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
Parts of Plant Cells 3D Diagram showing the cross section of a typical plant cell 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 watersaturated 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 Most plant cells contain plastids Surrounded by a cell wall and cell membrane Usually one, large storage vacuole present Generally have a regular shape Animal cells No plastids Surrounded by a cell membrane only No or few small specialised vacuoles present 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) Key Concepts Cell Cycle [10] 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. The Purpose of Mitosis Diagram showing the structure and parts of a chromosome 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 Interphase: Mitosis in Animal Cells 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