cell structure 1 - cells as the... · 2020-04-22 · cell (organelles). eukaryotc organisms are...

Cell Structure

• Inquiry question: What distinguishes one cell from another?

What is a cell?A cell is defned as the basic unit of all living things.

Some organisms are made of only one cell (unicellular) and some are made from many cells working together (multcellular).

W h a t is a ce l l ?Although there are diferent types of cells (blood cell, skin cell, bacteria etc), thecells of plants, animals and bacteria share a number of common structures. These include:• The cell membrane separate the interior of the cell from the outside

environment• Cytoplasm consists of the cytosol and, in eukaryotes, the

organelles.Cytosol is a gel-like substance, made up of more than 80% water, and contains ions, salts and organic molecules

• DNA carries hereditary informaton, directs the cell’s actvites and is passed from parents to ofspring

• Ribosomes are organelles responsible for the synthesis of proteins.

Prokaryotes v s EukaryotesCells can be classifed into two groups: prokaryotes and eukaryotes.• Prokaryotes are primitve and have a very simple structure

• Examples include bacteria• Eukaryotes are much more complex cells and have membrane-bound

internal structures.• Examples include paramecium, amoeba and cells that make up animals

and plants

P ro ka r y o t e sProkaryotes are organisms that are made up of a single cell (unicellular). Bacteria, cyanobacteria (photosynthetc bacteria), and archaea, such as methanogens, are examples of prokaryotes.

Prokaryotc organisms can be found everywhere—even in extreme environments such as volcanoes.

Most prokaryotc cells are small and therefore have a large surface area relatve to their volume. This allows the cells to take in and release materials efciently and replicate quickly.

P ro ka r y o t e s

As the size of a cell decreases, the surface-area-to-volume rato (SA:V) of the cell increases. A greater surface-area-to-volume rato increases the rate at which materials can move across the surface. Being small allows cells to efciently exchange materials with their environment.

Prokaryotc cells lack membrane-bound organelles, and their cytoplasm contains scatered ribosomes that are involved in the synthesis of proteins. The genetc material of prokaryotc cells is usually a single, circular DNA chromosome called the genophore.

EukaryotesThe cells of eukaryotes are much larger and more complex than prokaryotc cells.Eukaryotc cells have a cell (plasma) membrane that surrounds the cell’s cytoplasmand internal (non-plasma) membranes that form specialised compartments within the cell (organelles).Eukaryotc organisms are incredibly diverse. There are unicellular and multcellularforms and organisms that can reproduce asexually and sexually. Multcellularity andsexual reproducton are unique to eukaryotes. Eukaryotc organisms only represent a small proporton of all species on Earth, but eukaryotes are much larger in size.Eukaryotes are divided into the four kingdoms, Protsta, Fungi, Plantae and Animalia.The cells of these groups share many typical eukaryotc features, but they also have cell structures and functons that are unique

Comparing

Prokaryotc and eukaryotc cells difer in several ways:• Prokaryotc cells do not have membrane-bound organelles, while eukaryotc cells have

many diferent membrane-bound organelles, with specialised structures and functons.• Prokaryotc cells do not have a nucleus, and their DNA is in the form of a single, circular

chromosome and small, circular molecules called plasmids. The DNA of eukaryotc cells is in the form of linear chromosomes and is contained in the nucleus.

• Prokaryotc cell walls are made of peptdoglycan. The cells of some eukaryotes, such asplants, fungi (singular fungus) and protsts, are surrounded by a cell wall composed of carbohydrates.

• Prokaryotc and eukaryotc cells also difer signifcantly in size. The typical eukaryotccell is around 10 tmes larger than most prokaryotc cells.

Inside ce l l sAll eukaryotc cells have membrane-bound internal structures called organelles and they all have specialised functons.

S t r u c t u re and funct ionStructure• How something is arranged or put

together

• What looks like

• Organisaton of something

Functon

• What something does and why

• What it is used for

Nuc leusThe nucleus is a large organelle surrounded by a double-layered nuclear membrane. The nuclear membrane contains pores that link it with the cytosol. The nucleus contains most of the genetc material, which is formed as DNA and proteins. The informaton for the synthesis of new proteins is present in genes within the DNA. The nucleus is the control centre of the cell and coordinates all of the cell’s actvites.

