REVISION: CELL DIVISION 20 MARCH 2013 Lesson Description In this lesson we revise: The Cell Theory and the parts of plant and animal cells The process of mitosis The structure and function of different plant tissues Key Concepts The Cell Theory All living things are made up of cells and are either unicellular or multicellular. Cells are the smallest working units of all living things that show the characteristics and properties of life. All cells come from preexisting cells through cell division. Important Terms: Cell wall Golgi body Organelle Turgid Cell membrane Mitochondrion Ribosomes Cell sap Chromatin network Nucleus Vacuole Tonoplast Cytoplasm Nucleolus Chloroplast Vacuoles Endoplasmic reticulum Nuclear membrane Flaccid Plasmodesmata Typical Plant Cells Diagram showing the cross section of a plant cell
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REVISION: CELL DIVISION 20 MARCH 2013
Lesson Description
In this lesson we revise:
The Cell Theory and the parts of plant and animal cells
The process of mitosis
The structure and function of different plant tissues
Key Concepts
The Cell Theory
All living things are made up of cells and are either unicellular or multicellular.
Cells are the smallest working units of all living things that show the characteristics and
properties of life.
All cells come from preexisting cells through cell division.
Ribososmes are organelles that help in the synthesis of proteins.
Ribosomes are made up of two parts, called subunits.
They get their names from their size. One unit is larger than the other so they are called large
and small subunits.
Both these subunits are necessary for protein synthesis in the cell. When the two units are
docked together with a special information unit called messenger RNA, they make proteins.
Some ribosomes are found in the cytoplasm, but most are attached to the endoplasmic
reticulum. While attached to the ER, ribosomes make proteins that the cell needs and also
ones to be exported from the cell for work elsewhere in the body.
Endoplasmic Reticulum
Diagram showing the structure of the endoplasmic reticulum
It is a network of membranes throughout the cytoplasm of the cell.
There are two types of ER.
When ribosomes are attached it is called rough ER and smooth ER when there are no
ribosomes attached.
The rough endoplasmic reticulum is where most protein synthesis occurs in the cell.
The function of the smooth endoplasmic reticulum is to synthesize lipids in the cell.
The smooth ER is also helps in the detoxification of harmful substances in the cell.
Difference between Plant and Animals cells
Plants Cells Animal cells
Most plant cells contain plastids No plastids
Surrounded by a cell wall and cell membrane Surrounded by a cell membrane only
Usually one, large storage vacuole present No or few small specialised vacuoles present
Generally have a regular shape Have more irregular and diverse shapes
Questions
Question 1
The following flow chart illustrates the relationship between two important processes found in the cells of plants.
a.) Identify organelles X and Y (2) b.) Provide labels for parts A, B and C. (3) c.) Identify the metabolic processes that organelles X and Y control respectively. (2) d.) Organelle Y is called the “power house” of the cell. Suggest a reason for this. (2) e.) Name the carbohydrate that is formed by X and used by Y. (1) f.) In which cell would you expect to find more of organelle Y, in a skin cell or a liver cell?
Give a reason for your answer. (2) g.) Described the interrelatedness between organelles X and Y based on the waste products
formed by these organelles during their respective metabolic processes. (4) h.) Give ONE structural adaptation of each organelle and describe how this adaptation enables
the organelle to function efficiently. (4)
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Key Concepts
Cell Cycle
The cell cycle starts when the cell forms and ends when, as a mature cell, it divides into two daughter cells. Each cell has its own cycle.
The cell cycle has three parts. First is interphase which is cell growth, the second is mitosis which is cell division and the third is cytokinesis, the stage in which the cytoplasm divides into two parts at the end of cell division.
Pie graph showing the life cycle of a cell (Cell cycle)
Interphase is when the cell grows to its full size, the nuclear material is copied and ready for
a new division, and new organelles are made to fill the cytoplasm.
Mitosis is the division of the nuclear material into two identical sets.
Cytokinesis is the division of the cytoplasm into two half-sized parts again.
