CELL STRUCTURECELL STRUCTURE

A Liver cell

Liver cell TEM x9400

Lily Parenchyma cell x.s. (TEM x7210)

Cell and Organelle

All living organisms are made of cells.

A cell is made of living material called

protoplasm.

Different kinds of cells vary in size, shape,

colours and internal structures.

Cell TheoryCell Theory

• The cell is the basic unit of life

• All cells come from pre-existing cells

• Virus > Bacteria > Red blood cell

Prokaryotic Cell

Organization of a cell• Subatomic particle• atomic• molecule• molecular complex• organelle• Virus

• Cell• Tissue• Organ• System• Organism

Cell Structure

• Cell wall• Cell membrane• protoplasm - the living contents within the cell:

nucleus and cytoplasm• The cytoplasm contains many subcellular

organelles.•  Sub cellular organelles revealed are: Golgi apparatus, mitochondrion, food granules,

centriole, secretary granules and chromatin in nucleus.

Cell membrane

• Chemical components :consists of protein and lipid

• Structure: Two layers of phospholipid sandwiched between two protein layers

Davson and Danielli model

Unit membrane hypothesis ( proposed by Robertson )

  Under electron microscopy, membrane shows a characteristic trilaminar appearance.   The 3 layers are arranged as that of Davson Danielli’s model.

trilaminar appearance

Fluid mosaic model of cell membrane

A mosaic of protein molecules floats in a fluid lipid layer.

      In this model, lipid bilayer remains as the main backbone of the unit membrane.      The lipids spontaneously form a bilayer owing to their polar heads and non-polar tails.      Proteins act as islands floating in the lipids. Some proteins move about freely while others are fixed in position.    

Some proteins penetrate only part of the way into the membrane while others penetrate all the way through.

      Proteins are variable in function.      Sugars are incorporated with protein of lipid to form glycoprotein or glycolipid respectively. These sugars are important in recognition mechanism.      Both lipids and proteins show rapid lateral diffusion in the plane of the membrane.      Fluidity and permeability of the cell membrane are determined by the degree of saturation and the hydrocarbon chain of the lipids.

Function of cell membrane• Protein components:• Act as a partially permeable barrier controlling the

movement of substances between the cell and the surrounding

• Channels or pores are formed within a protein or between adjacent protein molecules

• Act as support to give strength• Act as enzyme which catalyzes the chemical

reaction within the cell membrane• Act as carriers in transporting substance across the

membrane• Act as recognition center• As a pump moving things across it

      Lipid component:  The presence of unsaturated lipids can prevent close packing of the molecules so as to make membrane fluid.  Fluidity is also increased with decreasing length of the fatty acid tails.       Some membranes are folded to increase surface area to facilitate exchange with environment. e.g. microvilli of intestinal epithelial cells.

      Cell membrane can also separate the contents of cells from their external environments, i.e. form compartments for individual metabolic reaction to take place within the cell.       Cell membrane is differentially permeable because cell membrane allows glucose, amino acid, fatty acid, glycerol to diffuse slowly and actively control what substances to pass through.       The ability of infolding and extension to form vesicles allows endocytosis and exocytosis to take place.

Nucleus• Contains chromatin which is involved in

nuclear division

• Contains a nucleolus

Structure of Nucleus

• Enclosed by and envelope of two membranes that is perforated by nuclear pores

Function of Nucleus

• Necessary for survival of a cell

• Controls all the activities of the cell, cellular function, cell division and heredity

Nuclear membrane

• Doubled layer• Similar structure as

cell membrane• Continuous with E.R.• With microscopic

pores for exchange of materials between nucleus and cytoplasm

Nucleus with nuclear pores

Nucleoplasm• Nuclear sap• Gel-like• Denser than cytoplasm• Contains proteins, nucleotides and ions

Chromatin

• Consists of DNA and protein ( histone )• Condense to rod-shape chromosome just prior to

nuclear division• Carry genetic materials which determine

organisms’ characteristics and transmit these characteristics to next generations

Nucleolus• Composed of DNA mainly• Act as the manufacturing site of ribosomal DNA

(rDNA) and ribosomes

Endoplasm Reticulum ( E.R.)

• A system of parallel flattened membrane-bounded sacs called cisternae

• Continuous with the outer membrane of the nuclear envelope

• Act as an intracellular transport system

• There are two types of E.R - Rough ER and smooth ER

Rough E.R.

• Ribosomes are attached to its surface

• Transports proteins made by the ribosomes through the cisternae to smooth ER and then to Golgi appartus for futher modification

Smooth E.R.

• Without ribosomes attached to its surface

• Transport lipids• Synthesis of lipids and

steriod

Diagram of Golgi

Golgi Apparatus

• Usually located near the nucleus• consists of stacks of flattened membrane bounded sacs

called cisternae and many vesicles• At one end of the stacks new cisternae are constantly

formed by fusion of vesicles pinched from smooth ER; at the other end, small Golgi vesicles are pinched off constantly

• Transport in vesicles of many cell materials, such as enzymes form ER

• Involved in secretion and lysosome formation

Ribosomes

• Particles synthesis in nucleolus and then pass through the nuclear pores to the cytoplasm

• Made of protein and rDNA

• The site for protein synthesis

Ribosomes and Polysomes

Mitochondrion I

• Surrounded by an envelope of two membranes, the inner being folded to form cristae

• Contains a matrix with respiratory enzymes for the Kreb’s cycle

• Rich in cell which require large amont of energy such as sperm tail, muscle cell

cristae

matrix

outer membrane

inner membrane

Mitochondion II• The cristae increase the surface area for attachment

of respiratory enzymes for the electron transfer reactions

• In aerobic respiration, cristae are the sites of oxidative phosporylation and electron transport