RibosomeRibosomes consist of two subunits joined together. Cells contain many thousands of ribosomes. These are only about 30 nanometres (nm) in diameter, and can therefore only be seen under an electron microscope. Ribosomes are composed of proteins and ribosomal RNA (rRNA), and are the sites of protein synthesis. They translate the sequence of amino acids specifed by the mRNA into proteins. Ribosomes do not have a membrane surrounding them; they arenon-membrane-bound organelles.

Endoplasmic reticulumThe endoplasmic retculum (ER) is a network of intracellular membranous sacs (cisternae) and tubules. It links with the cell membrane and other membranous organelles, including the nucleus.

The ER can be rough or smooth. Rough endoplasmic retculum (RER) has ribosomes atached. RER is abundant in cells that actvely produce and export proteins, such as pancreatc cells, which secrete digestve enzymes.

Smooth endoplasmic retculum (SER) does not have ribosomes atached. It contains the enzymes involved in the synthesis of molecules other than proteins, such as phospholipids and steroids.

Golgi Appa ra tusThe Golgi apparatus is also called the Golgi body or Golgi complex. It is a stack of fatened, smooth membrane sacs called cisternae.Unlike the cisternae found in the rough endoplasmic retculum, the cisternae in theGolgi apparatus are not connected. When proteins formed in the rough endoplasmicretculum reach the Golgi apparatus, vesicles are formed from each cisternae. Thevesicles transport the proteins from one cisternae to the next, where they are modifed for use by the cell, or for transport out of the cell.The cisternae then form transport vesicles to move the modifed proteins into thecytosol, into other organelles, or out of the cell. For example, digestve enzymes sentto lysosomes are not released from the cell, while secreted hormones are exportedfrom the cell. Other vesicles budding from the Golgi apparatus carry membrane- bound proteins to the cell membrane.

Ly s o s o m eLysosomes are the cell’s recycling units—specialised vesicles that digest unwanted mater.

Lysosomes fuse with vesicles containing unwanted mater, such as damaged organelles or foreign mater. The enzymes in the lysosome then digest the unwanted mater. Small molecules that the cell can reuse may difuse back into the cytoplasm. The rest are either retained in the lysosome, or released from thecell by the process of exocytosis.

MitochondriaMitochondria are organelles composed of two membranes – the inner and outer membranes.

Mitochondria is where cellular respiraton occurs, convertng the chemical energy in organic molecules (from food) into energy that cells can use. The inner mitochondrial membranes are the site of the chemical reactons of cellular respiraton. The highly folded structure of the inner membranes increases the surface area over which these chemical reactons can take place.

Chlorop las tsChloroplasts are organelles involved in photosynthesis. They contain large amounts of a green pigment called chlorophyll.

Chloroplasts are composed of three membranes: the outer membrane, the inner membrane and the thylakoid system. Thylakoids aredisc-shaped sacs that form compartments within the chloroplast.

Chloroplasts trap light energy, which is used to split water molecules into hydrogen and oxygen in the process of photosynthesis. The hydrogen then combines with carbon dioxide to make glucose, and the oxygen is released into the atmosphere as a waste product.

Va c u o l eA vacuole is a membrane-bound, liquid-flled space that stores enzymes and other organic and inorganic molecules.

Animal cells contain many small, temporary vacuoles, but most plant cells contain a single, large, permanent vacuole surrounded by a membrane called the tonoplast.

Plant vacuoles provide structural support by helping to maintain turgor. They also functon in a similar way to the lysosomes found in animal cells.

Cel l Wa l lThe cell wall is a rigid structure that surrounds the cell membrane of plant cells, fungal cells and some prokaryotc cells.

In plants, the cell wall is composed mainly of cellulose. Fungal cell walls are made of chitn.

