Sunday, November 23, 2008

CELLS





Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on Earth. The main purpose of a cell is to organize. Cells hold a variety of pieces and each cell has a different set of functions. It is easier for an organism to grow and survive when cells are present. If you were only made of one cell, you would only be able to grow to a certain size. You don't find single cells that are as large as a cow. Also, if you were only one cell you couldn't have a nervous system, no muscles for movement, and using the internet would be out of the question. The trillions of cells in your body make your life possible.

One Name, Many Types

There are many types of cells. In biology class, you will usually work with plant-like cells and animal-like cells. We say animal-like because an animal type of cell could be anything from a tiny microorganism to a nerve cell in your brain. Plant cells are easier to identify because they have a protective structure called a cell wall made of cellulose. Plants have the wall; animals do not. Plants also have organelles like the chloroplast (the things that make them green) or large water-filled vacuoles.

We said that there are many types of cells. Cells are unique to each type of organism. Humans may have hundreds of types of cells. Some cells are used to carry oxygen (O2) through the blood (red blood cells) and others might be specific to the heart. If you look at very simple organisms, you will discover cells that have no defined nucleus (prokaryotes) and other cells that have hundreds of nuclei (multinucleated). The thing they all have in common is that they are compartments surrounded by some type of membrane.
CELLS




Plasma Membrane
• Partially permeable: O2 and CO2 pass through by simple diffusion (also ethanol; drugs)
o Glucose, amino-acids, etc. enter by facilitated diffusion
o Water enters by osmosis
o Na+, K+, Cl – glucose etc, enter by active transport (sodium pump) – needs ATP
• Selectively permeable: controls entry and exit of specific materials.
• Keeps cell contents together allowing efficient coordination of its activity.
• Homeostasis - interior of cell maintained at optimum pH etc for efficient metabolism

Cytoplasm
• The living contents of a cell excluding the nucleus and large vacuoles.
• A complex solution in which the cell’s organelles are suspended.
• Many biochemical processes take place here, e.g., glycolysis, protein synthesis.

Nucleus

• Present in eukaryotes only – NOT prokaryotes (i.e. bacteria).
• Contains DNA (on chromosomes), the hereditary material.
• The genes are present in the DNA.
Passes on genes to next generation of cell (through mitosis)
• DNA (deoxyribonucleic acid) is visible as chromatin in active cells
• In chromosomes during mitosis and meiosis.
• Not present in a red blood cells (or bacteria!).

Nuclear Pores
• Large molecules can pass between the cytoplasm and the nucleus through these pores.
• E.g. mRNA from nucleus to cytoplasm. – and nucleotides the other way.

Chromatin
Chromatin is the very fine thread-like combinations of DNA and protein in non-dividing nuclei. The protein assists in the efficient packaging and regulation of DNA activity. A human nucleus contains 46 such fine threads of chromatin.

Chromosomes
A chromosome is a ‘condensed chromatin’ thread only visible during mitosis and meiosis.
Haploid nuclei (n) have one set of chromosomes i.e. one of each kind of chromosome. Diploid nuclei (2n) have two sets of chromosomes i.e. two of each chromosome.
The nuclei of human somatic (= body) cells are diploid (2n), with 46 chromosomes i.e. 23 x two sets.
One set from the mother in the haploid egg cell, the other from the father’s haploid sperm cell.
Sex Chromosomes: the 23rd pair. Female: XX Male XY
Each chromosome has a unique set of genes. Each gene has a specific locus – it is on a particular chromosome at a specific site.
Different (mutant) forms of a gene (A, B, O for the blood-group gene) are known as alleles.

Centrioles
Have a ‘9 + 2’ arrangement of protein tubes (tubulin).
Two sets of tubes, each at right-angles
Divides to from spindle at cell division.

