Download Unit 1 – Biology – Cells

Cell Biology
Summary
All living things are made up of cells. The structures of different types of cells are related to their
functions. To get into or out of cells, dissolved substances have to cross the cell membranes.
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Plant and Animal Cells (eukaryotic cells)
Cells are the smallest unit of life.
All living things are made of cells.
Most human cells, like most other animal cells, have the following parts:
o nucleus
o cytoplasm
o cell membrane
o mitochondria
o ribosomes
Plant and algal cells also have:
o cell wall
o chloroplasts
o permanent vacuole
Respiration
Summary
Respiration in cells can take place aerobically or anaerobically. The energy released is used in a
variety of ways. The human body needs to react to the increased demand for energy during exercise.
Respiration
 Definition: The process of releasing energy from food in every living cell.
 Aerobic respiration - uses oxygen
Word equation:
Glucose + Oxygen
Carbon dioxide + Water + Energy
Mitochondria
 Most of the reactions in respiration happen in the mitochondria.
 The inner surface of the mitochondria is highly folded to increase the surface area for enzymes.
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Bacteria (prokaryotic cells)
Bacterium is a single-celled organism.
Bacterial cells (prokaryotic cells) are much smaller in comparison.
They have cytoplasm and a cell membrane surrounded by a cell wall.
The genetic material is not enclosed in a nucleus, but float in the cytoplasm. It is a single DNA
loop and there may be one or more small rings of DNA called plasmids.
Cell differentiation
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Cells may be specialised to carry out a particular function.
As an organism develops, cells differentiate to form different types of cells.
Most types of animal cell differentiate at an early stage whereas many types of plant cells
retain the ability to differentiate throughout life.
As a cell differentiates it acquires different structures to enable it to carry out a certain
function. It has become a specialised cell.
Examples:
Microscopy
Instruments used to magnify cells.
Object = material placed under a microscope.
Image = the appearance of material when viewed under the microscope.
The magnification of an object is how many times bigger the image is when compared to the object.
Remember!!!
When doing any calculations the size of the object and image must be in the same units.
to convert all calculations into the smallest unit.
The units you need to know are:
It is best
The light microscope
Visible light is passed through an object and is focused on the eye using lenses. The light microscope
can view images in natural colours and can also view living and moving objects. The light microscope
can magnify an image up to 600x. Preparation of slides is cheap and easy.
The electron microscope
Beams of electrons are passed through an object and focused on a photographic plate using an
electromagnetic field
Electron microscopes allow objects to be magnified up to 500,000x. However the electron microscope
only allows black and white images to be seen and slides can be very difficult and expensive to
prepare.
An electron microscope has much higher magnification and resolving power than a light microscope.
This means that it can be used to study cells in much finer detail. This has enabled biologists to see
and understand many more sub-cellular structures.
Comparing the two microscopes…
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Resolution
The resolution or resolving power of a microscope is the minimum distance apart two objects
can be in order for them to appear as separate items.
e.g. the resolving power of a light microscope is 0.2 um. SO
- If two items are closer than 0.2 um apart they will appear as a single item.
- If two items are further apart than 0.2um they can be distinguished from each other
and will look like a single item.
The greater the resolution the greater the clarity.
The image produced is clearer and more precise.
Every microscope has a limit of resolution. Up to this point if you increase the magnification
you will see more detail, but beyond this point the object will appear larger but not clearer.
Low resolution
High resolution
Comparing the two types of microscopes
Calculations needed
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Growing Microoganisms
Microorganisms = organisms that can only be viewed with a microscope.
Eg bacteria, viruses and fungi.
Bacteria multiply by simple cell division (called binaryfission) as often as once every 20 minutes if
they have enough nutrients and a suitable temperature.
Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate.
Uncontaminated cultures of microorganisms are required for investigating the action of
disinfectants and antibiotics.
Antibiotics – drugs that can be taken to kill bacteria inside the body
Disinfectants – substances that can be used to kill bacteria on surfaces
It is important that the culture is not contaminated with other microorganisms that may compete
for nutrients or produce toxins.
Careful procedures are required to prevent potentially pathogenic microorganisms being released
into the environment.
Culturing microorganisms
 To study microorganisms, they need to be cultured.
 They need to be provided with the conditions they need to reproduce quickly:
o Nutrients
o Warmth
o Moisture
 Bacteria and fungi can be grown in special media called agar.
