Cell – the basic unit of life

Cells

• Smallest living unit• Most are microscopic

Discovery of Cells

• Robert Hooke (mid-1600s)– Observed bark of oak tree– Saw “row of empty boxes”– Coined the term cell

The origin and evolution of cells

• On the basis of presence of nucleus cells are divided into two

groups:

– Prokaryotic cells: lack a nuclear envelope

– Eukaryotic cells: have a nucleus in which the genetic material is

separated from the cytoplasm.

• Prokaryotic cells are generally smaller and simpler than

eukaryotic cells; their genomes are less complex and they do

not contain cytoplasmic organelles.

Cell Types

• Prokaryotic

• Eukaryotic

Prokaryotic Cells

• First cell type on earth• Cell type of Bacteria and Archaea

Prokaryotic Cells

• No membrane bound nucleus• Nucleoid = region of DNA concentration• Organelles not bound by membranes

Prokaryotic Cells• Features shared by all prokaryotic cells:

– All have a plasma membrane.

– All have a region called the nucleoid where the DNA is concentrated.

– The cytoplasm (the plasma-membrane enclosed region) consists of the nucleoid, ribosomes, and a liquid portion called the cytosol.

Prokaryotic Cells• Specialized features of some prokaryotic

cells:

– A cell wall just outside the plasma membrane.

– Some bacteria have an outermost slimy layer made of polysaccharides and referred to as a capsule.

• Some bacteria have flagella, locomotory structures.

• Some bacteria have cillia, threadlike structures that help bacteria adhere to one another during mating or to other cells for food and protection.

Figure 4.5 A Prokaryotic Cell

Structure of Prokaryotic cell

• Using bacteria as our sample prokaryote, the following structures can be found in bacterial cells:

• Capsule - Found in some bacterial cells, this additional outer covering protects the cell when it is engulfed by other organisms, assists in retaining moisture, and helps the cell adhere to surfaces and nutrients

• Cillia - Hair-like structures on the surface of the cell that attach to other bacterial cells. Flagella - Long, whip-like protrusion that aids in cellular locomotion.

• Ribosomes - Cell structures responsible for protein production.Plasmids - Gene carrying, circular DNA structures that are not involved in reproduction.Nucleiod Region - Area of the cytoplasm that contains the single bacterial DNA molecule.

Cell Wall - Outer covering of most cells that protects the bacterial cell and gives it shape.

Cytoplasm - A gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.

Cell Membrane or Plasma Membrane - Surrounds the cell's cytoplasm and regulates the flow of substances in and out of the cell.

Eukaryotic Cells• Nucleus bound by membrane• Include fungi,, plant, and animal cells• Possess many organelles

Protozoan

Eukaryotic Cells• Eukaryotes, animals, plants, fungi have a

membrane-enclosed nucleus in each of their cells.

• Eukaryotic cells:

– tend to be larger than prokaryotic cells.

– have a variety of membrane-enclosed compartments called organelles.

– have a protein scaffolding called the cytoskeleton.

Eukaryotic Cells• Compartmentalization is the key to

eukaryotic cell function. • Each organelle has a specific role

defined by chemical processes.• Membranes surrounding these

organelles keep away inappropriate molecules and also act as traffic regulators for raw materials into and out of the organelle.

Representative Animal Cell

Structure Animal cells Plant cells

cell membrane Yes yes

nucleus Yes yes

nucleolus yes yes

ribosomes yes yes

ER yes yes

Golgi yes yes

centrioles yes no

cell wall no yes

mitochondria yes yes

cholorplasts no yes

One big vacuole no yes

cytoskeleton yes Yes

Cell Structures and Their Functions

Things in common

Cell membraneCytoplasmGenetic MaterialEnergy CurrencyEnzymes

Plasma Membrane

• Every cell is surrounded by a plasma membrane, a continuous membrane composed of a lipid bilayer with proteins floating within it and protruding from it.

Plasma Membrane

• The current model of how membranes are constructed is known as Fluid-Mosaic Model.

• Proposes that various molecules of membrane are able to flow and move about.

Phospholipids

• Polar– Hydrophylic head– Hydrophobic tail

• Interacts with water

Proteins

• Can be found on the surface of the membrane or within the membrane among phospholipids.

• Roles of the plasma membrane:

– Acts as a selectively permeable barrier.

– Is an interface for cells where information is received from adjacent cells and extracellular signals.

– Allows cells to maintain a constant internal environment.

– Has molecules that are responsible for binding and adhering to adjacent cells.

CELL WALL

• The cell wall is the tough, fairly rigid layer that surrounds some types of cells.

