Cell Structure And Its Functions

CELLS AND IT’S STRUCTURES

• What is cell?
• Cell is the smallest basic unit of life.

These are building blocks of human body.

Microorganisms such as amoebas and bacteria are single-celled that function independently.

• Human cells work together and each of them function independently.
• These cells vary in size, shape and functions.

All cells are made of water, carbon, oxygen, hydrogen and nitrogen.

CELL STRUCTURE

Despite their differences in size, shape and functions, cells have several similar structures; a cell membrane, a nucleus, cytoplasm, and cell organelles ( Ribosomes, mitochondria, centrioles, lysosomes, vacuole, endoplasmic reticulum, golgi body, cilia and microvilli).

CELL MEMBRANE

Are also called plasma membrane,

Is the thin semi-permeable membrane of the cells that controls movement of substances in and out of the cell.

• It also maintain the shape of the cell and keep it intact.

It is made up of a double layer of lipids (fats) and proteins.

Cell membrane molecules and their functions

• Cell membrane is made up of three organic molecules ie,
• Phospholipids
• Cholesterol and
• Proteins.

A. Phospholipids

Organic molecule made of diglycerides that forms a bilayer or double layer and makes up most of the membrane.

Permit lipid-soluble materials to easily enter orleave the cell by diffusion through the cell membrane.

B. Cholesterol

Play role by decreasing the fluidity of the membrane, thus making it more stable.

C. Proteins

• Functions of proteins….
• Proteins form channels or pores to permit passage of materials such as water or ions.
• Proteins are carrier enzymes or transporters that also help substances enter the cell.
• Acts as antigenic markers that identify the cells of an individual as “self.”

Some proteins serves as hormone receptors

Composed of double layer of phospholipids and proteins

• Surrounds outside of ALL cells
• Controls what enters or leaves the cell
• Living layer
• Outside
• of cell
• Inside
• of cell
• (cytoplasm)
• Cell
• membrane
• Proteins
• Protein
• channel
• Lipid bilayer
• Carbohydrate

chains

NUCLEUS

• Is the control center of the cell

Nucleus is also called the brain of the cell without it cell would die.

The nucleus contains genetic information called DNA (deoxyribonucleic acid). These genetic information (DNA) is arranged in threads called chromosomes.

• Every normal cell contains 23 pairs of chromosomes (total 46).
• Surrounded by a double membrane called the nuclear membrane.

NUCLEOLUS

This is a small spherical body made of DNA, RNA and protein The nucleoli form a type of RNA called ribosomal RNA which becomes part of ribosomes (a cell organelle) involved in protein synthesis.

Each cell has fixed number of

• chromosomes that carry genes
• Genes control cell

characteristics

Functions of the nucleus and chromosomes.

• Controls the cells functions
• Store for DNA and / RNA
• Site for transcription- DNA is transcribed into mRNA
• Nucleolus makes ribosomes for protein synthesis
• Centre for cellular growth
• For cell division
• Control enzyme synthesis
• Play role in reproduction and

Controls hereditary traits

Diagram of nucleus and chromosomes.

• Inside nucleus
• Disappears when cell divides

Makes ribosomes that make proteins

CYTOPLASM

• Is the gel like fluid found within the cell that suspend cell organelles
• Made up of water and dissolved substances like gases, glucose, waste, salts and hormones.
• Cytosol
• Is the water portion of cytoplasm, and many chemical reactions take place within it.
• Nucleoplasm

Is a gel like fluid founs within the nucleus of the cell.

CELL ORGANELLES

Cell organelles are intracellular structures bounded by their own membrane with specific functions in metabolism.

Mitochondria

Are oval or spherical organelles bounded by double membrane where energy (ATP) production takes place. Its also called power house of the cell.

• This energy is provided through a process called respiration.

Site of aerobic cell respiration ( ATP production)

Ribosomes

Are very small structures made of protein and ribosomal RNA.

Some are found on the surface of rough Endoplasmic reticulum, while others float freely within the cytoplasm.

Ribosomes are the site of protein synthesis.

Endoplasmic Reticulum (Smooth and Rough)

• This consists of a network of tiny tubes, or channels running throughout the cytoplasm.
• These act as a passageways for transport of materials within the cell
• ER associate with ribosomes in protein production and transport.
• Site for most enzyme activities.