Function of Mitochondrion

• Act as power house of a cell

• The energy releasing reactions of respiration occur in matrix and on the cristae

Lysosomes

• A spherical sac bounded by a single membrane

• Contain digestive (hydrolytic) enzymes

• Intracellular digestion of food materials eg. Amoeba

• Destroy the worn-out organelles inside cell

• For self-destruction of cells in developmental process

Cell wall

• Only found in plant cells• Rigid and rather

permeable• Made of cellulose• Usually modified by lignin• with pores which are

penetrated by plasmmodesmata

Function of Cell wall

• Provides mechanical support and protection of the cell

• Allows a pressure potential to be developed which aids in support

• Prevent osmotic bursting of the cell

Microtubule

• Act as cytoskeleton which support the cell

• Involves in the movement of substances inside the cell

• Forming the spindle fibres which involve in the separation of chromatids and chromosome

Centrioles• Adjacent to nucleus• Internal structure of a centriole is similar to that of

basal body of a cilium, with 9 micotubules• Forming the spindle fibres and microtubules during

nuclear division to control the separation of chromatids and chromosome

Vacuole

• Absence or small in animal cells• Common and large in plant cells• The enclosing membrane is called

tonoplast• Contain the internal cell sap which

is a concentrated solution consists of water, sugar, salts, fat, oils, proteins and pigment

Function of Vacuole

• Store various substances eg. Food and wastes

• Maintenance of turgor for support

• Contain hydrolytic enzymes to acts as lysosomes during life and cause autolysis after death

Chloroplast

• Large plastid containing chlorophyll which absorb light for photosynthesis

• Bounded by two membrane

• Consists of chloroplast envelope, stroma, lamella and granum

Plant Histology

• Parenchyma

• Collenchyma

• Sclerenchyma

• Xylem Tissue

• Phloem Tissue

Parenchyma

• Plant cells with thin cell wall and living protoplasm

• Roughly isodiametric with intercellular spaces

• Found in cortex and pith of stems and root, mesophyll of leaves and packing tissues in xylem and phloem

Function of Parenchyma

• Act as packing tissues between more specialized tissues

• Turgidity of these cells can provide support in herbaceous plant

• Store food

• Intercellular air spaces allow gaseous exchange

• Metabolically active

• Their cell walls are important pathway for the water and mineral salts through the plant

Collenchyma

• Characterized by the deposition of extra cellulose at the corners of the cells so have thickening cell wall of their corners

• They are living cells

• Found in regions beneath the epidermis of stem (hypodermis) and near the vascular tissues, eg, midrib of leaves

Sclerenchyma

• Plant cells with uniformly thickened cell wall which is usually lignified

• They are dead cells• Support the cells• There are two types:

fibres and Sclerids

Fibre

• Long narrow cell shape with tapering ends, wall with few piths

• Found in cortex, pericycle, vascular tissues, surrounding vascular bundles

Sclereids

• Shorter, vary much in shape, may be spherical, polyhedral, elongated or branched with numerous pits

• Found in almost everywhere in plant body, especially in cortex, phloem of stems and roots, in fruit wall and seed coat

• Act as main cell type for mechanical support

Xylem Tissue

• Tracheary elements (tracheids, vessels) which are dead and empty cells for conducting water and support

Tracheids

• Narrow elongated cell with finely tapering ends, without protoplasm at maturity, with heavily lignified and pitted secondary cell wall

• Passage of water from cell to cell is facilitated through pit-pairs which allow lateral transport of water

• Act as the only water conducting elements in gymnosperms and primitive vascular plant; small amount in angiosperms

Vessel I

• Long, pipe-like cell shape, with complete or incomplete perforation at the end wall

• Without protoplasm at maturity

• Join each other at perforated end walls to form longitudinal conducting tubes

• Shorter, greater in diameter than tracheids

Vessel II

• Water moves from cell to cell through perforations and pit pairs

• Cell wall lignified and strengthened to prevent collapse

• More specialized for water conducting than tracheids

• Only present in angiosperms

Diagram of Phloem

Phloem Tissue

• Sieve elements (sieve cells, sieve tube elements) for conduction of food materials

• Sieve elements are elongated cells, cell wall with sieve areas

• Sieve elements is absence of nucleus, tonoplast, decrease in number of ER and ribosomes, with thin layer cytoplasm to facilitate the translocation of food

Sieve Cell

• With sieve areas evenly distributed

• Present in gymnosperms and lower vascular plant

Sieve Tube Element

• Located on the end walls called sieve plates

• Connected by sieve plates with each other to form sieve tube

• Present in angiosperms

• With companion cell to help translocation of food

Blood cell

• Is a specialized connective tissue for transportation of materials and body defence

• include fluid medium (plasma 55%) and cellular constituents (erythrocytes, leukocytes and thrombocytes 45%)

Skeletal muscle

• Innervated by the voluntary part of nervous system

• contraction is neurogenic required nervous stimulation

• contract and fatigue rapidly• attached to the skeleton• long, cylindrical with tapering or rounded

ends

Functions of skeleton muscle

• for the maintenance of posture

• for locomotion and movements of body parts

Neurones

• Including cell body and nerve fibres

• located in C.N.S

• cell body is stellate in shape

• nerve fibres are protoplasmic extensions of cell body, including dendrons and axon

Nerve

• a group of nerve fibres bound together by connective tissues lying outside C.N.S

• including sensory nerve, motor nerve and mixed nerve