Demonstrate understanding of life processes at the cellular level
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1
Life processes at the cellular level include:
·
photosynthesis
·
respiration
·
cell
division (DNA replication and mitosis as part of the cell cycle).
2
Biological
ideas, as they relate to each of the life processes at the cellular level, are
selected from:
·
movement
of materials (including diffusion, osmosis, active transport)
·
enzyme
activity (specific names of enzymes are not required)
·
factors
affecting the process
·
details
of the processes only as they relate to the overall functioning of the cell
(specific names of stages are not required)
·
reasons
for similarities and differences between cells such as cell size and shape, and
type and number of organelles present.
CELL THEORY AND THE CELL CYCLE
Although many variations on cell theory exist, there are three basic principals that provide a common understanding of what it means to be part of the living world:
- Cells are the basic functional units of life
- All living things are made up of cells and the products of cells
- All cells come form pre-existing cells
Plant and Animal Cells - Getting to know the organelles.
The video below outlines the main organelles of plant and animal cells. As you watch it, write down as many functions of organelles as you can.
Organelle list:
mitochondria - double membrane organelle that produces useable
energy in the form of ATP from glucose through cellular respiration
smooth endoplasmic reticulum – site where proteins are modified, lipids or fat are produced
ribosomes – synthesize new polypeptide chains (proteins) from amino acids by reading mRNA code
rough endoplasmic reticulum - transport, modification and storage of proteins
chloroplast – double membrane organelle that converts sunlight energy into chemical energy in the form of glucose
golgi apparatus – modifies and packages call products into vesicles
lysosomes - vesicle like structures containing enzymes for digestion
cilia - hair like structures usually found on the surface of the cell
flagellum – cell adaptation to facilitate movement of the cell in its environment
contractile vacuole – specialised pump to remove water from cells in hypotonic environments by active transport
vesicle – membrane bound structure that helps moves protein, lipid and carbohydrate in/out of the cell via active transport
vacuole – large space used for the storage of food or water (plant cells have a large vacuole)
cell membrane – selectively permeable phospholipid bilayer that separates cell contents from the environment
nucleus - information centre of the cell where DNA is kept.
A closer look at the Nucleus, Chloroplasts and Mitochondria
GENES - INSTRUCTIONS FOR LIFE
Since the discovery of DNA and the definition of genes as the basic units of inheritance, scientists have explored deeper into the innerworking of cells to unlock the secrets of life.
Starting with all the genetic potential of their predecessor, each new cell enters the beginning of a cell cycle through which it will access various parts of the genetic code as necessary for its unique and specific purpose in life. Many cells complete the cycle and give rise to new cells. Others become fixed in their function and survive only a defined length of time without entering a reproductive phase.
The following clip explains the cell cycle and revises the steps involved in the cell cycle.
The cell cycle describes how cells live, grow and divide to
produce new cells. This cycle consists of a working stage, interphase (G1, S,
G2), mitosis and cytokinesis.
G0: This is the working stage of a cell when it performs
it’s specialised job.
G1: During this stage of growth, the cell synthesises new
organelles and proteins (structural and catalytic) required for it to do its
job and also to prepare for division.
S stage: DNA is replicated through semi-conservative
replication.
G2: RNA and protein is synthesised and genetic code is
proof-read
M stage: Mitosis is the separation of homologous pairs of
chromosomes to ensure that each new cell has a full set of chromosomes.
Cytokensis: The cell divides, two new nuclei form and
chromosomes unravel.
DNA replication is necessary so that when a cell divides,
each new cell can receive a full set of chromosomes. Chromosomes are made of
genes which are the DNA code for synthesising proteins.
DNA replication takes place in the following way:
1) The enzyme helicase unwinds and unzips the
double stranded double helix. This exposes two single stranded template strands
of DNA which can be used to synthesise two complementary strands.
2) RNA primase synthesises primers to
identify starting points for the replication process
3) Free nucleotides line up along the template
according to the base-pairing rule due to the attractive hydrogen bonds
between complementary bases. A=T, G=C. The base pairing process ensures that
the genetic code of the chromosome is unaltered and so that both
chromosomes produced at the end of the process are identical.
4) DNA polymerase joins together the
sugar-phosphate backbone of the newly synthesised chain.
The following video explains what happens when the cell cycle is not kept in check.
DNA REPLICATION
DNA replication occurs in anticipation of cell division for reproduction, growth and repair. Bacteria cells preparing for binary fission, embryonic cells rapidly growing in number at the very start of each new life, and cells in the reproductive organs responsible for making gametes all follow the same basic rules when it comes to ensuring an exact copy of the genetic code is passed on.
