - PHAGOCYTOSIS AND PINOCYTOSIS

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Active transport is the movement of molecules and ions against the concentration gradient

• It requires energy supplied by respiration• It is controlled by carrier proteins (pumps) in the membrane• Ions may be actively pumped into the cell from the environment

or out of the cell to the environment• Ion uptake is selective i.e. each ion is concentrated to a specific

level

Active transport questions revolve around graphs. You can spot them as one axis gives an ion value and the other a factor associated with respiration - SEE GRAPHS AND EXPLANATIONS ON PAGE 9

You must learn to describe and explain what happens in graphs at higher level. Your teacher will help you practice.

Top Tip! – You must mention both axis and a number when describing graphs. This is very easy to learn!

Explaining or accounting for trends is more difficult and you must ‘talk’ your way from one axis to the other using your knowledge of Biology

Large particles require gross movements of the whole cell membrane e.g.

1. Endocytosis involves infolding of the membrane to form vesicles and includes :-

• Phagocytosis – ‘cell eating’ where large solid particles are engulfed - PAGE 10

• Pinocytosis – ‘cell drinking’ where water droplets are engulfed - PAGE 11

2 .Exocytosis . Vesicles inside the cell fuse with the membrane and secrete

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During active transport ions or molecules are moved against the concentration gradient. Active transport requires energy supplied by respiration. Any factor affecting respiration therefore affects the rate of active transport

GLUCOSE OXYGEN ENERGY WATER CARBON DIOXIDEEnzymes For active

transport

Substrate Concentrationa) Increased light in plants increases the rate of photosynthesis which produces higher sugar levels. This in turn increases the rate of respiration and Active Transportb) A further increase in light gives no further increase in Active Transport due to other limiting factors e.g. temperature

Increasing Oxygen Concentration

a) As oxygen levels increase the rate of active transport increasesb) A further increase in oxygen gives no further increase in Active Transport due to other limiting factors e.g. temperature

Increasing Temperaturea) As temperature increases the enzymes controlling respiration work more efficiently and the rate of Active Transport increasesb) Further increases in temperature denature the enzymes and Active Transport decreases

Increasing oxygen concentration (%)

Increasing temperature (◦C)

Increasing light intensity (lux)

a

b

a

b

a

b

Lysosomesecretesenzymes into vaculole

Senses chemical from food

Food Pseudopodia Food vacuole

Food vacuole forms

Engulfs food

Phagocytosis

Lysosome

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Pinocytosis

1.

2.

3.

4.

5.

1. Membrane enfolds droplet of water

2. Droplet moves into folds in the membrane which gradually lengthens into interior of cell.

3 – 4. Membrane start to isolate water droplet in vacuole.

5. Vacuole detaches and moves into cell

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