Introduction to plant & animal CELL STRUCTURE & FUNCTION

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 (a) CELLS

 All living things are made up of cells, the building blocks of life.

 A cell is the smallest unit of life able to control its own activities, BUT, it relies on the rest of the organism (if multicellular) or the surroundings (if unicellular) to provide it with raw materials i.e. nutrients and removal of waste material.

 You should know and understand  that the structures of different types of cells are related to their functions.

 You should know and understand the similarities and differences between animal cells, bacteria and plant cells.

 Appreciate that it is important that dissolved substances must be able to get in and out of a cell through the cell membranes, otherwise the cell could not live or reproduce!

Cells can be either eukaryotic or prokaryotic in character.

Eukaryotes are organisms made of eukaryotic cells, which are complex cells, and all plants and animals are made up of such cells. They are usually multi-cellular organisms.

Prokaryotes, are smaller and simpler prokaryotic cells and single celled organisms (unicellular) eg bacteria.

The different parts of a cell are referred to as subcellular structures.

A diagrammatic comparison of animal cells, plant cells and bacteria cells - their similarities and differences.


(b) ANIMAL CELLS including humans! (eukaryotes)

Most animal cells have the following five parts - subcellular structures, and, remember, plants cells usually have the same five components too.

1. Cell membrane

The cell contents i.e. the cytoplasm, nucleus, (small vacuoles), mitochondria etc. are all held together by the soft cell membrane which controls the passage of substances in and out of the cell. The cell membrane allows the free passage of water and gases but may act as a barrier to other chemicals.

2. Mitochondria

The energy releasing chemistry of respiration occurs in the mitochondria, which is where most energy is released in respiration - eg the aerobic 'burning' of glucose to release energy.

e.g. glucose + oxygen ==> carbon dioxide + water + energy

The equation of aerobic respiration, an exothermic chemical reaction and catalysed by the appropriate enzymes.

Mitochondria are power house of the cells and provides the chemical energy for any of the cells functions.

Liver cells carry out lots of metabolic reactions so lots of energy needed, so they contain a lot more mitochondria.

Similarly, muscle cells need lots of energy eg to contract, so again, they have a lot more mitochondria than other cells to supply the energy for the physical work animals perform.

3. Cytoplasm

Cytoplasm is a jelly like fluid (gel-like) in which most of the cells chemical reactions take place and most of these reactions are catalysed by enzymes (biological catalysts) which control the rate of these reactions. Anaerobic respiration take place in the cytoplasm, but aerobic respiration takes place in the mitochondria.

4. Nucleus

The cell nucleus contains all the genetic material, the DNA of the genes in the chromosomes which control the cells functions and the cell division in replication. The nucleus controls the activities of the cell by sending instructions to the cytoplasm. The chromosomal /DNA contains the instructions for making proteins eg that make up tissue or enzymes.

5. Ribosomes

The ribosomes are where protein synthesis from amino acids occurs in the cell. They are too small to be seen by a light microscope.

Other features

Glycogen granules

Stored food for respiration.

Small vacuoles

Some animal cells may have several small vacuoles

Some differences between animal and plant cells

Animal cells are much larger than bacterial cells, with important differences from plant cells.

Animal cells do not have (i) a rigid cell wall, (ii) a permanent vacuole and (iii) chloroplasts.


(c) PLANT CELLS (eukaryotes)

Plant cells are much larger than bacterial cells, with important differences from animal cells.

Like animal cells, plants cells have (1) a cell membrane, (2) mitochondria, (3) cytoplasm, (4) nucleus and (5) ribosomes, all of which perform the same functions as described above.

The three principal differences between them is that most plant cells have (i) a rigid cell wall, (ii) chloroplasts and (iii) a permanent vacuole, which animal cells do NOT have.

(i) Plant and algal cells have a rigid cell wall made of cellulose, which strengthens the cell, supports it and therefore the plant's structure as a whole.

chloroplasts, which absorb light energy to make food via chlorophyll in photosynthesis

Be able to describe the function of the components of a plant cell including chloroplast, large vacuole, cell wall, cell membrane, mitochondria, cytoplasm and nucleus (see diagram and notes below) and know the differences between plant and animal cells.

