Cell – the Unit

8/11/2019 Cell the Unit

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The Cell

Cell the unit, or the building block, of all living

things, plants and animals alike.

- Cells carry out all the chemical activities needed

to sustain life.

A cell is composed primarily of four (4)

elements:

Carbon

Hydrogen

Oxygen

Nitrogen

Trace elements

Living matter is over 60% water.

The major building material of the cell is

protein.

Cells vary in size, from microscopic to over a

meter in length.

Shape often reflects function.

For example:

muscle cells are elongated to allowshortening.

ANATOMY OF A GENERALIZED CELL

Cells have three (3) major regions

Nucleus, Cytoplasm and Plasma membrane.

A. The Nucleus

The nucleus or control center directs cell

activity and is necessary for reproduction.

The nucleus contains genetic material (DNA),

which carries instructions for protein

synthesis.

1. Nuclear Envelope or nuclear membrane, is a

double membrane barrier in which in between isa fluid filled space.

* These membranes fuse at some points

forming the nuclear pores.

* These is a selectively permeable membrane

but much freer because of the pores.

* This membrane encloses a jelly-like fluid

called nucleoplasm in which the nuclei and chromatin

are suspended.

2. Nucleoli any one or more small dark staining

essentially round bodies.

* These are the sites where ribosomes are

assembled before they migrate to thecytoplasm.

3. Chromatin these are loose network of bumpy

threads scattered throughout the nucleus.

* It contains DNA combined to proteins when

the cell is not dividing.

* But during mitosis, chromatin threads coil

and condense to form dense rod-like bodies called

as chromosomes.

B. The Plasma Membrane

- The plasma membrane limits and encloses

the cytoplasm.

- It acts as a selective barrier to themovement of substances into and out of the cell.

* It is composed of a lipid bilayer containing

proteins.

* The water-impermeable lipid portion forms

the basic membrane structure.

* The proteins (many of which are

glycoprotein):

- Act as enzymes or carriers in membrane

transport.

- Form membrane channels or pores.

- Provide receptor sites for hormones and

other chemicals

- Play a role in cellular recognition and

interactions during development and immune

reactions.

Specializations of the Plasma Membrane

1. Microvilli

- Finger-like projections that increase

surface area for absorption

2. Cell junctions

a. Tight Junctions

b. Desmosomes

c. Gap Junctions

C. The Cytoplasm

- It is where most cellular activities

occur.

Its fluid substance, the Cytosol, contains:

1. Inclusion bodies

- These are non-functioning

units in the cytoplasm containingstored or inactive materials.

a. Lipid droplets common in

fat cells

b. Glycogen granules abundant in liver

cells

c. Pigments such as melanin seen in

skin & hair cells

d. Mucus & other secretory products

e. Various types of crystals

2. Cytoplasmic Organelles

- These are specialized cellularcompartments each with a specific

function.

A. Mitochondria are tiny sausage-

shaped organelles.

- These are the sites of

ATP synthesis.

- The walls are double-

membrane, the outer layer is

smooth and the inner membrane has shelf-

like protrusions called cristae.

- Enzymes in the

cristae and those dissolved in themitochondrial fluid carry out the reactions in

which oxygen is used to break down

food.

- Energy is released,

most escape as heat some are

captured and used to form ATP molecules.

- Thus, the

mitochondria are referred as the

powerhouse of the cell.

- These are found

mostly on metabolically active

cells, like the liver and muscle cells. B. Ribosomes are tiny round dark

bodies made of

proteins and one variety of RNA

called ribosomal RNA.

* Are actual sites of

protein synthesis in the cell.

* Some ribosomes

float free in the cytoplasm, but

some are attached to membranes of


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endoplasmic reticulum collectively

called as rough endoplasmic

reticulum.

C. Endoplasmic Reticulum is a

system of fluid-filled cisterns

(tubules or canals) that coil and twist the

cytoplasm.

* These accounts

about half of all the membranes

in a cell.

* These serves as a

circulatory system for the cell

by providing a network of channels for

carrying substances from one part of the cell

to another.

There are two (2) forms of endoplasmic

reticulum:

i. Rough endoplasmic reticulum is

rough because of the

presence of ribosomes.

* This is where all ofthe building materials of

cellular membranes are formed the cells

membrane factory.

*In the tubules,

proteins produced from the

ribosomes are transported to other parts of

the cell.

* The number of ER a

cell has is proportional to the

amount of proteins it produces.

*Rough ER are especially abundant in

cells that export protein products like cellsof the pancreas which produces enzymes.

ii. Smooth endoplasmic reticulum is

the continuation of the

rough ER, without the ribosomes, thus

plays no role in protein

synthesis.

Instead it has functions in:

Cholesterol synthesis and

breakdown.

Fat metabolism.

Detoxification of drugs.