Interphase and Chromosomes
At the beginning of interphase the cell grows quickly. More organelles are made and there is an increase in the number of chemical reactions. The cell may become specialised for its function in the body or it may store nutrients and get ready for mitosis. Towards the end of interphase the chromatin material makes a copy of itself by replication.
The chromatin network coils up to make short chromosomes. There are chromosomes in the nucleus of every cell.
At the end of interphase, each chromosome is composed of two identical strands because it has made a copy of itself.
The two identical strands are called chromatids and they are joined at one point called the centromere.
Diagram showing the structure and parts of a chromosome
The Purpose of Mitosis
Mitosis has three purposes:
Growth: multicellular organisms need cell division to grow; they all start as a single cell and soon have a huge number of cells.
Repair: organisms constantly repair and renew themselves; worn out or dead cells are replaced through cell division.
Reproduction: single - celled organisms, such as bacteria and protists, also reproduce by cell division (binary fission and budding)
Location of Mitosis In plants, mitosis occurs in the apical meristem tissue behind the tip of the root or stem and in
buds and in the lateral meristem tissue underneath bark.
In animas, it happens in specific places in the organs, like bone marrow and skin basal layers.
Some tissues are continuously being replaced by mitosis. Examples include epithelium tissue and connective tissue. Others, like liver and skin cells, only divide when it is necessary to repair damage.
What is Mitosis?
Mitosis is linked to cell growth. It is the process of cell division – a mature cell divides into two identical new cells. Mitosis usually takes an hour or two. Mitosis is a continuous process.
The Stages of Mitosis in Animal and Plant Cells
Two division processes are important in mitosis: o Karyokinesis: is the division of the nucleus o Cytokinesis: is the division of the cytoplasm
To make it easier to describe, we divide mitosis into four phases. 1. PROPHASE 2. METAPHASE 3. ANAPHASE 4. TELOPHASE
Mitosis in Animal Cells Interphase:
Cells may appear inactive during this stage, but they are quite the opposite. This is the longest period of the complete cell cycle during which DNA replicates, the centrioles divide, and proteins are actively produced.
Prophase:
Centrosome is made of two separate centrioles.
Fibres form between the centrosomes to form spindle fibres.
Centrosomes move to the opposite of the cells.
Each chromosome is visible as two chromatids joined by a centromere. Metaphase:
The nuclear membrane has disintegrated.
Chromosomes line up at the equator of the cell.
Each chromosome becomes attached to a separate spindle fibre and starts to move towards the equator of the cell.
Anaphase:
Each chromosome separates into its sister chromatids by the action of spindle fibres pulling each towards a spindle pole.
Each chromatid (now called a daughter chromosome) is pulled to opposite sides (poles) of the cell.
Telophase:
Cytokinesis starts by the cell membrane starting to constrict at the equator of the cell.
A nuclear membrane and nucleolus form in each daughter cell.
Each daughter cell has the same number of chromosomes as the parent cell.
Mitosis in Plant Cells Interphase:
DNA in chromatin network duplicates.
DNA thickens into chromosomes. Prophase:
Spindle fibres form between the poles of the cells, without the use of centrosomes.
A spindle is found in the plant cells without centrioles. Metaphase:
The nucleus membrane is completely disintegrated.
Chromosomes line up at the equator of the cell.
A centromere joins two chromatids to form a chromosome.
Each chromatid of a chromosome becomes attached to a spindle fibre at the centromere.
Anaphase:
The centromere splits.
Each chromosome separates into its sister chromatids. This happens when spindle fibres pull each towards a pole.
Each chromatid (now called a daughter chromosome) is pulled to the opposite poles of the cell.
Telophase:
Cytokinesis starts by a cell plate (cell wall) forming at the equator.
The chromosomes unwind and lengthen to form a chromatin network.
A nuclear membrane and nucleolus form in each daughter cell.
Each daughter cell has the same number of chromosomes as the parent cell.