 This provides them with:
o Carbohydrate
o Protein or amino acids
o Water
 When agar is heated up it is liquid.
 It can be poured into a Petri dish.
o A circular plastic or glass dish with a lid:
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The agar solidifies when left to cool.
Petri dishes and culture media must be sterilised before use to kill unwanted microorganisms
Inoculating loops are used to transfer microorganisms to the media.
These must be sterilised by passing them through a flame:
 The lid of the Petri dish should be secured with adhesive tape to prevent microorganisms from the
air contaminating the culture.
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In school and college laboratories, cultures should be incubated at a maximum temperature of
25oC.
This greatly reduces the likelihood of growth of pathogens that might be harmful to humans.
In industrial conditions higher temperatures can produce more rapid growth.
Cell Division and Growth
Genetic material
 The nucleus of a cell contains chromosomes
 In body cells the chromosomes are normally found
in pairs.
 In the nucleus of a typical human body cell there
are 23 pairs of chromosomes.
 We inherit one set of 23 chromosomes from each
of our parents.
 Each chromosome carries a large number of genes.
 Chromosomes are made of DNA molecules.
 DNA molecules are large.
 DNA consists of two strands coiled into a double
helix structure.
 A gene is a small section of DNA.
 Each gene codes for a particular combination of
amino acids which make a specific protein.
 These proteins determine our characteristics.
Cell Division
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New body cells are produced:
o When the animal is growing.
o To repair damaged tissues.
o To replace worn out tissues.
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Cells divide in a series of stages called the cell cycle.
One of these stages is mitosis where the DNA, which has already been copied, divides.
Mitosis = nuclear division
The cell cycle
1. Before a cell can divide it needs to grow and increase the number of sub-cellular structures
such as ribosomes and mitochondria.
2. The genetic material is doubled - The DNA replicates to form two copies of each chromosome.
3. The DNA is divided - one set of chromosomes is pulled to each end of the cell and the nucleus
divides (mitosis).
4. Finally the cytoplasm and cell membranes divide to form two identical cells.
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The cell divides in 2 to form 2 genetically identical cells.
Some cells undergo cell division again and again.
Some cells carry out their function then die.
Asexual reproduction
The cells of the offspring produced by asexual reproduction are produced by mitosis from the
parental cells.
They contain the same genes as the parents.
Cell differentiation
Differentiation results when some genes are turned on, some are turned off during an organisms
development.
Once the cells are specialised they carry out their role.
Most types of animal cells differentiate at an early stage.
Many plant cells retain the ability to differentiate throughout life.
Stem cells
A stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more
cells of the same type, or can differentiate into any other cell type.
There are very few stem cells in an adult – they can be obtained from human embryos, umbilical cords
and adult bone marrow.
Stem cells can be cloned and made to differentiate into many different types of human cells.
How can stem cells be used?
There is currently a lot of research involving the use of stem cells to treat various diseases and
injuries.
 Repair of damaged tissues or organs
o Spinal cord injury (paralysis)
o Diabetes
o Heart failure
o Parkinson’s Disease
o Growth of intact organs for transplant
 Gene therapy for genetic diseases
 Scientific research into human development
 Drug and toxicity testing
Therapeutic cloning
In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the
embryo are not rejected by the patient’s body so they may be used for medical treatment.
Steps involve
• Cloning adult cells to make an embryo,.
• Using the embryo’s stem cells used to grow organs for transplant
• Perfectly matched to adult so no tissue rejection (no drugs needed!!).
In more detail…..
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However, many people are concerned about the use of human embryos to treat diseases.
They feel that all embryos have got the potential to become a baby, and that they should not be
used in this way.
ALSO
The use of stem cells has potential risks such as transfer of viral infection, and some people have
ethical or religious objections.
A more comprehensive table of the advantages and disadvantages are listed below…
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Cloning Plants
Stem cells from meristems in plants can be used to produce clones of plants quickly and
economically.
Plant cells have the ability to differentiate all their life not like in animals where embryos or bone
marrow cells can only be used.
It can divide by mitosis to produce identical copies.
It can change its function to do a different job if needed.
2 ways of cloning plants…
1. Taking cuttings
Cloning plants BY…Taking Cuttings
• Part of the plant is cut off – usually a leaf on its stalk.
• The end of the stem is dipped into hormone powder.