Lysosomes
Contain digestive enzymes for recycling macromolecules within the cell
Responsible for ‘cell death’ – burst to carry out ‘autolysis’


Mitochondria:
• The aerobic steps of respiration occur here – Krebs Cycle and Electron Transport System.
• 36 of the 38 ATPs from one molecule of glucose are produced in the mitochondrion.
• Liver, muscle and nerve cells are rich in mitochondria.
• Sperm cells have a ‘mid-piece’ containing many mitochondria
• Bone and fat cells have low numbers of mitochondria.
• Found in low numbers in plant cells



Flagellae / cilia
Composed of ‘9 + 2’ protein structure; have basal body at base, which provides power
Falgellae, few, long; cilia many, short, synchronised beating (metachronal rhythm)
Responsible for cell movement (sperm) or fluid movement (trachea and Fallopian tubes)

Ribosomes:
• Composed of RNA and protein.
• Function in protein synthesis – translation of mRNA into a specific sequence of amino acids.
• 70s – prokaryotes & mitochondria and chloroplasts – 80s all eukaryotes

Chloroplast
• Photosynthesis is the major function of the chloroplast.
• The light stage takes place in the green internal membranes.
• The dark stage occurs in liquid part of the chloroplast.
• Starch may be stored in the chloroplast



Large Plant Cell Vacuole
• Storage of water, food (sugar, amino acids), ions, wastes.
• Role in cell elongation during plant growth.

Saturday, November 15, 2008

GROUND TISSUES

Parenchyma
A generalized plant cell type, parenchyma cells are alive at maturity. They function in storage, photosynthesis, and as the bulk of ground and vascular tissues. Palisade parenchyma cells are elogated cells located in many leaves just below the epidermal tissue. Spongy mesophyll cells occur below the one or two layers of palisade cells. Ray parenchyma cells occur in wood rays, the structures that transport materials laterally within a woody stem. Parenchyma cells also occur within the xylem and phloem of vascular bundles. The largest parenchyma cells occur in the pith region, often, as in corn (Zea ) stems, being larger than the vascular bundles. In many prepared slides they stain green.


Collenchyma
Collenchyma cells support the plant. These cells are charcterized by thickenings of the wall, the are alive at maturity. They tend to occur as part of vascular bundles or on the corners of angular stems. In many prepared slides they stain red.



Sclerenchyma

Sclerenchyma cells support the plant. They often occur as bundle cap fibers. Sclerenchyma cells are characterized by thickenings in their secondary walls. They are dead at maturity. They, like collenchyma, stain red in many commonly used prepared slides.
A common type of schlerenchyma cell is the fiber.











A mature vascular plant, e.g., a tobacco plant, contains several differentiated cell types. These are grouped together in tissues. Some tissues contain only one type of cell. Some consist of several.

The main function of meristematic tissue is mitosis. The cells are small, thin-walled, with no central vacuole and no specialized features.
Meristematic tissue is located in the apical meristems at the growing points of roots and stems.
the secondary meristems (lateral buds) at the nodes of stems (where branching occurs) [View], and in some plants, a ring of meristematic tissue, called the cambium, that is found within the mature stem.
The cells produced in the meristems soon become differentiated into one or another of several types.
ProtectiveProtective tissue covers the surface of leaves and the living cells of roots and stems. Its cells are flattened with their top and bottom surfaces parallel. The upper and lower epidermis of the leaf are examples of protective tissue [View].

The cells of parenchyma are large, thin-walled, and usually have a large central vacuole. They are often partially separated from each other. They are usually stuffed with plastids.
In areas not exposed to light, colorless plastids predominate and food storage is the main function. The cells of the white potato are parenchyma cells. [View]
Where light is present, e.g., in leaves, chloroplasts predominate and photosynthesis is the main function. [View]
The walls of these cells are very thick and built up in a uniform layer around the entire margin of the cell. Often, the protoplasts die after the cell wall is fully formed. Sclerenchyma cells are usually found associated with other cells types and give them mechanical support. Sclerenchyma is found in stems and also in leaf veins. [View] Sclerenchyma also makes up the hard outer covering of seeds and nuts.

