Cell Structure

Note by eimearkelly3, updated more than 1 year ago
Created by eimearkelly3 about 8 years ago


Leaving Certificate Biology (Cell Structure) Note on Cell Structure, created by eimearkelly3 on 07/04/2013.

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Microscopes:The compound microscope uses two lenses, an objective lens, and an eyepiece lens.The simple microscope uses a single lens.Both of these microsocopes use light to show the image.

Cells are measured in units called micrometres.

Coarse adjustment - for rough focusingFine adjustment -  for precise focusingClip - To hold the slideStage height adjustment - To allow the long, high power lens to fit over the slideMirorr/ light source - To supply light to the objectCondenser - Focuses light on the object (is not always preset)Diaphragm lever - Controls the amount of light reaching the objectObjective lens - Magnifies the imageNosepiece - revolves to move the desired lens into positionEyepiece lens - Magnifies the image

Animal cells - methylene bluePlant cells - iodine

Animal cellsAnimal cells are surrounded my an outer membrane called the CELL OR PLASMA MEMBRANE. This membrane surrounds the PROTOPLASM Cytoplasm --> liquid surrounding the nucleus inculding organellesProtoplasm --> all living parts of the cell (nucleus, cytosol, organelles)Cytosol --> Cytoplasm without nucleus or organelles

<-- as seen under a light microscope

<-- ultrastructure of an animal cell (aslo include nuclear membrane, nuclear pores,nucleolus, chromatin, ribosomes)

Plant cellsPlat cells are enclosed by  rigid cell wall made of CELLULOSE. Cellulose is a strong structural carbohydrate.The function of a cell wall is support.The cell mebrane is usually found just inside the cell wall. The wall and membrane are often so close that the membrane is not seen clearly.Vacuoles contain a fluid called cell sap. This is a solution of salts, sugars, and pigments.The vacuole helps to give the cells strength and shape (turgor pressure) and may also store materials e.g. water, food (sugar, amino acids), ions, metabolic waste.

<-- as seen under a light microscope

<-- ultrastructure of a plant cell (omit the endoplasmic reticulum and golgi body, include; nuclear pores, nucleur membranes, nucleolus, and chromatin)

Cell ultrastructure Electron microscopes use a beam of electrons instead of light.A TEM (Transmission Electron Microscope) sends a beam of electrons through a thin section of the specimen.This shows the internal structure of the specimen in great detail.An SEM (Scanning Electron Microscope)  uses a beam of electrons to provide a surface view of the specimenUltrastructure - the fine detail of the cell as seen with an electron microscope

Cell membranes are composed of phospholipids and proteins.Phospholipids, which have a water-loving phosphate group and a water-hating lipid group, are arranged into double layers (bilayers) The phophates are on the exposed outer surfaces and the lpisd in the middle.

Protein molecules are completely or partially embedded in the phopsholipid bilayer. Some are attached to the bilayer, others are detachable and can move throughout the bilayer.

just include the glycoprotein and glycolipid, the phosphate head, lipid tail, protins, pores and phospholipid molecule

FUNCTIONS OF MEMBRANES The surface of the membrane contains receptors that receive chemical messages from other parts of the body and bring them to the cell. Membranes retain the cells contents Membranes control what enters and leaves the cell. Membranes can allow the free passage of some molecules and prevent the passage of others. In this way they are said to be SELECTIVELY PERMEABLE. e.g. water and oxygen can pass freely but sodium ions and large proteins have to be moved across using energy (ACTIVE TRANSPORT) Membranes give some support to the cell. Membranes recognise molecules that touch them. Membranes are in constant motion --> fluid mossaic model.

NUCLEUSThe nucleus is the control centre of the cell. It is surrounded by a double membrane with numerous nucleur pores. This allows the controlled entry and exit of molecules in and out of the nucleus.The nucleus contains strands of DNA (deoxyribonucleic acid). DNA is arranged into structures called chromosomes.Every organism has a definite number of chromosomes in each nucleus (humans have 46 chromosomes)Genes are located randomly along chromosomes. Genes are the structures that inform the cell how to make certain proteins. They are the units of inheritance.When the cell is not dividing i.e. most of the time, the chromosomes are very elongated and interwoven. In this form they are called chromatin.Nucleur pores allow a type of RNA (ribonucleic acid) called mRNA (messenger RNA) to pass in and out of the nucleus.LIVE PLANT CELLS WITHOUT A NUCLEUS: phloem sieve tube cellsLIVE ANIMAL CELLS WITHOUT A NUCLEUS: red blood cells

NUCLEOLUSThe nucleolus (pl. nucleoli) is an area in the nucleus which stains very darkly. It is where the ribosomes are made.As ribosomes are composed of RNA, the nucleolus makes and contains a lot of rRNA (ribosomal RNA)

CYTOPLASMThe cytoplasm is a jelly-like fluid in a cell that surrounds the nucleus. A number of small bodies called organelles (such as mitochondria, chloroplasts, and ribosomes) are suspended in the cytoplasm.

MITOCHONDRIAMitochondria (s.mitochondrion) supply energy to the cell. They are the sites of respiration (aerobic stage).Cells with many mitochondria produce lots of energy (e.g. muscle and liver cells in humans). Cells with few mitochondria (e.g. fat in humans) produce less energy.It is on the inner membrane, especially the infoldings, that energy is released. If more infoldings are present they will cause the mitochondria to produce more energy. Each mitchindrion has its own loop of DNA. (capable of reproduction to meet cell's needs - reprodction of mitochondria is by means of binary fission).Active mitochondria convert to the inactive form if the cell rests for too long. Exercise causes the number of infoldings to increase again.

CHLOROPLASTSChloroplasts are green structures in plants in which photsynthesis takes place.Chloroplasts contain the green pigment chorophyll.

granum - light stagestroma - dark stage

CELL WALLPlant cell walls are made of cellulose. Their function is to strengthen and support the cell. (Prevent cells from from bursting, if they are in a solution more dilute than their cytoplasm).Cell walls are fully permeable.The middle lamella of pectin glues neighbouring plant cell walls together.

RIBOSOMESRibosomes are tiny, bead-like structures. They are made of RNA and protein.The function of ribosomes is to make proteins (protein synthesis) . They do this by combining a sequence of amino acids to form the proteins.

PLASTIDSChloroplasts --> rich in chlorophyll, essential for photosynthesis.Glucose may be stored as starch.The light stage of photosynthesis takes place in the green internal membranes (grana)The dark stage takes place in the liquid part (stroma)Contain their own DNA (can replicate to suit the cell's needs by means of binary fission)Leucoplasts --> colourless, storing starch e.g. founds in potatoe cellsChromoplasts --> Used for strong coloured pigments e.g. petals.

PROKARYOTIC / EUKARYOTIC CELLSProkaryotic cells do not have a nucleus or membrane-enclosed organelles.Prokaryotes are single-celled organisms such as bacteria.The DNA of prokaryotic cells is found as circular loops in the cytoplasm. Prokaryotic cells are generally small and do not have structures such as mitchondria and chloroplasts. Reactions such as respiration and photosynthesis (if it occurs) are carried out on infoldings of the plasma membrane.Eukaryotic cells have a nucleus and cell organelles all of which are enclosed by membranes.Eukaryotes are more advanced than prokaryotes. Eukaryotic cells are larger then prokaryotic cells. Eukaryotic cells evolved from prokaryotic cells.

Cell structure




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