Golgi Body

This is a large organelle responsible for processing proteins and lipids. These may include hormones, or other proteins required elsewhere in the body.

The Golgi body packages and stores these in smaller organelles called the Golgi apparatus until they are required in the cell or other part of the body.

Main packaging centre of the cell

Cilia and

Are hair-like projections on their surface of the cell lining the nose and respiratory tract that sweep the materials across the cell surface.

• These cilia beat to trap dust, bacteria and viruses and push them out of the body.
• Flagella
• Hair like projections on the cell surface that provides motility, or movement.
• Forexample, flagellum in the sperm cell

Microvilli

Are finger like projections over the cell surface that increase the available surface area for absorption.

Cells in the intestinal surface layers typically have lots of microvilli.

Lysosomes

These single membrane organelles act as the ‘stomach’ or ‘waste disposal unit’ of the cell.

Contain the digestive enzymes which can digest ingested material or damaged tissue ( break down, foreign bodies and worn out cell parts) Liver cells contain the greatest concentration of lysosomes for removing waste products from the body.

Centrioles

• Are rod-like structures that lie at right angles to each other in the cytoplasma.

Are essential for cell division (cell mitosis).

FUNCTIONS OF THE CELL

Cells have many functions that help them to survive.

• Movement – whole cells can move (white blood cells) or parts of cells can move (cilia).
• Respiration – we take in oxygen into our lungs during breathing and expel carbon dioxide as waste. This is essential for us to survive. Cells too must take in oxygen and use it to create energy and to carry out work (metabolism).
• Sensitivity – cells can respond to stimuli (physical, chemical or thermal).

Cell Structure And Its Functions

• Reproduction – when human body cells have grown to their full capacity, they divide and reproduce by the process of mitosis.
• Excretion – cells must get rid of any waste produced from metabolism. The waste such as carbon dioxide, urea, is passed outside the cell via the cell membrane.
• Metabolism – this is the term to describe all the chemical reactions and physiological processes that occur in a cell.

CELLULAR TRANSPORT MECHANISMS.

Living cells constantly interact with the blood or tissue fluid around them, taking in some substances and secreting or excreting others. There are several mechanisms of transport that enable materials to move in and out of the cell.

• Diffusion
• Osmosis
• Facilitated diffusion
• Active transport
• Filtration
• Phagocytosis

Pinocytosis

1. DIFFUSION

Is the movement of materials ie gases such as carbon dioxide and oxygen across the semi-permeable membrane from a region of high concentration to an area of low concentration.

Forexample, the metabolism taking place inside the cell, alots of waste carbon dioxide is building up inside. So, it passes by diffusion from inside the cell where its concentration is high, to outside the cells where its concentration is low AND exchange of gases in the lungs or body tissues.

• OSMOSIS

Is the movement of water across a semi-permeable membrane from a region of high concentration to a region of low concentration until both sides are equal.

Forexample, the cells lining the small intestine absorb water from digested food by osmosis and in the kidneys water tend to be reabsorbed back to prevent its lose in urine,.

3. FACILITATED DIFFUSION

Is the movement of molecules from area of high concentration to area of low concentration but by the use of carrier enzyme or transporter.

Forexample, in the body intake of glucose by most cells reguire transporter.

Glucose will not diffuse through the cell membrane by itself regardless of concentration gradient. It requires glucose transporter which are also called carrier enzyme.

• ACTIVE TRANSPORT

Is the movement of molecules from lesser concentration to higher concentration by the use of energy.

Forexample, absorption of amino acids and glucose from the small intestine to the cell, requires energy and sodium and potassium pumps in muscle and nerve cell.

5. FILTRATION

Is the movement of water and dissolved substances from an area of higher pressure to an area of lower pressure (blood pressure).

• Forexample, formation of tissue fluids or formation of urine.
• PHAGOCYTOSIS

Is the process in which phagocyte ( white blood cell) engulf and destroys the moving large particle Forexample, white blood cells engulf bacteria

• PINOCYTOSIS

Is the process in which the cell may take in small molecules that become adsorbed or attached to their membranes.

Forexample, the cells of the kidney tubules reabsorb small so that proteins dont get lost in urine.

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