Collenchyma
Collenchyma cells have thick walls that are especially thick at their corners. These cells provide mechanical support for the plant. They are most often found in areas that are growing rapidly and need to be strengthened. The petiole ("stalk") of leaves is usually reinforced with collenchyma.
Xylem
Xylem conducts water and dissolved minerals from the roots to all the other parts of the plant.
Link to discussion of water and mineral transport in the xylem.
In angiosperms, most of the water travels in the xylem vessels. These are thick-walled tubes that can extend vertically through several feet of xylem tissue. Their diameter may be as large as 0.7 mm. Their walls are thickened with secondary deposits of cellulose and are usually further strengthened by impregnation with lignin. The secondary walls of the xylem vessels are deposited in spirals and rings and are usually perforated by pits.
Xylem vessels arise from individual cylindrical cells oriented end to end. At maturity the end walls of these cells dissolve away and the cytoplasmic contents die. The result is the xylem vessel, a continuous nonliving duct. The vessels carry water and some dissolved solutes, such as inorganic ions, up the plant.
Xylem also contains tracheids. These are individual cells tapered at each end so the tapered end of one cell overlaps that of the adjacent cell. Like xylem vessels, they have thick, lignified walls and, at maturity, no cytoplasm. Their walls are perforated so that water can flow from one tracheid to the next. The xylem of ferns and conifers contains only tracheids.
In woody plants, the older xylem ceases to participate in water transport and simply serves to give strength to the trunk. Wood is xylem. When counting the annual rings of a tree, one is counting rings of xylem.
The main components of phloem are sieve elements and companion cells.
Sieve elements are so-named because their end walls are perforated. This allows cytoplasmic connections between vertically-stacked cells. The result is a sieve tube that conducts the products of photosynthesis - sugars and amino acids - from the place where they are manufactured (a "source"), e.g., leaves, to the places ("sinks") where they are consumed or stored; such as
roots
growing tips of stems and leaves
flowers
fruits, tubers, corms, etc.
Sieve elements have no nucleus and only a sparse collection of other organelles. They depend on the adjacent companion cells for many functions. Companion cells move sugars and amino acids into and out of the sieve elements. In "source" tissue, such as a leaf, the companion cells use transmembrane proteins to take up - by active transport - sugars and amino acids from the cells manufacturing them. Water follows by osmosis. These materials then move into adjacent sieve elements by diffusion through plasmodesmata. The pressure created by osmosis drives the flow of materials through the sieve tubes.
In "sink" tissue, the sugars and amino acids leave the sieve tubes by diffusion through plasmodesmata connecting the sieve elements to the cells of their destination. Again, water follows by osmosis where it mayleave the plant by transpiration or increase the volume of the cells or move into the xylem for recycling through the plant.

Friday, November 14, 2008

HUMAN TISSUES



The human body is composed of four levels of organization: cells, tissues, organs and body systems.
Cells of similar structure & function are organized into tissues.
There are four primary types of tissue in the human body:
epithelial
connective
muscle
neural.

Epithelial Tissue

Epithelial tissue covers most internal & external surfaces of the body. In addition, epithelial tissue also forms many glandular structures as well.
Epithelium consists almost exclusively of cells with very little extracellular material between them.
Epithelial tissue often has one free surface that is not associated with other cells and a basal surface which is attached to a basement membrane.
Epithelial tissue are avascular. Blood vessels do not penetrate the basement membrane to the reach the epithelium. Therefore, all gases, nutrients & waste products must diffuse across the basement membrane to & from blood vessels in the underlying connective tissue.

Classification of Epithelium

Epithelial tissue are classified according to the arrangement of cell layers & cell shape.
Arrangement of layers:
Simple epithelium- consists of a single layer of cells with each cell extending from the basement membrane to the free surface.
Stratified epithelium- consists of more than one layer of cells & only one of which is adjacent to the basement membrane.
Pseudostratified epithelium- consists of a single layer of cells that are all attached to the basement membrane yet only some of the cells reach the free surface.
Squamous- cells are flatten or scale-like.
Cuboidal- cells are cube-shaped and are similar in width & height.
Columnar- cells are tall and thin.