The following animation helps to explain the role of the following essential enzymes involved in the successful replication of DNA:
- helicase
- primase/RNA polymerase
- DNA polymerase III
- DNA polymerase I
- DNA Ligase
The antiparallel nature of DNA (opposing 3' and 5' ends of complementary strands) requires differences in the replication process. As helicase unwinds and unzips the parent DNA strand by breaking the weak hydrogen bonds which hold base pairs aligned, regions where promoters can bind are exposed. Complementing one parental template strand DNA is continuously synthesised in the same direction that helicase exposes unpaired bases. On the other parental template strand, DNA is discontinuously synthesised as enzymes move away from the replication fork creating short fragments of DNA. As a result, one strand is referred to as the leading strand and the other is known as the lagging strand during replication.
The video below outlines the process of semi-conservative replication and details the role of each enzyme in the process.
DNA Replication
A slower, but more detailed step wise narrated animation of DNA replication
http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html
Now try talking through the process with a friend using the following link
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dna-rna2.swf
For a more accurate account please enjoy the following TED talk with fully animated nuclear processes.
CELL DIVISION
The semi-conservative nature of DNA replication ensures that each new cell produced possesses genes which have stood the test of evolutionary time. Daughter cells of multicellular organisms produced by mitosis are genetically identical to the parent cell. The nucleotide sequences in the nucleus and number of chromosomes remain unchanged following normal cell division by mitosis.
Meiosis, on the other hand, involves several processes which provide an opportunity for the creation of chromosomes with new genetic combinations of alleles and cells with differing numbers of sets of chromosomes. Gametes must carry the correct number of chromosomes to ensure that healthy fertile offspring result from their fertilisation.
The following video details meiosis following DNA replication.
PHOTOSYNTHESIS
Photosynthesis is the process by which plants use energy from the sun to produce chemical energy in the form of glucose.
Often this process is described as carbon dioxide reacting with water to produce glucose and oxygen.
In reality, a series of step-wise reactions involving: chlorophyll, trans-membrane proteins,
enzymes/co-enzymes, and the hydrogen carrier molecule NADP, energy in the form of ATP and NADPH is used to synthesize glucose molecules.
Photosynthesis can be split into two separate reactions
1) The light dependent phase - which involves the absorption of sunlight into chlorophyll molecules in the granna which excites several electrons and splits a molecule of water. As electrons are passed between transmembrane proteins in the thylakoids, hydrogen is also pumped out into the stroma, where it is picked up by NADPH. ATP is made in the same process.
2) Light independent phase - Energy from the ATP and NADPH is now used to convert carbon dioxide into longer hydrocarbon chains in the stroma. The Calvin Cycle does not require any additional light energy and converts carbon dioxide into many carbon based molecules ending with the formation of glucose.
CELLULAR RESPIRATION
Respiration is the process of breaking down chemical forms of energy to produce energy in the form of adenosine triphosphate (ATP).
Aerobic cellular respiration occurs in the mitochondria. It can be thought of as a type of "complete combustion" by which glucose reacts with oxygen to produce carbon dioxide and water.
In actual fact, through a series of step-wise reactions involving: trans-membrane proteins,
enzymes/co-enzymes, and hydrogen carrier molecules (NAD and FAD), ATP is made by the cell.
There are three main steps to the process, and each takes place in a specific part of the cell.
1) Glycolysis occurs in the cytoplasm of the cell. Glucose is converted into 2 molecules of pyruvate which then enter the matix of the mitochondira
2) The Krebs Cycle (Citric Acid Cycle) converts pyruvate into many carbon based molecules. During this process, hydrogen is picked up by carrier molecules and taken to the folded membrane which makes up the cristae of the mitochondria. Carbon dioxide is produced in the matrix.
3) Transmembrane proteins (enzymes and co-factors) in the cristae make up Electron Transport Chains. These molecules pass electrons down the line and pump H+ across the membrane in both directions. The energy released from these processes is used to synthesise ATP. (One of the final proteins in the chain is ATP synthase). It is during this stage that oxygen which has diffused into the mitochondria is converted into water.
The following video provides a detailed account of aerobic respiration:
Try using this diagram to help talk through your understanding of cellular respiration.
Do not worry about the exact names of compounds. Emphasize
a) which compartment of the cell each process takes place.
b) the product of each step of the process.
c) the role of proteins/enzymes/carrier molecules in the process.
CELL TRANSPORT
The plasma membrane is made up of a phospholipid bilayer
and transmembrane proteins. The bilayer consists of two layers of phospholipid
molecules. These phospholipid layers are arranged so that the hydrophilic
phosphate heads point outward and the hydrophobic fatty acid tails
face inward to create a semi-permeable membrane. Transmembrane
proteins embedded in the membrane create channels connecting the
cytoplasm on the interior of the cell to the fluid on the exterior of the cell.
