8/9/11
Biology Revision
Topic:From Cells to Organisms

What is a Cell?
Definition:
Cells are tiny room-like structures that all living things are made of.They are the basic units of living things.

All cells are made of matter,which are made up of the elements carbon,hydrogen and oxygen.

Unicellular and Multicellular Organisms

• Unicellular organisms are living things that are made up of only one cell
• Multicellular organisms are living things that are made up of many cells.

Inside cells
Organisms have many different kinds of cells which exist in various shapes and sizes.However,all cells share some common features.

Cell's Part
Parts of Animal Cells

• Cell Membrane
• Mitochondrion
• Cytoplasm
• Small Vacuoles
• Nucleus

Part of Plant Cells

• Cell Membrane
• Mitochondrion
• Cytoplasm
• Big Vacuoles
• Nucleus
• Cell Wall
• Chloroplasts

Function of Parts of Cells

• Cell Membrane-Thin layer around the cell,allowing shape of cell to change partially permeable
• Mitochondrion-Small spherical or rod-shaped & Involved in the release of energy from food substances during cell respiration
• Cytoplasm-Jelly-like substance that fills the cell & Contains water and many other substances(fats,carbohydrates and proteins) where chemical reaction occur
• Small Vacuoles-Small spaces containing air,liquid or food particles & Numerous in animals
• Big Vacuoles-Single Large space containing liquid(cell sap) & Cell sap contains water and dissolved substances(sugar and salt) & Cell sap keeps cell firm by taking in water
• Nucleus-Control cell activities & Contains chromosomes
• Cell Wall-Thick later around the cell & Made up of tough substance(cellulose) & Supports the cell and give it a regular shape
• Chloroplasts-Small disc-like structure found in the cell of green plants & Contains chlorophyll(which is needed in the process of photosynthesis)

Mitochondrion

• Sausage-shaped like organelles; double membrane with inner membrane thrown into folds called cristae
• Site of respiration to release energy from food used by cell for growth and reproduction

Ribosomes

• Small round structures that are either attached to the rough endo plasmic reticulum or lie freely in cytoplasm
• They are needed to make proteins in the cell

What are Chromosomes?
Definition:
They are the thread-like objects found in the nucleus in each cell.
They carry information which controls:

• How the cell works
• What the organism will look like

Different organisms contains different numbers of chromosomes in their cells

• In a human cell there are 46(or 23 pairs) of chromosomes
• Each chromosome is made up of many genes;each gene contains instructions for a different feature of an organism

Difference between animal cells and plant cells
Animal Cells

• Does it have a cell wall?     No
• Does it have a cell membrane?     Yes
• Does it have a nucleus?     Yes
• Does it have a cytoplasm?     Yes
• Does it have vacuoles?     Yes-many,but small
• Does it have chloroplasts?     No
• Shape?     Can Change

Plant Cells

• Does it have a cell wall?     Yes
• Does it have a cell membrane?     Yes
• Does it have a nucleus?     Yes
• Does it have a cytoplasm?     Yes
• Does it have vacuoles?     Yes-One or a few,Large
• Shape?     Regular(Doesn't change)
• Does it have chloroplasts?     Present only in cells of green plants

Dead cells
A living plant cell has a cell wall,nucleus and cytoplasm

When a cell dies,the nucleus and cytoplasm become part of the cell wall.The remainder of the cell becomes a large space containing only air

Example:cork(A secondary tissue produced by the cork cambium, and forms the outer part of the periderm in a woody plant)

Cells-Two dimensional or Three dimensional
Question:Are cells two-dimesional or threee-dimensional?
 Answer: Cells are actually three-dimensional,not flat

Types of Cells
Different types of cells in a multicellular organism come in different shapes and sizes

Each type of cells does a particular job
Fat cells-Stores fat
Nerve cells-Carry messages
Guard cells-Allow gases in and out of leaves

Modification of cell structure

• A cell can become specialized for a specific function.
• There are many different types of cells in organisms.They differ in size and shape and are adapted to perform specific functions.

1. Root hair cell
2. Red blood cell
3. Xylem cells

Root hair cell

• Each root hair is a projection from an epidermal cell of the root.It extends into the soil to absorb water and dissolved minerals
• Root hairs are adapted for efficient absorption of water and dissolved mineral because:

1. Each root has a large number of root hair-increase the total surface for absorption
2. Very thin cell walls-increase the speed of absorption
3. Large vacuole to absorb as much water as it can
4. It is long and narrow-increase the surface area to volume ratio of the cell for faster absorption of water and dissolved mineral salts from the soil