(ii) Chloroplasts the sites of photosynthesis

The chloroplasts contain the green chlorophyll molecules which are involved in the energy absorbing process of photosynthesis. The chlorophyll molecules absorb the light energy from the sun to promote the endothermic reaction below. The chloroplasts must also contain all the enzymes to catalyse the whole series of complex reactions to make sugars - the equation below is a greatly simplified summary!

sunlight energy + carbon dioxide + water ==> sugars (e.g. glucose) + oxygen

6H2O(l) + 6CO2(g) ====> C6H12O6(aq) + 6O2(g)

Therefore chloroplasts are the site of food production for the plant. The sugars may be used directly as a source of energy or converted to starch grains - the plant's food store (and part of our food store as well!).

Chlorophyll absorbs mainly in the violet-blue and orange-red regions of the visible spectrum, hence it appears green, the light NOT absorbed.

(iii) Large permanent vacuole

Most plant cells have a single large permanent vacuole containing cell sap, a dilute solution of mineral salts and sugars.

Other features

Starch grains

Stored food for respiration from the glucose made by photosynthesis.


(d) BACTERIA (prokaryotes)

A bacterial cell consists of cytoplasm within a membrane surrounded by a cell wall.

Bacteria do not have a real nucleus, chloroplasts or mitochondria.

Cell wall - the cell contents i.e. the cytoplasm, DNA etc. are all held together within the cell wall surface membrane which controls the passage of substances in and out of the cell.

Cytoplasm - the jelly like fluid in which most of the cells chemical reactions take place with the aid of enzyme catalysts. Although they do not have mitochondria, bacterial cells can still respire aerobically in the cytoplasm.

Bacterial cells, single-celled microorganisms, are much smaller than plant or animal cells with some quite distinct and different features.

Chromosomal DNA - the genetic material

The genes are not in a distinct true nucleus, the genetic material is a sort of jumbled cluster comprising of one long strand of DNA, sometimes in the form of connected loops floating in the cytoplasm.

The single chromosome that controls the cells functions and the cell division in replication.

The chromosomal DNA moves freely around in the cytoplasm and is not confined in a distinct nucleus as in plant and animal cells.

Plasmid DNA

Plasmids are small hoops of extra DNA that are separate from the chromosomal DNA.

Plasmids contain genes that help tolerance against drugs and can be passed from one bacteria to another.

This is how the dangerous bacteria MSRA have evolved.

Shape and Flagella

Bacteria come in all sorts of shapes eg rods, spirals etc. and some have a tail!

The flagellum is a long thin tail like structure that projects out of the body of the cell, and can rotate to move the bacteria along.

Some bacterial cells have more than one flagella (flagellum).

Each flagellum is effectively driven by a tiny biochemical electric motor with moving parts, mostly made of proteins!

It is quite a remarkable piece of biochemical engineering - bioengineering!


(d) YEAST CELLS (eukaryotes)

Yeast is used in the production of alcoholic beverages eg beer, wine etc. and in bread making.

A yeast cell, a single-cell microorganism, has a nucleus, cytoplasm, mitochondria enclosed in a cell membrane which is surrounded by a cell wall.

diagram?


(e) CELL SPECIALISM - a few examples

Know and understand that cells may be specialised to carry out a particular function e.g.

Animal cells

Red blood cells to carry oxygen via their haemoglobin molecules.

White blood cells of the immune system.

Gamete cells ie egg cells and sperm cells are the sexual reproduction cells.

Plant cells

Palisade leaf cell structure is adapted to support the sites of photosynthesis.

Guard cells can open and close the pores (stomata) in leaves - they must allow oxygen and carbon dioxide to pass in and out.


(f) MICROSCOPES - use and development

Plant and animal cells can be studied in greater detail with a light microscope.

Microscopes enable you to objects (like microorganisms) which you cannot see with the naked eye.

Microscopes using the visible part of the electromagnetic spectrum (visible light) were invented in the late 16th century and the optical lens systems have been improved through the following centuries even until today.

With these light microscopes you can see individual cells and smaller details such as nuclei and mitochondria in all cells, and chloroplasts in plant cells.

Changes in microscope technology have enabled us to see cells with more clarity and detail than in the past, including simple magnification calculations.

In the 20th century, with advances in atomic physics, the electron microscope was invented which works off beams of electrons instead of visible light.

This has enabled the magnification produced by a microscope to be considerably increased to the point where you can see even smaller structures such as the internal details of mitochondria, chloroplasts and plasmids (hoops of DNA).

magnification = length of image / length of object


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