Cell structure and organisation
Cell Theory; all living organisms are made up of cells – small building blocks. New cells are
formed when already existing cells divide.
Diagrams showing animal and plant cell as seen under a light microscope.
An animal cell (liver cell)
A plant cell (palisade cell)
Components of Animal Cells: cell membrane, cytoplasm, nucleus.
Extra Components present only on Plant Cells: cell wall, chloroplast, large central vacuole.
The term protoplasm is used to refer to the living components of the cell i.e. cell membrane,
cytoplasm & its contents and the nucleus. The large vacuole of plant cells is not part of the
protoplasm.
Cell structures and function
Cell Membrane;
It is semipermeable/ selectively permeable i.e. it allows small molecules such as water;
oxygen and carbon dioxide freely pass through it, but prevent large molecules from
passing through.
It controls movement of materials in and out of the cell.
It keeps cell contents together allowing efficient coordination of its activity.
It maintains the interior of the cell at a suitable constant composition for efficient
metabolism
Cytoplasm;
The living contents of a cell excluding the nucleus and cell membrane.
A complex solution (about 90% water) in which the cell’s organelles i.e. sub-cellular
structures are suspended.
Many biochemical reactions such as respiration & protein synthesis take place here.
Nucleus;
Contains deoxyribonucleic acid (DNA) - the hereditary material.
Controls all the cells’ metabolic activities.
Controls cell division & how cell develop.
A nucleus is not present in a red blood cell or phloem sieve element.
Large Cell Vacuole – sap vacuole;
Storage of water, food (sugar, amino acids), ions, wastes, pigments.
When full of water it maintains the shape & firmness of cells.
Plays a role in cell elongation during plant growth.
Cell Wall;
It is usually dead & composed of cellulose.
It is fully permeable to water and solutes – thus it allows them to pass freely
Protects and supports plant cells – stops them from bursting when they take in too much
water.
It gives plant cell the angular shape.
Chloroplast;
Contains chlorophyll, the green pigment that traps light energy.
Site for biochemical reactions of photosynthesis.
Store starch.
Differences in structure between typical animal and plant cells
Feature
Cellulose cell
wall
Shape
Chloroplast
Cell vacuole
Plant cell
Animal cell
present
absent
Usually permanent & angular
Present in some cells
Large, centrally placed, permanent &
contain cell sap
Varies & not angular
absent
Usually lacking, if present they are
small, non-permanent & don’t contain
cell sap
Nucleus
Usually centrally placed
Carbohydrate Often has starch grains
storage
Found anywhere within the cell
Starch grain absent, sometimes may
have glycogen granules
Specialised cells:
Relate the structure of the following specialized cells to their functions:
Specialized cells
Function(s)
ciliated cells
movement of
mucus plus
trapped
pathogens and
dust particles up
the respiratory
tract
also movement
of egg along the
oviduct
root hair cells
absorption of
water & mineral
ions
xylem vessels
conduction of
water & mineral
and support of
stem / leaves in
woody plants
muscle cells
contraction to
bring about
movement of the
body parts
red blood cells
transport of
oxygen/carbon
dioxide
Structural adaptation(s) to their
function
 have cilia on their surface which move
back & forth creating a current that
move the mucus and trapped solid
particles
 presence of many mitochondria to
provide energy for movement of cilia
 long and thin extension to provides
large surface area for absorption
 large cell vacuole with cell sap (a high
concentration of solutes) to creates low
water potential for absorption of water
by osmosis
 thin cell wall to provide a short
diffusion distance
 has cylindrical empty dead cells
arranged into columns like pipes to
allow free flow of water and dissolved
mineral ions
 cell walls are thickened with spiral
rings of cellulose and waterproof
material called lignin
 presence of fibrils for contraction &
relaxation
 presence of many mitochondria to
provide energy for contraction of fibrils
 contain haemoglobin that carries
oxygen
 they are bi-concave discs to provide
large surface area for efficient diffusion
of oxygen
 they lack nucleus to allow more space
to load with haemoglobin
Levels of organisation:
Tissue; is a group of cells with similar structures, working together to perform a shared
function e.g. muscle, epidermis, blood, xylem, phloem, mesophyll etc.
Organ; is a structure made up of a group of tissues, working together to perform specific
functions e.g. heart, stomach, kidney, lung, leaf, stem, root etc.
organ system; is a group of organs with related functions, working together to perform
body functions e.g. Skeletal system, nervous system, circulatory system,
reproductive system etc.
Size of specimens:
Magnification and size of biological specimens can be calculated using the formula;
Magnification =
𝑠𝑖𝑧𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑖𝑚𝑎𝑔𝑒
𝑠𝑖𝑧𝑒 𝑜𝑓𝑡ℎ𝑒 𝑎𝑐𝑡𝑢𝑎𝑙 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛
Fig. 1.1 shows the external appearance of animal A.
Fig. 1.1
(i)
Make a large, labelled drawing of animal A.
(ii)
Measure the length of animal A in Fig. 1.1 and in your drawing. Calculate the
magnification of your drawing. (Show your working).
length of animal A: in Fig. 1.1 .............................................................................................
in drawing ...........................................................................................
magnification .......................................................................................................................