Download - Biology Lesson Notes Cells
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Cells
Under microscope, plant or animal parts are seen to be made of small distinct units.
These units are called cells.
A cell is a distinct mass of living material (protoplasm) enclosed by a membrane.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Some details about cells
There are different shapes and sizes of cells.
Cells are very small, usually too small to see
with naked eye.
They are small enough for thousands to fit on
full-stop.
But sometimes they can be as large as a tennis
ball.
The unit of measurement used is
the micron (equal to one millionth of a metre)
(1m = 1000,000 micons, 1mm = 1,000 microns).
The average human cell is about 10 microns in
diameter.
Smallest bacteria are about 0.2 microns in
diameter (2 thousandths of a millimetre).
Each cell is adapted to carry out a particular
function.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Different types of Animal Cells
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Different types of Plant Cells
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Red Blood Cells White Blood Cell Muscle Cells Root Hair Cell
Since there are so many differences between cells, it is difficult to speak of
a “typical” cell. However, there are certain common features.
Nerve Cell Sperm Cell Egg Cell
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
A Typical Animal Cell
Cell Membrane
Cytoplasm Contains glycogen granules)
Nucleus (contains chromatin)
Mitochondria (visible only under electron microscope)
Nuclear Membrane
Vacuoles (food or contractile)
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
A Typical Plant Cell
Mitochondria
Chloroplasts (contains chlorophyll)
Vacuole (contains cell sap)
Nucleus
Cellulose Cell Wall
Cytoplasm (contains starch granules)
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Parts of a Typical Cell
Cell Cell
Membrane
Cytoplasm
Nucleus
Vacuoles
Mitochondria
Chloroplasts
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
The Cell Membrane
The Cell Membrane is a semi-permeable layer which holds the cell together and controls what goes in and out of the cell.
• The cell membrane is a semi-permeable layer. • This means that some substances can pass through it, but others cannot. • The cell can take up and excrete certain small chemicals through these “holes” in the membrane.
• It is very thin but tough • It is made of protein and phospholipid (a kind of fat) molecules. • It encloses the other cell structures.
• In plant cells only, the cell membrane is also called the plasmalemma • It is surrounded by the Cell Wall, which is is a rigid, protective wall made of cellulose. • It gives the plant its stiffness and helps it grow tall. • It is secreted by the plant cell's cytoplasm. • Cell walls may contain lignin (a complex aromatic compound).
This is an important component of wood.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
The Cytoplasm
The Cytoplasm is the jelly-like, transparent substance inside the cell. It is the site where most reactions take place.
• A jelly-like, transparent substance.
• Consists mainly of water (between 65% and 95%).
• Solids present in cytoplasm include protein granules, carbohydrates, droplets of fat, and pigments (e.g. chlorophyll).
• The cytoplasm is either watery (liquid) or syrupy (semi-solid), depending on the concentration of solids dispersed in the fluid.
• Much of the chemical work of the cell is done in the cytoplasm.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
The Nucleus
The Nucleus is an organelle which contains the genetic material of the cell. It controls most of the functions of the cell.
• The nucleus normally has a round or oval shape.
• The nucleus contains chromatin. These consist of strands of DNA and protein.
• The chromatin is enclosed in a nuclear membrane in most cells.
• A strand of DNA is a long series of genes or “instructions” used to control all the functions of plants and animals.
• Genes determine the characteristics of a cell.
• The nucleus is the control centre of the cell.
• It controls growth and division
• It also controls most of the chemical reactions taking place in the cell.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
The Vacuoles
• Vacuoles are non-living, fluid-filled cavities in the cytoplasm.
• In animal cells, they are small and change size and position.
• Food vacuoles contain food molecules
• Contractile vacuoles used to pump out water. (e.g. Amoeba)
• Plant cells have one large central vacuole.
• It is filled with a fluid called cell sap.
• It contains salts, sugars and pigments dissolved in water.
