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

3D Diagram showing the cross section of a typical plant cell

Parts of Plant Cells A typical plant cell consists of the following parts:

A Cell Membrane

Cell Wall

Nucleus

The Cytoplasm

The Various Organelles

The Cell Membrane

Diagram showing the structure of a cell membrane

The fluid mosaic model describes the structure of the plasma membrane.

Different kinds of cell membrane models have been proposed, and one of the most useful is

the Fluid-mosaic model. In this model the membrane is seen as a bilayer of phospholipids

in which protein molecules are embedded.

Cell Wall

Diagram showing the structure of a cell wall

One of the most important distinguishing features of plant cells is the presence of a cell wall.

Structure: The cell wall is formed from fibrils of cellulose molecules, embedded in a water-

saturated matrix of polysaccharides and structural glycoprotein.

Functions: The cell wall protects the cellular contents; gives rigidity to the plant structure;

provides a porous medium for the circulation and distribution of water, minerals, and other

small nutrient molecules; and contains specialised molecules that regulate growth and protect

the plant from disease. It provides the cell with great tensile strength.

Cell Wall & Plasmodesmata:

Diagram showing the structure of a cell wall and plasmodesmata

Unlike cell membranes materials cannot get through cell walls. This would be a problem for

plant cells if not for special openings called plasmodesmata.

These openings are used to communicate and transport materials between plant cells

because the cell membranes are able touch and therefore exchange needed materials.

Nucleus

Diagram showing the structure on a nucleus

The nucleus is the control center of the cell. It is the largest organelle in the cell and it contains the

DNA of the cell. The DNA of all cells is made up of chromosomes.

DNA (Deoxyribonucleic Acid) contains all the information for cells to live, perform their

functions and reproduce.

Inside the nucleus is another organelle called the nucleolus. The nucleolus is responsible for

making ribosomes.

The circles on the surface of the nucleus are the nuclear pores. These are where ribosomes,

and other materials move in and out of the cell.

The Cytoplasm

Diagram showing the internal contents of Cytoplasm

Cytoplasm refers to the jelly-like material with organelles in it.

If the organelles were removed, the soluble part that would be left is called the cytosol.

It consists mainly of water with dissolved substances such as amino acids, vitamins and

nutrients in it.

Other Cellular Organelles

Chloroplast

Diagram showing the internal structure of a chloroplast

It is an oval structure surrounded by a double unit membrane.

It has an internal medium called the stroma.

Stacks of thylakoids form granum which contain the pigment for photosynthesis.

These grana are connected to other grana by lamellae.

The chloroplast is a cell organelle in which photosynthesis takes place. In this organelle the

light energy of the sun is converted into chemical energy.

Chloroplasts are found only in plant cells and not animal cells. The chemical energy that is

produced by chloroplasts is finally used to make carbohydrates like starch that get stored in

the plant.

Chloroplasts contain tiny pigments called chlorophylls. Chlorophylls are responsible for

trapping the light energy from the sun.

Vacuoles

Diagram showing the structure of a plant vacuole

Vacuoles and vesicles are storage organelles in cells. Vacuoles are larger than vesicles.

Functions: These structures may store water, waste products, food, and other cellular

materials.

In plant cells, the vacuole may take up most of the cell's volume.

The membrane surrounding the plant cell vacuole is called the tonoplast.

When a cell has its vacuole filled with cell sap it is referred to as a turgid cell. A cell in which

the vacuole has no or little water is referred to as a flaccid cell.

Mitochondrion

Diagram showing the electron micrograph of a mitochondrion

Mitochondria are membrane-enclosed organelles distributed throughout the cytosol of most

eukaryotic cells.

Their main function is cellular respiration in which y convert the potential energy of food

molecules into ATP.

Every type of cell has a different amount of mitochondria. There are more mitochondria in

cells that have to perform lots of work, for example - your leg muscle cells, heart muscle cells

etc. Other cells need less energy to do their work and have less mitochondrion.

Diagram showing the internal structure on a mitochondrion

Structure of Mitochondrion

Mitochondria have:

o an outer membrane that encloses the entire structure

o an inner membrane that encloses a fluid-filled matrix

o between the two is the intermembrane space the inner membrane is elaborately folded

with shelf like cristae projecting into the matrix.

Ribosome

Diagram showing the structure of a ribosome

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)

[10]

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.