• The cutting is placed in moist soil and left in a damp environment.
• The cutting will grow roots and will develop into new plants.
• The new plants are clones of the original plant.
• This method can produce a lot of plants more quickly than traditional plant sexual
reproduction.
Advantages of taking Cuttings
• You can make clones of a plant which is desirable/superior.
• No specialist equipment is needed – anyone can do it.
Limitations of taking cuttings
• You can make more plants than traditional methods but the numbers are still limited.
• Diseases can be passed onto the offspring plants.
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2. Tissue Culture
A few cells from the meristems (tips of roots or shoots) of a desirable plants are taken.
The cells are placed on a jelly that contains nutrients and hormones to stimulate growth.
The cells divide to form a ball of cells called a callus.
The large callus can be divided to make lots of smaller calluses.
Each small callus can be placed onto another jelly with different hormones to cause them to
grow roots and shoots, little plantlets are formed.
When the plantlets grow into larger plants they are placed into soil and grow into clones of the
desirable plant.
Advantages of Tissue Culture
• You can make an unlimited number of clones of a plant which is desirable/superior.
• Meristems are disease free so the clones will also be disease free.
Limitations of Tissue Culture
• It is more expensive than taking cuttings
• Specialist equipment is needed.
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All plants produced by taking cuttings and tissue culture will be identical to the parent plant.
What will we clone?
Rare species can be cloned to protect from extinction.
Large numbers of identical crop plants with special features such as disease resistance.
Movement into and out of cells
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To get into or out of cells, dissolved substances have to cross the cell membranes.
Solutes = particles in solution eg glucose, sodium ions, chloride ions.
Solvent = liquid in which the particles are dissolved eg water.
Solute and solvent molecules move around randomly.
Solutes can move into and out of cells by diffusion.
Diffusion
Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in a
net movement from a region where they are of a higher concentration.
Oxygen required for respiration passes through cell membranes by diffusion.
Carbon dioxide (the waste product of respiration diffuses out of cells.
The waste product urea diffuses from cells into the blood plasma for excretion in the kidney.
Factors which affect the rate of diffusion are:
1. the difference in concentrations (concentration gradient)- The greater the difference in
concentration, the faster the rate of diffusion.
2. the temperature – diffusion is faster if it is warmer.
3. the surface area of the membrane – the larger the surface area the faster the diffusion.
Single-celled organism has a relatively large surface area to volume ratio. This allows sufficient
transport of molecules into and out of the cell to meet the needs of the organism.
In multicellular organisms the smaller surface area to volume ratio means surfaces and organ systems
are specialised for exchanging materials. This is to allow sufficient molecules to be transported into
and out of cells for the organism’s needs.
Specialised exchange surfaces
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Many organ systems are specialised for exchanging materials.
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Gas and solute exchange surfaces in humans and other organisms are adapted to maximise
effectiveness.
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The size and complexity of an organism increases the difficulty of exchanging materials.
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Larger organisms have more cells, so have greater requirement for exchange.
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However, the larger an organism, the smaller their surface area to volume ratio.
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Therefore, they require even more complex exchange surfaces to supply their requirements.
The effectiveness of an exchange surface is increased by:
• having a large surface area
• a membrane that is thin, to provide a short diffusion path
• (in animals) having an efficient blood supply
• (in animals, for gaseous exchange) being ventilated (air being refreshed in the lungs.
Osmosis:
Water may move across cell membranes via osmosis.
DEFINITION
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially
permeable membrane.
It can also be described as movement from a high water concentration to a lower water
concentration. This is because a concentrated solution of sugar or salt (or any solute) has a low water
concentration and a dilute solution of sugar or salt (or any solute) has a high water concentration.
Osmosis in animal cells:
If animal cells are placed in a solution that has a higher solute concentration than the
cytoplasm, then water will leave the cell by osmosis, until it shrinks and dies.
If animal cells are placed in a solution that has a lower solute concentration
the cytoplasm, then water will enter the cell by osmosis until it bursts.
than
This is why it is vital that we maintain the concentration of our body fluids at an
equal solute concentration to our cells’ cytoplasm.
Osmosis in plant cells:
If plant cells are placed in a solution that has a higher solute concentration than the cytoplasm, then
water will leave the cell by osmosis, and the cell membrane separates from the cell wall. This will
cause a plant to wilt.