Liver cells are full of smoothER and cells that produce steroid-based

hormones (e.g. cells of the testes).

D. Golgi apparatus appears as a stack of

flattened

membranous sacs, associated with swarms

of tiny vesicles.

* Its major function is modifying and

packaging proteins or substances sent by the RER

via transport vesicles.

* When proteins accumulate in the

Golgi apparatus, the sacs swell and pinch off

to form vesicles.* It is the principal controller of the

cellular proteins directions or

destinations.

Three (3) ways of packaging and transporting the

products of the RER:

Vesicles containing proteins to be secreted

becomes a secretory vesicle, where it fuses

with the plasma membrane and eject it to the

outside of the cell.

Vesicles containing membrane components

fuse with the plasma membrane and become

part of the cell wall.

Vesicles containing packaged hydrolytic

enzymes become a lysosome that remains in

the cytoplasm.

E. Lysosomes are membranous sacs

containing powerful

digestive enzymes.

* They are capable of digesting

worn-out or nonusable cell

structures and most foreign substances that

enters the cell.

* They function in carrying out

intracellular digestion or the cells

demolition sites.

* They are abundant in white

blood cells that engulf bacteria and other

harmful substances.

HOMEOSTATIC IMBALANCE: The membrane

of the lysosomal sac isordinarily quite stable but becomes

fragile in certain circumstances such

as:

Cell

injury

Cellular oxygen deprivation

Excessive amounts of vitamin A

*Consequently, the lysosomes

rupture and result in self- digestion of

the cell.F. Peroxisomes are membranous sacs

containing powerful

oxidase enzymes that use

molecular oxygen to detoxify a number of

harmful substances such as alcohol

and formaldehyde.

Their most important function is to clean free

radicals which are highly reactive chemicals

with unpaired electrons that can scramble the

structure of proteins and nucleic acids.

They convert free radicals to hydrogen

peroxide (H202), and then hydrogen peroxideis converted by the enzyme catalase into

water.

They are especially numerous in liver and

kidney cells which are active in detoxification.

They do not originate from the Golgi

apparatus; they replicate themselves by

pinching into halves.

F. Cytoskeleton this elaborate network of protein

structures that extends throughout

the cytoplasm.

This act as the cells internal framework that:

Determines the cells shape.Supports other organelles.

Provides the machinery needed for

intracellular transport.

Provides the machinery for various

types of cellular movements.

This is made up three (3) elements (from largest to

smallest):


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Intermediate filaments these are strong

stable ropelike

filaments.

They help form Desmosomes and

provide internal meshwork to resist pulling

forces on the cell.

Microfilaments such as actin and myosin,

are most involved in cell motility

and in producing changes in cell shape.

Microtubules are tube like filaments that

determine the overall shape of a cell

and the distribution of organelles.

They are important during cell

division.

G.Centriolesthese are paired rod-shaped bodies

that lie at right angles to each other.

Internally they are made up of fine

microtubules

They play a role in cell division which they

direct the formation of the mitotic spindle.

They form the bases of cellular projections:i. Cilia these are whip like cellular

extensions that move substances

along the cell surface.

The ciliated cells of the respiratory system

lining move mucus up and away from the

lungs.

It is formed when centrioles multiply and

then line up beneath the plasma membrane at

the free cell surface.

Then microtubules begin to sprout from the

centrioles and put pressure on the membrane

forming the projections.ii. Flagella these are projections

formed by the centrioles but are

substantially longer.

The only example of a flagellated cell in the

human body is the sperm cell.

CELL DIVERSITY

* There are trillions of cells in the human body which

are made up of 200 different cell types.

* So the above is only a description of the average

cell.

* Cells vary greatly in size, shape and function.A. Cells that connect body parts:

Fibroblasts these are elongated shaped cells

which are attached along the

cable-like fibers that it secretes.

*Contains abundant rough ER and large

Golgi apparatus to make and

secrete the protein building blocks of

the fibers.

Erythrocytes (red blood cells) these are

biconcave disk shaped cells which provides extra

surface area for the uptake of oxygen.

*Its size and shape alsostreamlines the cell so it flows easily through

the bloodstream.

*Contains almost no organelles,

almost of the cell is occupied by

the oxygen-carrying pigment called

hemoglobin.

B. Cells that cover and line body organs:

Epithelial cells these are flat hexagonal shaped cells

that allow them to pack together in

sheets.

These cells have abundant intermediate

filaments that resist tearing when the epithelium is

rubbed or pulled.

C. Cells that move organs and body parts:

Skeletal muscle and Smooth muscle cells these are

elongated cells that can shorten

forcefully and move the bones or

change the size of internal organs.

Contains abundant contractile filaments.

D. Cells that stores nutrients:

Fat cells these are huge spherical shaped cells that

contains a large lipid droplet in its

cytoplasm.