The cell wall provides support, prevents expansion of the cell, and allows water and dissolved substances to pass freely through it. Lignin in the cell walls of woody plants, especially in the xylem, gives them additonal strength.

TECHNOLOGIES USED TO SEE

CELLS

Light microscopesLight microscopes are those we use in the school labs. They work by passing a beam of light through a condenser lens then through the thin specimen (usually placed on a slide). The light passes through a convex objectve lens where the image is magnifed and can be seen through the ocular lens.

• Produces images up to x1500 depending on the lenses (ours only go up to x400)

• Both living and non-living specimens can be viewed under a compound light microscope.

Fluorescence microscopesA fuorescence microscope is used to examine cells,cellular structures or any fuorescing material, such as stains, dyes or antbodies, with fuorescent molecules.Fluorescent cells contain molecules that absorb light ata partcular wavelength (called the excitng wavelength) and emit light at another wavelength.Fluorescence techniques allow scientsts to visualisestructures and materials inside cells that are usually toosmall to view. They can also target and detect partcular proteins, and diagnose disease.

E lec t ron microscope

•Since the 1950s the electron microscope has revolutonised studies of microscope organisms.

•It uses an electron beam rather than light, allowing us to see structures in far more detail than is possible using light microscopy. An electron microscope produces a narrow beam of electrons. Electrons striking the specimen are either absorbed, scatered, or pass through it.

E lec t ron microscopeThe image obtained with an electron microscope has a much higher resoluton and a greater depth of feld than an image from a light microscope.

Electron microscopy produces only black and white images, but these are ofen coloured later to highlight important features.

Fluid Mosaic Model

Cel l Membra neThe cell membrane controls the exchange of material between the internal and external environments of the cell.

It is selectvely permeable meaning only certain molecules or ions are allowed in or out.

We cannot see the cell membrane with any microscope available today, however, we use the fuid mosaic model proposed in 1972 by Singer and Nicholson.

Cel l Memb raneThe cell membrane encloses the contents of a cell and controls the movement of substances between the extracellular fuid outside the cell and the intracellular fuid (or cytosol) inside the cell.

The cell membrane therefore helps maintain an environment within the cell that difers from the external environment.

As well as controlling the transport of molecules into and out of the cell, the cell membrane is also involved in cell recogniton and communicaton with other cells.

Fluid mosa ic modelAccording to this model, cell membranes consist of a bilayer (two layers) of phospholipid molecules. Other molecules, such as proteins, carbohydrates and cholesterol, are scatered throughout the bilayer.

The phospholipid bilayer of the cell membrane is called a bilayer because it has two layers of phospholipids. Phospholipid molecules have a hydrophilic (water-atractng) ‘head’ and two hydrophobic (water-repelling) ‘tails’.

The hydrophilic heads form the outside and inside lining of the cell membrane, and the hydrophobic tails of the two layers of phospholipids meet in the middle.

Fluid mosaic model

Membrane proteinsProtein molecules are suspended in the bilayer and some can form channels that allow material to cross the membrane. Some are fxed and some can move freely.

These proteins allow cell to cell interactons and communicaton.

Transport proteins act like passageways allowing specifc substances to move across the membrane. Receptor proteins cause the cell to respond to certain signals such as hormones. Glycoproteins act to identfy the cell and are called antgens or marker molecules.

Cell FunctionHOW DO CELLS COORDINATE ACTIVITIES WITHIN THEIR INTERNAL ENVIRONMENT AND THE EXTERNAL ENVIRONMENT?

Part 1:Movement of Materials In

and Out of Cells

Introduction to Cell Transport

The movement of materials in and out of a cell is critcal to its functon and survival, allowing essental materials to enter while keeping waste materials out.

Investgate the way in which materials can move into and out of cells


PASSIVE TRANSPORT• Does not require energy• Generally high to low

concentraton

ACTIVE TRANSPORT• Requires energy• Generally low to high

concentraton



PASSIVE TRANSPORT• Simple Difusion• Facilitated Difusion• Osmosis

ACTIVE TRANSPORT• Endocytosis• Exocytosis


Difusion and Osmosis

Cell membrane is permeable to small molecules and lipid-soluble molecules due to phospholipids being made of lipids.