Check out this video which outlines how different forms of transport take place across and involving the plasma membrane:
The following text explains how passive diffusion and facilitated diffusion are involved in the process of aerobic respiration:
Aerobic respiration is the conversion of chemical
energy in the form of glucose into usable energy in the form of ATP in
the presence of oxygen.
Glucose + oxygen --> carbon dioxide + water + ATP
The mitochondria is the organelle responsible for aerobic
respiration. In order to produce energy in the cell, oxygen and glucose are
required by the cell.
Oxygen molecules are small and are able to move by simple
diffusion into the cell by sliding past the phospholipids. No energy
is required for this process to occur provided that there is a higher
concentration of oxygen outside of the cell, than inside the cell.
Glucose is a larger molecule compared to oxygen.
Protein channels in the plasma membrane provide a pathway for glucose molecules
to move into the cell by facilitated diffusion. Glucose moves along the
concentration gradient, from an area of higher concentration (outside
the cell) to an area of lower concentration (inside the cell), with the
assistance of the protein channel, but without the need for the cell to expend
energy on the process.
ATP molecules produced through aerobic respiration provide
the energy needed for the cell for enzyme-catalyzed reactions and to
perform all of the life processes (movement, reproduction (mitosis),
excretion/nutrition (active transport). Since these processes all require
energy, the cell needs a constant supply of glucose and oxygen to replenish
ATP as it is used. Using up glucose and oxygen in the cell maintains the
concentration gradient required to continue the movement of glucose and
oxygen into the cell.
Differences in the plasma membranes, or number of organelles (such as mitochondria) between cell types can relate to their job or function.
Cells such as muscle cells and sperm cells require a larger
amount of energy than cells such as fat cells or skin cells since movement
requires more energy than acting as a storage container or physical barrier.
Muscle cells are constantly repairing themselves and use ATP every time they
are used, whereas fat cells only require energy when catalysing reactions which
store or release energy in times of need. Sperm cells have a continuous journey
requiring a lot of energy to seek out the egg, whereas skin cells, once formed,
act as a protective layer and are shed on a regular basis. Muscle cells and sperm cells may have a greater number of transmembrane proteins in their plasma membranes to assist with glucose transport and they will have more mitochondria than cells with a smaller energy demand.
ENZYME ACTIVITY
Enzymes are biological catalysts which increase the rate of reaction in cells by:
Enzymes are biological catalysts which increase the rate of reaction in cells by:
Reducing the activation energy required for the
reaction, ensuring the correct orientation of reactants during
collisions and providing and alternative pathway for the reaction.
Enzymes are protein molecules that have an active site
that can bind to a substrate or substrates. They can assist in forming
new products by synthesising or breaking down larger molecules.
Enzymes are specific, meaning that their active site
can only bind substrates with a specific complementary shape. Enzymes
and substrates can be thought of as a lock and a key.
The induced fit model of enzyme activity explains
that once a substrate and enzyme combine to form a complex, a change of shape
occurs in the enzyme which catalyses the chemical reaction. After this, the
product or products are released.
Temperature is an important factor which influences the rate
of chemical reactions. Generally higher temperature speeds up chemical
reactions and colder temperatures slow down chemical reactions.
First, increasing temperature increased the kinetic energy
of the reactants and enzymes. This means that substrates and enzymes combine
quicker and collisions occur with more energy. Overall this will
result in a faster rate of successful collisions. The opposite is true
when temperatures are colder.
However, since enzymes are proteins, they can also change
shape when the temperature is changed. When temperatures are too cold, the
enzyme may take on a form that prevents its active site from binding with the
specific substrate. This will slow the rate of reaction. When temperatures are
too hot, a similar change may occur, also preventing the enzyme from performing
its task. When temperatures are too hot, permanent damage can occur to
the active site of an enzyme preventing it from being able to bind with its
substrate. The enzyme is said to be denatured.
Photosynthesis happens more quickly in summer months
due to the increased warmth (and light intensity). This is because enzymes
present in the thylakoid membrane have an optimal temperature at which
their shape can most efficiently catalyse the chemical reactions
necessary to synthesise glucose from carbon dioxide and water.
Respiration is also most efficient at specific temperatures
due to the role of enzymes in the electron transport chain of the cristae
in the mitochondria which assist in the breakdown of glucose to form ATP.
For humans, the optimal temperature is around 37 degrees, body temperature.
Above and below this temperature, enzyme shape is altered which slows
down the rate of respiration.
DNA replication also relies on enzymes. Most plants show an
increased growth rate in summer due to the return of warmer temperatures.
This is due to the shape of enzymes involved in DNA replication and cell
division being most efficient during warmer months. For seeds, this helps in
the timing of germination when many new cells are produced requiring a
rapid rate of DNA replication.
Very good and helpful blog Mr cooper
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