CELL STRUCTURE AND ORGANISATION.
Cells- are the small building blocks that make up all living organisms.
SIMILLARITIES AND DIFFERENCES BETWEEN PLANT AND ANIMAL
CELLS.
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Activity: Nov 2012 P21 Q3 The shaded area of the diagram represents structures
found in both plant and animal cells.
Write down the structures on the shaded area?
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STRUCTURE OF A CELL AS SEEN USING AN ELECTRON MICROSCOPE.
• An electron microscopes gives more detailed structure of a cell..
• Additional
structures
like,
mitochondria,
rough
endoplasmic
reticulum, ribosomes were seen.
FUNCTIONS OF CELL STRUCTURES.
1.Cell Membrane.
• Forms a barrier between the cell and its surrounding.
• Keeps contents inside the cell.
• Controls movement of substances into and out of the cell, the membrane is
described as partially permeable.
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2.Nucleus.
• Contains the genetic material(DNA) that controls all activities of the cell.
• Controls cell development.
3.Cytoplasm.
• It is the site for chemical reactions eg protein synthesis.
4. Chloroplast (plant cells only).
Contains chlorophyll which absorbs light energy and converts it to chemical
energy (ATP).
•
Site for photosynthesis.
• Stores starch.
5. Cell wall (plant cells only).
• Stops cell from bursting when they fill with water.
• Gives cell shape.
Allows water and dissolved substances to pass through freely (it is described as fully
permeable).
6. Vacuole (plant cells only)
• Full of water to maintain shape and firmness of cell.
• Stores salts and sugars.
7. Rough endoplasmic reticulum (RER).
• It is complex system of membranes with fluid filled space between the
membranes which allow materials to be transported throughout the cell.
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• Some ribosomes are dotted on the membranes making the membranes rough.
Function.
• Packaging of proteins made by the ribosomes.
Their tips pinch off giving vesicle which transports proteins around the cell.
NB: Cell that synthesis proteins for instance those making digestive
enzymes have a lot of RER
8. Ribosome.
Ribosomes are small organelles (about 20nanometers).
They are found free in the cytoplasm or attached to the rough endoplasmic
reticulum.
• The function of ribosomes is to synthesize proteins, such as those enzymes
involved in respiration.
9.Mitochondria.
• Mitochondria are relatively large organelles (5micrometers) found in
eukaryotic cells.
• The membrane is sausage –shaped.
• Each mitochondrion has two membranes , the inner and outer membrane.
• The inner membrane is folded to increase surface area for
chemical reactions of respiration to take place.
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Function.
• Mitochondria are the site for aerobic respiration which releases a lot of
energy.
NB: Cells with high metabolism require large number of mitochondria to
provide sufficient energy. e.g. muscle cells .
10. Golgi body.
• A system of flat membranes.
Functions.
• Receives proteins from the RER and process them.
• Its tips pinch off to form vesicles.
11. Vesicles.
• Vesicle carry proteins to the cell membrane for secretion
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SAMPLE QUESTIONS.
1 A researcher used a light microscope to observe epithelial cells from a human cheek.
Fig. 1.1 is a photograph that the researcher made of these cells.
Fig. 1.1
(a) (i) Name the parts labeled A and B.
[2]
(ii) The cells in Fig. 1.1 each have a cell membrane.
State one of the functions of a cell membrane.
[1]
(iii) State how the shape of the cells shown in Fig. 1.1 differs from the shape of a palisade
mesophyll cell in a leaf.
[1]
(b) Fig. 1.2 shows an electron micrograph of a mitochondrion.
Fig. 1.2
Mitochondria have two membranes, an inner membrane and an outer membrane. The inner
membrane is folded and used in respiration.
Suggest why the inner membrane of mitochondria is folded.
[1]
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(c) Table 1.1 shows different specialised cells and the average number of mitochondria each
cell contains.
Table 1.1
Explain the differences between the average numbers of mitochondria in the cells shown in
Table 1.1.
[4]
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LEVELS OF ORGANISATION.
SPECIALISED CELLS.
• After cell division and growth a cell specialization occurs.
• When a cell is specialized,
✓ it performs one particular function
✓ It develops a distinct shape
✓ Special kinds of chemical change take place in its cytoplasm.
SPECIALISED PLANT CELLS.
1. Palisade cell.
• The cell is specialized to carry out photosynthesis.
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•
•
•
•
Activity: Label parts 1-5.
Adaptions of the Palisade cell.
Has large number of chloroplasts, for maximum absorption of light.
Has large vacuole to push the chloroplasts to the edges of the cell for
them to absorb as much light as possible.
Cell wall is relatively thin to reduce the diffusion distance.
Its chloroplasts are able to move towards the upper surface in dim
light in order to absorb more light.
SAMPLE QUESTION.
NOV 2012 P21Q8 Fig. 8.1 shows a cell from the palisade
layer of a leaf.
Fig. 8.1
(a) In Table 8.1 tick (_) the numbers that label the three
features of the palisade cell which are also found in animal
cells.
Table 8.1
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(b) State and describe the function of two features of the
palisade cell that are only found in plant cells.
[4]
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2.Root hair cell.