E. Cells that fight disease:

Macrophage these are irregularly shaped cells

which extend long pseudopods used to

move and ingest infectious

microorganisms. Contains many lysosomes to digest foreign

harmful bodies.

F. Cells that gather information and controls body

functions:

Nerve cells (neurons) these are cells that

have long processes for receiving and

transmitting messages to other structures

of the body.

Contains extensive plasma membranes and

plenty of rough ER to synthesize membrane

components.

G. Cells of reproduction:Oocytes (female) these are the largest cells

in the body.

These egg cells contain many copies of all

organelles for distribution to the daughter

cells during fertilization.

Sperm cells (male) these are long and

streamlined cells, built for swimming to the egg for

fertilization.

Contains a flagellum that acts as a motile

whip to propel the sperm.

CELL PHYSIOLOGY

All cells exhibit irritability, digest foods,excrete wastes, able to reproduce, grow,

move and metabolize.

But before a cell reproduce itself and grow, it

should be in homeostasis meaning it should

have the right

environment, adequate nutrients and

substances used for protein

synthesis, adequate

oxygen and water and rid out of

wastes.

All the above requires transport across the

plasma membrane and all are suspended bythe fluid environment on both sides of the

membranes.

1. Intracellular fluid collectively

the cytoplasm or

cytosol, is a solution containing

small amounts of gases (oxygen and

carbon dioxide), nutrients and

salts, dissolved in water.


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2. Interstitial fluid is the fluid that

continuously bathes the

exterior of the cells.

It is a rich, nutritious fluid containing

thousands of ingredients including:

Nutrients amino acids,

sugars, fatty acids and

vitamins.

Regulatory substances

hormones and

neurotransmitters.

Salts sodium, potassium and

calcium.

Waste products

The plasma membrane is a selectively

permeable membrane - meaning that the

barrier or the membrane

allows some substances to pass through it

while it exclude others.

Plasma membrane allows nutrients to enter

the cell but keeps wastes products out. And at the same time, valuable cell proteins

are kept within the cell and wastes are

allowed to pass to the outside.

Transport of substances through the cell

membrane:

Movement of substances through the plasma

membrane happens basically in two (2) ways

1. Passive Transport Processes substances are

transported across the membrane

without any energy input from

the cell.

a. Diffusion is the movement of a substancefrom an area of its higher concentration

to an area of its lower concentration

moving down the concentration

gradient.

It occurs because of kinetic energy of the

molecules themselves.

The speed of diffusion is affected by the size

of the molecules (the smaller the faster) and

temperature (the warmer the faster).

Molecules will move passively through the plasma

membrane by diffusion if:

- they are small enough to passthrough its pores.

- they can dissolve in the fatty portion

of the membrane.

The unassisted diffusion of dissolved solutes

through the plasma membrane is simple

diffusion.

The diffusion of water through the plasma

membrane is osmosis.

Diffusion that requires a protein carrier is

facilitated diffusion.

Particles (molecules & ions) tend to distribute

themselves (throughout the available space)evenly within a solution

Movement is

from high

concentration

to low

concentration,

or down a

concentration

gradient

b. Filtration is the movement of substances

through a membrane from an

area of high hydrostatic

pressure to an area of lower fluid pressure.

This generally occurs only across capillary

walls.

In the body, the driving force of filtration is

blood pressure.

This is needed by the kidneys to filter blood

that passes by.

2. Active Transport Processes uses energy (ATP)

provided by the cell.

Substances move against concentration

gradient

a. Solute Pumping substances are moved

across the membrane

against an electrical or a

concentration gradient by proteins called

solute pumps.

This accounts for the transport of amino

acids, some sugars and most ions.

b. Bulk Transport there are two (2) types

of this kind of transport:

1. Exocytosis moves secretions and

other substances out of cells.

A membrane-bound vesicle fuses with the

plasma membrane, ruptures and ejects its

contents to the cell exterior.

2. Endocytosis particles are taken

up by enclosure in a plasmamembrane sac.

These includes two types of processes:

Phagocytosis uptake of solid particles.

Pinocytosis or bulk-phase endocytosis is the

uptake of fluids.

Osmotic pressure

Determines whether cells will gain or lose

water.

Reflects the volume concentration of a

solution.

There are three (3) kinds of solutions with different

osmotic pressures:Hypertonic solutions contain more solutes (and

Less water) than the cells.

In these solutions, cells lose water by osmosis

and cells will crenate.

Hypotonic solutions contain fewer solutes (and

more water) than do the cells.

In these solutions, cells swell and may

rupture (lyse) as water rushes in by osmosis.

Isotonic solutions contain the same solute-to

solvent ratio as with the cells.

In these solutions, there are no changes in cell

size and shape.


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Endocytosis

Exocytosis

Diffusion through the Plasma Membrane


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Cell – the Unit

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