However, their lipid nature also makes cell membranes impermeable to:• Water-soluble molecules• Ions• Polar molecules


Difusion and Osmosis


Simple Difusion

• Does NOT require energy.• Movement of molecules from high to low concentraton.• Can move in and out of cell through phospholipid layer.

membrane


Simple Difusion


Simple Difusion

Three factors that can afect the rate of difusion:

1


ConcentratonsThe greater the diference in conc. Gradient, the faster the rate of difusion.

2TemperaturesHigh temp. = High rate of difusion Low temp. = Slow rate of difusion

3Partcle SizeSmall partcle = fast rate of difusion Big partcles = slow rate of difusion

Facilitated Difusion• Does NOT require energy.• Occur through transport proteins• Proteins are specifc for partcular partcles, so

transport is selectve• Faster than simple difusion• Transport of one partcle may be hindered by

the presence of another partcle using the same transport protein


OsmosisOsmosis is the net difusion of water molecules across a semipermeable membrane.

If a diluted and a concentrated soluton are separated by a semipermeable membrane that allows the movement of free water molecules will move across the membrane from the dilutes to the concentrated soluton.

Nothing but water can move across the membrane!


Osmosis


Osmosis in cells


Isotonic Soluton

• solute conc. inside = solute conc. outside• water molecules move in and out of cell freely• cells stay normal

Hypotonic Soluton

• solute conc. inside > solute conc. outside• water molecules move into the cell• cells swell

Hypertonic Soluton

• solute conc. inside < solute conc. outside• water molecules move out of the cell• cells shrink

Active Transport

The movement of molecules from a low concentraton to a region of high concentraton and requires an input of energy (ATP).

Eg. When kidney cells reabsorb glucose and amino acids so they are not lost in urine.

Investgate the way in which materials can move into and out of cells, including but not limited to:• Examining the roles of actve transport, endocytosis and exocytosis

Actve Transport

The carrier protein is required to actvely move these chemicals.


Transport of Large MoleculesEndocytosis

When a large partcle has to be moved into a cell, the cell membrane will change its shape to surround the partcle and engulf it by the process of endocytosis.

If a solid partcle is engulfed, the process is called phagocytosis (cell eatng).

If a fuid is engulfed, the process is calledpinocytosis (cell drinking).



Example:Unicellular amoeba feeds on a smaller organism.


Transport of Large MoleculesExocytosis

How:A membrane-bound vesicle moves towards the cell membrane, fuses with it and then releases its contents to the exterior of the cell. The vesicle membrane becomes part of the cell membrane.


Transport of Large MoleculesExocytosis

Why:• Specialised animal and plant cells

produce a variety of substances (antbodies, enzymes, neurotransmiters) that have important functons elsewhere in the organism.

• Get rid of waste produced inside cells


Factors Afecting the Exchange of Materials Across MembranesChemical Factors

• Uncharged molecules (eg. Ethanol) and lipid-soluble substances can easily penetrate the cell membrane because they dissolve in the phospholipid bilayer.

Investgate the way in which materials can move into and out of cells, including but not limited to:• Relatng the exchange of materials across membranes to the surface-area-to-volumes rato,

concentraton gradients and characteristcs of the materials being exchanged


• Hydrophilic charged ions (eg Sodium Na+) and water-soluble substances cannot cross through the hydrophobic centre of the membrane. Protein channels must be used.




• Water is not lipid-soluble therefore cannot move through the hydrophobic tails in the cell membrane. They must go through special aquaporins.



Factors Afecting the Exchange of Materials Across MembranesPhysical Factors (size and shape)

• Small molecules = difuse easily• Large molecules = difusion more difcult

• Examples:• Glucose and amino acids are large molecules that use carrier proteins to move

through membrane.