Function of Epithelial Tissu
Functionally, epithelial tissues play an important role in diffusion, filtration, secretion, absorption, protection, movement of mucus and are also capable of great stretching

Connective Tissue

Structure of Connective Tissue
The essential characteristic that separates connective tissue from the other three tissue types is the presence of an abundant amount of extracellular matrix or material between its cells.
The extracellular matrix has three major components:
Protein fibers (ie. Collagen, reticular & elastin)
Ground substances consisting of nonfibrous proteins and other materials

Classification of Connective Tissue
The extracellular matrix is the basis for the classification of connective tissue into the three following subgroups:
Matrix with mainly protein fibers (ie. Adipose)
Matrix with protein fibers and ground substance (ie. Bone)
Matrix with mainly fluid (ie. Blood)

Adipose Tissue
Structure: Consists of adipocytes which are specialized to store triglycerides & lipids. Cells are so full of lipids that the nucleus is pushed to the periphery of the cell.
Function: Thermal insulator, energy storage, supports & protects underlying structures.


Bone
Bone is a hard connective tissue that consists of living cells ( osteocytes) & mineralized matrix. Bone tissue is classified into two types:
Compact (Dense) bone
Structure: Hard bony matrix predominates with almost no space between the many thin layers of bone.
Function: Provides great strength & support, protects delicate structures and works in conjunction with skeletal muscles to generate movement.
Location: Outer portion of all bones and the shafts of long bones.

Blood
Structure: Various blood cells and a fluid matrix.
Function: Transports O2, CO2, hormones, nutrients & waste products. Protects the body from infections, and is involved in temperature regulation and the clotting process.

Muscle Tissue

Structure of Muscle Tissue
Muscle tissue consists of long, thin cells or fibers known as myocytes which are constructed to generate a force for contraction.
Based on location and certain structural & functional characteristics, muscle tissue is classified into three categories:

Smooth Muscle Tissue
Structure: Cells are spindle-shaped, nonstriated fibers with one centrally located nucleus.
Function: Regulates the size of organs, forces fluid through tubes, controls the amount of light entering the eye. Under involuntary control.
Location: Walls of hollow organs, glands & iris of the eye.

Skeletal Muscle Tissue
Structure: Cells are large, long, striated & cylindrical with multiple peripherally located nuclei in each cell.
Function: Movement of the body, posture & heat production. Under voluntary control.
Location: Attached to bones by tendons.
Cardiac Muscle Tissue
Structure: Cells are cylindrical, branching and are striated with a single, centrally located nucleus. Cells are connected to one another largely by specialized gap junctions called intercalated disks.
Function: Pumps blood. Under mostly involuntary control.
Location: Heart

Nerve Tissue

Structure of Nerve Tissue
Nerve tissue is composed of highly specialized tissue called neurons.
Neurons are composed of three major parts:
Cell body- contains the nucleus & is the site of general cellular function.
Dendrites- receives electric signals (action potentials) and then conduct these signals toward the cell body.
Axons- conducts action potentials away from the cell body.
Structure: Relatively large cells in a variety of shapes. Characterized mostly by cell processes.
Function: Conducts action potentials, store data and integrate & evaluate data.
Location: Primarily in the CNS yet some cell processes extend to all parts of the human body.





Monday, November 3, 2008

STRUCTURED QUESTIONS

1. Diagram 1 shows organelles P, Q, R and S that are found in plant cells.












DIAGRAM 1

a. (i) Name the organelles P, Q, R and S.

P : ………………………………..

Q : ………………………………

R : ………………………………

S : ………………………………


(ii) State the roles of organelles Q and S.

Q : ……………………………………………………………………………………..

…………………………………………………………………………………….

S: ……………………………………………………………………………………..

……………………………………………………………………………………..


b. In which part of a leaf can organelle R be found the most?

………………………………………………………………………………………


c. What is produced from structure Y in organelle Q.

……………………………………………………………………………………….


d. Diagram 2 shows a cell T which can be found in human body. The cell contains

abundant of organelles P.


























DIAGRAM 2

Explain the need of that amount of organelle P in cell T.

…………………………………………………………………………………………….

……………………………………………………………………………………………

……………………………………………………………………………………………