If plant cells are placed in a solution that has a lower solute concentration than the
cytoplasm, then water will enter the cell by osmosis until it is fully turgid, and the
cell wall prevents any more water entering. This is important in enabling plants to
remain upright. The cell wall prevents the plant cell from bursting.
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Sports Drinks
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Most soft drinks contain water, sugar and ions.
Sports drinks contain sugars to replace the sugar used in energy release during the activity.
They also contain water and ions to replace the water and ions lost during sweating.
If water and ions are not replaced, the ion / water balance of the body is disturbed and the cells
do not work as efficiently.
Energy drinks
 Contain the same concentration of ions as the body fluids, and a high concentration of glucose.
 This enables rapid uptake of glucose.
Rehydrating drinks
 These contain lower concentration of ions than in body fluids.
 This enables rapid uptake of water by osmosis.
 These drinks enable people to become quickly rehydrated after exercise.
Dissolved substances
Dissolved substances move by diffusion and by active transport.
Active Transport:
Active transport moves substances from a more dilute solution to a more concentrated solution
(against a concentration gradient). This requires energy from respiration.
Active transport occurs
Sequence of events…
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The substances being transported attaches to large protein molecules in the cell membrane.
Energy released through respiration is used to change the shape of the protein.
This releases the substance on the other side of the membrane.
This process enables cells to absorb substances from very dilute solutions (against a
concentration gradient).
2 examples:
1. Absorption of mineral ions by the root hair cells in roots in plants. –
Plants require mineral ions for healthy growth. Minerals are in really low concentrations in the soil so
plants cannot obtain minerals by diffusion through their membranes as minerals would move out of
their cells.
Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in
the soil.
Root Hair Cell Structure
• A root hair cell has a long and narrow protrusion (may also be referred to as hair-like
structure).
• A root hair cell has a large vacuole with lots of mitochondria in the cytoplasm.
• A root hair cells have no chloroplasts!
Function of root hair cell:
• To absorb water by osmosis and mineral salts by active transport.
ADAPTATIONS of root hair cells:
• The hair-like structure helps to increase the surface area of the root hair cell, thus helps the
root hair cell to absorb more water and mineral salts.
• The hair-like structure which is long and narrow helps the root hair cell to penetrate in
between soil particles in search of water and mineral salts.
• Thin walls to reduce the distance needed for exchange
• Many transport proteins in the membranes to allow lots of active transport
• The presence of mitochondria in large number in the cytoplasm of the root hair cell, helps for
more absorption of mineral salts by active transport (Remember, active transport will only
occur in the presence of energy provided by the mitochondria).
• The large vacuole enable more water and mineral salts to be stored after being absorbed.
(Note: the absence of chloroplasts actually helps the vacuole to stretch out)
2. Absorption of glucose by epithelial cells in the small intestine.
It allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which
has a higher sugar concentration.
Sugar molecules are used for cell respiration.
Epithelial cells lining the small intestine need to bring glucose made available from digestion into the
body and must prevent the reverse flow of glucose from the cells back into the intestines. We need a
mechanism to ensure that glucose always flows into intestinal cells and gets transported into the
bloodstream, no matter what the concentration of glucose is in the small intestine. Imagine what
would happen if this were not so, and intestinal cells used diffusion for glucose. Immediately after
you ate a chocolate bar or other food rich in sugar, the concentration of glucose in the gut would be
high, and glucose would flow "downhill" from the small intestine into your body. But an hour later,
when your intestines were empty and glucose concentrations in the intestines were lower than in your
blood and tissues, diffusion would allow the glucose in the intestinal cells to flow back into the small
intestine. Because this situation would be biologically wasteful and probably lethal, it is worth the
additional energy cost of active transport to make sure that glucose transport is a one-way process.
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Intestinal Cell Structure
They have microvilli on the surface facing the lumen of the small intestine.
They have many transport proteins in their cell membrane.
They have many mitochondria in the cytoplasm.
Function of intestinal cell:
To absorb products of digestion e.g. absorption of glucose by active transport.
ADAPTATIONS of intestinal cells:
The microvilli help to increase the surface area of the cell, thus helps the cell to absorb more
glucose.
Many transport proteins in the membranes to allow maximum possible active transport
The large number of mitochondria in the cytoplasm of the cell, helps for more absorption of
glucose by active transport (Remember, active transport will only occur in the presence of
energy provided by the mitochondria).
Membrane
Carrier