• The cell is adapted to absorb water and mineral ions from the cell.
Activity: Label parts A,B and C
QUESTION JUN 2016 P32 Q4.
Adaptations of the root hair cell.
• Has long extension to increase surface area for absorption of water and
mineral ions.
• Has relatively thin cell wall to reduce diffusion distance.
• Has many mitochondria in its cytoplasm to provide energy needed for active
transport of mineral ions.
• Has many special carrier proteins on its cell membrane for active uptake of
mineral ions..
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SAMPLE QUESTION.
J19 P32 Q3(c) The list shows some of the structures in a plant.
Write the structures in order of size from smallest to largest.
chloroplast , palisade cell, phloem tissue
root, whole plant
3.Xylem vessels.
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• Xylem vessels are specialized to transport water and mineral ions and for
support.
NB: Xylem vessel is a tissue made of many xylem elements joined together.
A transverse section through a plant stem. The xylem vessels are
clearly visible.
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Adaptations of the Xylem vessels.
• The walls are lignified to make xylem strong.
• They have hollow lumen to allow smooth flow water and mineral ions.
• Have pits on the walls to allow lateral movement of water.
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SPECIALISED ANIMAL CELLS.
1.Red Blood Cell.
• It is specialized to transport oxygen.
Adaptations.
• Has many hemoglobin molecules to combine with oxygen.
• Mature red blood cell does not have large organelles e.g. nucleus,
mitochondria and rough endoplasmic reticulum in order to have more
space for hemoglobin.
• It’s biconcave in shape in order to increase surface area for the diffusion
of oxygen.
• Has very flexible membrane which enables it to squeeze through narrow
blood capillaries.
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Questions: Nov 2012 p21 Q8 (c)
2.Nerve cell.
• Cell is specialized to conduct electrical impulses.
Activity: Label the parts A-D
Adaptations of the nerve cell.
✓ Has myelin sheath which gives electrical insulation to the nerve cell.
✓ Has many mitochondria to release energy for the transmission of electrical
impulses
✓ The cells are long to connect distant parts.
✓ Nodes of Ranvier increase the speed of conduction as the nerve impulses
jump from one node to another.
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3.Sperm Cell.
• Its adapted to fertilise the egg cell.
•
•
•
•
Adaptations of the sperm cell.
Has a tail to enable it to swim towards the egg for fertilization.
Its middle –piece has many mitochondria to provide energy for swimming.
In the head has an acrosome(special vesicle) which contain digestive
enzymes to digest path for the sperm to the egg.
Its nucleus has one set of chromosomes so that when the egg is fertilized the
normal number of chromosomes is maintained.
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4.Egg cell.
• Its for reproduction.
Adaptation.
• Has large amount of cytoplasm containing york droplets made up of
protein and fat –nutrients for the zygote.
5. Ciliated cell.
• Ciliated cell is adapted to move mucus in the trachea and bronchi back to
the throat.
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Adaptation of ciliated cell.
• Has tiny ‘hairs’ called cilia which are in continual flicking movement
which creates a stream of mucus that carries dust and bacteria through the
bronchi and trachea away from the lungs.
TISSUES AND ORGANS..
• A tissue is a group of cells with similar structures, working together to
perform a shared function.e.g Xylem, palisade mesophyll, epidermis,
muscle.
• An organ is group of different tissues that work to perform specific
functions.
• An organ system is a group of organs with related functions, working
together to perform the body function.
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LEVELS OF ORGANISATION IN A PLANT.
SAMPLE QUESTIONS.
JUN 2009 P2 Q5 Five types of animal and plant cells and five possible functions of such cells are shown
below.
Draw one straight line from each type of cell to a function of that cell.
NOV 2014 P32 Q1 Fig. 1.1 is a photomicrograph of a leaf of the tea plant, Camellia sinensis.
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Fig. 1.1
(a) Name A to E.[5]
(b) Fig. 1.2 shows a cell from region B of the leaf shown in Fig. 1.1.
Fig. 1.2
Use the letters from Fig. 1.2 to complete Table 1.1.
Write one letter only in each box to identify the function. You may use each letter once, more
than once or not at all.
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SIZE OF SPECIMENS.
• A microscope enlarge the specimen i.e magnification.
• Magnification is the number of times the specimen has been enlarged.
FORMULA FOR CALCULATING MAGNIFICATION.
UNITS.
1mm=100 micrometers(µm)
Magnification= Image size
Actual size
SAMPLE QUESTIONS.
Jun 2009 P6 Q3 Fig. 3.1 shows a photomicrograph of a human blood smear.
Fig. 3.1
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(b) (i) Measure the diameter of the blood cell labelled A. [1]
(ii) The photomicrograph has been enlarged by x 800, calculate the actual size of
cell A.
show your working
[2]
(iii) State the function of cell A.
[1]
NOV 2017 P22 Q4 The diagram shows a magnified image of a human liver cell with a
mitochondrion labelled. The actual size of the liver cell is 20µm.
The image size of the liver cell is 40 mm and the image size of the mitochondrion is 4 mm.
What is the actual size of the mitochondrion shown in the diagram?
Show your working.
A 0.002 mm B 0.02 mm C 0.2 mm D 2 mm
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