• Very large molecules = endocytosis/exocytosis



Factors Afecting the Exchange of Materials Across Membranes

Surface-Area-to-Volume Rato

• Small cells• more surface in relaton to its volume – higher

SA:V• Faster movement of substances between centre

of cell and surface

• Large cells• less surface in relaton to its volume – lower

SA:V• Slower movement of substances between centre

of cell and surfaceInvestgate the way in which materials can move into and out of cells, including but not limited to:• Relatng the exchange of materials across membranes to the surface-area-to-volumes rato,


Part 2:Cell Requirements

Cell Requirements

•Wherever life exists, it depends on a source of energy and a supply of mater. Cells need to obtain nutrients in the form of organic substances such as glucose, amino acids, faty acids and glycerol, nucleotdes and vitamins. Organic substances are synthesised by living things and contain carbon and hydrogen atoms.

Cell Requirements

•Cells also need inorganic nutrients such as gases (oxygen and carbon dioxide), minerals (for example phosphates, sodium ions and chloride ions) and water. These inorganic nutrients are part of the non-living world and do not contain carbon and hydrogen in long chains

• The substances needed by living cells for their functoning, are used in two main ways:

1. As essental building blocks from which cells and living tssues are made or

2. As a source of stored energy for the cell.

• Organic nutrients are the main supply of stored energy in living things, but they are also used in the structure of cells. Inorganic nutrients are essental as structural parts of cells and tssues

Cell Requirements

Part 3:Biochemical Processes

Autotrophs and HeterotrophsLiving things are divided into:

•Autotrophs:Living things that produce own organic materials

•Heterotrophs:Living things that consume organic material produced by others

Investgate the biochemical processes of photosynthesis, cell respiraton and the removal of cellularproducts and wastes in eukaryotc cells

Autotrophs and Heterotrophs

Chloroplast


Mitochondria

Carbon Dioxide and water Glucose and oxygen

Photosynthesis


Photosynthesis

Chloroplasts

• Photosynthesis occurs

• Where light energy isconverted to glucose (food) for the plant and oxygen is released.

• Chlorophyll found inside the chloroplast is green pigment absorbs the energy from sunlight.


PhotosynthesisStages of Photosynthesis

1. Light-dependent reactons

2. Light-independent reactons


Photosynthesis

Stage 1: Light-dependent reactons

• Chlorophyll captures solar energy and uses it to produce adenosine triphosphate (ATP)

• Photolysis occurs (water splits into H+ and O )2 (g)


Photosynthesis

Stage 2: Light-independent reactons

• Don’t need light in this step• ATP from stage 1 provides energy

2• Combine CO with

H+• Produce glucose, water and adenosine diphosphate (ADP)• Takes place in the Stroma

Stroma: Fluid part of the chloroplast


Biomechanical processes(beside photosynthesis)

Part 4:Enzymes

Enzymes

Enzymes are protein molecules that control all metabolic reactons in living cells.

Without them, chemical reactons in your body would be very slow.

They are catalysts

Control the rate of reactons

Enzymes

Enzymes are composed of protein molecules that are ofen highly folded to create a partcular shape. The surface of the enzymes with a specifc shape is called the ‘actve site’ which is where the reactants (substrates) in a chemical reacton bind to.

EnzymesModel 1: Lock and Key Model

EnzymesModel 2: Induced Fit Model

Enzymes- Factors Afecting Enzyme Reactions• Enzymes are temperature sensitve and functon best at the body temperature of the

organism in which they occur (usually 40°C)

• Rate of reacton increases as temperature increases untl the optmal temperature is reached.

• Above it, enzyme actvity slows untl it stops altogether at temperatures of 50–60°C(approx.).

• At high temperatures the moton associated with heat energy can make the protein structure change the shape of the actve site causing the enzyme to denature. Change is irreversible.

• Excessive cold also causes the enzyme to change shape and its functoning to slow down or stop. This change is reversible.

• Enzymes are pH sensitve Investgate the efects of the environment on enzyme actvity through the collecton of primary orsecondary data

cell structure 1 - cells as the... · 2020-04-22 · cell (organelles). eukaryotc organisms are...

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