• It has a high pressure to keep the plant rigid and upright.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
The Mitochondria
The Chloroplasts
• Mitochondria are sausage-shaped structures with semi-permeable membranes.
• Mitochondria function as energy production centres
• Glucose (sugar) in the cytoplasm is oxidized (burnt) in the mitochondria for energy.
• This process is called respiration.
• Chloroplasts are green, oval structures containing chlorophyll (a green pigment).
• They are found only in plants (and some one-celled animals)
• In the presence of sunlight, the chloroplasts carry out photosynthesis.
• All life on Earth depends on photosynthesis.
• Without green plants animals would not have food.
• Fossil fuels are formed from green plants of the past that died and changed into oils/coal by large pressures as they became buried
• These form today’s major fuels (coal, gas, and oil).
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Differences between Plant and Animal Cells
Animal Cells Plant Cells
Usually smaller than plant cells Usually larger than animal cells
No cell wall and no cellulose (shape is often irregular and outline less visible)
Cellulose cell wall (gives cells a well-defined outline and regular shape)
Consist almost entirely of cytoplasm Thin lining of cytoplasm
Small vacuoles which change shape, size and position (concerned only with excretion, secretion or food intake)
Large central vacuole filled with fluid (sap) (concerned with rigidity of cell and therefore whole plant)
No chloroplasts (and no chlorophyll) Chloroplasts (containing chlorophyll)
Food stored as glycogen granules Food stored as starch granules
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Levels of Organisation
All Living organisms are made of cells.
But living organisms can be grouped into several levels by considering the amount of complexity and organisation they show.
There are four levels of organisation:
Organisms exist at all of the 4 levels
Level of organisation
Cellular
Tissue Organ
System
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Cellular
Level
• Organism consists of a single cell.
• The cell is capable of carrying out all the functions necessary for living.
• E.g. unicellular animals Amoeba, Paramecium and most Bacteria.
Tissue Level
• A tissue consists of hundreds of cells of one or few types.
• Each cell has more or less the same structure and function.
• E.g. hydra, jellyfish, and most plants (higher plants have leaves and reproductive structures, but most of their body functions are still carried out by cells and tissues).
• Higher organisms also have tissues e.g. bone, muscle & nervous tissue.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Organ Level
• Organ a functional unit consisting of several tissues grouped together
• An organ usually has a particular function(s).
• Examples earthworm, higher plants (have simple organs like leaves and reproductive structures)
• Higher organisms also have organs e.g. kidney, pancreas, muscles, heart.
Organ
System Level
• Organ system several organs which together perform a specific function.
• Examples the great majority of animals (insects, fish, mammals)
• Organ systems e.g. the digestive system includes structures like the mouth, stomach, intestines, liver, pancreas, blood, nervous system, etc. The circulatory system includes the heart, arteries, veins and capillaries.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Comparing Unicellular and Multicellular Organisms
Unicellular Organisms Multicellular Organisms
Growth is limited.
A cell can only grow up to a certain size.
Then it has to divide into 2 smaller organisms.
Organism can grow much bigger.
This makes them stronger, faster, and more difficult to kill.
A cell is non-specialized.
It has to be able to carry out all the functions of living.
This means that it cannot be particularly good at one job. Its functions remain primitive and inefficient.
Different groups of cells can specialize at particular jobs so they are more efficient at that particular job.
This “division of labour” makes the whole organism more efficient, allowing it to develop better structures and functions.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Unicellular Organisms Multicellular Organisms
Cells are independent since they can carry out all functions of living.
Cells lose their independence.
This means that they cannot survive without each other.
This is beneficial to whole organism but a disadvantage to individual cells.
Each cell has easy access to nutrients and respiratory gases through its surface
This is because it has a large surface area-to-volume ratio.
Cells in the centre of the body cannot get nutrients and respiratory gases directly from surroundings.
The larger the organism, the greater the problem.
This is because large organisms have a small surface area-to-volume ratio.