Key Concepts

Terminology

blood epidermal nerve stem cell

chlorenchyma epithelium tissue palisade parencyhma vascular tissue

companion cell ground tissue phloem xylem

connective tissue lignin sclerenchyma

cuticle mesophyll sieve tube

dermal tissue muscle spongy mesophyll

The Organisation of Life

Diagram showing the organisation of Life

Plant Tissue

Plant cells with similar structure and functions form plant tissue.

Diagram showing the where different tissue are found in plants

Plant tissue can be divided into two main types: 1. Meristematic tissue 2. Permanent tissue

Meristematic Tissue

Meristematic tissue is actively dividing to produce new cells. Meristematic tissue consists of

undifferentiated small cell, with dense cytoplasm and large nuclei. The cells differentiate into new

tissue of the plant.

Meristematic tissue is found at the meristems of plants:

Apical Meristem: are located at the growing points at the tips of roots and stems and results

in an increase in the length of these structures.

Diagram showing the different types of meristematic tissue

Lateral Meristem: results in the growth in thickness or width of woody roots and stems. This

tissue is also called cambium; cork cambium divides to form the cork cells that form the outer

bark of a woody plant. Vascular cambium divides to make xylem and phloem tissue.

Permanent Tissue

Permanent tissue are specialised in function and do not divide constantly. Differentiation of cells

begins as soon as cells have been formed by cell division, and results in changes in structure. There

are three groups of permanent tissue:

1. Epidermal

2. Vascular tissue

3. Ground

Epidermal Tissue

This is the outermost layer of cells that covers the roots, stems and leaves. Epidermal cells are tightly

packed, with no intercellular air spaces. The main function of the epidermal cells is to protect the

underlying tissue from injury.

Some epidermal cells are modified to perform a specific function. Specialised epidermal cells of the

stem and leaves secrete a waxy layer, called the cuticle, to prevent water loss. Other examples of

specialised cells are guard cells and root hair cells.

Guard Cells

Micrograph of an Guard cell showing

Guard cells are bean- shaped epidermal cells that occur on either side of a stoma- which is

the opening that occurs on the surface of a leaf. The guard cells function to open and close

the stoma, thus controlling the loss of water by transpiration.

Hair cells

Micrograph of a Guard cell showing

The hair cells of an epidermal root hair cell are formed by an extension of the cell wall. The

hair functions to increase the surface area of the root to maximise the uptake of water and

nutrients.

Vascular Tissue

Vascular tissue functions to transport and support.

Xylem Tissue:

Xylem tissue transport water and mineral salts from the ground water through the roots to the

stems and leaves.

Xylem tissue consists of vessels and tracheids- both cells have cell walls that are

strengthened with lignin and both types of cells are dead at maturity.

Xylem vessels and tracheids do not contain cytoplasm and cross walls are perforated with

pits to enable the sideways movement of water.

Xylem vessels are elongated and hollow and form long tubes that are joined end to end to

allow water to flow from one cell to the next.

Tracheids are long and tapered at the ends. Tracheids function to strengthen the plant.

Phloem Tissue:

Plants have phloem tissue to transport food from the leaves, where photosynthesis takes

place, to areas undergoing growth or storage sites.

Phloem tissue consists of long columns of sieve tubes and companion cells.

Sieve tubes are elongated, hollow cells. Sieve tubes remain living, although the nuclei in the

cells die.

The end walls (called sieve plates) are perforated and hollow phloem sap to flow from one cell

to the next.

Each sieve tube is found next to a companion cell.

Companion cells keep the sieve tubes alive by regulating and performing their metabolic

activities

.

Ground Tissue

Ground tissue forms the body of the plant and is responsible for support, storage and photosynthesis.

There are three types of ground tissue:

1. Parenchyma

2. Collenchyma

3. Sclerenchyma

Table showing the structure and function of parenchyma, collenchyma and sclerenchyma

Diagram showing the different types of parenchyma cells

Parenchyma – thin walled & alive at maturity; often multifaceted.

Diagram showing the different types of collenchymas cells

Collenchyma – thick walled & alive at maturity

Sclerids Fibers

Diagram showing the different types of simple tissue – consisting of one cell type

Sclerenchyma – thick walled and dead at maturity

Sclerids or stone cells – cells as long as they are wide

Fibers – cells longer than they are wide