Cell structure and Organization

 

• All living things are made of cells.
• A cell is the smallest (they could be seen only through a microscope) unit that carries most processes.
• There are two types of cells:
  • plant cell;
  • animal cell

 

All typical cells have the following features.

 

• Cell membrane: differentially or partially permeable to allow certain substances to enter and leave the cell ie it controls movement of substances in and out of the cell.
• Cytoplasm: This is a jelly like substance. It is made up of mostly water and protein. Metabolic reactions occur in the cytoplasm
• Nucleus: contains DNA and controls the cell
• Mitochondria: organelle where aerobic respiration happens
• Ribosome: makes protein

 

cell walls

• permeable to salts and water through diffusion
• cell walls in plants is made up of cellulose fiber which is tough
• it supports the plant cell and maintain its shape.
• prevents the cell from bursting during turgor pressure

 

cell membranes

• partially permeable to different substances
• water passes through membranes by osmosis
• salts and mineral ions by active uptake which requires some energy
• some soluble substances pass through membranes by diffusion

 

Features that are only fount in plant cells;

• Vacuole: it stores food (ie salts and sugars) and water-helps to maintain shape of cell and controls movement of water in and out of the cell
• Cell wall: rigid to keep shape of cell
• Chloroplasts: contain chlorophyll, which absorbs light energy for photosynthesis

 

Specialized Cells

a) Red Blood Cells

they are found in the blood of animals, its function is to transport oxygen from the lungs to all the body cells, and carbon dioxide from the body cells to the lungs.

Adaptations of red blood cells;

• They have a biconcave disc shape that gives it a large surface area to carry more oxygen.
• They contain hemoglobin that combines with oxygen and carbon dioxide.
• They have no nucleus to carry more oxygen and CO2
• They are tiny enough to squeeze through capillaries.

 

b) Ciliated Cells

Ciliated cells are present in the trachea and bronchi of out respiratory system. They move mucus from the trachea up to the throat. The mucus traps bacteria and dust particles. The mucus is secreted by goblet cells which are next to ciliated cells.

Adaptations of ciliated cells;

• the tiny hair like projections called cilia which sweeps the contaminated mucus upwards

 

c) Muscle Cells

They are found in muscles in animals, they contract and relax together to move the organisms. Their function is to contract to support and move the body

Adaptations of ciliated cells;

• they are made of contractile filament to help in contraction
• it contains lots of mitochondria to supply the cell with energy

 

d) Root Hair Cells

They are found in the roots of the plant cells. They absorb water and minerals from the soil.

Adaptations of root hair cells;

• root hair elongates and increases the surface area for absorption of water and mineral solution
• they have a concentrated vacuole to help absorbing water by osmosis.
• they have a large number of mitochondria for respiration to become more active

 

e) Xylem Vessels

These are dead lignified cells which transport water and support plant

Adaptations of root hair cells;

• they are hollow to allow water and minerals to pass through them with no resistance
• they are strong and lignified to support the plant
• No cross walls so cells connect to form tube

 

 

All typical cells have:

• • Cell membrane: differentially or partially permeable to allow certain substances to enter and leave the cell.
  • Cytoplasm: where chemical reactions take place
  • Nucleus: contains DNA and controls the cell
  • Mitochondria: organelle where aerobic respiration happens
  • Ribosome: makes protein and can be found floating within the cytoplasm
• A typical animal cell (e.g. the liver cell) has all above

 

Only plant cells have:

• • Vacuole: stores food & water & helps to maintain shape of cell
  • Cell wall: rigid to keep shape of cell
  • Chloroplasts: contain chlorophyll, which absorbs light energy for photosynthesis
• A typical plant cell (e.g. the palisade cell) has all the above things.

 

Cell Organelles

1. The cell surface membrane or plasma membrane is a partially permeable membrane surrounding the cytoplasm of the cell. It controls substances entering
or leaving the cell.

2. The cytoplasm is the gel-like matrix embedded with organelles. It is the site of most cellular activities.

3. The cell vacuoles are small fluid-filled spaces bound by a membrane. In animal cells they store water and food substances. They are usually not permanent.

4. The nucleus is an organelle surrounded by an envelope called the nuclear envelope. It contains darker bodies called nucleoli (singular: nucleolus) and thread-like structures called chromatin which are made of DNA. The nucleus controls cellular activities such as growth, repair, and cell division.

6. The endoplasmic reticulum (ER) is a network of membranes forming tubes and flattened spaces. There are two types of ER:

(a) The smooth endoplasmic reticulum (SER) does not have ribosomes attached to it. It synthesises fats and steroids such as sex hormones. It also contains enzymes that detoxify drugs and poisons.

(b) The rough endoplasmic reticulum (RER) is studded with ribosomes. Ribosomes in the cell can either be free ribosomes (i.e. they lie freely in the cytoplasm) or be attached to the membrane of the RER. Ribosomes synthesize proteins.

7. All proteins made in the RER depart in membrane-bound vesicles to the Golgi
apparatus.

8. The Golgi apparatus resembles a stack of flattened disc-shaped spaces surrounded by membranes. It stores, sorts and modifies substances made by the ER, and packages them in vesicles to be secreted out of the cell.

9. The mitochondria (singular: mitochondrion) are small, elongated organelles with folded inner membranes. Aerobic respiration takes place in the mitochondria. Aerobic respiration is the process where energy is extracted from food substances in the presence of oxygen. This energy is used by the cell to perform cellular activities such as growth and cell division.

10. The centrioles are a pair of barrel-shaped structures at right angles to each other. They play a role in cell division. Centrioles are usually absent in plants.

 

Levels of Organization

Cell	Function	Adaptation(s)
Red blood cell	Transport of oxygen	Biconcave shape
		No nucleus
		Flexible
		Has hemoglobin
Muscle cell	Contracts to get structures closer together	Long
		Many protein fibres in cytoplasm to shorten cell when energy available
Ciliated cell	Move and push mucus	Tiny hairs called cilia
Root hair cell	Absorb minerals and water	Elongated shape for more surface area
Xylem vessel	Transport water and support plant	No cytoplasm so water passes freely
		No cross walls so cells connect to form tube
		Lignin makes it strong and waterproof
Palisade cell	Photo-synthesizes	Regular shape so many can fit in a small space
		Many chloroplasts

 

Levels of Organization

• Organelle: a specialized part of a cell that has its own function, e.g. the nucleus
• Cell: the smallest part of a living structure that can operate as an independent unit e.g. red blood cell
• Tissue: a group of cells with similar structures, working together to perform a shared function e.g. muscle tissue
• Organ: a structure made up of a group of tissues, working together to perform specific functions e.g. heart
• Organ system: group of organs with related functions, working together to perform body functions e.g. respiratory system

 

Organization of a multicellular organism

• The cell is the most basic unit of a living organism that can be classified as
  living.
•  A group of cells of the same type that are found near each other and carry
  out the same function comprises a tissue.
• An organ is made up of different tissues working together to perform a specific
  function or a group of functions within an organism. An organ has a distinct
  shape which allows it to carry out its function well.
• A group of functionally related organs form an organ system.
• Many organ systems working together make up a multicellular organism.