• It is located outside the cell membrane.• In bacteria, peptidoglycan forms the cell wall.• Fungi possess cell walls made of chitin.• In plants: cellulose

Functions

• provides these cells with structural support and protection.

• A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the cell.

Membranous Organelles

• Endoplasmic Reticulum• Golgi Apparatus• Lysosomes• Chloroplast• Mitochondrion• Peroxisomes• Nucleus

Endoplasmic Reticulum

• Is a set of folded membranes and tubes throughout the system.

• The general structure :membranous network of cisternae.

• Two types: rough endoplasmic reticulum smooth endoplasmic reticulum

Functions

• Provides a large surface area on which chemical reactions can take place.

• It synthesizes lipids, phospholipids and steroid.• It also carries out the metabolism of

carbohydrates, drug detoxification.• attachment of receptors on cell membrane

proteins.• RER involved in protein synthesis

Golgi Bodies

• The Golgi is composed of stacks of membrane-bound structures known as cisternae.

• Each cisternae comprises a flat, membrane enclosed disc that includes special Golgi enzymes.

• The cisternae stack has two main functional regions: the cis-Golgi network and trans-Golgi network.

• Vesicles from the endoplasmic reticulum fuse with the network and subsequently progress through the stack to the trans Golgi network, where they are packaged and sent to their destination

Functions

• Site of synthesis and packaging of molecules produced in cells.

• Involved in transportation of molecules s/a carbohydrates, proteins, insulin outside the cell.

Lysosomes

• Lysosomes are cellular organelles that contain hydrolase enzymes that break down waste materials and cellular debris.

Used in four major ways

• Cell damaged- membranes of Lysosomes break-enzymes release- components of cells broken- available for surrounding cells.

• Also play part in normal development of an organism.

• Lysosomes combine with food vacuoles. Eg: Paramecium

• Used in destruction of engulfed harmful microbes

Peroxisomes

• Peroxisomes are small membrane bound vesicles.

• In human cells, peroxisomes produce hydrogen peroxides that is used to destroy invading microbes.

• They contain enzymes responsible for breakdown of fatty acids and cholesterol synthesis.

Mitochondria• Have their own DNA- semiautonomous Structure :1.the outer mitochondrial membrane2.the intermembrane space3.the inner mitochondrial membrane4.the cristae space5.the matrix Located on their surface are enzymes

involved involved in respiration

Figure 4.14 A Mitochondrion Converts Energy from Fuel Molecules into ATP (Part 2)

Mitochondria

• Break down fuel molecules (cellular respiration)

– Glucose– Fatty acids

• Release energy– ATP

Mitochondria found in both plants and animals are called the power houses of the cell. They convert the energy in the bonds of glucose and oxygen to readily available energy in the form of ATP. This process is called aerobic respiration.

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Chloroplasts

Chloroplasts-organelles present in cells of plants and some other eukaryotes-contain chlorophyll for photosynthesis-surrounded by 2 membranes-thylakoids are membranous sacs within the inner membrane-grana are stacks of thylakoids

Nucleus

• Known as the “ control centre of the Cell”– a membrane-bound sac evolved to store the cell’s

chromosomes(DNA)– stores the genetic material of the cell in the form of

multiple, linear chromosomes– To control the activity of cell by regulating gene

expression

– surrounded by a nuclear envelope composed of 2 phospholipid bilayers

– in chromosomes – DNA is organized with proteins to form chromatin

• Nuclear membrane: • separates the nucleoplasm, liquid material in

the nucleus, from the cytoplasm• Nuclear pores: • Exchange of materials• allows thousands of large molecules such as

RNA to pass into and out of the nucleus each minute.

• Nucleolus:• Inside nucleus• Location of ribosome factory• Made of RNA

NON MEMBRANEOUS

• Ribosomes• Cytoskeleton• Centrioles• Cillia and Flagella

Ribosomes

• A ribosome is made from complexes of RNAs and proteins -ribonucleoprotein.

• Each ribosome is divided into two subunits: the smaller subunit and the larger subunit

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Cytoskeleton

Cytoskeleton-network of protein fibers found in all eukaryotic cells-supports the shape of the cell -keeps organelles in fixed locations-helps move materials within the cell

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Cytoskeleton

Cytoskeleton fibers include- microfilaments – responsible for cellular contractions, crawling, “pinching”-microtubules – provide organization to the cell and move materials within the cell-intermediate filaments – provide structural stability

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Cytoskeleton

• both cilia and flagella are constructed from microtubules

• both provide either locomotion for the cell or move fluid pass the cell

Cilia and flagella