Transport systems are needed to supply these cells.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
The diagrams below explain the effect of the surface area-to-volume ratio:
Cell can easily take in and remove substances from its
surroundings.
The outer cells have less surface available for exchange and the central cell has no contact
with the surroundings.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Calculating the surface area-to-volume ratio:
S. Area = 16 cm2
Volume = 2x2x1 = 4 cm
3
SA to Vol. Ratio = 16/4
= 4:1
S. Area = 24 cm2
Volume = 2x2x2 = 8 cm
3
SA to Vol. Ratio = 24/8
= 3:1
2
2 1
2 1
2
2
2 2
2
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Fill in the blanks The larger the organism, the smaller the surface area to volume ratio.
Elephants have a small surface area to volume ratio. To make up for this, they have
large flat ears so that they can lose heat from them.
Artic foxes have small ears. This prevents them from losing too much heat. Desert
foxes have large ears. This helps them to loose excess heat from their bodies.
A large penguin is better adapted at living in very cold conditions than a small penguin
since it has a smaller surface area to volume ratio. This means it loses less heat.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
The Light Microscope
Fine Focusing
Adjustment
Rotating
Nosepiece
Objective
Lenses
Slide with
Specimen
Stage Iris Diaphragm
Mirror
Stand
Eyepiece Lens
Rough Focusing
Adjustment
Tube
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Parts of the Microscope
The Iris Diaphragm controls the intensity
(amount) of light which enters the Microscope.
The Mirror directs light into the Microscope. Some Microscopes have an in-built light below the stage instead of
an adjustable mirror.
The Stage has two Clips (not shown) which hold
the slide in place.
The Small Objective Lens is the Low Power Lens while the Large Objective Lens is the High Power Lens.
When using the high power lens, be careful that you do not crack the slide!
The Magnifying Power of the microscope is calculated by multiplying the magnifying power of the eyepiece by the
magnifying power of the objective lens being used. You can calculate this for your microscope.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Using the Microscope
Switch on the microscope.
Place the slide on the stage and clip it in place.
Turn the nosepiece to the low power objective lens (x10). Be careful not to break the slide.
Turn the rough focusing knob until the LP objective lens is at its lowest position.
Focus by turning the rough focusing knob to raise the lens upwards until a sharp image is observed.
To use high power, turn the nosepiece to the high power objective lens (x40 or x60).
Use ONLY the fine focusing knob when using high power.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Taking Care of the Microscope
Handle the microscope only from the stand.
Be careful not to scratch the lenses - clean them only with lens tissue.
Keep the microscope covered when not in use.
If possible, store the microscope in a cupboard.
Make sure that the objective lenses do not make contact with the slide.
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Calculating the actual size of an object from a scale drawing
Actual size = Size of diagram
Magnification
Measure this distance on your paper in cm or mm, then change to metres
(Magnification x 4000)
Actual size = 0.067 m
= 0.00001675 m
(or 1.675 x 10-5
m) 4000
Actual size in µm (x1 000,000) = 16.75 microns
Biology Lesson Notes
Form 3/SS+NS/ss/12-13 Cells
Summary
Nearly all plants and animals are made up of thousands or millions of microscopic cells.
All cells contain cytoplasm enclosed in a cell membrane.
Most cells have a nucleus.
Cytoplasm contains organelles such as mitochondria and chloroplasts.
Many chemical reactions take place in the cytoplasm to keep the cell alive.
The nucleus directs the chemical reactions in the cell and also controls cell division.
Plant cells have a cellulose cell wall and a large central vacuole.
Cells are often specialized in their shapes and activities to carry out particular jobs.
Large number of similar cells packed together form a tissue.
Different tissues arranged together form organs.
A group of related organs make up a system.
As a cell’s size increases its surface area to volume ratio gets less. Eventually the surface area to volume ratio becomes too small for enough food and oxygen to get into the cell. Transport systems are then required.