Cells and tissues: Class 9 Biology

Key points

The cell is the basic unit of structure and function of living things. Robert Hooke first saw cells in 1665 in thin slices of cork. Cell theory (Schleiden, Schwann and Virchow) says that all organisms are made of one or more cells, the cell is the basic unit of life, and new cells arise only from existing cells.

The light microscope uses light and glass lenses and can magnify up to about 1500 times. The electron microscope uses a beam of electrons and magnifies hundreds of thousands of times, showing fine detail of organelles. Magnification = size of image ÷ actual size of object. Resolving power is the ability to show two close points as separate. Electron microscopes have a much higher resolving power, but they cannot be used on living specimens.

Parts of the cell and their functions:

• Cell wall (plants only): made of cellulose, it is a strong, fully permeable outer layer that gives shape and support.
• Cell membrane: a thin, selectively permeable layer of lipid and protein that controls entry and exit of substances. It is described by the fluid mosaic model.
• Cytoplasm: the jelly like material in which the organelles lie and many chemical reactions occur.
• Nucleus: has a double membrane with pores, chromatin (DNA with protein) and a nucleolus. It controls the activities of the cell and holds hereditary information.
• Mitochondria: the sites of aerobic respiration, where ATP is made. They have a folded inner membrane (cristae).
• Ribosomes: tiny granules where proteins are made.
• Endoplasmic reticulum (ER): a network of membranes. Rough ER has ribosomes and makes and transports proteins. Smooth ER makes lipids.
• Golgi apparatus: stacks of flattened sacs that modify, pack and secrete substances.
• Lysosomes: sacs of digestive enzymes that break down worn out parts and foreign matter.
• Chloroplasts (plants): contain chlorophyll and carry out photosynthesis.
• Vacuole: a sac of cell sap. It is large and central in a mature plant cell and gives turgidity.
• Centrioles (animal cells): help in forming the spindle during cell division.

Prokaryotic cells (bacteria) are small, have no true nucleus, no membrane bound organelles and a single circular DNA in the cytoplasm. Eukaryotic cells (plants, animals, fungi, protists) are larger and have a true nucleus and membrane bound organelles.

Plant and animal cells differ. Plant cells have a cellulose wall, chloroplasts and a large central vacuole, and usually a fixed regular shape. Animal cells have no wall or chloroplasts, have small vacuoles if any, have centrioles and are usually round or irregular in shape.

Movement of substances across the membrane:

• Diffusion: net movement of particles from a region of higher concentration to lower concentration, down a concentration gradient. It needs no energy.
• Osmosis: movement of water from a region of higher water potential (dilute solution) to lower water potential (concentrated solution) through a selectively permeable membrane.
• Active transport: movement of substances against the concentration gradient using energy (ATP) and carrier proteins.
• Endocytosis and exocytosis: bulk movement into and out of the cell by vesicles.

Osmosis explains the state of cells. In a hypotonic solution an animal cell takes in water and may burst, while a plant cell becomes turgid, which is good for support. In a hypertonic solution the cell loses water: an animal cell shrinks and a plant cell becomes flaccid, and when the membrane pulls away from the wall it is called plasmolysis. In an isotonic solution there is no net movement.

In multicellular organisms cells become specialised for particular jobs, for example red blood cells for carrying oxygen and nerve cells for sending messages. A tissue is a group of similar cells that work together. Organs are made of several tissues.

Animal tissues:

• Epithelial tissue: covers and lines surfaces, for example the skin and the lining of the gut. It protects, absorbs and secretes.
• Connective tissue: joins and supports other tissues, for example bone, cartilage, tendons, ligaments and blood.
• Muscular tissue: contracts to cause movement. It is skeletal, smooth or cardiac.
• Nervous tissue: carries electrical impulses and coordinates the body.

Plant tissues:

• Meristematic tissue: cells that keep dividing, found at root and shoot tips and in the cambium. It causes growth.
• Permanent tissue: made of cells that have stopped dividing. It includes epidermis, parenchyma (storage and photosynthesis), collenchyma and sclerenchyma (support), xylem (carries water) and phloem (carries food).

Study tip: Practise labelled diagrams of a plant cell and an animal cell, with a one line function for each part. For magnification questions, write the formula, convert to the same units and then divide. In osmosis answers always use the words water potential and selectively permeable membrane.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Numericals with worked steps

Common mistakes

• Saying that osmosis is the movement of any particles. It is only the movement of water through a selectively permeable membrane.
• Writing that the cell wall controls what enters the cell. The cell wall is fully permeable, and the membrane is the controlling layer.
• Forgetting to convert units (cm and µm) before calculating magnification.
• Calling the mitochondria the place where energy is made. Energy is released from food and stored in ATP.
• Mixing up turgid and flaccid, or hypotonic and hypertonic.

Practise this chapter

Practice questions and timed test Printable worksheet